Decrypt AES with OpenSSL & Qt 5.5.1 Win32 VS2013

qDebug

Yes. It is the only one i found so far. But the iv and key are random generated, it does not show how to use an already existing key and iv correctly. I really don't know what i am missing here.

SGaist

Just replace the random key and iv by yours.

qDebug

I did. That was my first idea, did not work. Id did check the key and iv output, same as i use with openssl, does work in openssl, does not work in Qt / QCA - at least not the way i do it. I don't know, maybe i miss something in general or just just a tiny mistake, but after days and hours, i can confirm, it won't work for me.

SGaist

I took the example as is, replaced key and iv by

QByteArray key("098f6bcd4621d373cade4e832627b4f6");
QByteArray iv("d8e8fca2dc0f896fd7cb4cb0031ba249");

and it's working fine.

qDebug

If you can tell me why this code

QString decodedString;
QCA::Initializer init;
QByteArray array(encodedString.left(16).toStdString().c_str(), encodedString.left(16).size());
QCA::SecureArray iv = array.toHex();
QCA::Base64 decoder(QCA::Decode);
QCA::SecureArray decoded = decoder.decodeString(encodedString).toStdString().c_str();
QCA::SecureArray key = QByteArray(QCryptographicHash::hash(password.toUtf8(), QCryptographicHash::Md5).toHex());
QCA::Cipher cipher(QString("aes128"), QCA::Cipher::CBC, QCA::Cipher::NoPadding, QCA::Decode, key, iv);
QCA::SecureArray plainText = cipher.update(decoded);
if(!cipher.ok())
{
	qDebug() << "update Fail";
}
plainText  = cipher.final();
if(!cipher.ok())
{
	qDebug() << "final fail";
}
qDebug() << "process: " << QCA::SecureArray(cipher.process(decoded)).data();
QString decodedString = plainText.data();
qDebug() << "Decoded: " << decodedString;

is not working, it may help me. If you tell me some code is working for you, it does not. This is btw. taken from the example, i did not change anything beside adding key and iv.

SGaist

The first thing that looks strange is your iv creation. You take 16 bytes of your encoded string and turn it to hexadecimal. Why ?

Also, why all the conversions ? Just use QByteArray.

qDebug

098f6bcd4621d373cade4e832627b4f6 is "test" in md5. all the strings i have to decode are encoded with a different iv, the iv is always the first 16 bytes in hex from the encoded string, d8e8fca2dc0f896fd7cb4cb0031ba249 in this case.

So i have to hash the key "test" and get the iv from the encoded string.

And of course, i did test the key and iv just as QByteArray, before and after you posted the example, but the decoding so far did always fail.

SGaist

That's something that is really not clear. You are telling me that you are using as iv for the decoding a part of the alrey encoded string ? So what did you use as iv for the encoding part ?

qDebug

The iv is taken from the still encrypted and base64 encoded string, first 16 bytes hex. iv 507055722b4c4d4876614b6d66307136 is QByteArray array(encodedString.left(16).toStdString().c_str(), encodedString.left(16).size()); qDebug() << "iv: " << array.toHex();

Proof:

echo PpUr+LMHvaKmf0q6J7Oyzo4jbFO5kfWyXl0d8nD3hyM= | openssl enc -d -a -A -aes-128-cbc -iv 507055722b4c4d4876614b6d66307136 -K 098f6bcd4621d373cade4e832627b4f6
8°&¦=YaÌ?{Äa+Dr. Test

SGaist

And what iv did you use to encrypt the string ?

qDebug

If you scroll up a few posts, there is the source code.

SGaist

Do you mean the VB code ? Then you generate an IV in there, and use a different one when decrypting your string or am I missing something there ?

qDebug

It is the same iv. There is no problem with the iv i believe, it does work to decrypt in openssl and VB. It is a common way to use the first 16 bytes of a string for iv. Like salting passwords, kind of, not really but i'm no expert and can't explain it better. Defending on the software and api you may have to remove the iv from the string before decrypting but i don't thing it is the case in qca - but i did try it anyways - no luck.

