c - Incompatible AES implementations?

Question

I've compiled some AES implementation code from this site, it's supposed to perfrom a 128 bits key encryption. I tested the encryption/decryption programs which work OK together.

However if I encrypt anything with the mentioned code and then try to decrypt it by openssl tool built-in in linux, I just can't get decrypt it, it even logs me the bad magic number error. Same, if I encrypt anything with openssl and try to decrypt with the code won't work. I tried with both cbc ecb.

If they're both implementing AES, shouldn't it work same way?

Answer 1

it looks like the C code is using ECB and does no padding. so try encrypting a message (a multiple) of 16 bytes followed by 16 bytes of value 16 (pkcs#7 padding). or use a message (a multiple) of 16 bytes and --nopad in openssl (more likely to work). also, use aes-128-ecb or whatever it's called.

a block cipher works on "chunks" of text - in this case, it's 16 characters long. so if you don't want to worry about padding you need to give an exact number of chunks.

also, ecb mode (doing each chunk in turn with no extra processing) isn't secure for many uses. see the wikipedia article (look at the penguin photos).

[edit:] [edit 2:]

> echo -n "abcdabcdabcdabcd" > msg
> wc msg
 0  1 16 msg
> openssl enc -aes-128-ecb -nopad -in msg -K 0 -S "" -iv ""
[noise]
> openssl enc -aes-128-ecb -nopad -in msg -K 0 -S "" -iv "" | wc
 0 1 16

try the above yourself and see if the other code decrypts it (edit 2 sets the key explicitly, and removes IV and salt - not sure what the latter two are for in this case).

[edit 3:]

as far as i can tell, the problem is related to the way that the password is converted to a key. openssl seems to be doing something extra that i can't get rid of unless i specify a key as hex (-K 0). and if i do that, the other program doesn't work (needs a password).

sorry, i'm out of ideas.