by Dudley Pope
‘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?’
Jenson raised an eyebrow. ‘I doubt it, but anyway, don’t forget the “switchboard” plugs. There are thirteen pairs — thirteen, half the total number of letters of the alphabet. Each pair of plugs allows one letter to be linked to another — A to P, for example. On a really important cipher, all thirteen pairs could be used, so that there are a few more permutations…’
Ned noted the slight emphasis on ‘few’. ‘How many?’ he asked.
‘Well, the thirteen plugs means you have to multiply 25 by 23 by 21 and so on, and the answer is about eight followed by a dozen noughts, but there are other ways of introducing more permutations, so the final answer is about two hundred followed by a dozen noughts.’
‘I’m beginning to see why everyone this side is worried about Triton!’ Ned exclaimed.
‘Not so much just Triton,’ Jenson corrected. ‘We want to look at the cross-wiring on the fourth rotor they are going to put to work on the Mark III. A choice of eight rotors… I’ll spare you the mathematics. Now, any questions?’
‘Yes, ’fraid so,’ said Jemmy. ‘I don’t see the need for the first three letters in the preamble after the unit’s call sign, whether an Army, Air Force or Navy unit.’
‘Ah, we assume that’s so that no one German service (or even unit) can read a signal intended for someone else. Himmler’s Gestapo wouldn’t want Goering’s Luftwaffe to read their signals. So each unit has its own monthly manual giving the rotor settings which change every midnight and the operator or Enigma clerk knows at once whether he has the manual for that particular group, or key. If he hasn’t, I expect he goes off for a smoke or continues reading his magazine. If he has, then he consults the manual and sets up his rotors.’
‘You seem to know a lot about the Triton business?’ Ned commented cautiously.
‘Yes, Triton and Mark III. In fact I’m probably to blame for the three of you getting your present orders. My report went to the Prime Minister, and my old friend Henry Watts was sent for, as I guessed would be the case.’
‘I went with him,’ Ned said. ‘Now, with Hydra, you know more or less exactly what’s going on in the Atlantic?’
‘Yes. There are delays at times because of the random settings until we break the individual signals, but rarely longer than twenty-four hours.’
‘So when the Germans bring in Mark III with its extra rotor, and new manual, and start using Triton…?’
Jenson took his glasses off and began polishing them again. ‘It will be a disaster. You understand by now that Triton (or Hydra, or any other cipher for that matter) is a particular setting of the rotors and of the plugs. What we have to know about a new Enigma with four rotors is how the four rotors and four alternative rotors are each wired up to their discs, or cross-wired, rather. Four rotors can be arranged eight ways…’
‘Yes,’ Jemmy said, ‘so eight way with four rotors, and four more to choose from is — ’ he stopped, defeated by the mathematics.
‘There are a lot of permutations of the cross-wiring,’ Jenson agreed, replacing his spectacles. ‘With three rotors it is more than two hundred followed by twelve noughts. Just think of a three-figure number and add a dozen or so noughts…’
‘Without a Mark III Enigma, the eight rotors and manual, what are you going to do?’ Ned asked out of curiosity.
Jenson’s face suddenly went blank, like a hurt child refusing to cry out. ‘I have a wife and three small children, and there are more than fifty million other people on this island quite apart from all those in Occupied Europe,’ he said simply. ‘To stay sane, I don’t think about it. I can only wish you luck, gentlemen: whether all these millions live or die now depends on you.’
The Marine sergeant instructor held up a black object the size of a large orange. ‘And this,’ he said, ‘is — ’
‘A black banger!’ exclaimed Jemmy.
The sergeant was not intimidated by naval lieutenants, even if they wore medal ribbons and were — as in the case of this smart alec here — famous submarine commanders. Anyone fool enough to serve in submarines when it was hard enough to stay alive on the top of the water deserved what came to him.
