"You get the challenges in batches of eight by emailing cryptopals at Matasano, and solve them at your own pace, in the programming language of your choice. Once you finish a set, you send in the solutions and Sean unlocks the next eight. (Curiously, after the third set, Gmail started rejecting my tarball as malware.) "
Not really, but sort of. There's a set 8, and it's by far the best of all 8 sets (I'd rank the sets 8->6->2->3->4->7->5->1), but you have to mail Sean to get it, and you should tell him you've done at least sets 1-6.
I haven't checked. Do you mean can you receive some sort of certificate of successful completion? I would think successful solutions with sharaebale code (e.g. a github repo) would be adequate for recognition of completion and competency.
The Challenges: Cryptopals --> http://cryptopals.com/