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I have a java version to do an encryption using AES GCM. When converting to C# I found the result is not the same. The C# code generates no error but resulted different value from Java version. I really have no idea about how to correct it.

    public static void main(String args[]) {
      String iv = "349ED607B1BDF85B";
      String str = "secret";
      String encrypted = encrypt(str,iv);
      String decrypted = decrypt(encrypted,iv);
      System.out.println("Encrypted = " + encrypted);
      System.out.println("Decrypted = " + decrypted);
    public static String encrypt(String plaintext, String iv) {
        try {
            Cipher instance = Cipher.getInstance("AES/GCM/NoPadding");
            instance.init(1, key(), new GCMParameterSpec(128, iv.getBytes(StandardCharsets.UTF_8)));
            byte[] doFinal = instance.doFinal(plaintext.getBytes(StandardCharsets.UTF_8));
            return Base64.getEncoder().encodeToString(doFinal);
        } catch (Exception e) {
            return "";
    public static String decrypt(String cipherText, String iv) {
        byte[] decodedBytes = Base64.getDecoder().decode(cipherText);
        try {
            Cipher instance = Cipher.getInstance("AES/GCM/NoPadding");
            instance.init(2, key(), new GCMParameterSpec(128, iv.getBytes(StandardCharsets.UTF_8)));
            byte[] doFinal = instance.doFinal(decodedBytes);
            return new String(doFinal);
        } catch (Exception e) {
            return "";
    private static SecretKeySpec key() {
        String key = "8645eb9e5e59a7e021cf50c5b44346fa";
        byte[] bytes = key.getBytes(StandardCharsets.UTF_8);
        return new SecretKeySpec(bytes, 0, bytes.length, "AES");
C# version
    public static string Encrypt(string plainText, string iv)
        byte[] key = HexToByteArray("8645eb9e5e59a7e021cf50c5b44346fa");
        byte[] ivBytes = AesGcm.HexToByteArray(iv); ;
        var instance = CipherUtilities.GetCipher("AES/GCM/NoPadding");
        instance.Init(true, new AeadParameters(new KeyParameter(key), 128, ivBytes));
        var plainTextData = Encoding.ASCII.GetBytes(plainText);
        var cipherText = instance.DoFinal(plainTextData);
        return Convert.ToBase64String(cipherText);
                First of all, the C# code doesn't compile, so no repro is possible. Apart from that, there seems to be a similar issue as with your other question: In the Java code, key and IV are UTF-8 encoded, in the C# code (probably) hex decoded. Btw, does the PHP to C# port work? You didn't give any final feedback.
– Topaco
                Dec 12, 2021 at 12:24
                You should UTF-8 encode key and IV with Encoding.UTF8.GetBytes(...) in the C# code (instead of HexToByteArray()). ASCII encoding as for the plaintext also works.
– Topaco
                Dec 12, 2021 at 13:06
        
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