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to be a monster.”
    “And it was.” Logie puffed on his pipe. He was starting to
disappear in a bank of fumes. The cheap wartime tobacco smelled
like burning hay. “And it is.”
    Something in the way he delivered that last word—some slight
hesitation—made Jericho look up sharply.
    ♦
    The Germans called it Triton, after the son of Poseidon, the
demigod of the ocean who blew through, a twisted seashell to raise
the furies of the deep. “German humour,” Puck had groaned when they
discovered the code name, “German fucking humour.” But at Bletchley
they stuck to Shark. It was a tradition, and they were British and
they liked their traditions. They named all the enemy’s ciphers
after sea creatures. The main German naval cipher they called
Dolphin. Porpoise was the Enigma key for Mediterranean surface
vessels and shipping in the Black Sea. Oyster was an “officer only”
variation on Dolphin. Winkle was the “officer only” variant of
Porpoise.
    And Shark? Shark was the operational cipher of the U-boats.
    Shark was unique. Every other cipher was produced on a standard
three-rotor Enigma machine. But Shark came out of an Enigma with a
specially adapted fourth rotor which made it twenty-six times more
difficult to break. Only U-boats were allowed to carry it.
    It came into service on 1 February 1942 and it blacked out
Bletchley almost completely.
    Jericho remembered the months that followed as a prolonged
nightmare. Before the advent of Shark, the cryptanalysts in 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