SAFE: Sealed, Algorithm-Flexible Envelope
draft-sullivan-safe-01

Network Working Group N. Sullivan
Internet-Draft Cryptography Consulting LLC
Intended status: Standards Track 15 March 2026
Expires: 16 September 2026

 SAFE: Sealed, Algorithm-Flexible Envelope
 draft-sullivan-safe-01

Abstract

 SAFE defines an encryption envelope that encrypts a payload once for
 multiple recipients. Decryption can require multiple credentials in
 sequence (public keys, passphrases, or other registered methods), so
 that no single compromise reveals content. The format targets large,
 writable files: it supports streaming decryption, random-access reads
 at block granularity, and selective re-encryption of modified blocks
 without re-keying. Per-block Authenticated Encryption with
 Associated Data (AEAD) provides confidentiality and integrity and
 detects reordering, truncation, and extension. SAFE accommodates
 post-quantum key encapsulation without format changes, provides
 algorithm agility through IANA registries, supports recipient
 privacy, and defines application profiles for common deployment
 scenarios.

Status of This Memo

 This Internet-Draft is submitted in full conformance with the
 provisions of BCP 78 and BCP 79.

 Internet-Drafts are working documents of the Internet Engineering
 Task Force (IETF). Note that other groups may also distribute
 working documents as Internet-Drafts. The list of current Internet-
 Drafts is at https://datatracker.ietf.org/drafts/current/.

 Internet-Drafts are draft documents valid for a maximum of six months
 and may be updated, replaced, or obsoleted by other documents at any
 time. It is inappropriate to use Internet-Drafts as reference
 material or to cite them other than as "work in progress."

 This Internet-Draft will expire on 16 September 2026.

Copyright Notice

 Copyright (c) 2026 IETF Trust and the persons identified as the
 document authors. All rights reserved.

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 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents (https://trustee.ietf.org/
 license-info) in effect on the date of publication of this document.
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document. Code Components
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Table of Contents

 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
 2. Comparison with Related Formats . . . . . . . . . . . . . . . 6
 3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 9
 4. Conventions and Notation . . . . . . . . . . . . . . . . . . 10
 4.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 10
 4.2. Text Encoding . . . . . . . . . . . . . . . . . . . . . . 11
 4.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 12
 5. Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . 12
 5.1. Default Parameters . . . . . . . . . . . . . . . . . . . 12
 5.2. Algorithm Summary Tables . . . . . . . . . . . . . . . . 13
 5.2.1. AEAD Algorithms . . . . . . . . . . . . . . . . . . . 13
 5.2.2. Key Encapsulation Mechanisms . . . . . . . . . . . . 14
 5.2.3. Step Types . . . . . . . . . . . . . . . . . . . . . 15
 5.3. Algorithm Requirements . . . . . . . . . . . . . . . . . 15
 5.4. Hash Function and KDF . . . . . . . . . . . . . . . . . . 15
 5.4.1. SafeDerive . . . . . . . . . . . . . . . . . . . . . 16
 5.4.2. Random Generation (SafeRandom) . . . . . . . . . . . 16
 5.5. Encryption Parameters . . . . . . . . . . . . . . . . . . 17
 5.6. Steps . . . . . . . . . . . . . . . . . . . . . . . . . . 18
 5.6.1. Step Interface . . . . . . . . . . . . . . . . . . . 19
 5.6.2. Passphrase Step . . . . . . . . . . . . . . . . . . . 20
 5.6.3. HPKE Step . . . . . . . . . . . . . . . . . . . . . . 21
 5.7. Key Schedules . . . . . . . . . . . . . . . . . . . . . . 27
 5.7.1. KEK Schedule . . . . . . . . . . . . . . . . . . . . 28
 5.7.2. Sealing Encrypted-CEK . . . . . . . . . . . . . . . . 29
 5.7.3. Payload Schedule . . . . . . . . . . . . . . . . . . 30
 5.7.4. Epoch Key Derivation . . . . . . . . . . . . . . . . 31
 5.7.5. Per-block Nonces . . . . . . . . . . . . . . . . . . 31
 5.7.6. Nonce Constructions . . . . . . . . . . . . . . . . . 32
 5.7.7. Block Rewrite Rules . . . . . . . . . . . . . . . . . 33
 5.7.8. Snapshot Accumulator . . . . . . . . . . . . . . . . 34
 5.7.9. Block AAD . . . . . . . . . . . . . . . . . . . . . . 35
 6. File Layout . . . . . . . . . . . . . . . . . . . . . . . . . 36
 6.1. SAFE CONFIG . . . . . . . . . . . . . . . . . . . . . . . 36
 6.1.1. Block-Size Selection . . . . . . . . . . . . . . . . 38
 6.2. SAFE LOCK . . . . . . . . . . . . . . . . . . . . . . . . 39
 6.2.1. Readable Format . . . . . . . . . . . . . . . . . . . 39

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 6.2.2. Armored Format . . . . . . . . . . . . . . . . . . . 41
 6.2.3. LOCK Selection . . . . . . . . . . . . . . . . . . . 42
 6.3. Data Encoding . . . . . . . . . . . . . . . . . . . . . . 44
 6.3.1. Armored Encoding . . . . . . . . . . . . . . . . . . 44
 6.3.2. Binary Encoding . . . . . . . . . . . . . . . . . . . 45
 6.4. Payload Layouts . . . . . . . . . . . . . . . . . . . . . 45
 6.4.1. Linear Layout . . . . . . . . . . . . . . . . . . . . 46
 6.4.2. Aligned Layout . . . . . . . . . . . . . . . . . . . 47
 7. Compatibility and Migration . . . . . . . . . . . . . . . . . 49
 7.1. Handling Unknown Elements . . . . . . . . . . . . . . . . 49
 7.2. Versioning . . . . . . . . . . . . . . . . . . . . . . . 49
 7.3. Extension Points . . . . . . . . . . . . . . . . . . . . 50
 7.4. Application Profiles . . . . . . . . . . . . . . . . . . 50
 7.4.1. Objects . . . . . . . . . . . . . . . . . . . . . . . 50
 7.4.2. Streaming . . . . . . . . . . . . . . . . . . . . . . 50
 7.4.3. Edit . . . . . . . . . . . . . . . . . . . . . . . . 51
 7.4.4. FIPS Edit . . . . . . . . . . . . . . . . . . . . . . 51
 8. Security Considerations . . . . . . . . . . . . . . . . . . . 52
 8.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 53
 8.2. Sender Authentication Properties . . . . . . . . . . . . 53
 8.3. Integrity and Authenticity . . . . . . . . . . . . . . . 54
 8.4. Implementation Considerations . . . . . . . . . . . . . . 54
 8.5. Passphrase KDF Selection . . . . . . . . . . . . . . . . 55
 8.6. Recipient Anonymity and Trial Decryption . . . . . . . . 56
 8.6.1. Privacy Benefits . . . . . . . . . . . . . . . . . . 56
 8.6.2. Sender Anonymity . . . . . . . . . . . . . . . . . . 56
 8.6.3. Trial Complexity . . . . . . . . . . . . . . . . . . 57
 8.7. Denial of Service Considerations . . . . . . . . . . . . 57
 8.8. Hint Assignment . . . . . . . . . . . . . . . . . . . . . 58
 8.9. Nonce Generation and CEK Reuse . . . . . . . . . . . . . 58
 8.9.1. File Extension . . . . . . . . . . . . . . . . . . . 59
 8.9.2. Derived Nonces . . . . . . . . . . . . . . . . . . . 60
 8.10. Selective Editing Security . . . . . . . . . . . . . . . 60
 8.11. Key Identifier Collisions . . . . . . . . . . . . . . . . 61
 8.12. Key Commitment . . . . . . . . . . . . . . . . . . . . . 61
 8.13. AEAD Usage Bounds . . . . . . . . . . . . . . . . . . . . 62
 8.13.1. Lifetime Encryption Budget . . . . . . . . . . . . . 63
 8.13.2. Per-AEAD Analysis . . . . . . . . . . . . . . . . . 64
 8.13.3. Epoch Key Rotation . . . . . . . . . . . . . . . . . 65
 8.13.4. Relationship to Key Commitment . . . . . . . . . . . 67
 8.14. Algorithm Agility and Post-Quantum Support . . . . . . . 67
 8.15. Security Level and Design Notes . . . . . . . . . . . . . 68
 8.16. Downgrade Resistance . . . . . . . . . . . . . . . . . . 68
 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 69
 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 69
 10.1. SAFE AEAD Identifiers Registry . . . . . . . . . . . . . 69
 10.2. SAFE KEM Identifiers Registry . . . . . . . . . . . . . 70
 10.3. SAFE KDF Identifiers Registry . . . . . . . . . . . . . 71

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 10.4. SAFE Step Names Registry . . . . . . . . . . . . . . . . 72
 10.5. SAFE Config Options Registry . . . . . . . . . . . . . . 74
 10.6. SAFE Block Types Registry . . . . . . . . . . . . . . . 75
 10.7. Media Type Registration . . . . . . . . . . . . . . . . 76
 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 77
 11.1. Normative References . . . . . . . . . . . . . . . . . . 77
 11.2. Informative References . . . . . . . . . . . . . . . . . 79
 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 81
 Appendix B. Implementation Guide . . . . . . . . . . . . . . . . 87
 B.1. Encryptor Processing . . . . . . . . . . . . . . . . . . 87
 B.2. Decryptor Processing . . . . . . . . . . . . . . . . . . 88
 Appendix C. Error Codes for Testing . . . . . . . . . . . . . . 89
 Appendix D. Armored Data Arithmetic . . . . . . . . . . . . . . 91
 Appendix E. Selective Decryption . . . . . . . . . . . . . . . . 93
 E.1. Example: Armored Selective Block Decryption . . . . . . . 93
 Appendix F. Design Rationale . . . . . . . . . . . . . . . . . . 94
 F.1. Two-Tier Key Hierarchy . . . . . . . . . . . . . . . . . 94
 F.1.1. Benefits . . . . . . . . . . . . . . . . . . . . . . 94
 F.1.2. Trade-offs . . . . . . . . . . . . . . . . . . . . . 95
 F.2. Minimal Block AAD . . . . . . . . . . . . . . . . . . . . 95
 F.2.1. Rationale . . . . . . . . . . . . . . . . . . . . . . 95
 F.2.2. Security Properties . . . . . . . . . . . . . . . . . 95
 F.2.3. Alternative Designs Considered . . . . . . . . . . . 96
 F.3. Fixed HKDF Salt . . . . . . . . . . . . . . . . . . . . . 96
 Appendix G. Test Vectors . . . . . . . . . . . . . . . . . . . . 97
 Appendix H. X25519 Test Vector . . . . . . . . . . . . . . . . . 99
 Appendix I. Auth Mode Test Vector . . . . . . . . . . . . . . . 101
 Appendix J. Multi-Block Test Vector . . . . . . . . . . . . . . 103
 Appendix K. SafeDerive Test Vectors . . . . . . . . . . . . . . 105
 K.1. Hash=sha-256, L=32 . . . . . . . . . . . . . . . . . . . 105
 K.2. Hash=sha-256, L=16 . . . . . . . . . . . . . . . . . . . 105
 K.3. Hash=turboshake256, L=32 . . . . . . . . . . . . . . . . 105
 K.4. Hash=turboshake256, L=16 . . . . . . . . . . . . . . . . 106
 Appendix L. Defining New Step Types . . . . . . . . . . . . . . 106
 L.1. Example: Privacy Pass Steps . . . . . . . . . . . . . . . 106
 L.1.1. Shared Parameters . . . . . . . . . . . . . . . . . . 107
 L.1.2. ppkdf Step . . . . . . . . . . . . . . . . . . . . . 107
 L.1.3. ppkdf-pass Step . . . . . . . . . . . . . . . . . . . 108
 L.1.4. IANA Registry Entries . . . . . . . . . . . . . . . . 108
 L.1.5. Security Considerations for Privacy Pass Steps . . . 109
 L.2. Example: WebAuthn PRF Step . . . . . . . . . . . . . . . 109
 L.2.1. Step Definition . . . . . . . . . . . . . . . . . . . 110
 L.2.2. IANA Registry Entry . . . . . . . . . . . . . . . . . 111
 L.2.3. Security Considerations for WebAuthn PRF Step . . . . 112
 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 112

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1. Introduction

 SAFE is an encryption format for files and objects. A SAFE-encoded
 file contains an encrypted payload and one or more LOCK blocks. Each
 LOCK wraps a Content-Encryption Key (CEK) for one recipient; multiple
 LOCKs allow multiple recipients to decrypt the same payload without
 duplicating ciphertext. A LOCK can require several credentials in
 sequence (a passphrase AND a private key, for example), so neither
 factor alone suffices.

 The payload is split into fixed-size blocks, each encrypted with
 Authenticated Encryption with Associated Data (AEAD) [RFC5116] and a
 per-block random nonce. Blocks can be decrypted individually or
 streamed sequentially. An aligned binary layout (Section 6.4.2)
 places each ciphertext block at a predictable offset, enabling O(1)
 random reads. Per-block random nonces (Section 5.7.5) allow
 individual blocks to be re-encrypted without re-wrapping the CEK or
 touching other blocks.

 Existing formats address subsets of these capabilities; Section 2
 surveys the differences. SAFE provides algorithm agility through
 IANA registries (Section 10) and accommodates post-quantum key
 encapsulation mechanisms without format changes. Recipient privacy
 modes (Section 8.6) allow Hybrid Public Key Encryption (HPKE) steps
 to omit key identifiers, preventing passive observers from linking
 files to recipients. Application profiles (Section 7.4) provide
 baseline algorithm guidance for common deployment scenarios.

 For example, a deployment might require that documents be decryptable
 only with both a passphrase AND a recipient private key. The LOCK
 block for such a recipient would contain two Step lines:

 Step: pass(kdf=argon2id, salt=...)
 Step: hpke(kem=p-256, id=..., kemct=...)

 Both steps are evaluated with the known passphrase and private key to
 derive the key that wraps the CEK. Neither factor alone suffices.
 See Section 5.7.1.1 for the cryptographic details.

 The payload layer and LOCK layer serve distinct roles. The LOCK
 layer manages key encapsulation and multi-factor access control,
 wrapping the CEK independently for each recipient. The payload layer
 provides per-block authenticated encryption with support for random-
 access reads and selective re-encryption of modified blocks. A file-
 level commitment value precedes independently decryptable fixed-size
 blocks. Confidentiality and integrity are enforced per block. Block
 AAD authenticates both the block index and a final-block indicator,
 so a Decryptor can detect block substitution, reordering, truncation,

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 and extension (Section 5.7.9). A 32-octet commitment prefix derived
 from the CEK, the negotiated algorithm parameters, and a per-file
 salt provides uniform key commitment across all AEAD choices
 (Section 8.12). [FLOE] formalizes a similar set of properties under
 the term random-access authenticated encryption (raAE).

 SAFE's registered AEADs fall into two usage classes. Nonce-Misuse-
 Resistant (NMR) and non-NMR suites. Non-NMR suites (AES-256-GCM,
 ChaCha20-Poly1305, AEGIS-256, AEGIS-256X2) use stored per-block
 random nonces; their security depends on nonce uniqueness and the
 total number of block encryptions under a given payload key. The NMR
 suite (AES-256-GCM-SIV, where SIV denotes Synthetic Initialization
 Vector) uses derived nonces and tolerates nonce reuse, degrading to
 deterministic encryption rather than permitting plaintext recovery.
 Because block rewrites consume additional encryptions under the same
 payload key, the relevant usage budget is not just the current file
 size but the total lifetime block encryptions, including all
 rewrites. See Section 8.13 for per-AEAD analysis.

2. Comparison with Related Formats

 The following table compares SAFE with existing encryption formats on
 the capabilities most relevant to encrypted file storage. X
 indicates native support, P indicates that the capability is
 achievable but is not a design goal of the format, and - indicates no
 support.

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 +==============+====+===+===+=======+====+======+===+===+====+=======+
 |Capability |SAFE|JWE|CMS|OpenPGP| S/ |SFrame|Age|AEA|FLOE|Chunked|
 | | | | | |MIME| | | | | OHTTP |
 +==============+====+===+===+=======+====+======+===+===+====+=======+
 |Large-file | X | P | P | P | P | - | X | X | X | X |
 |framing | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Streaming | X | P | X | X | X | X | X | X | X | P |
 |decrypt | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Random-access | X | - | - | - | - | P | - | X | X | - |
 |reads | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Random-access | X | - | - | - | - | P | - | - | X | - |
 |writes | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Random per- | X | - | - | - | - | - | - | - | X | - |
 |block nonces | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Multi- | X | X | X | X | X | - | P | - | - | - |
 |recipient | | | | | | | | | | |
 |(single | | | | | | | | | | |
 |ciphertext) | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Multi-factor | X | P | P | - | P | - | - | - | - | - |
 |per recipient | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Algorithm | X | X | X | X | X | X | - | - | P | P |
 |agility | | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+
 |Restricts | X | P | P | P | P | P | X | X | X | - |
 |insecure | | | | | | | | | | |
 |configurations| | | | | | | | | | |
 +--------------+----+---+---+-------+----+------+---+---+----+-------+

 Table 1

 JWE ([RFC7516]) encrypts the entire plaintext as a single AEAD
 operation; its JSON Serialization wraps the Content-Encryption Key
 per recipient but defines no block structure for streaming or random
 access.

 CMS ([RFC5652]) wraps a content-encryption key per recipient and
 supports streaming, but defines no fixed-size blocks for random
 access or selective rewrite.

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 OpenPGP ([RFC9580]) similarly wraps a session key per recipient via
 PKESK packets and supports streaming through partial body lengths.
 Like CMS, it provides no block structure for random access or
 selective rewrite. Both CMS and OpenPGP provide recipient-level
 composition (multiple recipients, each with one key), not per-
 recipient multi-factor.

 SFrame ([RFC9605]) targets real-time media: low-latency per-frame
 AEAD for conferencing, not stored-object encryption.

 Age [AGE] streams fixed-size chunks with counter-derived nonces and
 wraps keys per recipient but deliberately avoids algorithm agility
 (single cipher suite, no registries). Counter-based nonces prevent
 selective editing; modifying any block requires re-encrypting the
 entire payload. Age's payload AEAD (ChaCha20-Poly1305) is not key-
 committing; [AGE-COMMIT] demonstrates that a malicious encryptor can
 craft recipient stanzas that unwrap to different file keys while the
 same ciphertext decrypts under each, limiting multi-recipient
 robustness against targeted attacks.

 AEA [AEA] encrypts segments (1 MB default, 256 per cluster) using
 hierarchical HKDF-SHA256 key derivation and AES-256-CTR with HMAC-
 SHA256 per segment (encrypt-then-MAC). Per-segment keys are derived
 deterministically from the main key and segment index, enabling
 parallel decryption and random-access reads without decrypting
 preceding segments. Key encapsulation uses Elliptic Curve Diffie-
 Hellman Ephemeral (ECDHE) P-256 for a single recipient or scrypt for
 passwords; there is no multi-recipient key wrapping and no algorithm
 agility (the cipher suite is fixed per profile).

 FLOE [FLOE] defines a rewritable authenticated encryption scheme
 (raAE) with epoch-based key rotation for segment-level random access.
 SAFE adapts FLOE's epoch key rotation (Section 5.7.4) but adds multi-
 recipient key wrapping, multi-factor authentication, and algorithm
 agility.

 Chunked OHTTP [I-D.ietf-ohai-chunked-ohttp] splits HTTP message
 bodies into individually encrypted chunks for incremental processing
 through an oblivious relay. It targets live HTTP streaming, not
 stored-object encryption, and provides no random access or multi-
 recipient support.

 SAFE's block construction builds on the STREAM [STREAM] streaming
 AEAD pattern (truncation detection via a last-block indicator in
 Section 5.7.9) and extends it with per-block nonces (Section 5.7.5)
 so that individual blocks can be re-encrypted without re-wrapping the
 CEK. Non-NMR suites use stored random nonces, which are compatible
 with AEAD implementations that provide only random IV generation,

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 including FIPS-validated AES-GCM modules per [NIST-SP-800-38D]
 Section 8.2.2. The NMR suite uses deterministic per-block nonces
 derived from the key schedule and block index, tolerating nonce reuse
 during rewrites (Section 8.9.2).

3. Protocol Overview

 This section summarizes the encryption and decryption procedures.
 Normative details appear in the referenced sections.

 Given a plaintext and a set of recipients (each defined by one or
 more credentials), an Encryptor produces a SAFE object:

 1. Select AEAD, Block-Size, and Hash (or use defaults).

 2. Generate a random 32-octet CEK using SafeRandom (Section 5.4.2).

 3. For each recipient, build a LOCK: generate step artifacts (salts,
 KEM ciphertexts), derive a KEK, and wrap the CEK.

 4. Generate a 32-octet per-file salt: SafeRandom(32, "SAFE-SALT").

 5. Derive payload_key and acc_key from CEK and salt (Section 5.7.3).

 6. Split plaintext into blocks; encrypt each with a per-block nonce.

 7. Compute the snapshot accumulator from block tags (Section 5.7.8).

 8. Write the file: optional CONFIG, LOCK blocks, and payload (salt
 is the first 32 octets of the DATA block).

 Given a SAFE object and the appropriate credentials (private keys,
 passphrases, or other step inputs), a Decryptor recovers the
 plaintext:

 1. Parse CONFIG, LOCK, and DATA blocks.

 2. Try each LOCK until one succeeds: evaluate its steps with the
 recipient's credentials to derive a KEK, then unwrap the CEK.

 3. Read the 32-octet salt from the start of the DATA block.

 4. Verify the commitment prefix.

 5. Derive payload_key and acc_key from CEK and salt (Section 5.7.3).

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 6. If all N block nonces and tags are available (i.e., not streaming
 or partial random-access reads), verify the snapshot accumulator
 (Section 5.7.8).

 7. Decrypt requested blocks.

 The CEK enables multi-recipient encryption (wrap once per recipient).
 The KEK binds each recipient's credentials to the CEK. The
 payload_key is derived from the CEK, the encryption_parameters (the
 AEAD, Block-Size, and Hash; Section 5.5), and the per-file salt,
 providing domain separation: distinct salts produce distinct payload
 keys even under the same CEK.

4. Conventions and Notation

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

 Header text is UTF-8. Base64 means [RFC4648] with padding equals
 signs and no line wrapping in the source value. When Base64 values
 appear in SAFE blocks, Encryptors SHOULD wrap lines at 64 characters;
 Decryptors MUST accept any line length and MUST ignore line breaks
 within Base64 values. LF denotes the newline U+000A; Encryptors MUST
 use LF, Decryptors MUST accept LF and MAY accept CRLF.

 String constants used in Encode AAD labels are ASCII and begin with
 SAFE- (e.g., SAFE-DATA, SAFE-STEP). SafeDerive labels are ASCII
 (e.g., commit, kek); the protocol prefix SAFE-v1 is added
 automatically.

 ABNF follows [RFC5234]. The following common ABNF rule is used
 throughout this document:

 BASE64CHAR = ALPHA / DIGIT / "+" / "/" / "="

 This rule is a loose character-class approximation. Implementations
 MUST validate Base64 encoding per [RFC4648] Section 4, including
 correct padding placement.

4.1. Notation

 This document uses the following notation:

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 +============+===============================================+
 | Symbol | Meaning |
 +============+===============================================+
 | || | Octet string concatenation |
 +------------+-----------------------------------------------+
 | XOR | Bitwise exclusive-or of equal-length octet |
 | | strings |
 +------------+-----------------------------------------------+
 | len(x) | Length of x in octets |
 +------------+-----------------------------------------------+
 | x[i:j] | Slice of x from octet i (inclusive) to j |
 | | (exclusive), zero-indexed |
 +------------+-----------------------------------------------+
 | uint8(n) | 8-bit unsigned integer n (single octet) |
 +------------+-----------------------------------------------+
 | I2OSP(n, | w-octet big-endian encoding of non-negative |
 | w) | integer n |
 +------------+-----------------------------------------------+
 | lp16(x) | I2OSP(len(x), 2) || x — 2-octet length- |
 | | prefixed encoding |
 +------------+-----------------------------------------------+
 | Encode(x1, | lp16(x1) || ... || lp16(xn) — multi-value |
 | ..., xn) | length-prefixed encoding |
 +------------+-----------------------------------------------+
 | ...x | List expansion: if x is a list, each element |
 | | becomes a separate argument; if x is a single |
 | | octet string, it is passed as one argument |
 +------------+-----------------------------------------------+
 | uint64(n) | 64-bit unsigned integer n in network byte |
 | | order (big-endian) |
 +------------+-----------------------------------------------+
 | floor(x) | Largest integer less than or equal to x |
 +------------+-----------------------------------------------+
 | ceil(x) | Smallest integer greater than or equal to x |
 +------------+-----------------------------------------------+

 Table 2

 All integers serialized in binary are unsigned and use network byte
 order (big-endian). Multi-byte integer fields are serialized most-
 significant byte first.

4.2. Text Encoding

 SAFE header lines (fence markers, field names, field values) MUST
 contain only ASCII printable characters (0x20-0x7E) plus LF (0x0A).
 Derive info strings and AEAD AAD prefixes use ASCII. Decryptors MUST
 reject malformed UTF-8 in text fields.

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 SAFE uses standard Base64 per Section 4 of [RFC4648]. Padding is
 REQUIRED. Base64 values in headers MAY wrap across lines;
 continuation lines MUST begin with whitespace. Decryptors MUST strip
 leading whitespace from continuation lines before decoding. In
 armored encoding, the DATA block's Base64 MAY contain line breaks;
 Decryptors MUST ignore them.

 Encryptors MUST use LF (0x0A) line terminators. Decryptors MUST
 accept LF and MAY accept CRLF. Decryptors MUST strip trailing
 whitespace from header lines.

 Case sensitivity: All field names, identifiers, and fence markers
 are case-sensitive.

4.3. Terminology

 CEK (Content-Encryption Key): A randomly generated 32-octet key used
 to derive the payload encryption key. The CEK is wrapped
 independently for each recipient.

 KEK (Key-Encryption Key): A 32-octet key derived from a LOCK's step
 sequence. Used to wrap or unwrap the CEK.

 Encryptor: The party that creates a SAFE-encoded file.

 Decryptor: The party that recovers plaintext from a SAFE-encoded
 file using appropriate credentials.

5. Algorithms

5.1. Default Parameters

 The following defaults apply whenever a CONFIG block is absent or
 when a field is omitted from the CONFIG block:

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 +===============+===============+
 | Field | Default Value |
 +===============+===============+
 | AEAD | aes-256-gcm |
 +---------------+---------------+
 | Block-Size | 65536 |
 +---------------+---------------+
 | Hash | sha-256 |
 +---------------+---------------+
 | Key-Epoch | (absent) |
 +---------------+---------------+
 | Lock-Encoding | armored |
 +---------------+---------------+
 | Data-Encoding | armored |
 +---------------+---------------+

 Table 3

 Implementations MUST use these values for any omitted fields. CONFIG
 need only include fields that differ from the defaults; see
 Section 6.1.

5.2. Algorithm Summary Tables

 This section provides a quick reference of all cryptographic
 algorithms and identifiers used in SAFE. Detailed specifications
 appear in later sections.