Maybe i don't use the API correctly. The example code:

cipher.setup( QCA::Decode, key, iv );

QCA::SecureArray cipherText = u.append(f);
QCA::SecureArray plainText = cipher.update(cipherText);
if (!cipher.ok()) {
	printf("Update failed\n");
}

printf("Decryption using AES128 of [0x%s] is %s\n",
	   qPrintable(QCA::arrayToHex(cipherText.toByteArray())), plainText.data());

plainText = cipher.final();
if (!cipher.ok()) {
	printf("Final failed\n");
}

printf("Final decryption block using AES128 is %s\n", plainText.data());
printf("One step decryption using AES128: %s\n",
	   QCA::SecureArray(cipher.process(cipherText)).data() );

So cipherText here is u.append(f); the update and the final results together.

Or maybe i found a bug. Who knows? But chances are i'm doing it wrong, somehow.

SGaist

That's what I'm trying to clear up with you.

What you wrote is that you were using a part of your already encoded string as IV when setting up decoding and that's that part that is puzzling me. AFAIK, you should use the same IV that you generated, whatever the means, when you encoded that string. So basically (pseudo code):

Get secret key as sk
Get password
Generate IV as iv: 16 first chars of md5 of password
Encode "Dr. Test" with sk + iv as encoded_str

and later:

Get secret key as sk
Get password
Generate IV as iv: 16 first char of md5 of password
Decode encoded_str with sk + iv
Get Dr. Test

What I understand of what you wrote for the decoding part is:

Get secret key as sk
Get password
Generate IV as iv: 16 first char of encoded_str
Decode encoded_str with sk + iv
Get garbage + Dr. Test

qDebug

I did not wrote the VB code nor the encoder or decoder for this part. I just try to decrypt strings in XML files encoded using this VB code. For now i use a bash script on Linux but i fail to implement it in my Qt app so far.

Bash:

passmd5="$(echo -n "$1" | md5sum | cut -d '-' -f1 | tr -d '[[:space:]]')"
theiv="$(echo $2 | xxd -l 16 -ps)"
echo $2 | openssl enc -d -a -A -aes-128-cbc -iv $theiv -K $passmd5 | tail -c +17

Works perfectly. No luck in Qt.

SGaist

Can you give a sample of input you pass to that script and what you should have as output ?

qDebug

Of course:

#!/bin/bash
password="test" # the password
cstring="PpUr+LMHvaKmf0q6J7Oyzo4jbFO5kfWyXl0d8nD3hyM=" # the aes-128-cbc and base64 encoded string
passmd5="$(echo -n "$password" | md5sum | cut -d '-' -f1 | tr -d '[[:space:]]')" # md5 hash the password
theiv="$(echo $cstring | xxd -l 16 -ps)" # get the iv
decoded="$(echo $cstring | openssl enc -d -a -A -aes-128-cbc -iv $theiv -K $passmd5 | tail -c +17)" # decode in openssl

# expected: Dr. Test
echo decoded: $decoded

SGaist

Based on your script

QByteArray data = "PpUr+LMHvaKmf0q6J7Oyzo4jbFO5kfWyXl0d8nD3hyM=";
QByteArray iv = data.left(16).toHex();
QCA::Hash hash( "md5" );
hash.update("test");
QCA::SecureArray key = hash.final();
QCA::Cipher cipher(QString("aes128"),QCA::Cipher::CBC,
                                 // use Default padding, which is equivalent to PKCS7 for CBC
                                QCA::Cipher::DefaultPadding,
                                // this object will encrypt
                                QCA::Decode,
                                key, iv);

        QCA::SecureArray decryptedData = cipher.process(QByteArray::fromBase64(data));
        if (!cipher.ok()) {
            qDebug() << "Decryption failed !  ";
        }
        qDebug() << decryptedData.toByteArray().mid(16);

Note however that using tail like that looks suspicious. I'm still not convinced of your initial vector handling. It should be the same used when encoding the string. Here it clearly is not.

qDebug

@SGaist Perfekt, thank you!

I was total stuck. Password hash, update and final before. Thanks again!

SGaist

But again and I insist the IV handling is wrong. You should use the same IV for both encoding and decoding and following your code it's clearly not the case.

The IV should change each time you encode your data though to avoid repeat attack.