public static string EncryptSHA256(string str)
        {
            byte[] plainTextBytes = Encoding.UTF8.GetBytes(str);
            HashAlgorithm hash = new SHA256Managed();
            byte[] hashBytes = hash.ComputeHash(plainTextBytes);
            StringBuilder stringBuilder = new StringBuilder();

            foreach (byte b in hashBytes)
                stringBuilder.Append(b.ToString("x2"));

            return stringBuilder.ToString();
        }