A fellow mod from crackmes.de has posted an interesting crackme that will exercise your number theory knowledge. It's short and not too hard, have fun!
Funny hint: the color of the crackme may be some clue!
EDIT: freesoul freaking solved it in record time! (I admittedly had a head start as it rested in the moderator queue). So the spoiler is that it's Goldbach's conjecture. The serial has four pairs of numbers that must be prime and sum to four CRC's generated from the username (which are incremented if odd).
Cyclops said that not all usernames have serials, but I think they do. The CRC's are 32-bits, and the conjecture has been tested way up above 2^32. Since the addition check happens in the confines of a 32-bit register, you can even generate serials for when a CRC is 0 or 2 (aim for 2^32 and 2^32+2).