Error while sending encrypted data with Boost::asio::async_send_to

Question

I am developing an encrypted version of a realtime communication application. The issue I have is, that the encrypted data pakets sent to the receiver are faulty. An example from the error log: (hex encoded data, the original data is pure byte code).

sent: 262C1688215232656B5235B691826A21C51D37A99413050BAEADB81D8892493FC0DB519250199F5BE73E18F2703946593C4F6CEA396A168B3313FA689DE84F380606ED3C322F2ADFC561B9F1571E29DF5870B59D2FCF497E01D9CD5DFCED743559C3EE5B00678966C8D73EA3A5CD810BB848309CDF0F955F949FDBA618C401DA70A10C36063261C5DBAB0FC0F1

received: 262C1688215232656B5235B691826A21C51D37A99413050BAEADB81D8892493FC0DB519250199F5BE73E18F2703946593C4F6CEA396A168B3313FA689DE84F380606ED3C322F2ADFC561B9F1571E29DF5870B59D2FCF497E01D9CD5DFCED743559C3EE5B00CDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCDCD

This is the call of the send-method:

string encSendBuffer = sj->cipherAgent->encrypt(sj->dFC->sendBuffer, sj->dFC->sendBytes);    

char* newSendBuffer = new char[encSendBuffer.length() + 1];
strcpy(newSendBuffer, encSendBuffer.c_str());

sj->dFC->s->async_send_to(boost::asio::buffer(newSendBuffer, encSendBuffer.length()),
    *sj->dFC->f,
    boost::bind(&sender::sendHandler, this,
    boost::asio::placeholders::error,
    boost::asio::placeholders::bytes_transferred)
)

sj->dFC->s is a UDP-Socket and sj->dFC->f is an UDP Endpoint. The error code of the sendHandler is always system: 0

This is how I do the encryption using the Crypto++ library: (extract)

string cipherEngine::encrypt(char* input, int length)
{
    string cipher = "";

    CTR_Mode<AES>::Encryption e;
    e.SetKeyWithIV(key, keyLength, iv);

    ArraySource as((byte*)input, length, true,
        new StreamTransformationFilter(e,
            new StringSink(cipher)
        )
    );

    return cipher;
}

UPDATE: Code of the receive function:

void receiver::receive(){
    int maxLength = 4096;

    sj->dFC->s->async_receive_from(boost::asio::buffer(input,maxLength),
                                   senderEndpoint, 
                                   boost::bind(&receiver::handleReceiveFrom, this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
}

After the Data is received, it is stored in the char buffer input and decrypted in the handleReceiveFrom function.

Without encryption everything is fine. The number of bytes that are sended is always correct, on receiver side too. The length of de "CD"- blocks are quite random. I already checked the encryption and the decrypted data is the same as the original plain text.

Does any know where this behavior comes from?

Answer 1

The key here is that the erroneous data begins after the first null (0x00) value in your encrypted data array. The following line:

strcpy(newSendBuffer, encSendBuffer.c_str());

...looks like it's only copying up to the data until that null byte into newSendBuffer. The send function is sending that buffer contents just fine; the buffer just doesn't have the data you expect. You'll need to load newSendBuffer in a different way, not using strcpy(), that can handle null bytes. Try std::memcpy().