Enigma Machine and Bletchley Park
The Enigma machine was invented by a German engineer Arthur Scherbius at the end of World War I. It was a rotor cipher machine that could be used for encrypting and decrypting secret messages. When a letter is typed into the enigma machine it will substitute that letter with a new letter (e.g. A becomes B). The rotors advance a position after each letter, changing the substitution cipher being used (i.e. if you typed another A, this time it might become a G in your encrypted message). This system makes messages very difficult to decrypt unless you have another enigma machine and you know which substitution cipher with which to begin. During World War II the Nazis reset their cipher keys daily, making it even harder to decode their messages.
During World War II teams of mathematicians and cryptologists at Bletchley Park worked tirelessly decoding enemy military and naval transmissions.