Misplaced trust in signal security

In the summer of 1943, as Rebecca Ratcliff reveals in this fine study of Second World War code breaking, German counterintelligence "discovered the biggest secret of the war". That August, the German U-boat command logged a warning from an agent inside the US Navy: the Americans, the agent said, were reading secret signals encrypted by German Enigma machines.

Little action was taken. Only in 1974, when the British Government finally announced that Enigma had indeed been broken, did Germany's wartime code makers accept that their supposedly unbreakable codes had been compromised.

Today, the war-winning role of Ultra, the intelligence derived from Enigma signals intercepted and decrypted by the Allies, and the astonishing achievements of the men and women involved in that success are well known and deservedly so. Less well known is the other side of the signals war and, more especially, how the Germans lost that struggle through oversight, complacency and excessive faith in their ciphers and systems.

Germany was not without success at reading Allied signals. It is possible that the Germans came to read up to 70 per cent of Soviet Air Force messages. Decrypts gave them forewarning of British landings in Norway in 1940 and at Dieppe two years later. Between 1940 and 1943, decrypts also revealed the location of up to 60 per cent of Allied North Atlantic convoys, making the merciless role of U-boat wolf packs that bit easier.

Yet the German code-breaking effort was inefficient, ill-focused and badly supported. With the emphasis placed on tactical needs, different service arms had different intelligence divisions but the degree of collaboration between them was poor. "One division might spend years attempting to break a system currently being read by another." And continuing deficiencies in "financial, human and technological resources" led to competition and "a vicious circle": "Fewer resources meant diminished intelligence results, which lowered the standing of the intelligence departments and staff." Had intelligence been given a higher priority, these problems might have been solved. As it was, "the greatest limitation on Germany's military signals intelligence was its perceived lack of importance".

Significant, too, Ratcliff observes, was the Germans' "unshakeable belief"

that Allied machines such as the sophisticated American Sigaba could not be broken, which meant that little effort was put into attacking them. "Hence, they had no luck breaking British and American high-grade cipher machines."

Such strength of belief was matched by the Germans' misplaced faith in the security of Enigma. When Ultra helped the British to find and sink the Bismarck , the Germans blamed loose talk and secret agents. When Ultra allowed convoys to be rerouted round the lurking wolf packs, the Germans fell back on their entrenched belief in the high quality of British detection devices, such as radar. They did so again as U-boat losses rose.

Underpinning all this was the Germans' steady conviction in the impenetrability of their ciphers: "German cryptanalysts trusted Enigma and believed it could not be broken."

The Allies, on the other hand, gave top priority to the task of cracking the enemy's codes, believing from the start that it could be done.

Encouragement and overriding support were forthcoming from the highest levels. Care was taken to recruit those thought best suited to the job: forward thinkers and problem-solvers; chess players and classicists; individuals with "the ability to recognise, categorise, and contextualise unfamiliar material".

Extraordinary care was taken to protect the central secret. Those who knew it were made well aware of its significance: "Telling them the truth made them realise the tremendous damage that a security lapse could cause." The success of the British and Americans in breaking Enigma also spurred them on to focus more closely on the security of their own secret systems. Great investment was made in developing ever more advanced technology. Specialist teams were even set up with the task of attacking their own side's ciphers and signals, monitoring traffic for flaws and errors and initiating action when necessary.

Drawing on a mass of recently declassified material, Ratcliff, a former scholar-in-residence at the National Security Agency, has produced a groundbreaking analysis that underlines the Allies' achievement in reading the enemy's ciphers and illuminates, for the first time, the long hidden story of the Germans' failings. Delusions of Intelligence is well written and accessible and is indispensable to any student of wartime intelligence. For the general reader, it is an excellent introduction to the topic of wartime code breaking.

Roderick Bailey is a historian attached to the Imperial War Museum, London, specialising in the study of the Special Operations Executive.