John Herivel

John William Jamieson Herivel (29 August 1918 – 18 January 2011) was a British science historian and World War II codebreaker at Bletchley Park. As a codebreaker concerned with Cryptanalysis of the Enigma, Herivel is remembered chiefly for the discovery of what was soon dubbed the Herivel tip or Herivelismus. Herivelismus consisted of the idea, the Herivel tip and the method of establishing whether it applied using the Herivel square. It was based on Herivel's insight into the habits of German operators of the Enigma cipher machine that allowed Bletchley Park to easily deduce part of the daily key. For a brief but critical period after May 1940, the Herivel tip in conjunction with "cillies" (another class of operator error) was the main technique used to solve Enigma. After the war, Herivel became an academic, studying the history and philosophy of science at Queen's University Belfast, particularly Isaac Newton, Joseph Fourier, Christiaan Huygens. In 1956, he took a brief leave of absence from Queen's to work as a scholar at the Dublin Institute for Advanced Studies. In retirement, he wrote an autobiographical account of his work at Bletchley Park entitled Herivelismus and the German Military Enigma.

Recruitment to Bletchley Park John Herivel was born in Belfast, and attended Methodist College Belfast from 1924 to 1936. In 1937 he was awarded a Kitchener Scholarship to study mathematics at Sidney Sussex College, Cambridge, where his supervisor was Gordon Welchman. Welchman recruited Herivel to the Government Code and Cypher School at Bletchley Park. Welchman worked with Alan Turing in the newly formed Hut 6 section created to solve Army and Air Force Enigma. Herivel, then aged 21, arrived at Bletchley on 29 January 1940, and was briefed on Enigma by Turing and Tony Kendrick.

Enigma

At the time that Herivel started work at Bletchley Park, Hut 6 was having only limited success with Enigma-enciphered messages, mostly from the Luftwaffe Enigma network known as "Red". He was working alongside David Rees, another Cambridge mathematician recruited by Welchman, in nearby Elmers School, testing candidate solutions and working out plugboard settings. The process was slow, however, Herivel was determined to find a method to improve their attack, and he would spend his evenings trying to think up ways to do so. Intercepted Morse coded messages had been enciphered by the Germans' Enigma, an electro-mechanical rotor cipher machine that implemented a polyalphabetic cipher. The main model in use in 1940 had three rotors that set an electrical pathway from the keyboard to the lampboard. Pressing a key caused one lamp to light and the right-most rotor to advance by one letter position. This changed the electrical pathway so that pressing the same key again caused a different letter to light up. At one of the 26 positions, a notch on the right-most rotor engaged with the middle rotor so that the two rotors advanced together, and similarly the middle rotor would engage with the left-most rotor, giving a very long period before the sequence repeated (26 × 26 × 26 = 17,576). The ring on the rotor that contained the notch and so caused the next rotor to advance, could be set to any one of the 26 positions. The three rotors were selected from a set of five, giving 60 different ways of mounting rotors in the machine. However, because the Germans laid down the rule that no rotor should be in the same position on successive days, if the previous days's rotors and their positions were known, this number was reduced to 32. The Enigma machine worked reciprocally so that an identical machine with identical settings would, if fed the enciphered letters, show the deciphered letters on the lampboard. Hut 6 had Enigma replica machines that were logically identical to the machines that the Germans were using. To decipher the intercepted messages required that the selection of rotors, the ring settings and the plugboard connections were known. At this time, the first three letters of the prelude to the message were used as an indicator to tell the receiving operator the letters that should appear in the windows for this particular message.

Herivel tip

Herivel had an insight in February 1940 that some lazy German code clerks might give away the Enigma's ring settings (Ringstellung) in their first message of the day. If there were several lazy clerks, the first message Grundstellungs would not be random but would have a clustering around the Ringstellung. The insight became known as the Herivel tip. It was not needed at the time because the Luftwaffe was doubly-enciphering their message keys so techniques such as Zygalski sheets could be used. In May 1940, the Germans stopped the doubly-enciphered keys. Other methods becoming ineffective, Bletchley Park started using the Herivel tip to break Luftwaffe traffic. It continued to be the main method until the bombe was delivered in August 1940.

Enigma enciphering procedure The rotors and the positioning of the ring containing the notch were changed daily. The settings were defined in a codebook that was common to all operators on that network. At the start of each day, before any messages were sent or received, Enigma operators implemented the day's rotor selection and ring settings. Having selected the three rotors, they adjusted the ring settings. That could be done before the rotors were mounted on their axle or after they had been inserted into the machine. It was possible to adjust the ring settings of the loaded rotors by moving the spring-loaded retaining pin to the right and turning the rotor to display the specified letter. Herivel thought it likely that at least some of the operators would adjust the rings after they had mounted the rotors in the machine. Having set the alphabet rings and closed the lid, the operator should then have moved the rotors well away from the positions that displayed the three letters of the ring setting in the windows, but some operators did not.

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