John Robert Fisher Jeffreys (25 January 1916 – 13 January 1944) [1] was a British mathematician and World War II codebreaker.
Jeffreys was educated at Brentwood School, Essex, and Downing College, Cambridge, where he graduated as a Wrangler in Part II of the mathematics tripos in 1936. [2] Appointed a research fellow at Downing, [3] he 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. [4] These three, together with Peter Twinn and working under Dilly Knox, formed the research section working on the German Enigma machine, [5] and were housed in "The Cottage" at Bletchley Park. [6]
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. [7] 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". [7] 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. [8]
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. [9] 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". [10]
In May 1940, [11] Jeffreys took a vacation, but became ill and was diagnosed with tuberculosis and diabetes. [12] He died in January 1944.
Gordon Welchman later recalled: