Here is the modified version with custom HMAC generation. Main thing to consider is that Input Block Size referred in the K, is the hash algorithm block size; not returned hashed byte length. For SHA256, block size is 64 bytes. I believe you were using 32byte block size. You can find different block size references here: http://en.wikipedia.org/wiki/Secure_Hash_Algorithm.
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;
}