5.2.1. AEAD Algorithms

 +===================+===================+==+==+=====+=============+
 | Algorithm | Identifier |Nk|Nn| NMR | Key-Epoch |
 +===================+===================+==+==+=====+=============+
 | AES-256-GCM | aes-256-gcm |32|12| No | Recommended |
 +-------------------+-------------------+--+--+-----+-------------+
 | ChaCha20-Poly1305 | chacha20-poly1305 |32|12| No | Required |
 +-------------------+-------------------+--+--+-----+-------------+
 | AES-256-GCM-SIV | aes-256-gcm-siv |32|12| Yes | Not |
 | | | | | | Applicable |
 +-------------------+-------------------+--+--+-----+-------------+
 | AEGIS-256 | aegis-256 |32|32| No | Not |
 | | | | | | Recommended |
 +-------------------+-------------------+--+--+-----+-------------+
 | AEGIS-256X2 | aegis-256x2 |32|32| No | Not |
 | | | | | | Recommended |
 +-------------------+-------------------+--+--+-----+-------------+

 Table 4

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 Nk/Nn are key/nonce sizes in octets. "NMR" indicates nonce-misuse
 resistance (see Section 8.9). "Key-Epoch" indicates whether
 Encryptors should include Key-Epoch in CONFIG: Required means MUST,
 Recommended means SHOULD, Not Applicable means MUST NOT (NMR AEADs),
 Not Recommended means SHOULD NOT (256-bit nonce AEADs gain no
 practical benefit). See Section 6.1 and Section 5.7.4 for details.
 AEADs without NMR permit plaintext recovery under nonce reuse;
 Encryptors SHOULD select an NMR AEAD when nonce reuse cannot be ruled
 out. All AEADs provide [RFC5116] semantics with 16-octet tags. All
 SAFE DATA payloads begin with a 32-octet salt followed by a 32-octet
 commitment prefix derived per Section 5.7.3 that binds the ciphertext
 to the CEK, the negotiated algorithm parameters, and the per-file
 salt (see Section 8.12).

5.2.2. Key Encapsulation Mechanisms

 +============+============+=============+=============+======+
 | KEM | Identifier | HPKE KEM ID | Encap Size | Auth |
 +============+============+=============+=============+======+
 | X25519 | x25519 | 0x0020 | 32 octets | Yes |
 +------------+------------+-------------+-------------+------+
 | P-256 | p-256 | 0x0010 | 65 octets | Yes |
 +------------+------------+-------------+-------------+------+
 | ML-KEM-768 | ml-kem-768 | 0x0041 | 1088 octets | No |
 +------------+------------+-------------+-------------+------+

 Table 5

 HPKE KEM IDs are defined in Section 7.1 of [RFC9180] and the IANA
 HPKE KEM Identifiers registry. ML-KEM-768 enables hybrid post-
 quantum constructions via multi-step step sequences (see
 Section 5.6.3.2).

 All KEMs use HPKE [RFC9180] in export-only mode (AEAD ID 0xFFFF).
 The encryptor calls SetupBaseS (or SetupAuthS when sid or shint is
 present) to produce a KEM ciphertext and an HPKE context, then calls
 Export on the context to derive the step secret (Section 5.6.3.4).
 When a sender parameter is present, HPKE Auth mode is used (see
 Section 5.6.3.1). The KEM identifier appears in hpke(...) step
 tokens (Section 5.6.3). SAFE maintains a registry mapping string
 identifiers to HPKE KEM IDs (Section 10.2).

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5.2.3. Step Types

 +============+=========================+=================+========+
 | Step Type | Token Format | Parameters | Secret |
 +============+=========================+=================+========+
 | Passphrase | pass(kdf=..., salt=...) | kdf, salt | 32 |
 | | | | octets |
 +------------+-------------------------+-----------------+--------+
 | HPKE | hpke(kem=X, kemct=..., | kem, kemct, id/ | 32 |
 | | ...) | hint, sid/shint | octets |
 +------------+-------------------------+-----------------+--------+

 Table 6

 Step types are composed via multiple Step lines within a LOCK block,
 with AND semantics: all steps are required to derive the KEK. Each
 step conforms to the interface defined in Section 5.6 and produces a
 32-octet secret that contributes to KEK derivation (Section 5.7.1).
 Additional step types MAY be registered per Section 10.4.

5.3. Algorithm Requirements

 AEADs used with SAFE MUST provide [RFC5116] semantics with a 16-octet
 authentication tag. KEMs MUST use HPKE export-only mode (AEAD ID
 0xFFFF) as specified in Section 5.6.3. When sid or shint is present,
 HPKE Auth mode is used (Section 5.6.3.1).

5.4. Hash Function and KDF

 SAFE piggybacks on the HPKE KDF interface. HPKE [RFC9180] and
 [I-D.ietf-hpke-pq] define two KDF classes:

 Two-stage KDFs (e.g., HKDF-SHA256, KDF ID 0x0001):

 * KDF.Nh: output size of Extract (32 for HKDF-SHA256)

 * KDF.Extract(salt, ikm) -> prk (Nh octets)

 * KDF.Expand(prk, info, L) -> okm (L octets)

 Single-stage KDFs (e.g., TurboSHAKE256, [I-D.ietf-hpke-pq]):

 * KDF.Derive(ikm, L) -> okm (L octets). For TurboSHAKE256:
 TurboSHAKE256(M=ikm, D=0x1F, L).

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 The Hash config parameter selects both the KDF and its class. Any
 KDF registered for SAFE MUST be registered in the HPKE KDF
 Identifiers registry [RFC9180] and MUST implement either the two-
 stage (Extract/Expand/Nh) or single-stage (Derive) interface.

5.4.1. SafeDerive

 SafeDerive binds every derivation to the protocol version and a call-
 site label. The ikm and info parameters accept a single octet string
 or a list; list elements are individually lp16-encoded by Encode().

 Callers that require suite binding include encryption_parameters in
 the info argument. Config-independent derivations such as key
 identifiers (Section 5.6.3.3) omit it.

 Two-stage (HKDF):

 SafeDerive(label, ikm, info, L):
 prk = Extract(
 "SAFE-v1",
 Encode("SAFE-v1", label, ...ikm))
 return Expand(prk,
 Encode("SAFE-v1", label, ...info,
 I2OSP(L, 2)), L)

 Single-stage (Extendable Output Function (XOF)):

 SafeDerive(label, ikm, info, L):
 return Derive(
 Encode("SAFE-v1", label, ...ikm,
 I2OSP(L, 2), ...info), L)

 SafeDerive labels MUST be unique across all call sites within a
 single protocol version. The labels defined by this specification
 are reserved; future extensions and step type registrations MUST NOT
 reuse them. Similarly, SafeRandom labels (Section 5.4.2) MUST be
 unique and MUST NOT be reused by extensions.

5.4.2. Random Generation (SafeRandom)

 SafeRandom is the random generation function used for all encryptor-
 generated random values in SAFE. It requires a cryptographically
 secure pseudorandom number generator (CSPRNG).

5.4.2.1. Base Construction

 SafeRandom(n, label):
 return CSPRNG(n)

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 When no private key is available, SafeRandom returns raw CSPRNG
 output. The label parameter is reserved for use by the hedged
 construction (Section 5.4.2.2).

5.4.2.2. Hedged Construction

 When a long-term private key sk is available, the Encryptor SHOULD
 mix it into SafeRandom to defend against a weak or attacker-
 influenced RNG. The construction follows [RFC8937]: hedge_key is a
 deterministic function of sk (replacing the signature in [RFC8937]
 with a KDF, since SAFE does not require a signature scheme), and
 SafeRandom combines it with CSPRNG output via SafeDerive.

 hedge_key = SafeDerive("SAFE-HEDGE", sk, "", 32)

 SafeRandom(n, label):
 return SafeDerive(label,
 [hedge_key, CSPRNG(n)], "", n)

 An adversary who can predict CSPRNG output but does not know sk
 cannot predict the hedged values. Hedging does not prevent repeated
 output from RNG state duplication (VM snapshot restore, process fork
 without reseed); identical CSPRNG output produces identical hedged
 output regardless of the private key.

 Suitable private keys include any long-term key held by the Encryptor
 (an HPKE sender private key, an application-provided signing key, or
 similar). The key need not correspond to any LOCK step.

 Encryptors MUST use SafeRandom for all random values generated during
 SAFE encoding: CEK, payload salt, passphrase salt, lock_nonce, and
 block nonce base. HPKE internal randomness (Encap) is not hedged by
 default. Implementations whose HPKE library accepts an external
 randomness source SHOULD supply SafeRandom(Nrand, "SAFE-ENCAP")
 instead of raw CSPRNG output, where Nrand is the randomness length
 required by the KEM.

 A functioning CSPRNG is REQUIRED when no private key is available.
 See Section 8.9 for the security analysis.

5.5. Encryption Parameters

 The encryption_parameters is an ordered list of the effective
 parameters (defaults augmented by any config overrides):

 encryption_parameters = [aead_id, block_size, hash_id]

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 When Key-Epoch is present in CONFIG, the list includes it as a fourth
 element:

 encryption_parameters = [aead_id, block_size, hash_id,
 key_epoch_str]

 where key_epoch_str is the decimal ASCII representation of Key-Epoch
 with no leading zeros. When Key-Epoch is absent,
 encryption_parameters has three elements, preserving backward
 compatibility with files that predate this feature. Because Key-
 Epoch changes encryption_parameters, adding or removing it requires
 re-encryption of the payload and re-wrapping of the CEK in new LOCKs.

 aead_id, block_size, and hash_id are the ASCII string forms of the
 AEAD, Block-Size, and Hash parameters respectively. SafeDerive
 splices this list via ...encryption_parameters and applies lp16
 framing to each element (Section 5.4.1).

 AEAD identifiers MUST be lowercase ASCII and match exactly the
 registered values in Section 10.1. Block-Size MUST be rendered as a
 decimal string with no leading zeros (except for the value 0 itself).
 Hash identifiers MUST be lowercase ASCII and match exactly the
 registered values in Section 10.3.

 SafeDerive binds encryption_parameters throughout the key schedule:
 the KEK aggregator initialization and final derivation
 (Section 5.7.1), and each payload schedule call (Section 5.7.3). See
 Appendix G for a worked example.

5.6. Steps

 Step terminology at a glance:

 Step: <type>(<key>=<val>, ...)
 '--+--''------+------'
 | |
 step type step parameters
 | | |
 +---+---+ |
 | |
 v v
 step_token step_secret
 (Encode) (32 octets)

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5.6.1. Step Interface

 A step is a registered cryptographic operation that produces a
 32-octet step secret from user-supplied credentials or cryptographic
 material. Each step type MUST define:

 Step name: A unique ASCII identifier used in step tokens (e.g.,
 "pass", "hpke").

 Parameters: An ABNF grammar for step-specific parameters appearing
 in the token (e.g., kem=x25519, id=...).

 Derivation: A deterministic algorithm that produces exactly 32
 octets. The algorithm MUST be reproducible given the same inputs.
 The step definition MUST specify all required inputs for both
 encryption and decryption.

 KEK schedule integration: The step secret and binding step_token
 feed into the KEK schedule (Section 5.7.1) via
 SafeDerive("kek_step", [agg, step_secret], step_token, 32). The
 step secret (ikm) enters Extract; the step token (info) enters
 Expand. The on-wire step token appears verbatim in the LOCK
 block. Each step type defines an Encode form: the canonical
 binary encoding of its cryptographically relevant fields via
 Encode() (Section 4.1). The Encode form serves as the binding
 step_token in the KEK schedule. Display-only fields (label, hint,
 shint) are excluded. The binding forms for the built-in step
 types are:

 +=============+=============================================+
 | Step | Binding step_token |
 +=============+=============================================+
 | pass | Encode("pass", kdf, salt) |
 +-------------+---------------------------------------------+
 | hpke | Encode("hpke", kem, kemct, id) |
 +-------------+---------------------------------------------+
 | hpke (auth) | Encode("hpke", kem, kemct, id, "auth", sid) |
 +-------------+---------------------------------------------+

 Table 7

 String values (kdf, kem) are UTF-8; binary values (salt, kemct,
 id, sid) are raw decoded octets, not Base64.

 Fields computed for binding (hpke id, hpke sid, webauthn-prf rpid)
 may be omitted on-wire for privacy but are deterministically
 reconstructed during decryption and always appear in the binding
 form. For hpke, id is always present in the binding step_token

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 even when omitted from the on-wire token; it is computed during
 trial decryption. Similarly, sid is optional on-wire but always
 present in auth-mode binding; when omitted, it is computed from
 the candidate sender public key.

 Registration: New step types are registered via the IANA SAFE Step
 Names registry (Section 10.4) with Specification Required policy.
 The registration MUST include: step name, parameters grammar,
 inputs, derivation algorithm, Encode binding form, and any step-
 specific parameter definitions. See Appendix L for an example.

 label (OPTIONAL, any step): A human-readable display name intended
 to help users identify which passphrase, credential, or key to use
 during decryption (e.g., "Work laptop", "Recovery key"). The
 label is always excluded from the binding step_token and has no
 cryptographic effect. Encryptors MAY include a label in any step
 token; Decryptors MUST ignore it for binding purposes. The label
 value MUST match the grammar 1*(ALPHA / DIGIT / "-").

 Steps are registered in the IANA SAFE Step Names registry
 (Section 10.4). The following subsections define the initial
 registered steps.

5.6.2. Passphrase Step

 The passphrase step derives a 32-octet step secret from a user
 passphrase using a password-based KDF. The kdf parameter selects the
 algorithm:

 +==========+===============================+=======================+
 | KDF | Algorithm | Parameters |
 +==========+===============================+=======================+
 | argon2id | Argon2id [RFC9106] | m=65536 KiB, t=2, p=1 |
 +----------+-------------------------------+-----------------------+
 | pbkdf2 | PBKDF2-HMAC-SHA-256 [RFC8018] | iter=600000 |
 +----------+-------------------------------+-----------------------+

 Table 8

 The step token format is:

 pass(kdf=<kdf>,salt=<Base64>)
 pass(kdf=<kdf>,salt=<Base64>,label=<text>)

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 The kdf parameter is REQUIRED. The label parameter is OPTIONAL and
 is for display only; it is not included in the binding step_token
 (Section 5.6). Encryptors MUST generate a fresh 16-octet salt using
 SafeRandom(16, "SAFE-PASS-SALT") for each pass(...) step in a LOCK.
 Decryptors MUST reject pass(...) steps whose salt value does not
 decode to exactly 16 octets.

 Grammar:

 pass-step = "pass(" pass-params ")"
 pass-params = "kdf=" kdf-name "," "salt=" salt
 [ "," "label=" label-value ]
 kdf-name = "argon2id" / "pbkdf2"
 salt = 1*BASE64CHAR
 label-value = 1*( ALPHA / DIGIT / "-" )

 Encode form:

 Encode("pass", kdf, salt)

 Binding step_token: Encode("pass", kdf, salt).

 The step secret is computed as follows:

 For kdf=argon2id: Argon2id(passphrase, salt, m=65536, t=2, p=1,
 T=32) per Section 3.1 of [RFC9106].

 For kdf=pbkdf2: PBKDF2(PRF=HMAC-SHA-256, Password=passphrase,
 Salt=salt, c=600000, dkLen=32).

 In both cases, salt is the decoded value of the salt parameter.

 Implementations SHOULD prefer argon2id for its memory-hardness
 properties. Implementations MAY support pbkdf2 for environments
 where Argon2id is not permitted by policy.

5.6.3. HPKE Step

 The HPKE step token has three forms:

 hpke(kem=x25519, kemct=<Base64>, id=<Base64>) ; Identified mode
 hpke(kem=x25519, kemct=<Base64>, hint=<digits>) ; Hinted mode
 hpke(kem=x25519, kemct=<Base64>) ; Anonymous mode

 The parameters are:

 kem (REQUIRED): The KEM algorithm. Supported values: x25519, p-256,
 ml-kem-768.

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 kemct (REQUIRED): The Base64-encoded HPKE KEM encapsulated key
 material (the KEM ciphertext). This value MUST decode to the
 encapsulated key length for the selected KEM (see Section 5.2.2).
 Decryptors MUST reject hpke steps whose decoded kemct length does
 not match the KEM's encapsulated key length.

 id (OPTIONAL): The key identifier computed as SafeDerive("SAFE-SPKI-
 v1", spki_der, "", 32) using the configured Hash (default: sha-
 256). When present, Decryptors match this value against their
 local keys. When omitted, Decryptors perform trial decryption.
 See Section 5.6.3.3 and Section 8.6.

 hint (OPTIONAL): A 4-digit decimal value (0000-9999) assigned by the
 recipient out-of-band; not solely dependent on the public key.
 When present, Decryptors filter candidate keys to those associated
 with this hint in their local key storage. Mutually exclusive
 with id.

 Encryptors MUST include exactly one of: id, hint, or neither (but not
 both id and hint).

5.6.3.1. HPKE Auth Mode

 In Base mode, any party who knows a recipient's public key can create
 a valid SAFE object for that recipient. Auth mode [RFC9180] uses
 SetupAuthS/SetupAuthR, which bind the HPKE context to the sender's
 private key so that the Decryptor can verify who produced the object.
 This is useful for offline encrypted file exchange where the
 recipient needs assurance of origin (for example, encrypted firmware
 images, signed-then-encrypted document workflows, or air-gapped key
 escrow) without requiring a separate signature layer.

 The presence of sid or shint selects HPKE Auth mode (mode_auth)
 instead of Base mode. Auth mode MUST only be used with Diffie-
 Hellman KEM (DHKEM) based KEMs (x25519, p-256). Encryptors MUST NOT
 include sid or shint with ml-kem-768 or other non-DHKEM KEMs, because
 these KEMs do not define AuthEncap/AuthDecap.

 sid (OPTIONAL): The sender's key identifier, computed as
 SafeDerive("SAFE-SPKI-v1", spki_der, "", 32) using the same Hash
 as id. When present with a Base64 value, Decryptors match it
 against known sender public keys. The special value anon
 indicates anonymous sender Auth mode: the sender's key is not
 identified, and Decryptors perform trial decryption across
 candidate sender keys. Mutually exclusive with shint.

 shint (OPTIONAL): A 4-digit decimal value (0000-9999) assigned by

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 the sender out-of-band; parallels hint for recipient keys. When
 present, Decryptors filter candidate sender keys to those
 associated with this value. Mutually exclusive with sid.

 Encryptors MUST include exactly one of sid or shint (but not both)
 when using Auth mode.

 Auth mode token forms extend the base forms:

 hpke(kem=x25519, kemct=<B64>, id=<B64>, sid=<B64>)
 hpke(kem=x25519, kemct=<B64>, sid=<B64>)
 hpke(kem=x25519, kemct=<B64>, sid=anon)
 hpke(kem=x25519, kemct=<B64>, shint=1234)

 All combinations of recipient identification (id, hint, or anonymous)
 and sender identification (sid, shint, or sid=anon) are valid.

 The hpke step token refines the step-token grammar in
 Section 6.2.1.1. OWS (optional whitespace) is permitted after each
 comma separator, consistent with step-params:

 hpke-step = "hpke(" hpke-params ")"
 hpke-params = "kem=" kem-name "," OWS "kemct=" kemct
 [ "," OWS recipient-id ]
 [ "," OWS sender-id ]
 kem-name = 1*( ALPHA / DIGIT / "-" )
 ; registered in SAFE KEM Identifiers
 ; registry ({{iana-kem}})
 kemct = 1*BASE64CHAR
 recipient-id = "id=" 1*BASE64CHAR / "hint=" hint-value
 sender-id = "sid=" ( 1*BASE64CHAR / "anon" )
 / "shint=" hint-value
 hint-value = 4DIGIT

 Encryptors MUST NOT include both id and hint, and MUST NOT include
 both sid and shint.

 Each HPKE step uses HPKE [RFC9180] in export-only mode with
 ciphersuite (KEM_ID, KDF_ID, 0xFFFF) constructed from the KEM's
 registered identifiers (Section 5.6.3.2). AEAD ID 0xFFFF disables
 Seal/Open; only Export is used.

 For Base mode (default):

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 ;; Encryptor
 (kemct, ctx) = SetupBaseS(pkR, info="SAFE-v1")
 step_secret = ctx.Export(exporter_context, L=32)

 ;; Decryptor
 ctx = SetupBaseR(kemct, skR, info="SAFE-v1")
 step_secret = ctx.Export(exporter_context, L=32)

 For Auth mode (sid or shint present):

 ;; Encryptor
 (kemct, ctx) = SetupAuthS(pkR, skS, info="SAFE-v1")
 step_secret = ctx.Export(exporter_context, L=32)

 ;; Decryptor
 ctx = SetupAuthR(kemct, skR, pkS, info="SAFE-v1")
 step_secret = ctx.Export(exporter_context, L=32)

 The kemct value is the KEM ciphertext (enc in HPKE terminology). The
 exporter_context is defined in Section 5.6.3.4.

 This design uses HPKE's standardized key schedule and export
 interface for KDF agility, while SAFE's own SafeDerive function
 handles KEK aggregation, payload key derivation, and nonce
 constructions.

5.6.3.2. Supported KEMs

 The following table lists the KEMs defined in the IANA HPKE KEM
 Identifiers registry [RFC9180] that are recognized by SAFE:

 +============+========+========+==================+==============+
 | KEM | KEM ID | KDF ID | HPKE Ciphersuite | Key Encoding |
 +============+========+========+==================+==============+
 | x25519 | 0x0020 | (see | (0x0020, KDF_ID, | [RFC8410] |
 | | | below) | 0xFFFF) | |
 +------------+--------+--------+------------------+--------------+
 | p-256 | 0x0010 | (see | (0x0010, KDF_ID, | [RFC5480] |
 | | | below) | 0xFFFF) | |
 +------------+--------+--------+------------------+--------------+
 | ml-kem-768 | 0x0041 | (see | (0x0041, KDF_ID, | (see below) |
 | | | below) | 0xFFFF) | |
 +------------+--------+--------+------------------+--------------+

 Table 9

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 The HPKE Ciphersuite column shows the (KEM_ID, KDF_ID, AEAD_ID)
 triple used with HPKE's Setup functions. AEAD ID 0xFFFF selects
 export-only mode per Section 5.3 of [RFC9180].

 * All KEMs, including DHKEM-based KEMs (x25519, p-256), use the KDF
 selected by the Hash parameter, both for the HPKE key schedule and
 for DHKEM's internal operations per [I-D.ietf-hpke-hpke]: HKDF-
 SHA256 (KDF ID 0x0001) when Hash=sha-256, or TurboSHAKE256 (KDF ID
 0x0013) when Hash=turboshake256.

 * When Hash=turboshake256, the HPKE implementation MUST conform to
 the one-stage key schedule defined in [I-D.ietf-hpke-hpke].

 ML-KEM-768 key encoding follows [I-D.ietf-lamps-kyber-certificates].
 Auth mode requires AuthEncap/AuthDecap, which are defined only for
 DHKEM-based KEMs. ML-KEM-768 MUST NOT be used with Auth mode.
 Additional KEMs from the IANA HPKE KEM Identifiers registry MAY be
 supported following the process defined in Section 10.2.

5.6.3.3. Key Identifier Computation

 The id parameter in hpke(...) steps identifies the intended recipient
 public key. Key identifiers hash the SubjectPublicKeyInfo (SPKI)
 Distinguished Encoding Rules (DER) encoding rather than raw key
 octets. This ensures key identifiers are consistent with certificate
 fingerprint practices and include the algorithm Object Identifier
 (OID), preventing collisions between keys of different types.

 spki_der = DER-encode SubjectPublicKeyInfo for pk
 per the KEM's registered SPKI Encoding ({{iana-kem}})
 DER encoding MUST be canonical.

 id = Base64( SafeDerive("SAFE-SPKI-v1",
 spki_der, "", 32) ) per {{RFC4648}}

 The resulting Base64 string is the value of the id parameter (44
 characters for the 32-octet output).

5.6.3.4. HPKE Step Secret Derivation

 When the step sequence includes one or more hpke(...) steps, the LOCK
 MUST include a corresponding kemct parameter value for each HPKE
 step, in the same order as they appear in the step sequence.
 Encryptors MUST generate a fresh encapsulation per LOCK; reusing a
 prior encapsulation is prohibited.

 For Auth mode, the Decryptor resolves the sender public key as
 follows:

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 * If sid is present: match against known sender public keys using
 the configured Hash.

 * If shint is present: filter candidate sender keys by the hint
 value.

 * If neither is present: try all locally known sender keys matching
 the kem type.

 The step secret is derived via HPKE's Export interface:

 exporter_context = SafeDerive("SAFE-STEP",
 step_token, "", 32)

 step_secret = ctx.Export(exporter_context, L=32)

 where ctx is the HPKE context returned by SetupBaseS/R (or
 SetupAuthS/R for Auth mode) as described in Section 5.6.3.2, and
 step_token is the binding form defined in Section 5.6.

 When id or sid is omitted from the on-wire token, the Decryptor
 reconstructs it during trial decryption per Section 5.6.

 The KEM binds the shared secret to the recipient key (and for Auth
 mode, the sender key). The exporter_context binds the step secret to
 the step token, preventing key-confusion attacks where an attacker
 substitutes one recipient's encapsulation for another's. The HPKE
 info parameter "SAFE-v1" binds the key schedule to this protocol,
 preventing cross-protocol reuse of the same KEM keys from producing
 valid SAFE step secrets. Suite binding is not needed here because
 the final KEK derivation commits to encryption_parameters
 (Section 5.7.1).

 Encode form:

 Encode("hpke", kem, kemct, id)
 Encode("hpke", kem, kemct, id,
 "auth", sid) ; auth

 Binding step_token: Encode("hpke", kem, kemct, id) for Base mode;
 Encode("hpke", kem, kemct, id, "auth", sid) for Auth mode. Display-
 only fields (hint, shint) are not included. The id and sid fields
 are reconstructed per Section 5.6 when omitted on-wire.

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5.7. Key Schedules

 SAFE uses two applications of SafeDerive (Section 5.4). The KEK
 derivation produces a LOCK-specific KEK from its ordered step
 secrets. The payload derivation produces the per-file payload key,
 commitment, accumulator key, and (for NMR AEADs) nonce base from the
 CEK, encryption parameters, and per-file salt.

 The following diagram shows the two independent chains:

 CEK (32 octets)
 |
 +-----------+-----------+
 | |
 (sealed by) (derives from)
 | |
 v v
 KEK chain (per LOCK) Payload chain (per file)
 +----------------------+ +----------------------+
 | agg_0 = SafeDerive | | payload_info = |
 | ("kek_init", "", | | enc_params + salt |
 | enc_params, 32) | | |
 | | | | commitment = |
 | secret | step_token | | SafeDerive("commit",|
 | \ | | | CEK, |
 | v v | | payload_info, 32) |
 | agg_1 = SafeDerive | | |
 | ("kek_step", | | payload_key = |
 | [agg_0, secret], | | SafeDerive( |
 | step_token, 32) | | "payload_key", CEK,|
 | | | | payload_info, Nk) |
 | ... | | |
 | v | | nonce_base = |
 | kek = SafeDerive | | SafeDerive( |
 | ("kek", agg_N, | | "nonce_base", CEK, |
 | enc_params, Nk) | | payload_info, Nn) |
 +--------+-------------+ | (NMR suites) |
 | | |
 v | acc_key = |
 Encrypted-CEK = | SafeDerive( |
 lock_nonce || | "acc_key", CEK, |
 AEAD.Seal(kek, | payload_info, Nh) |
 lock_nonce, "", CEK) +----------------------+

 Payload encryption is performed once under the CEK and per-file salt
 and does not depend on lock structure. Locks are independent
 wrappers of the same CEK and can be added or removed without touching
 payload ciphertext.

