Secure Frame (SFrame): Lightweight Authenticated Encryption for Real-Time Media
RFC 9605



Internet Engineering Task Force (IETF) E. Omara
Request for Comments: 9605 Apple
Category: Standards Track J. Uberti
ISSN: 2070-1721 Fixie.ai
 S. G. Murillo
 CoSMo Software
 R. Barnes, Ed.
 Cisco
 Y. Fablet
 Apple
 August 2024

 Secure Frame (SFrame): Lightweight Authenticated Encryption for
 Real-Time Media

Abstract

 This document describes the Secure Frame (SFrame) end-to-end
 encryption and authentication mechanism for media frames in a
 multiparty conference call, in which central media servers (Selective
 Forwarding Units or SFUs) can access the media metadata needed to
 make forwarding decisions without having access to the actual media.

 This mechanism differs from the Secure Real-Time Protocol (SRTP) in
 that it is independent of RTP (thus compatible with non-RTP media
 transport) and can be applied to whole media frames in order to be
 more bandwidth efficient.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in Section 2 of RFC 7841.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9605.

Copyright Notice

 Copyright (c) 2024 IETF Trust and the persons identified as the
 document authors. All rights reserved.

 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 extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1. Introduction
 2. Terminology
 3. Goals
 4. SFrame
 4.1. Application Context
 4.2. SFrame Ciphertext
 4.3. SFrame Header
 4.4. Encryption Schema
 4.4.1. Key Selection
 4.4.2. Key Derivation
 4.4.3. Encryption
 4.4.4. Decryption
 4.5. Cipher Suites
 4.5.1. AES-CTR with SHA2
 5. Key Management
 5.1. Sender Keys
 5.2. MLS
 6. Media Considerations
 6.1. Selective Forwarding Units
 6.1.1. RTP Stream Reuse
 6.1.2. Simulcast
 6.1.3. Scalable Video Coding (SVC)
 6.2. Video Key Frames
 6.3. Partial Decoding
 7. Security Considerations
 7.1. No Header Confidentiality
 7.2. No Per-Sender Authentication
 7.3. Key Management
 7.4. Replay
 7.5. Risks Due to Short Tags
 8. IANA Considerations
 8.1. SFrame Cipher Suites
 9. Application Responsibilities
 9.1. Header Value Uniqueness
 9.2. Key Management Framework
 9.3. Anti-Replay
 9.4. Metadata
 10. References
 10.1. Normative References
 10.2. Informative References
 Appendix A. Example API
 Appendix B. Overhead Analysis
 B.1. Assumptions
 B.2. Audio
 B.3. Video
 B.4. Conferences
 B.5. SFrame over RTP
 Appendix C. Test Vectors
 C.1. Header Encoding/Decoding
 C.2. AEAD Encryption/Decryption Using AES-CTR and HMAC
 C.3. SFrame Encryption/Decryption
 Acknowledgements
 Contributors
 Authors' Addresses

1. Introduction

 Modern multiparty video call systems use Selective Forwarding Unit
 (SFU) servers to efficiently route media streams to call endpoints
 based on factors such as available bandwidth, desired video size,
 codec support, and other factors. An SFU typically does not need
 access to the media content of the conference, which allows the media
 to be encrypted "end to end" so that it cannot be decrypted by the
 SFU. In order for the SFU to work properly, though, it usually needs
 to be able to access RTP metadata and RTCP feedback messages, which
 is not possible if all RTP/RTCP traffic is end-to-end encrypted.

 As such, two layers of encryption and authentication are required:

 1. Hop-by-hop (HBH) encryption of media, metadata, and feedback
 messages between the endpoints and SFU

 2. End-to-end (E2E) encryption (E2EE) of media between the endpoints

 The Secure Real-Time Protocol (SRTP) is already widely used for HBH
 encryption [RFC3711]. The SRTP "double encryption" scheme defines a
 way to do E2E encryption in SRTP [RFC8723]. Unfortunately, this
 scheme has poor efficiency and high complexity, and its entanglement
 with RTP makes it unworkable in several realistic SFU scenarios.

 This document proposes a new E2EE protection scheme known as SFrame,
 specifically designed to work in group conference calls with SFUs.
 SFrame is a general encryption framing that can be used to protect
 media payloads, agnostic of transport.

2. Terminology

 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.

 MAC: Message Authentication Code

 E2EE: End-to-End Encryption

 HBH: Hop-by-Hop

 We use "Selective Forwarding Unit (SFU)" and "media stream" in a less
 formal sense than in [RFC7656]. An SFU is a selective switching
 function for media payloads, and a media stream is a sequence of
 media payloads, regardless of whether those media payloads are
 transported over RTP or some other protocol.

3. Goals

 SFrame is designed to be a suitable E2EE protection scheme for
 conference call media in a broad range of scenarios, as outlined by
 the following goals:

 1. Provide a secure E2EE mechanism for audio and video in conference
 calls that can be used with arbitrary SFU servers.

 2. Decouple media encryption from key management to allow SFrame to
 be used with an arbitrary key management system.

 3. Minimize packet expansion to allow successful conferencing in as
 many network conditions as possible.

 4. Decouple the media encryption framework from the underlying
 transport, allowing use in non-RTP scenarios, e.g., WebTransport
 [WEBTRANSPORT].

 5. When used with RTP and its associated error-resilience
 mechanisms, i.e., RTX and Forward Error Correction (FEC), require
 no special handling for RTX and FEC packets.

 6. Minimize the changes needed in SFU servers.

 7. Minimize the changes needed in endpoints.

 8. Work with the most popular audio and video codecs used in
 conferencing scenarios.

4. SFrame

 This document defines an encryption mechanism that provides effective
 E2EE, is simple to implement, has no dependencies on RTP, and
 minimizes encryption bandwidth overhead. This section describes how
 the mechanism works and includes details of how applications utilize
 SFrame for media protection as well as the actual mechanics of E2EE
 for protecting media.

4.1. Application Context

 SFrame is a general encryption framing, intended to be used as an
 E2EE layer over an underlying HBH-encrypted transport such as SRTP or
 QUIC [RFC3711][MOQ-TRANSPORT].

 The scale at which SFrame encryption is applied to media determines
 the overall amount of overhead that SFrame adds to the media stream
 as well as the engineering complexity involved in integrating SFrame
 into a particular environment. Two patterns are common: using SFrame
 to encrypt either whole media frames (per frame) or individual
 transport-level media payloads (per packet).

 For example, Figure 1 shows a typical media sender stack that takes
 media from some source, encodes it into frames, divides those frames
 into media packets, and then sends those payloads in SRTP packets.
 The receiver stack performs the reverse operations, reassembling
 frames from SRTP packets and decoding. Arrows indicate two different
 ways that SFrame protection could be integrated into this media
 stack: to encrypt whole frames or individual media packets.

 Applying SFrame per frame in this system offers higher efficiency but
 may require a more complex integration in environments where
 depacketization relies on the content of media packets. Applying
 SFrame per packet avoids this complexity at the cost of higher
 bandwidth consumption. Some quantitative discussion of these trade-
 offs is provided in Appendix B.

 As noted above, however, SFrame is a general media encapsulation and
 can be applied in other scenarios. The important thing is that the
 sender and receivers of an SFrame-encrypted object agree on that
 object's semantics. SFrame does not provide this agreement; it must
 be arranged by the application.

 +------------------------------------------------------+
 | |
 | +--------+ +-------------+ +-----------+ |
 .-. | | | | | | HBH | |
| | | | Encode |----->| Packetize |----->| Protect |----------+
 '+' | | | ^ | | ^ | | | |
 /|\ | +--------+ | +-------------+ | +-----------+ | |
/ + \ | | | ^ | |
 / \ | SFrame SFrame | | |
/ \ | Protect Protect | | |
Alice | (per frame) (per packet) | | |
 | ^ ^ | | |
 | | | | | |
 +---------------|-------------------|---------|--------+ |
 | | | v
 | | | +------+-+
 | E2E Key | HBH Key | Media |
 +---- Management ---+ Management | Server |
 | | | +------+-+
 | | | |
 +---------------|-------------------|---------|--------+ |
 | | | | | |
 | V V | | |
 .-. | SFrame SFrame | | |
| | | Unprotect Unprotect | | |
 '+' | (per frame) (per packet) | | |
 /|\ | | | V | |
/ + \ | +--------+ | +-------------+ | +-----------+ | |
 / \ | | | V | | V | HBH | | |
/ \ | | Decode |<-----| Depacketize |<-----| Unprotect |<---------+
 Bob | | | | | | | |
 | +--------+ +-------------+ +-----------+ |
 | |
 +------------------------------------------------------+

Figure 1: Two Options for Integrating SFrame in a Typical Media Stack

 Like SRTP, SFrame does not define how the keys used for SFrame are
 exchanged by the parties in the conference. Keys for SFrame might be
 distributed over an existing E2E-secure channel (see Section 5.1) or
 derived from an E2E-secure shared secret (see Section 5.2). The key
 management system MUST ensure that each key used for encrypting media
 is used by exactly one media sender in order to avoid reuse of
 nonces.

4.2. SFrame Ciphertext

 An SFrame ciphertext comprises an SFrame header followed by the
 output of an Authenticated Encryption with Associated Data (AEAD)
 encryption of the plaintext [RFC5116], with the header provided as
 additional authenticated data (AAD).

 The SFrame header is a variable-length structure described in detail
 in Section 4.3. The structure of the encrypted data and
 authentication tag are determined by the AEAD algorithm in use.

 +-+----+-+----+--------------------+--------------------+<-+
 |K|KLEN|C|CLEN| Key ID | Counter | |
 +->+-+----+-+----+--------------------+--------------------+ |
 | | | |
 | | | |
 | | | |
 | | | |
 | | Encrypted Data | |
 | | | |
 | | | |
 | | | |
 | | | |
 +->+-------------------------------------------------------+<-+
 | | Authentication Tag | |
 | +-------------------------------------------------------+ |
 | |
 | |
 +--- Encrypted Portion Authenticated Portion ---+

 Figure 2: Structure of an SFrame Ciphertext

 When SFrame is applied per packet, the payload of each packet will be
 an SFrame ciphertext. When SFrame is applied per frame, the SFrame
 ciphertext representing an encrypted frame will span several packets,
 with the header appearing in the first packet and the authentication
 tag in the last packet. It is the responsibility of the application
 to reassemble an encrypted frame from individual packets, accounting
 for packet loss and reordering as necessary.

4.3. SFrame Header

 The SFrame header specifies two values from which encryption
 parameters are derived:

 * A Key ID (KID) that determines which encryption key should be used

 * A Counter (CTR) that is used to construct the nonce for the
 encryption

 Applications MUST ensure that each (KID, CTR) combination is used for
 exactly one SFrame encryption operation. A typical approach to
 achieve this guarantee is outlined in Section 9.1.

 Config Byte
 |
 .-----' '-----.
 | |
 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+------------+------------+
 |X| K |Y| C | KID... | CTR... |
 +-+-+-+-+-+-+-+-+------------+------------+

 Figure 3: SFrame Header

 The SFrame header has the overall structure shown in Figure 3. The
 first byte is a "config byte", with the following fields:

 Extended KID Flag (X, 1 bit): Indicates if the K field contains the
 KID or the KID length.

 KID or KID Length (K, 3 bits): If the X flag is set to 0, this field
 contains the KID. If the X flag is set to 1, then it contains the
 length of the KID, minus one.

 Extended CTR Flag (Y, 1 bit): Indicates if the C field contains the
 CTR or the CTR length.

 CTR or CTR Length (C, 3 bits): This field contains the CTR if the Y
 flag is set to 0, or the CTR length, minus one, if set to 1.

 The KID and CTR fields are encoded as compact unsigned integers in
 network (big-endian) byte order. If the value of one of these fields
 is in the range 0-7, then the value is carried in the corresponding
 bits of the config byte (K or C) and the corresponding flag (X or Y)
 is set to zero. Otherwise, the value MUST be encoded with the
 minimum number of bytes required and appended after the config byte,
 with the KID first and CTR second. The header field (K or C) is set
 to the number of bytes in the encoded value, minus one. The value
 000 represents a length of 1, 001 a length of 2, etc. This allows a
 3-bit length field to represent the value lengths 1-8.

 The SFrame header can thus take one of the four forms shown in
 Figure 4, depending on which of the X and Y flags are set.

