Not a skill tree, but this reminds me of the challenge map from http://hacker.org.

EDIT: To add, the intent of the challenge map is that many of the challenges build directly or indirectly on past challenges. Basic crypto challenges (by basic, I mean simple substitution ciphers) to simple XOR ciphers with increasing levels of difficulty (intended, sometimes there are shortcuts the designer didn't perceive). Programming challenges using their own esoteric languages or solving problems with potential exponential algorithms (until you figure out the shortcuts) and the like.