Decoy (11 page)

‘Where does Enigma come into all this?’ Jemmy asked. ‘Just a superior crystal ball?’

Ned decided that Jenson was completely humourless: no joke or wry remark ever intruded into his life (or, rather, was ever recognized).

In the flat tone of a true academic, Jenson began: ‘Enciphering a message which has to be transmitted by a means the enemy can overhear (by wireless, for instance) means that the words of the message have to be scrambled in a completely
random
way, and the chances of it being broken are lessened if each time the same letter is repeated in the message it becomes a different one in the scramble.

‘For example, say A is Z the first time it is used, K the second, R the third and so on.

‘But war today is fast moving, particularly where armies are concerned. There isn’t time to decipher long signals by turning the pages of a cipher book, particularly while you are being bombed or shelled, and it will probably be raining, too.’

‘Enigma?’ Jemmy prompted hopefully.

‘So the ideal way to encipher a message before transmitting it by wireless is a mechanical means: in other words something like a special typewriter. You type the actual letters of the message in plain language on to the keyboard, but what you read as you hit each key is
another
letter, so instead of the original plain message you now have an apparently meaningless scramble. And a special refinement would be that, as you type each, it not only produces the enciphered letter but then changes, so that the next time that particular letter comes up, it is enciphered differently.’

‘And that’s what Enigma does?’ Ned asked.

‘That was my bird,’ Jemmy complained.

‘Basically that’s what the Germans did when they produced Enigma,’ Jenson said pedantically. ‘They added a battery, a nice mahogany box, a manual for each cipher — and here you have it.’ He patted the box. ‘Just like a portable typewriter — which is what is really is.’

‘But what happens when someone picks up the signal after Enigma has scrambled it?’ the Croupier asked.

‘Ah, that’s quite simple. That person would need to be listening on an assigned frequency, have a manual for the particular cipher, and an Enigma machine — which have been issued by the hundred.

‘Let’s suppose Hans Schmidt is the wireless operator of the 15th Panzer Division in the Western Desert and the
Ober Kommando der Wehrmacht
in Berlin has a signal for the general commanding.

‘The OKW signals people get the message at headquarters in plain language and will insert the prefix for that particular panzer division, say COD, then type the message on the Enigma machine in the appropriate Wehrmachit cipher, take the scrambled (and thus enciphered) result, and transmit it on that panzer wireless frequency.

‘Out in the Western Desert, wireless operator Schmidt listening on his frequency hears the letters COD in Morse, recognizes it as the call sign of the 15th Panzer division and copies down the Morse signal. It is, of course, the letters COD followed by what seems to be gibberish.

‘That signal is then taken to the 15th Panzer division’s Enigma. Right at the beginning of the message, following COD, is another three-letter group which tells what the machine should be set at, and, if it is very secret, an indication that it must be deciphered by an officer. I’ll show you all that on the machine in a moment.’

‘As easy as that, eh?’ Jemmy grunted.

‘If you have an Enigma, and
if
you have the appropriate manual for the cipher being used and can set the machine correctly. For the eavesdropper — us, in fact, — it’s more complex. We can usually work out for whom the signal is intended. Our problem is to work out the three-letter setting for the Enigma. Of course, a message can be
coded
first, and then sent by
cipher
using the Enigma. It’s rare, but it really provides a headache. A double headache,’ he said and smiled to indicate he had made a joke.

Ned said: ‘I thought there were dozens of different ciphers — Hydra, Medusa, Thetis, Neptun and so on for the Navy…’

‘Oh yes, for all the German services. These are the three-letter settings. The more important the cipher, the more difficult the setting. Look at the machine and you’ll realize that one three-letter setting is not necessarily as easy as another.’

The trio grouped round him at the little table. ‘Just a simple wooden box, and when you open the lid you see — ’ he swung it up and back on its hinge as though to reveal a white rabbit, ‘what looks like a complicated portable typewriter. And in many ways that’s what it is, electrically operated.

‘In front, you have an ordinary typewriter keyboard — Q,W,E,R,T,Y and so on. Then behind it, where you’d expect to find the platen, is this board with all the letters reproduced again, each behind a tiny glass window and in the same order. And then behind that you see the three small slots with metal wheels showing, each with a sort of thin disc, with a cogwheel rim, next to it. Now look carefully — ’

Ned leaned over as Jenson opened a small cover.

‘What do you see, Commander?’

‘Letters of the alphabet are engraved round the rim of each wheel.’

‘Exactly, and let’s call that the rotor. But you can see — ’ he turned the rotors beside the engraved rims, ‘that each rotor is fitted to it own disc. Actually, we call the rim and disc together a rotor. You see three are fitted together here in the middle, but there are two more over in this rack on the right: they are spares or, rather, alternatives: the machine has five rotors but only uses three at any given time.’

‘How does wireless operator Schmidt know which three to use?’ the Croupier asked.

‘The sequence is changed every twenty-four hours, so his manual tells him that for, say, the fifth of November he uses rotors number one, four and five. So he fits those — it’s a simple pull-out and drop-in affair — depending on the date. Schmidt then picks up the call sign COD which shows the signal is for his own Panzer Division, and he copies it down as it comes over the wireless in Morse.

‘Then — I’m guessing now — he takes his handwritten signal to the Enigma operator, who will set his machine to the same setting as the OKW Enigma was when it scrambled — enciphered, rather — this particular signal. There’s another step too, but I’ll explain that later.’

‘How do the Germans actually pass messages to U-boats on this thing?’ Ned asked.

‘Well, let’s look at it from the point of view of the U-boat at sea and of Dönitz’s headquarters. The U-boat has the regular U-boat wireless frequency assigned to it, a call sign for that boat, a Mark II Enigma machine and a Hydra manual.

‘At the start of the day — technically at 00.01 — the person responsible for Enigma (let’s call him the U-boat Enigma man) looks at his manual for that day. Now, he knows that any signals sent to him will be in the Hydra cipher, so all he needs to listen for is the call sign for his own boat.

‘The manual tells him which three of the five rotors he is to use, and the order (left to right) they are to be fitted. It also gives him three numbers — say 425.

‘These tell him what letter on the wheel is to be set against the zero setting on each disc. The three numbers correspond to the letters in the alphabet, thus 425 means DBE. So he twists each of the three engraved rims one after the other until D, B and E are set against the zeros on the discs to which they are attached.

‘Plug and socket settings are also given, and he puts each one into the right socket, making a pair which links one letter with another. I’ll explain them later.’

Jenson looked round at the three men, taking off his spectacles and polishing them with toilet paper. ‘Any questions?’

They shook their heads, though Ned wanted to ask if the paper did a good job, and Jenson continued. ‘Very well, the U-boat is on patrol, but Dönitz has just received a sighting report of a convoy from another U-boat and wants to assemble a pack, including our U-boat, so he drafts a suitable signal. This is passed to his Enigma man, who looks up the particular U-boat’s own call sign, and puts that in the Enigma signal preamble along with a three-letter group indicating that the Hydra cipher is being used. He then decides (at random) on a rotor setting, say ARP, and taps it out. This lights up three more letters, say SWL.

‘Now he puts down SWL as the last group in the preamble, none of which is enciphered. He then sets his rotors to ARP — which does not appear anywhere in the signal — and types out the message, with someone noting down each letter as it lights up on the light board. That — with the preamble including the U-boat’s call sign — is the Enigma-enciphered signal given to the wireless operator.

‘The operator now sends it off in Morse, and thousands of miles out in the Atlantic our U-boat picks it up. There is the usual preamble telling the U-boat that
B der U
has a signal for them. Then the last group of three letters in the unenciphered part is, of course, SWL.

‘The Enigma operator now types out SWL with the Enigma on the day’s setting of DBE and lights up the letters ARP. He resets his rotors to ARP, and since he now has the correct setting for the message, types out the enciphered signal and someone else copies down the letters — which are of course now the deciphered signal — as light after light comes on.