 KID < 8, CTR < 8:
 +-+-----+-+-----+
 |0| KID |0| CTR |
 +-+-----+-+-----+

 KID < 8, CTR >= 8:
 +-+-----+-+-----+------------------------+
 |0| KID |1|CLEN | CTR... (length=CLEN) |
 +-+-----+-+-----+------------------------+

 KID >= 8, CTR < 8:
 +-+-----+-+-----+------------------------+
 |1|KLEN |0| CTR | KID... (length=KLEN) |
 +-+-----+-+-----+------------------------+

 KID >= 8, CTR >= 8:
 +-+-----+-+-----+------------------------+------------------------+
 |1|KLEN |1|CLEN | KID... (length=KLEN) | CTR... (length=CLEN) |
 +-+-----+-+-----+------------------------+------------------------+

 Figure 4: Forms of Encoded SFrame Header

4.4. Encryption Schema

 SFrame encryption uses an AEAD encryption algorithm and hash function
 defined by the cipher suite in use (see Section 4.5). We will refer
 to the following aspects of the AEAD and the hash algorithm below:

 * AEAD.Encrypt and AEAD.Decrypt - The encryption and decryption
 functions for the AEAD. We follow the convention of RFC 5116
 [RFC5116] and consider the authentication tag part of the
 ciphertext produced by AEAD.Encrypt (as opposed to a separate
 field as in SRTP [RFC3711]).

 * AEAD.Nk - The size in bytes of a key for the encryption algorithm

 * AEAD.Nn - The size in bytes of a nonce for the encryption
 algorithm

 * AEAD.Nt - The overhead in bytes of the encryption algorithm
 (typically the size of a "tag" that is added to the plaintext)

 * AEAD.Nka - For cipher suites using the compound AEAD described in
 Section 4.5.1, the size in bytes of a key for the underlying
 encryption algorithm

 * Hash.Nh - The size in bytes of the output of the hash function

4.4.1. Key Selection

 Each SFrame encryption or decryption operation is premised on a
 single secret base_key, which is labeled with an integer KID value
 signaled in the SFrame header.

 The sender and receivers need to agree on which base_key should be
 used for a given KID. Moreover, senders and receivers need to agree
 on whether a base_key will be used for encryption or decryption only.
 The process for provisioning base_key values and their KID values is
 beyond the scope of this specification, but its security properties
 will bound the assurances that SFrame provides. For example, if
 SFrame is used to provide E2E security against intermediary media
 nodes, then SFrame keys need to be negotiated in a way that does not
 make them accessible to these intermediaries.

 For each known KID value, the client stores the corresponding
 symmetric key base_key. For keys that can be used for encryption,
 the client also stores the next CTR value to be used when encrypting
 (initially 0).

 When encrypting a plaintext, the application specifies which KID is
 to be used, and the CTR value is incremented after successful
 encryption. When decrypting, the base_key for decryption is selected
 from the available keys using the KID value in the SFrame header.

 A given base_key MUST NOT be used for encryption by multiple senders.
 Such reuse would result in multiple encrypted frames being generated
 with the same (key, nonce) pair, which harms the protections provided
 by many AEAD algorithms. Implementations MUST mark each base_key as
 usable for encryption or decryption, never both.

 Note that the set of available keys might change over the lifetime of
 a real-time session. In such cases, the client will need to manage
 key usage to avoid media loss due to a key being used to encrypt
 before all receivers are able to use it to decrypt. For example, an
 application may make decryption-only keys available immediately, but
 delay the use of keys for encryption until (a) all receivers have
 acknowledged receipt of the new key, or (b) a timeout expires.

4.4.2. Key Derivation

 SFrame encryption and decryption use a key and salt derived from the
 base_key associated with a KID. Given a base_key value, the key and
 salt are derived using HMAC-based Key Derivation Function (HKDF)
 [RFC5869] as follows:

 def derive_key_salt(KID, base_key):
 sframe_secret = HKDF-Extract("", base_key)

 sframe_key_label = "SFrame 1.0 Secret key " + KID + cipher_suite
 sframe_key =
 HKDF-Expand(sframe_secret, sframe_key_label, AEAD.Nk)

 sframe_salt_label = "SFrame 1.0 Secret salt " + KID + cipher_suite
 sframe_salt =
 HKDF-Expand(sframe_secret, sframe_salt_label, AEAD.Nn)

 return sframe_key, sframe_salt

 In the derivation of sframe_secret:

 * The + operator represents concatenation of byte strings.

 * The KID value is encoded as an 8-byte big-endian integer, not the
 compressed form used in the SFrame header.

 * The cipher_suite value is a 2-byte big-endian integer representing
 the cipher suite in use (see Section 8.1).

 The hash function used for HKDF is determined by the cipher suite in
 use.

4.4.3. Encryption

 SFrame encryption uses the AEAD encryption algorithm for the cipher
 suite in use. The key for the encryption is the sframe_key. The
 nonce is formed by first XORing the sframe_salt with the current CTR
 value, and then encoding the result as a big-endian integer of length
 AEAD.Nn.

 The encryptor forms an SFrame header using the CTR and KID values
 provided. The encoded header is provided as AAD to the AEAD
 encryption operation, together with application-provided metadata
 about the encrypted media (see Section 9.4).

 def encrypt(CTR, KID, metadata, plaintext):
 sframe_key, sframe_salt = key_store[KID]

 # encode_big_endian(x, n) produces an n-byte string encoding the
 # integer x in big-endian byte order.
 ctr = encode_big_endian(CTR, AEAD.Nn)
 nonce = xor(sframe_salt, CTR)

 # encode_sframe_header produces a byte string encoding the
 # provided KID and CTR values into an SFrame header.
 header = encode_sframe_header(CTR, KID)
 aad = header + metadata

 ciphertext = AEAD.Encrypt(sframe_key, nonce, aad, plaintext)
 return header + ciphertext

 For example, the metadata input to encryption allows for frame
 metadata to be authenticated when SFrame is applied per frame. After
 encoding the frame and before packetizing it, the necessary media
 metadata will be moved out of the encoded frame buffer to be sent in
 some channel visible to the SFU (e.g., an RTP header extension).

 +---------------+
 | |
 | |
 | plaintext |
 | |
 | |
 +-------+-------+
 |
 .- +-----+ |
 | | +--+--> sframe_key ----->| Key
 Header | | KID | | |
 | | | +--> sframe_salt --+ |
 +--+ +-----+ | |
 | | | +---------------------+->| Nonce
 | | | CTR | |
 | | | | |
 | '- +-----+ |
 | |
 | +----------------+ |
 | | metadata | |
 | +-------+--------+ |
 | | |
 +------------------+----------------->| AAD
 | |
 | AEAD.Encrypt
 | |
 | SFrame Ciphertext |
 | +---------------+ |
 +-------------->| SFrame Header | |
 +---------------+ |
 | | |
 | |<----+
 | ciphertext |
 | |
 | |
 +---------------+

 Figure 5: Encrypting an SFrame Ciphertext

4.4.4. Decryption

 Before decrypting, a receiver needs to assemble a full SFrame
 ciphertext. When an SFrame ciphertext is fragmented into multiple
 parts for transport (e.g., a whole encrypted frame sent in multiple
 SRTP packets), the receiving client collects all the fragments of the
 ciphertext, using appropriate sequencing and start/end markers in the
 transport. Once all of the required fragments are available, the
 client reassembles them into the SFrame ciphertext and passes the
 ciphertext to SFrame for decryption.

 The KID field in the SFrame header is used to find the right key and
 salt for the encrypted frame, and the CTR field is used to construct
 the nonce. The SFrame decryption procedure is as follows:

 def decrypt(metadata, sframe_ciphertext):
 KID, CTR, header, ciphertext = parse_ciphertext(sframe_ciphertext)

 sframe_key, sframe_salt = key_store[KID]

 ctr = encode_big_endian(CTR, AEAD.Nn)
 nonce = xor(sframe_salt, ctr)
 aad = header + metadata

 return AEAD.Decrypt(sframe_key, nonce, aad, ciphertext)

 If a ciphertext fails to decrypt because there is no key available
 for the KID in the SFrame header, the client MAY buffer the
 ciphertext and retry decryption once a key with that KID is received.
 If a ciphertext fails to decrypt for any other reason, the client
 MUST discard the ciphertext. Invalid ciphertexts SHOULD be discarded
 in a way that is indistinguishable (to an external observer) from
 having processed a valid ciphertext. In other words, the SFrame
 decrypt operation should take the same amount of time regardless of
 whether decryption succeeds or fails.

 SFrame Ciphertext
 +---------------+
 +---------------| SFrame Header |
 | +---------------+
 | | |
 | | |-----+
 | | ciphertext | |
 | | | |
 | | | |
 | +---------------+ |
 | |
 | .- +-----+ |
 | | | +--+--> sframe_key ----->| Key
 | | | KID | | |
 | | | | +--> sframe_salt --+ |
 +->+ +-----+ | |
 | | | +---------------------+->| Nonce
 | | | CTR | |
 | | | | |
 | '- +-----+ |
 | |
 | +----------------+ |
 | | metadata | |
 | +-------+--------+ |
 | | |
 +------------------+----------------->| AAD
 |
 AEAD.Decrypt
 |
 V
 +---------------+
 | |
 | |
 | plaintext |
 | |
 | |
 +---------------+

 Figure 6: Decrypting an SFrame Ciphertext

4.5. Cipher Suites

 Each SFrame session uses a single cipher suite that specifies the
 following primitives:

 * A hash function used for key derivation

 * An AEAD encryption algorithm [RFC5116] used for frame encryption,
 optionally with a truncated authentication tag

 This document defines the following cipher suites, with the constants
 defined in Section 4.4:

 +============================+====+=====+====+====+====+
 | Name | Nh | Nka | Nk | Nn | Nt |
 +============================+====+=====+====+====+====+
 | AES_128_CTR_HMAC_SHA256_80 | 32 | 16 | 48 | 12 | 10 |
 +----------------------------+----+-----+----+----+----+
 | AES_128_CTR_HMAC_SHA256_64 | 32 | 16 | 48 | 12 | 8 |
 +----------------------------+----+-----+----+----+----+
 | AES_128_CTR_HMAC_SHA256_32 | 32 | 16 | 48 | 12 | 4 |
 +----------------------------+----+-----+----+----+----+
 | AES_128_GCM_SHA256_128 | 32 | n/a | 16 | 12 | 16 |
 +----------------------------+----+-----+----+----+----+
 | AES_256_GCM_SHA512_128 | 64 | n/a | 32 | 12 | 16 |
 +----------------------------+----+-----+----+----+----+

 Table 1: SFrame Cipher Suite Constants

 Numeric identifiers for these cipher suites are defined in the IANA
 registry created in Section 8.1.

 In the suite names, the length of the authentication tag is indicated
 by the last value: "_128" indicates a 128-bit tag, "_80" indicates an
 80-bit tag, "_64" indicates a 64-bit tag, and "_32" indicates a
 32-bit tag.

 In a session that uses multiple media streams, different cipher
 suites might be configured for different media streams. For example,
 in order to conserve bandwidth, a session might use a cipher suite
 with 80-bit tags for video frames and another cipher suite with
 32-bit tags for audio frames.

4.5.1. AES-CTR with SHA2

 In order to allow very short tag sizes, we define a synthetic AEAD
 function using the authenticated counter mode of AES together with
 HMAC for authentication. We use an encrypt-then-MAC approach, as in
 SRTP [RFC3711].

 Before encryption or decryption, encryption and authentication
 subkeys are derived from the single AEAD key. The overall length of
 the AEAD key is Nka + Nh, where Nka represents the key size for the
 AES block cipher in use and Nh represents the output size of the hash
 function (as in Section 4.4). The encryption subkey comprises the
 first Nka bytes and the authentication subkey comprises the remaining
 Nh bytes.

 def derive_subkeys(sframe_key):
 # The encryption key comprises the first Nka bytes
 enc_key = sframe_key[..Nka]

 # The authentication key comprises Nh remaining bytes
 auth_key = sframe_key[Nka..]

 return enc_key, auth_key

 The AEAD encryption and decryption functions are then composed of
 individual calls to the CTR encrypt function and HMAC. The resulting
 MAC value is truncated to a number of bytes Nt fixed by the cipher
 suite.

 def truncate(tag, n):
 # Take the first `n` bytes of `tag`
 return tag[..n]

 def compute_tag(auth_key, nonce, aad, ct):
 aad_len = encode_big_endian(len(aad), 8)
 ct_len = encode_big_endian(len(ct), 8)
 tag_len = encode_big_endian(Nt, 8)
 auth_data = aad_len + ct_len + tag_len + nonce + aad + ct
 tag = HMAC(auth_key, auth_data)
 return truncate(tag, Nt)

 def AEAD.Encrypt(key, nonce, aad, pt):
 enc_key, auth_key = derive_subkeys(key)
 initial_counter = nonce + 0x00000000 # append four zero bytes
 ct = AES-CTR.Encrypt(enc_key, initial_counter, pt)
 tag = compute_tag(auth_key, nonce, aad, ct)
 return ct + tag

 def AEAD.Decrypt(key, nonce, aad, ct):
 inner_ct, tag = split_ct(ct, tag_len)

 enc_key, auth_key = derive_subkeys(key)
 candidate_tag = compute_tag(auth_key, nonce, aad, inner_ct)
 if !constant_time_equal(tag, candidate_tag):
 raise Exception("Authentication Failure")

 initial_counter = nonce + 0x00000000 # append four zero bytes
 return AES-CTR.Decrypt(enc_key, initial_counter, inner_ct)

5. Key Management

 SFrame must be integrated with an E2E key management framework to
 exchange and rotate the keys used for SFrame encryption. The key
 management framework provides the following functions:

 * Provisioning KID / base_key mappings to participating clients

 * Updating the above data as clients join or leave

 It is the responsibility of the application to provide the key
 management framework, as described in Section 9.2.

