Enigma

transmission, even as the U-boats were preparing to dive, they were emerging via a tunnel into Hut 8's Registration Room. Jericho helped himself to the contents of a wire basket labelled 'Shark' and carried them to the nearest light. The hours immediately after midnight were usually the busiest time. Sure enough, six messages had been intercepted in the last eighteen minutes. Three consisted of just eight letters: he guessed they were weather reports. Even the longest of the other cryptograms was no more than a couple of dozen four-letter groups:
    JRLO GOPL DNRZ LOBT -
    Puck made a weary face at him, as if to say: What can you do?
    Jericho said: 'What's the volume?'
    'It varies. One hundred and fifty, perhaps two hundred messages a day. And rising.'
    The Registration Room didn't just handle Shark. There was Porpoise and Dolphin and all the other different Enigma keys to log and then pass across the corridor to the Crib Room. Here, the cribsters sifted them for clues—radio station call signs they recognised (Kiel was JDU, for example, Wilhelmshaven KYU), messages whose contents they could guess at, or cryptograms that had already been enciphered in one key and then retransmitted in another (they marked these 'XX' and called them 'kisses'). Atwood was the champion cribster and the Wrens said cattily behind his back that these were the only kisses he had ever had.
    It was in the big room next door—which they called, with their solemn humour, the Big Room -that the cryptanalysts used the cribs to construct possible solutions that could be tested on the bombes. Jericho took in the rickety tables, the hard chairs, the weak lighting, the fug of tobacco, the college-library atmosphere, the night chill (most of the cryptanalysts were wearing coats and mittens) and he wondered why—why?—he had been so ready to come back. Kingcome and Proudfoot were there, and Upjohn and Pinker and de Brooke, and maybe half a dozen newcomers whose faces he didn't recognise, including one young man sitting bold as you please in the seat which had once been reserved for Jericho. The tables were stacked with cryptograms, like ballot papers at an election count.
    Puck was muttering something about back-breaks but Jericho, fascinated by the sight of someone else in his place, lost track and had to interrupt him. 'I'm sorry, Puck. What was that?'
    'I was saying that from twenty minutes ago we are up to date. Shark is now fully read to the point of the code change. So that there is nothing left to us. Except history.' He gave a weak smile and patted Jericho's shoulder. 'Come. I'll show you.'
    When a cryptanalyst believed he'd glimpsed a possible break into a message, his guess was sent out of the hut to be tested on a bombe. And if he'd been skilful enough, or lucky enough, then in an hour, or a day, the bombe would churn through a million permutations and reveal how the Enigma machine had been set up. That information was relayed back from the bombe bays to the Decoding Room.
    Because of its noise, the Decoding Room was tucked away at the far end of the hut. Personally, Jericho liked the clatter. It was the sound of success. His worst memories were of the nights when the building was silent. A dozen British Type-X enciphering machines had been modified to mimic the actions of the German Enigma. They were big, cumbersome devices -typewriters with rotors, a plugboard and a cylinder—at which sat young and well-groomed debutantes.
    Baxter, who was the hut's resident Marxist, had a theory that Bletchley's workforce (which was mainly female) was arranged in what he called 'a paradigm of the English class system'. The wireless interceptors, shivering in their coastal radio stations, were generally working-class and laboured in ignorance of the Enigma secret. The bombe operators, who worked in the grounds of some nearby country houses and in a big new installation just outside London, were petit-bourgeois and had a vague idea. And the Decoding Room girls, in the heart of the Park, were mostly upper-middle-class, even aristocratic, and they saw it all—the secrets literally passed through their fingers. They typed out the letters of the original cryptogram, and from the cylinder on the right of the Type-X a strip of sticky-backed paper, the sort you saw gummed down on telegram forms, slowly emerged, bearing the decrypted plaintext.
    'Those three are doing Dolphin,' said Puck, pointing across the room, 'and the two by the door are just starting on