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Qstring to unsigned char conversion function



  • Dear all,

    i have this variable:

    unsigned char plain[] = { 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff };
    

    and using this function to convert it to QString:

    QString CharToString(unsigned char *str)
    {
        QString result = "";
        int lengthOfString = strlen(reinterpret_cast<const char*>(str));
    
        // print string in reverse order
        QString s;
        for(int i = 0; i < lengthOfString; i++)
        {
            s = QString("%1").arg(str[i], 0, 16);
    
            // account for single-digit hex values (always must serialize as two digits)
            if(s.length() == 1)
                result.append("0");
    
            result.append(s);
        }
    
        return result;
    }
    

    now I am trying to figure out SAME function to convert the QString again to unsigned char.

    Im very lost and tried many solutions i found online, nothing worked...

    Can you pleas ehelp me to make new funciton StringToChar() to convert the Qstring into unsigned char?

    Thank you!!


  • Lifetime Qt Champion

    Hi @shokarta,

    if I get it right, all you want to do is to convert the array plain[] to a hexadecimal string.

    You can easily do this with: QString s = QByteArray(reinterpret_cast<const char*>(plain)).toHex() instead your complicated function.

    The other way round can be done with QByteArray::fromHex():

    const QString s ="0123456789ABCDEF";
    const QByteArray ba = QByteArray::fromHex(s.toLatin1());
    const unsigned char *plain = reinterpret_cast<const unsigned char *>(ba.constData());
    

    Regards



  • @aha_1980
    Hello,

    many thanks for helping, so I have this then:

    unsigned char StringToChar(QString *str)
    {
        const QByteArray ba = QByteArray::fromHex(str->toLatin1());
        const unsigned char result[] = reinterpret_cast<const unsigned char*>(ba.constData());
    
        return result;
    }
    

    which brings error on line const unsigned char:
    error: array initializer must be an initializer list or string literal


  • Lifetime Qt Champion

    @shokarta

    Sorry, I missed a star, I meant const unsigned char *plain = reinterpret_cast<const unsigned char *>(ba.constData());

    Fixed my example.

    unsigned char StringToChar(QString *str)

    Why do you pass str by pointer here?

    const unsigned char result = reinterpret_cast<const unsigned char>(ba.constData());
    return result;

    Note that you cannot do that! the local variable is destroyed when you leave the function. You can only copy the result into a buffer provided from outside.

    Regards



  • @aha_1980

    the pointer was placed because QT gives me error and suggest to do that...

    so basicaly when u do this:

    unsigned char StringToChar(QString *str)
    {
        const QByteArray ba = QByteArray::fromHex(str.toLatin1());
        const unsigned char *result = reinterpret_cast<const unsigned char *>(ba.constData());
        return result;
    }
    

    i have error on line const QbyteArray ba:

    main.cpp:23:50: error: member reference type 'QString *' is a pointer; did you mean to use '->'?
    

    and another error on line return result:

    main.cpp:25:12: error: cannot initialize return object of type 'unsigned char' with an lvalue of type 'const unsigned char *'
    

  • Lifetime Qt Champion

    @shokarta

    the pointer was placed because QT gives me error and suggest to do that...

    That is no reason to do it like this. Fix it please. Either use QString str or (better) const QString &str as parameter.

    main.cpp:25:12: error: cannot initialize return object of type 'unsigned char' with an lvalue of type 'const unsigned char *'

    That is because the return value does not match the functions signature.

    But again: you cannot return a pointer to a local variable! You can use it within the function, but not outside.

    Better explain first why you need const unsigned char * at all, then we can find a solution.

    Regards



  • basicaly i hae dowloaded and used huge function to AES encode and decode string.
    the function is loaded by unsigned char:

            const unsigned int BLOCK_BYTES_LENGTH = 16 * sizeof(unsigned char);
            unsigned char plain[] = { 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff };
            unsigned char iv[] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
            unsigned char key[] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11,
              0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f };
            unsigned int len;
    
            AES aes(256);
            unsigned char *out = aes.EncryptCBC(plain, BLOCK_BYTES_LENGTH, key, iv, len);
    

    as well as it outputs the encrypted string in unsigned char

    Hoewer because its a huge function which i understand just on basic logical concept i prefer not to use modify the function to insert string instead of char, as well as output, so i would prefer to convert it (i will be using this on many places, therefore the function is desired)

    so then i could only use:

    unsigned char *out = aes.EncryptCBC(StringToChat(plain), BLOCK_BYTES_LENGTH, StringToChat(key), StringToChat(iv), len);
    

  • Lifetime Qt Champion

    @shokarta said in Qstring to unsigned char conversion function:

    aes.EncryptCBC

    can you please show the function signature of that function?



  • its downloaded and used from here:
    https://github.com/SergeyBel/AES

    i have done no futher changes on these files


  • Lifetime Qt Champion

    @shokarta

    So the signature is:

    unsigned char *AES::EncryptCBC(unsigned char in[], unsigned int inLen, unsigned char key[], unsigned char * iv, unsigned int &outLen)

    None of these parameters is const, so that means the function can modify in, key, and iv (not sure if it does modify these parameters, but it could).

    That means, you have to create local buffers for these parameters, and copy the data into the buffers.

    Especially, this

    unsigned char *out = aes.EncryptCBC(StringToChat(plain), BLOCK_BYTES_LENGTH, StringToChat(key), StringToChat(iv), len);

    is not possible!

    Regards



  • @aha_1980
    Would it be very dificult to modify those aes.* in order to put inside the signature not unsigned char but qstring directly?
    For skilled one it could be matrer of several minutes only :/.
    I know its not nice to ask for such a modification, but this would be extreme helpfull :(.
    Any chance you and i can have any deal on that?
    Only thus one aes-cbc-256 function would be needed, other methods not at all...


  • Lifetime Qt Champion

    @shokarta

    If you want to use security functions, you should stick with well known and good tested libraries like openssl.

    Using a not widely used lib or modifying it yourself is just a hazard game.

    That said, I'm not even sure if Qt SLL would provide something useful for you.

    Regards


  • Lifetime Qt Champion

    @shokarta said in Qstring to unsigned char conversion function:

    Would it be very dificult to modify those aes.* in order to put inside the signature not unsigned char but qstring directly?

    Why? This library does not use Qt. Simply use these functions the way they are supposed to be used.
    Also, https://github.com/SergeyBel/AES/blob/master/README.md shows how to use the functions.



  • @shokarta
    If you want a changed signature in a library, especially if it's just one function, the usual way is you write the desired wrapper function and go via that. Is there any reason you cannot do this for your usage? Over the years I have found it's often a good idea to write wrapper functions/classes for more or less every external library one uses, for all sorts of reasons. Heck, I don't even call the Qt classes/methods directly, I tend to provide wrappers for all of them!


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