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5.7.1. KEK Schedule

 The KEK schedule derives a KEK from an ordered sequence of step
 secrets using a running aggregator.

 Algorithm:

 agg = SafeDerive("kek_init", "",
 encryption_parameters, 32)

 for each (step_token_i, step_secret_i) in order:
 agg = SafeDerive("kek_step",
 [agg, step_secret_i],
 step_token_i, 32)

 derived_kek = SafeDerive("kek", agg,
 encryption_parameters, Nk)

 Each step folds the aggregator and step secret into ikm (Extract) as
 an Encode-framed array. The step token is placed in info (Expand).
 Suite binding enters at kek_init and again at the final kek
 derivation, where encryption_parameters commits the aggregator to the
 negotiated AEAD, Block-Size, and Hash.

5.7.1.1. Multi-Step Example

 Consider a LOCK block with two steps requiring both a passphrase and
 a private key:

 Step: pass(kdf=argon2id, salt=<Base64>)
 Step: hpke(kem=p-256, id=<Base64>, kemct=<Base64>)
 Encrypted-CEK: <Base64>

 Evaluation proceeds per Section 5.7.1:

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 agg = SafeDerive("kek_init", "",
 encryption_parameters, 32)

 // Step 1: passphrase
 step_secret_1 = Argon2id(passphrase, salt)
 agg = SafeDerive("kek_step",
 [agg, step_secret_1],
 step_token_1, 32)

 // Step 2: HPKE (export-only mode)
 ctx = SetupBaseR(kemct, sk, info="SAFE-v1")
 step_secret_2 = ctx.Export(
 exporter_context =
 SafeDerive("SAFE-STEP",
 step_token_2, "", 32),
 L = 32)
 agg = SafeDerive("kek_step",
 [agg, step_secret_2],
 step_token_2, 32)

 derived_kek = SafeDerive("kek", agg,
 encryption_parameters, Nk)

 The derived_kek depends on both factors. Each step is bound to its
 position via the aggregator chaining through ikm, preventing step
 reordering.

5.7.2. Sealing Encrypted-CEK

 The per-step salt and kemct values MUST be unique per LOCK. Reusing
 these values with the same credentials produces the same step_secret,
 weakening KEK uniqueness.

 With derived_kek computed per Section 5.7.1, the Encrypted-CEK field
 is:

 Encrypted-CEK = lock_nonce || AEAD.Seal(
 key = derived_kek,
 nonce = lock_nonce,
 aad = "",
 pt = CEK )

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 The AEAD algorithm is the one specified in the CONFIG block (or the
 default aes-256-gcm when CONFIG is absent); the same AEAD is used for
 both CEK wrapping and payload encryption. Empty AAD is sufficient
 because derived_kek already binds encryption_parameters (via kek_init
 and the final kek derivation, Section 5.7.1) and all step_tokens (via
 kek_step chaining); repeating this binding in the AEAD AAD would be
 redundant.

 Encryptors MUST generate lock_nonce using SafeRandom(Nn, "SAFE-LOCK-
 NONCE"); each LOCK requires a fresh value.

5.7.3. Payload Schedule

 The payload schedule derives the commitment, payload key, accumulator
 key, and (for NMR AEADs) nonce base from the CEK using SafeDerive.
 The salt is a 32-octet random value generated using SafeRandom(32,
 "SAFE-SALT") and stored at the start of the DATA block. It is
 appended to encryption_parameters in the info input:

 payload_info = [...encryption_parameters, salt]

 commitment = SafeDerive("commit", CEK,
 payload_info, 32)
 payload_key = SafeDerive("payload_key", CEK,
 payload_info, Nk)
 acc_key = SafeDerive("acc_key", CEK,
 payload_info, Nh)

 For NMR AEADs (NMR=Yes in Section 5.2.1), the payload schedule also
 derives nonce_base:

 nonce_base = SafeDerive("nonce_base", CEK,
 payload_info, Nn)

 For non-NMR AEADs, nonce_base is not derived. The accumulator key
 acc_key is always derived and is used for the snapshot accumulator
 (Section 5.7.8).

 The commitment prefix is always 32 octets for all AEADs. Decryptors
 MUST verify the commitment before decrypting any block and MUST
 reject the file if the derived commitment does not match the stored
 value (see Section 8.12).

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 Each call independently derives from CEK with distinct labels. Using
 the wrong AEAD identifier or Block-Size when attempting decryption
 produces different SafeDerive outputs, causing AEAD verification to
 fail on every block. The per-file salt ensures that even when the
 same CEK is reused across files, each file produces unique derived
 keys. This binding provides implicit integrity assurance that the
 decryptor is using the correct encryption parameters.

5.7.4. Epoch Key Derivation

 When Key-Epoch is present in CONFIG, per-block encryption uses an
 epoch key derived from the payload key. Key-Epoch is a non-negative
 integer r; each epoch spans 2^r consecutive blocks.

 block_key(i):
 if key_epoch is absent:
 return payload_key
 epoch_index = i >> key_epoch
 return SafeDerive("epoch_key", payload_key,
 [I2OSP(epoch_index, 8)], Nk)

 Blocks 0 through 2^r - 1 share epoch 0, blocks 2^r through 2^(r+1) -
 1 share epoch 1, and so on. When Key-Epoch is 0, every block has its
 own key (epoch size 1).

 Since Key-Epoch is included in encryption_parameters when present
 (Section 5.5), the commitment and all payload schedule outputs change
 when the epoch configuration changes. Implementations SHOULD cache
 epoch keys across consecutive blocks in the same epoch.

 This construction is inspired by [FLOE], adapted to SAFE's key
 schedule.

5.7.5. Per-block Nonces

 Nonces are unique per block; their size Nn is determined by the AEAD
 algorithm (see Section 10.1). Encryptors MUST ensure nonce
 uniqueness within each block_key(i)'s lifetime (Section 5.7.4).

 The per-block encryption produces:

 (ciphertext_i, tag_i) = AEAD.Seal(key = block_key(i),
 nonce = nonce_i,
 aad = aad_i,
 pt = plaintext_block_i)

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 See Section 5.7.4 for block_key(i). AEAD.Seal returns (ciphertext,
 tag) where tag is the final 16 octets of the combined AEAD output per
 [RFC5116]. Implementations receiving a single output MUST split at
 offset len(output) - 16.

 For non-NMR AEADs, Encryptors MUST use one of the nonce constructions
 defined in Section 5.7.6. Each block's nonce and tag are stored:

 block_metadata_i = nonce_i || tag_i (Nn + 16 octets)

 Where nonce_i is Nn random octets and aad_i is defined below. In
 armored mode, blocks are stored as nonce_i || ciphertext_i || tag_i.
 In binary mode, the block metadata (nonce + tag) is stored separately
 from the ciphertext to enable single-seek block access.

 For NMR AEADs, per-block nonces are derived deterministically from
 nonce_base (Section 5.7.3):

 nonce_i = nonce_base XOR uint64(i)

 The XOR is applied to the last 8 octets of nonce_base; leading octets
 are unchanged. Nonces are not stored; only the authentication tag is
 kept:

 block_metadata_i = tag_i (16 octets)

 See Section 8.9 for security rationale.

 This design enables re-encryption of the payload without re-wrapping
 the CEK for each recipient, and supports selective editing of
 individual blocks. For non-NMR AEADs, per-block random nonces allow
 CEK reuse across payload revisions while maintaining AEAD security.

5.7.6. Nonce Constructions

 Two nonce constructions are defined for non-NMR block encryption.
 Both produce Nn-octet nonces suitable for use with the configured
 AEAD. Decryptors read the stored nonce from each block and do not
 need to know which construction was used.

5.7.6.1. Base-XOR Construction

 base = SafeRandom(Nn, "SAFE-NONCE")
 nonce_i = base XOR uint64(i)

 The XOR is applied to the last 8 octets of base; leading octets are
 unchanged. Within a single file, the XOR with distinct block indices
 guarantees nonce uniqueness.

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 This construction is one-pass and supports parallel block encryption.

5.7.6.2. Plaintext-Bound Construction

 Encryptors SHOULD incorporate plaintext into per-block nonce
 derivation for SIV-like nonce-misuse resistance when RNG state
 duplication is a concern (e.g., VM snapshots, process forks without
 reseed). This requires two passes over each block (hash then
 encrypt).

 The construction proceeds in two steps:

 ;; Pass 1: hash the plaintext block
 pt_hash_i = SafeDerive("SAFE-NONCE",
 plaintext_i, encryption_parameters, 32)

 ;; Pass 2: derive the nonce
 nonce_ctx = Encode("SAFE-NONCE",
 I2OSP(i, 8), pt_hash_i)
 nonce_i = SafeDerive("nonce",
 [SafeRandom(Nn, "SAFE-NONCE"),
 payload_key],
 [...encryption_parameters, nonce_ctx],
 Nn)

 Pass 1 commits to the plaintext via a SafeDerive hash bound to the
 suite. Pass 2 mixes fresh randomness, the payload key, and the
 plaintext hash into the final nonce. Under RNG state duplication,
 this construction produces different nonces when plaintext differs,
 limiting exposure to leaking equality of identical blocks at the same
 index.

5.7.7. Block Rewrite Rules

 When re-encrypting a modified block, the procedure depends on the
 AEAD's nonce-misuse resistance property.

 For non-NMR AEADs (stored nonce required):

 Rewriting block i:
 old_tag = stored tag_i
 nonce_i = SafeRandom(Nn, "SAFE-NONCE")
 (ct, new_tag) = AEAD.Seal(block_key(i), nonce_i,
 data_aad(i, is_final),
 new_plaintext)
 Update stored nonce_i and tag_i
 Update accumulator ({{snapshot-accumulator}})

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 For NMR AEADs (derived nonce; Key-Epoch MUST be absent):

 Rewriting block i:
 old_tag = stored tag_i
 nonce_i = nonce_base XOR uint64(i)
 (ct, new_tag) = AEAD.Seal(block_key(i), nonce_i,
 data_aad(i, is_final),
 new_plaintext)
 Update stored tag_i
 Update accumulator ({{snapshot-accumulator}})

 NMR rewrites reuse the derived nonce for that block index. Because
 the AEAD is nonce-misuse resistant, this degrades to deterministic
 encryption: identical plaintext at the same index produces identical
 ciphertext, leaking equality but not content. See Section 8.9.2.

5.7.8. Snapshot Accumulator

 The snapshot accumulator binds all block authentication tags into a
 single value, providing file-level integrity without decrypting every
 block. It uses SafeDerive and XOR; no additional primitives are
 needed.

 For each block i with authentication tag tag_i, the per-block
 contribution is:

 contrib_i = SafeDerive("acc_contrib", acc_key,
 [uint64(i), tag_i], Nh)

 The accumulator is the XOR of all contributions:

 accumulator = contrib_0 XOR contrib_1
 XOR ... XOR contrib_{N-1}

 XOR is commutative and associative: blocks may be processed in any
 order, and a single contribution can be replaced without reprocessing
 other blocks. The result is Nh octets (32 for all registered Hash
 algorithms).

 When re-encrypting block i with new tag new_tag_i (replacing old tag
 old_tag_i):

 old_c = SafeDerive("acc_contrib", acc_key,
 [uint64(i), old_tag_i], Nh)
 new_c = SafeDerive("acc_contrib", acc_key,
 [uint64(i), new_tag_i], Nh)
 accumulator = accumulator XOR old_c XOR new_c

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 This update costs two SafeDerive calls and two XOR operations,
 independent of the total block count.

 Verification:

 1. Derive acc_key from the CEK and salt (Section 5.7.3).

 2. Read all stored tags from block metadata.

 3. Compute the accumulator by XOR-combining per-block contributions.

 4. Compare the result to the stored accumulator. If they differ, at
 least one block has been modified, replaced, or is missing.

 The accumulator is a PRF-based XOR-checksum over the set of (index,
 tag) pairs. Each contribution is a PRF output under acc_key with
 unique inputs; an adversary who does not know acc_key cannot predict
 any individual contribution and therefore cannot forge a valid
 accumulator value. Decryptors MUST verify the accumulator before
 per-block decryption when the full set of block tags is available.
 Streaming Decryptors that process blocks incrementally MUST verify
 the accumulator once all blocks have been processed.

5.7.9. Block AAD

 For block index i in the range 0 <= i < N, the associated data binds
 each block to its position and indicates whether it is the final
 block:

 data_aad(i, is_final) = Encode("SAFE-DATA",
 I2OSP(i, 8), I2OSP(is_final, 1))

 Where I2OSP(i, 8) is the block index in network byte order and
 is_final is 0x01 for the last block (index N-1) and 0x00 for all
 preceding blocks. Encode provides prefix-free framing via lp16
 (Section 4.1). No KDF call is needed: payload_key already depends on
 encryption_parameters (Section 5.7.3), and the AEAD tag authenticates
 the AAD under that key.

 The is_final flag provides truncation detection: if a Decryptor
 decrypts block i with is_final=0 and no subsequent block exists,
 truncation has occurred. Decryptors MUST abort when truncation is
 detected (is_final=0 with no successor block). For random-access
 reads, Decryptors SHOULD verify that the block count N in the
 metadata matches the actual number of blocks. The flag also prevents
 extension attacks: appending blocks after a block marked is_final=1
 will fail AEAD verification.

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 For streaming writes where the total block count is unknown,
 Encryptors buffer the last block until more data arrives or the
 stream ends. All emitted blocks use is_final=0; only when the stream
 closes does the Encryptor encrypt the final block with is_final=1.

 Encryptors MUST ensure block indices remain below 2^64. Encryptors
 SHOULD limit i to at most 2^48 to avoid Base64 strings exceeding
 typical filesystem or object store limits; this is a practical
 recommendation, not a protocol limit. Decryptors MUST reject block
 indices i where i >= 2^64.

6. File Layout

 A SAFE encoding MUST consist of an optional config header
 (Section 6.1), followed by one or more LOCK blocks (Section 6.2),
 followed by exactly one DATA block (Section 6.4). Blocks MUST appear
 in this order; Decryptors MUST reject encodings that violate it.
 There is no version marker in the fences. Multiple LOCK blocks
 provide multi-recipient encryption; the DATA block is shared.

6.1. SAFE CONFIG

 The config header may be omitted when all defaults apply. When
 present, it lists only non-default parameters. The config does not
 need to be parsed before attempting decryption if the Decryptor
 already knows or can infer the default parameters.

 The header is:

 -----BEGIN SAFE CONFIG-----
 AEAD: aes-256-gcm | chacha20-poly1305 | ...
 Block-Size: 16384 | 65536
 Hash: sha-256 | turboshake256
 Key-Epoch: <non-negative integer>
 Lock-Encoding: armored | readable
 Data-Encoding: armored | binary | binary-linear
 -----END SAFE CONFIG-----

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 All CONFIG fields are optional. Omitted fields use default values
 (Section 5.1): AEAD defaults to aes-256-gcm, Block-Size to 65536,
 Hash to sha-256, Lock-Encoding to armored, and Data-Encoding to
 armored. Hash selects the hash function used throughout the protocol
 (Section 5.4); any identifier in the SAFE KDF Identifiers registry
 (Section 10.3) is valid. Lock-Encoding specifies the LOCK block
 representation (Section 6.2). Data-Encoding specifies the payload
 format (Section 6.3): "armored" uses Base64 within DATA fence
 markers, "binary" uses block-aligned raw binary after the last LOCK,
 and "binary-linear" uses sequential raw binary after the last LOCK.
 The encoding fields are presentational choices that do not affect
 cryptographic operations.

 Key-Epoch enables per-epoch key derivation (Section 5.7.4). When
 absent, all blocks use the payload key directly. When present, it is
 a non-negative integer r where r MUST be less than 64; Decryptors
 MUST reject files where Key-Epoch is 64 or greater. Block i uses an
 epoch key derived from the payload key with epoch index i >> r,
 giving each epoch 2^r blocks and limiting AEAD invocations per key
 (Section 8.13). Key-Epoch MUST NOT be present for NMR AEADs;
 Decryptors MUST reject files where Key-Epoch is present and the AEAD
 has NMR=Yes in Section 5.2.1. Encryptors using chacha20-poly1305
 MUST include Key-Epoch; Section 4 of [RFC8439] requires nonce
 uniqueness per key, and epoch keys limit the collision domain to
 rewrites within each epoch. Encryptors using aes-256-gcm SHOULD
 include Key-Epoch for rewritable files to stay within the per-key
 invocation limits of [NIST-SP-800-38D]; see Section 8.13.3 for budget
 calculations. Key-Epoch: 0 is RECOMMENDED for maximum security; Key-
 Epoch: 5 is a practical alternative when key-derivation overhead
 matters.

 NMR AEADs (NMR=Yes in Section 5.2.1) implicitly use derived nonces:
 per-block nonces are computed from the key schedule and block index,
 and the format nonce length is 0 (nonces are not serialized). Non-
 NMR AEADs use stored random nonces. See Section 5.7.5 for details.

 Implementations MUST support Lock-Encoding: armored and Data-
 Encoding: armored. Support for Lock-Encoding: readable and Data-
 Encoding: binary or binary-linear is OPTIONAL.

 A conforming SAFE file MAY omit the SAFE CONFIG block entirely;
 parsers MUST treat this identically to a CONFIG block with all
 defaults. When CONFIG is present, it MAY contain any subset of
 fields. Implementations MUST construct encryption_parameters using
 defaults for any omitted fields.

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 Field names within the SAFE CONFIG are case-sensitive. Encryptors
 MUST NOT include duplicate field names; Decryptors MUST reject SAFE
 config blocks containing duplicate fields. Decryptors MUST reject
 SAFE CONFIG blocks containing field names not listed in the SAFE
 Config Options registry (Section 10.5).

 The order of fields within a CONFIG block is not significant.
 Encryptors MAY emit fields in any order; Decryptors MUST accept
 fields in any order.

 All header lines MUST contain only ASCII characters (octets 0x20-0x7E
 and LF). Encryptors MUST NOT include non-ASCII characters in field
 names or values. Decryptors MUST reject SAFE CONFIG blocks
 containing non-ASCII octets or malformed UTF-8 sequences.

 Implementations SHOULD bound SAFE CONFIG size; Decryptors MAY reject
 SAFE CONFIG headers exceeding 64 KiB.

 Field values MAY wrap across multiple lines using the same rules as
 LOCK blocks (Section 6.2.1.2): continuation lines MUST be indented
 with at least two spaces, and Decryptors MUST concatenate
 continuation lines (stripping leading whitespace) before processing.

6.1.1. Block-Size Selection

 SAFE defines two Block-Size values: 16384 and 65536. Only these
 values are valid; Decryptors MUST reject any other Block-Size.
 Adding new Block-Size values requires Standards Action.

 65536 (default): Larger blocks amortize per-block AEAD overhead and
 reduce I/O syscalls, yielding higher throughput for sequential
 encrypt and decrypt. This value is appropriate when the payload
 will be decrypted in full or streamed sequentially.

 16384: Smaller blocks reduce the cost of partial updates. Re-
 encrypting a modified block requires reading and rewriting only
 that block; at 16384 octets the I/O cost per edit is one quarter
 of the default. Applications that perform random-access writes to
 encrypted data SHOULD use Block-Size 16384.

 Both values align to hardware page boundaries. Block-Size 16384 is
 one page on systems with 16 KiB pages (e.g., Apple Silicon) and four
 pages on systems with 4 KiB pages (e.g., x86-64). Block-Size 65536
 is four pages and sixteen pages, respectively. This alignment avoids
 page-crossing penalties with direct I/O or memory-mapped access.

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 The Encryptor selects Block-Size at encryption time. The value is
 recorded in the SAFE CONFIG and applies to all recipients. There is
 no mechanism to change Block-Size after encryption without re-
 encrypting the entire payload.

6.2. SAFE LOCK

 A LOCK block defines the unlock steps for a single recipient and
 carries the artifacts needed to recover the CEK. Each LOCK contains
 one or more steps and exactly one Encrypted-CEK.

 Steps are evaluated in the order they appear. Step-specific inputs
 are carried as parameters (e.g., salt= for pass, kemct= for hpke).
 See Section 5.6.2 and Section 5.6.3 for step-specific requirements.

 The Encrypted-CEK is the concatenation of lock_nonce and the AEAD
 ciphertext of the CEK under the derived KEK with empty associated
 data (Section 5.7.2). The lock_nonce length is the AEAD's nonce size
 (Nn) as specified in Section 10.1. Encryptors MUST generate a fresh
 lock_nonce per LOCK using SafeRandom(Nn, "SAFE-LOCK-NONCE")
 (Section 5.4.2). Decryptors MUST reject Encrypted-CEK values that do
 not decode to exactly Nn + 32 + 16 octets (lock_nonce, CEK
 ciphertext, AEAD tag).

 Decryptors MUST skip LOCK blocks containing unknown KEM identifiers
 or unknown step types, and attempt other LOCKs (if available).
 Decryptors MUST reject step tokens containing parameter names not
 defined for the step type in its registration.

 Implementations SHOULD bound the number of LOCK blocks; Decryptors
 MAY reject files containing more than 1024 LOCK blocks to prevent
 resource exhaustion.

 Two Lock-Encoding values are defined: readable (text) and armored
 (binary). Both produce the same binding step_tokens for the KEK
 schedule (Section 5.6).

6.2.1. Readable Format

 The readable format uses text step tokens and colon-delimited fields:

 -----BEGIN SAFE LOCK-----
 Step: pass(kdf=argon2id, salt=<Base64>)
 Step: hpke(kem=x25519, id=<Base64>, kemct=<Base64>)
 Encrypted-CEK: <Base64>
 -----END SAFE LOCK-----

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 Field names are case-sensitive. Encryptors MUST NOT include fields
 other than Step and Encrypted-CEK. Encryptors MUST include at least
 one Step line and exactly one Encrypted-CEK line. Decryptors MUST
 reject LOCK blocks containing multiple Encrypted-CEK lines or unknown
 field names. Optional whitespace (OWS) after colons and commas is
 permitted for readability.

6.2.1.1. Step Syntax

 Each Step line declares a single cryptographic step. Multiple steps
 form an ordered sequence with AND semantics: all steps MUST be
 satisfied to derive the KEK. The syntax follows this ABNF, which
 applies after Decryptors perform line unfolding (concatenating
 continuation lines and stripping leading whitespace per
 Section 6.2.1.2):

 step-line = "Step:" OWS step-token LF
 step-token = step-name "(" step-params ")"
 step-name = 1*( ALPHA / DIGIT / "-" )
 step-params = param *( "," OWS param )
 param = param-name "=" param-value
 param-name = 1*( ALPHA / DIGIT / "-" )
 param-value = 1*PCHAR
 PCHAR = %x21-28 / %x2A-2B / %x2D-7E
 ; VCHAR except SP, ")", ","
 OWS = *( SP / HTAB )

 The binding step_token used in the KEK schedule (Section 5.7.1) is
 derived per Section 5.6: extract the binding fields and encode them
 with Encode().

 Each Step line contains exactly one step token. LOCK blocks with
 multiple steps use multiple Step lines. Step-specific inputs are
 carried as step token parameters (e.g., salt= for pass, kemct= for
 hpke).

 The param-value production forbids spaces (SP, 0x20) and tabs (HTAB,
 0x09). Percent-encoding is not supported; all parameter values MUST
 be literal UTF-8 printable characters excluding whitespace.

 Encryptors MUST emit parameters in the order specified by the step
 definition. Decryptors MUST reject step tokens whose parameters are
 not in the specified order. Decryptors MUST reject step tokens
 containing duplicate parameter names within a single step.

 See Section 5.7 for how step secrets are combined to derive the KEK.
 See Section 5.6.3 for the HPKE step format and Section 5.6.2 for the
 passphrase step format.

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6.2.1.2. Line Wrapping

 Field values MAY wrap across multiple lines. Continuation lines MUST
 be indented with at least two spaces. Decryptors MUST unfold wrapped
 values by concatenating continuation lines and stripping leading
 whitespace.

 * Step tokens MAY wrap at parameter boundaries (after commas).
 Encryptors SHOULD insert a space after the comma at each wrap
 point. Continuation lines use 4-space indent.

 * Encrypted-CEK values use 2-space indent for continuation lines.

 Encryptors SHOULD wrap lines at 64 characters. Decryptors MUST
 accept any line length.

 A field value extends from immediately after the colon (and any
 following whitespace) until one of:

 1. A line starting with "Step:" (unindented)

 2. A line starting with "Encrypted-CEK:" (unindented)

 3. A fence line "-----END SAFE LOCK-----"

 Trailing whitespace on individual lines SHOULD be avoided; Decryptors
 MUST strip trailing spaces and tabs from each line before
 concatenation.

 Example with wrapped HPKE step token:

 Step: hpke(kem=ml-kem-768, hint=4217,
 kemct=bWxrZW03NjhrZW1jaXBoZXJ0ZXh0
 ZXh0cmVtZWx5bG9uZ2JhY2U2NGVuY29kZW
 RkYXRhYXBwcm94aW1hdGVseTEwODhvY3Rl
 dHNmb3JwcXNlY3VyaXR5)

 Decryptors parse the text, extract field values, and produce the step
 Encode form for binding.

6.2.2. Armored Format

 When Lock-Encoding is armored, the SAFE LOCK block contains a single
 Base64 value. The value is the Base64 encoding of the Encode-
 serialized LOCK:

 armored_lock = Base64(
 Encode(step_1, step_2, ..., encrypted_cek))

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 Each step_i is the binding Encode form (excluding display-only
 fields) as defined in the step's specification. The encrypted_cek is
 the raw Encrypted-CEK octets (lock_nonce || ciphertext).

 Decryptors decode the Base64, split the outer Encode into lp16-framed
 elements. The last element is the Encrypted-CEK; preceding elements
 are step tokens. Each step token is itself an Encode whose first
 element is the step name.

 Encryptors MUST use Base64 with padding per [RFC4648]. The Base64
 value MAY wrap across multiple lines; continuation lines MUST be
 indented with at least two spaces. Decryptors MUST concatenate
 continuation lines (stripping leading whitespace) before decoding.

6.2.3. LOCK Selection

 A decryptor determines candidate LOCK blocks without touching the
 SAFE data block.

6.2.3.1. Candidate Selection

 For each LOCK block, determine candidacy as follows:

 1. Parse the Step tokens. If any token references an unsupported
 step type, the LOCK is not a candidate.

 2. For each hpke(...) step, determine candidate keys based on mode:

 Identified (id present): Compute the key identifier for locally
 available public keys using SafeDerive("SAFE-SPKI-v1",
 spki_der, "", 32) with the configured Hash (default: sha-256).
 Keys whose identifier matches the id parameter are candidates.

