Enigma

by. He waited two hours, then surfaced.
    The convoy was already so far ahead as to be barely visible in the faint dawn light—just the masts of the ships and a few smudges of smoke on the horizon, and then, occasionally, when a high wave lifted the U-boat, the ironwork of bridges and funnels.
    Feiler's task under standing orders was not to attack—impossible in any case, given his lack of torpedoes -but to keep his quarry in sight while drawing in every other U-boat within a radius of 100 miles.
    'Convoy steering 070 degrees,' said Feiler. 'Naval grid square BD 1491.'
    The first officer made a scrawled note in pencil then dropped down the conning tower to collect the Short Signal Code Book. In his cubbyhole next to the captain's berth the radioman pressed his switches. The Enigma came on with a hum.

2

    At 7 a.m., Logie had sent Pinker, Proudfoot and Kingcome back to their digs to get some decent rest. 'Sod's law will now proceed to operate,' he predicted, as he watched them go, and sod's law duly did. Twenty-five minutes later, he was back in the Big Room with the queasy expression of guilty excitement which would characterise the whole of that day. 'It looks like it may have started.' St Erith, Scarborough and Flowerdown had all reported an E-bar signal followed by eight Morse letters, and within a minute one of the Wrens from the Registration Room was bringing in the first copies. Jericho placed his carefully in the centre of his trestle table.
    RGHC DMIG. His heart began to accelerate. 'Hubertus net,' said Logie. '4601 kilocycles.' Cave was listening to someone on the telephone. He put his hand over the mouthpiece. 'Direction finders have a fix.' He clicked his fingers. 'Pencil. Quick.' Baxter threw him one. '49.4 degrees north,' he repeated. '38.8 degrees west. Got it. Well done.' He hung up.
    Cave had spent all night plotting the convoys' courses on two large charts of the North Atlantic—one issued by the Admiralty, the other a captured German naval grid, on which the ocean was divided into thousands of tiny squares. The cryptanalysts gathered round him. Cave's finger came down on a spot almost exactly midway between Newfoundland and the British Isles. 'There she is. She's shadowing HX-229.' He made a cross on the map and wrote 0725 beside it.
    Jericho said: 'What grid square is that?'
    'BD1491.'
    'And the convoy course?'
    '070'
    Jericho went back to his desk and in less than two minutes, using the Short Signal Code Book and the current Kriegsmarine address book for encoding naval grid squares ('Alfred Krause, Blucherplatz 15': Hut 8 had broken that just before the blackout) he had a five-letter crib to slide under the contact report.
    R G H C D M I G
    D D F G R X ? ?
    The first four letters announced that a convoy had been located steering 070 degrees, the next two gave the grid square, the final two represented the code name of the U-boat, which he didn't have. He circled R-D and D-R. A four-letter loop on the first signal.
    'I get D-R/R-D,' said Puck a few seconds later.
    'So do I.'
    'Me too,'said Baxter.
    Jericho nodded and doodled his initials on the pad. 'A good omen.'
    After that, the pace of events began to quicken.
    At 8.25, two long signals were intercepted emanating from Magdeburg, which Cave at once surmised would be U-boat headquarters ordering every submarine in the North Atlantic into the attack zone; At 9.20, he put down the telephone to announce that the Admiralty had just signalled the convoy commander with a warning that he was probably being shadowed. Seven minutes later, the telephone rang again. Flowerdown intercept station. A second E-bar flash from almost the same location as the first. The Wrens hurried in with it: KLYS QNLP.
    'The same hearse,' said Cave. 'Following standard operating procedure. Reporting every two hours, or near as damn it.'
    'Grid square?'
    'The same.'
    'Convoy course?'
    'Also the same. For now.'
    Jericho went back to his desk and manipulated the original crib under the new cryptogram.
    K L Y S Q N L P
    D D F G R X ? ?
    Again, there were no letter clashes. The golden rule of Enigma, its single, fatal weakness: nothing is ever itself—A can never be A, B can never be B... It was working. His feet performed a little tap dance of delight beneath the table. He glanced up to find Baxter staring at him and he realised, to his horror, that he was smiling.
    'Pleased?'
    'Of course not.'
    But such was his shame that when, an hour later, Logie came through to say that a