JWE AES-256-GCM encoding

[Nemo64]

I had a situation where I got a JWE object from an external source that used A256GCM encoding.

Most other php libraries had trouble even reading my private key, and since this here is using openssl directly, it could. But the encoding was unknown.
So I digged a bit and managed to add decoding support.

$context = (new DefaultContextFactory())->get();
$context->jweEncryptions()->add('A256GCM', new AESGCM(256));

class AESGCM implements JweEncryption
{
    private $keySize;

    public function __construct(int $keySize)
    {
        $this->keySize = $keySize;
    }

    public function getKeySize()
    {
        return $this->keySize;
    }

    private function getIVSize()
    {
        return 96;
    }

    public function encrypt($aad, $plainText, $cek)
    {
    }

    public function decrypt($aad, $cek, $iv, $cipherText, $authTag)
    {
        if (strlen($cek) !== $this->getKeySize() / 8) {
            throw new JoseJwtException('Incorrect key length ' . strlen($cek));
        }

        if (strlen($iv) !== $this->getIVSize() / 8) {
            throw new JoseJwtException('Incorrect IV length ' . strlen($iv));
        }

        if (strlen($authTag) !== 16) {
            throw new JoseJwtException('Incorrect authentication tag length ' . strlen($authTag));
        }

        $plaintext = openssl_decrypt($cipherText, "aes-$this->keySize-gcm", $cek, OPENSSL_RAW_DATA, $iv, $authTag, $aad);
        if ($plaintext === false) {
            throw new JoseJwtException('Authentication tag does not match: ' . openssl_error_string());
        }

        return $plaintext;
    }
}

It's probably pretty easy to go from there but I don't have time to write tests and am happy enough that it works at all.
Maybe it'll help someone.