New archive research has revealed how intercepted German high command messages decoded by the Colossus codebreaking machine at Bletchley Park confirmed that the deception operation had successfully fooled the enemy about the Allies landing plans and provided many more insights that would be vital for the success of the Allied invasion.

Colossus Mk I became operational at Bletchley on 5 February 1944 and was so successful in tackling Lorenz-encrypted messages between Hitler and his generals that more were ordered. A deadline for another was set for 1 June, although Tommy Flowers and his team who built the machines would not then have known that this may have been to coincide with D-Day preparations. Colossus Mk II arrived in May and was in partial operation by 1 June 1944.

During the war, the Wrens who operated Colossus had little idea of the importance of their work.  For decades afterwards, secrecy bound them not to speak of what they did at Bletchley. Only in recent years have they come to realise the impact of their work and even today, now in their 90s, they know few of the details.

Speakers at this week’s 75^th anniversary event at the National Museum of Computing, which houses the reconstructed Colossus machine at Bletchley, spoke of  the pressures codebreakers worked under in the run-up to D-Day and the difficult choices they had to make in deciding what Lorenz messages to tackle. Of the 2000 or so messages intercepted per month, only 10 per cent could be decoded so they had to carefully prioritise different streams of traffic from Europe at different times.

Research in the National Archives at Kew has unearthed some of the key pieces of intelligence that Colossus and the codebreakers were able to uncover and that gave the architects of D-Day the confidence to know that the enemy had swallowed the vast campaign of deception over the place, timing and scale of the Allied landings, the deception codenamed operation Fortitude.

Decoded messages also revealed German plans for invasion defences, including the anti-tank obstacles of reinforced concrete the Germans called Rommel’s Teeth and the 6.5 million mines the Germans had laid by June 1944. There were important bombing impact assessments and the locations of the most effective German forces. Even messages about the transfer of less experienced officers away from France to southern Europe would be useful in planning methods of attack, knowing more about the less experienced officers commanding enemy forces that they would be likely to encounter. Allied landing forces were anxious wherever possible to avoid running into the feared German Panzer divisions but couldn’t know the locations of the units stationed in France until German general Heinz Guderian decided to tour all the units in May 1944 and file reports in messages that were cracked at Bletchley to reveal the locations of the all the Panzer units there.

Most of those working at Bletchley Park during the war had no or little idea of the enormity of what they were doing at the time. Each was given only the little information they had to on a ‘need to know’ basis and virtually nothing was written down for security reasons. One of the veteran WREN operators, Irene Dixon, said they only knew their own hut at the time but she remembered once a senior colleague giving her a peak into another hut where she felt something momentous was happening and very soon after discovered that was probably the D-Day signals. With today’s shortage of cybersecurity skills, she said it is still important to “encourage young people into working in this domain to find out what the enemy is all up to”.

German teleprinter signals encrypted by Lorenz machines were first heard in Britain by police officers on the south coast listening for possible spy transmissions in 1940. In August 1941, a procedural error by a German operator enabled Colonel John Tiltman, a top codebreaker at Bletchley Park, to decipher a message.

Brilliant mathematician Bill Tutte  began working on the case and was able to deduce the complete logical structure of the cipher machine we now know as Lorenz. Codebreakers in the Testery, under Colonel Tester, began breaking the codes by hand, but this was very time consuming. The head of the Newmanry, Max Newman, a mathematician at Bletchley Park, believed certain aspects of the decryption process could be automated. Initially, electromechanical Robinson machines (named after the cartoonist Heath Robinson) were used to find the start positions of the Lorenz wheels to speed up codebreaking. But the Robinson machines had shortcomings.

Tommy Flowers, an ingenious Post Office electronics engineer at Dollis Hill, was asked to improve upon the Robinsons but instead designed Colossus, the world’s first electronic computer, which enabled the Lorenz start-wheel positions to be found in a few hours. This greatly shortened the codebreaking process and enabled larger numbers of messages to be broken.

Colossus Mark I began operating on 5 February 1944, and was supplemented in June of that year by the Mark II. By the end of the war there were ten functioning Colossi working around the clock helping to decipher the messages of German High Command.

The deciphered messages provided the Allies with crucial intelligence on what enemy armed forces were plotting. They knew for example that Hitler had swallowed the bait that the D-Day landings in June 1944 would be at Calais rather than Normandy. This gave the Allies a decisive advantage.

As a direct result of Colossus, the war was shortened and countless lives saved. By the end of the war, 63 million characters of high-grade German messages had been decrypted by the 550 people working on the Colossi at Bletchley Park.

At the Museum, housed in Block H where six of the ten Colossi were operational at the end of the war, and became the world’s first purpose-built computer centre, original German Lorenz encryption devices are on display alongside working reconstructions of the British technology that intercepted and deciphered the messages: Colossus, Heath Robinson (the predecessor to Colossus) and the Tunny machine. The Bombe reconstruction showed how the key of the day for D-Day was discovered to reveal the Enigma-encrypted messages about German operations on that crucial day of 6 June 1944.

The National Museum of Computing, located on Bletchley Park in Block H, one of England’s ‘irreplaceable places’, is an independent charity housing the world’s largest collection of functional historic computers, including reconstructions of the wartime code-breaking Colossus and the Bombe, and the WITCH, the world’s oldest working digital computer. The Museum enables visitors to follow the development of computing from the ultra-secret pioneering efforts of the 1940s through the large systems and mainframes of the 1950s, ’60s and ’70s, and the rise of personal computing in the 1980s and beyond.

