MacData Generation

static string hashMacData(string rrn, string creditAmount, string creditAccount, string key)
        {
            if(string.IsNullOrWhiteSpace(rrn)) throw new ArgumentNullException(nameof(rrn));

            if(string.IsNullOrWhiteSpace(creditAmount)) throw new ArgumentNullException(nameof(creditAmount));

            if(string.IsNullOrWhiteSpace(creditAccount)) throw new ArgumentNullException(nameof(creditAccount));

            if(string.IsNullOrWhiteSpace(key)) throw new ArgumentNullException(nameof(key));

            var entireStringToHash = rrn + "&" + creditAmount + "&" + creditAccount + "&" + key;

            var saltbytes = Encoding.UTF8.GetBytes(key);

            var entireStringToHashByte = Encoding.UTF8.GetBytes(entireStringToHash);

            byte[] concatenatedByteWithSaltArray = new byte[saltbytes.Length + entireStringToHashByte.Length];

            string result = string.Empty;

            using (HashAlgorithm  shaM = new SHA512Managed()){
                saltbytes.CopyTo(concatenatedByteWithSaltArray, 0);
                entireStringToHashByte.CopyTo(concatenatedByteWithSaltArray, saltbytes.Length);

                result = Convert.ToBase64String(shaM.ComputeHash(concatenatedByteWithSaltArray));
            }

            return result;
        }