 Hinted (hint present): Look up keys in local storage associated
 with this hint value. Keys with matching hint are candidates.

 Anonymous (neither id nor hint): All local keys matching the kem
 type are candidates.

 If no local recipient keys are candidates, the LOCK is not a
 candidate.

 For auth-mode hpke(...) steps (sid or shint present), also
 determine candidate sender keys:

 Identified (sid with Base64 value): Keys whose identifier
 matches sid are candidates.

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 Hinted (shint present): Keys with matching shint are candidates.

 Anonymous (sid=anon): All locally known sender keys matching the
 kem type are candidates.

 If no sender keys are candidates for an auth-mode step, the LOCK
 is not a candidate.

 3. For pass(...) steps: the LOCK is a candidate if the
 implementation supports passphrase entry.

 4. For other registered steps: the LOCK is a candidate if the
 implementation supports them and local policy permits.

6.2.3.2. Attempt Order

 Among remaining candidates, Decryptors SHOULD attempt LOCKs in order
 of confidence:

 1. LOCKs where all hpke steps are identified or hinted; the
 decryptor has confirmed it holds matching keys.

 2. LOCKs with anonymous hpke steps; requires trial decryption across
 all keys of the matching KEM type.

 3. LOCKs with pass steps; may require user interaction, so defer
 until key-only LOCKs are exhausted.

 Encryptors MAY include multiple pass(...)-only LOCK blocks if they
 use different KDF variants (e.g., one pass(kdf=argon2id, ...) and one
 pass(kdf=pbkdf2, ...) for the same passphrase). This enables
 interoperability between implementations with different passphrase
 KDF support. Encryptors MUST NOT include duplicate pass(...)-only
 LOCKs with the same KDF variant. Decryptors MUST stop at the first
 successful CEK recovery. Decryptors MAY attempt multiple candidates
 in parallel.

6.2.3.3. Trial Decryption

 For hinted or anonymous step sequences, Decryptors iterate through
 candidate key combinations. For composable step sequences (multiple
 hpke steps with AND semantics), trial decryption MUST consider the
 combinatorial product of candidates for each step. For auth-mode
 steps, the product includes sender key candidates in addition to
 recipient key candidates. For each combination:

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 1. Establish an HPKE context for each hpke(...) step via SetupBaseR
 (or SetupAuthR with the candidate sender key for auth-mode steps)
 and call Export per Section 5.6.3.4

 2. Derive the KEK by aggregating step secrets per Section 5.7.1

 3. Attempt to open Encrypted-CEK with the derived KEK

 A candidate succeeds when AEAD tag verification passes on Encrypted-
 CEK. If the KEK is wrong, the tag will not verify.

6.3. Data Encoding

 The Data-Encoding CONFIG field specifies how the payload is
 represented. SAFE defines two payload layouts: linear concatenates
 encrypted blocks sequentially, while aligned adds padding for block-
 aligned random access. Three encoding values are defined:

 +===============+=========+=============================+
 | Value | Layout | Representation |
 +===============+=========+=============================+
 | armored | linear | Base64 within fence markers |
 +---------------+---------+-----------------------------+
 | binary | aligned | Raw binary, block-aligned |
 +---------------+---------+-----------------------------+
 | binary-linear | linear | Raw binary, sequential |
 +---------------+---------+-----------------------------+

 Table 10

 The default is "armored" when Data-Encoding is omitted.

6.3.1. Armored Encoding

 Armored encoding wraps the linear layout (Section 6.4.1) in Base64
 with fence markers:

 -----BEGIN SAFE DATA-----
 <Base64: linear_payload>
 -----END SAFE DATA-----

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 The Base64 string MAY be wrapped across multiple lines for
 readability. When wrapped, each line break MUST be a single LF
 character. For length calculations and random-access arithmetic,
 Decryptors MUST first remove all line breaks (LF and CRLF) and CR
 octets (0x0D), then strip trailing whitespace from the result. The
 normalized string length determines padding and block offset
 computations. Decryptors MUST ignore these characters during Base64
 decoding and concatenate all lines before decoding.

 Encryptors SHOULD wrap Base64 lines at 64 characters. Decryptors
 MUST accept any line length.

 Implementations MAY enforce an upper bound on payload size to prevent
 over-allocation; Decryptors MAY reject payloads exceeding 64 TiB of
 ciphertext.

 Armored data arithmetic (computing block count, byte-to-Base64
 offsets, and per-block decryption) is detailed in Appendix D.

6.3.2. Binary Encoding

 Binary encoding omits fence markers. Binary data begins at the octet
 immediately following the LF (0x0A) that terminates the last -----END
 SAFE ...----- line (typically the final LOCK block). Binary data
 ends at EOF.

 Two variants exist:

 Data-Encoding: binary Uses the aligned layout (Section 6.4.2).
 Optimized for random access to large files via memory-mapped I/O
 or O_DIRECT.

 Data-Encoding: binary-linear Uses the linear layout (Section 6.4.1).
 Suitable for streaming or simple implementations that do not
 require block-aligned random access.

 Implementations that do not support binary encoding MUST fail when
 encountering Data-Encoding: binary or Data-Encoding: binary-linear,
 consistent with the handling of unknown AEAD or Hash values.

 Encryptors SHOULD prefer armored encoding for maximum compatibility.
 Binary encoding is intended for performance-critical applications or
 programmatic access where human readability is not required.

6.4. Payload Layouts

 SAFE defines two payload layouts that describe how encrypted blocks
 are structured.

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6.4.1. Linear Layout

 The linear layout concatenates encrypted blocks sequentially with no
 padding or alignment constraints:

 [salt] [commitment] [accumulator] [eb_0] [eb_1] ... [eb_{N-1}]

 For non-NMR AEADs:

 eb_i = nonce_i || ciphertext_i || tag_i (Nn + len(pt_i) + 16)

 For NMR AEADs:

 eb_i = ciphertext_i || tag_i (len(pt_i) + 16)

 The payload begins with a 32-octet salt, a 32-octet commitment (both
 derived per Section 5.7.3), and the Nh-octet accumulator
 (Section 5.7.8), followed by encrypted blocks.

 For non-NMR AEADs, each encrypted block (eb_i) is Nn +
 len(plaintext_i) + 16 octets. For NMR AEADs, each encrypted block is
 len(plaintext_i) + 16 octets (nonces are derived, not stored).

 Decryptors MUST verify the commitment before decryption and MUST
 reject the file if the derived commitment does not match the stored
 value. See Section 8.12.

 All blocks except the final block contain Block-Size octets of
 plaintext. The final block MAY be smaller. For non-NMR AEADs, each
 encrypted block consists of a nonce (Nn octets), ciphertext (same
 length as the plaintext), and authentication tag (16 octets). For
 NMR AEADs, each encrypted block consists of ciphertext and
 authentication tag only.

 Zero-length plaintexts are allowed. A zero-length plaintext produces
 N = 1, L_final = 0. For non-NMR AEADs with Nn = 12, the minimum
 payload is 124 octets (32-octet salt + 32-octet commitment + 32-octet
 accumulator + 28-octet encrypted block). For NMR AEADs, the minimum
 is 112 octets (32 + 32 + 32 + 16).

 Decryptors MUST reject payloads with unexpected structure: incorrect
 commitment length, missing or invalid accumulator, or block
 boundaries that do not align with expected sizes.

 In binary-linear encoding the block count N is not stored explicitly.
 Streaming readers determine N by reading blocks sequentially until
 EOF: each non-final block is exactly C octets (Nn + B + 16 for non-
 NMR AEADs; B + 16 for NMR AEADs). The final block is smaller than C.

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6.4.2. Aligned Layout

 The aligned layout structures the file so that every ciphertext block
 begins at an offset that is an exact multiple of the Block-Size B.
 This alignment enables efficient memory-mapped I/O and O_DIRECT
 access, since the operating system can read any block without copying
 data across page boundaries.

 The file begins with a header section containing the text headers
 (CONFIG and LOCK blocks) followed by binary fields: salt, commitment,
 block count N, first ciphertext index D, per-block metadata (nonces
 and tags), and the snapshot accumulator. The header is padded with
 zeros to a block boundary, followed by zero or more padding blocks
 for append growth, then ciphertext blocks.

 Let B denote the Block-Size in octets (16384 or 65536). Let Nn be
 the nonce size (12 for AES-GCM and ChaCha20, 32 for AEGIS-256 and
 AEGIS-256X2). N is the block count (uint32), and D is the first
 ciphertext block index (uint32). Let meta_len be the per-block
 metadata size: Nn + 16 for non-NMR AEADs, or 16 for NMR AEADs.

 Each row below represents one Block-Size:

 +-------------+------+------------+---+---+------------+
 | CONFIG+LOCK | salt | commitment | N | D | metadata...|
 +------------------------------------------------------+
 | ...metadata (cont.) | accumulator | padding |
 +------------------------------------------------------+
 | padding (optional append growth) |
 +------------------------------------------------------+
 | ct0 |
 +------------------------------------------------------+
 | ct1 |
 +------------------------------------------------------+
 | ... |
 +------------------------------------------------------+

 Per-block metadata entry:

 Non-NMR AEADs: +-------+------+ Nn + 16 octets
 | nonce | tag |
 +-------+------+

 NMR AEADs: +------+ 16 octets
 | tag |
 +------+

 The binary portion immediately follows the text headers:

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 * salt (32 octets)

 * commitment (32 octets)

 * N: block count (uint32, big-endian)

 * D: first ciphertext block index (uint32, big-endian)

 * metadata: N entries, each meta_len octets

 * accumulator: Nh octets (Section 5.7.8)

 * padding to block boundary, then zero or more padding blocks

 * ciphertext blocks starting at offset (D × B)

 The uint32 block count limits aligned-layout files to 2^32 - 1
 blocks. Files exceeding this count MUST use linear layout.

6.4.2.1. Writing

 To write an aligned-layout file, the Encryptor computes D as follows.
 Let H be the total header octet count: all CONFIG and LOCK text
 (including fence markers and newlines), plus 32 (salt) + 32
 (commitment) + 4 (N) + 4 (D) + N * meta_len (metadata) + Nh
 (accumulator). Then:

 D = ceil(H / B)

 Padding blocks for append growth MAY be added by increasing D beyond
 the minimum value. The encryptor writes the header, pads to D * B
 octets, then writes ciphertext blocks at offsets D * B, (D+1) * B,
 and so on.

 The aligned layout requires a seekable output. For streaming writes
 where N is unknown at the start, the Encryptor estimates a maximum
 block count, computes D from that estimate, reserves space for D, N,
 and metadata in the header, then seeks back to fill the actual values
 once the stream closes.

6.4.2.2. Reading

 To read an aligned-layout file:

 1. Parse CONFIG and LOCK text to determine AEAD and Block-Size.

 2. Read the 32-octet salt, 32-octet commitment, N, and D.

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 3. Read N metadata entries, each meta_len octets.

 4. Read the Nh-octet accumulator.

 5. Verify the accumulator (Section 5.7.8).

 6. Ciphertext block i is at offset (D + i) × B.

 To read only block i: for NMR AEADs, compute nonce_i from nonce_base
 and the block index; for non-NMR AEADs, read the nonce from the
 metadata entry. Read the tag from the metadata entry in both cases.
 Then read B octets (or fewer for the final block) at offset (D + i) x
 B. The final block's ciphertext length is file_size - (D + N - 1) *
 B octets.

7. Compatibility and Migration

7.1. Handling Unknown Elements

 Decryptors processing SAFE-encoded data MUST:

 * Fail if they encounter an unrecognized or unimplemented value for
 any CONFIG field (AEAD, Hash, Data-Encoding, Lock-Encoding, Block-
 Size). Implementations MUST NOT silently ignore CONFIG values
 they do not support.

 * Reject Block-Size values other than 16384 or 65536.

 * Skip LOCKs containing unknown field names, KEM identifiers, or
 step types and attempt other LOCKs (if available).

 * Fail if a CONFIG block contains field names not listed in the SAFE
 Config Options registry (Section 10.5).

 * Fail if the payload has unexpected structure (wrong commitment
 length, trailing octets, misaligned block boundaries).

 * Skip unknown block types if the IANA SAFE Block Types registry
 (Section 10.6) marks them as Ignorable; otherwise fail.

7.2. Versioning

 This document defines SAFE version 1, identified by fence markers ("
 -----BEGIN SAFE CONFIG-----", etc.). Future incompatible versions
 would use different fence markers or a new media type. New features
 SHOULD be added through IANA registries rather than format version
 changes.

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7.3. Extension Points

 SAFE provides IANA registries for AEADs (Section 10.1), KEMs
 (Section 10.2), step types (Section 10.4), and block types
 (Section 10.6).

 Unknown block types are critical by default: Decryptors MUST fail if
 they encounter an unrecognized block. The IANA SAFE Block Types
 registry (Section 10.6) MAY mark specific block types as Ignorable,
 enabling forward-compatible optional extensions such as metadata or
 signatures that older implementations can safely skip.

7.4. Application Profiles

 This section is informative. It describes three parameter
 combinations for common deployment scenarios. These profiles compose
 the CONFIG fields defined in Section 6.1; they do not introduce new
 protocol elements.

7.4.1. Objects

 Applications that prioritize text-safe output and maximum
 interoperability SHOULD use the default parameters (Section 5.1). No
 CONFIG block is required. The resulting SAFE object is entirely
 printable ASCII and can be embedded in email, JSON, YAML, or version-
 controlled files. AES-256-GCM is the default AEAD (Section 5.1).

7.4.2. Streaming

 Applications that process data sequentially at high throughput SHOULD
 consider:

 -----BEGIN SAFE CONFIG-----
 AEAD: aegis-256
 Data-Encoding: binary-linear
 -----END SAFE CONFIG-----

 AEGIS-256 offers high throughput and a 32-octet nonce that simplifies
 nonce management. Combined with binary-linear encoding
 (Section 6.4.1), this yields minimal framing overhead and sequential
 I/O without alignment padding. Encryptors using this profile SHOULD
 apply the hedged nonce construction (Section 5.4.2.2) or plaintext-
 bound nonce construction (Section 5.7.6.2) per Section 8.9.

 Encryptors targeting broad interoperability SHOULD verify recipient
 support before selecting this profile.

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7.4.3. Edit

 Applications that perform random-access reads and writes on encrypted
 data SHOULD consider:

 -----BEGIN SAFE CONFIG-----
 AEAD: aes-256-gcm-siv
 Block-Size: 16384
 Data-Encoding: binary
 -----END SAFE CONFIG-----

 AES-256-GCM-SIV is nonce-misuse resistant (Section 5.2.1), so per-
 block nonces are derived rather than stored (Section 5.7.5). This
 reduces per-block metadata from Nn + 16 octets to 16 octets. Block-
 Size 16384 aligns each block to a single page on 16 KiB-page systems,
 minimizing page faults per edit (Section 6.1.1). Binary aligned
 encoding (Section 6.4.2) enables O(1) random access to any block via
 memory-mapped I/O.

 Re-encrypting a modified block reuses the derived nonce for that
 block index. Because AES-256-GCM-SIV is nonce-misuse resistant, this
 degrades to deterministic encryption for unchanged blocks rather than
 catastrophic nonce reuse (Section 8.9.2).

7.4.4. FIPS Edit

 Applications that require FIPS 140-validated algorithms for random-
 access editing SHOULD consider:

 -----BEGIN SAFE CONFIG-----
 AEAD: aes-256-gcm
 Data-Encoding: binary
 Key-Epoch: 5
 -----END SAFE CONFIG-----

 AES-256-GCM is FIPS 140-validated; ChaCha20-Poly1305, AES-256-GCM-
 SIV, AEGIS-256, and AEGIS-256X2 are not currently covered by a FIPS
 140 validation program. Key-Epoch: 5 limits each epoch key to 32
 blocks, giving 2^27 - 1 (over 134 million) rewrites per block before
 exhausting the per-key nonce budget (Section 8.13.3). This is
 sufficient for virtually all editing workloads while adding
 negligible key-derivation overhead. The default Block-Size of 65536
 reduces the number of epoch key derivations relative to smaller block
 sizes.

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 Unlike the Edit profile (Section 7.4.3), this profile stores random
 per-block nonces (Nn + 16 octets per block vs. 16 octets for GCM-
 SIV). Applications that can tolerate block-level equality leakage on
 rewrite SHOULD prefer the Edit profile for its lower per-block
 overhead.

8. Security Considerations

 SAFE provides:

 Confidentiality: Indistinguishability under adaptive chosen-
 ciphertext attack (IND-CCA2) security for the payload, assuming
 IND-CCA2-secure AEAD. This follows from the standard reduction:
 SafeDerive is a PRF, so payload_key is indistinguishable from
 random; under a random key, the registered AEADs provide IND-CCA2
 per their specifications.

 Authentication: Each LOCK's Encrypted-CEK is authenticated via AEAD
 under derived_kek, which binds step tokens and step secrets
 through the KEK schedule. Block AEAD with index-bound AAD
 (Section 8.3) prevents reordering, modification, and splicing.
 The snapshot accumulator (Section 5.7.8) provides file-level
 integrity over all block tags.

 Binding: The KEK schedule binds encryption_parameters at
 initialization and final derivation (Section 5.7.1). Step tokens
 and per-step secrets are folded into the aggregator via sequential
 SafeDerive chaining. Payload keys inherit suite binding from
 their own SafeDerive calls (Section 5.7.3).

 SAFE does not provide:

 Encryptor authentication (Base mode): Without a sender parameter
 (sid or shint), any party with recipient public keys can create
 SAFE-encoded data. See Section 8.2 for Auth mode authentication
 properties.

 Forward secrecy: CEK compromise exposes all recipients' copies.
 This is inherent to stored-object encryption, which has no
 interactive key exchange.

 Unlinkability: Key identifiers enable linking SAFE-encoded data to
 the same recipient. See Section 8.6 for privacy modes.

 SAFE assumes secure key storage, side-channel resistant
 implementations, and trusted cryptographic primitives. A functioning
 CSPRNG is REQUIRED. See Section 8.9 for defenses against RNG
 weakness or state duplication.

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8.1. Threat Model

 SAFE defends against:

 Compromised storage provider (confidentiality): An adversary with
 read access to stored SAFE-encoded data cannot decrypt without
 valid credentials (passphrase or private key) for at least one
 LOCK. The adversary can observe approximate file size, recipient
 count, and key identifier linkability. SAFE detects block
 corruption (AEAD failure) and truncation (is_final mismatch), but
 does not detect LOCK removal, LOCK addition, or whole-file
 replacement by an adversary with write access.

 SAFE does not defend against:

 Compromised recipient: If a recipient's credentials (passphrase or
 private key) are compromised, the adversary can decrypt the
 payload. All recipients share the same CEK; compromise of one
 recipient's KEK does not expose other recipients' KEKs, but does
 expose the shared CEK and payload. The weakest LOCK determines
 the effective security of the entire file: an attacker who can
 satisfy any single LOCK recovers the CEK.

 Active attacker with key compromise: If an attacker compromises a
 recipient's private key and can modify files, they can create
 valid SAFE-encoded data for that recipient (in Base mode). Auth
 mode (Section 5.6.3.1) mitigates this for steps where the attacker
 does not also hold the sender's private key; see Section 8.2.

 Side-channel attacks: SAFE assumes implementations do not
 intentionally leak secrets. Timing attacks on Argon2id, HPKE, or
 AEAD operations are out of scope for this document.

 Malicious Encryptor: Any party with a recipient's public key can
 create valid SAFE-encoded data for that recipient. SAFE does not
 constrain what an Encryptor can encrypt or for whom. Applications
 MUST validate decrypted content independently of the encryption
 envelope.

8.2. Sender Authentication Properties

 When sid or shint is present in an hpke(...) step, SAFE uses HPKE
 Auth mode (mode_auth, [RFC9180]). Auth mode defends against forgery
 by parties who do not hold the sender's private key: a decryptor who
 successfully processes an auth-mode step is assured that the
 encapsulation was produced by a holder of skS. This closes the
 "encryptor authentication" gap identified above for Base mode, within
 the following limits:

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 Non-repudiation: Auth mode authenticates the sender only to the
 holder of the recipient's private key. The recipient cannot prove
 to a third party that the sender created the SAFE object.
 Applications requiring non-repudiation MUST use external
 signatures.

 Sender identity confidentiality: The sid parameter (when a Base64
 value) reveals the sender's key identifier to any observer. shint
 narrows the sender's identity. Anonymous Auth mode (sid=anon)
 avoids explicit sender identification, at the cost of trial
 decryption across all candidate sender keys.

 Sender key trust: SAFE does not define a trust model for sender
 public keys. Decryptors MUST independently verify that a sender's
 public key is authentic (e.g., via a certificate, trust on first
 use (TOFU), or out-of-band verification) before relying on auth-
 mode authentication.

8.3. Integrity and Authenticity

 The KEK schedule binds encryption_parameters at initialization and
 final derivation, with step tokens and per-step secrets folded via
 sequential SafeDerive chaining. The payload schedule binds
 payload_key to encryption_parameters independently, tying block
 encryption to the negotiated AEAD and Block-Size. Payload AEAD
 authenticates each block with index-bound AAD, preventing reordering
 and cross-file splicing. SAFE detects truncation and extension at
 block boundaries via is_final in block AAD (Section 5.7.9).

 The snapshot accumulator (Section 5.7.8) provides file-level
 integrity: a single Nh-octet value that binds all block tags under
 acc_key. Decryptors can verify whole-file consistency from metadata
 alone, without decrypting every block. The accumulator updates in
 O(1) when individual blocks are rewritten.

 Applications requiring third-party verifiability (e.g., signatures)
 MUST use external signatures.

8.4. Implementation Considerations

 The step sequence has AND semantics: an attacker must break every
 step to recover the CEK, so security is at least that of the
 strongest step. Compromising one step secret (e.g., a passphrase)
 allows the attacker to compute intermediate aggregator values up to
 that step; this is inherent to sequential chaining and does not
 weaken subsequent steps. Nonces MUST be unique: fresh lock_nonce per
 LOCK, and fresh random nonce per block. Implementations MUST zeroize
 sensitive values (CEK, KEK, PRKs, payload_key, acc_key, nonce_base)

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 immediately after use. Long-lived processes that retain a CEK (e.g.,
 for incremental writes) SHOULD store it in swap-protected memory
 (e.g., mlock). To prevent error oracles, implementations exposing
 decryption to untrusted callers (e.g., network services, APIs) MUST
 return a single generic "decryption failed" error rather than
 distinguishing between wrong passphrase, wrong key, commitment
 mismatch, or AEAD failure. Local tools (e.g., CLI applications, test
 harnesses) MAY use the detailed error codes in Appendix C for
 diagnostics. Implementations MUST use constant-time AEAD, KEM, KDF,
 commitment comparison, and accumulator verification operations. This
 extends to passphrase KDF evaluation and Base64 decoding of secret
 material (Encrypted-CEK, step parameters carrying key material).
 Commitment and accumulator comparisons MUST use a constant-time
 equality function (e.g., CRYPTO_memcmp or equivalent). Trial
 decryption loops (Section 8.6.3) MUST NOT leak timing information
 about which candidate key succeeded.

8.5. Passphrase KDF Selection

 SAFE supports two passphrase KDF variants with different security
 properties:

 pass(kdf=argon2id, ...): Memory-hard function that resists GPU and
 ASIC attacks. The default parameters (64 MiB memory, 2
 iterations) provide strong resistance to offline attacks.
 Recommended for most deployments.

 pass(kdf=pbkdf2, ...): Widely deployed function using PBKDF2-HMAC-
 SHA-256. Lacks memory-hardness, making it more vulnerable to GPU
 and ASIC attacks than Argon2id. The 600,000 iteration count
 provides equivalent CPU-based attack resistance but does not
 mitigate hardware-based attacks. Use only when policy prohibits
 memory-hard KDFs.

 Encryptors targeting Decryptors with mixed policy constraints MAY
 include two pass(...) LOCK blocks: one with pass(kdf=argon2id, ...)
 and one with pass(kdf=pbkdf2, ...), using the same passphrase but
 fresh salts for each.

 Applications SHOULD enforce a minimum passphrase complexity policy
 (e.g., at least 20 characters or equivalent entropy). For high-value
 data, Encryptors SHOULD combine a pass(...) step with an hpke(...)
 step in the same LOCK, so that compromise of the passphrase alone is
 insufficient.

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 Multiple LOCK blocks allow observers to infer shared payload access.
 HPKE key identifiers link files to the same recipient across objects.
 The Base64 length reveals approximate payload size; LOCK count
 reveals recipient count. Applications concerned about traffic
 analysis SHOULD pad payloads.

8.6. Recipient Anonymity and Trial Decryption

 SAFE supports three levels of recipient identification for hpke(...)
 steps:

 Identified mode: The id parameter uniquely identifies the
 recipient's public key. Observers can link SAFE-encoded data
 encrypted to the same recipient.

 Hinted mode: The hint parameter is a recipient-assigned value (not
 cryptographically derived). It filters candidates locally while
 revealing nothing about the key itself. Multiple keys may share
 the same hint.

 Anonymous mode: No identifier is present. Decryptors MUST trial-
 decrypt against all local keys matching the kem type. Provides
 maximum privacy at the cost of increased decryptor computation.

8.6.1. Privacy Benefits

 Omitting or replacing the key identifier with a hint prevents passive
 observers from mapping SAFE-encoded data to specific public keys.
 This is valuable when file-recipient associations are sensitive
 metadata.

8.6.2. Sender Anonymity

 Auth-mode hpke(...) steps support the same three levels of sender
 identification via sid and shint:

 Identified (sid present): The sid parameter identifies the sender's
 public key using the same Hash as id. Observers can link SAFE
 objects to the same sender across files.

 Hinted (shint present): The shint parameter is a sender-assigned
 value that filters candidates locally. It reveals less than sid
 but still narrows the sender's identity.

 Anonymous (neither sid nor shint): Decryptors trial-decrypt against
 all locally known sender keys matching the kem type. Provides
 sender privacy at the cost of increased trial decryption (see
 Section 8.6.3).

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 Encryptors SHOULD prefer sid unless sender privacy is required. The
 same trade-offs between identification, hinting, and anonymity apply
 to sender keys as to recipient keys.

8.6.3. Trial Complexity

 Anonymous mode with composable step sequences (multiple hpke steps)
 requires combinatorial trial decryption. For a step sequence with
 two anonymous hpke(...) steps, where the Decryptor holds K1 keys for
 step 1 and K2 keys for step 2, up to K1 x K2 combinations may be
 attempted. Auth-mode steps add a further multiplicative factor: if
 an auth-mode step has no sid or shint, the Decryptor MUST try all S
 candidate sender keys, multiplying the search space by S.

 Implementations MUST set a MaxTrialAttempts limit to bound
 computation and MUST reject LOCK blocks that would exceed this limit.
 A value of 1024 is RECOMMENDED; implementations MAY adjust based on
 deployment constraints.

