(C#) 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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// 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=";

Chilkat.Crypt2 crypt = new Chilkat.Crypt2();

crypt.CryptAlgorithm = "aes";
crypt.CipherMode = "gcm";
crypt.KeyLength = 256;

bool 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..
Chilkat.BinData bdEncrypted = new Chilkat.BinData();
Chilkat.BinData bdAuthTag = new Chilkat.BinData();
success = bdEncrypted.AppendEncoded(cipherBase64Url,"base64url");

int numBytes = bdEncrypted.NumBytes;
string authTagHex = bdEncrypted.GetEncodedChunk(numBytes - 16,16,"hex");

Debug.WriteLine("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.EncodingMode = "base64";
string originalText = crypt.DecryptStringENC(bdEncrypted.GetEncoded("base64"));
if (crypt.LastMethodSuccess == false) {
    Debug.WriteLine(crypt.LastErrorText);
}
else {
    Debug.WriteLine(originalText);
    Debug.WriteLine("Success.");
}

// Decrypted text