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Extending the KeyedHashAlgorithm Class 

The cryptographic classes provided by the .NET Framework are extremely extensible. This section provides an example of how to extend these cryptography classes by creating a keyed hash algorithm class that implements the MD5 hash algorithm.

A keyed hash algorithm is a key-dependent, one-way hash function used as a message authentication code. The .NET Framework provides two such keyed hash algorithm classes (HMACSHA1 and MACTripleDES), both of which derive from the abstract KeyedHashAlgorithm Class. If you want to implement a keyed hash algorithm class that implements a hash algorithm different from the ones provided in the .NET Framework, you can create a new class derived from KeyedHashAlgorithm that uses your preferred hash algorithm.

The following example creates a new keyed hash algorithm class using the MD5 hash algorithm. Because this class inherits from the KeyedHashAlgorithm class, it can easily be used with other managed cryptography classes like the CryptoStream class.

Imports System
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 
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 == false) 
        {
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0);
            bHashing = true;                
        }
        hash1.TransformBlock(rgb, ib, cb, rgb, ib);
    }

    protected override byte[] HashFinal() 
    {
        if (bHashing == false) 
        {
            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;
    }        
}

This example extends the KeyedHashAlgorithm class to use the MD5 hash algorithm in a class called HMACMD5. When executed, this code creates two instances of the HMACMD5 class, passing different keys to each constructor. It then hashes the values of two strings and prints the following to the console.

Length: 16
92    94    72    7A    36    38    BB    1C
13    F4    8E    F8    15    8B    FC    9D
Length: 16
1    10    3A    9    C2    10    6E    E1
1    13    9B    7D    DA    EA    A3    EC

See Also

Other Resources

Cryptographic Tasks
Cryptographic Services