KeyedHashAlgorithm Class

Definition

public ref class KeyedHashAlgorithm abstract : System::Security::Cryptography::HashAlgorithm

public abstract class KeyedHashAlgorithm : System.Security.Cryptography.HashAlgorithm

[System.Runtime.InteropServices.ComVisible(true)]
public abstract class KeyedHashAlgorithm : System.Security.Cryptography.HashAlgorithm

type KeyedHashAlgorithm = class
 inherit HashAlgorithm

[<System.Runtime.InteropServices.ComVisible(true)>]
type KeyedHashAlgorithm = class
 inherit HashAlgorithm

Public MustInherit Class KeyedHashAlgorithm
Inherits HashAlgorithm

Inheritance

Object

HashAlgorithm

KeyedHashAlgorithm

Derived

System.Security.Cryptography.HMAC

System.Security.Cryptography.HMACSHA1

System.Security.Cryptography.MACTripleDES

Attributes

ComVisibleAttribute

Examples

The following code example demonstrates how to derive from the KeyedHashAlgorithm class.

using System;
using System.Security.Cryptography;

public class TestHMACMD5
{
 static private void PrintByteArray(Byte[] arr)
 {
 int i;
 Console.WriteLine("Length: " + arr.Length);
 for (i = 0; i < arr.Length; i++)
 {
 Console.Write("{0:X}", arr[i]);
 Console.Write(" ");
 if ((i + 9) % 8 == 0) Console.WriteLine();
 }
 if (i % 8 != 0) Console.WriteLine();
 }
 public static void Main()
 {
 // Create a key.
 byte[] key1 = { 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b };
 // Pass the key to the constructor of the HMACMD5 class.
 HMACMD5 hmac1 = new HMACMD5(key1);

 // Create another key.
 byte[] key2 = System.Text.Encoding.ASCII.GetBytes("KeyString");
 // Pass the key to the constructor of the HMACMD5 class.
 HMACMD5 hmac2 = new HMACMD5(key2);

 // Encode a string into a byte array, create a hash of the array,
 // and print the hash to the screen.
 byte[] data1 = System.Text.Encoding.ASCII.GetBytes("Hi There");
 PrintByteArray(hmac1.ComputeHash(data1));

 // Encode a string into a byte array, create a hash of the array,
 // and print the hash to the screen.
 byte[] data2 = System.Text.Encoding.ASCII.GetBytes("This data will be hashed.");
 PrintByteArray(hmac2.ComputeHash(data2));
 }
}
public class HMACMD5 : KeyedHashAlgorithm
{
 private MD5 hash1;
 private MD5 hash2;
 private bool bHashing = false;

 private byte[] rgbInner = new byte[64];
 private byte[] rgbOuter = new byte[64];

 public HMACMD5(byte[] rgbKey)
 {
 HashSizeValue = 128;
 // Create the hash algorithms.
 hash1 = MD5.Create();
 hash2 = MD5.Create();
 // Get the key.
 if (rgbKey.Length > 64)
 {
 KeyValue = hash1.ComputeHash(rgbKey);
 // No need to call Initialize; ComputeHash does it automatically.
 }
 else
 {
 KeyValue = (byte[])rgbKey.Clone();
 }
 // Compute rgbInner and rgbOuter.
 int i = 0;
 for (i = 0; i < 64; i++)
 {
 rgbInner[i] = 0x36;
 rgbOuter[i] = 0x5C;
 }
 for (i = 0; i < KeyValue.Length; i++)
 {
 rgbInner[i] ^= KeyValue[i];
 rgbOuter[i] ^= KeyValue[i];
 }
 }

 public override byte[] Key
 {
 get { return (byte[])KeyValue.Clone(); }
 set
 {
 if (bHashing)
 {
 throw new Exception("Cannot change key during hash operation");
 }
 if (value.Length > 64)
 {
 KeyValue = hash1.ComputeHash(value);
 // No need to call Initialize; ComputeHash does it automatically.
 }
 else
 {
 KeyValue = (byte[])value.Clone();
 }
 // Compute rgbInner and rgbOuter.
 int i = 0;
 for (i = 0; i < 64; i++)
 {
 rgbInner[i] = 0x36;
 rgbOuter[i] = 0x5C;
 }
 for (i = 0; i < KeyValue.Length; i++)
 {
 rgbInner[i] ^= KeyValue[i];
 rgbOuter[i] ^= KeyValue[i];
 }
 }
 }
 public override void Initialize()
 {
 hash1.Initialize();
 hash2.Initialize();
 bHashing = false;
 }
 protected override void HashCore(byte[] rgb, int ib, int cb)
 {
 if (!bHashing)
 {
 hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0);
 bHashing = true;
 }
 hash1.TransformBlock(rgb, ib, cb, rgb, ib);
 }

 protected override byte[] HashFinal()
 {
 if (!bHashing)
 {
 hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0);
 bHashing = true;
 }
 // Finalize the original hash.
 hash1.TransformFinalBlock(new byte[0], 0, 0);
 // Write the outer array.
 hash2.TransformBlock(rgbOuter, 0, 64, rgbOuter, 0);
 // Write the inner hash and finalize the hash.
 hash2.TransformFinalBlock(hash1.Hash, 0, hash1.Hash.Length);
 bHashing = false;
 return hash2.Hash;
 }
}