5.1. Sender Keys

 If the participants in a call have a preexisting E2E-secure channel,
 they can use it to distribute SFrame keys. Each client participating
 in a call generates a fresh base_key value that it will use to
 encrypt media. The client then uses the E2E-secure channel to send
 their encryption key to the other participants.

 In this scheme, it is assumed that receivers have a signal outside of
 SFrame for which client has sent a given frame (e.g., an RTP
 synchronization source (SSRC)). SFrame KID values are then used to
 distinguish between versions of the sender's base_key.

 KID values in this scheme have two parts: a "key generation" and a
 "ratchet step". Both are unsigned integers that begin at zero. The
 key generation increments each time the sender distributes a new key
 to receivers. The ratchet step is incremented each time the sender
 ratchets their key forward for forward secrecy:

 base_key[i+1] = HKDF-Expand(
 HKDF-Extract("", base_key[i]),
 "SFrame 1.0 Ratchet", CipherSuite.Nh)

 For compactness, we do not send the whole ratchet step. Instead, we
 send only its low-order R bits, where R is a value set by the
 application. Different senders may use different values of R, but
 each receiver of a given sender needs to know what value of R is used
 by the sender so that they can recognize when they need to ratchet
 (vs. expecting a new key). R effectively defines a reordering
 window, since no more than 2^R ratchet steps can be active at a given
 time. The key generation is sent in the remaining 64 - R bits of the
 KID.

 KID = (key_generation << R) + (ratchet_step % (1 << R))

 64-R bits R bits
 <---------------> <------------>
 +-----------------+--------------+
 | Key Generation | Ratchet Step |
 +-----------------+--------------+

 Figure 7: Structure of a KID in the Sender Keys Scheme

 The sender signals such a ratchet step update by sending with a KID
 value in which the ratchet step has been incremented. A receiver who
 receives from a sender with a new KID computes the new key as above.
 The old key may be kept for some time to allow for out-of-order
 delivery, but should be deleted promptly.

 If a new participant joins in the middle of a session, they will need
 to receive from each sender (a) the current sender key for that
 sender and (b) the current KID value for the sender. Evicting a
 participant requires each sender to send a fresh sender key to all
 receivers.

 It is the application's responsibility to decide when sender keys are
 updated. A sender key may be updated by sending a new base_key
 (updating the key generation) or by hashing the current base_key
 (updating the ratchet step). Ratcheting the key forward is useful
 when adding new receivers to an SFrame-based interaction, since it
 ensures that the new receivers can't decrypt any media encrypted
 before they were added. If a sender wishes to assure the opposite
 property when removing a receiver (i.e., ensuring that the receiver
 can't decrypt media after they are removed), then the sender will
 need to distribute a new sender key.

5.2. MLS

 The Messaging Layer Security (MLS) protocol provides group
 authenticated key exchange [MLS-ARCH] [MLS-PROTO]. In principle, it
 could be used to instantiate the sender key scheme above, but it can
 also be used more efficiently directly.

 MLS creates a linear sequence of keys, each of which is shared among
 the members of a group at a given point in time. When a member joins
 or leaves the group, a new key is produced that is known only to the
 augmented or reduced group. Each step in the lifetime of the group
 is known as an "epoch", and each member of the group is assigned an
 "index" that is constant for the time they are in the group.

 To generate keys and nonces for SFrame, we use the MLS exporter
 function to generate a base_key value for each MLS epoch. Each
 member of the group is assigned a set of KID values so that each
 member has a unique sframe_key and sframe_salt that it uses to
 encrypt with. Senders may choose any KID value within their assigned
 set of KID values, e.g., to allow a single sender to send multiple,
 uncoordinated outbound media streams.

 base_key = MLS-Exporter("SFrame 1.0 Base Key", "", AEAD.Nk)

 For compactness, we do not send the whole epoch number. Instead, we
 send only its low-order E bits, where E is a value set by the
 application. E effectively defines a reordering window, since no
 more than 2^E epochs can be active at a given time. To handle
 rollover of the epoch counter, receivers MUST remove an old epoch
 when a new epoch with the same low-order E bits is introduced.

 Let S be the number of bits required to encode a member index in the
 group, i.e., the smallest value such that group_size <= (1 << S).
 The sender index is encoded in the S bits above the epoch. The
 remaining 64 - S - E bits of the KID value are a context value chosen
 by the sender (context value 0 will produce the shortest encoded
 KID).

 KID = (context << (S + E)) + (sender_index << E) + (epoch % (1 << E))

 64-S-E bits S bits E bits
 <-----------> <------> <------>
 +-------------+--------+-------+
 | Context ID | Index | Epoch |
 +-------------+--------+-------+

 Figure 8: Structure of a KID for an MLS Sender

 Once an SFrame stack has been provisioned with the
 sframe_epoch_secret for an epoch, it can compute the required KID
 values on demand (as well as the resulting SFrame keys/nonces derived
 from the base_key and KID) as it needs to encrypt or decrypt for a
 given member.

 ...
 |
 |
 Epoch 14 +--+-- index=3 ---> KID = 0x3e
 | |
 | +-- index=7 ---> KID = 0x7e
 | |
 | +-- index=20 --> KID = 0x14e
 |
 |
 Epoch 15 +--+-- index=3 ---> KID = 0x3f
 | |
 | +-- index=5 ---> KID = 0x5f
 |
 |
 Epoch 16 +----- index=2 --+--> context = 2 --> KID = 0x820
 | |
 | +--> context = 3 --> KID = 0xc20
 |
 |
 Epoch 17 +--+-- index=33 --> KID = 0x211
 | |
 | +-- index=51 --> KID = 0x331
 |
 |
 ...

 Figure 9: An Example Sequence of KIDs for an MLS-based SFrame
 Session (E=4; S=6, Allowing for 64 Group Members)

6. Media Considerations

6.1. Selective Forwarding Units

 SFUs (e.g., those described in Section 3.7 of [RFC7667]) receive the
 media streams from each participant and select which ones should be
 forwarded to each of the other participants. There are several
 approaches for stream selection, but in general, the SFU needs to
 access metadata associated with each frame and modify the RTP
 information of the incoming packets when they are transmitted to the
 received participants.

 This section describes how these normal SFU modes of operation
 interact with the E2EE provided by SFrame.

6.1.1. RTP Stream Reuse

 The SFU may choose to send only a certain number of streams based on
 the voice activity of the participants. To avoid the overhead
 involved in establishing new transport streams, the SFU may decide to
 reuse previously existing streams or even pre-allocate a predefined
 number of streams and choose in each moment in time which participant
 media will be sent through it.

 This means that the same transport-level stream (e.g., an RTP stream
 defined by either SSRC or Media Identification (MID)) may carry media
 from different streams of different participants. Because each
 participant uses a different key to encrypt their media, the receiver
 will be able to verify the sender of the media within the RTP stream
 at any given point in time. Thus the receiver will correctly
 associate the media with the sender indicated by the authenticated
 SFrame KID value, irrespective of how the SFU transmits the media to
 the client.

 Note that in order to prevent impersonation by a malicious
 participant (not the SFU), a mechanism based on digital signature
 would be required. SFrame does not protect against such attacks.

6.1.2. Simulcast

 When using simulcast, the same input image will produce N different
 encoded frames (one per simulcast layer), which would be processed
 independently by the frame encryptor and assigned an unique CTR value
 for each.

6.1.3. Scalable Video Coding (SVC)

 In both temporal and spatial scalability, the SFU may choose to drop
 layers in order to match a certain bitrate or to forward specific
 media sizes or frames per second. In order to support the SFU
 selectively removing layers, the sender MUST encapsulate each layer
 in a different SFrame ciphertext.

6.2. Video Key Frames

 Forward security and post-compromise security require that the E2EE
 keys (base keys) are updated any time a participant joins or leaves
 the call.

 The key exchange happens asynchronously and on a different path than
 the SFU signaling and media. So it may happen that when a new
 participant joins the call and the SFU side requests a key frame, the
 sender generates the E2EE frame with a key that is not known by the
 receiver, so it will be discarded. When the sender updates his
 sending key with the new key, it will send it in a non-key frame, so
 the receiver will be able to decrypt it, but not decode it.

 The new receiver will then re-request a key frame, but due to sender
 and SFU policies, that new key frame could take some time to be
 generated.

 If the sender sends a key frame after the new E2EE key is in use, the
 time required for the new participant to display the video is
 minimized.

 Note that this issue does not arise for media streams that do not
 have dependencies among frames, e.g., audio streams. In these
 streams, each frame is independently decodable, so a frame never
 depends on another frame that might be on the other side of a key
 rotation.

6.3. Partial Decoding

 Some codecs support partial decoding, where individual packets can be
 decoded without waiting for the full frame to arrive. When SFrame is
 applied per frame, partial decoding is not possible because the
 decoder cannot access data until an entire frame has arrived and has
 been decrypted.

7. Security Considerations

7.1. No Header Confidentiality

 SFrame provides integrity protection to the SFrame header (the KID
 and CTR values), but it does not provide confidentiality protection.
 Parties that can observe the SFrame header may learn, for example,
 which parties are sending SFrame payloads (from KID values) and at
 what rates (from CTR values). In cases where SFrame is used for end-
 to-end security on top of hop-by-hop protections (e.g., running over
 SRTP as described in Appendix B.5), the hop-by-hop security
 mechanisms provide confidentiality protection of the SFrame header
 between hops.

7.2. No Per-Sender Authentication

 SFrame does not provide per-sender authentication of media data. Any
 sender in a session can send media that will be associated with any
 other sender. This is because SFrame uses symmetric encryption to
 protect media data, so that any receiver also has the keys required
 to encrypt packets for the sender.

7.3. Key Management

 The specifics of key management are beyond the scope of this
 document. However, every client SHOULD change their keys when new
 clients join or leave the call for forward secrecy and post-
 compromise security.

7.4. Replay

 The handling of replay is out of the scope of this document.
 However, senders MUST reject requests to encrypt multiple times with
 the same key and nonce since several AEAD algorithms fail badly in
 such cases (see, e.g., Section 5.1.1 of [RFC5116]).

7.5. Risks Due to Short Tags

 The SFrame cipher suites based on AES-CTR allow for the use of short
 authentication tags, which bring a higher risk that an attacker will
 be able to cause an SFrame receiver to accept an SFrame ciphertext of
 the attacker's choosing.

 Assuming that the authentication properties of the cipher suite are
 robust, the only attack that an attacker can mount is an attempt to
 find an acceptable (ciphertext, tag) combination through brute force.
 Such a brute-force attack will have an expected success rate of the
 following form:

 attacker_success_rate = attempts_per_second / 2^(8*Nt)

 For example, a gigabit Ethernet connection is able to transmit
 roughly 2^20 packets per second. If an attacker saturated such a
 link with guesses against a 32-bit authentication tag (Nt=4), then
 the attacker would succeed on average roughly once every 2^12
 seconds, or about once an hour.

 In a typical SFrame usage in a real-time media application, there are
 a few approaches to mitigating this risk:

 * Receivers only accept SFrame ciphertexts over HBH-secure channels
 (e.g., SRTP security associations or QUIC connections). If this
 is the case, only an entity that is part of such a channel can
 mount the above attack.

 * The expected packet rate for a media stream is very predictable
 (and typically far lower than the above example). On the one
 hand, attacks at this rate will succeed even less often than the
 high-rate attack described above. On the other hand, the
 application may use an elevated packet arrival rate as a signal of
 a brute-force attack. This latter approach is common in other
 settings, e.g., mitigating brute-force attacks on passwords.

