自己实现HMAC Hash 256 算法

public static string MyHMACHash(string key, string message)
{
    Encoding encoding = Encoding.UTF8;

    //Reference http://en.wikipedia.org/wiki/Secure_Hash_Algorithm
    //SHA256 block size is 512 bits => 64 bytes.
    const int HashBlockSize = 64;

    var keyBytes = encoding.GetBytes(key);
    var opadKeySet = new byte[HashBlockSize];
    var ipadKeySet = new byte[HashBlockSize];


    if (keyBytes.Length > HashBlockSize)
    {
        keyBytes = GetHash(keyBytes);
    }

    // This condition is independent of previous
    // condition. If previous was true
    // we still need to execute this to make keyBytes same length
    // as blocksize with 0 padded if its less than block size
    if (keyBytes.Length < HashBlockSize)
    {
        var newKeyBytes = new byte[HashBlockSize];
        keyBytes.CopyTo(newKeyBytes, 0);
        keyBytes = newKeyBytes;
    }


    for (int i = 0; i < keyBytes.Length; i++)
    {
        opadKeySet[i] = (byte)(keyBytes[i] ^ 0x5C);
        ipadKeySet[i] = (byte)(keyBytes[i] ^ 0x36);
    }

    var hash = GetHash(ByteConcat(opadKeySet, 
        GetHash(ByteConcat(ipadKeySet, encoding.GetBytes(message)))));

    // Convert to standard hex string 
    return hash.Select<byte, string>(a => a.ToString("x2"))
                .Aggregate<string>((a, b) => string.Format("{0}{1}", a, b));            
}

public static byte[] GetHash(byte[] bytes)
{
    using (var hash = new SHA256Managed())
    {
        return hash.ComputeHash(bytes);
    }
}

public static byte[] ByteConcat(byte[] left, byte[] right)
{
    if (null == left)
    {
        return right;
    }

    if (null == right)
    {
        return left;
    }

    byte[] newBytes = new byte[left.Length + right.Length];
    left.CopyTo(newBytes, 0);
    right.CopyTo(newBytes, left.Length);

    return newBytes;
}