John Herivel - Exploiting The Tip

Exploiting The Tip

The next day, Herivel's colleagues agreed that his idea was a possible way into Enigma. Hut 6 began looking for the effect predicted by the Herivel tip, and arranged to have the first messages of the day from each transmitting station to be sent to them early. They plotted the indicators in a grid termed a "Herivel square", an example of which is shown below. The rows and columns of the grid are labelled with the alphabet. Each ground setting received would be entered into the grid by finding the column corresponding to the first letter, the row corresponding to the second letter, and entering the third letter into the cell where the row and column intersected. For example, GKX would be recorded by entering a X in the cell in column G and row K.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ---------------------------------------------------------- Z| |Z Y| S |Y X| |X W| L |W V| |V U| E |U T| |T S| |S R| K |R Q| S |Q P| |P O| |O N| N |N M| X |M L| W T |L K| X Y |K J| W X |J I| |I H| Q |H G| |G F| |F E| A |E D| |D C| V |C B| J |B A| P |A ---------------------------------------------------------- A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

The Herivel tip suggested that there would be a cluster of entries close together, such as the cluster around GKX in the above example. This would narrow the options for the ring settings down from 17,576 (263) to a small set of possibilities, perhaps 6–30, which could be tested individually.

The effect predicted by Herivel was not immediately apparent in the Enigma traffic, however, and Bletchley Park relied chiefly on a different technique to get into Enigma: the method of "perforated sheets", which had been passed on by Polish cryptologists. The situation changed on 1 May 1940, when the Germans changed their indicating procedure, rendering the perforated sheets obsolete. Hut 6 was suddenly unable to decrypt Enigma.

Fortunately for the codebreakers, the pattern predicted by the Herivel tip began to manifest itself. David Rees spotted a cluster in the indicators, and on 22 May an Air Force message sent on 20 May was decoded, the first since the change in procedure. The Herivel tip was used in combination with another class of operator mistake, known as "cillies", in order to solve the settings and decipher the messages. This method was used for several months until specialised codebreaking machines designed by Alan Turing, the so-called "bombes", were ready for use.

Gordon Welchman speculates that the Herivel tip was a vital part of breaking Enigma at Bletchley Park, writing, "If Herivel had not been recruited in January 1940, who would have thought of the Herivel tip, without which we whould have been defeated in May 1940 — unable to maintain continuity until the bombes began to arrive many months later? Let there be no misconceptions about this last point. Loss of continuity would, at all stages, have been very serious, if not disastrous."

Because of the importance of his contribution, Herivel was singled out and introduced to Winston Churchill during a visit to Bletchley Park. He also taught Enigma cryptanalysis to a party of Americans assigned to Hut 6 in an intensive two-week course. Herivel later worked in administration in the "Newmanry", the section responsible for solving German teleprinter ciphers using machine methods such as the Colossus computers, as assistant to the head of the section, mathematician Max Newman.

In 2005, researchers studying a set of Enigma-encrypted messages from World War II noted the occurrence of the Herivel tip in messages from August 1941.