8.7. Denial of Service Considerations

 An attacker can craft SAFE-encoded data with many anonymous LOCK
 blocks to force Decryptors into expensive cryptographic operations.
 Implementations MUST:

 * Limit the number of LOCK blocks processed per object

 * Prioritize identified blocks over hinted blocks over anonymous
 blocks

 * Abort early when resource limits are exceeded

 Implementations MUST also limit the number of steps per LOCK block.
 A limit of 16 steps per LOCK is RECOMMENDED; this prevents an
 attacker from crafting a single LOCK block that forces evaluation of
 an excessive number of passphrase KDF computations. Implementations
 SHOULD limit the total number of passphrase KDF evaluations to 8 per
 file; an attacker who crafts multiple LOCKs with pass(...) steps can
 otherwise force expensive Argon2id computations proportional to the
 LOCK count.

 ML-KEM decapsulation is significantly more expensive than X25519;
 anonymous ML-KEM steps amplify the DoS potential.

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8.8. Hint Assignment

 The hint is a 4-digit decimal value (0000-9999) assigned by the
 recipient; it is not solely dependent on the public key. Recipients
 communicate their hint to Encryptors out-of-band. Multiple keys MAY
 share the same hint.

 Encryptors MUST NOT assume the hint uniquely identifies a key.
 Decryptors MAY reassign hints at any time; Encryptors SHOULD refresh
 hint values periodically through out-of-band communication.

 Encryptors SHOULD prefer identified mode unless recipient privacy is
 required.

8.9. Nonce Generation and CEK Reuse

 Encryptors SHOULD use the hedged construction (Section 5.4.2.2) when
 a private key is available, the plaintext-bound nonce construction
 (Section 5.7.6.2) when RNG state duplication is a concern, and an NMR
 AEAD (Section 5.2.1) for additional protection. The following cases
 describe the resulting security properties.

 With private key, working RNG: Full protection. The block nonce
 base is derived from both fresh randomness and the hedge key.
 Nonces are unique across files and within files.

 With private key, duplicated RNG state: Deterministic encryption per
 Encryptor. Different Encryptors (with different private keys)
 produce different hedge keys and therefore different CEKs and
 nonce bases. Within a single file, block indices guarantee nonce
 uniqueness. Across files from the same encryptor, the CEK and
 salt repeat, producing identical payload keys and ciphertext for
 identical plaintext blocks at the same index. This leaks equality
 but not content. The plaintext-bound nonce construction
 (Section 5.7.6.2) further limits exposure: nonces differ when
 plaintext differs, even across files.

 Without private key, working RNG: Full protection. SafeRandom
 returns raw CSPRNG output. Nonce uniqueness depends on the RNG.

 Without private key, duplicated RNG state: No defense. CEKs and

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 salts repeat across files, producing identical payload keys and
 nonce bases. Within a file, block indices still provide distinct
 nonces. Across files, nonce reuse under distinct plaintext
 permits key recovery attacks against AES-GCM, ChaCha20-Poly1305,
 AEGIS-256, and AEGIS-256X2. AES-256-GCM-SIV limits the damage to
 deterministic encryption (leaks equality). The plaintext-bound
 nonce construction also limits nonce reuse to identical blocks at
 the same index.

 A functioning CSPRNG is REQUIRED when no private key is available.

 Encryptors operating in environments where RNG state duplication is
 possible (VM snapshots, process forks without reseed, container
 cloning) SHOULD use the plaintext-bound nonce construction
 (Section 5.7.6.2). Because the plaintext-bound construction
 incorporates a plaintext-dependent derivation via SafeDerive("SAFE-
 NONCE", plaintext_i, encryption_parameters, 32) into nonce
 derivation, two instances that share identical key material still
 produce distinct nonces whenever plaintext differs. The two-pass
 cost of this construction is justified by the defense it provides
 against state duplication.

 Within a single file, block indices are bounded by the plaintext
 length and Block-Size. Encryptors MUST ensure block indices remain
 below 2^64. Practical implementations SHOULD enforce a lower bound;
 for example, rejecting plaintexts exceeding 2^48 blocks
 (approximately 16 EiB at the default Block-Size of 65536 octets)
 provides a conservative margin while supporting files far larger than
 current storage systems.

8.9.1. File Extension

 Appending data to an existing SAFE file requires re-encrypting the
 old final block (which had is_final=1) with is_final=0, then
 encrypting the new blocks. Encryptors MUST NOT generate a new CEK or
 salt; the existing LOCKs and salt are reused. Encryptors MUST verify
 the snapshot accumulator before extending the file; extending a
 corrupted file propagates undetected damage. The procedure is:

 1. Decrypt the current final block (index N-1) and verify
 is_final=1.

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 2. Re-encrypt block N-1 with is_final=0. For non-NMR AEADs,
 Encryptors MUST generate a fresh nonce using SafeRandom
 (Section 5.4.2). For NMR AEADs, re-encrypting block N-1 at the
 same index produces the same nonce (nonce_base XOR uint64(N-1)).
 The AAD differs because is_final changes from 1 to 0; nonce-
 misuse resistance ensures security despite the repeated nonce.
 See Section 8.9.2.

 3. Encrypt new blocks N through N+K-1 with is_final=0.

 4. Encrypt block N+K (the new final block) with is_final=1.

 5. Update the metadata (nonces, tags) and block count N.

 6. Recompute the snapshot accumulator.

 For NMR AEADs, re-encryption of block N-1 at the same index reuses
 the same derived nonce. Because the AAD differs (is_final changed
 from 1 to 0), the NMR AEAD produces different ciphertext. This is
 the standard NMR equality-leakage property: an observer can detect
 that block N-1 was re-encrypted, but content is not revealed.

8.9.2. Derived Nonces

 For NMR AEADs, per-block nonces are derived deterministically from
 nonce_base and the block index rather than generated randomly and
 stored. This is restricted to NMR AEADs for the following reasons:

 Uniqueness: The SafeDerive output is unique per CEK (each CEK
 produces a distinct nonce_base). XOR with distinct block indices
 yields distinct nonces for all i < 2^64.

 Nonce reuse tolerance: Re-encrypting block i with the same CEK
 reuses nonce_i. NMR AEADs degrade gracefully to deterministic
 encryption: identical plaintext at the same index produces
 identical ciphertext, but no additional information is leaked.
 Non-NMR AEADs would suffer catastrophic nonce reuse, which is why
 derived nonces are not used with them.

8.10. Selective Editing Security

 Per-block random nonces and the is_final flag enable selective
 editing: individual blocks can be re-encrypted without affecting
 other blocks or LOCK blocks. When editing:

 * Generate a fresh random nonce for any re-encrypted block

 * Update the is_final flag if the last block changes

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 * Update the snapshot accumulator (Section 5.7.8)

 * Blocks not being edited retain their original nonces and
 ciphertexts

 The is_final flag prevents truncation and extension attacks:

 * Truncation: decrypting a block with is_final=0 when no successor
 exists indicates malicious or accidental truncation

 * Extension: appending blocks after a block with is_final=1 will
 fail AEAD verification because the original final block's AAD
 included is_final=1

8.11. Key Identifier Collisions

 Key identifiers are 32-octet hashes of SPKI DER encodings
 (Section 5.6.3.3). Both registered Hash algorithms (sha-256 and
 turboshake256) produce 32-octet output, giving a birthday bound on
 collision probability of approximately N^2 / 2^257 for a deployment
 with N keys. This is negligible for any practical key population.

 Implementers MUST NOT rely solely on key identifier matching for
 authorization; successful HPKE decapsulation and AEAD verification of
 Encrypted-CEK are required.

8.12. Key Commitment

 SAFE supports multiple LOCK blocks that can be added or removed
 independently without re-encrypting the payload, because each LOCK
 wraps the same CEK. However, removing a LOCK does not revoke access:
 any party who previously decrypted the CEK retains the ability to
 decrypt the payload. Applications requiring revocation MUST generate
 a new CEK and re-encrypt.

 Without key commitment, an adversary could craft LOCK blocks that
 decrypt to different CEKs and exploit AEAD malleability to create
 payload ciphertext valid under multiple keys:

 1. Creates LOCK₁ that wraps CEK₁ for recipient A

 2. Creates LOCK₂ that wraps CEK₂ for recipient B

 3. Crafts payload ciphertext C that decrypts to plaintext P₁ under
 payload_key₁ (derived from CEK₁) and to P₂ under payload_key₂
 (derived from CEK₂)

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 None of the AEADs registered for SAFE provide key commitment from the
 AEAD mechanism alone at the 128-bit security level (AES-GCM and
 ChaCha20-Poly1305 are well-known to lack this property; AEGIS-256
 with 128-bit tags provides only birthday-bound commitment at
 approximately 2^64). SAFE's external 32-octet commitment prefix
 (Section 5.7.3) supersedes the AEAD's native commitment properties,
 providing uniform 2^128 key-commitment security across all registered
 suites.

 The commitment prefix in the SAFE DATA block (Section 6.4.1) provides
 uniform key commitment for all AEAD choices. The commitment is
 always 32 octets, derived via SafeDerive("commit", CEK, payload_info,
 32) where payload_info = [...encryption_parameters, salt], per
 Section 5.7.3. Recipients verify that the derived commitment equals
 the prefix before block decryption. This binds the ciphertext to the
 CEK, the negotiated algorithm parameters, and the per-file salt,
 providing 2^128 key-commitment security and preventing cross-
 algorithm commitment collisions. The security relies on collision
 resistance of SafeDerive: the Encode() framing ensures unambiguous
 parsing of all inputs, so distinct (encryption_parameters, salt, CEK)
 tuples cannot produce the same commitment. Formally, for any two
 distinct input tuples the probability of a commitment collision is at
 most 2^(-128), under the assumption that the underlying KDF is a PRF.

8.13. AEAD Usage Bounds

 The security properties described in this section address distinct
 threats and are provided by separate mechanisms. Key commitment
 (Section 8.12) prevents an adversary from crafting ciphertext that
 decrypts to different plaintexts under different keys; the 32-octet
 commitment prefix solves this uniformly for all registered AEADs and
 is independent of nonce discipline. Block-level integrity
 (Section 5.7.9) prevents reordering, truncation, and extension; the
 block index and final-block indicator in each block's AAD solve this
 independently of nonce collision risk.

 Nonce collision risk is a separate concern. For non-NMR AEADs with
 stored random nonces, the primary practical constraint is the total
 number of block encryptions performed under one payload key over the
 file's lifetime. This total includes both initial writes and all
 subsequent rewrites of individual blocks. See Section 8.9 for
 defenses against RNG weakness and state duplication, including the
 hedged nonce construction (Section 5.4.2.2, [RFC8937]).

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8.13.1. Lifetime Encryption Budget

 Let q denote the total number of block encryptions under one payload
 key, counting every initial block write and every block rewrite.
 Because all recipients of a SAFE object share one CEK and therefore
 one payload key, the per-key analysis applies regardless of recipient
 count; adding recipients does not increase q. For AEADs with Nn-
 octet random nonces, the probability of at least one nonce collision
 among q encryptions is approximately:

 P_collision ≈ q^2 / 2^(8*Nn + 1)

 For 96-bit nonces (Nn = 12), this simplifies to approximately q^2 /
 2^97. This is a standard birthday-bound approximation, not a formal
 proof of the AEAD's concrete multi-user security.

 The following table illustrates q for representative workloads at the
 default Block-Size of 65536 octets:

 +======================+===============+
 | Total encrypted data | Approximate q |
 +======================+===============+
 | 1 TiB | 2^24 |
 +----------------------+---------------+
 | 16 TiB | 2^28 |
 +----------------------+---------------+
 | 256 TiB | 2^32 |
 +----------------------+---------------+

 Table 11

 The same values of q can be reached by smaller files that are
 rewritten many times. A 1 GiB file contains 2^14 blocks at the
 default block size; rewriting the entire file 1024 times produces
 approximately 2^24 total block encryptions, the same budget as
 encrypting 1 TiB once. The relevant quantity is lifetime encrypted
 blocks per payload key, not current file size.

 When Key-Epoch is present (Section 8.13.3), each epoch key covers at
 most 2^r blocks and their rewrites. The birthday bound then applies
 per epoch key rather than file-wide, so the effective q per key is
 bounded by the epoch size plus rewrites, not total file size. See
 Section 8.13.3 for per-epoch budget calculations.

 In a multi-user setting where an attacker targets any one of U
 independently keyed SAFE files, the effective collision probability
 is approximately U * q^2 / 2^(8*Nn + 1). For 96-bit nonces with U =
 2^20 files each at q = 2^32, this is approximately 2^(-13), which is

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 not negligible. Key-Epoch (Section 8.13.3) mitigates this by
 reducing the per-key q to the per-epoch budget; with Key-Epoch: 0
 each key encrypts a single block, making per-key q equal to the
 number of rewrites of that block. At more typical workloads without
 epochs (q = 2^24 per file), the multi-user bound is approximately
 2^(-29). The 256-bit nonce AEADs (AEGIS-256, AEGIS-256X2) render
 multi-user bounds operationally irrelevant (approximately 2^(-173) at
 the same parameters).

8.13.2. Per-AEAD Analysis

 AES-256-GCM: AES-256-GCM [NIST-SP-800-38D] [RFC5116] is nonce-
 respecting with 96-bit nonces. SAFE uses a fresh random nonce per
 block for this suite. With stored random nonces, the practical
 limit is birthday-bound nonce collision under a single payload
 key: approximately q^2 / 2^97. At q = 2^32 (approximately 256 TiB
 at the default block size, or equivalent rewrite volume), the
 collision probability is approximately 2^(-33), which is
 negligible for most applications. Nonce reuse under AES-GCM
 permits authentication-key recovery and full plaintext recovery
 for the affected blocks. Encryptors SHOULD include Key-Epoch
 (Section 8.13.3) for rewritable files to limit per-key invocations
 within the bounds of [NIST-SP-800-38D]. Key-Epoch: 0 is
 RECOMMENDED; Key-Epoch: 5 is a practical alternative when key-
 derivation overhead matters (see Section 8.13.3 for the tradeoff).
 Alternatively, implementations MAY generate a fresh CEK and re-
 wrap it in new LOCKs.

 ChaCha20-Poly1305: ChaCha20-Poly1305 [RFC8439] is nonce-respecting
 with 96-bit nonces. The ChaCha20-Poly1305 specification
 (Section 4 of [RFC8439]) requires nonce uniqueness per key and
 notes the collision risk of random nonces; SAFE's use of stored
 random nonces gives the same birthday-style collision accounting
 as AES-256-GCM (approximately q^2 / 2^97). Nonce reuse under
 ChaCha20-Poly1305 permits XOR of plaintexts for the affected
 blocks and compromises Poly1305 authentication. Block rewrites
 are especially relevant because the same logical block may be
 encrypted multiple times under the same payload key, each time
 consuming budget. Because [RFC8439] requires nonce uniqueness per
 key, Encryptors MUST include Key-Epoch when using
 chacha20-poly1305 (Section 6.1). Key-Epoch: 0 is RECOMMENDED;
 Key-Epoch: 5 is a practical alternative when key-derivation
 overhead matters. Key-Epoch (Section 8.13.3) confines the
 collision domain to rewrites within each epoch.

 AES-256-GCM-SIV: AES-256-GCM-SIV [RFC8452] is nonce-misuse resistant

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 (NMR). Under nonce reuse, it degrades to deterministic
 encryption: identical plaintext at the same block index produces
 identical ciphertext, leaking equality but not content. The
 collision analysis is therefore qualitatively different from AES-
 256-GCM and ChaCha20-Poly1305: the practical question is whether
 leaking block-level equality across rewrites is acceptable for a
 given application, not whether a nonce collision permits plaintext
 recovery. SAFE uses derived nonces for NMR AEADs (Section 5.7.5),
 which are deterministic per block index by design. Key-Epoch MUST
 NOT be present for this suite (Section 6.1). Unique nonces remain
 preferred to avoid equality leakage; misuse resistance does not
 provide unlimited safety. The concrete security bound for AES-
 256-GCM-SIV (Section 6 of [RFC8452]) includes a message-length-
 dependent term: the distinguishing advantage is bounded by
 approximately (q * l)^2 / 2^128, where q is the number of queries
 and l is the maximum message length in 128-bit blocks. At SAFE's
 default Block-Size of 65536 (l = 4096 blocks), this term is
 negligible for practical query counts.

 AEGIS-256: AEGIS-256 [I-D.irtf-cfrg-aegis-aead] is nonce-respecting
 with 256-bit nonces. The birthday bound for 256-bit random nonces
 is approximately q^2 / 2^257. At q = 2^48 (approximately 16 EiB
 at the default block size), the collision probability is
 approximately 2^(-161), which is negligible for any foreseeable
 SAFE workload. The larger nonce space makes random-nonce
 collision operationally irrelevant at SAFE scale. Key-Epoch adds
 no practical benefit for this suite (Section 8.13.3).
 Implementations MUST still generate fresh random nonces for each
 block encryption.

 AEGIS-256X2: AEGIS-256X2 shares the 256-bit nonce of AEGIS-256. The
 same analysis applies: the birthday bound of q^2 / 2^257 makes
 nonce collision negligible for any practical SAFE deployment. As
 with AEGIS-256, Key-Epoch adds no practical benefit. AEGIS-256X2
 provides higher throughput on wide-vector hardware; the nonce
 discipline requirements are identical to AEGIS-256.

8.13.3. Epoch Key Rotation

 Key-Epoch (Section 5.7.4) limits AEAD invocations per key. With Key-
 Epoch = r, each epoch key covers at most 2^r blocks (plus their
 rewrites). The birthday bound applies per epoch key, not file-wide.

 For 96-bit nonce AEADs, each epoch key's budget is 2^32 encryptions
 (P < 2^(-33)). The total rewrite budget per epoch key is 2^32 - 2^r
 encryptions, shared across all 2^r blocks in the epoch. Assuming
 uniform rewrite distribution:

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 avg_rewrites_per_block = 2^(32-r) - 1

 +===========+============+====================+
 | Key-Epoch | Epoch size | Avg rewrites/block |
 +===========+============+====================+
 | 0 | 1 | 2^32 - 1 |
 +-----------+------------+--------------------+
 | 5 | 32 | 2^27 (134M) |
 +-----------+------------+--------------------+
 | 8 | 256 | 2^24 (16M) |
 +-----------+------------+--------------------+
 | 16 | 65536 | 2^16 (65K) |
 +-----------+------------+--------------------+

 Table 12

 In a multi-user setting with U files, the per-epoch multi-user bound
 is U * q_epoch^2 / 2^(8*Nn + 1), where q_epoch is the total
 encryptions under one epoch key (at most 2^r initial blocks plus
 their rewrites). With Key-Epoch: 0 (r=0) and W rewrites per block,
 q_epoch = 1 + W; even at U = 2^20 files and W = 2^20 rewrites, the
 bound is approximately 2^20 * 2^40 / 2^97 = 2^(-37).

 If rewrites concentrate on a single block within an epoch, that block
 can consume the full epoch budget. With r=0 each key encrypts a
 single block, so the cross-block collision risk is eliminated and the
 only collision risk is across rewrites of that block — over 2^32
 rewrites to reach the threshold. [FLOE] Section 8 benchmarks the
 overhead of epoch key derivation across segment sizes and rotation
 masks; at r=0 the overhead is noticeable for small segments but
 decreases rapidly with segment size, and by r=5 it is dominated by
 the cost of processing the plaintext. Each epoch key at r=5 still
 permits over 134 million rewrites per block.

 Implementations SHOULD set Key-Epoch: 0 for maximum security. When
 throughput is critical (e.g., Hardware Security Module (HSM)-backed
 key derivation or constrained devices), Key-Epoch: 5 provides a
 practical alternative with strong security margins.

 For 256-bit nonce AEADs (AEGIS-256, AEGIS-256X2), epoch rotation adds
 no practical benefit. Implementations SHOULD NOT include Key-Epoch
 for these suites.

 Epoch keys are derived, not stored, so no LOCK re-wrapping is needed.
 This construction adapts the epoch-based key rotation of [FLOE] to
 SAFE's key schedule.

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8.13.4. Relationship to Key Commitment

 The nonce-collision analysis above is orthogonal to key commitment.
 As noted in Section 8.12, none of the AEADs registered for SAFE
 provide key commitment from the AEAD mechanism alone at the target
 security level. SAFE therefore provides a uniform file-level
 commitment prefix that is verified before block decryption.
 Commitment addresses wrong-key and cross-parameter ambiguity; the
 nonce analysis in this section addresses per-key AEAD lifetime and
 rewrite safety. The choice of layout (linear versus aligned,
 Section 6.4) does not change the cryptographic analysis; it affects
 only framing and on-disk storage layout.

8.14. Algorithm Agility and Post-Quantum Support

 SAFE accommodates post-quantum KEMs without format changes. ML-
 KEM-768 (HPKE KEM ID 0x0041) is registered and MAY be used as kem=ml-
 kem-768.

 Hybrid post-quantum constructions require no protocol extensions. An
 encryptor lists both a classical and a post-quantum hpke(...) step in
 the same step sequence:

 Step: hpke(kem=x25519, kemct=<Base64>, id=<classical-id>)
 Step: hpke(kem=ml-kem-768, kemct=<Base64>, id=<pq-id>)

 The KEK schedule (Section 5.7.1) folds each step's secret into the
 aggregator in order, so the derived KEK depends on both the X25519
 and ML-KEM-768 shared secrets. An attacker must break both KEMs to
 recover the KEK. Because the KEK schedule folds each step secret
 sequentially, the derived KEK's security is conjunctive: an attacker
 must break every step. Hybrid post-quantum protection follows
 naturally from combining classical and post-quantum steps.

 The decryptor evaluates both decapsulations during CEK recovery.
 Each KEM ciphertext is carried in the kemct parameter of its
 corresponding hpke(...) step token. See Appendix A for a complete
 example.

 Implementations planning PQ migration SHOULD ensure kemct parsing
 does not impose unnecessary length limits (ML-KEM-768 ciphertexts are
 1088 octets).

 Auth mode (Section 5.6.3.1) relies on DHKEM AuthEncap/AuthDecap,
 which requires a CDH-hard group. No post-quantum KEM currently
 supports Auth mode; ML-KEM-768 MUST NOT be used with Auth mode
 (Section 5.6.3.2). Applications requiring post-quantum sender
 authentication MUST use external signatures.

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8.15. Security Level and Design Notes

 SAFE derives all symmetric keys at 256 bits. The AEAD tag length
 (128 bits), accumulator contribution length (Nh = 256 bits), and
 commitment length (256 bits) provide at least 128-bit security
 against forgery and collision attacks.

 The KEK schedule initializer kek_init = SafeDerive("kek_init", "",
 encryption_parameters, 32) uses an empty ikm. This is a public
 derivation: its output is deterministic and computable by anyone who
 knows the encryption parameters. Security of the KEK relies on the
 step secrets folded in subsequent aggregator rounds, not on kek_init
 being secret.

 The Encode function is injective for a fixed number of arguments:
 distinct input tuples produce distinct outputs because each field is
 length-prefixed. This ensures that binding step_tokens are
 unambiguous.

 Per-block nonces are unique within a file by construction: for non-
 NMR AEADs, each nonce is generated via SafeRandom; for NMR AEADs,
 nonce_base XOR block_index is unique because block indices are
 unique. The relevant birthday bound for nonce collisions is across
 files or across block rewrites under the same payload key; see
 Section 8.13 for per-AEAD analysis.

8.16. Downgrade Resistance

 SAFE has no algorithm negotiation: the Encryptor selects
 encryption_parameters unilaterally, and the Decryptor either accepts
 them or fails. An active attacker who modifies encryption_parameters
 (e.g., substituting a weaker AEAD) changes the derived KEK
 (Section 5.7.1) and payload keys (Section 5.7.3), causing CEK
 unwrapping or block decryption to fail. Forging a valid SAFE object
 with altered parameters requires the attacker to also hold valid
 credentials for the target LOCK.

 Decryptors SHOULD enforce a locally configured allowlist of
 acceptable encryption parameters. Rejecting algorithms outside the
 allowlist limits the attack surface to supported primitives.

 The symmetric components of SAFE (HPKE export-only key schedules,
 SafeDerive-based KEK aggregation, commitment prefixes, and AEAD
 encryption) are not vulnerable to quantum attacks. Grover's
 algorithm provides at most a quadratic speedup against symmetric
 primitives, leaving all symmetric operations at or above 128-bit
 security. The post-quantum migration surface is limited to KEM
 selection.

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 A deployment using Hash=turboshake256 eliminates all SHA-256
 dependencies: per [I-D.ietf-hpke-hpke], all KEMs use the Hash-
 selected KDF internally.

9. Privacy Considerations

 This section addresses the privacy properties of SAFE per [RFC6973].
 SAFE is primarily a data-at-rest format; it does not define a
 transport protocol, so many [RFC6973] considerations (correlation by
 IP, traffic analysis of flows) do not apply directly.

 SAFE-encoded data reveals the following metadata to passive
 observers: approximate payload size (from Base64 or binary length),
 recipient count (from the number of LOCK blocks), and — when key
 identifiers are present — linkability across files encrypted to the
 same recipient (Section 8.6). Hinted mode (Section 8.6.1) reduces
 linkability; anonymous mode eliminates key-identifier-based linking
 at the cost of increased trial decryption. Auth-mode sender
 identifiers (sid, shint) create analogous sender linkability
 (Section 8.6.2). Applications concerned about metadata leakage
 SHOULD pad payloads and SHOULD prefer hinted or anonymous modes.

 Beyond the metadata listed above, the step types and KEM identifiers
 in each LOCK are visible in cleartext, revealing the authentication
 factors required (e.g., passphrase, X25519, ML-KEM-768). In multi-
 recipient files, the set of LOCK blocks reveals co-recipient
 relationships. For NMR AEADs, block rewrites at the same index
 produce identical ciphertext when the plaintext is unchanged, leaking
 equality (Section 8.9.2). A comprehensive [RFC6973] privacy analysis
 is deferred to a future revision.

10. IANA Considerations

10.1. SAFE AEAD Identifiers Registry

 IANA is requested to create a SAFE AEAD Identifiers registry.
 Registration policy is Specification Required. Designated Experts
 should verify that proposed AEADs provide [RFC5116] semantics with a
 16-octet authentication tag and Nk of 32 octets (SAFE derives all
 keys at 256 bits). Experts MUST reject registrations where Nk is not
 32. Identifiers MUST consist of lowercase ASCII letters, digits, and
 hyphens, and MUST NOT exceed 255 octets. The algorithm MUST be
 appropriate for general-purpose use in encrypted data formats.