 * Media applications typically do not provide feedback to media
 senders as to which media packets failed to decrypt. When media-
 quality feedback mechanisms are used, decryption failures will
 typically appear as packet losses, but only at an aggregate level.

 * Anti-replay mechanisms (see Section 7.4) prevent the attacker from
 reusing valid ciphertexts (either observed or guessed by the
 attacker). A receiver applying anti-replay controls will only
 accept one valid plaintext per CTR value. Since the CTR value is
 covered by SFrame authentication, an attacker has to do a fresh
 search for a valid tag for every forged ciphertext, even if the
 encrypted content is unchanged. In other words, when the above
 brute-force attack succeeds, it only allows the attacker to send a
 single SFrame ciphertext; the ciphertext cannot be reused because
 either it will have the same CTR value and be discarded as a
 replay, or else it will have a different CTR value and its tag
 will no longer be valid.

 Nonetheless, without these mitigations, an application that makes use
 of short tags will be at heightened risk of forgery attacks. In many
 cases, it is simpler to use full-size tags and tolerate slightly
 higher bandwidth usage rather than to add the additional defenses
 necessary to safely use short tags.

8. IANA Considerations

 IANA has created a new registry called "SFrame Cipher Suites"
 (Section 8.1) under the "SFrame" group registry heading.

8.1. SFrame Cipher Suites

 The "SFrame Cipher Suites" registry lists identifiers for SFrame
 cipher suites as defined in Section 4.5. The cipher suite field is
 two bytes wide, so the valid cipher suites are in the range 0x0000 to
 0xFFFF. Except as noted below, assignments are made via the
 Specification Required policy [RFC8126].

 The registration template is as follows:

 * Value: The numeric value of the cipher suite

 * Name: The name of the cipher suite

 * Recommended: Whether support for this cipher suite is recommended
 by the IETF. Valid values are "Y", "N", and "D" as described in
 Section 17.1 of [MLS-PROTO]. The default value of the
 "Recommended" column is "N". Setting the Recommended item to "Y"
 or "D", or changing an item whose current value is "Y" or "D",
 requires Standards Action [RFC8126].

 * Reference: The document where this cipher suite is defined

 * Change Controller: Who is authorized to update the row in the
 registry

 Initial contents:

 +========+============================+===+===========+============+
 | Value | Name | R | Reference | Change |
 | | | | | Controller |
 +========+============================+===+===========+============+
 | 0x0000 | Reserved | - | RFC 9605 | IETF |
 +--------+----------------------------+---+-----------+------------+
 | 0x0001 | AES_128_CTR_HMAC_SHA256_80 | Y | RFC 9605 | IETF |
 +--------+----------------------------+---+-----------+------------+
 | 0x0002 | AES_128_CTR_HMAC_SHA256_64 | Y | RFC 9605 | IETF |
 +--------+----------------------------+---+-----------+------------+
 | 0x0003 | AES_128_CTR_HMAC_SHA256_32 | Y | RFC 9605 | IETF |
 +--------+----------------------------+---+-----------+------------+
 | 0x0004 | AES_128_GCM_SHA256_128 | Y | RFC 9605 | IETF |
 +--------+----------------------------+---+-----------+------------+
 | 0x0005 | AES_256_GCM_SHA512_128 | Y | RFC 9605 | IETF |
 +--------+----------------------------+---+-----------+------------+
 | 0xF000 | Reserved for Private Use | - | RFC 9605 | IETF |
 | - | | | | |
 | 0xFFFF | | | | |
 +--------+----------------------------+---+-----------+------------+

 Table 2: SFrame Cipher Suites

9. Application Responsibilities

 To use SFrame, an application needs to define the inputs to the
 SFrame encryption and decryption operations, and how SFrame
 ciphertexts are delivered from sender to receiver (including any
 fragmentation and reassembly). In this section, we lay out
 additional requirements that an application must meet in order for
 SFrame to operate securely.

 In general, an application using SFrame is responsible for
 configuring SFrame. The application must first define when SFrame is
 applied at all. When SFrame is applied, the application must define
 which cipher suite is to be used. If new versions of SFrame are
 defined in the future, it will be the application's responsibility to
 determine which version should be used.

 This division of responsibilities is similar to the way other media
 parameters (e.g., codecs) are typically handled in media
 applications, in the sense that they are set up in some signaling
 protocol and not described in the media. Applications might find it
 useful to extend the protocols used for negotiating other media
 parameters (e.g., Session Description Protocol (SDP) [RFC8866]) to
 also negotiate parameters for SFrame.

9.1. Header Value Uniqueness

 Applications MUST ensure that each (base_key, KID, CTR) combination
 is used for at most one SFrame encryption operation. This ensures
 that the (key, nonce) pairs used by the underlying AEAD algorithm are
 never reused. Typically this is done by assigning each sender a KID
 or set of KIDs, then having each sender use the CTR field as a
 monotonic counter, incrementing for each plaintext that is encrypted.
 In addition to its simplicity, this scheme minimizes overhead by
 keeping CTR values as small as possible.

 In applications where an SFrame context might be written to
 persistent storage, this context needs to include the last-used CTR
 value. When the context is used later, the application should use
 the stored CTR value to determine the next CTR value to be used in an
 encryption operation, and then write the next CTR value back to
 storage before using the CTR value for encryption. Storing the CTR
 value before usage (vs. after) helps ensure that a storage failure
 will not cause reuse of the same (base_key, KID, CTR) combination.

9.2. Key Management Framework

 The application is responsible for provisioning SFrame with a mapping
 of KID values to base_key values and the resulting keys and salts.
 More importantly, the application specifies which KID values are used
 for which purposes (e.g., by which senders). An application's KID
 assignment strategy MUST be structured to assure the non-reuse
 properties discussed in Section 9.1.

 The application is also responsible for defining a rotation schedule
 for keys. For example, one application might have an ephemeral group
 for every call and keep rotating keys when endpoints join or leave
 the call, while another application could have a persistent group
 that can be used for multiple calls and simply derives ephemeral
 symmetric keys for a specific call.

 It should be noted that KID values are not encrypted by SFrame and
 are thus visible to any application-layer intermediaries that might
 handle an SFrame ciphertext. If there are application semantics
 included in KID values, then this information would be exposed to
 intermediaries. For example, in the scheme of Section 5.1, the
 number of ratchet steps per sender is exposed, and in the scheme of
 Section 5.2, the number of epochs and the MLS sender ID of the SFrame
 sender are exposed.

9.3. Anti-Replay

 It is the responsibility of the application to handle anti-replay.
 Replay by network attackers is assumed to be prevented by network-
 layer facilities (e.g., TLS, SRTP). As mentioned in Section 7.4,
 senders MUST reject requests to encrypt multiple times with the same
 key and nonce.

 It is not mandatory to implement anti-replay on the receiver side.
 Receivers MAY apply time- or counter-based anti-replay mitigations.
 For example, Section 3.3.2 of [RFC3711] specifies a counter-based
 anti-replay mitigation, which could be adapted to use with SFrame,
 using the CTR field as the counter.

9.4. Metadata

 The metadata input to SFrame operations is an opaque byte string
 specified by the application. As such, the application needs to
 define what information should go in the metadata input and ensure
 that it is provided to the encryption and decryption functions at the
 appropriate points. A receiver MUST NOT use SFrame-authenticated
 metadata until after the SFrame decrypt function has authenticated
 it, unless the purpose of such usage is to prepare an SFrame
 ciphertext for SFrame decryption. Essentially, metadata may be used
 "upstream of SFrame" in a processing pipeline, but only to prepare
 for SFrame decryption.

 For example, consider an application where SFrame is used to encrypt
 audio frames that are sent over SRTP, with some application data
 included in the RTP header extension. Suppose the application also
 includes this application data in the SFrame metadata, so that the
 SFU is allowed to read, but not modify, the application data. A
 receiver can use the application data in the RTP header extension as
 part of the standard SRTP decryption process since this is required
 to recover the SFrame ciphertext carried in the SRTP payload.
 However, the receiver MUST NOT use the application data for other
 purposes before SFrame decryption has authenticated the application
 data.

10. References

10.1. Normative References

 [MLS-PROTO]
 Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
 Omara, E., and K. Cohn-Gordon, "The Messaging Layer
 Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
 July 2023, <https://www.rfc-editor.org/info/rfc9420>.

 [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/info/rfc2119>.

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

 [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/info/rfc5869>.

 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
 Writing an IANA Considerations Section in RFCs", BCP 26,
 RFC 8126, DOI 10.17487/RFC8126, June 2017,
 <https://www.rfc-editor.org/info/rfc8126>.

 [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/info/rfc8174>.

10.2. Informative References

 [MLS-ARCH] Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., and
 A. Duric, "The Messaging Layer Security (MLS)
 Architecture", Work in Progress, Internet-Draft, draft-
 ietf-mls-architecture-15, 3 August 2024,
 <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
 architecture-15>.

 [MOQ-TRANSPORT]
 Curley, L., Pugin, K., Nandakumar, S., Vasiliev, V., and
 I. Swett, Ed., "Media over QUIC Transport", Work in
 Progress, Internet-Draft, draft-ietf-moq-transport-05, 8
 July 2024, <https://datatracker.ietf.org/doc/html/draft-
 ietf-moq-transport-05>.

 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
 Norrman, "The Secure Real-time Transport Protocol (SRTP)",
 RFC 3711, DOI 10.17487/RFC3711, March 2004,
 <https://www.rfc-editor.org/info/rfc3711>.

 [RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the
 Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
 September 2012, <https://www.rfc-editor.org/info/rfc6716>.

 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
 for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
 DOI 10.17487/RFC7656, November 2015,
 <https://www.rfc-editor.org/info/rfc7656>.

 [RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
 DOI 10.17487/RFC7667, November 2015,
 <https://www.rfc-editor.org/info/rfc7667>.

 [RFC8723] Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
 "Double Encryption Procedures for the Secure Real-Time
 Transport Protocol (SRTP)", RFC 8723,
 DOI 10.17487/RFC8723, April 2020,
 <https://www.rfc-editor.org/info/rfc8723>.

 [RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
 Session Description Protocol", RFC 8866,
 DOI 10.17487/RFC8866, January 2021,
 <https://www.rfc-editor.org/info/rfc8866>.

 [RTP-PAYLOAD]
 Murillo, S. G., Fablet, Y., and A. Gouaillard, "Codec
 agnostic RTP payload format for video", Work in Progress,
 Internet-Draft, draft-gouaillard-avtcore-codec-agn-rtp-
 payload-01, 9 March 2021,
 <https://datatracker.ietf.org/doc/html/draft-gouaillard-
 avtcore-codec-agn-rtp-payload-01>.

 [TestVectors]
 "SFrame Test Vectors", commit 025d568, September 2023,
 <https://github.com/sframe-wg/sframe/blob/025d568/test-
 vectors/test-vectors.json>.

 [WEBTRANSPORT]
 Vasiliev, V., "The WebTransport Protocol Framework", Work
 in Progress, Internet-Draft, draft-ietf-webtrans-overview-
 08, 25 August 2024,
 <https://datatracker.ietf.org/api/v1/doc/document/draft-
 ietf-webtrans-overview/>.

Appendix A. Example API

 *This section is not normative.*

 This section describes a notional API that an SFrame implementation
 might expose. The core concept is an "SFrame context", within which
 KID values are meaningful. In the key management scheme described in
 Section 5.1, each sender has a different context; in the scheme
 described in Section 5.2, all senders share the same context.

 An SFrame context stores mappings from KID values to "key contexts",
 which are different depending on whether the KID is to be used for
 sending or receiving (an SFrame key should never be used for both
 operations). A key context tracks the key and salt associated to the
 KID, and the current CTR value. A key context to be used for sending
 also tracks the next CTR value to be used.

 The primary operations on an SFrame context are as follows:

 * *Create an SFrame context:* The context is initialized with a
 cipher suite and no KID mappings.

 * *Add a key for sending:* The key and salt are derived from the
 base key and used to initialize a send context, together with a
 zero CTR value.

 * *Add a key for receiving:* The key and salt are derived from the
 base key and used to initialize a send context.

 * *Encrypt a plaintext:* Encrypt a given plaintext using the key for
 a given KID, including the specified metadata.

 * *Decrypt an SFrame ciphertext:* Decrypt an SFrame ciphertext with
 the KID and CTR values specified in the SFrame header, and the
 provided metadata.

