Anna was surprised. How could they have forgotten to pass on this important detail? The surprise showed on her face. Alastair apparently misinterpreted her expression, thinking she was unaware of this peculiarity of the Enigma.
“The reason is quite simple. If, at a randomly chosen setting of the wheels, G comes out T, then it's reversible. T will come out G. To make the machine reversible—so it can be used for both coding and decoding—the designers not only used the three rotors, but also a reflector at the end. Without getting into the mechanics, the reflector not only ensures reversibility, but also ensures that a letter must come out as some other letter. This is so even with the steckerboard, which is an integral part of the wiring.”
Anna thought it would be impolite to interrupt his explanation; she saw no gracious way of saying that she already knew. Also, with Alastair's explanation, she might actually learn something new.
“This idiosyncrasy turns out to be surprisingly useful. For example, suppose we suspect that a German message is addressed: ANDIEGRUPPE. We line these eleven letters up directly below the beginning of the encoded German message. If any one of the letters corresponds—for example, if the second letter of the encoded message is also N—then we know the encoded message
doesn't
have A
N
DIEGRUPPE at the very beginning. We slide ANDIEGRUPPE along one space, to find if we now have possible solution, with the Germans having put one random, dummy letter at the beginning. As long as we find a matching letter in the top and bottom lines, we know we're not in the right place, and can continue to slide ANDIEGRUPPE along, one letter at a time, until we get a possible solution. If we move a dozen spaces to the right, with a matching letter each time, we can be pretty sure the message isn't addressed ANDIEGRUPPE, and can turn to a different message.
“There's also a whole set of additional clues that Welchman and others in Hut 6 have been working on. They occur when the German operators do something stupid. In fact, you may know about this type of clue because Rejewski was working on them.”
“Yes, of course. But I'd also be interested in the specific ones you've found.”
“The most obvious cribs come from operators who use their girlfriends' names. One of the first was a girl named Cilli. Her boyfriend often uses CIL and other parts of her name for the wheel settings.
“Not surprisingly, our decoders quickly corrupted 'Cilli' into 'silly.' But it's just one example of a whole series of sillies. Come on into my office, and I'll show you what I mean.” Alastair led her to what was apparently a closet, but now concealed an Enigma machine. He pointed to the keyboard:
Q W E R T Z U I O
A S D F G H J K
P Y X C V B N M L
“We occasionally find a really stunned operator who just works his way along the top row. He picks QWE as the setting for the first message, and RTZ for the second. The chances are pretty good that the next one will be UIO. Or, if he starts diagonally with QAY and then WSX, the next ones are likely to be EDC and RFV.
“Then, there's the Herivel Tip, named after John Herivel, whom you'll undoubtedly meet. John noticed that the enigma operators may be just plain lazy. When picking a setting, they sometimes just move each wheel one or two notches. Thus, for example, if the initial settings is GNU, the new setting for the first wheel is likely to be within one or two letters of G, and so on. Occasionally, we're really lucky. An operator will set the wheels at AAA for the first message, BBB for the second, and so on.
“Finally—and I've got to go. I mentioned we have a new version of the bombe coming along. Alan Turing was already designing the machine before our Pyry Forest meeting. He recently went to see Marian Rejewski and Henryk Zygalski; they got out of Poland and are working with Bertrand near Paris. Marian and Henryk were a great help, proving once again that three heads are better than one.
“Oh, by the way, don't even
dream
of contacting Marian or Henryk. Just so there are no misunderstandings: you are flatly forbidden to leave the country, and you're also forbidden to contact Poles outside Britain—or
anyone
outside Britain, for that matter. Also, you should check with security before you have contacts with Poles even within Britain.”
“So those two, at least, got away,” said Anna, obviously relieved.
“So did Jerzy Rozycki and some of the others.”
Anna was delighted, but also puzzled. Alastair seemed almost embarrassed as he gave her the good news. She waited to see if she would get more information. It came.
