As of summer 2016, the Cryptopal working group no longer meets regularly. If you are interested in organizing regular meet ups, feel free to contact the people listed.
a group of hackers working through the cryptopal cryptography challenges by Matasano, in the hopes of learning about practical attacks on real world cryptography.
No background knowledge is required, just a curious mind and a desire to break some crypto! Even the matasano folks say that all you need is some 9th grade math.
every second Thursday @ 7pm.