 Figure 10 shows an example of the types of structures and methods
 that could be used to create an SFrame API in Rust.

 type KeyId = u64;
 type Counter = u64;
 type CipherSuite = u16;

 struct SendKeyContext {
 key: Vec<u8>,
 salt: Vec<u8>,
 next_counter: Counter,
 }

 struct RecvKeyContext {
 key: Vec<u8>,
 salt: Vec<u8>,
 }

 struct SFrameContext {
 cipher_suite: CipherSuite,
 send_keys: HashMap<KeyId, SendKeyContext>,
 recv_keys: HashMap<KeyId, RecvKeyContext>,
 }

 trait SFrameContextMethods {
 fn create(cipher_suite: CipherSuite) -> Self;
 fn add_send_key(&self, kid: KeyId, base_key: &[u8]);
 fn add_recv_key(&self, kid: KeyId, base_key: &[u8]);
 fn encrypt(&mut self, kid: KeyId, metadata: &[u8],
 plaintext: &[u8]) -> Vec<u8>;
 fn decrypt(&self, metadata: &[u8], ciphertext: &[u8]) -> Vec<u8>;
 }

 Figure 10: An Example SFrame API

Appendix B. Overhead Analysis

 Any use of SFrame will impose overhead in terms of the amount of
 bandwidth necessary to transmit a given media stream. Exactly how
 much overhead will be added depends on several factors:

 * The number of senders involved in a conference (length of KID)

 * The duration of the conference (length of CTR)

 * The cipher suite in use (length of authentication tag)

 * Whether SFrame is used to encrypt packets, whole frames, or some
 other unit

 Overall, the overhead rate in kilobits per second can be estimated
 as:

 OverheadKbps = (1 + |CTR| + |KID| + |TAG|) * 8 * CTPerSecond / 1024

 Here the constant value 1 reflects the fixed SFrame header; |CTR|
 and |KID| reflect the lengths of those fields; |TAG| reflects the
 cipher overhead; and CTPerSecond reflects the number of SFrame
 ciphertexts sent per second (e.g., packets or frames per second).

 In the remainder of this section, we compute overhead estimates for a
 collection of common scenarios.

B.1. Assumptions

 In the below calculations, we make conservative assumptions about
 SFrame overhead so that the overhead amounts we compute here are
 likely to be an upper bound of those seen in practice.

 +==============+=======+============================+
 | Field | Bytes | Explanation |
 +==============+=======+============================+
 | Config byte | 1 | Fixed |
 +--------------+-------+----------------------------+
 | Key ID (KID) | 2 | >255 senders; or MLS epoch |
 | | | (E=4) and >16 senders |
 +--------------+-------+----------------------------+
 | Counter | 3 | More than 24 hours of |
 | (CTR) | | media in common cases |
 +--------------+-------+----------------------------+
 | Cipher | 16 | Full authentication tag |
 | overhead | | (longest defined here) |
 +--------------+-------+----------------------------+

 Table 3: Overhead Analysis Assumptions

 In total, then, we assume that each SFrame encryption will add 22
 bytes of overhead.

 We consider two scenarios: applying SFrame per frame and per packet.
 In each scenario, we compute the SFrame overhead in absolute terms
 (kbps) and as a percentage of the base bandwidth.

B.2. Audio

 In audio streams, there is typically a one-to-one relationship
 between frames and packets, so the overhead is the same whether one
 uses SFrame at a per-packet or per-frame level.

 Table 4 considers three scenarios that are based on recommended
 configurations of the Opus codec [RFC6716] (where "fps" stands for
 "frames per second"):

 +==============+==============+=====+======+==========+==========+
 | Scenario | Frame length | fps | Base | Overhead | Overhead |
 | | | | kbps | kbps | % |
 +==============+==============+=====+======+==========+==========+
 | Narrow-band | 120 ms | 8.3 | 8 | 1.4 | 17.9% |
 | speech | | | | | |
 +--------------+--------------+-----+------+----------+----------+
 | Full-band | 20 ms | 50 | 32 | 8.6 | 26.9% |
 | speech | | | | | |
 +--------------+--------------+-----+------+----------+----------+
 | Full-band | 10 ms | 100 | 128 | 17.2 | 13.4% |
 | stereo music | | | | | |
 +--------------+--------------+-----+------+----------+----------+

 Table 4: SFrame Overhead for Audio Streams

B.3. Video

 Video frames can be larger than an MTU and thus are commonly split
 across multiple frames. Tables 5 and 6 show the estimated overhead
 of encrypting a video stream, where SFrame is applied per frame and
 per packet, respectively. The choices of resolution, frames per
 second, and bandwidth roughly reflect the capabilities of modern
 video codecs across a range from very low to very high quality.

 +=============+=====+===========+===============+============+
 | Scenario | fps | Base kbps | Overhead kbps | Overhead % |
 +=============+=====+===========+===============+============+
 | 426 x 240 | 7.5 | 45 | 1.3 | 2.9% |
 +-------------+-----+-----------+---------------+------------+
 | 640 x 360 | 15 | 200 | 2.6 | 1.3% |
 +-------------+-----+-----------+---------------+------------+
 | 640 x 360 | 30 | 400 | 5.2 | 1.3% |
 +-------------+-----+-----------+---------------+------------+
 | 1280 x 720 | 30 | 1500 | 5.2 | 0.3% |
 +-------------+-----+-----------+---------------+------------+
 | 1920 x 1080 | 60 | 7200 | 10.3 | 0.1% |
 +-------------+-----+-----------+---------------+------------+

 Table 5: SFrame Overhead for a Video Stream Encrypted per
 Frame

 +==========+=====+==============+======+==========+==========+
 | Scenario | fps | Packets per | Base | Overhead | Overhead |
 | | | Second (pps) | kbps | kbps | % |
 +==========+=====+==============+======+==========+==========+
 | 426 x | 7.5 | 7.5 | 45 | 1.3 | 2.9% |
 | 240 | | | | | |
 +----------+-----+--------------+------+----------+----------+
 | 640 x | 15 | 30 | 200 | 5.2 | 2.6% |
 | 360 | | | | | |
 +----------+-----+--------------+------+----------+----------+
 | 640 x | 30 | 60 | 400 | 10.3 | 2.6% |
 | 360 | | | | | |
 +----------+-----+--------------+------+----------+----------+
 | 1280 x | 30 | 180 | 1500 | 30.9 | 2.1% |
 | 720 | | | | | |
 +----------+-----+--------------+------+----------+----------+
 | 1920 x | 60 | 780 | 7200 | 134.1 | 1.9% |
 | 1080 | | | | | |
 +----------+-----+--------------+------+----------+----------+

 Table 6: SFrame Overhead for a Video Stream Encrypted per
 Packet

 In the per-frame case, the SFrame percentage overhead approaches zero
 as the quality of the video improves since bandwidth is driven more
 by picture size than frame rate. In the per-packet case, the SFrame
 percentage overhead approaches the ratio between the SFrame overhead
 per packet and the MTU (here 22 bytes of SFrame overhead divided by
 an assumed 1200-byte MTU, or about 1.8%).

B.4. Conferences

 Real conferences usually involve several audio and video streams.
 The overhead of SFrame in such a conference is the aggregate of the
 overhead across all the individual streams. Thus, while SFrame
 incurs a large percentage overhead on an audio stream, if the
 conference also involves a video stream, then the audio overhead is
 likely negligible relative to the overall bandwidth of the
 conference.

 For example, Table 7 shows the overhead estimates for a two-person
 conference where one person is sending low-quality media and the
 other is sending high-quality media. (And we assume that SFrame is
 applied per frame.) The video streams dominate the bandwidth at the
 SFU, so the total bandwidth overhead is only around 1%.

 +=====================+===========+===============+============+
 | Stream | Base Kbps | Overhead Kbps | Overhead % |
 +=====================+===========+===============+============+
 | Participant 1 audio | 8 | 1.4 | 17.9% |
 +---------------------+-----------+---------------+------------+
 | Participant 1 video | 45 | 1.3 | 2.9% |
 +---------------------+-----------+---------------+------------+
 | Participant 2 audio | 32 | 9 | 26.9% |
 +---------------------+-----------+---------------+------------+
 | Participant 2 video | 1500 | 5 | 0.3% |
 +---------------------+-----------+---------------+------------+
 | Total at SFU | 1585 | 16.5 | 1.0% |
 +---------------------+-----------+---------------+------------+

 Table 7: SFrame Overhead for a Two-Person Conference

B.5. SFrame over RTP

 SFrame is a generic encapsulation format, but many of the
 applications in which it is likely to be integrated are based on RTP.
 This section discusses how an integration between SFrame and RTP
 could be done, and some of the challenges that would need to be
 overcome.

 As discussed in Section 4.1, there are two natural patterns for
 integrating SFrame into an application: applying SFrame per frame or
 per packet. In RTP-based applications, applying SFrame per packet
 means that the payload of each RTP packet will be an SFrame
 ciphertext, starting with an SFrame header, as shown in Figure 11.
 Applying SFrame per frame means that different RTP payloads will have
 different formats: The first payload of a frame will contain the
 SFrame headers, and subsequent payloads will contain further chunks
 of the ciphertext, as shown in Figure 12.

 In order for these media payloads to be properly interpreted by
 receivers, receivers will need to be configured to know which of the
 above schemes the sender has applied to a given sequence of RTP
 packets. SFrame does not provide a mechanism for distributing this
 configuration information. In applications that use SDP for
 negotiating RTP media streams [RFC8866], an appropriate extension to
 SDP could provide this function.

 Applying SFrame per frame also requires that packetization and
 depacketization be done in a generic manner that does not depend on
 the media content of the packets, since the content being packetized
 or depacketized will be opaque ciphertext (except for the SFrame
 header). In order for such a generic packetization scheme to work
 interoperably, one would have to be defined, e.g., as proposed in
 [RTP-PAYLOAD].

 +---+-+-+-------+-+-----------+------------------------------+<-+
 |V=2|P|X| CC |M| PT | sequence number | |
 +---+-+-+-------+-+-----------+------------------------------+ |
 | timestamp | |
 +------------------------------------------------------------+ |
 | synchronization source (SSRC) identifier | |
 +============================================================+ |
 | contributing source (CSRC) identifiers | |
 | .... | |
 +------------------------------------------------------------+ |
 | RTP extension(s) (OPTIONAL) | |
 +->+-------------------+----------------------------------------+ |
 | | SFrame header | | |
 | +-------------------+ | |
 | | | |
 | | SFrame encrypted and authenticated payload | |
 | | | |
 +->+------------------------------------------------------------+<-+
 | | SRTP authentication tag | |
 | +------------------------------------------------------------+ |
 | |
 +--- SRTP Encrypted Portion SRTP Authenticated Portion ---+

 Figure 11: SRTP Packet with SFrame-Protected Payload

 +----------------+ +---------------+
 | frame metadata | | |
 +-------+--------+ | |
 | | frame |
 | | |
 | | |
 | +-------+-------+
 | |
 | |
 V V
 +--------------------------------------+
 | SFrame Encrypt |
 +--------------------------------------+
 | |
 | |
 | V
 | +-------+-------+
 | | |
 | | |
 | | encrypted |
 | | frame |
 | | |
 | | |
 | +-------+-------+
 | |
 | generic RTP packetize
 | |
 | +----------------------+--------.....--------+
 | | | |
 V V V V
 +---------------+ +---------------+ +---------------+
 | SFrame header | | | | |
 +---------------+ | | | |
 | | | payload 2/N | ... | payload N/N |
 | payload 1/N | | | | |
 | | | | | |
 +---------------+ +---------------+ +---------------+

 Figure 12: Encryption Flow with per-Frame Encryption for RTP

Appendix C. Test Vectors

 This section provides a set of test vectors that implementations can
 use to verify that they correctly implement SFrame encryption and
 decryption. In addition to test vectors for the overall process of
 SFrame encryption/decryption, we also provide test vectors for header
 encoding/decoding, and for AEAD encryption/decryption using the AES-
 CTR construction defined in Section 4.5.1.

 All values are either numeric or byte strings. Numeric values are
 represented as hex values, prefixed with 0x. Byte strings are
 represented in hex encoding.

 Line breaks and whitespace within values are inserted to conform to
 the width requirements of the RFC format. They should be removed
 before use.

 These test vectors are also available in JSON format at
 [TestVectors]. In the JSON test vectors, numeric values are JSON
 numbers and byte string values are JSON strings containing the hex
 encoding of the byte strings.

