Herivelismus

In 1940, a young promising codebreaker named John Herivel was put to work at Bletchley Park. He’d been recruited by Gordon Welchman and trained by Alan Turing. His task was to assist with breaking the ‘Red’ Enigma cypher, used by the German army and Luftwaffe. Things hadn’t been going so well. A key factor was the method the German forces used to set up the Enigma machines in use. Although they had daily settings for Enigma, they were required to use the three rotor settings to send a further three settings, picked by the operator at random. So, if the day’s settings were ABC, the machine would be set up this way and then used to encrypt a further three letters, say, HSN. The receiving operator would set up their machine the same way, then enter the encrypted three letters to discover the rotor settings for the rest of the message.

If the codebreakers knew what the second group of letters may be, it would drastically simplify the process of breaking the day’s settings. John Herivel decided to place himself in the mind of a German operator. You’re in the field, under gunfire, it’s cold, wet and dark (as the first messages went out at midnight, when the day’s key settings changed). Would you waste time choosing a good random sequence? Maybe cypher security is not foremost on your mind. After all, you’re just an operator, not a cryptanalyst. Herivel concluded that the operator would probably just move the rotors a few positions in either direction of the start settings and that precious time could be saved by checking these first.

He was right. It was exactly what the in-field operators were doing. Many just moved the rotors one notch (or sometime not at all) to create the so-called random settings.

The technique was so effective that it was adopted and given one of the Bletchley’s much-loved ‘names’: Herivelismus (also known as The Herivel Tip). John had a long and distinguished career at Bletchley Park, later on working on the ‘Fish’ cyphers including Colossus.

I had the great honour of meeting John Herivel in October last year at the annual Enigma Reunion. It was a humbling experience. So, it is with great sadness that I heard that he had passed away last week aged an impressive 92. This was a man who, through a feat of lateral thinking, saved time and therefore saved lives. We all owe his memory a debt of gratitude.

Attached are two pictures of John, one from his ID card and another taken by Gordon Tant at the very reunion where I met him.

 

PJ