While struggling my way through documentation of 3DES encryption, I found a site explaining in detail about the Enigma, a typewriter like machine, used by the Nazi''s in World War II to encrypt military messages. This article is a lengthy piece which appeared originaly in Nautical Brass magazine, and reads like a book :
The Enigma could have been unbreakable, at least with the methods available at the time, had the machine been used properly. The biggest mistake the Germans made was their blind belief in the invincibility of Enigma. Procedural errors in using the machine, combined with occasional operator laziness, allowed the Poles and, subsequently the British, to crack the "unbreakable" codes.