reverse engineering.
Here're a couple of essays I've written on reverse engineering software protections. Note that I don't advocate any form of piracy, have never released any kind of "crack" and only do this for fun.
Writing a Keygen for Magic DVD Copier 4.7.1
2/16/08 | 3 MB
Writing a Keygen for Magic DVD Ripper 5.2.1
2/9/08 | 2.6 MB