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

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

Chilkat PHP Downloads PHP Extension for Windows, Linux, MacOS, Alpine Linux, Solaris

<?php

// The version number (9_5_0) should match version of the Chilkat extension used, omitting the micro-version number.
// For example, if using Chilkat v9.5.0.48, then include as shown here:
include("chilkat_9_5_0.php");

// 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 = new CkCrypt2();

$crypt->put_CryptAlgorithm('aes');
$crypt->put_CipherMode('gcm');
$crypt->put_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 = new CkBinData();
$bdAuthTag = new CkBinData();
$success = $bdEncrypted->AppendEncoded($cipherBase64Url,'base64url');

$numBytes = $bdEncrypted->get_NumBytes();
$authTagHex = $bdEncrypted->getEncodedChunk($numBytes - 16,16,'hex');

print 'Auth tag in hex: ' . $authTagHex . "\n";

$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');
$originalText = $crypt->decryptStringENC($bdEncrypted->getEncoded('base64'));
if ($crypt->get_LastMethodSuccess() == false) {
    print $crypt->lastErrorText() . "\n";
}
else {
    print $originalText . "\n";
    print 'Success.' . "\n";
}

// Decrypted text

?>