C.1. Header Encoding/Decoding

 For each case, we provide:

 * kid: A KID value

 * ctr: A CTR value

 * header: An encoded SFrame header

 An implementation should verify that:

 * Encoding a header with the KID and CTR results in the provided
 header value

 * Decoding the provided header value results in the provided KID and
 CTR values

 kid: 0x0000000000000000
 ctr: 0x0000000000000000
 header: 00

 kid: 0x0000000000000000
 ctr: 0x0000000000000001
 header: 01

 kid: 0x0000000000000000
 ctr: 0x00000000000000ff
 header: 08ff

 kid: 0x0000000000000000
 ctr: 0x0000000000000100
 header: 090100

 kid: 0x0000000000000000
 ctr: 0x000000000000ffff
 header: 09ffff

 kid: 0x0000000000000000
 ctr: 0x0000000000010000
 header: 0a010000

 kid: 0x0000000000000000
 ctr: 0x0000000000ffffff
 header: 0affffff

 kid: 0x0000000000000000
 ctr: 0x0000000001000000
 header: 0b01000000

 kid: 0x0000000000000000
 ctr: 0x00000000ffffffff
 header: 0bffffffff

 kid: 0x0000000000000000
 ctr: 0x0000000100000000
 header: 0c0100000000

 kid: 0x0000000000000000
 ctr: 0x000000ffffffffff
 header: 0cffffffffff

 kid: 0x0000000000000000
 ctr: 0x0000010000000000
 header: 0d010000000000

 kid: 0x0000000000000000
 ctr: 0x0000ffffffffffff
 header: 0dffffffffffff

 kid: 0x0000000000000000
 ctr: 0x0001000000000000
 header: 0e01000000000000

 kid: 0x0000000000000000
 ctr: 0x00ffffffffffffff
 header: 0effffffffffffff

 kid: 0x0000000000000000
 ctr: 0x0100000000000000
 header: 0f0100000000000000

 kid: 0x0000000000000000
 ctr: 0xffffffffffffffff
 header: 0fffffffffffffffff

 kid: 0x0000000000000001
 ctr: 0x0000000000000000
 header: 10

 kid: 0x0000000000000001
 ctr: 0x0000000000000001
 header: 11

 kid: 0x0000000000000001
 ctr: 0x00000000000000ff
 header: 18ff

 kid: 0x0000000000000001
 ctr: 0x0000000000000100
 header: 190100

 kid: 0x0000000000000001
 ctr: 0x000000000000ffff
 header: 19ffff

 kid: 0x0000000000000001
 ctr: 0x0000000000010000
 header: 1a010000

 kid: 0x0000000000000001
 ctr: 0x0000000000ffffff
 header: 1affffff

 kid: 0x0000000000000001
 ctr: 0x0000000001000000
 header: 1b01000000

 kid: 0x0000000000000001
 ctr: 0x00000000ffffffff
 header: 1bffffffff

 kid: 0x0000000000000001
 ctr: 0x0000000100000000
 header: 1c0100000000

 kid: 0x0000000000000001
 ctr: 0x000000ffffffffff
 header: 1cffffffffff

 kid: 0x0000000000000001
 ctr: 0x0000010000000000
 header: 1d010000000000

 kid: 0x0000000000000001
 ctr: 0x0000ffffffffffff
 header: 1dffffffffffff

 kid: 0x0000000000000001
 ctr: 0x0001000000000000
 header: 1e01000000000000

 kid: 0x0000000000000001
 ctr: 0x00ffffffffffffff
 header: 1effffffffffffff

 kid: 0x0000000000000001
 ctr: 0x0100000000000000
 header: 1f0100000000000000

 kid: 0x0000000000000001
 ctr: 0xffffffffffffffff
 header: 1fffffffffffffffff

 kid: 0x00000000000000ff
 ctr: 0x0000000000000000
 header: 80ff

 kid: 0x00000000000000ff
 ctr: 0x0000000000000001
 header: 81ff

 kid: 0x00000000000000ff
 ctr: 0x00000000000000ff
 header: 88ffff

 kid: 0x00000000000000ff
 ctr: 0x0000000000000100
 header: 89ff0100

 kid: 0x00000000000000ff
 ctr: 0x000000000000ffff
 header: 89ffffff

 kid: 0x00000000000000ff
 ctr: 0x0000000000010000
 header: 8aff010000

 kid: 0x00000000000000ff
 ctr: 0x0000000000ffffff
 header: 8affffffff

 kid: 0x00000000000000ff
 ctr: 0x0000000001000000
 header: 8bff01000000

 kid: 0x00000000000000ff
 ctr: 0x00000000ffffffff
 header: 8bffffffffff

 kid: 0x00000000000000ff
 ctr: 0x0000000100000000
 header: 8cff0100000000

 kid: 0x00000000000000ff
 ctr: 0x000000ffffffffff
 header: 8cffffffffffff

 kid: 0x00000000000000ff
 ctr: 0x0000010000000000
 header: 8dff010000000000

 kid: 0x00000000000000ff
 ctr: 0x0000ffffffffffff
 header: 8dffffffffffffff

 kid: 0x00000000000000ff
 ctr: 0x0001000000000000
 header: 8eff01000000000000

 kid: 0x00000000000000ff
 ctr: 0x00ffffffffffffff
 header: 8effffffffffffffff

 kid: 0x00000000000000ff
 ctr: 0x0100000000000000
 header: 8fff0100000000000000

 kid: 0x00000000000000ff
 ctr: 0xffffffffffffffff
 header: 8fffffffffffffffffff

 kid: 0x0000000000000100
 ctr: 0x0000000000000000
 header: 900100

 kid: 0x0000000000000100
 ctr: 0x0000000000000001
 header: 910100

 kid: 0x0000000000000100
 ctr: 0x00000000000000ff
 header: 980100ff

 kid: 0x0000000000000100
 ctr: 0x0000000000000100
 header: 9901000100

 kid: 0x0000000000000100
 ctr: 0x000000000000ffff
 header: 990100ffff

 kid: 0x0000000000000100
 ctr: 0x0000000000010000
 header: 9a0100010000

 kid: 0x0000000000000100
 ctr: 0x0000000000ffffff
 header: 9a0100ffffff

 kid: 0x0000000000000100
 ctr: 0x0000000001000000
 header: 9b010001000000

 kid: 0x0000000000000100
 ctr: 0x00000000ffffffff
 header: 9b0100ffffffff

 kid: 0x0000000000000100
 ctr: 0x0000000100000000
 header: 9c01000100000000

 kid: 0x0000000000000100
 ctr: 0x000000ffffffffff
 header: 9c0100ffffffffff

 kid: 0x0000000000000100
 ctr: 0x0000010000000000
 header: 9d0100010000000000

 kid: 0x0000000000000100
 ctr: 0x0000ffffffffffff
 header: 9d0100ffffffffffff

 kid: 0x0000000000000100
 ctr: 0x0001000000000000
 header: 9e010001000000000000

 kid: 0x0000000000000100
 ctr: 0x00ffffffffffffff
 header: 9e0100ffffffffffffff

 kid: 0x0000000000000100
 ctr: 0x0100000000000000
 header: 9f01000100000000000000

 kid: 0x0000000000000100
 ctr: 0xffffffffffffffff
 header: 9f0100ffffffffffffffff

 kid: 0x000000000000ffff
 ctr: 0x0000000000000000
 header: 90ffff

 kid: 0x000000000000ffff
 ctr: 0x0000000000000001
 header: 91ffff

 kid: 0x000000000000ffff
 ctr: 0x00000000000000ff
 header: 98ffffff

 kid: 0x000000000000ffff
 ctr: 0x0000000000000100
 header: 99ffff0100

 kid: 0x000000000000ffff
 ctr: 0x000000000000ffff
 header: 99ffffffff

 kid: 0x000000000000ffff
 ctr: 0x0000000000010000
 header: 9affff010000

 kid: 0x000000000000ffff
 ctr: 0x0000000000ffffff
 header: 9affffffffff

 kid: 0x000000000000ffff
 ctr: 0x0000000001000000
 header: 9bffff01000000

 kid: 0x000000000000ffff
 ctr: 0x00000000ffffffff
 header: 9bffffffffffff

 kid: 0x000000000000ffff
 ctr: 0x0000000100000000
 header: 9cffff0100000000

 kid: 0x000000000000ffff
 ctr: 0x000000ffffffffff
 header: 9cffffffffffffff

 kid: 0x000000000000ffff
 ctr: 0x0000010000000000
 header: 9dffff010000000000

 kid: 0x000000000000ffff
 ctr: 0x0000ffffffffffff
 header: 9dffffffffffffffff

 kid: 0x000000000000ffff
 ctr: 0x0001000000000000
 header: 9effff01000000000000

 kid: 0x000000000000ffff
 ctr: 0x00ffffffffffffff
 header: 9effffffffffffffffff

 kid: 0x000000000000ffff
 ctr: 0x0100000000000000
 header: 9fffff0100000000000000

 kid: 0x000000000000ffff
 ctr: 0xffffffffffffffff
 header: 9fffffffffffffffffffff

 kid: 0x0000000000010000
 ctr: 0x0000000000000000
 header: a0010000

 kid: 0x0000000000010000
 ctr: 0x0000000000000001
 header: a1010000

 kid: 0x0000000000010000
 ctr: 0x00000000000000ff
 header: a8010000ff

 kid: 0x0000000000010000
 ctr: 0x0000000000000100
 header: a90100000100

 kid: 0x0000000000010000
 ctr: 0x000000000000ffff
 header: a9010000ffff

 kid: 0x0000000000010000
 ctr: 0x0000000000010000
 header: aa010000010000

 kid: 0x0000000000010000
 ctr: 0x0000000000ffffff
 header: aa010000ffffff

 kid: 0x0000000000010000
 ctr: 0x0000000001000000
 header: ab01000001000000

 kid: 0x0000000000010000
 ctr: 0x00000000ffffffff
 header: ab010000ffffffff

 kid: 0x0000000000010000
 ctr: 0x0000000100000000
 header: ac0100000100000000

 kid: 0x0000000000010000
 ctr: 0x000000ffffffffff
 header: ac010000ffffffffff

 kid: 0x0000000000010000
 ctr: 0x0000010000000000
 header: ad010000010000000000

 kid: 0x0000000000010000
 ctr: 0x0000ffffffffffff
 header: ad010000ffffffffffff

 kid: 0x0000000000010000
 ctr: 0x0001000000000000
 header: ae01000001000000000000

 kid: 0x0000000000010000
 ctr: 0x00ffffffffffffff
 header: ae010000ffffffffffffff

 kid: 0x0000000000010000
 ctr: 0x0100000000000000
 header: af0100000100000000000000

 kid: 0x0000000000010000
 ctr: 0xffffffffffffffff
 header: af010000ffffffffffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0000000000000000
 header: a0ffffff

 kid: 0x0000000000ffffff
 ctr: 0x0000000000000001
 header: a1ffffff

 kid: 0x0000000000ffffff
 ctr: 0x00000000000000ff
 header: a8ffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0000000000000100
 header: a9ffffff0100

 kid: 0x0000000000ffffff
 ctr: 0x000000000000ffff
 header: a9ffffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0000000000010000
 header: aaffffff010000

 kid: 0x0000000000ffffff
 ctr: 0x0000000000ffffff
 header: aaffffffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0000000001000000
 header: abffffff01000000

 kid: 0x0000000000ffffff
 ctr: 0x00000000ffffffff
 header: abffffffffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0000000100000000
 header: acffffff0100000000

 kid: 0x0000000000ffffff
 ctr: 0x000000ffffffffff
 header: acffffffffffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0000010000000000
 header: adffffff010000000000

 kid: 0x0000000000ffffff
 ctr: 0x0000ffffffffffff
 header: adffffffffffffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0001000000000000
 header: aeffffff01000000000000

 kid: 0x0000000000ffffff
 ctr: 0x00ffffffffffffff
 header: aeffffffffffffffffffff

 kid: 0x0000000000ffffff
 ctr: 0x0100000000000000
 header: afffffff0100000000000000

 kid: 0x0000000000ffffff
 ctr: 0xffffffffffffffff
 header: afffffffffffffffffffffff

