Everyone knows the story of Enigma and secret codebreaking in the Second World War: the triumph of Bletchley Park over world-class cipher technology. Except that excellence in codebreaking was nearly betrayed by incompetence in codemaking.

German codebreakers were effective and Allied codes and ciphers were weak. With both sides reading each other’s codes, the biggest secret of all – that the codes had been broken – was now at risk. Sooner or later, on one side or the other, the cipher failures would become known, the systems would be changed and the most valuable source of intelligence would dry up.

Were it not for obstinacy, overconfidence and ostrichism. On both sides. The Germans demanded that the traitors be rooted out; the British stifled cipher questions beneath a tangle of committees. The codebreakers’ contest became a struggle to lose the cipher war.

From the very outset, the Enigma secret was one of treachery, betrayal and deception. This is the story of the people who fought behind the scenes for cipher security – and of the Enigma traitors.