(Java) Decrypt 256-bit AES GCM Produced by Something Unknown

Demonstrates how to decrypt something produced elsewhere (unknown) with 256-bit AES GCM.

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import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // We have the following to decrypt:

    // Key (Base64): 
    String keyBase64 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";

    // IV (Base64Url):
    String ivBase64Url = "xrvaINMLqotAbWRK";

    // ciphertext (base64url):
    String cipherBase64Url = "RtGNAS-zQOxSB8W0HfqJjCoyt9KgImW_l-HjVC40hOOl-RNfRF3hzDIT1kvFVF8i_KX9XmqAftb6lyq-jLCEc_MSgqt3q1ixv3Ez4SbS3G5e3qGzLwxIMi2sCt00aDNwK2ipsJ4aw8s7ePPnl4oY-y1st9rwCWR0rrgEZwS9jmS4uJWGPn9K3jbKRnMslznDbtFLNJctMVXBTP-cv47JelxLCBOQSlK29rMuEFrhHR_VQrPq6gtZaBVSXZSYT0XOklp7nu9mVhrMCRtBCC5oiu5MPE5JYx4ANo3hUY7_NyQl2bpn9GfRXrdvqRGE-gy2upj-cDkm0t_tV8xmYge9DBQTH3B_4BGl2qTk_o-m7pEmKkS8XSdQhGcuFlykqrkE8SzB5I8esfzWOM0pwxbz0H_VaylKYHY=";

    CkCrypt2 crypt = new CkCrypt2();

    crypt.put_CryptAlgorithm("aes");
    crypt.put_CipherMode("gcm");
    crypt.put_KeyLength(256);

    boolean success = crypt.SetEncodedAad("random","ascii");
    crypt.SetEncodedKey(keyBase64,"base64");
    crypt.SetEncodedIV(ivBase64Url,"base64url");

    // The cipher text contains the 16-byte auth tag at the end.
    // get it separately..
    CkBinData bdEncrypted = new CkBinData();
    CkBinData bdAuthTag = new CkBinData();
    success = bdEncrypted.AppendEncoded(cipherBase64Url,"base64url");

    int numBytes = bdEncrypted.get_NumBytes();
    String authTagHex = bdEncrypted.getEncodedChunk(numBytes - 16,16,"hex");

    System.out.println("Auth tag in hex: " + authTagHex);

    success = bdAuthTag.AppendEncoded(authTagHex,"hex");
    bdEncrypted.RemoveChunk(numBytes - 16,16);

    // Use this special value to tell Chilkat to ignore the auth tag.
    success = crypt.SetEncodedAuthTag("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF","hex");

    // Decrypt
    crypt.put_EncodingMode("base64");
    String originalText = crypt.decryptStringENC(bdEncrypted.getEncoded("base64"));
    if (crypt.get_LastMethodSuccess() == false) {
        System.out.println(crypt.lastErrorText());
        }
    else {
        System.out.println(originalText);
        System.out.println("Success.");
        }

    // Decrypted text
  }
}