Enigma

Jericho.
    'Praise the Lord!' said Atwood, throwing up his hands like a revivalist preacher. 'The Oracle has spoken.'
    'All right, Frank. Just a minute. What about it, old love?'
    Jericho could see it all much faster than he could convey it. Indeed, it was quite hard to formulate it in words at all. He spoke slowly, as if translating from a foreign language, reordering it in his mind, turning it into a narrative.
    'Do you remember, in November, when we got the Short Weather Cipher Book off the U-459? When we also got the Short Signal Book? Only we decided not to concentrate on the Short Signal Book at the time, because it never yielded anything long enough to make a worthwhile crib? I mean, a convoy contact signal on its own, it isn't worth a damn, is it? It's just five letters once in a blue moon.' Jericho withdrew the little pink pamphlet carefully from his pocket. 'One letter for the speed of the convoy, a couple for its course, a couple more for the grid reference
    Baxter stared at the code book as if hypnotised. 'You've removed that from the safe without permission?
    'But if Lieutenant Cave is correct, and whichever U-boat finds the convoy is going to send a contact signal every two hours, and if it's going to shadow it till nightfall, then it's possible—theoretically possible—it might send as many as four, or even five signals, depending on what time of day it makes its first sighting.' Jericho sought out the only uniform in the room. 'How long does daylight last in the North Atlantic in March?'
    'About twelve hours,' said Kramer.
    'Twelve hours, you see? And if a number of other U-boats attach themselves to the same convoy, on the same day, in response to the original signal, and they all start sending contact signals every two hours
    Logie, at least, could see what he was driving at. He withdrew his pipe slowly from his mouth. 'Bloody hell!'
    'Then again, theoretically, we could have, say, twenty letters of crib off the first boat, fifteen off the second—I don't know, if it's an attack by eight boats, let's say, we could easily get to a hundred letters. It's just as good as the weather crib.' Jericho felt as a proud as a father, offering the world a glimpse of his newborn child. 'It's beautiful, don't you see?' He gazed at each of the crytanalysts in turn: Kingcome and Logie were begin-to look excited, de Brooke and Proudfoot seemed thoughtful, Baxter, Atwood and Puck appeared down-right hostile. 'It was never possible till this moment, because until now the Germans have never been able to throw so many U-boats against such a mass of shipping. It's the whole story of Enigma in a nutshell. The very scale of the Germans' achievement breeds such a mass of malrial for us, it'll sow the seeds of their eventual defeat.'
    He paused.
    Aren't there rather a lot of ifs there?' said Baxter drily. 'If the U-boat finds the convoy early enough in the day, if it reports every two hours, if the others all do the same, manage to intercept every transmission . . .'
    'And if' said Atwood, 'the Short Signal Book we pinched in November wasn't changed last week at the same time as the Weather Cipher Book
    That was a possibility Jericho hadn't considered. He felt his enthusiasm crumble slightly.
    Now Puck joined in the attack. 'I agree. The concept is quite brilliant, Thomas. I applaud your—inspiration, I suppose. But your strategy depends on failure, does it not We will only break Shark, on your admission, if the U-Boats find the convoy, which is exactly what we want to avoid. And suppose we do come up with that day's Shark settings—so what? Marvellous. We can read all the U-boats' signals to Berlin, boasting to Donitz about how many Allied ships they've sunk. And twenty-four hours later, we're blacked out again.'
    Several of the cryptanalysts groaned in agreement.
    'No, no,' Jericho shook his head emphatically. 'Your logic is flawed, Puck. What we hope, obviously, is that the U-boats don't find the convoys. Yes—that's the whole point of the exercise. But if they do, we can at least turn it to our advantage. And it won't just be one day, not if we're lucky. If we break the Shark settings for twenty-four hours, then we'll pick up the encoded weather messages for that entire period. And, remember, we'll have our own ships in the area, able to give us the precise weather data the U-boats are encoding. We'll have the plaintext, we'll have the Shark cipher settings, so we'll be able to make a start on reconstructing the new