John Robert Fisher Jeffreys (25 January 1916 – 13 January 1944)[1] was a British mathematician and World War II codebreaker.
A research fellow at Downing College, Cambridge,[2] Jeffreys joined the codebreaking efforts at Bletchley Park in September 1939 alongside fellow Cambridge mathematicians Gordon Welchman, with whom he had previously worked closely, and Alan Turing.[3] These three, together with Peter Twinn and working under Dilly Knox, formed the research section working on the German Enigma machine,[4] and were housed in "The Cottage" at Bletchley Park.[5]
Jeffreys was put in charge of a small section manufacturing perforated sheets for use in the cryptanalysis of the Enigma, a task which took over three months, completed on 7 January 1940.[6] One type were the Zygalski sheets, known as Netz at Bletchley Park, a technique revealed to the British by Polish cryptologists. Another type, named "Jeffreys sheets", were different, and were a "catalogue of the effect of any two Enigma rotors and the reflector".[6] Jeffreys's perforated sheets were used by Polish cryptologists in exile in France to make the first wartime decryption of an Enigma message on 17 January 1940.[7]
In early 1940, a section called "Hut 6" — named after the building in which it was initially housed — was created to work on solving German Army and Air Force Enigma messages. Jeffreys was chosen to run the hut alongside Welchman.[8] Jeffreys was in charge of "Sheet-Stacking and Machine Room activities", while Welchman handled "Registration, Intercept Control, Decoding, and relations with the intelligence people in Hut 3".[9]
In May 1940,[10] Jeffreys took a vacation, but became ill and was diagnosed with tuberculosis and diabetes.[11] He died in January 1944.
Gordon Welchman later recalled: