I started doing the Cryptopals challenges (https://cryptopals.com, formerly the Matasano challenges) in Prolog and it is really fun - so satisfying to see nonsense suddenly resolve itself into plain text.
I was worried Prolog might not be a good fit for this sort of bit-munging thing, but it is actually working very well.
Love (SWI-)Prolog; it has definitely become my current favourite programming language (sorry Clojure).
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