John Herivel was born in Belfast on 29 August 1918. Twenty-one years later, he arrived at Bletchley Park and devised a way of attacking German Enigma traffic by looking at the people operating the machines.
Herivel attended Methodist College Belfast before winning a scholarship to study mathematics at Sidney Sussex College, Cambridge. One of his tutors, Gordon Welchman, recruited him to Bletchley Park after the outbreak of the Second World War. Herivel joined Hut 6 in January 1940.
Hut 6 worked on Enigma traffic used by the German Army and Luftwaffe. Herivel began thinking about what happened when an operator prepared an Enigma machine at the start of the day.
German procedures were intended to prevent predictable settings. Herivel reasoned that a quick operator might take a shortcut after setting the machine’s rings, leaving the rotors at or near the same positions instead of moving them far enough. If several operators behaved in this way, their first transmitted indicators could cluster around the day’s ring settings.
The idea became known as the Herivel tip, or Herivelismus. Hut 6 began looking for the pattern, but for weeks it produced nothing useful.
That changed in May 1940. A change in German procedures made another British method ineffective. Soon afterwards, as German forces attacked in western Europe, the clustering Herivel had predicted began to appear. It reduced sharply the number of possible settings that needed to be tested. It helped the codebreakers recover Luftwaffe Enigma traffic.
The Herivel tip was especially useful during the months before machine-assisted methods became more widely available. It did not break Enigma by itself, but it gave Hut 6 a practical route into important traffic when other methods had stopped working.
After the war, Herivel returned to Belfast and later joined Queen’s University Belfast, where his work moved towards the history and philosophy of science. He wrote on scientists including Isaac Newton and Joseph Fourier. Decades after the secrecy surrounding Bletchley Park began to lift, he published his own account of Herivelismus and the German military Enigma.