Encryption/decryption issue with QString.

Aditya1309

Hi,

I am able to encrypt and decrypt using symmetric key using AES algo.
std::string Encrypt(const void * const rszBuffer)
std::string Decrypt(const void * const rszBuffer)

This works fine with .txt file.
But while I am trying to save it in xml using Qstring decryption fails.

I am converting string returned from Encrypt and converting to QString and then saving to XML.
While decrypting I am doing the reverse but I am unable to construct the plain text.

// getting from xml
std::string szpasswd = Obj.getPassword().toStdString();
qDebug()<<"Password is:"<<QString::fromStdString(szpasswd);
std::string szciphertext = Encrypt(szpasswd.data());

qDebug()<<"ciphertext value is:"<<QString(szciphertext.data());
qDebug()<<"Saving cipher text:"<<QString::fromStdString(szciphertext.data());

std::string plaintext = Decrypt(szciphertext.data()); //cipher is correct from and plaintext is constructed

qDebug()<<"saving plaintext:"<<QString(plaintext.data()); // get the correct plaintext

qDebug()<<"Converting again to plaintext:";
plaintext = Decrypt(QString(szciphertext.data()).toStdString().data()); //cipher is wrong while using QString
qDebug()<<"saving plaintext using Qstring:"<<QString(plaintext.data()); // plaintext is not constructed

I am not able to figure out the problem, but looks like issue while converting from std::String to QString.

sierdzio

@Aditya1309 said in Encryption/decryption issue with QString.:

I am converting string returned from Encrypt and converting to QString and then saving to XML

That moment looks suspicious. The encryption can produce any combination of bits. Converting them to UTF-16 (QString) and then to XML (which uses it's own escaping for special characters etc.) is likely introducing the issues here.

Instead, do this:

Work on QByteArrays, not QStrings.
Encrypt, then enconde into base64 (QByteArray has special methods to do it).
Then, when data is in base64, you can do whatever with it: convert to QString, XML, Json - anything.
Now reversing the process should work.

Aditya1309

@sierdzio said in Encryption/decryption issue with QString.:

base64

It would be helpful if you just elaborate then I can implement the same and revert back.
Currently my encrypt and decrypt returns std::string. Do you mean I should change them to return QbyteArray ?

JonB

@Aditya1309
Yes, (assuming std::string Encrypt() is your own code) you should have that return a QByteArray, or std::byte or equivalent.

http://doc.qt.io/qt-5/qbytearray.html#toBase64 & http://doc.qt.io/qt-5/qbytearray.html#fromBase64 convert to/from base64.

Aditya1309

@sierdzio Thanks for the help, I used bytearray for saving my QString and I did the following and it worked fine.

For saving ciphertext into XML.

QByteArray byteArray;
byteArray.append(Obj.getPassword().toLatin1());
// Converting to ciphertext
QByteArray ciphertext = Encrypt(byteArray);
newNodeText = m_Doc.createTextNode(QString::fromLatin1(ciphertext));

For getting the ciphertext into plaintext and set the lineedit.
// Getting the encrypted password and setting the password
QByteArray plaintext = Decrypt(nodeValue.toLatin1());
Obj.setPassword(QString(plaintext));

The above code is working as expected.

Aditya1309

@JonB Thanks I used bytearray and it worked fine.

JonB

@Aditya1309
Maybe it works fine on your particular password content, I don't know. Don't blame us if it goes wrong on other input. As @Aditya1309 & I have said, we would have thought you would be best/safest storing it as hex or base64, not as arbitrary, unknown bytes which you treat as a string. Saving as hex/base64 would require one extra line in your encryption, and one extra line in your decryption. It's up to you....