Enigma

Hut 8 had been able to break most U-boat transmissions within a day of interception, allowing ample time to re-route convoys around the wolf packs of German submarines. But in the ten months after the introduction of Shark they read the traffic on just three occasions, and even then it took them seventeen days each time, so that the intelligence, when it did arrive, was virtually useless, was ancient history.
    To encourage them in their labours a graph was posted in the code-breakers' hut, showing the monthly tonnages of shipping sunk by the U-boats in the North Atlantic. In January, before the blackout, the Germans destroyed forty-eight Allied ships. In February they sank seventy-three. In March, ninety-five. In May, one hundred and twenty . . .
    'The weight of our failure,' said Skynner, the head of the Naval Section, in one of his portentous weekly addresses, 'is measured in the bodies of drowned men.'
    In September, ninety-five ships were sunk. In November, ninety-three . . .
    And then came Fasson and Grazier.
    Somewhere in the distance the college clock began to toll. Jericho found himself counting the chimes.
    'Are you all right, old thing? You've gone terribly silent.'
    'Sorry. I was just thinking. Do you remember Fasson and Grazier?'
    'Fasson and who? Sorry, I don't think I ever met them.'
    'No. Nor did I. None of us did.'
    Fasson and Grazier. He never knew their Christian names. A first lieutenant and an able-bodied seaman. Their destroyer had helped trap a U-boat, the U-459, in the eastern Mediterranean. They had depth-charged her and forced her to the surface. It was about ten o'clock at night. A rough sea, a wind blowing up. After the surviving Germans had abandoned the submarine, the two British sailors had stripped off and swum out to her, lit by searchlights. The U-boat was already low in the waves, holed in the conning tower by cannon fire, shipping water fast. They'd brought off a bundle of secret papers from the radio room, handing them to a boarding party in a boat alongside, and had just gone back for the Enigma machine itself when the U-boat suddenly went bows up and sank. They went down with her—half a mile down, the Navy man had said when he told them the story in Hut 8. 'Let's just hope they were dead before they bit the bottom.'
    And then he'd produced the code books. This was on 24 November 1942. More than nine and a half months into the blackout.
    At first glance they scarcely looked worth the cost of two men's lives: two little pamphlets, the Short Signal Book and the Short Weather Cipher, printed in soluble ink on pink blotting paper, designed to be dropped into water by the wireless operator at the first sign of trouble. But to Bletchley they were beyond price, worth more than all the sunken treasure ever raised in history. Jericho knew them by heart even now. He closed his eyes and the symbols were still there, burned into the back of his retina.
    T = Lufttemperatur in ganzen Celsius-Graden. -28C = a. -27C = b. -26C ...
    U-boats made daily weather reports: air temperature, barometric pressure, wind-speed, cloud-cover . . .
    The Short Weather Cipher book reduced that data to a half-dozen letters. Those half-dozen letters were enciphered on the Enigma. The message was then broadcast from the submarine in Morse code and picked up by the German Navy's coastal weather stations. The weather stations used the U-boats' data to compile meteorological reports of their own. These reports were then re-broadcast, 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