GenerateHash

public class GenerateHash
    {
        private string key;
        private string secret;

        public GenerateHash(string key, string secret)
        {
            this.key = key;
            this.secret = secret;
        }

        public int GetEpoch()
        {
            return (int)(DateTime.UtcNow - new DateTime(1970, 1, 1)).TotalSeconds;
        }

        public string GetAuthorizationHeader()
        {
            string epoch = GetEpoch().ToString();
            string stringTosign = string.Concat(key, epoch);

            byte[] secretBytes = Encoding.ASCII.GetBytes(secret);
            using (HMACSHA256 myhmacsha1 = new HMACSHA256(secretBytes))
            {
                byte[] byteArray = Encoding.ASCII.GetBytes(stringTosign);
                byte[] hashValue = myhmacsha1.ComputeHash(byteArray);
                string hashToBase64 = Convert.ToBase64String(hashValue);

                string keyHashEpoch = string.Format("{0}:{1}:{2}", key, hashToBase64, epoch);
                return string.Concat("A5-API ", keyHashEpoch);
            }
        }
    }