New SimpleCrypt page

mnosov

[quote author="Andre" date="1317372274"]Your application is bound to have some documentation or help system, right? I'd think there is enough opportunity to fullfil the licence terms that way. Licences do not get much more liberal than the one I have used...[/quote]
Ok, thanks, i'll add this notice to the global "copyright" file in the sources. I just needed confirmation that it is ok that this notice will not appear anywhere on the application's UI

andre

The relevant part of the licence states:
[quote]Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.[/quote]

So, if you are providing other materials/documentation with the binairy application, then you must basically repeat the licence block in that documentation or additional materials. That can also occur in in-application help or something like that. If you distribute your application as source, then you can place it there (basically: just keep the source file as-is - at least in terms of the licence header - and you are ok.)

erwanage

Hi,

First, Thanks for your work.
I'd like to use your class but i need some more help.
The constructor of SimpleCrypt only accept a quint64 as key.
But in my project, i have to convert a given passphrase (like "password") to this quint64 and cannot manage to do this.
You said in your topic "you can also use other means to get to a quint64 key, such as using some hash of a password and reducing that to 64 bits."
But i don't know how to do it.

Is there some easy way to do this?
Is there some existing class or wathever that do it?

regards.

koahnig

welcome to devnet

Did you see already "qHash":http://developer.qt.nokia.com/doc/qt-4.8/qhash.html#qHash-22 ?

andre

Well, the qHash function comes to mind, or perhaps you could use [[doc:QCryptographicHash]]. You add a bit a salt to your pass string, generate a hash, and combine the bits in the resulting hash to create a 64 bit value. If you use MD5, you get a 128bits hash. You could do something like this:

@
quint64 passToKey(const QString& password) {
QByteArray hash;
QCryptographicHash hasher(Md5);
hasher.addData(salt); //salt is a QByteArray with some random data
hasher.addData(password.toUtf8());
hash = hasher.result();

//we now have a QByteArray that is too long with a hash of the password.

//get the contents of the byte array into two quint64's. There are other ways...
quint64 part1, part2;
QBuffer buffer(hash);
QDataStream stream(&buffer);
stream >> part1 >> part2;
key = part1 ^ part2; //combine the two parts

return key;
}
@

Note: brain to editor, not tested.

Edit:
Note that qHash returns a 32 bits result, so you will have to combine two of these to get a 64 bits version. Perhaps using part of the password for hash1, and the other part for hash2, and then combining the result to get a 64 bits key.

erwanage

Thanks for your help.
I'm not at ease with with hashing and combining stuff :/
How can you do that?
How do you combine 2 unsigned int?

andre

In the same way I do in the snippet I posted, for instance. I use bitshifting, but that is the same as multiplication by 2-to-the-power-of-n. Basically, what you do is:

put the values of your ints in 64 bits variables

shift one of the values 32 bits by either:

multiplying by 0xFFFFFFFF, or

bitshifting

add the two numbers by either:

simply adding the numbers, or

using a binary OR operation like I did in my sample.

rich

Just a note that this appears to be a Vigenere cipher scheme (see http://en.wikipedia.org/wiki/Vigenère_cipher for details) if you simply want basic scrambling of data to prevent trivial access to the plain text then this could well be sufficient, but it's not very strong. Particularly be careful of using this for long texts.

andre

Thanks for the note. If I understand the page you link, I'm not sure that the class implements what qualifies as a Vigenere cipher, but I will agree that it does not provide strong cryptography.

The small additional trick is that the code uses the value of the previous code block as part of the key for the next block. That will hinder the kinds of analysis described in the article, if I understand it correctly. The key length is known in this case: 8 bytes, but because the key is mixed with the previously generated cypher text, it does not work to just decrypt the text as eight different cesar cyphers.

rich

Yes, this is the auto-key variant of vigenere cipher, and is a lot stronger than the basic one.

andre

Interesting stuff. Perhaps I should try to make a new version (still: keeping it simple!) that is a bit stronger.

rich

If you do, I'd be tempted to use something like RC4 which while not perfect, is very simple to implement.

zester

Andre I think you did a great job, at documenting not only usage but also the algorithm. I wish all of Qt's examples were so well thought out ;)

katropine

Great job, easy to use. One question, I tested my app on Debian and Mint and no problem, but on Fedora 17 and Arch
I get "Invalid version or not a cyphertext."

@ QByteArray ba = cypher;

char version = ba.at(0);
if (version !=3) {  //we only work with version 3
    m_lastError = ErrorUnknownVersion;
    qWarning() << "Invalid version or not a cyphertext.";
    return QByteArray();
}@

andre

Sorry, no idea. I did not test on these sytems, but I have no clue why it would go wrong on a different linux system. That seems unlikely somehow. Perhaps the data you feed into SimpleCrypt is corrupted somehow?

katropine

Yes, sorry something else went wrong on those systems and indeed corrupted the settings string.
My Bad.

SimpleCrypt works perfectly.

amanjit

Hi,

[quote author="Andre" date="1300457411"]I have just added a "page":http://developer.qt.nokia.com/wiki/Simple_encryption in the Snippets category [/quote]

Thanks for this class. I am invoking it's constructor with my predefined key (my secret) and I am wondering why qsrand() is initialized with currentTimeMillis or similar (in the constructor code)? I don't get the same encryption results on multiple invocations so I used my quint64 key to initialize qsrand (in the constructor), then it works..

@
SimpleCrypt c1(Q_UINT64_C(0x0c2ad4a4acb9f023)); //some random number
SimpleCrypt c2(Q_UINT64_C(0x0c2ad4a4acb9f023)); //some random number

qDebug() << "Crypt1 " << c1.encryptToString(QString("justatest"));
qDebug() << "Crypt2 " << c2.encryptToString(QString("justatest"));
@

Output
@
Crypt1 "AwLLXV+ZSO+x3Ise1Aw="
Crypt2 "AwIUgoBGlzBuA1TBC9M="
@

Just wondering :)

andre

Why would you want to have the same cypher text when using the same clear text and key? As long as the decrypted plain text from these cypher texts is the same, what is the problem with having different cypher texts? The algorithm uses a randomization of the string on purpose. It makes it much harder to leak part of the key because analysis is much harder this way.

An explanation is in the "details page":/wiki/SimpleCrypt_algorithm_details#2d478ba9ee3cf03e338b506b1a0292dc that has more on the idea of using a random number as a leading byte.

You replacing that they way you did partly negates this, and thus makes the cypher weaker by a couple of bits. Note that even with your change, encrypting the same plain text using the same SimpleCrypt instance twice will result in different cypher texts.

amanjit

Hi Andre,

Thanks for answering; I wasn't looking for reasonably strong encryption - I just wanted to always get the same encrypted string for the same input (private key+string_to_be_encrypted); its just for private use anyway, and non-critical.

I am using QCryptographicHash for that now, it solves my problem

andre

Eh, no, that class does not solve your problem, if you are indeed looking for encryption rather than hashing. There is a big difference between the two...