AES encryption on QStrings, libcrypto++ - c++

Question

I have the followind piece of code that encrypts and decrypts the message.
QString AesUtils::encrypt(QString message, QString aesKey)
{
string plain = message.toStdString();
qDebug() << "Encrypt" << plain.data() << " " << plain.size();
string ciphertext;
// Hex decode symmetric key:
HexDecoder decoder;
string stdAesKey = aesKey.toStdString();
decoder.Put((byte*)stdAesKey.data(), aesKey.size());
decoder.MessageEnd();
word64 size = decoder.MaxRetrievable();
char *decodedKey = new char[size];
decoder.Get((byte *)decodedKey, size);
// Generate Cipher, Key, and CBC
byte key[ AES::MAX_KEYLENGTH ], iv[ AES::BLOCKSIZE ];
StringSource( reinterpret_cast<const char *>(decodedKey), true,
new HashFilter(*(new SHA256), new ArraySink(key, AES::MAX_KEYLENGTH)) );
memset( iv, 0x00, AES::BLOCKSIZE );
CBC_Mode<AES>::Encryption Encryptor( key, sizeof(key), iv );
StringSource( plain, true, new StreamTransformationFilter( Encryptor,
new HexEncoder(new StringSink( ciphertext )) ) );
return QString::fromStdString(ciphertext);
}
QString AesUtils::decrypt(QString message, QString aesKey)
{
string plain;
string encrypted = message.toStdString();
// Hex decode symmetric key:
HexDecoder decoder;
string stdAesKey = aesKey.toStdString();
decoder.Put( (byte *)stdAesKey.data(), aesKey.size() );
decoder.MessageEnd();
word64 size = decoder.MaxRetrievable();
char *decodedKey = new char[size];
decoder.Get((byte *)decodedKey, size);
// Generate Cipher, Key, and CBC
byte key[ AES::MAX_KEYLENGTH ], iv[ AES::BLOCKSIZE ];
StringSource( reinterpret_cast<const char *>(decodedKey), true,
new HashFilter(*(new SHA256), new ArraySink(key, AES::MAX_KEYLENGTH)) );
memset( iv, 0x00, AES::BLOCKSIZE );
try {
CBC_Mode<AES>::Decryption Decryptor
( key, sizeof(key), iv );
StringSource( encrypted, true,
new HexDecoder(new StreamTransformationFilter( Decryptor,
new StringSink( plain )) ) );
}
catch (Exception &e) { // ...
qDebug() << "Exception while decrypting " << e.GetWhat().data();
}
catch (...) { // ...
}
qDebug() << "decrypt" << plain.data() << " " << AES::BLOCKSIZE;
return QString::fromStdString(plain);
}
The problem is that I randomly get:
StreamTransformationFilter: invalid PKCS #7 block padding found
When decrypting the content. The encryption should fully support QString,
since it may contain some Unicode data. But it doesn't work even with a basic,
string which contains only [A-z][a-z][0-9]
The aesKey size is 256.
Following some answers on Stack Overflow, somebody suggested the use of HexDecoder / HexEncoder, but it does not solve the problem in my case.

Answer 1

The fist problem with my code was that I was feeding normal string in aesKey QString.
So instead of "1231fsdf$5r4" you need to give a key in hex format: [0-9][A-F]
Then, the problem was here:
char *decodedKey = new char[size];
decoder.Get((byte *)decodedKey, size);
I guess the string was full 64 bytes and there was no space for NULL at the end. The problem dissapeared after I changed to:
char *decodedKey = new char[size+2];
Now the code works fine. Hope this will help somebody in the future.