Roamer: Young Ulsterman helped break Enigma code during WWII

A recent note in Roamer’s mailbox from local historian and author George Busby described Herivel as “the unsung hero of World War Two.” While writing his book ‘The Spies at Gilnahirk’, about a top-secret WWII wireless station in Belfast’s Castlereagh Hills, George “became aware of John Herivel” who, according to prominent Bletchley Park codebreaker Gordon Welchman “saved the UK from defeat in May 1940.”

George outlined Herivel’s role in the extremely complicated (and often very confusing) Bletchley Park codebreaking process. Formerly the Dean of Sydney Sussex College, Cambridge, Welchman led Bletchley Park’s secret decryption centre and established the famous Hut 6.

Whilst other teams translated, analysed and dispatched decoded messages, Welchman’s élite team, including John Herivel, focused on breaking Germany’s Enigma code.

A student in Methodist College Belfast, John won a scholarship in 1936 to Sydney Sussex College where he met Gordon Welchman. Prior to recruiting Herivel onto his team, Welchman had been analysing the ‘preambles’ of German signals.

Sent in plain text at the beginning of the message, the ‘preamble’ contained details of the sender, the receiver, and the receiver’s German unit. Welchman discerned relationships between units, like a genealogy revealing the command-and-control structure of a network, from the top down.

As he studied the flow of signals traffic, Welchman could, without breaking the encryption of a message, predict what was about to happen. This became known as ‘Traffic Analysis.’

It’s important to understand that the German enigma codes were changed at midnight every night - so today’s successes were defunct tomorrow. Also, the Enigma machine could generate over one hundred and fifty million million (17 digits!) variations of encryption.

To break the Enigma code a ‘crib’ was needed - a word or a phrase within a message. With a correct ‘crib’ the codebreaker gained a way into the code of that day, allowing him or her to read a percentage of the German unit’s signals traffic.

Some of these ‘cribs’ were known as ‘Cillies’ - words like ‘Heil Hitler’ or ‘Berlin’ at the end of a message - and some operators sent the same message day and daily, like ‘Nothing to Report.’ A further technique called ‘gardening’ was used to ‘plant’ words in a message; if an aeroplane flew past the German unit the operator might include the word ‘aircraft’ in his message.

As John Herivel studied these ‘Cillies’ he realised that some German operators failed to follow proper procedures when sending their second, third and fourth message of the day.

John had discovered the so-called ‘Herivel Tip’ - a method of breaking the Enigma. Coincidentally, Hitler invaded France, Belgium, and the low countries on the 10 May 1940.

That same day, Winston Churchill became the UK’s Prime Minister and that same evening John Herivel arrived at Hut 6 for his night watch and was told “the Herivel Tip is laying the golden eggs” - his codebreaking formulae was working!

Churchill paid a surprise visit to Bletchley Park in September 1941 and entering Hut 6, Gordon Welchman told the Prime Minister: “I would like to introduce John Herivel, who broke the Enigma codes last year.”

John recalled this meeting with Churchill years later, “We looked silently at each other for a moment or two before he moved on with his entourage.”

Afterwards Churchill spoke “with deep emotion” to an outdoor gathering of Bletchley staff - “I want to thank you all for what you have done for the war effort.”

“That was our finest hour at Bletchley Park” John recalled.

Gordon Welchman published his book ‘The Hut Six Story’ in 1982 where he wrote “If John Herivel had not been recruited in January 1940 we would have been defeated in May 1940.” On the morning of the 11 November 2022, at Methodist College, the Ulster History Circle unveiled a Blue Paque to John Herivel, Mathematician and Bletchley Code Breaker - final recognition after 82 years for snatching victory from the jaws of certain defeat.