[difficulty: 2] [protection: 1/2 of RSA]
Check out Trevil's (a cool guy from #c4n) keygenme. It's old, and does some basic RSA stuff. Just encrypt(user) == serial. But the fact that its in VB made it a little difficult. I probably would have been lost without Googling for some of the strings. See if you can do it without searching!