‘You see that Dönitz’s Enigma man choosing three letters at random — in this case ARP — allows him in fact to encipher the actual setting that he is going to use as SWL. But of course both sender and receiver must each have copies of the same manual, and each must set up his Enigma machine — the right three rotors and the correct rotor-disc settings — as the manual lays down.

‘Each cipher has a different manual which gives the settings for one month. Thus an Atlantic U-boat with the up-to-date Hydra manual can read only Hydra traffic. He could copy down a Neptun signal intended for a major war vessel but because he didn’t have the Neptun manual he could not set up his Enigma to decipher and thus read it.

‘Remember, the Enigma is only the machine for mechanically enciphering and deciphering a signal. The manual is what gives the settings. It’s like going round the world with a perfectly accurate watch. Unless you have something that tells you your longitude you still don’t know the local time.

‘And remember, gentlemen, the one vital thing about an Enigma machine: it also works in reverse. If you tap A on the keyboard and P comes up on the lampboard, when you tap P on the keyboard A will come up on the lampboard. And if you keep on tapping P on the keyboard you’ll get a different letter each time on the lampboard because a rotor turns one position with each letter and changes the circuit.’

‘Seems unnecessarily complex,’ the Croupier grumbled.

‘It isn’t really. If one didn’t have an Enigma machine to experiment with, it’d be almost unbreakable —
providing
the users choose letters at random. With each new message the sender must select a new setting and choose at random the three letters for enciphering that setting. If, for example, he uses his girlfriend’s initials a few times, his enemy’s cryptographers will eventually break the cipher.’

‘Do some operators do that — repeat themselves, in effect?’

Jenson smiled and said evasively: ‘That kind of thing is one of BP’s most closely guarded secrets. All I’ve told you is how Enigma works. You don’t know how we read Hydra; you don’t know exactly why Triton could take us a year or more to break unless we get a Mark III machine
and
a manual.’

‘Even an out-of-date one?’ Jemmy asked.

‘Everything helps. Bring us a Mark III and last month’s edition of the manual and I’m sure we would invite you in for a cup of tea and a slice of sponge cake. You can use your imagination to see just how much we could learn from even an out-of-date manual since we read Hydra.’

Jenson looked round at the three of them, as if trying to assess their intelligence, and asked: ‘Can you picture the circuit?’

‘I think I can,’ said the Croupier, pointing with his finger at the right-hand end of the rotor assembly. ‘The current comes in here on one wire of the disc corresponding to a letter of the alphabet, say Y, crosses the rotor and comes out the other side — ’

‘Cross-wired, of course, so that it is changed.’

‘ — yes, so it then leaves as, say, B and enters one side of the next disc as J, goes across the rotor and comes out as R because of the cross-wiring. To the third rotor the same way, in perhaps as D and out as X and then out through the disc at the other end still as X.’

Jenson shook his head. ‘You were more or less correct up to the last disc. It doesn’t exactly come out there — that last disc really reverses it, turning it round again so the circuit goes
back
through the cross-wire discs and rotors, so that there is in effect a double zigzag in the circuit. Now let me round it off for you all.

‘As you press a key the first disc and rotor turns, and they eventually rotate the second disc and rotor one position, which in sequence turns the third rotor one position. So if you multiply 26 by 26 by 26, you get the variations — 17,576 in fact.

‘There are also thirteen plugs in the circuit — I mentioned them earlier. They are here in front, with the cover over them, looking like a telephone switchboard in miniature but which I won’t bother you with other than to say each represents a letter connected to another, adding to the scramble. For deciphering, remember that the machine has five rotors, and three are used at any one time. Remembering that the chosen three have to be turned so that a particular combination is showing, there are six possible ways the rotors can be installed. Let us say rotors 1, 2 and 3 are to be used, leaving 4 and 5 in the rack. You can put them in as 123, 132, 213, 231, 312 or 321, so in fact you have actually many possible rotor positions. You also have 17,576 different circuits through the rotors, multiplied by the rotor possibilities.’

Jemmy sighed, overwhelmed by the figures. ‘But surely this is all beyond the average German wireless operator or Enigma machine juggler?’