Substitution ciphers that encode a message by substituting one
character for another go back at least as far as Julius Caesar,
who used a rotating character scheme to encode military orders.
This simple type of encryption is vulnerable to statistical
attacks, however, as anyone who has solved CRYPTOGRAM puzzles
can attest. In World War II, the Nazi military employed an
encryption scheme that addressed this weakness of simple
substitution ciphers. This scheme, implemented by typewriter-
sized devices known as Enigma machines, gave the Nazis a
tactical advantage that greatly contributed to their early
success in the war. In fact, the eventual breaking of this
coding scheme by researchers at Bletchley Park, England
(including Alan Turing) is hailed as one of the turning points
of the war. Enigma machines used interchangeable rotors that
could be placed in different orientations to obtain different
substitution patterns. More significantly, the rotors rotated
after each character was encoded, changing the substitu