The Nazis went to great lengths to encipher their secret messages, building mechanical scramblers. The most famous of these was the Enigma machine, which was secretly cracked by Alan Turing and his team of Bletchley Park codebreakers. Here is a functional papercraft Enigma Machine to print, fold and assemble.
(Thanks, Zed!)
Update: Teece sez, "In reality, most of the credit for cracking Enigma goes to a Polish man named Marian Rejewski (Wikipedia article).
"He led the Polish team that cracked the early Enigma machine, using machines they called bombes. It's a fascinating story. The Polish shared their intelligence with the British when the German switched to a beefed up system, which they didn't have the resources to crack, as it required a bigger machine. And then, of course, Poland was invaded (which is another reason the Poles shared their info — they knew it was coming, as they had been reading German messages)."