“It's shameful, what happened in Romania. When your colleagues got out of Poland, they headed straight for the British embassy in Bucharest. The imbeciles there didn't realize what they were being offered. Brushed off your colleagues, suggesting they come back in a few days.”
Anna couldn't help but think of her experience at Naval Intelligence, the first day she arrived.
“But, of course, they couldn't wait around. They had entered Romania illegally, without papers, and were afraid that the police might pick them up and put them in an internment camp. Naturally, they went to the French embassy and were greeted with open arms. In fact, Bertrand was there waiting for them; he made a special trip to Romania in the hope that they would show up. So they immediately got French visas. It worked out all right for the allies, I suppose. But we were fools.”
During the next few weeks, Anna worked away methodically, spending the mornings with a “tabula rasa,” so to speak—starting with a clean sheet of paper and jotting down ideas when they came to her, whether they seemed good, bad, or indifferent. During the afternoons, she would work through the pile of unbroken messages, not quite sure what she was looking for.
Then, one afternoon, she thought she found something. Two messages, both from the same source, and both with exactly the same number of letters—92. Could this be what she was after? A repeat of a message, on different days, using different basic settings? Her reason—or excuse?—for contacting Alastair in the first place? She looked at the messages themselves; the letters did not correspond at all. Then she looked at the times of transmission: 11:31 Dec. 29, 1939 and 12:14 Dec. 29, 1939. Damn. They weren't sent on different days. Was it possible that the Huns were picking new settings twice a day, once at midnight and once at noon? Unlikely. Changing a code in the middle of the day and in the middle of action would cause too much confusion. Probably just a coincidence. After all, if messages are generally less than 100 letters, it would be surprising if she
didn't
occasionally find two different messages with the same number of letters. And yet….
She sat there, staring at the wall. More precisely, at the place where water had seeped in from a leak in the roof, leaving a stain on the wallpaper. An informal Rorschach test. She wondered: did it look more like a frog or a butterfly?
She then started leafing slowly back through the unbroken messages of Dec. 29. She found what she was looking for. A very brief return message to the original sender, at 11:47. She got up thoughtfully, and walked toward Alastair's door. She was about to turn the knob absentmindedly when she caught herself. She detoured through the secretary's office. Yes, Mr. Denniston would see her.
“Suppose,” she said, laying the three messages before him, “that the operator was having a bad morning. He forgot to enter new wheel settings for the day, and simply transmitted, using the basic settings from the previous day. Then comes the abrupt response: 'We can't read your message. Please transmit again.' Or whatever. Then the original chap does retransmit, this time using the current day's settings.”
It was now Alastair's turn to sit and think. After a few minutes, he said simply, “Sounds promising—an idea worth pursuing. I'll try it out on Alan Turing and Gordon Welchman over in Hut 6; they may find it useful.
"You know, Anna, it brings back one of our most notable successes in the Great War, when I was a junior member of the codebreaking unit. One Christmas eve, a German commander in the Middle East had much too much to drink. In a mellow, inebriated haze, he decided to send season's greetings to all his stations. Apparently his wireless operators had quite a bit to drink, too. They should have sent the greetings in the clear, but instead sent exactly the same message in six different ciphers. We could scarcely believe our good luck. It was a time when there was practically no other traffic, and it was obvious what had happened. We had already broken one of the ciphers, and the drunken commander gave us a key to break the other five. Quite a Christmas gift. At the time, we called him the Snookered Santa. Maybe we could revive that name."
"That reminds me, Alastair. I think that we might be a bit more obscure and nondescript in our code names. I overheard a discussion in the cafeteria the other day. These two young chaps were talking about FUN clues, and sillies. They wondered if there was an office game going on, and if so, how they could join it. As they were leaving, I walked up beside them and said: For your information, FUN doesn't have anything to do with fun. That's just how the German word V-O-N—meaning from—is pronounced. So it's all business, not fun. Just keep your mouths shut.
“Perhaps, Alastair, we should use some other shorthand besides VON. As far as I can see, the words 'sillies' and 'snookered santa' don't convey any hints. But VON may."
