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an hour or two later, in a standard three-rotor Enigma weather cipher—a cipher Bletchley could break—for the use of every German vessel.
    It was the back door into Shark.
    First, you read the weather report. Then you put the weather report back into the short weather cipher. And what you were left with, by a process of logical deduction, was the text that had been fed into the four-rotor Enigma a few hours earlier. It was a perfect crib. A cryptanalyst's dream.
    But still they couldn't break it.
    Every day the code-breakers, Jericho among them, fed their possible solutions into the bombes—immense electro-mechanical computers, each the size of a walk-in wardrobe, which made a noise like a knitting machine—and waited to be told which guess was correct. And every day they received no answer. The task was simply too great. Even a message enciphered on a three-rotor enigma might take twenty-four hours to decode, as the bombes clattered their way through the billions of permutations. A four-rotor Enigma, multiplying the numbers by a factor of twenty-six, would theoretically take the best part of a month.
    For three weeks Jericho worked round the clock, and when he did grab an hour or two's sleep it was only to dream fitfully of drowning men. 'Let's just hope they were dead before they hit the bottom. . . 'His brain was beyond tiredness. It ached physically, like an overworked muscle. He began to suffer blackouts. These only lasted a matter of seconds but they were frightening enough. One moment he might be working in the Hut, bent over his slide-rule, and the next everything around him had blurred and jumped on, as if a film had slipped its sprockets in a projector. He managed to beg some Benzedrine off the camp doctor but that only made his mood swings worse, his frenzied highs followed by increasingly protracted lows.
    Curiously enough, the solution, when it came, had nothing to do with mathematics, and afterwards he was to reproach himself furiously for becoming too immersed in detail. If he had not been so tired, he might have stepped back and seen it earlier.
    It was a Saturday night, the second Saturday in December. At about nine o'clock Logie had sent him home. Jericho had tried to argue, but Logie had said: 'No, you're going to kill yourself if you go on at this rate, and that won't be any use to anyone, old love, especially you.' So Jericho had cycled wearily back to his digs above the pub in Shenley Church End and had crawled beneath the bedclothes. He heard last orders called downstairs, listened as the final few regulars departed and the bar was closed up. In the dead hours after midnight he lay looking at the ceiling wondering if he would ever sleep again, his mind churning like a piece of machinery he couldn't switch off.
    It had been obvious from the moment Shark had first surfaced that the only acceptable, long-term solution was to redesign the bombes to take account of the fourth rotor. But that was proving a nightmarishly slow process. If only they could somehow complete the mission Fasson and Grazier had begun so heroically and steal a Shark Enigma. That would make the redesign easier. But Shark Enigmas were the crown jewels of the German Navy. Only the U-boats had them. Only the U-boats and, of course, U-boat communication headquarters in Sainte-Assise, southeast of Paris.
    A commando raid on Sainte-Assise, perhaps? A parachute drop? He played with the image for a moment and then dismissed it. Impossible. And, in any case, useless. Even if, by some miracle, they got away with a machine, the Germans would know about it, and switch to a different system of communications. Bletchley's future rested on the Germans continuing to believe that Enigma was impregnable. Nothing could ever be done which might jeopardise that confidence. Wait a minute. Jericho sat upright. Wait a bloody minute.
    If only the U-boats and their controllers in Saint-Assise were allowed to have four-rotor Enigmas—and Bletchley knew for