 Initial entries:

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 +=================+==+==+===+===========+==========================+==========+
 |Identifier |Nk|Nn|NMR|Key-Epoch |Reference |Change |
 | | | | | | |Controller|
 +=================+==+==+===+===========+==========================+==========+
 |aes-256-gcm |32|12|No |Recommended|[NIST-SP-800-38D] |IETF |
 +-----------------+--+--+---+-----------+--------------------------+----------+
 |chacha20-poly1305|32|12|No |Required |[RFC8439] |IETF |
 +-----------------+--+--+---+-----------+--------------------------+----------+
 |aes-256-gcm-siv |32|12|Yes|Not |[RFC8452] |IETF |
 | | | | |Applicable | | |
 +-----------------+--+--+---+-----------+--------------------------+----------+
 |aegis-256 |32|32|No |Not |[I-D.irtf-cfrg-aegis-aead]|IETF |
 | | | | |Recommended| | |
 +-----------------+--+--+---+-----------+--------------------------+----------+
 |aegis-256x2 |32|32|No |Not |[I-D.irtf-cfrg-aegis-aead]|IETF |
 | | | | |Recommended| | |
 +-----------------+--+--+---+-----------+--------------------------+----------+

 Table 13

 Nk/Nn are key/nonce sizes in octets. Nn is required to compute block
 boundaries (Section 5.7.5). "NMR" indicates nonce-misuse resistance.
 "Key-Epoch" indicates whether Encryptors should include Key-Epoch in
 CONFIG; see Section 6.1 for normative requirements and Section 5.7.4
 for details.

10.2. SAFE KEM Identifiers Registry

 IANA is requested to create a SAFE KEM Identifiers registry. This
 registry maps SAFE's string identifiers (used in kem= parameters) to
 HPKE KEM IDs. Registration policy is Specification Required.
 Designated Experts should verify that the KEM is registered in the
 IANA HPKE KEM Identifiers registry, is compatible with HPKE export-
 only mode (AEAD ID 0xFFFF) as specified in Section 5.6.3, and that a
 specification for SPKI encoding of public keys is provided. The Auth
 column indicates whether the KEM supports AuthEncap/AuthDecap for
 HPKE Auth mode. Encryptors MUST NOT include sid or shint with KEMs
 where Auth=No.

 Initial entries:

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 +==========+======+======+====+===================================+==========+
 |Identifier|HPKE |Encap |Auth|SPKI Encoding |Change |
 | |KEM ID|Size | | |Controller|
 +==========+======+======+====+===================================+==========+
 |x25519 |0x0020|32 |Yes |[RFC8410] |IETF |
 | | |octets| | | |
 +----------+------+------+----+-----------------------------------+----------+
 |p-256 |0x0010|65 |Yes |[RFC5480] |IETF |
 | | |octets| | | |
 +----------+------+------+----+-----------------------------------+----------+
 |ml-kem-768|0x0041|1088 |No |[I-D.ietf-lamps-kyber-certificates]|IETF |
 | | |octets| | | |
 +----------+------+------+----+-----------------------------------+----------+

 Table 14

 HPKE KEM IDs are defined in the IANA HPKE KEM Identifiers registry
 established by [RFC9180]. Identifiers MUST consist of lowercase
 ASCII letters, digits, and hyphens, and MUST NOT exceed 255 octets.

10.3. SAFE KDF Identifiers Registry

 IANA is requested to create a SAFE KDF Identifiers registry.
 Registration policy is Specification Required. Designated Experts
 should verify that identifiers consist of lowercase ASCII letters,
 digits, and hyphens, do not exceed 255 octets, that the underlying
 KDF provides at least 128-bit security, and that the entry references
 a KDF registered in the HPKE KDF Identifiers registry (Section 7.2 of
 [RFC9180]).

 Each entry MUST reference a KDF registered in the HPKE KDF
 Identifiers registry. Two-stage KDFs provide Extract(salt, ikm),
 Expand(prk, info, L), and Nh. Single-stage KDFs ([I-D.ietf-hpke-pq])
 provide Derive(ikm, L). The Nh column specifies the output size in
 octets used for accumulator contributions and commitment derivation;
 Nh MUST be 32 for all registrations. The Class column determines
 which SafeDerive instantiation is used (see Section 5.4).

 The CONFIG field name remains "Hash" for wire compatibility; the IANA
 registry is named "SAFE KDF Identifiers" because the identifiers
 select a KDF (and its underlying hash function) rather than a bare
 hash algorithm.

 All conforming implementations MUST implement sha-256, which is the
 default when Hash is omitted from the SAFE CONFIG. Implementations
 MAY implement turboshake256.

 Initial entries:

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 +=============+======+============+==+==================+==========+
 |Identifier |HPKE |Class |Nh|Reference |Change |
 | |KDF ID| | | |Controller|
 +=============+======+============+==+==================+==========+
 |sha-256 |0x0001|two-stage |32|[RFC5869] |IETF |
 +-------------+------+------------+--+------------------+----------+
 |turboshake256|TBD |single-stage|32|[I-D.ietf-hpke-pq]|IETF |
 +-------------+------+------------+--+------------------+----------+

 Table 15

 [RFC Editor: The HPKE KDF ID for turboshake256 is pending allocation
 in the HPKE KDF Identifiers registry per [I-D.ietf-hpke-pq]. Replace
 "TBD" with the assigned value before publication.]

10.4. SAFE Step Names Registry

 IANA is requested to create a SAFE Step Names registry. Each
 registration defines a step type conforming to the interface in
 Section 5.6. The registry has the following columns:

 Step Name: Unique ASCII identifier for the step type (e.g., "pass",
 "hpke").

 Parameters Grammar: ABNF grammar for step-specific parameters in the
 token, or "None" if no parameters.

 Inputs: Description of required inputs (e.g., "user passphrase,
 salt", "recipient private key, kemct").

 Secret Length: MUST be 32 octets for all registered steps.

 Reference: Document specifying the step's derivation algorithm.

 Registration policy is Specification Required. Designated Experts
 MUST verify:

 * The derivation algorithm is deterministic and produces exactly 32
 octets

 * Parameter names do not conflict with existing registrations

 * The specification provides complete implementation guidance
 including the Encode binding form

 Step names MUST match the grammar 1*(ALPHA / DIGIT / "-") and MUST
 NOT exceed 255 octets.

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 Initial entries:

 +====+==============+=============+========+===========+============+
 |Step| Parameters | Inputs | Secret | Reference | Change |
 |Name| Grammar | | Length | | Controller |
 +====+==============+=============+========+===========+============+
 |pass| kdf, salt, | passphrase, | 32 | Section | IETF |
 | | (t, p, m | salt | octets | 5.6.2 | |
 | | for | | | | |
 | | argon2id; c | | | | |
 | | for pbkdf2) | | | | |
 +----+--------------+-------------+--------+-----------+------------+
 |hpke| kem, kemct, | private | 32 | Section | IETF |
 | | id, (sid, | key, kemct | octets | 5.6.3 | |
 | | shint for | | | | |
 | | auth) | | | | |
 +----+--------------+-------------+--------+-----------+------------+

 Table 16

 The pass step's algorithm variant (argon2id or pbkdf2) is specified
 in the step token's kdf parameter. The default KDF is argon2id;
 pbkdf2 is available for environments where policy prohibits Argon2id.

 The hpke step additionally requires the step token itself as input.
 When sid or shint is present, the sender's private key (for
 encryption) or public key (for decryption) is also required.
 Supported kem values are defined in Section 5.6.3.2; key identifier
 computation is defined in Section 5.6.3. hpke(...) with sid or shint
 is OPTIONAL and limited to DHKEM-based KEMs (x25519, p-256).

 Future registrations MAY define additional step types (e.g., hardware
 token, Oblivious Pseudorandom Function (OPRF)) or variant algorithms
 for existing step names (subject to Designated Expert review for
 interoperability impact). A registration request MUST include:

 * Step Name and Parameters Grammar (ABNF)

 * Complete list of Inputs with their sources

 * Derivation algorithm producing exactly 32 octets

 * Definition of any step-specific parameters (name, encoding,
 semantics)

 * Security considerations for the step type

 See Appendix L for an illustrative example.

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10.5. SAFE Config Options Registry

 IANA is requested to create a SAFE Config Options registry. Each
 registration defines a CONFIG field name and the registry or value
 set that defines its legal values. The registry has the following
 columns:

 Field Name: Case-sensitive ASCII field name used in SAFE CONFIG.

 Value Definition: Registry or fixed set of values allowed for the
 field.

 Reference: Document specifying the field and its semantics.

 Registration policy is Specification Required. Designated Experts
 MUST verify that the field name does not conflict with existing
 registrations, that the specification defines default behavior when
 the field is absent, and that the value definition is unambiguous.

 Initial entries:

 +===============+=======================+===========+============+
 | Field Name | Value Definition | Reference | Change |
 | | | | Controller |
 +===============+=======================+===========+============+
 | AEAD | SAFE AEAD Identifiers | Section | IETF |
 | | registry | 10.1 | |
 +---------------+-----------------------+-----------+------------+
 | Block-Size | 16384, 65536 | Section | IETF |
 | | | 6.1 | |
 +---------------+-----------------------+-----------+------------+
 | Hash | SAFE KDF Identifiers | Section | IETF |
 | | registry | 10.3 | |
 +---------------+-----------------------+-----------+------------+
 | Key-Epoch | Non-negative integer; | Section | IETF |
 | | absent when disabled | 5.7.4 | |
 +---------------+-----------------------+-----------+------------+
 | Lock-Encoding | armored (default), | Section | IETF |
 | | readable | 6.2 | |
 +---------------+-----------------------+-----------+------------+
 | Data-Encoding | armored (default), | Section | IETF |
 | | binary, binary-linear | 6.3 | |
 +---------------+-----------------------+-----------+------------+

 Table 17

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 The default encoding is "armored" when the Data-Encoding field is
 omitted. The default Lock-Encoding is "armored" when omitted. Key-
 Epoch is absent (disabled) when omitted. Block-Size values are fixed
 by this specification; adding new values requires a Standards Track
 document update.

10.6. SAFE Block Types Registry

 IANA is requested to create a SAFE Block Types registry. This
 registry lists the block types that may appear in SAFE-encoded data,
 identified by their fence markers. The registry has the following
 columns:

 Block Type: The block type name as it appears in fence markers
 (e.g., "CONFIG", "LOCK", "DATA").

 Fence Marker: The opening fence marker string (e.g., "-----BEGIN
 SAFE CONFIG-----").

 Ignorable: "Yes" if Decryptors MAY skip unrecognized instances of
 this block type without failing; "No" if Decryptors MUST fail when
 encountering an unrecognized block of this type.

 Reference: Document defining the block's semantics.

 Registration policy is Specification Required. Designated Experts
 MUST verify:

 * The block type name does not conflict with existing registrations

 * The specification clearly defines the block's syntax and semantics

 * Blocks marked Ignorable=Yes do not affect security or correctness
 if omitted

 Block type names MUST consist of uppercase ASCII letters and hyphens
 and MUST NOT exceed 32 octets.

 Initial entries:

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 +============+=================+===========+===========+============+
 | Block | Fence Marker | Ignorable | Reference | Change |
 | Type | | | | Controller |
 +============+=================+===========+===========+============+
 | CONFIG | -----BEGIN SAFE | No | Section | IETF |
 | | CONFIG----- | | 6.1 | |
 +------------+-----------------+-----------+-----------+------------+
 | LOCK | -----BEGIN SAFE | No | Section | IETF |
 | | LOCK----- | | 6.2 | |
 +------------+-----------------+-----------+-----------+------------+
 | DATA | -----BEGIN SAFE | No | Section | IETF |
 | | DATA----- | | 6.3.1 | |
 +------------+-----------------+-----------+-----------+------------+

 Table 18

 All initial block types are critical (Ignorable=No). Future
 extensions MAY register new block types with Ignorable=Yes for
 optional features such as detached signatures, metadata, or recipient
 hints.

10.7. Media Type Registration

 IANA is requested to register the following media type per [RFC6838]:

 Type name: application

 Subtype name: safe

 Required parameters: None

 Optional parameters: None

 Encoding considerations: Binary or 7bit. Armored-encoded SAFE data
 (the default) consists of ASCII printable characters and line
 feeds, with Base64 encoding for payload data. Binary-encoded SAFE
 data have ASCII headers followed by raw binary payload data.

 Security considerations: SAFE-encoded data contain encrypted
 content. See Section 8 of this document.

 Interoperability considerations: SAFE-encoded data are ASCII-armored
 with PEM-style fence markers. Line wrapping of Base64 content is
 permitted; Decryptors MUST accept any line length.

 Published specification: This document

 Applications that use this media type: File encryption, secure file

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 sharing, encrypted backups

 Fragment identifier considerations: None

 Additional information: Deprecated alias names for this type: None

 Magic number(s): Files begin with "-----BEGIN SAFE" (ASCII)

 File extension(s): .safe

 Macintosh file type code(s): None

 Person & email address to contact for further information: N.
 Sullivan (nicholas.sullivan+ietf@gmail.com)

 Intended usage: COMMON

 Restrictions on usage: None

 Author: See Authors' Addresses section

 Change controller: IETF

11. References

11.1. Normative References

 [I-D.ietf-hpke-hpke]
 Barnes, R., Bhargavan, K., Lipp, B., and C. A. Wood,
 "Hybrid Public Key Encryption", Work in Progress,
 Internet-Draft, draft-ietf-hpke-hpke-03, 2 March 2026,
 <https://datatracker.ietf.org/doc/html/draft-ietf-hpke-
 hpke-03>.

 [I-D.ietf-hpke-pq]
 Barnes, R. and D. Connolly, "Post-Quantum and Post-
 Quantum/Traditional Hybrid Algorithms for HPKE", Work in
 Progress, Internet-Draft, draft-ietf-hpke-pq-04, 2 March
 2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
 hpke-pq-04>.

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 [I-D.ietf-lamps-kyber-certificates]
 Turner, S., Kampanakis, P., Massimo, J., and B.
 Westerbaan, "Internet X.509 Public Key Infrastructure -
 Algorithm Identifiers for the Module-Lattice-Based Key-
 Encapsulation Mechanism (ML-KEM)", Work in Progress,
 Internet-Draft, draft-ietf-lamps-kyber-certificates-11, 22
 July 2025, <https://datatracker.ietf.org/doc/html/draft-
 ietf-lamps-kyber-certificates-11>.

 [I-D.irtf-cfrg-aegis-aead]
 Denis, F. and S. Lucas, "The AEGIS Family of Authenticated
 Encryption Algorithms", Work in Progress, Internet-Draft,
 draft-irtf-cfrg-aegis-aead-18, 5 October 2025,
 <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
 aegis-aead-18>.

 [NIST-SP-800-38D]
 Dworkin, M., "Recommendation for Block Cipher Modes of
 Operation: Galois/Counter Mode (GCM) and GMAC",
 NIST Special Publication 800-38D, November 2007,
 <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
 nistspecialpublication800-38d.pdf>.

 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/rfc/rfc2119>.

 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
 <https://www.rfc-editor.org/rfc/rfc4648>.

 [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
 Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
 <https://www.rfc-editor.org/rfc/rfc5116>.

 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
 Specifications: ABNF", STD 68, RFC 5234,
 DOI 10.17487/RFC5234, January 2008,
 <https://www.rfc-editor.org/rfc/rfc5234>.

 [RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
 "Elliptic Curve Cryptography Subject Public Key
 Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
 <https://www.rfc-editor.org/rfc/rfc5480>.

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 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
 Key Derivation Function (HKDF)", RFC 5869,
 DOI 10.17487/RFC5869, May 2010,
 <https://www.rfc-editor.org/rfc/rfc5869>.

 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
 Specifications and Registration Procedures", BCP 13,
 RFC 6838, DOI 10.17487/RFC6838, January 2013,
 <https://www.rfc-editor.org/rfc/rfc6838>.

 [RFC8018] Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
 Password-Based Cryptography Specification Version 2.1",
 RFC 8018, DOI 10.17487/RFC8018, January 2017,
 <https://www.rfc-editor.org/rfc/rfc8018>.

 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

 [RFC8410] Josefsson, S. and J. Schaad, "Algorithm Identifiers for
 Ed25519, Ed448, X25519, and X448 for Use in the Internet
 X.509 Public Key Infrastructure", RFC 8410,
 DOI 10.17487/RFC8410, August 2018,
 <https://www.rfc-editor.org/rfc/rfc8410>.

 [RFC8439] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
 Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
 <https://www.rfc-editor.org/rfc/rfc8439>.

 [RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
 Nonce Misuse-Resistant Authenticated Encryption",
 RFC 8452, DOI 10.17487/RFC8452, April 2019,
 <https://www.rfc-editor.org/rfc/rfc8452>.

 [RFC9106] Biryukov, A., Dinu, D., Khovratovich, D., and S.
 Josefsson, "Argon2 Memory-Hard Function for Password
 Hashing and Proof-of-Work Applications", RFC 9106,
 DOI 10.17487/RFC9106, September 2021,
 <https://www.rfc-editor.org/rfc/rfc9106>.

 [RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
 Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
 February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

11.2. Informative References

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 [AEA] Apple Inc., "Apple Encrypted Archive (AEA)", 2024,
 <https://support.apple.com/guide/security/protecting-app-
 access-to-user-data-secddd150c21/web>.

 [AGE] Valsorda, F., "The age file encryption format", 2025,
 <https://age-encryption.org/v1>.

 [AGE-COMMIT]
 Stäuble, M., "Actually Good Encryption? Confusing Users by
 Changing Nonces", ETH Zürich Semester Project, 2022,
 <https://ethz.ch/content/dam/ethz/special-interest/infk/
 inst-infsec/appliedcrypto/education/theses/semester-
 project_mirco-stauble.pdf>.

 [FLOE] Fábrega, A., Len, J., Ristenpart, T., and G. Rubin,
 "Random-Access AEAD for Fast Lightweight Online
 Encryption", IACR ePrint 2025/2275, 2025,
 <https://eprint.iacr.org/2025/2275>.

 [I-D.ietf-ohai-chunked-ohttp]
 Pauly, T. and M. Thomson, "Chunked Oblivious HTTP
 Messages", Work in Progress, Internet-Draft, draft-ietf-
 ohai-chunked-ohttp-08, 18 February 2026,
 <https://datatracker.ietf.org/doc/html/draft-ietf-ohai-
 chunked-ohttp-08>.

 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
 RFC 5652, DOI 10.17487/RFC5652, September 2009,
 <https://www.rfc-editor.org/rfc/rfc5652>.

 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
 Morris, J., Hansen, M., and R. Smith, "Privacy
 Considerations for Internet Protocols", RFC 6973,
 DOI 10.17487/RFC6973, July 2013,
 <https://www.rfc-editor.org/rfc/rfc6973>.

 [RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
 RFC 7516, DOI 10.17487/RFC7516, May 2015,
 <https://www.rfc-editor.org/rfc/rfc7516>.

 [RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
 and C. Wood, "Randomness Improvements for Security
 Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
 <https://www.rfc-editor.org/rfc/rfc8937>.

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 [RFC9497] Davidson, A., Faz-Hernandez, A., Sullivan, N., and C. A.
 Wood, "Oblivious Pseudorandom Functions (OPRFs) Using
 Prime-Order Groups", RFC 9497, DOI 10.17487/RFC9497,
 December 2023, <https://www.rfc-editor.org/rfc/rfc9497>.

 [RFC9578] Celi, S., Davidson, A., Valdez, S., and C. A. Wood,
 "Privacy Pass Issuance Protocols", RFC 9578,
 DOI 10.17487/RFC9578, June 2024,
 <https://www.rfc-editor.org/rfc/rfc9578>.

 [RFC9580] Wouters, P., Ed., Huigens, D., Winter, J., and Y. Niibe,
 "OpenPGP", RFC 9580, DOI 10.17487/RFC9580, July 2024,
 <https://www.rfc-editor.org/rfc/rfc9580>.

 [RFC9605] Omara, E., Uberti, J., Murillo, S. G., Barnes, R., Ed.,
 and Y. Fablet, "Secure Frame (SFrame): Lightweight
 Authenticated Encryption for Real-Time Media", RFC 9605,
 DOI 10.17487/RFC9605, August 2024,
 <https://www.rfc-editor.org/rfc/rfc9605>.

 [STREAM] Hoang, V. T., Reyhanitabar, R., Rogaway, P., and D. Vizár,
 "Online Authenticated-Encryption and its Nonce-Reuse
 Misuse-Resistance", IACR ePrint 2015/189, 2015,
 <https://eprint.iacr.org/2015/189>.

 [W3C.webauthn-3]
 "Web Authentication: An API for accessing Public Key
 Credentials - Level 3", W3C WD webauthn-3, W3C webauthn-3,
 <https://www.w3.org/TR/webauthn-3/>.

Appendix A. Examples

 This appendix is informative.

 Note: The examples in this section illustrate structure and
 formatting only. The Base64 values are placeholders and do not
 represent valid cryptographic outputs. Implementers requiring test
 vectors with known inputs and outputs should consult the Test Vectors
 appendix (Appendix G).

 A minimal object with readable LOCK format (Lock-Encoding set to
 readable; aes-256-gcm, commitment prefix):

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 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: pass(kdf=argon2id, salt=c2FsdHZhbHVlMTIzNDU2Nzg=)
 Encrypted-CEK:
 MTIzNDU2Nzg5MDEyM0FCQ0RFRkdISUpLTE1OT1BRUlNU
 VVZXWFlaYWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXowMTIzNA==
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 cGxhY2Vob2xkZXJjZWtjb21taXRtZW50aGFzaDEyMzQ1Njc4YWJjZGVmZ2hpamts
 bW5vcHFyc3R1dnd4eXpBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWjAxMjM0NTY3
 ODkrLz09
 -----END SAFE DATA-----

 A LOCK block encoded as armored (default Lock-Encoding):

 -----BEGIN SAFE LOCK-----
 U3RlcDpwYXNzKGtkZj1hcmdvbjJpZCxzYWx0PWMyRnNkSFpoYkhWbE1USXpORFUy
 TnpnPSlFbmNyeXB0ZWQtQ0VLOk1USXpORFUyTnpnNU1ERXlNMEZDUTBSRlJrZElT
 VXBMVEUxT1QxQlJVbE5VVlZaWFdGbGFZV0pqWkdWbVoyaHBhbXRzYlc1dmNIRnlj
 M1IxZG5kNGVYb3dNVEl6TkE9PQ==
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 YmFzZTY0bG9ja2V4YW1wbGVkYXRhY2h1bmtwbGFjZWhvbG
 RlcmV4YW1wbGVkYXRhMTIzNDU2
 -----END SAFE DATA-----

 HPKE recipient in armored format (derived from the readable example
 above):

 -----BEGIN SAFE CONFIG-----
 Block-Size: 16384
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 U3RlcDpocGtlKGtlbT14MjU1MTksaWQ9Wmk2bVFuWTVqOHBJWXEzbzZyV2dOdz09
 LGtlbWN0PVlXSmpaR1ZtWjJocGFtdHNiVzV2Y0hGeWMzUjFkbmQ0ZVhveE1qTTBO
 VFkzT0RrdylFbmNyeXB0ZWQtQ0VLOmNHRnpjM2R2Y21ReE1qTTBOVFkzT0Rrd1lX
 SmpaR1ZtWjJocGFtdHNiVzV2Y0hGeWMzUjFkbmQ0ZVhveE1qTTBOVFkzT0Rrd1lX
 SmpaR1ZtWjJocGFnPT0=
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 aHBrZWV4YW1wbGVjZWtjb21taXRtZW50aGFzaHZhbHVlMTJWR2hwY3lCcGN5QmhJ
 SE5oYlhCc1pTQmxibU55ZVhCMFpXUWdjR0Y1Ykc5aFpDQjNhWFJvSUcxMWJIUnBj
 R3hsSUdOb2RXNXJjdz09
 -----END SAFE DATA-----

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 A HPKE recipient with non-default Block-Size (AEAD omitted, uses
 default aes-256-gcm with commitment prefix):

 -----BEGIN SAFE CONFIG-----
 Block-Size: 16384
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: hpke(kem=x25519, id=Zi6mQnY5j8pIYq3o6rWgNw==,
 kemct=YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXoxMjM0NTY3ODkw)
 Encrypted-CEK:
 cGFzc3dvcmQxMjM0NTY3ODkwYWJjZGVmZ2hpamtsbW5vcHFy
 c3R1dnd4eXoxMjM0NTY3ODkwYWJjZGVmZ2hpag==
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 aHBrZWV4YW1wbGVjZWtjb21taXRtZW50aGFzaHZhbHVlMTJWR2hwY3lCcGN5QmhJ
 SE5oYlhCc1pTQmxibU55ZVhCMFpXUWdjR0Y1Ykc5aFpDQjNhWFJvSUcxMWJIUnBj
 R3hsSUdOb2RXNXJjdz09
 -----END SAFE DATA-----

 HPKE recipient with Hash: turboshake256 (32-octet key identifier,
 default aes-256-gcm with commitment prefix):

 -----BEGIN SAFE CONFIG-----
 Hash: turboshake256
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: hpke(kem=x25519,
 id=dHVyYm9zaGFrZTI1NmV4YW1wbGVoYXNoMzJvY3RldHM=,
 kemct=dHVyYm9zaGFrZWtlbWNpcGhlcnRleHRleGFtcGxlMTIz)
 Encrypted-CEK:
 dHVyYm9ub25jZTEyMzQ1Njc4OWFiY2RlZmdoaWprbG1u
 b3BxcnN0dXZ3eHl6MDEyMzQ1Njc4OTBhYmNkZWZnaGk=
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 dHVyYm9zaGFrZWNla2NvbW1pdG1lbnRleGFtcGxlaGFzaDFSeGhiWEJzWlNCMWMy
 bHVaeUJ6Y0d0cExYUjFjbUp2YzJoaGEyVXlOVFlnYTJWNUlHbGtaVzUwYVdacFpY
 SWdkMmwwYUNBek1pMXZZM1JsZENBPT0=
 -----END SAFE DATA-----

 AEGIS-256 with TurboShake key identifier (commitment prefix in DATA):

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 -----BEGIN SAFE CONFIG-----
 AEAD: aegis-256
 Hash: turboshake256
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: hpke(kem=x25519,
 id=YWVnaXN0dXJib3NoYWtlaWRlbnRpZmllcjMyYnl0ZXM=,
 kemct=YWVnaXNrZW1jaXBoZXJ0ZXh0ZXhhbXBsZTEyMzQ1Njc4)
 Encrypted-CEK:
 YWVnaXNub25jZTEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIz
 YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXowMTIzNDU2
 Nzg5MGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 YWVnaXNub2NvbW1pdG1lbnRjaHVua2NpcGhlcnRleHRvbmx5ZXhhbXBsZWRhdGE=
 -----END SAFE DATA-----

 Anonymous X25519 recipient (trial decryption required):

 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: hpke(kem=x25519,
 kemct=YW5vbnltb3VzZW5jYXBzdWxhdGVka2V5bWF0ZXJpYWwx
 MjM0NTY3ODkwYWJjZGVmZ2hpamtsbW5vcHFyc3R1dg==)
 Encrypted-CEK:
 YW5vbnltb3VzZW5jcnlwdGVkY2VrZGF0YTEyMzQ1Njc4
 OTBhYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3h5ejAxMg==
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 YW5vbnltb3VzY2VrY29tbWl0bWVudGhhc2h2YWx1ZXBheWxvYWRjaHVua2RhdGE=
 -----END SAFE DATA-----

 Hinted ML-KEM-768 recipient (4-digit hint for candidate filtering):

