encrypt method
Implementation
Uint8List encrypt(Uint8List data, int objectNumber, int generation, {required bool isString, Uint8List? iv}) {
if (fileKey.isEmpty) return data;
final UPdfCryptMethod method = isString ? stringMethod : streamMethod;
if (method == UPdfCryptMethod.none) return data;
final Uint8List key = _objectKey(objectNumber, generation, method);
if (method == UPdfCryptMethod.rc4) return UPdfRc4.apply(key, data);
final Uint8List vector = iv ?? _randomIv();
return UPdfAes(key).encryptCbc(data, vector);
}