Alan Turing (1912-1954)
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Founder of computer science, mathematician, philosopher, 
codebreaker, strange visionary and a gay man before his time: 

Known for many brilliant things (was a logician):
-- Developed formal theory of computing before computers (Turing machine)
-- World War II, breaking the Enigma code
-- Father of Artificial Intelligence
-- Developed the Turing test to determine machine intelligence

1912 (23 June): Birth, Paddington, London
1926-31: Sherborne School
1930:    Death of friend Christopher Morcom
1931-34: Undergraduate at King's College, Cambridge University
1932-35: Quantum mechanics, probability, logic
1935:    Elected fellow of King's College, Cambridge
1936:    The Turing machine, computability, universal machine
1936-38: Princeton University. Ph.D. Logic, algebra, number theory
1938-39: Return to Cambridge. Introduced to German Enigma cipher machine
1939-40: The Bombe, machine for Enigma decryption
1939-42: Breaking of U-boat Enigma, saving battle of the Atlantic
1943-45: Chief Anglo-American crypto consultant. Electronic work.
1945:    National Physical Laboratory, London
1946:    Computer and software design leading the world.
1947-48: Programming, neural nets, and artificial intelligence
1948:    Manchester University
1949:    First serious mathematical use of a computer
1950:    The Turing Test for machine intelligence
1951:    Elected FRS. Non-linear theory of biological growth
1952:    Arrested as a homosexual, loss of security clearance
1953-54: Unfinished work in biology and physics
1954 (7 June): Death (suicide) by cyanide poisoning, Wilmslow, Cheshire.

The Enigma
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Most German communications were enciphered on the Enigma cipher machine.
It was based on rotors whose movement produced ever-changing alphabetic
substitutions. In its military use, the basic machine was greatly
enhanced by a plugboard, visible on the front of the machine. 

The ciphers it produced were supposed to be unbreakable even by someone
in possession of the machine. Ideas of great logical ingenuity were
needed to defeat it. 

The Enigma had to be broken afresh over and over again. The German use of
the Enigma depended on systems for setting the keys for each message
transmitted, and it was these key-systems that had to be broken. There were
many such systems, often changing, and the hardware was changed as well from
time to time. The brilliant pre-war work by Polish mathematicians enabled them
to read Enigma messages on the simplest key-systems. The information they gave
to Britain and France in 1939 may have been crucial, but it was not sufficient
for the continuation and extension of Enigma breaking over the next six years.
New ideas were essential. 

In 1939-40 Alan Turing and another Cambridge mathematician, Gordon Welchman,
designed a new machine, the British Bombe. The basic property of the Bombe
was that it could break any Enigma-enciphered message, provided that the
hardware of the Enigma was known and that a plain-text 'crib' of about
20 letters could be guessed accurately.