She had a point. Thereafter, the denizens of BP would refer to VON and other standard introductions simply as one of the "sillies."
And so it was. They had a number of cribs and clues that might lead to the Enigma settings—sillies, snookered santas, Herivel tips, and others that were yet to come. Soon, however, santa was retired. When two identical messages with different keys were matched up, the match came to be known simply as a "kiss.”
13
Lady Luck
You do your worst
—
we will do our best.
Churchill, pretending to address Hitler
12 February, 1940. Aboard minesweeper,
HMS Gleaner.
I
t was a typical February day. A blanket of fog covered the waters near the Firth of Clyde, off the western coast of Scotland. As Lt.-Commander Hugh Price strained to see past the bow of his minesweeper, his ship abruptly came out of the fog bank. There, about 100 yards, almost dead ahead, was a faint, sinister gray form—a German submarine, lying low in the water.
Price immediately sounded Battle Stations, thoughts racing through his head. "Now we're in for it. They've got a bigger gun than we do. They're a smaller target, and they're more solidly built. There's nothing for it but to have a go at them. With luck, we may put them off their stroke.” He ordered the wheel turned slightly to port, heading straight for the U-boat, and signaled full power. If they could only get up to 12 knots!
He did have luck. The dozen men on the sub's deck quickly sprang to action, but were confused. Some headed for the conning tower, hoping to submerge before the minesweeper was upon them. Others rushed toward the deck gun. As they prepared for action, an invisible stopwatch clicked in Price's head. 75 yards; 15 seconds to collision. 60 yards, 12 seconds; 50 yards, 10 seconds. His deck gun fired, but the shot was high. There went the gunner's only chance; they wouldn't be able to lower their gun enough to get a second shot.
25 yards, 5 seconds. The sub's gun had now swung around, and Price felt the peculiar sensation of looking straight down the barrel. It fired; Price was surprised to be alive. As the front of his ship pitched upward, the German shell struck the very top of the bow, sending shrapnel across the deck and wounding two gunners.
One of U-boat gunners was apparently thinking of reloading, but for them, also, it was too late; they too had taken their one and only shot. The minesweeper would soon be upon them. Price ordered the wheel adjusted slightly to starboard, aiming directly for the conning tower. He braced for a collision. The German gunners dove for safety over the far side of the sub.
A crunch was followed by a grinding sound as the minesweeper came to rest, partially supported by the sub. Price called to his Number One, asking him to send a Tommy gunner up to the deck. On the double. He then proceeded to the right side of the bow and looked down at the sub. The front of its conning tower had a five-foot gash; it was in no condition to submerge.
Several minutes passed while Price—and presumably the sub commander, too—pondered the situation. Neither ship was in a position to move. With the larger ship draped over its deck, the only direction the sub could go was down. And it couldn't do that because of damage to the conning tower; it would sink. On the other hand, if Price backed off—assuming he could—the sub might slip away on the surface; the fog was beginning to close in again.
By now, two—not one—Tommy gunners had appeared on deck, along with another half dozen of the crew. Price decided that the time had come for action; he didn't want to give the U-boat commander more time to think. Able Seaman Skinner was on deck; he spoke some German. Price quickly gave him instructions and sent him over the side on a rope, carrying a hammer and several grenades.
Skinner got to the top of the conning tower and banged on the hatch with the hammer. As it slowly opened, he slid around behind the hatch to avoid presenting a target for anyone inside with a firearm. He shouted an ultimatum: the Germans had two minutes to abandon ship and surrender. They would be taken aboard
HMS Gleaner
as prisoners. They had just two minutes, he repeated; then the submarine would be sunk. Meanwhile, the
Gleaner's
crew had brought forward a rope ladder, which was being lowered toward the conning tower.
Price hoped the bluff would work; he wasn't sure what he would do after two minutes if it didn't. After 30 seconds, nothing had happened, and Price nodded to Skinner, who thereupon inched his hand over the top of the hatch. In it was a hand grenade, with the pin pulled.