 kid: 0x0000000001000000
 ctr: 0x0000000000000000
 header: b001000000

 kid: 0x0000000001000000
 ctr: 0x0000000000000001
 header: b101000000

 kid: 0x0000000001000000
 ctr: 0x00000000000000ff
 header: b801000000ff

 kid: 0x0000000001000000
 ctr: 0x0000000000000100
 header: b9010000000100

 kid: 0x0000000001000000
 ctr: 0x000000000000ffff
 header: b901000000ffff

 kid: 0x0000000001000000
 ctr: 0x0000000000010000
 header: ba01000000010000

 kid: 0x0000000001000000
 ctr: 0x0000000000ffffff
 header: ba01000000ffffff

 kid: 0x0000000001000000
 ctr: 0x0000000001000000
 header: bb0100000001000000

 kid: 0x0000000001000000
 ctr: 0x00000000ffffffff
 header: bb01000000ffffffff

 kid: 0x0000000001000000
 ctr: 0x0000000100000000
 header: bc010000000100000000

 kid: 0x0000000001000000
 ctr: 0x000000ffffffffff
 header: bc01000000ffffffffff

 kid: 0x0000000001000000
 ctr: 0x0000010000000000
 header: bd01000000010000000000

 kid: 0x0000000001000000
 ctr: 0x0000ffffffffffff
 header: bd01000000ffffffffffff

 kid: 0x0000000001000000
 ctr: 0x0001000000000000
 header: be0100000001000000000000

 kid: 0x0000000001000000
 ctr: 0x00ffffffffffffff
 header: be01000000ffffffffffffff

 kid: 0x0000000001000000
 ctr: 0x0100000000000000
 header: bf010000000100000000000000

 kid: 0x0000000001000000
 ctr: 0xffffffffffffffff
 header: bf01000000ffffffffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0000000000000000
 header: b0ffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0000000000000001
 header: b1ffffffff

 kid: 0x00000000ffffffff
 ctr: 0x00000000000000ff
 header: b8ffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0000000000000100
 header: b9ffffffff0100

 kid: 0x00000000ffffffff
 ctr: 0x000000000000ffff
 header: b9ffffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0000000000010000
 header: baffffffff010000

 kid: 0x00000000ffffffff
 ctr: 0x0000000000ffffff
 header: baffffffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0000000001000000
 header: bbffffffff01000000

 kid: 0x00000000ffffffff
 ctr: 0x00000000ffffffff
 header: bbffffffffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0000000100000000
 header: bcffffffff0100000000

 kid: 0x00000000ffffffff
 ctr: 0x000000ffffffffff
 header: bcffffffffffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0000010000000000
 header: bdffffffff010000000000

 kid: 0x00000000ffffffff
 ctr: 0x0000ffffffffffff
 header: bdffffffffffffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0001000000000000
 header: beffffffff01000000000000

 kid: 0x00000000ffffffff
 ctr: 0x00ffffffffffffff
 header: beffffffffffffffffffffff

 kid: 0x00000000ffffffff
 ctr: 0x0100000000000000
 header: bfffffffff0100000000000000

 kid: 0x00000000ffffffff
 ctr: 0xffffffffffffffff
 header: bfffffffffffffffffffffffff

 kid: 0x0000000100000000
 ctr: 0x0000000000000000
 header: c00100000000

 kid: 0x0000000100000000
 ctr: 0x0000000000000001
 header: c10100000000

 kid: 0x0000000100000000
 ctr: 0x00000000000000ff
 header: c80100000000ff

 kid: 0x0000000100000000
 ctr: 0x0000000000000100
 header: c901000000000100

 kid: 0x0000000100000000
 ctr: 0x000000000000ffff
 header: c90100000000ffff

 kid: 0x0000000100000000
 ctr: 0x0000000000010000
 header: ca0100000000010000

 kid: 0x0000000100000000
 ctr: 0x0000000000ffffff
 header: ca0100000000ffffff

 kid: 0x0000000100000000
 ctr: 0x0000000001000000
 header: cb010000000001000000

 kid: 0x0000000100000000
 ctr: 0x00000000ffffffff
 header: cb0100000000ffffffff

 kid: 0x0000000100000000
 ctr: 0x0000000100000000
 header: cc01000000000100000000

 kid: 0x0000000100000000
 ctr: 0x000000ffffffffff
 header: cc0100000000ffffffffff

 kid: 0x0000000100000000
 ctr: 0x0000010000000000
 header: cd0100000000010000000000

 kid: 0x0000000100000000
 ctr: 0x0000ffffffffffff
 header: cd0100000000ffffffffffff

 kid: 0x0000000100000000
 ctr: 0x0001000000000000
 header: ce010000000001000000000000

 kid: 0x0000000100000000
 ctr: 0x00ffffffffffffff
 header: ce0100000000ffffffffffffff

 kid: 0x0000000100000000
 ctr: 0x0100000000000000
 header: cf01000000000100000000000000

 kid: 0x0000000100000000
 ctr: 0xffffffffffffffff
 header: cf0100000000ffffffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0000000000000000
 header: c0ffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0000000000000001
 header: c1ffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x00000000000000ff
 header: c8ffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0000000000000100
 header: c9ffffffffff0100

 kid: 0x000000ffffffffff
 ctr: 0x000000000000ffff
 header: c9ffffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0000000000010000
 header: caffffffffff010000

 kid: 0x000000ffffffffff
 ctr: 0x0000000000ffffff
 header: caffffffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0000000001000000
 header: cbffffffffff01000000

 kid: 0x000000ffffffffff
 ctr: 0x00000000ffffffff
 header: cbffffffffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0000000100000000
 header: ccffffffffff0100000000

 kid: 0x000000ffffffffff
 ctr: 0x000000ffffffffff
 header: ccffffffffffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0000010000000000
 header: cdffffffffff010000000000

 kid: 0x000000ffffffffff
 ctr: 0x0000ffffffffffff
 header: cdffffffffffffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0001000000000000
 header: ceffffffffff01000000000000

 kid: 0x000000ffffffffff
 ctr: 0x00ffffffffffffff
 header: ceffffffffffffffffffffffff

 kid: 0x000000ffffffffff
 ctr: 0x0100000000000000
 header: cfffffffffff0100000000000000

 kid: 0x000000ffffffffff
 ctr: 0xffffffffffffffff
 header: cfffffffffffffffffffffffffff

 kid: 0x0000010000000000
 ctr: 0x0000000000000000
 header: d0010000000000

 kid: 0x0000010000000000
 ctr: 0x0000000000000001
 header: d1010000000000

 kid: 0x0000010000000000
 ctr: 0x00000000000000ff
 header: d8010000000000ff

 kid: 0x0000010000000000
 ctr: 0x0000000000000100
 header: d90100000000000100

 kid: 0x0000010000000000
 ctr: 0x000000000000ffff
 header: d9010000000000ffff

 kid: 0x0000010000000000
 ctr: 0x0000000000010000
 header: da010000000000010000

 kid: 0x0000010000000000
 ctr: 0x0000000000ffffff
 header: da010000000000ffffff

 kid: 0x0000010000000000
 ctr: 0x0000000001000000
 header: db01000000000001000000

 kid: 0x0000010000000000
 ctr: 0x00000000ffffffff
 header: db010000000000ffffffff

 kid: 0x0000010000000000
 ctr: 0x0000000100000000
 header: dc0100000000000100000000

 kid: 0x0000010000000000
 ctr: 0x000000ffffffffff
 header: dc010000000000ffffffffff

 kid: 0x0000010000000000
 ctr: 0x0000010000000000
 header: dd010000000000010000000000

 kid: 0x0000010000000000
 ctr: 0x0000ffffffffffff
 header: dd010000000000ffffffffffff

 kid: 0x0000010000000000
 ctr: 0x0001000000000000
 header: de01000000000001000000000000

 kid: 0x0000010000000000
 ctr: 0x00ffffffffffffff
 header: de010000000000ffffffffffffff

 kid: 0x0000010000000000
 ctr: 0x0100000000000000
 header: df0100000000000100000000000000

 kid: 0x0000010000000000
 ctr: 0xffffffffffffffff
 header: df010000000000ffffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0000000000000000
 header: d0ffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0000000000000001
 header: d1ffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x00000000000000ff
 header: d8ffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0000000000000100
 header: d9ffffffffffff0100

 kid: 0x0000ffffffffffff
 ctr: 0x000000000000ffff
 header: d9ffffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0000000000010000
 header: daffffffffffff010000

 kid: 0x0000ffffffffffff
 ctr: 0x0000000000ffffff
 header: daffffffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0000000001000000
 header: dbffffffffffff01000000

 kid: 0x0000ffffffffffff
 ctr: 0x00000000ffffffff
 header: dbffffffffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0000000100000000
 header: dcffffffffffff0100000000

 kid: 0x0000ffffffffffff
 ctr: 0x000000ffffffffff
 header: dcffffffffffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0000010000000000
 header: ddffffffffffff010000000000

 kid: 0x0000ffffffffffff
 ctr: 0x0000ffffffffffff
 header: ddffffffffffffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0001000000000000
 header: deffffffffffff01000000000000

 kid: 0x0000ffffffffffff
 ctr: 0x00ffffffffffffff
 header: deffffffffffffffffffffffffff

 kid: 0x0000ffffffffffff
 ctr: 0x0100000000000000
 header: dfffffffffffff0100000000000000

 kid: 0x0000ffffffffffff
 ctr: 0xffffffffffffffff
 header: dfffffffffffffffffffffffffffff

 kid: 0x0001000000000000
 ctr: 0x0000000000000000
 header: e001000000000000

 kid: 0x0001000000000000
 ctr: 0x0000000000000001
 header: e101000000000000

 kid: 0x0001000000000000
 ctr: 0x00000000000000ff
 header: e801000000000000ff

 kid: 0x0001000000000000
 ctr: 0x0000000000000100
 header: e9010000000000000100

 kid: 0x0001000000000000
 ctr: 0x000000000000ffff
 header: e901000000000000ffff

 kid: 0x0001000000000000
 ctr: 0x0000000000010000
 header: ea01000000000000010000

 kid: 0x0001000000000000
 ctr: 0x0000000000ffffff
 header: ea01000000000000ffffff

 kid: 0x0001000000000000
 ctr: 0x0000000001000000
 header: eb0100000000000001000000

 kid: 0x0001000000000000
 ctr: 0x00000000ffffffff
 header: eb01000000000000ffffffff

 kid: 0x0001000000000000
 ctr: 0x0000000100000000
 header: ec010000000000000100000000

 kid: 0x0001000000000000
 ctr: 0x000000ffffffffff
 header: ec01000000000000ffffffffff

 kid: 0x0001000000000000
 ctr: 0x0000010000000000
 header: ed01000000000000010000000000

 kid: 0x0001000000000000
 ctr: 0x0000ffffffffffff
 header: ed01000000000000ffffffffffff

 kid: 0x0001000000000000
 ctr: 0x0001000000000000
 header: ee0100000000000001000000000000

 kid: 0x0001000000000000
 ctr: 0x00ffffffffffffff
 header: ee01000000000000ffffffffffffff

 kid: 0x0001000000000000
 ctr: 0x0100000000000000
 header: ef010000000000000100000000000000

 kid: 0x0001000000000000
 ctr: 0xffffffffffffffff
 header: ef01000000000000ffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0000000000000000
 header: e0ffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0000000000000001
 header: e1ffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x00000000000000ff
 header: e8ffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0000000000000100
 header: e9ffffffffffffff0100

 kid: 0x00ffffffffffffff
 ctr: 0x000000000000ffff
 header: e9ffffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0000000000010000
 header: eaffffffffffffff010000

 kid: 0x00ffffffffffffff
 ctr: 0x0000000000ffffff
 header: eaffffffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0000000001000000
 header: ebffffffffffffff01000000

 kid: 0x00ffffffffffffff
 ctr: 0x00000000ffffffff
 header: ebffffffffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0000000100000000
 header: ecffffffffffffff0100000000

 kid: 0x00ffffffffffffff
 ctr: 0x000000ffffffffff
 header: ecffffffffffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0000010000000000
 header: edffffffffffffff010000000000

 kid: 0x00ffffffffffffff
 ctr: 0x0000ffffffffffff
 header: edffffffffffffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0001000000000000
 header: eeffffffffffffff01000000000000

 kid: 0x00ffffffffffffff
 ctr: 0x00ffffffffffffff
 header: eeffffffffffffffffffffffffffff

 kid: 0x00ffffffffffffff
 ctr: 0x0100000000000000
 header: efffffffffffffff0100000000000000

 kid: 0x00ffffffffffffff
 ctr: 0xffffffffffffffff
 header: efffffffffffffffffffffffffffffff

 kid: 0x0100000000000000
 ctr: 0x0000000000000000
 header: f00100000000000000

 kid: 0x0100000000000000
 ctr: 0x0000000000000001
 header: f10100000000000000

 kid: 0x0100000000000000
 ctr: 0x00000000000000ff
 header: f80100000000000000ff

 kid: 0x0100000000000000
 ctr: 0x0000000000000100
 header: f901000000000000000100

 kid: 0x0100000000000000
 ctr: 0x000000000000ffff
 header: f90100000000000000ffff

 kid: 0x0100000000000000
 ctr: 0x0000000000010000
 header: fa0100000000000000010000

