I first learned what little I know about it in some kid’s book of my youth, you know the kind, there’s a small group of brainy kids who try to outsmart everyone around them, yet they’re often spending the middle chunk of each story being outsmarted by the bully kid, who turns out to be the only real genius (an unintended feature of the genre), who of course gets his in the end. From that book I learned about frequency tables used to break ciphers (which are simple letter substitution systems) and how to build cipher wheels and such. What fun. ;^) ETAON...
Sherdlu