Having trouble decrypting a well-formed cipher text using Crypto++

Question

Background

I've been struggling with decrypting an apparently well-formed cipher text for about a day. Assume we've got the following hex-encoded cipher text which contains exactly 160 characters thereby having 80 bytes.

QString c = "1BFAC407AF0D440A2D6176C0B5D125AA96088490299AC18C74623C0EF1BB1372E554FC4150A8066220E943697BE2491D8AE13AA036B298425AC510A8A917D59EBB69708B9040AB3A84C63043EAD4AB07";
QString k = CryptoUtils::hexEncode("abc");
QString p = CryptoUtils::decrypt(c, k);

qDebug() << p;

Provided we're using AES 256, AFAIK, the key must be of length 32 bytes and cipher text of a length of multiple of 16 bytes, which all these consditions are met regarding my snippet code.

Please note that I'm using SHA256 feeding with a pass phrase to generate a 32 bytes key. So, this ensures that all keys are of length 32 bytes.

Full source codes of those function can be found on my repo on GitHub (at branch Part1).

---

My Question

When I want to run this code, my app crashes. Here's the exception:

terminate called after throwing an instance of 'CryptoPP::InvalidCiphertext'
  what():  StreamTransformationFilter: invalid PKCS #7 block padding found
The program has unexpectedly finished.

I searched around about this problem and figured out it could be because of the trailing \0 once you encrypted the plain text. However, I couldn't just solve the problem. Please help me out, it's just driving me crazy.

Answer 1

The hexEncode function seems to misbehave:

QString CryptoUtils::hexEncode(QString text)
{
    byte *bytearray = (byte *) text.toLatin1().data();
    int length = text.toLatin1().length();

    return hexEncode(bytearray, length);
}

Should be replaced with:

QString CryptoUtils::hexEncode(QString text)
{
    byte *bytearray = (byte *) text.toStdString().data();
    int length = text.length();

    return hexEncode(bytearray, length);
}