using crypto++ on iphone sdk with pycrypto on app engine

Question

Hi,

I'm trying to encrypt http requests using crypto++ and decrypt them with pycrypto on the app engine server end. Using Arc4 encryption, I can successfully encrypt and decrypt on the iphone end but when I try decrypting on app engine, the result is garbled. The ciphertext after encrypting on the client is the same as the text received on the server when I check logging, so if they are visually the same, why would decrypting fail?

I thought maybe it has something to do with the encoding of the NSString, as I find I need to call encode() on the cipher on the server end before decrypting just to avoid decrypt() failing on it attempting to encode the cipher in ascii. I have a separate post that delves a bit into this. Can anyone offer some advice?

http://stackoverflow.com/questions/2794942/crypto-pycrypto-with-google-app-engine

Update: I have discovered that the ciphertext resulting from encrypting in C with Crypto++ is not the same as the ciphertext from encrypting in python with PyCrypto. Could there be something I'm doing wrong with initializing the keys? I do something like:

    ARC4::Encryption enc("a");

in C. And in python I do:

testobj=ARC4.new('a')

The %-encoded resulting cipher is different in C than in python. I noticed that in C, I can pass a 2nd parameter for keylength, which I guessed should be 1 for "a", resulting in a different cipher than when putting no parameter. The %-encoded result was still different from the python encoding, though.

Does anything look particularly amiss with my init perhaps?

Answer 1

I've discovered that the problem was not with the init of either crypto impl but rather mistakenly trying to stuff the encrypted cipher text into an NSString which cant simply take raw binary data with no particular encoding. The trick was to encode the data in base64 or base16 so that it is is readable, then use unhexlify on the server end before decrypting.