Imports System.Security.Cryptography
 _

Public Class TestHMACMD5

 Private Shared Sub PrintByteArray(ByVal arr() As [Byte])
 Dim i As Integer
 Console.WriteLine(("Length: " + arr.Length.ToString()))
 For i = 0 To arr.Length - 1
 Console.Write("{0:X}", arr(i))
 Console.Write(" ")
 If (i + 9) Mod 8 = 0 Then
 Console.WriteLine()
 End If
 Next i
 If i Mod 8 <> 0 Then
 Console.WriteLine()
 End If
 End Sub

 Public Shared Sub Main()
 ' Create a key.
 Dim key1 As Byte() = {&HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB}
 ' Pass the key to the constructor of the HMACMD5 class. 
 Dim hmac1 As New HMACMD5(key1)

 ' Create another key.
 Dim key2 As Byte() = System.Text.Encoding.ASCII.GetBytes("KeyString")
 ' Pass the key to the constructor of the HMACMD5 class. 
 Dim hmac2 As New HMACMD5(key2)

 ' Encode a string into a byte array, create a hash of the array,
 ' and print the hash to the screen.
 Dim data1 As Byte() = System.Text.Encoding.ASCII.GetBytes("Hi There")
 PrintByteArray(hmac1.ComputeHash(data1))

 ' Encode a string into a byte array, create a hash of the array,
 ' and print the hash to the screen.
 Dim data2 As Byte() = System.Text.Encoding.ASCII.GetBytes("This data will be hashed.")
 PrintByteArray(hmac2.ComputeHash(data2))
 End Sub
End Class
 _

Public Class HMACMD5
 Inherits KeyedHashAlgorithm
 Private hash1 As MD5
 Private hash2 As MD5
 Private bHashing As Boolean = False

 Private rgbInner(64) As Byte
 Private rgbOuter(64) As Byte


 Public Sub New(ByVal rgbKey() As Byte)
 HashSizeValue = 128
 ' Create the hash algorithms.
 hash1 = MD5.Create()
 hash2 = MD5.Create()
 ' Get the key.
 If rgbKey.Length > 64 Then
 KeyValue = hash1.ComputeHash(rgbKey)
 ' No need to call Initialize; ComputeHash does it automatically.
 Else
 KeyValue = CType(rgbKey.Clone(), Byte())
 End If
 ' Compute rgbInner and rgbOuter.
 Dim i As Integer = 0
 For i = 0 To 63
 rgbInner(i) = &H36
 rgbOuter(i) = &H5C
 Next i
 i = 0
 For i = 0 To KeyValue.Length - 1
 rgbInner(i) = rgbInner(i) Xor KeyValue(i)
 rgbOuter(i) = rgbOuter(i) Xor KeyValue(i)
 Next i
 End Sub


 Public Overrides Property Key() As Byte()
 Get
 Return CType(KeyValue.Clone(), Byte())
 End Get
 Set(ByVal Value As Byte())
 If bHashing Then
 Throw New Exception("Cannot change key during hash operation")
 End If
 If value.Length > 64 Then
 KeyValue = hash1.ComputeHash(value)
 ' No need to call Initialize; ComputeHash does it automatically.
 Else
 KeyValue = CType(value.Clone(), Byte())
 End If
 ' Compute rgbInner and rgbOuter.
 Dim i As Integer = 0
 For i = 0 To 63
 rgbInner(i) = &H36
 rgbOuter(i) = &H5C
 Next i
 For i = 0 To KeyValue.Length - 1
 rgbInner(i) ^= KeyValue(i)
 rgbOuter(i) ^= KeyValue(i)
 Next i
 End Set
 End Property


 Public Overrides Sub Initialize()
 hash1.Initialize()
 hash2.Initialize()
 bHashing = False
 End Sub


 Protected Overrides Sub HashCore(ByVal rgb() As Byte, ByVal ib As Integer, ByVal cb As Integer)
 If bHashing = False Then
 hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0)
 bHashing = True
 End If
 hash1.TransformBlock(rgb, ib, cb, rgb, ib)
 End Sub


 Protected Overrides Function HashFinal() As Byte()
 If bHashing = False Then
 hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0)
 bHashing = True
 End If
 ' Finalize the original hash.
 hash1.TransformFinalBlock(New Byte(0) {}, 0, 0)
 ' Write the outer array.
 hash2.TransformBlock(rgbOuter, 0, 64, rgbOuter, 0)
 ' Write the inner hash and finalize the hash.
 hash2.TransformFinalBlock(hash1.Hash, 0, hash1.Hash.Length)
 bHashing = False
 Return hash2.Hash
 End Function
End Class

Remarks

Hash functions map binary strings of an arbitrary length to small binary strings of a fixed length. A cryptographic hash function has the property that it is computationally infeasible to find two distinct inputs that hash to the same value. Small changes to the data result in large, unpredictable changes in the hash.

A keyed hash algorithm is a key-dependent, one-way hash function used as a message authentication code. Only someone who knows the key can verify the hash. Keyed hash algorithms provide authenticity without secrecy.

Hash functions are commonly used with digital signatures and for data integrity. The HMACSHA1 class is an example of a keyed hash algorithm.

Due to collision problems with SHA-1, Microsoft recommends a security model based on SHA-256 or better.

See also

• Cryptographic Services