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

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

Chilkat for Delphi Downloads Chilkat ActiveX DLL for Delphi Chilkat non-ActiveX DLL for Delphi * The examples here use the ActiveX DLL.

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Chilkat_TLB;

...

procedure TForm1.Button1Click(Sender: TObject);
var
keyBase64: WideString;
ivBase64Url: WideString;
cipherBase64Url: WideString;
crypt: TChilkatCrypt2;
success: Integer;
bdEncrypted: TChilkatBinData;
bdAuthTag: TChilkatBinData;
numBytes: Integer;
authTagHex: WideString;
originalText: WideString;

begin
// 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): 
keyBase64 := 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';

// IV (Base64Url):
ivBase64Url := 'xrvaINMLqotAbWRK';

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

crypt := TChilkatCrypt2.Create(Self);

crypt.CryptAlgorithm := 'aes';
crypt.CipherMode := 'gcm';
crypt.KeyLength := 256;

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..
bdEncrypted := TChilkatBinData.Create(Self);
bdAuthTag := TChilkatBinData.Create(Self);
success := bdEncrypted.AppendEncoded(cipherBase64Url,'base64url');

numBytes := bdEncrypted.NumBytes;
authTagHex := bdEncrypted.GetEncodedChunk(numBytes - 16,16,'hex');

Memo1.Lines.Add('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';
originalText := crypt.DecryptStringENC(bdEncrypted.GetEncoded('base64'));
if (crypt.LastMethodSuccess = 0) then
  begin
    Memo1.Lines.Add(crypt.LastErrorText);
  end
else
  begin
    Memo1.Lines.Add(originalText);
    Memo1.Lines.Add('Success.');
  end;

// Decrypted text
end;