New archive research has revealed how intercepted German high command messages decoded by the Colossus codebreaking machine at Bletchley Park confirmed that the deception operation had successfully fooled the enemy about the Allies landing plans and provided many more insights that would be vital for the success of the Allied invasion.

Colossus Mk I became operational at Bletchley on 5 February 1944 and was so successful in tackling Lorenz-encrypted messages between Hitler and his generals that more were ordered. A deadline for another was set for 1 June, although Tommy Flowers and his team who built the machines would not then have known that this may have been to coincide with D-Day preparations. Colossus Mk II arrived in May and was in partial operation by 1 June 1944.

During the war, the Wrens who operated Colossus had little idea of the importance of their work.  For decades afterwards, secrecy bound them not to speak of what they did at Bletchley. Only in recent years have they come to realise the impact of their work and even today, now in their 90s, they know few of the details.

Speakers at this week’s 75^th anniversary event at the National Museum of Computing, which houses the reconstructed Colossus machine at Bletchley, spoke of  the pressures codebreakers worked under in the run-up to D-Day and the difficult choices they had to make in deciding what Lorenz messages to tackle. Of the 2000 or so messages intercepted per month, only 10 per cent could be decoded so they had to carefully prioritise different streams of traffic from Europe at different times.

Research in the National Archives at Kew has unearthed some of the key pieces of intelligence that Colossus and the codebreakers were able to uncover and that gave the architects of D-Day the confidence to know that the enemy had swallowed the vast campaign of deception over the place, timing and scale of the Allied landings, the deception codenamed operation Fortitude.

Decoded messages also revealed German plans for invasion defences, including the anti-tank obstacles of reinforced concrete the Germans called Rommel’s Teeth and the 6.5 million mines the Germans had laid by June 1944. There were important bombing impact assessments and the locations of the most effective German forces. Even messages about the transfer of less experienced officers away from France to southern Europe would be useful in planning methods of attack, knowing more about the less experienced officers commanding enemy forces that they would be likely to encounter. Allied landing forces were anxious wherever possible to avoid running into the feared German Panzer divisions but couldn’t know the locations of the units stationed in France until German general Heinz Guderian decided to tour all the units in May 1944 and file reports in messages that were cracked at Bletchley to reveal the locations of the all the Panzer units there.

Most of those working at Bletchley Park during the war had no or little idea of the enormity of what they were doing at the time. Each was given only the little information they had to on a ‘need to know’ basis and virtually nothing was written down for security reasons. One of the veteran WREN operators, Irene Dixon, said they only knew their own hut at the time but she remembered once a senior colleague giving her a peak into another hut where she felt something momentous was happening and very soon after discovered that was probably the D-Day signals. With today’s shortage of cybersecurity skills, she said it is still important to “encourage young people into working in this domain to find out what the enemy is all up to”.

German teleprinter signals encrypted by Lorenz machines were first heard in Britain by police officers on the south coast listening for possible spy transmissions in 1940. In August 1941, a procedural error by a German operator enabled Colonel John Tiltman, a top codebreaker at Bletchley Park, to decipher a message.

Brilliant mathematician Bill Tutte  began working on the case and was able to deduce the complete logical structure of the cipher machine we now know as Lorenz. Codebreakers in the Testery, under Colonel Tester, began breaking the codes by hand, but this was very time consuming. The head of the Newmanry, Max Newman, a mathematician at Bletchley Park, believed certain aspects of the decryption process could be automated. Initially, electromechanical Robinson machines (named after the cartoonist Heath Robinson) were used to find the start positions of the Lorenz wheels to speed up codebreaking. But the Robinson machines had shortcomings.

Tommy Flowers, an ingenious Post Office electronics engineer at Dollis Hill, was asked to improve upon the Robinsons but instead designed Colossus, the world’s first electronic computer, which enabled the Lorenz start-wheel positions to be found in a few hours. This greatly shortened the codebreaking process and enabled larger numbers of messages to be broken.

Colossus Mark I began operating on 5 February 1944, and was supplemented in June of that year by the Mark II. By the end of the war there were ten functioning Colossi working around the clock helping to decipher the messages of German High Command.

The deciphered messages provided the Allies with crucial intelligence on what enemy armed forces were plotting. They knew for example that Hitler had swallowed the bait that the D-Day landings in June 1944 would be at Calais rather than Normandy. This gave the Allies a decisive advantage.

As a direct result of Colossus, the war was shortened and countless lives saved. By the end of the war, 63 million characters of high-grade German messages had been decrypted by the 550 people working on the Colossi at Bletchley Park.

At the Museum, housed in Block H where six of the ten Colossi were operational at the end of the war, and became the world’s first purpose-built computer centre, original German Lorenz encryption devices are on display alongside working reconstructions of the British technology that intercepted and deciphered the messages: Colossus, Heath Robinson (the predecessor to Colossus) and the Tunny machine. The Bombe reconstruction showed how the key of the day for D-Day was discovered to reveal the Enigma-encrypted messages about German operations on that crucial day of 6 June 1944.

The National Museum of Computing, located on Bletchley Park in Block H, one of England’s ‘irreplaceable places’, is an independent charity housing the world’s largest collection of functional historic computers, including reconstructions of the wartime code-breaking Colossus and the Bombe, and the WITCH, the world’s oldest working digital computer. The Museum enables visitors to follow the development of computing from the ultra-secret pioneering efforts of the 1940s through the large systems and mainframes of the 1950s, ’60s and ’70s, and the rise of personal computing in the 1980s and beyond.