If you haven't seen it yet, the Poe Cipher is a fun thing.

I broke that... unfortunately the winner of the contest broke it 6 weeks before me and 3 or so other people on a mailing list that I ran broke it prior to me as well.
I largely used Perl and was pretty much how I learned Perl. (the ones that solved it faster were all using C)

It was a great deal of fun and allowed me to learn a lot about cryptography. Along the way I also learned about genetic and hillclimber algorithms, which I have as of late been using a lot in financial analysis (and from there learned Neural Nets).

Things like this are great to me - just when I feel that I have hit a wall in things to learn - I find something new and it spawns off a series of new things to learn. Keeps it interesting.

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There are some odd things afoot now, in the Villa Straylight.