Encryption.cs

private static String Encryption(String plain)
{
    byte[] plainTextByte = Encoding.ASCII.GetBytes(plain);
    byte[] sharedSecret = GetSharedSecret();
    byte[] sessionKey = GetSessionKey(sharedSecret);
    SecureRandom secureRandom = new SecureRandom();
    byte[] nonce = new byte[CryptoConstant.NonceSize];
    byte[] aad = new byte[CryptoConstant.AadSize];
    secureRandom.NextBytes(nonce);
    aad = Encoding.ASCII.GetBytes(GetKeyId());
    GcmBlockCipher cipherSpec = new GcmBlockCipher(new AesEngine());
    AeadParameters cipherParams = new AeadParameters(new KeyParameter(sessionKey), CryptoConstant.AadSize * 8, nonce, aad);
    cipherSpec.Init(true, cipherParams);
    byte[] cipherText = new byte[cipherSpec.GetOutputSize(plainTextByte.Length)];
    int offset = cipherSpec.ProcessBytes(plainTextByte, 0, plainTextByte.Length, cipherText, 0);
    cipherSpec.DoFinal(cipherText, offset);
    return Conversion.HexToString(nonce.Concat(cipherText).ToArray());
}