published 31/03/2014 at 18:20 BST by Michael Smith
Top codebreaker at Bletchley Park who helped to decipher Hitler's messages
Jerry Roberts in 2009. Photograph: Martyn Goddard/Rex
Jerry Roberts, who has died aged 93, was one of the leading Bletchley Park codebreakers working on the Fish
teleprinter messages sent between Hitler and his generals which revealed that the Germans had been fooled by British deception, so ensuring the success of the D-day landings. Roberts worked in the Testery at the Buckinghamshire estate, where communications between Field Marshal Gerd von Rundstedt, the German commander in France, and Hitler were deciphered. The codebreakers named the teleprinter ciphers Fish, and the link between Von Rundstedt and Hitler was known as Jellyfish.
"Many messages were signed by top field marshals like Von Rundstedt," Roberts wrote in Colossus: Bletchley Park's Greatest Secret (2006). "Occasionally there were messages signed by Hitler himself. I can remember deciphering at least one message − he called himself: 'Adolf Hitler, Führer'."
MI5 had captured a number of German spies sent to Britain and was using them to feed fake intelligence to the Abwehr,
German military intelligence, in an operation known as the Double Cross system. The British used them to build up a completely false picture of allied plans ahead of D-day to give the impression
that the invasion force would land on the Pas de Calais rather than in Normandy.
Although the Bletchley Park codebreakers had broken the Abwehr Enigma machine, itself an astonishing feat, and knew that German intelligence believed the fake messages, they could not
predict how Hitler would react to the D-day deception. The Jellyfish messages deciphered in the Testery not only
revealed that Hitler believed the invasion would be in the Pas de Calais but that even as the allies were landing
in Normandy he overruled Von Rundstedt's plans to reinforce his positions there at the expense
of those around Calais. Hitler's decision to order two key armoured divisions back to Calais was revealed in the
messages deciphered by Roberts and his Testery colleagues; this depleting of German troops is widely seen as
having helped to ensure that allied forces were not driven back into the sea.
Jerry was born in Wembley, north-west London, to a pharmacist father and a mother who played the organ in the
local chapel. He was educated at Latymer upper school, Hammersmith, before studying modern languages at University College London. Shortly before he graduated in 1941, he was recommended to
Bletchley Park as a German linguist of ability by Professor Leonard Willoughby, who had himself been a codebreaker during the first world war. As a result, he was recruited, initially as a civilian, into the Bletchley Park research
section which dealt with ciphers no one else was working on.
His first assignment, under Ralph Tester, who had been recruited from the BBC monitoring service, was deciphering the messages sent back to Berlin by German police troops operating on the eastern
front. These messages revealed the killing of many thousands of Jews in Belorussia (now Belarus), Ukraine and the Baltic republics in what is recognised as the early stages of the Holocaust. The
messages led Winston Churchill to denounce the killings publicly as "a crime without a name", despite the
risk of revealing the codebreakers' work.
In July 1942, Tester and his five codebreakers at the Testery, including Roberts, were put to work on the
highest-grade cipher ever to be broken at Bletchley. The Lorenz SZ40 machine used to encipher messages on the teleprinter links between Berlin and all the major German military fronts had two
sets of five cipher rotors. Even the most complex Enigma machines had only four. The Lorenz SZ40 messages were unlocked
by two spectacular pieces of codebreaking. First, in October 1941, Bletchley's chief cryptographer, John Tiltman,
broke into one of the teleprinter links when a German operator re-sent one message, but with abbreviations, to save time. The difference between this and the original gave Tiltman a way into the
code. With this knowledge of the enciphering, Bill Tutte, then a young Cambridge chemistry graduate, was able to reconstruct the entire workings of the Lorenz SZ40 machine.
One of the new recruits to the Testery was Max Newman, who had been Alan Turing's tutor at Cambridge. He realised that the process of breaking the Fish ciphers could be speeded up by a
computation machine of the kind conceived by Turing in his work at Cambridge. The result was Colossus, the world's first large-scale electronic digital computer, built by a team led by GPO
telecommunications engineer Tommy Flowers. By the end of the war the Testery team was 118-strong and included Roy Jenkins, later a Labour chancellor, and Peter Benenson, the Amnesty International
founder. Other important messages they intercepted included the German plans ahead of the 1943 Battle of Kursk.
"I can remember myself breaking messages about Kursk," Roberts recalled in an interview. "We were able to warn
the Russians that the attack was going to be launched and the fact that it was going to be a pincer movement. We were able to warn them what army groups were going to be used and most important,
what tank units were going to be used."
The German plans were passed officially to Moscow disguised as reports from British spies. They were also given to Soviet intelligence by John Cairncross, a member of the Cambridge spy ring, who
was then working in Hut 3, which wrote the intelligence reports on the Fish intercepts. Stalin rarely believed
intelligence and particularly that supplied by his western allies, but the confirmation from two sources ensured it was believed, allowing the Red Army to defeat the Germans in what was one of
the war's major turning points. After 1945 Roberts was sent to Germany to investigate war crimes. He then built a highly successful career in market research, working for companies that included British
Gas, American Airlines and Chrysler.
He retired in the early 1990s after meeting his third wife, Mei Li, an artist and book illustrator. They married in 1995.
Recently Roberts campaigned for greater recognition of the groundbreaking work of both Tutte and Flowers. He was
appointed MBE in 2013.
He is survived by Mei Li, and by two daughters and a son from his previous marriages.
Jerry (Raymond Clarke) Roberts, codebreaker, born 18 November 1920; died 25 March 2014
This article was amended on 1 April 2014. The original stated that Roberts was survived by a daughter from his
third marriage. This has been corrected.