 kid: 0x0100000000000000
 ctr: 0x0000000000ffffff
 header: fa0100000000000000ffffff

 kid: 0x0100000000000000
 ctr: 0x0000000001000000
 header: fb010000000000000001000000

 kid: 0x0100000000000000
 ctr: 0x00000000ffffffff
 header: fb0100000000000000ffffffff

 kid: 0x0100000000000000
 ctr: 0x0000000100000000
 header: fc01000000000000000100000000

 kid: 0x0100000000000000
 ctr: 0x000000ffffffffff
 header: fc0100000000000000ffffffffff

 kid: 0x0100000000000000
 ctr: 0x0000010000000000
 header: fd0100000000000000010000000000

 kid: 0x0100000000000000
 ctr: 0x0000ffffffffffff
 header: fd0100000000000000ffffffffffff

 kid: 0x0100000000000000
 ctr: 0x0001000000000000
 header: fe010000000000000001000000000000

 kid: 0x0100000000000000
 ctr: 0x00ffffffffffffff
 header: fe0100000000000000ffffffffffffff

 kid: 0x0100000000000000
 ctr: 0x0100000000000000
 header: ff010000000000000001000000000000
 00

 kid: 0x0100000000000000
 ctr: 0xffffffffffffffff
 header: ff0100000000000000ffffffffffffff
 ff

 kid: 0xffffffffffffffff
 ctr: 0x0000000000000000
 header: f0ffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0000000000000001
 header: f1ffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x00000000000000ff
 header: f8ffffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0000000000000100
 header: f9ffffffffffffffff0100

 kid: 0xffffffffffffffff
 ctr: 0x000000000000ffff
 header: f9ffffffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0000000000010000
 header: faffffffffffffffff010000

 kid: 0xffffffffffffffff
 ctr: 0x0000000000ffffff
 header: faffffffffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0000000001000000
 header: fbffffffffffffffff01000000

 kid: 0xffffffffffffffff
 ctr: 0x00000000ffffffff
 header: fbffffffffffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0000000100000000
 header: fcffffffffffffffff0100000000

 kid: 0xffffffffffffffff
 ctr: 0x000000ffffffffff
 header: fcffffffffffffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0000010000000000
 header: fdffffffffffffffff010000000000

 kid: 0xffffffffffffffff
 ctr: 0x0000ffffffffffff
 header: fdffffffffffffffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0001000000000000
 header: feffffffffffffffff01000000000000

 kid: 0xffffffffffffffff
 ctr: 0x00ffffffffffffff
 header: feffffffffffffffffffffffffffffff

 kid: 0xffffffffffffffff
 ctr: 0x0100000000000000
 header: ffffffffffffffffff01000000000000
 00

 kid: 0xffffffffffffffff
 ctr: 0xffffffffffffffff
 header: ffffffffffffffffffffffffffffffff
 ff

C.2. AEAD Encryption/Decryption Using AES-CTR and HMAC

 For each case, we provide:

 * cipher_suite: The index of the cipher suite in use (see
 Section 8.1)

 * key: The key input to encryption/decryption

 * enc_key: The encryption subkey produced by the derive_subkeys()
 algorithm

 * auth_key: The encryption subkey produced by the derive_subkeys()
 algorithm

 * nonce: The nonce input to encryption/decryption

 * aad: The aad input to encryption/decryption

 * pt: The plaintext

 * ct: The ciphertext

 An implementation should verify that the following are true, where
 AEAD.Encrypt and AEAD.Decrypt are as defined in Section 4.5.1:

 * AEAD.Encrypt(key, nonce, aad, pt) == ct

 * AEAD.Decrypt(key, nonce, aad, ct) == pt

 The other values in the test vector are intermediate values provided
 to facilitate debugging of test failures.

 cipher_suite: 0x0001
 key: 000102030405060708090a0b0c0d0e0f
 101112131415161718191a1b1c1d1e1f
 202122232425262728292a2b2c2d2e2f
 enc_key: 000102030405060708090a0b0c0d0e0f
 auth_key: 101112131415161718191a1b1c1d1e1f
 202122232425262728292a2b2c2d2e2f
 nonce: 101112131415161718191a1b
 aad: 4945544620534672616d65205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 6339af04ada1d064688a442b8dc69d5b
 6bfa40f4bef0583e8081069cc60705

 cipher_suite: 0x0002
 key: 000102030405060708090a0b0c0d0e0f
 101112131415161718191a1b1c1d1e1f
 202122232425262728292a2b2c2d2e2f
 enc_key: 000102030405060708090a0b0c0d0e0f
 auth_key: 101112131415161718191a1b1c1d1e1f
 202122232425262728292a2b2c2d2e2f
 nonce: 101112131415161718191a1b
 aad: 4945544620534672616d65205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 6339af04ada1d064688a442b8dc69d5b
 6bfa40f4be6e93b7da076927bb

 cipher_suite: 0x0003
 key: 000102030405060708090a0b0c0d0e0f
 101112131415161718191a1b1c1d1e1f
 202122232425262728292a2b2c2d2e2f
 enc_key: 000102030405060708090a0b0c0d0e0f
 auth_key: 101112131415161718191a1b1c1d1e1f
 202122232425262728292a2b2c2d2e2f
 nonce: 101112131415161718191a1b
 aad: 4945544620534672616d65205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 6339af04ada1d064688a442b8dc69d5b
 6bfa40f4be09480509

C.3. SFrame Encryption/Decryption

 For each case, we provide:

 * cipher_suite: The index of the cipher suite in use (see
 Section 8.1)

 * kid: A KID value

 * ctr: A CTR value

 * base_key: The base_key input to the derive_key_salt algorithm

 * sframe_key_label: The label used to derive sframe_key in the
 derive_key_salt algorithm

 * sframe_salt_label: The label used to derive sframe_salt in the
 derive_key_salt algorithm

 * sframe_secret: The sframe_secret variable in the derive_key_salt
 algorithm

 * sframe_key: The sframe_key value produced by the derive_key_salt
 algorithm

 * sframe_salt: The sframe_salt value produced by the derive_key_salt
 algorithm

 * metadata: The metadata input to the SFrame encrypt algorithm

 * pt: The plaintext

 * ct: The SFrame ciphertext

 An implementation should verify that the following are true, where
 encrypt and decrypt are as defined in Section 4.4, using an SFrame
 context initialized with base_key assigned to kid:

 * encrypt(ctr, kid, metadata, plaintext) == ct

 * decrypt(metadata, ct) == pt

 The other values in the test vector are intermediate values provided
 to facilitate debugging of test failures.

 cipher_suite: 0x0001
 kid: 0x0000000000000123
 ctr: 0x0000000000004567
 base_key: 000102030405060708090a0b0c0d0e0f
 sframe_key_label: 534672616d6520312e30205365637265
 74206b65792000000000000001230001
 sframe_salt_label: 534672616d6520312e30205365637265
 742073616c7420000000000000012300
 01
 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
 ceff8cceee34f574d23909eb314c40c0
 sframe_key: 3f7d9a7c83ae8e1c8a11ae695ab59314
 b367e359fadac7b9c46b2bc6f81f46e1
 6b96f0811868d59402b7e870102720b3
 sframe_salt: 50b29329a04dc0f184ac3168
 metadata: 4945544620534672616d65205747
 nonce: 50b29329a04dc0f184ac740f
 aad: 99012345674945544620534672616d65
 205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 9901234567449408b6f490086165b9d6
 f62b24ae1a59a56486b4ae8ed036b889
 12e24f11

 cipher_suite: 0x0002
 kid: 0x0000000000000123
 ctr: 0x0000000000004567
 base_key: 000102030405060708090a0b0c0d0e0f
 sframe_key_label: 534672616d6520312e30205365637265
 74206b65792000000000000001230002
 sframe_salt_label: 534672616d6520312e30205365637265
 742073616c7420000000000000012300
 02
 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
 ceff8cceee34f574d23909eb314c40c0
 sframe_key: e2ec5c797540310483b16bf6e7a570d2
 a27d192fe869c7ccd8584a8d9dab9154
 9fbe553f5113461ec6aa83bf3865553e
 sframe_salt: e68ac8dd3d02fbcd368c5577
 metadata: 4945544620534672616d65205747
 nonce: e68ac8dd3d02fbcd368c1010
 aad: 99012345674945544620534672616d65
 205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 99012345673f31438db4d09434e43afa
 0f8a2f00867a2be085046a9f5cb4f101
 d607

 cipher_suite: 0x0003
 kid: 0x0000000000000123
 ctr: 0x0000000000004567
 base_key: 000102030405060708090a0b0c0d0e0f
 sframe_key_label: 534672616d6520312e30205365637265
 74206b65792000000000000001230003
 sframe_salt_label: 534672616d6520312e30205365637265
 742073616c7420000000000000012300
 03
 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
 ceff8cceee34f574d23909eb314c40c0
 sframe_key: 2c5703089cbb8c583475e4fc461d97d1
 8809df79b6d550f78eb6d50ffa80d892
 11d57909934f46f5405e38cd583c69fe
 sframe_salt: 38c16e4f5159700c00c7f350
 metadata: 4945544620534672616d65205747
 nonce: 38c16e4f5159700c00c7b637
 aad: 99012345674945544620534672616d65
 205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 990123456717fc8af28a5a695afcfc6c
 8df6358a17e26b2fcb3bae32e443

 cipher_suite: 0x0004
 kid: 0x0000000000000123
 ctr: 0x0000000000004567
 base_key: 000102030405060708090a0b0c0d0e0f
 sframe_key_label: 534672616d6520312e30205365637265
 74206b65792000000000000001230004
 sframe_salt_label: 534672616d6520312e30205365637265
 742073616c7420000000000000012300
 04
 sframe_secret: d926952ca8b7ec4a95941d1ada3a5203
 ceff8cceee34f574d23909eb314c40c0
 sframe_key: d34f547f4ca4f9a7447006fe7fcbf768
 sframe_salt: 75234edefe07819026751816
 metadata: 4945544620534672616d65205747
 nonce: 75234edefe07819026755d71
 aad: 99012345674945544620534672616d65
 205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 9901234567b7412c2513a1b66dbb4884
 1bbaf17f598751176ad847681a69c6d0
 b091c07018ce4adb34eb

 cipher_suite: 0x0005
 kid: 0x0000000000000123
 ctr: 0x0000000000004567
 base_key: 000102030405060708090a0b0c0d0e0f
 sframe_key_label: 534672616d6520312e30205365637265
 74206b65792000000000000001230005
 sframe_salt_label: 534672616d6520312e30205365637265
 742073616c7420000000000000012300
 05
 sframe_secret: 0fc3ea6de6aac97a35f194cf9bed94d4
 b5230f1cb45a785c9fe5dce9c188938a
 b6ba005bc4c0a19181599e9d1bcf7b74
 aca48b60bf5e254e546d809313e083a3
 sframe_key: d3e27b0d4a5ae9e55df01a70e6d4d28d
 969b246e2936f4b7a5d9b494da6b9633
 sframe_salt: 84991c167b8cd23c93708ec7
 metadata: 4945544620534672616d65205747
 nonce: 84991c167b8cd23c9370cba0
 aad: 99012345674945544620534672616d65
 205747
 pt: 64726166742d696574662d736672616d
 652d656e63
 ct: 990123456794f509d36e9beacb0e261d
 99c7d1e972f1fed787d4049f17ca2135
 3c1cc24d56ceabced279

Acknowledgements

 The authors wish to specially thank Dr. Alex Gouaillard as one of the
 early contributors to the document. His passion and energy were key
 to the design and development of SFrame.

Contributors

 Frédéric Jacobs
 Apple
 Email: frederic.jacobs@apple.com

 Marta Mularczyk
 Amazon
 Email: mulmarta@amazon.com

 Suhas Nandakumar
 Cisco
 Email: snandaku@cisco.com

 Tomas Rigaux
 Cisco
 Email: trigaux@cisco.com

 Raphael Robert
 Phoenix R&D
 Email: ietf@raphaelrobert.com

Authors' Addresses

 Emad Omara
 Apple
 Email: eomara@apple.com

 Justin Uberti
 Fixie.ai
 Email: justin@fixie.ai

 Sergio Garcia Murillo
 CoSMo Software
 Email: sergio.garcia.murillo@cosmosoftware.io

 Richard Barnes (editor)
 Cisco
 Email: rlb@ipv.sx

 Youenn Fablet
 Apple
 Email: youenn@apple.com