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 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: hpke(kem=ml-kem-768, hint=4217,
 kemct=bWxrZW1jaXBoZXJ0ZXh0d2l0aGhpbnRlZHJlY2lwaWVu
 dGZpbHRlcmluZ2V4YW1wbGVkYXRhMTIzNDU2Nzg5MA==)
 Encrypted-CEK:
 aGludGVkZW5jcnlwdGVkY2VrZGF0YWV4YW1wbGUxMjM0
 NTY3ODkwYWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXo=
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 aGludGVkY2VrY29tbWl0bWVudGhhc2h2YWx1ZXBheWxvYWRjaHVua2RhdGFoZXJl
 -----END SAFE DATA-----

 Two recipients, one passphrase-only and one HPKE (default aes-
 256-gcm):

 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: pass(kdf=argon2id, salt=cHdkc2FsdDEyMzQ1Njc4OTA=)
 Encrypted-CEK:
 cHdkbm9uY2UxMjM0NTY3ODkwYWJjZGVmZ2hpamtsbW5vcHFy
 c3R1dnd4eXowMTIzNDU2Nzg5MGFiY2RlZmdoaWo=
 -----END SAFE LOCK-----
 -----BEGIN SAFE LOCK-----
 Step: hpke(kem=p-256, id=Z1d0u6QG0cB2a4nM3Kp2Ww==,
 kemct=QUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVphYmNkZWZnaGlq
 a2xtbm9wcXJzdHV2d3h5ejAxMjM0NTY3ODkwYWJjZGVmZ2hp
 amtsbW5vcHFyc3R1dnd4eXo=)
 Encrypted-CEK:
 aHBrZW5vbmNlMTIzNDU2Nzg5MGFiY2RlZmdoaWprbG1ub3Bx
 cnN0dXZ3eHl6MDEyMzQ1Njc4OTBhYmNkZWZnaGlq
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 bXVsdGlyZWNpcGllbnRjZWtjb21taXRtZW50aGFzaHZhbHVlVFhWc2RHa3RjbVZq
 YVhCcFpXNTBJR1Y0WVcxd2JHVWdkMmwwYUNCemFHRnlaV1FnY0dGNWJHOWhaQ0Js
 Ym1OeWVYQjBaV1FnYjI1alpRPT0=
 -----END SAFE DATA-----

 Multi-step sequence with AND semantics (passphrase AND HPKE,
 chacha20-poly1305 with commitment prefix):

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 -----BEGIN SAFE CONFIG-----
 AEAD: chacha20-poly1305
 Block-Size: 16384
 Key-Epoch: 0
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: pass(kdf=argon2id, salt=bXVsdGlzdGVwc2FsdDEyMzQ=)
 Step: hpke(kem=x25519, id=eEF3bXlyT3BWbXpLUjRCdz09,
 kemct=bXVsdGlzdGVwZXhhbXBsZWtlbWNpcGhlcnRleHQxMjM0NTY3)
 Encrypted-CEK:
 bXVsdGlzdGVwbm9uY2UxMjM0NTY3ODkwYWJjZGVmZ2hpamts
 bW5vcHFyc3R1dnd4eXowMTIzNDU2Nzg5MGFiYw==
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 bXVsdGlzdGVwY2VrY29tbWl0bWVudGhhc2h2YWx1ZTEyMzQ1VW1WeGRXbHlaWE1n
 WW05MGFDQndZWE56ZDI5eVpDQkJUa1FnV0RJMU5URTVJSEJ5YVhaaGRHVWdhMlY1
 SUhSdklHUmxZM0o1Y0hRPQ==
 -----END SAFE DATA-----

 Same multi-step example in armored format (default Lock-Encoding):

 -----BEGIN SAFE CONFIG-----
 AEAD: chacha20-poly1305
 Block-Size: 16384
 Key-Epoch: 0
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 U3RlcDpwYXNzKGtkZj1hcmdvbjJpZCxzYWx0PWJYVnNkR2x6ZEdWd2MyRnNkREV5
 TXpRPSlTdGVwOmhwa2Uoa2VtPXgyNTUxOSxpZD1lRUYzYlhseVQzQldiWHBMVWpS
 Q2R6MDksa2VtY3Q9YlhWc2RHbHpkR1Z3WlhoaGJYQnNaV3RsYldOcGNHaGxjblJs
 ZUhReE1qTTBOVFkzKUVuY3J5cHRlZC1DRUs6YlhWc2RHbHpkR1Z3Ym05dVkyVXhN
 ak0wTlRZM09Ea3dZV0pqWkdWbVoyaHBhbXRzYlc1dmNIRnljM1IxZG5kNGVYb3dN
 VEl6TkRVMk56ZzVNR0ZpWXc9PQ==
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 bXVsdGlzdGVwY2VrY29tbWl0bWVudGhhc2h2YWx1ZTEyMzQ1VW1WeGRXbHlaWE1n
 WW05MGFDQndZWE56ZDI5eVpDQkJUa1FnV0RJMU5URTVJSEJ5YVhaaGRHVWdhMlY1
 SUhSdklHUmxZM0o1Y0hRPQ==
 -----END SAFE DATA-----

 Note: The armored LOCK examples above use placeholder Base64 values.
 The armored payload is Base64(Encode(step_1, ..., encrypted_cek)) per
 Section 6.2.2.

 Two-passphrase step sequence (requires both passphrases to decrypt):

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 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: pass(kdf=argon2id, salt=Zmlyc3RwYXNzd29yZHNhbHQ=)
 Step: pass(kdf=pbkdf2, salt=c2Vjb25kcGFzc3dvcmRzYWx0)
 Encrypted-CEK:
 dHdvcGFzc3dvcmRub25jZTEyMzQ1Njc4OTBhYmNkZWZnaGlq
 a2xtbm9wcXJzdHV2d3h5ejAxMjM0NTY3ODkwYWI=
 -----END SAFE LOCK-----

 Hybrid post-quantum step sequence (X25519 AND ML-KEM-768, default
 aes-256-gcm with commitment prefix):

 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: hpke(kem=x25519, id=cHF6RmlucHJpbnQxMjM0NTY3,
 kemct=eDI1NTE5a2VtY2lwaGVydGV4dDEyMzQ1Njc4OTBhYmNkZWY=)
 Step: hpke(kem=ml-kem-768, id=bWxrZW1maW5nZXJwcmludDEyMw==,
 kemct=bWxrZW03NjhrZW1jaXBoZXJ0ZXh0ZXh0cmVtZWx5bG9uZ2Jh
 c2U2NGVuY29kZWRkYXRhYXBwcm94aW1hdGVseTEwODhvY3Rl
 dHNmb3JwcXNlY3VyaXR5dGhpc2lzZHVtbXlkYXRhZm9yZGVt
 ...
 NzY4a2VtY2lwaGVydGV4dGVuY2Fwc3VsYXRpb25kYXRh)
 Encrypted-CEK:
 aHlicmlkbm9uY2UxMjM0NTY3ODkwYWJjZGVmZ2hpamtsbW5v
 cHFyc3R1dnd4eXowMTIzNDU2Nzg5MGFiY2RlZmc=
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 aHlicmlkcHFjZWtjb21taXRtZW50aGFzaHZhbHVlMTIzNDU2VUc5emRDMXhkV0Z1
 ZEhWdElHaDVZbkpwWkNCbGVHRnRjR3hsSUdSbGJXOXVjM1J5WVhScGJtY2dZMjl0
 WW1sdVpXUWdXREkxTlRFNUlHRnVaQ0JOVEMxTFJVMHQ=
 -----END SAFE DATA-----

Appendix B. Implementation Guide

 This appendix is informative. For a concise summary of the encryptor
 and decryptor flows, see Section 3.

B.1. Encryptor Processing

 Encryptor processing proceeds in three phases: setup, recipient key
 wrapping, and content encryption.

 During setup, the Encryptor:

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 1. Selects an AEAD algorithm and Block-Size (or accepts defaults)

 2. Emits a SAFE CONFIG block if using non-default values

 3. Generates a random 32-octet CEK: SafeRandom(32, "SAFE-CEK")

 4. Generates a 32-octet salt: SafeRandom(32, "SAFE-SALT")

 Encryptors should generate a fresh CEK for each file. When the same
 CEK is reused across files, the per-file salt ensures payload keys
 are unique: each salt produces distinct commitment, payload_key,
 acc_key, and nonce_base values via Section 5.7.3.

 For each recipient, the Encryptor wraps the CEK by:

 1. Emitting Step lines with required step parameters (salt for pass
 steps, kemct for hpke steps)

 2. Generating a fresh lock_nonce: SafeRandom(Nn, "SAFE-LOCK-NONCE")

 3. Deriving step secrets and computing the KEK per Section 5.7.1

 4. Emitting the Encrypted-CEK field

 To encrypt content, the Encryptor:

 1. Derives payload_key from the CEK, encryption_parameters, and salt
 per Section 5.7.3

 2. Splits the plaintext into Block-Size blocks (N blocks total)

 3. For NMR AEADs, derives nonce_base per Section 5.7.3 and computes
 nonce_i = nonce_base XOR uint64(i) for each block. For non-NMR
 AEADs, generates per-block nonces using one of the constructions
 in Section 5.7.6.

 4. For each block i: a. Computes nonce_i per the chosen
 construction b. Determines is_final = 1 if i == N - 1, else 0 c.
 Constructs data_aad(i, is_final) per Section 5.7.9 d. Computes
 key_i = block_key(i) (Section 5.7.4). e. For non-NMR AEADs:
 emits nonce_i || AEAD.Seal(key_i, nonce_i, aad, block). For NMR
 AEADs: emits AEAD.Seal(key_i, nonce_i, aad, block) (ciphertext
 and tag only; nonce is not stored).

B.2. Decryptor Processing

 Decryptor processing proceeds in three phases: configuration, CEK
 recovery, and content decryption.

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 During configuration, the Decryptor reads the SAFE CONFIG (if
 present) to learn non-default values and constructs
 encryption_parameters from AEAD, Block-Size, and Hash.

 To recover the CEK, the Decryptor:

 1. Selects a LOCK block per Section 6.2.3

 2. Parses lock_nonce (first Nn octets) from the Encrypted-CEK field

 3. Evaluates steps to derive the KEK per Section 5.7.1

 4. Decrypts Encrypted-CEK to recover the 32-octet CEK

 Encrypted-CEK size validation is specified in Section 6.2.

 To decrypt content, the Decryptor reads the 32-octet salt from the
 start of the DATA block, then derives payload_key from the CEK,
 encryption_parameters, and salt per Section 5.7.3. For each block i,
 the Decryptor computes key_i = block_key(i) (Section 5.7.4). For NMR
 AEADs, the Decryptor derives nonce_base and computes nonce_i =
 nonce_base XOR uint64(i). For non-NMR AEADs, the Decryptor reads
 each nonce from the stored metadata. Each block is decrypted using
 key_i, its nonce, ciphertext, tag, and data_aad(i, is_final) per
 Section 5.7.9. The location of these components depends on the
 layout:

 * Linear layout (Section 6.4.1): for non-NMR AEADs, each encrypted
 block contains nonce, ciphertext, and tag concatenated; for NMR
 AEADs, ciphertext and tag only.

 * Aligned layout (Section 6.4.2): for non-NMR AEADs, nonces and tags
 are in the header metadata array; for NMR AEADs, only tags are
 stored. Ciphertext blocks are at aligned offsets.

 See Appendix E for offset calculations.

Appendix C. Error Codes for Testing

 This appendix is informative.

 For interoperability testing, implementations MAY use the following
 error identifiers to categorize failures:

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 +=============================+=====================+=============+
 | Error Code | Description | When to |
 | | | Emit |
 +=============================+=====================+=============+
 | ERR_UNSUPPORTED_AEAD | Unknown AEAD | Parsing |
 | | algorithm | SAFE CONFIG |
 +-----------------------------+---------------------+-------------+
 | ERR_UNSUPPORTED_KEM | Unknown KEM | Parsing |
 | | identifier | hpke(...) |
 | | | step |
 +-----------------------------+---------------------+-------------+
 | ERR_INVALID_BLOCK_SIZE | Invalid Block-Size | Parsing |
 | | value | SAFE CONFIG |
 +-----------------------------+---------------------+-------------+
 | ERR_HPKE_NO_MATCH | No matching private | Recipient |
 | | key | discovery |
 +-----------------------------+---------------------+-------------+
 | ERR_HPKE_DECAP_FAILED | HPKE decapsulation | CEK |
 | | error | recovery |
 +-----------------------------+---------------------+-------------+
 | ERR_LOCK_AEAD_FAILED | Encrypted-CEK | CEK |
 | | decryption failed | recovery |
 +-----------------------------+---------------------+-------------+
 | ERR_PAYLOAD_AEAD_FAILED | Block decryption | Content |
 | | failed | decryption |
 +-----------------------------+---------------------+-------------+
 | ERR_BLOCK_OUT_OF_RANGE | Block index invalid | Content |
 | | | decryption |
 +-----------------------------+---------------------+-------------+
 | ERR_MALFORMED_BASE64 | Base64 decoding | Any Base64 |
 | | error | field |
 +-----------------------------+---------------------+-------------+
 | ERR_DUPLICATE_FIELD | Repeated field name | Parsing |
 | | | SAFE CONFIG |
 +-----------------------------+---------------------+-------------+
 | ERR_DUPLICATE_PARAM | Repeated step | Parsing |
 | | parameter | Step lines |
 +-----------------------------+---------------------+-------------+
 | ERR_MISSING_SALT | pass(...) without | Parsing |
 | | salt | LOCK |
 +-----------------------------+---------------------+-------------+
 | ERR_MISSING_KEMCT | hpke(...) without | Parsing |
 | | kemct | LOCK |
 +-----------------------------+---------------------+-------------+
 | ERR_MULTIPLE_PASS_ONLY_LOCK | Multiple same- | Discovery |
 | | variant pass-only | |
 | | LOCKs | |
 +-----------------------------+---------------------+-------------+

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 | ERR_NON_ASCII_HEADER | Non-ASCII in header | Parsing any |
 | | | header |
 +-----------------------------+---------------------+-------------+
 | ERR_RESOURCE_LIMIT | Size/count limit | Parsing any |
 | | exceeded | block |
 +-----------------------------+---------------------+-------------+
 | ERR_INVALID_SALT_LENGTH | salt not exactly 16 | Parsing |
 | | octets | salt |
 +-----------------------------+---------------------+-------------+
 | ERR_COMMITMENT_MISMATCH | Commitment | Key |
 | | verification failed | schedule |
 +-----------------------------+---------------------+-------------+
 | ERR_ACCUMULATOR_MISMATCH | Accumulator | Integrity |
 | | verification failed | check |
 +-----------------------------+---------------------+-------------+
 | ERR_TRUNCATION | Missing final block | Content |
 | | or block with | decryption |
 | | is_final=0 at EOF | |
 +-----------------------------+---------------------+-------------+

 Table 19

Appendix D. Armored Data Arithmetic

 This appendix is informative.

 In armored mode, Decryptors compute the block count N and final block
 size from the Base64 payload length. Let S_b64 be the payload string
 between the fences, len_b64 its length in characters, pad the number
 of trailing = signs (0, 1, or 2), and len_bin_total = 3 *
 floor(len_b64 / 4) - pad. The header region is 32 (salt) + 32
 (commitment) + Nh (accumulator) octets. The encrypted block region
 is len_bin_ciphertext = len_bin_total - 64 - Nh. Let B = Block-Size
 and Nn = AEAD nonce length. For NMR AEADs, nonces are derived (not
 stored), so the per-block overhead excludes Nn. Define:

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 if NMR:
 C = B + 16
 C_min = 16 # minimum: tag only
 else:
 C = Nn + B + 16
 C_min = Nn + 16 # minimum: nonce + tag

 N_nonfinal = floor( len_bin_ciphertext / C )
 rem = len_bin_ciphertext - N_nonfinal * C
 if rem == 0:
 N = N_nonfinal
 C_final = C
 else if rem < C_min:
 reject as malformed
 else:
 N = N_nonfinal + 1
 C_final = rem

 if NMR:
 L_final = C_final - 16
 else:
 L_final = C_final - Nn - 16

 A decryptor decrypting block index i computes octet offsets relative
 to the start of the encrypted block region (after salt, commitment,
 and accumulator):

 block_byte_start = i * C
 block_byte_len = C if i < N - 1 else C_final
 byte_start = 64 + Nh + block_byte_start
 byte_len = block_byte_len
 char_start = 4 * floor( byte_start / 3 )
 char_end = 4 * ceil( (byte_start + byte_len) / 3 )

 For each block index i, the Decryptor:

 1. Extracts S_b64[char_start:char_end]

 2. Base64-decodes to a temporary buffer tmp

 3. Computes skip = byte_start mod 3

 4. Selects encrypted_block = tmp[skip : skip + byte_len]

 5. For non-NMR AEADs: parses nonce_i = encrypted_block[0:Nn] and
 ciphertext_i = encrypted_block[Nn:]. For NMR AEADs: ciphertext_i
 = encrypted_block and nonce_i = nonce_base XOR uint64(i) per
 Section 5.7.5

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 6. Determines is_final = 1 if i == N - 1 else 0

 7. Constructs data_aad(i, is_final) per Section 5.7.9

 8. Computes key_i = block_key(i) (Section 5.7.4)

 9. AEAD-opens ciphertext_i under key_i with nonce_i and data_aad

Appendix E. Selective Decryption

 This appendix is informative.

 To decrypt block index i from a long object, a Decryptor first
 selects a candidate LOCK per Section 6.2.3. The decryptor constructs
 encryption_parameters from the SAFE CONFIG or from defaults, parses
 lock_nonce from Encrypted-CEK, evaluates the step sequence to derive
 the KEK, and opens Encrypted-CEK to recover the CEK. The decryptor
 reads the 32-octet salt from the start of the DATA block, then
 derives payload_key and acc_key from the CEK, encryption_parameters,
 and salt. It then locates block i in the payload. No other blocks
 need to be read or decoded.

 For binary encoding, read N and D from the header, then compute block
 i's ciphertext offset as (D + i) × B. The nonce and tag are at
 offset header_len + 64 + 8 + i × meta_len, where meta_len is Nn + 16
 for non-NMR AEADs or 16 for NMR AEADs (Section 6.4.2).

 For armored encoding, the Decryptor must compute the Base64 character
 window covering block i and decode only that window, as described
 below.

E.1. Example: Armored Selective Block Decryption

 This example uses a non-NMR AEAD (AES-256-GCM, Nn=12). For NMR
 AEADs, omit Nn from per-block sizes (C = B + 16 instead of Nn + B +
 16).

 Consider Block-Size=16384, Nn=12, and three blocks: two full blocks
 plus a 5000-octet final block.

 C = Nn + B + 16 = 12 + 16384 + 16 = 16412 octets (full block)
 C_final = Nn + 5000 + 16 = 12 + 5000 + 16 = 5028 octets (final block)
 total_binary = 32 + 32 + 16412 + 16412 + 5028 + 32 = 37948 octets
 Base64 len = ceil(37948 / 3) * 4 = 50600 characters

 To decrypt block i=0, compute octet and character offsets (salt,
 commitment, and accumulator occupy the first 96 octets):

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 byte_start = 96 + (0 * C) = 96
 byte_len = C = 16412
 char_start = floor(byte_start / 3) * 4 = 128
 char_end = ceil((byte_start + byte_len) / 3) * 4 = 22012
 skip = byte_start mod 3 = 0

 Extract S_b64[128:22012], Base64-decode, then take 16412 octets as
 the encrypted block (skip = 0, no leading octets to discard). Parse
 the first Nn octets (12 for AES-256-GCM) as the stored nonce, compute
 block_key(i) (Section 5.7.4), and AEAD-open the remaining octets
 under block_key(i) with the extracted nonce and block AAD. For NMR
 AEADs, derive the nonce from nonce_base instead (Section 5.7.5).

Appendix F. Design Rationale

 This appendix is informative.

 SAFE's design choices reflect trade-offs between flexibility,
 performance, and simplicity. This section explains the rationale
 behind key architectural decisions.

F.1. Two-Tier Key Hierarchy

 SAFE separates the Content-Encryption Key (CEK) from the Key-
 Encryption Key (KEK) to enable multi-recipient encryption without
 duplicating payload ciphertexts.

F.1.1. Benefits

 A single CEK is generated once and used to encrypt the payload; each
 recipient's LOCK derives a KEK that wraps the same CEK. This design
 offers several advantages:

 1. Storage and bandwidth efficiency: Adding recipients requires only
 adding LOCK blocks (typically < 1 KB each), not duplicating the
 entire payload. For large files, this is critical.

 2. Key rotation: Recipients can be added or removed by re-wrapping
 the CEK under new KEKs without re-encrypting the payload.

 3. Operational flexibility: The CEK remains constant while KEKs
 rotate, simplifying key management.

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F.1.2. Trade-offs

 This design implies that all recipients share the same payload_key.
 Encryptors who require per-recipient payload keys (e.g., for fine-
 grained access control that survives CEK compromise) would need to
 encrypt multiple independent payloads.

 If recipients directly decrypted the payload with their KEK, each
 recipient would require a distinct copy of the ciphertext,
 multiplying storage and bandwidth costs.

F.2. Minimal Block AAD

 Block associated data is defined as Encode("SAFE-DATA", I2OSP(i, 8),
 I2OSP(is_final, 1)), binding each block to its position and finality.
 Suite parameters and LOCK-specific data are excluded from block AAD.

F.2.1. Rationale

 This choice prioritizes simplicity and O(1) random access:

 1. Selective decryption: payload_key already depends on
 encryption_parameters, so block AAD need not repeat them. This
 avoids requiring every block decryption to reference the SAFE
 CONFIG.

 2. Multi-recipient caching: Including LOCK-specific data (Step
 lines, kemct) would couple block decryption to a specific LOCK,
 preventing efficient caching of payload_key across multiple
 recipients.

F.2.2. Security Properties

 Suite and LOCK binding is indirect through the key hierarchy:

 * The KEK schedule binds encryption_parameters at initialization and
 final derivation, with all step_tokens folded between; Encrypted-
 CEK AEAD authenticates the CEK under this KEK.

 * The payload schedule binds payload_key to encryption_parameters
 via SafeDerive.

 * Block AAD includes the block index and finality flag, preventing
 reordering, splicing, truncation, and extension within a file.

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 * The snapshot accumulator (Section 5.7.8) binds all block tags
 under acc_key, providing file-level integrity without per-block
 decryption. Each contribution is a PRF output under acc_key; XOR
 accumulation is unforgeable without knowledge of the CEK.

F.2.3. Alternative Designs Considered

 An alternative design could include a "recipient_id" in block AAD,
 but this would require additional per-recipient metadata and
 complicate multi-recipient scenarios. SAFE's choice favors
 performance and simplicity for the common case of single-recipient or
 trust-equivalent multi-recipient files, while accepting that
 ciphertext blocks alone do not directly identify which LOCK unlocked
 them.

 SAFE provides built-in truncation and extension detection via the
 is_final flag in block AAD (Section 5.7.9). The final block is
 marked with is_final=1; all preceding blocks use is_final=0. This
 design, inspired by the STREAM construction [STREAM], enables:

 * Truncation detection: a block with is_final=0 and no successor
 indicates truncation

 * Extension prevention: appending after is_final=1 fails AEAD
 verification

 * Streaming writes: encryptors buffer the last block until the
 stream closes, then encrypt it with is_final=1

 Per-block random nonces (Section 5.7.5) enable selective editing:
 individual blocks can be re-encrypted without affecting LOCK blocks
 or other blocks. This design trades a small storage overhead (Nn
 octets per block) for flexibility in payload modification.

F.3. Fixed HKDF Salt

 SafeDerive (Section 5.4.1) uses a fixed string ("SAFE-v1") as the
 HKDF salt in its Extract step. The same string appears in the
 Encode-framed Input Keying Material (IKM), making its presence in the
 salt redundant but harmless. This design is intentional: the fixed
 salt binds every Extract call to the protocol version at the HMAC key
 position, while security relies on the entropy of the IKM, not the
 salt. All security-critical IKMs (CEK, step secrets, hedged random
 values) have sufficient min-entropy for this to hold. Krawczyk's
 analysis of HKDF shows that Extract with a fixed salt is a good
 randomness extractor when the IKM has high min-entropy. For the XOF
 single-stage variant, there is no separate salt; the version string
 appears only within the Encode frame.

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Appendix G. Test Vectors

 This appendix is informative.

 This appendix provides a complete known-answer test for a passphrase-
 based SAFE encoding using default parameters (aes-256-gcm, Block-
 Size=65536, Hash=sha-256, Data-Encoding=armored). All values are hex
 unless noted.

## Inputs
Passphrase: "correct horse battery staple" (UTF-8, 28 octets)
Salt: 01010101010101010101010101010101
CEK: aaaaaaaa...aa (32 octets of 0xAA)
lock_nonce: 020202020202020202020202
payload_salt: 04040404...04 (32 octets of 0x04)
Plaintext: "Hello, SAFE!" (12 octets)

## encryption_parameters
encryption_parameters = ["aes-256-gcm", "65536", "sha-256"]

## KEK Schedule
step_token: 00047061737300086172676f6e326964
 001001010101010101010101010101010101
 Encode("pass", "argon2id", salt)
step_secret: 7d3491ac8af1b54526792869b7257f5dbf7cc3c20929417bb193e396c51d7965
agg_init: 1b257512ce57328cbb04bbf80b4b3aa220d875832c8439c0cdda85e1e4f8428b
 SafeDerive("kek_init", "",
 encryption_parameters, 32)
agg_step: 596a483b938ad11da3369007f1b7f073502101879eb257f0f4b22c0758fdee21
 SafeDerive("kek_step",
 [agg, secret], token, 32)
derived_kek: bfedcafd41d9da3c1c77f73358b973a4ececfbc212ae558eed0dfba709cdc24e
 SafeDerive("kek", agg,
 encryption_parameters, 32)

## Encrypted-CEK
aad: "" (empty)
Encrypted-CEK: 020202020202020202020202352cbe85
 a8e4434e5cd98d6507c80759dfe41fbe
 13a649df57a9f7f46d1a7f90c60e1531
 92ecb8c83a649656a6785487

## Payload Schedule
payload_info = [...encryption_parameters, payload_salt]
commitment: 42330a7379357f4f369f0271369546047f702ff37c53a8e17eb2342731683905
 SafeDerive("commit", CEK,
 payload_info, 32)
payload_key: 01a830b8a79a687b784109020b70d58dd53e3b51260d468c8c5ba05181ae09d8

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 SafeDerive("payload_key", CEK,
 payload_info, 32)
acc_key: 9ce7a1a28f00e17c601b49ef3959a797088c8872ec7dd33f7b1258c0362da6ec
 SafeDerive("acc_key", CEK,
 payload_info, 32)

## Block Encryption (block 0, is_final=1)
raw_random: 030303030303030303030303
uint64(0): 0000000000000000
nonce0: 030303030303030303030303 (= raw_random XOR 0)
data_aad: 0009534146452d4441544100080000000000000000000101
 Encode("SAFE-DATA",
 I2OSP(0, 8), I2OSP(1, 1))
ciphertext+tag: b42ba125cc5d376aef03f1ec3ecbc9c96c4264265f94dea4a1312bbd
encrypted_block: 030303030303030303030303b42ba125
 cc5d376aef03f1ec3ecbc9c96c426426
 5f94dea4a1312bbd

## Snapshot Accumulator
tag_0: 3ecbc9c96c4264265f94dea4a1312bbd
contrib_0: 4b4160f1af84bd74fbb1cf7fdccae69b2027c7ffdc67a7a03bc33c1b9489a4e8
 SafeDerive("acc_contrib", acc_key,
 [uint64(0), tag_0], 32)
accumulator: 4b4160f1af84bd74fbb1cf7fdccae69b2027c7ffdc67a7a03bc33c1b9489a4e8
 = contrib_0 (single block)

## Complete SAFE Object
SAFE DATA = salt (32) + commitment (32) + accumulator (32)
 + encrypted_block (40)
= 136 octets, 184 Base64 chars.

