using System;

using System.Globalization;

using System.Security.Cryptography;

using System.Text;

public class Program

{

    public static void Main()

    {   

        Console.WriteLine("HashHMACBase64: " + new DigitalHMACSignature().HashHMACHex("825bb9ad1548d8f418253d42aed07415520d46f6f55c98026376387a81e36ec2", "ABCD123456ABCD123456ABCD123456ABCD123456ABCD123456"));

        Console.WriteLine("ComputeHash: " + DigitalHMACSignature.ComputeHash("825bb9ad1548d8f418253d42aed07415520d46f6f55c98026376387a81e36ec2", "ABCD123456ABCD123456ABCD123456ABCD123456ABCD123456"));

    }

}

​

    public class DigitalHMACSignature

    {

        public static string ComputeHash(string key, string message)

        {

            //Changes the hexadecimal string to a byte array by parsing the string byte-by-byte.

            byte[] keyBytes = new byte[key.Length / 2];

            for (int i = 0; i < keyBytes.Length; i++)

            {

                keyBytes[i] = byte.Parse(key.Substring(i * 2, 2), NumberStyles.HexNumber);

            }

            //Encodes a message into a sequence of bytes.

            byte[] encodedMessage = new ASCIIEncoding().GetBytes(message);

            //Computes a Hash-based Message Authentication Code (HMAC) by using the SHA256 hash function.

            byte[] hash = HashHMAC(keyBytes, encodedMessage);

            //Converts the numeric value of each element of a specified array of bytes to its equivalent hexadecimal string representation

            var computedString = BitConverter.ToString(hash).Replace("-", "").ToLower();

​

            return computedString;

        }

        public string HashHMACHex(string keyHex, string message)

        {

            byte[] hash = HashHMAC(HexDecode(keyHex), StringEncode(message));

            return HashEncode(hash);

        }

​

        private static byte[] HashHMAC(byte[] key, byte[] message)

        {

            var hash = new HMACSHA256(key);

            return hash.ComputeHash(message);

        }

​

        private static byte[] StringEncode(string text)

        {

            var encoding = new ASCIIEncoding();

            return encoding.GetBytes(text);

        }

​

        private static string HashEncode(byte[] hash)

        {

            return BitConverter.ToString(hash).Replace("-", "").ToLower();

        }

​

        public static byte[] HexDecode(string hex)

        {

            var bytes = new byte[hex.Length / 2];

            for (int i = 0; i < bytes.Length; i++)

            {

                bytes[i] = byte.Parse(hex.Substring(i * 2, 2), NumberStyles.HexNumber);

            }

            return bytes;

        }

        public static byte[] ConvertHexStringToByteArray(string hexString)

        {

            if (hexString.Length % 2 != 0)

            {

                throw new ArgumentException(String.Format(CultureInfo.InvariantCulture, "The binary key cannot have an odd number of digits: {0}", hexString));

            }

​

            byte[] data = new byte[hexString.Length / 2];

            for (int index = 0; index < data.Length; index++)

            {

                string byteValue = hexString.Substring(index * 2, 2);

                data[index] = byte.Parse(byteValue, NumberStyles.HexNumber, CultureInfo.InvariantCulture);

            }

​

            return data; 

        }

    }