Making CryptoJS and pycrypto compatible

Question

I'm trying to figure out how to use the CryptoJS library for AES but the documentation is somewhat scarce.

I've followed their example but I can't seem to decode something that I encoded in PyCrypto

I've formatted as follows:

key = '0f3f0fe38eb5f216cb4ef0649f81d761'
ciphertext = 'Y+gwBI6R37Ko1lQmwZ57gg==FqUSqQ=='

The ciphertext has two components

iv: Y+gwBI6R37Ko1lQmwZ57gg==
encoded message: FqUSqQ==

I tried running the code below but it didn't work. It logs "" in the console. It should be resolving to 'Test'

var decrypted =CryptoJS.AES.decrypt(ciphertext, key);
console.log( decrypted.toString(CryptoJS.enc.Utf8) );

Any pointers would great. Thanks

Answer 1

I ran into this problem as well and unfortunately it was very tricky to get pycrypto to talk to crypto-js. Here are the combination that worked for me:

1. Use MODE_CFB with segment_size=128 when encrypting in Python
2. You must pad your string with Pkcs7 as per RFC 2315 [Page 21]
3. Once you have received the base64 version of ciphertext in JS, you need to wrap it in CryptoJS.lib.CipherParams.create({ ... })

Take a look at the code that works for me (Python 2.7.x):
https://gist.github.com/marcoslin/8026990

Answer 2

I don't have direct experience with CryptoJS, but many years of experience with a variety of other crypto libraries. It's almost certainly the case the CryptoJS.AES.decrypt expects a different ciphertext format than the one you're using. Your ciphertext is the contanenation of two separate Base64-encoded entities:

Base64(IV) + Base64(Enc(Message))

If CryptoJS.AES.decrypt expects the IV to be specified as part of the ciphertext (which it may not), and expects Base64-encoded ciphertext input at all (which it may not), then I would try passing:

Base64( (binary IV) + (binary Enc(Message)) )

Looking at the CryptoJS doc, I share your frustration with it. But there's an encryption example with "manually specified IV" that may provide a clue.

<script src="http://crypto-js.googlecode.com/svn/tags/3.1.2/build/rollups/aes.js"></script>
<script>
    var key = CryptoJS.enc.Hex.parse('000102030405060708090a0b0c0d0e0f');
    var iv  = CryptoJS.enc.Hex.parse('101112131415161718191a1b1c1d1e1f');

    var encrypted = CryptoJS.AES.encrypt("Message", key, { iv: iv });
</script>

Sorry I can't be more helpful-