Starting soon:

If you’re curious how the real world cryptography works and how it is being broken, join us in solving the Matasano Crypto challenge exercises. We code individually and can recap the previous exercises, so feel free to just drop in and start anytime! 

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/