The challenge: https://cryptopals.com/sets/1/challenges/7 My solution: https://github.com/CJStadler/cryptopals-rust/commit/32e4762

This was a very short challenge but I managed to have some difficulty with it so maybe it’s still worth writing a blog. The instructions say

Do this with code.
You can obviously decrypt this using the OpenSSL command-line tool, but we’re having you get ECB working in code for a reason.

But I didn’t listen to this and tried to solve it using the openssl command-line tool first (mostly so that I could then use the solution in the test for my Rust implementation). Having never used openssl before how to do this turned out to be far from “obvious” to me. Eventually I resorted to googling and got this incantation from reddit:

curl http://cryptopals.com/static/challenge-data/7.txt 2>/dev/null|openssl enc -aes-128-ecb -a -d -K '59454c4c4f57205355424d4152494e45' -nosalt

The thing getting passed to -K there is the hex representation of the ASCII encoded key (“YELLOW SUBMARINE”).1

For the Rust implementation I used the openssl crate which made the solution very simple:

use self::openssl::symm::{decrypt, Cipher};

fn decode_aes_128_ecb(ciphertext: &[u8], key: &[u8]) -> Vec<u8> {
    decrypt(Cipher::aes_128_ecb(), key, None, ciphertext).unwrap()
}

Hmm I should probably get rid of the unwrap and return a Result instead if I’m going to be using this function in future challenges…

  1. For example (because I find this confusing), the first letter of the key, “Y”, is coded as the bits 1011001 (most significant on the left) in ASCII. The bits 101 decode to “5” in hex, and 1001 to “9”. ↩