I spent the afternoon talking cryptanalysis with my lovely neighbours. The husband was a codebreaker during WWII. His job was to break the really hard codes - stuff that makes the Enigma machine look like a Fisher Price toy. The Enigma machine had three wheels; the machine whose code he was trying to break (the SZ40) had twelve. On a good day, he'd have decoded the message before Hitler read it over his morning coffee.
Under the Official Secrets Act he couldn't tell anyone what he was doing. His brother was fighting out in the field, and all he told his family was that he was 'pushing paper'. Churchill called the women and men who worked as codebreakers at Bletchley Park 'my geese who laid the golden eggs and never cackled'.
A fascinating afternoon!