 Readable format: ~~~~ -----BEGIN SAFE CONFIG----- Lock-Encoding:
 readable -----END SAFE CONFIG----- -----BEGIN SAFE LOCK----- Step:
 pass(kdf=argon2id, salt=AQEBAQEBAQEBAQEBAQEBAQ==) Encrypted-CEK:
 AgICAgICAgICAgICNSy+hajkQ05c2Y1lB8gHWd/kH74TpknfV6n39G0af5DGDhUx
 kuy4yDpkllameFSH -----END SAFE LOCK----- -----BEGIN SAFE DATA-----
 BAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBARCMwpzeTV/TzafAnE2lUYE
 f3Av83xTqOF+sjQnMWg5BUtBYPGvhL10+7HPf9zK5psgJ8f/3GenoDvDPBuUiaTo
 AwMDAwMDAwMDAwMDtCuhJcxdN2rvA/HsPsvJyWxCZCZflN6koTErvQ== -----END
 SAFE DATA----- ~~~~

 Same object in armored format (default Lock-Encoding). The armored
 LOCK body is Base64(Encode(step, ecek)) per Section 6.2.2:

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 -----BEGIN SAFE LOCK-----
 ACIABHBhc3MACGFyZ29uMmlkABABAQEBAQEBAQEBAQEBAQEBADwCAgICAgICAgIC
 AgI1LL6FqORDTlzZjWUHyAdZ3+QfvhOmSd9Xqff0bRp/kMYOFTGS7LjIOmSWVqZ4
 VIc=
 -----END SAFE LOCK-----
 -----BEGIN SAFE DATA-----
 BAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBARCMwpzeTV/TzafAnE2lUYE
 f3Av83xTqOF+sjQnMWg5BUtBYPGvhL10+7HPf9zK5psgJ8f/3GenoDvDPBuUiaTo
 AwMDAwMDAwMDAwMDtCuhJcxdN2rvA/HsPsvJyWxCZCZflN6koTErvQ==
 -----END SAFE DATA-----

Appendix H. X25519 Test Vector

 This appendix is informative.

 This appendix provides a complete known-answer test for a single
 X25519 recipient using DHKEM(X25519, HKDF-SHA256) per Section 5.2.2.
 All values are hex unless noted. The ephemeral and recipient keys
 are from RFC 9180, Appendix A.7.1 (DHKEM(X25519, HKDF-SHA256), HKDF-
 SHA256, AES-128-GCM), so the shared_secret value can be independently
 verified against that appendix.

 ## Inputs
 Recipient skR: 4612c550263fc8ad58375df3f557aac531d26850
 903e55a9f23f21d8534e8ac8
 Recipient pkR: 3948cfe0ad1ddb695d780e59077195da6c56506b
 027329794ab02bca80815c4d
 Ephemeral skE: 52c4a758a802cd8b936eceea314432798d5baf2d
 7e9235dc084ab1b9cfa2f736
 Ephemeral pkE: 37fda3567bdbd628e88668c3c8d7e97d1d1253b6
 d4ea6d44c150f741f1bf4431 (= enc)
 CEK: aaaaaaaa...aa (32 octets of 0xAA)
 lock_nonce: 020202020202020202020202
 payload_salt: 04040404...04 (32 octets of 0x04)
 Plaintext: "Hello, SAFE!" (12 octets)

 ## encryption_parameters
 encryption_parameters = ["aes-256-gcm", "65536", "sha-256"]

 ## DHKEM(X25519, HKDF-SHA256)
 dh = DH(skE, pkR):
 b3b5c19eab3f088ac18f23f774ff6414
 ba4fde45404d10085efc3e4dc9c72e35
 kem_context = enc || pkR:
 37fda3567bdbd628e88668c3c8d7e97d
 1d1253b6d4ea6d44c150f741f1bf4431
 3948cfe0ad1ddb695d780e59077195da
 6c56506b027329794ab02bca80815c4d

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 prk = LabeledExtract("", "eae_prk", dh):
 d8b4a0e70fe904de0f643c91e091903c
 f3c628994f51a025a7306bd4d4eb03c3
 shared_secret = LabeledExpand(prk,
 "shared_secret", kem_context, 32):
 fe0e18c9f024ce43799ae393c7e8fe8f
 ce9d218875e8227b0187c04e7d2ea1fc

 ## SAFE Step Secret
 key_id = SafeDerive("SAFE-SPKI-v1",
 [SPKI(pkR)], [""], 32):
 98cdd10b776ac15ed78f5520bed9f3e6
 ffdf682fe3ecb68163b4f1dd8b1dfefa
 step_token = Encode("hpke", "x25519",
 enc, key_id):
 000468706b650006783235353139
 002037fda3567bdbd628e88668c3c8d7
 e97d1d1253b6d4ea6d44c150f741f1bf
 4431002098cdd10b776ac15ed78f5520
 bed9f3e6ffdf682fe3ecb68163b4f1dd
 8b1dfefa
 exporter_context = SafeDerive(
 "SAFE-STEP", [token], [""], 32):
 3be7e9568086a415ada19b306534bc64
 6fcf0efc90e5faaaca358305c3ab7360
 exporter_secret = KeySchedule(Base,
 shared_secret, info="SAFE-v1"):
 3b2120e20d71e9e93c01b659c4835c72
 d0e43c660dca0dba8439b64cc78a9b2b
 step_secret = Export(exporter_secret,
 exporter_context, 32):
 42a4a3f299e1a71a97b04a3d9a7e9ae6
 7cd1b8ea3dec017e26fa1e369ee6f85b

 ## KEK Schedule
 agg_init:
 1b257512ce57328cbb04bbf80b4b3aa2
 20d875832c8439c0cdda85e1e4f8428b
 agg_step:
 16dfec67e5ab80a27b5c338de616c453
 c0660ed1c030dc29e6f4d46fadbad3a1
 derived_kek:
 c88d0730216dd222c4d64ffceac8be3e
 25815bdf067cc3a27760928ab0cc82f6

 ## Encrypted-CEK
 Encrypted-CEK:
 020202020202020202020202

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 8865cde5f682dcd6155b30ffbcd80bd9
 879d6663ac56b340dfc0e082e78f23ea
 a44944abc2e4cb1bd2fba5ebffd08a8f

 ## Payload Schedule
 payload_info = [...encryption_parameters, payload_salt]
 commitment:
 42330a7379357f4f369f0271369546047f702ff3
 7c53a8e17eb2342731683905
 payload_key:
 01a830b8a79a687b784109020b70d58dd53e3b51
 260d468c8c5ba05181ae09d8
 acc_key:
 9ce7a1a28f00e17c601b49ef3959a797088c8872
 ec7dd33f7b1258c0362da6ec

 ## Block Encryption (block 0, is_final=1)
 ciphertext+tag:
 b42ba125cc5d376aef03f1ec3ecbc9c9
 6c4264265f94dea4a1312bbd
 encrypted_block:
 030303030303030303030303b42ba125
 cc5d376aef03f1ec3ecbc9c96c426426
 5f94dea4a1312bbd

 ## Snapshot Accumulator
 tag_0:
 3ecbc9c96c4264265f94dea4a1312bbd
 contrib_0:
 4b4160f1af84bd74fbb1cf7fdccae69b
 2027c7ffdc67a7a03bc33c1b9489a4e8
 SafeDerive("acc_contrib", acc_key,
 [uint64(0), tag_0], 32)
 accumulator:
 4b4160f1af84bd74fbb1cf7fdccae69b
 2027c7ffdc67a7a03bc33c1b9489a4e8
 = contrib_0 (single block)

 ## Complete SAFE Object
 SAFE DATA = salt (32) + commitment (32) + accumulator (32)
 + encrypted_block (40)
 = 136 octets, 184 Base64 chars.

Appendix I. Auth Mode Test Vector

 This appendix is informative.

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 This appendix provides a known-answer test for Auth mode HPKE
 (X25519) with a single recipient. The recipient keys are the same as
 Appendix H. All values are hex unless noted.

 ## Inputs
 Recipient skR: 4612c550263fc8ad58375df3f557aac531d26850
 903e55a9f23f21d8534e8ac8
 Recipient pkR: 3948cfe0ad1ddb695d780e59077195da6c56506b
 027329794ab02bca80815c4d
 Sender skS: 6b7298af684f45181f80ac5cb3d9a3713abb62cb
 ecd21db5dba0eb2a8bfb3a05
 Sender pkS: ccc340219b8098b48749f1c36e2c336faefb87f9
 cbe3463e59e3b8ec18c44c49
 Ephemeral skE: 52c4a758a802cd8b936eceea314432798d5baf2d
 7e9235dc084ab1b9cfa2f736
 Ephemeral pkE: 37fda3567bdbd628e88668c3c8d7e97d1d1253b6
 d4ea6d44c150f741f1bf4431 (= enc)
 CEK: aaaaaaaa...aa (32 octets of 0xAA)
 lock_nonce: 020202020202020202020202

 ## encryption_parameters
 encryption_parameters = ["aes-256-gcm", "65536", "sha-256"]

 ## DHKEM Auth Mode (X25519)
 shared_secret:
 8c1cd5a4e40708af36bf6446d65f62ed
 7a7b7f1286d966d9bc96b96808021ba8

 ## SAFE Step Secret (Auth mode)
 id:
 98cdd10b776ac15ed78f5520bed9f3e6
 ffdf682fe3ecb68163b4f1dd8b1dfefa
 sid:
 d9b9d59d0f10a55a8365b2f440dbf787
 f280e0f400427beaaf17de1f8662fcdc
 step_token = Encode("hpke", "x25519",
 enc, id, "auth", sid):
 000468706b650006783235353139
 002037fda3567bdbd628e88668c3c8d7
 e97d1d1253b6d4ea6d44c150f741f1bf
 4431002098cdd10b776ac15ed78f5520
 bed9f3e6ffdf682fe3ecb68163b4f1dd
 8b1dfefa0004617574680020d9b9d59d
 0f10a55a8365b2f440dbf787f280e0f4
 00427beaaf17de1f8662fcdc
 exporter_context:
 6da02fe479338d9a1798f8c02e09f601
 6dc3e0c49e2dff50c63ba273eabd2382

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 exporter_secret = KeySchedule(Auth,
 shared_secret, info="SAFE-v1"):
 c35c97b436c388b766bd65008d1c7c5f
 0f52aad653b4072cbc8bb70e98c035bf
 step_secret:
 24c50a152128baaa3f77c255b0e5b895
 2cece2e7680b21bd7dc24aa1807e9374

 ## KEK Schedule
 agg_init:
 1b257512ce57328cbb04bbf80b4b3aa2
 20d875832c8439c0cdda85e1e4f8428b
 agg_step:
 2cb2cd8062e86b7dda6580d6bf8ba351
 25ef38644af50a8b1f6ab7eb53bae59c
 derived_kek:
 4a2b4a469f961e824c8c1681723393f0
 3937ede4bb824fc507200f932ef9c739

 ## Encrypted-CEK
 Encrypted-CEK:
 020202020202020202020202
 e29d2e1e0502fdbadaa1232a013a401c
 309e30dd7db132692a8dc03dfc64375d
 effa4696385b308f1093e41cf9f407f4

Appendix J. Multi-Block Test Vector

 This appendix is informative.

 This appendix provides a known-answer test with two blocks,
 demonstrating block index > 0, is_final transitions, and accumulator
 XOR of multiple contributions. Uses a passphrase LOCK with default
 parameters. All values are hex unless noted.

 ## Inputs
 Passphrase: "correct horse battery staple" (28 octets)
 Salt: 01010101...01 (32 octets of 0x01)
 CEK: aaaaaaaa...aa (32 octets of 0xAA)
 lock_nonce: 020202020202020202020202
 payload_salt: 04040404...04 (32 octets of 0x04)
 Block 0: "Block zero data!" (16 octets)
 Block 1: "Final block." (12 octets, final)

 ## Step Secret
 step_secret:
 2bab4c606fb5c84123e48f0bb5eeb2a1
 39a6dd996c6cff2efad2ad78a12143e0

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 ## KEK Schedule
 derived_kek:
 152c31bd033c76fc1d4987681b7ca76a
 8948514d6e5b7c2a5d8a4b681873a56f

 ## Encrypted-CEK
 Encrypted-CEK:
 020202020202020202020202
 3dbace72c0486e28026e6be8c1f0fc12
 dfc833d4c76b76a40dd68be3f423568c
 b4fb3d2ef43a2374f5a2fc0642e90cd1

 ## Payload Schedule
 payload_info = [...encryption_parameters, payload_salt]
 commitment:
 42330a7379357f4f369f027136954604
 7f702ff37c53a8e17eb2342731683905
 payload_key:
 01a830b8a79a687b784109020b70d58d
 d53e3b51260d468c8c5ba05181ae09d8
 acc_key:
 9ce7a1a28f00e17c601b49ef3959a797
 088c8872ec7dd33f7b1258c0362da6ec

 ## Block 0 (is_final=0)
 nonce_0: 030303030303030303030303
 data_aad_0: 0009534146452d44415441
 00080000000000000000000100
 Encode("SAFE-DATA", I2OSP(0, 8), I2OSP(0, 1))
 ciphertext+tag_0:
 be22a22ac8516d5cdc2a94a9863ced1c
 712ded5352105fddab8539c9570eda40
 tag_0: 712ded5352105fddab8539c9570eda40

 ## Block 1 (is_final=1)
 nonce_1: 050505050505050505050505
 data_aad_1: 0009534146452d44415441
 00080000000000000001000101
 Encode("SAFE-DATA", I2OSP(1, 8), I2OSP(1, 1))
 ciphertext+tag_1:
 128cb7c8a035399b40d0a69d
 866cbbc0f49d8f85ce6b1883a0f0c028
 tag_1: 866cbbc0f49d8f85ce6b1883a0f0c028

 ## Snapshot Accumulator (2 blocks)
 contrib_0:
 c1ad6f915fa1babef344e5307b62bc7f
 33d6bf7269cdb84dbe5c76889204220d

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 contrib_1:
 4862e6122d79464e22a033c2b8dd17e5
 c8922f35898f35638802ae4b9e4f0bb9
 accumulator = contrib_0 XOR contrib_1:
 89cf898372d8fcf0d1e4d6f2c3bfab9a
 fb449047e0428d2e365ed8c30c4b29b4

Appendix K. SafeDerive Test Vectors

 This appendix is informative. These vectors validate the SafeDerive
 function in isolation, independent of the SAFE protocol. Inputs are
 deliberately short for readability. See Section 5.4.1 for the
 definition.

 Common inputs for all vectors:

 label: "SAFE-TEST" (9 octets, hex: 534146452d54455354)
 ikm: 0a0b0c0d0e0f (6 octets)
 info: "" (0 octets)

K.1. Hash=sha-256, L=32

 Extract salt = "SAFE-v1":
 534146452d7631

 Extract ikm = Encode("SAFE-v1",
 label, ...ikm):
 0007534146452d76310009534146452d5445535400060a0b0c0d0e0f

 prk = Extract(salt, ikm):
 983d59830192955caf33fff4056ed415e2cd1cef7fe3072e075cf90903c97146

 Expand info = Encode("SAFE-v1",
 label, ...info,
 I2OSP(32, 2)):
 0007534146452d76310009534146452d54455354000000020020

 output = Expand(prk, info, 32):
 d7413c70bb7bde999f5e543c0796d63a0af6839ebbe5203cc526776b978ba147

K.2. Hash=sha-256, L=16

 output = SafeDerive(label, ikm, info, 16):
 e190628e91995808047c49a7269b9d3b

K.3. Hash=turboshake256, L=32

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Derive(Encode("SAFE-v1",
 label, ...ikm,
 I2OSP(32, 2), ...info), 32):
 input: 0007534146452d76310009534146452d5445535400060a0b0c0d0e0f00020020
 0000
 output: 0394158419b3a24cb4f29376ada39b833f100a1825bfac7bc64f4f2ca9ff8a50

K.4. Hash=turboshake256, L=16

 output = SafeDerive(label, ikm, info, 16):
 f1c0c2cd38c6b0cbd0fa6a986b12c82d

Appendix L. Defining New Step Types

 This appendix is informative.

 New step types can be defined and registered to extend SAFE with
 additional authentication mechanisms. This section illustrates the
 process using three hypothetical steps: two Privacy Pass Key
 Derivation Function (PPKDF) steps for token-gated key derivation
 (Appendix L.1), and a WebAuthn PRF step for hardware token
 authentication.

L.1. Example: Privacy Pass Steps

 Privacy Pass [RFC9578] type 0x0001 tokens use a Verifiable Oblivious
 Pseudorandom Function (VOPRF) [RFC9497]. The client constructs a
 TokenInput containing a nonce, blinds it, and sends the blinded
 element to the issuer. The issuer evaluates its PRF on the blinded
 element and returns the result. The client unblinds to obtain the
 authenticator, which is a deterministic function of the TokenInput
 and the issuer's secret key.

 In normal issuance the nonce is random, making each token unique.
 PPKDF sets the nonce to a deterministic value derived from the SAFE
 context, so the authenticator is reproducible and serves as step key
 material. The issuer's behavior is unchanged and cannot distinguish
 a PPKDF request from normal token issuance.

 Two step types use this mechanism:

 * ppkdf: token-gated key derivation with application-supplied
 context.

 * ppkdf-pass: token-gated key derivation with password-derived
 context for online throttling.

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L.1.1. Shared Parameters

 issuer (REQUIRED): Server name (host or host:port) of the Privacy
 Pass token issuer. The issuer holds the VOPRF key pair; its
 public key pkI must be known to the client.

 salt (REQUIRED): Base64-encoded salt; must decode to exactly 32
 octets. Generated at encryption time using SafeRandom.

 The binding step_token uses the Encode form (Section 5.6):

 * ppkdf: Encode("ppkdf", issuer, salt)

 * ppkdf-pass: Encode("ppkdf-pass", issuer, kdf, salt)

 where issuer and kdf are UTF-8 strings and salt is the raw decoded 32
 octets.

L.1.2. ppkdf Step

 The ppkdf step provides token-gated key derivation with application-
 supplied context.

 Token form:

 ppkdf(issuer=tokens.example.com,salt=<Base64>)

 Grammar:

 ppkdf-step = "ppkdf(" ppkdf-params ")"
 ppkdf-params = "issuer=" pp-host "," "salt=" salt
 pp-host = host [ ":" port ]
 host = 1*( ALPHA / DIGIT / "-" / "." )
 port = 1*DIGIT
 salt = 1*BASE64CHAR ; 44 chars = 32 octets

 Derivation:

 Decode salt to salt_bytes (32 octets). Compute a deterministic nonce
 for the TokenInput:

 nonce = SafeDerive("SAFE-PPKDF",
 "", [...encryption_parameters,
 binding_step_token, salt_bytes], 32)

 where binding_step_token is the Encode-based binding form defined in
 Section 5.6. Construct a type 0x0001 TokenInput ([RFC9578],
 Section 5.1) with nonce as above. Set challenge_digest to

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 SafeDerive("SAFE-PPKDF", TokenChallenge, encryption_parameters, 32),
 using a TokenChallenge with issuer_name = issuer and empty
 redemption_context and origin_info.

 Blind the TokenInput and send in a standard type 0x0001 TokenRequest.
 The issuer evaluates the VOPRF and returns a TokenResponse. Verify
 the VOPRF proof and unblind to obtain the authenticator (32 octets).
 Set step_secret = authenticator.

L.1.3. ppkdf-pass Step

 The ppkdf-pass step adds password-derived input. Each password guess
 requires a VOPRF evaluation from the issuer.

 Token form:

 ppkdf-pass(issuer=tokens.example.com,kdf=argon2id,salt=<Base64>)

 Grammar:

 ppkdf-pass-step = "ppkdf-pass(" ppkdf-pass-params ")"
 ppkdf-pass-params = "issuer=" pp-host ","
 "kdf=" kdf-name "," "salt=" salt
 kdf-name = "argon2id" / "pbkdf2"

 The kdf parameter selects the password KDF using the same algorithms
 and default parameters as the pass() step (Section 5.6.2).

 Derivation:

 Decode salt to salt_bytes (32 octets). Derive pw32 from the user's
 password using the KDF indicated by the kdf parameter, with
 salt_bytes as salt input and the default parameters defined in
 Section 5.6.2. Compute the deterministic nonce:

 nonce = SafeDerive("SAFE-PPKDF-PASS",
 pw32, [...encryption_parameters,
 binding_step_token], 32)

 Execute the PPKDF protocol as in the ppkdf step using this nonce.
 Set step_secret = authenticator.

L.1.4. IANA Registry Entries

 Registration of ppkdf and ppkdf-pass is deferred to a separate
 document; this appendix is informative. A registration for these
 steps would include:

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 +============+==================+==============+===========+=======+
 | Step Name | Parameters | Inputs | Secret | Ref |
 +============+==================+==============+===========+=======+
 | ppkdf | issuer=X, salt=X | PPKDF token | 32 octets | (this |
 | | | | | doc) |
 +------------+------------------+--------------+-----------+-------+
 | ppkdf-pass | issuer=X, kdf=X, | PPKDF token, | 32 octets | (this |
 | | salt=X | password | | doc) |
 +------------+------------------+--------------+-----------+-------+

 Table 20

L.1.5. Security Considerations for Privacy Pass Steps

 For ppkdf:

 The issuer sees only the blinded_element. It cannot learn context,
 step_secret, or anything about the SAFE file. The VOPRF proof lets
 the client detect the wrong issuer key. Compromise of skI enables
 offline computation of step_secret for any context, breaking the
 online-gating property.

 For ppkdf-pass:

 Each password guess requires a VOPRF evaluation; issuer rate limits
 control guessing frequency. The issuer never sees the password-
 derived context. Compromise of skI still requires inverting the
 memory-hard KDF to recover the password.

 Example LOCK block:

 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: ppkdf(issuer=tokens.example.com,salt=<Base64>)
 Encrypted-CEK:
 Base64-encoded encrypted CEK
 -----END SAFE LOCK-----

L.2. Example: WebAuthn PRF Step

 A WebAuthn-based step would allow hardware token authentication using
 the PRF extension defined in Web Authentication Level 3
 [W3C.webauthn-3]. Unlike WebAuthn assertions (signatures), the PRF
 extension provides deterministic output suitable for SAFE's step
 model.

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L.2.1. Step Definition

 Step name: webauthn-prf

 The webauthn-prf step token has three forms:

 webauthn-prf(rpid=example.com,salt=xyz...) ; Identified RP
 webauthn-prf(salt=xyz...) ; Anonymous RP
 webauthn-prf(rpid=example.com,salt=xyz...,label=YubiKey) ; With label

 The parameters are:

 rpid (OPTIONAL): The WebAuthn relying party identifier. When
 present, the Decryptor uses this rpId for the WebAuthn ceremony.
 When omitted, selects anonymous RP mode.

 salt (REQUIRED): The Base64-encoded PRF salt; must decode to exactly
 32 octets. Generated at encryption time using SafeRandom.

 label (OPTIONAL): A human-readable display name for this credential
 (e.g., "YubiKey", "Phone"). Not included in the binding
 step_token; see Section 5.6.

 Credential selection is delegated to the authenticator via WebAuthn's
 allowCredentials mechanism. The Decryptor passes all candidate
 credential IDs for the rpId; the authenticator selects the matching
 credential internally.

 Grammar:

 webauthn-prf-step = "webauthn-prf(" webauthn-params ")"
 webauthn-params = [ "rpid=" rpid "," ] "salt=" salt
 [ "," "label=" label-value ]
 rpid = 1*( ALPHA / DIGIT / "-" / "." )
 salt = 1*BASE64CHAR ; 44 chars = 32 octets
 label-value = 1*( ALPHA / DIGIT / "-" )

 Anonymous RP mode: When rpid is omitted from the token, the
 Decryptor tries each rpId for which it holds credentials. Each
 rpId requires a separate WebAuthn ceremony (and potentially a user
 prompt). Privacy benefit: hides the relying party from passive
 observers. Cost: one ceremony per candidate rpId.

 Derivation: The authenticator evaluates the PRF extension with the
 selected credential and the decoded salt:

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 prf_salt = decode(salt) ; 32 octets

 prf_output = WebAuthn_PRF(credential, prf_salt)

 step_secret = SafeDerive(
 "webauthn-prf", prf_output,
 encryption_parameters, 32)

 Inputs: credential (local, selected by authenticator for the
 rpId), prf_salt (from token).

 Encode form: Encode("webauthn-prf", rpid, salt). rpid is UTF-8; salt
 is the raw decoded 32 octets. rpid is always present in the
 binding form even when omitted on-wire; when omitted, the
 Encryptor or Decryptor uses the rpId from the WebAuthn ceremony.
 Label is not included in binding.

 Validation: salt must decode to exactly 32 octets. rpid, when
 present, must match the hostname grammar 1*(ALPHA / DIGIT / "-" /
 ".").

 Example LOCK:

 -----BEGIN SAFE CONFIG-----
 Lock-Encoding: readable
 -----END SAFE CONFIG-----
 -----BEGIN SAFE LOCK-----
 Step: webauthn-prf(rpid=example.com,salt=xyz...)
 Encrypted-CEK:
 Base64-encoded encrypted CEK
 -----END SAFE LOCK-----

L.2.2. IANA Registry Entry

 +==============+================+==================+========+=======+
 | Step Name | Parameters | Inputs | Secret | Ref |
 +==============+================+==================+========+=======+
 | webauthn-prf | rpid=X, salt=X | Credential, | 32 | (this |
 | | | rpId | octets | doc) |
 +--------------+----------------+------------------+--------+-------+

 Table 21

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L.2.3. Security Considerations for WebAuthn PRF Step

 The WebAuthn PRF step provides hardware-bound key material (the
 authenticator holds the secret), user presence verification (touch
 required), phishing resistance (rpid binding), and offline decryption
 capability once the PRF output is computed.

 The PRF extension requires WebAuthn Level 3 support in the browser.
 Non-discoverable credentials wrap key material in the credential ID;
 losing the credential ID means losing access to the encrypted file.
 Unlike the Privacy Pass steps, no server-side rate limiting is
 possible.

 Privacy: rpid in cleartext reveals the relying party to passive
 observers. Anonymous RP mode (rpid omitted) hides this but the rpId
 may still be guessable from context.

 Trial bounds: anonymous RP mode requires one WebAuthn ceremony per
 candidate rpId. Decryptors should impose a local bound on the number
 of rpIds to try. Prefer identified mode when privacy is not
 required.

Author's Address

 Nick Sullivan
 Cryptography Consulting LLC
 Email: nicholas.sullivan+ietf@gmail.com

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