And, struth, what a bloody twitch; wonder he didn’t break his bloody neck. There he goes, nearly twitches himself off the bloody chair. Black bangers, indeed; all that farting around beneath the waves must have sent him round the twist. Come to think of it, all three of ’em are a bit off, like Aunt Alice’s piano: all the bloody notes are flat, no matter what key you hit, white or black, though he was not quite sure what the black ones did.
Anyway, the sarn’t major, for all his winks and nods and nose-tapping with his finger, had not given much away yesterday. ‘Sergeant Gill,’ he had said, ‘we ’ave a special task assigned to us.’ That comes of working with the Americans for a week or two; every job is a ‘task’ and they’re always ‘assigning’ things. Anyway, the sarn’t major had said three naval officers were coming for a week’s special training in close quarters combat. ‘You’re to assume they have to fight themselves into or out of a large chicken house filled with Jerries, and that they’ve got to kill, wound or otherwise render useless said Jerries without riddling the chicken house with bullets, bits of Mills grenades or other debris.’
Bloody debris, that’s what he said. But why should a couple of lieutenants and a to-and-a-half ringer be fooling round in a hen house with a crowd of Jerries? Well, it must be something special because the sarn’t major swore him to secrecy.
‘You could call it a “black banger”, sir,’ the sergeant said, with what he considered just the right emphasis on ‘could’ and ‘sir’ to put the bloody fool in his place. ‘Certainly five seconds after the pin has been pulled out and the handle allowed to fly up to release the detonator, it will hexplode with a very loud bang. Sufficient,’ he quoted parrot fashion, ‘to hinduce complete deafness, stunned sensation and considerable disorientation to anyone within a radius of twelve feet and alarm and discomfort to fifteen.’
‘What happens if a piece of that Bakelite actually hits someone?’ the Croupier asked.
‘Cut, bruises, abrasions,’ the sergeant said airily. ‘this grenade, however, is not intended to be lethal. It is purely a training weapon. It could also be used as a psychological one, too.’
Ned considered the sergeant’s pronunciation of the ‘p’ in the word and decided it sounded more effective.
‘Panic, gentlemen, that’s what the grenade cater
s for. Panic in confined quarters. If you want to kill ’em, then you use the standard Mills 36 fragmentation grenades –’
‘With which we are familiar,’ Jemmy said. ‘How far can you chuck a “black banger”?’
The sergeant was not used to dealing with pupils adopting a light-hearted manner. ‘Both the Bakelite grenade and the Mills are delivered with a bowling movement, as with a cricket ball, gentlemen.’
‘Unless there’s not much room and a quick lob is all that you can manage,’ Jemmy said, and realized he had fallen into a trap.
‘Ah, sir,’ the sergeant said triumphantly, ‘there you have the problem with grenades. Hit his himportant,’ he said, being generous with his aspirates to emphasize the point, ‘to hinsure the hoptimum distance between the hexploding grenade and the hurler thereof.’
And that, the sergeant thought to himself, was put over word-perfect: page seventeen of the manual, paragraph four, the third and fourth lines, and with three smart alecs it was just as well to get it right. Not, he admitted, that he wanted to see a two-and-a-half ringer with two good gongs stunned stupid by his own grenade, and that twitchy bloke’s name for it, ‘black banger’, was good; in fact, he decided, he would work it into his next course.
‘So for close-range work you have the black Bakelite grenade designed to cause panic, and the Mills grenade which will cause death and destruction. Now we come to the Sten sub-machine gun.’
He picked up a Sten and held it up. ‘Has any of you gentlemen ever used one? No? Well, I’ll start with a bit of ’istory.
‘The gun itself is made of bits of cast-off gas pipe delicately welded together by beautiful young ladies conscripted into the munitions factories. Made in Britain. But the hammunition his a different story. Nine millimetre.’ His eyes opened wider and his eyebrows lifted as he said it, and his voice dropped. Thus, Ned thought, would the rector’s wife refer to the village’s fallen woman.
