AES function giving not the output that I expected

Question

I am currently doing a project for school (comparison between runntime of two cryptographic functions). So I searched for AES. I found this nice looking solution: https://www.c-plusplus.net/forum/148732

Now when I am using as a key aes and as plaintext also aes, then I get a strange result with characters that are not a-z.

My full project can be viewed at: https://gogs.4seul.de/root/CryptLaufzeit

void MainWindow::on_pushButtonAesCalc_clicked()
{
    //Check if key and plaintext is given (if not -> display alert,if yes -> save in vars)
    QString key=ui->lineEditAesKey->text();
    QString plaintext=ui->lineEditAesPlaintext->text();
    if (key != "" && plaintext != "") {
        //Time measure start
        std::chrono::time_point<std::chrono::system_clock> start, end;
        start = std::chrono::system_clock::now();
        //Create AES Object
        //char array for key and plaintext; length is size_t type and length of char array
        size_t sizeKey = key.size();
        size_t sizePlain = plaintext.size();
        char *plainCharArr = new char [sizePlain];
        char *keyCharArr = new char [sizeKey];
        memcpy( keyCharArr, key.toStdString().c_str() ,sizeKey);
        memcpy( plainCharArr, plaintext.toStdString().c_str() ,sizePlain);
        aes AES;
        sizePlain = AES.encrypt(&plainCharArr,sizePlain,keyCharArr);
        //Time measure end
        end = std::chrono::system_clock::now();

        std::chrono::duration<double> elapsed_seconds = end-start;
        std::time_t end_time = std::chrono::system_clock::to_time_t(end);

        //Rerun everything and get steps in between (optional)
        //Set values
        QMessageBox msgBoxTest;
        msgBoxTest.setText(plaintext + "|" + plainCharArr);
        msgBoxTest.exec();
    } else {
        //Display Dialog
        QMessageBox msgBox;
        msgBox.setText("Bitte fülle alle Felder aus!");
        msgBox.exec();
    }
}

score 2 · Accepted Answer · edited May 23 '17 at 12:32

2

AES is an encryption algorithm. The function is not closed on [a-zA-Z0-9]. Meaning you can get any value between 0 and 256 in the encrypted string.

If you still want to visualize the encrypted output (for comparison and verification purpose), I will suggest convert each byte to hexadecimal representation which contains only readable characters.

See this, to know how to convert byte buffers to hexadecimal.

edited May 23 '17 at 12:32

Community

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answered Mar 01 '17 at 09:54

Ajay Brahmakshatriya

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Is the conversation done in my codepiece which I provided here or in the aes algorithm? The functionallity should be sth like this: http://www.cryptool-online.org/index.php?Itemid=135 – school_guy Mar 01 '17 at 10:01
You will have to convert the plainCharArr to hex before displaying it on the msgBox. – Ajay Brahmakshatriya Mar 01 '17 at 10:04
Thx, will try your solution and when its working I mark it as the correct answer – school_guy Mar 01 '17 at 10:42
Thinking of aes... How can I prevent aes from giving back non printable values, without losing the correct de- and encryption? – school_guy Mar 01 '17 at 11:02
Putting simple, you can't. If you want printable output, you will have to use some other encryption scheme. Or make the output of AES "printable" by converting to hexadecimal like I said above. – Ajay Brahmakshatriya Mar 01 '17 at 11:05
But when I understand hexadecimal right, its just another representation. I can't display some hex chars also. – school_guy Mar 01 '17 at 11:29
Hexadecimal characters only comprise of 0-9 and A-F (or a-f). All of which are printable. Each character in the output will be displayed with 2 hex characters. The hex of 0 is 00 and that of 255 is FF. – Ajay Brahmakshatriya Mar 01 '17 at 11:33
If you do not want to use hexadecimal you can simply convert it to decimal and put spaces between the consecutive letters. This will print numbers from 0 to 256. – Ajay Brahmakshatriya Mar 01 '17 at 11:33
ok, that would be an acaptable solution, when converting them back, I would have to convert the hex into char back, right? – school_guy Mar 01 '17 at 12:00
You can keep the char as it is, just convert it for displaying purpose. Keep the original copy of chars. – Ajay Brahmakshatriya Mar 01 '17 at 12:01
Glad I could help. – Ajay Brahmakshatriya Mar 01 '17 at 12:05

AES function giving not the output that I expected

1 Answers1