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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.

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
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