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

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, BinData, Crypt2;

...

procedure TForm1.Button1Click(Sender: TObject);
var
keyBase64: PWideChar;
ivBase64Url: PWideChar;
cipherBase64Url: PWideChar;
crypt: HCkCrypt2;
success: Boolean;
bdEncrypted: HCkBinData;
bdAuthTag: HCkBinData;
numBytes: Integer;
authTagHex: PWideChar;
originalText: PWideChar;

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 := CkCrypt2_Create();

CkCrypt2_putCryptAlgorithm(crypt,'aes');
CkCrypt2_putCipherMode(crypt,'gcm');
CkCrypt2_putKeyLength(crypt,256);

success := CkCrypt2_SetEncodedAad(crypt,'random','ascii');
CkCrypt2_SetEncodedKey(crypt,keyBase64,'base64');
CkCrypt2_SetEncodedIV(crypt,ivBase64Url,'base64url');

// The cipher text contains the 16-byte auth tag at the end.
// get it separately..
bdEncrypted := CkBinData_Create();
bdAuthTag := CkBinData_Create();
success := CkBinData_AppendEncoded(bdEncrypted,cipherBase64Url,'base64url');

numBytes := CkBinData_getNumBytes(bdEncrypted);
authTagHex := CkBinData__getEncodedChunk(bdEncrypted,numBytes - 16,16,'hex');

Memo1.Lines.Add('Auth tag in hex: ' + authTagHex);

success := CkBinData_AppendEncoded(bdAuthTag,authTagHex,'hex');
CkBinData_RemoveChunk(bdEncrypted,numBytes - 16,16);

// Use this special value to tell Chilkat to ignore the auth tag.
success := CkCrypt2_SetEncodedAuthTag(crypt,'FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF','hex');

// Decrypt
CkCrypt2_putEncodingMode(crypt,'base64');
originalText := CkCrypt2__decryptStringENC(crypt,CkBinData__getEncoded(bdEncrypted,'base64'));
if (CkCrypt2_getLastMethodSuccess(crypt) = False) then
  begin
    Memo1.Lines.Add(CkCrypt2__lastErrorText(crypt));
  end
else
  begin
    Memo1.Lines.Add(originalText);
    Memo1.Lines.Add('Success.');
  end;

// Decrypted text

CkCrypt2_Dispose(crypt);
CkBinData_Dispose(bdEncrypted);
CkBinData_Dispose(bdAuthTag);

end;