If you’re curious how the real world cryptography works and how it is being broken, join us in solving the Matasano Crypto challenge exercise set 1 – we’re at ex.7 at the moment. We’ll recap the previous exercises if necessary, so feel free to join even if you missed the first meetups.

All you need is a basic familiarity with at least one programming language, 9th grade math knowledge and a laptop.
More info: http://cryptopals.com/
https://blog.pinboard.in/2013/04/the_matasano_crypto_challenges/