Read The Secrets of Station X Online
Authors: Michael Smith
Around the same time as the Lofoten Islands ‘pinch’, the codebreakers discovered that messages sent on Enigma were also being sent using a lower level hand cypher, known by the Germans as the
Werftschlüssel
, the Dockyard Key,
providing
a potentially useful source of cribs. The
Dolphin
‘Home Waters’ Naval Enigma traffic, sent in four-letter groups, was now teleprinted direct to Hut 8 from the Royal Navy intercept site at Scarborough. But there was still no continuous break. The search went on for a ‘cunning scheme’ that would find the crucial ‘pinch’ to help Turing and his team, now strengthened by the arrival of Hugh Alexander from Hut 6 with Hut 8 moving on to a 24-hour shift system in an indication of the growing belief that the vital breakthrough was not far away.
It came when Hinsley found messages to and from German weather ships among the Enigma traffic. The Germans encoded the weather messages using the
Wetterkurzschlüssel
, or short weather key, and they were then sent as short weather signals. They were made as short as possible to cut down the amount of time spent transmitting and make it more difficult for the British DF sites to locate the ships sending the weather messages. These weather ships, stationed in two places, north of Iceland and in the mid-Atlantic, would need to have exactly the same equipment and keys as any other ship using Enigma but would be far more vulnerable to raids designed to furnish a ‘pinch’. Just as importantly, the same encoded short weather messages were then being encyphered using the Naval Enigma when they were sent on to other ships, providing a rich source of potential cribs.
The German weather-ship the
München
was captured in early May, providing the settings for June. A few days later, more material was captured when the
U-110
was forced to surface off Iceland. A second weather-ship, the
Lauenberg
, captured at the end of June gave Hut 8 the settings for July. Turing and his team read through June and July using the captured cyphers. From the beginning of August they were on their own. But
as a result of the continuity established over the previous two months they had built up a library of cribs that, together with those provided by the weather messages, gardening, and a
process
known as Banburismus, allowed them to decypher
Dolphin
with only a few days missing in August and September and from 20 September 1941 every day until the end of the war.
The break into
Dolphin
was followed by success against the
Offizier
system, which allowed Naval Enigma cyphers to be double-encyphered by officers for confidential messages. Leslie Yoxall, another mathematician from Sidney Sussex, Cambridge, devised a way of breaking the
Offizier
cyphers. Alexander recalled that Yoxall ‘caused a considerable sensation in breaking an “example” that had been set him on a length of eighty letters. This was particularly striking as it happened about a day after Turing – rightly recognised by all of us as the authority on any theoretical matter connected with the machine – had stated his opinion that 200 letters constituted a theoretical minimum. Yoxall must indeed have been inspired on this occasion as none of us ever succeeded in after days on a length of under 100 letters without some sort of crib.’ The system for breaking
Offizier
became known as Yoxallismus.
Initially, Banburismus was absolutely vital to the
breaking
of Naval Enigma. Its name derived from the fact that it involved the use of long strips of paper, which were known to the codebreakers as ‘Banburies’ for the simple reason that they were printed in Banbury. It was devised by Turing as a means of cutting down the number of possibilities of wheel orders in order to cut the amount of time the Bombes would need to find the wheel order. The Bombe could test all the 17,576 possible positions on a single wheel order in about twenty minutes. So to test all 336 possible wheel orders would have taken one Bombe 112 hours, or nearly five days. Even if five Bombes were put to work on the problem it would take a full twenty-four hours. Banburismus aimed to cut the possible wheel orders to around twenty, which reduced the time for a single Bombe to a more
manageable six hours and forty minutes. With so few Bombes in existence at this stage, the reduction in wheel orders obtained from Banburismus was extremely important. Noskwith said:
The aim was to identify the right-hand and middle wheels because you could locate the turnover point of the wheel. Most wheels had different turnover points, so if you could show that the middle wheel turned over between E and F that would be, I think, wheel two. Identifying the right-hand and middle wheels meant you had to try fewer combinations on the Bombes.
A Banburismus section was set up in Hut 8 to look for messages containing similar streams of letters which had been sent with the wheel positions relatively close to each other giving streams of the same letters. These coinciding streams of letters were known as ‘fits’, said Patrick Mahon, another of the Hut 8 codebreakers.
Banburismus aims first of all at setting messages in depth with the help of ‘fits’ and of a repeat rate much higher than the random expectation. The idea behind Banburismus is based on the fact that if two rows of letters of the alphabet, selected at random, are placed on top of each other the repeat rate between them will be one in twenty-six, while if two stretches of German Naval plain language are compared in the same way the repeat rate will be one in seventeen. Cypher texts of Enigma signals are in effect a selection of random letters and if compared in this way the repeat rate will be one in twenty-six but if, by any chance, both cypher texts were encyphered at the same position of the machine and then written level under each other, the repeat rate will be one in seventeen because, wherever there was a plain language repeat, there will also be a cypher repeat. Two messages thus aligned are said to be set in depth.
The initial searches for ‘fits’ were done using tabulating machines which picked up coincidental streams of four or more letters, Mahon said. ‘At the same time messages were punched by hand onto Banburies, long strips of paper with alphabets printed vertically, so that any two messages could be compared together and the number of repeats be recorded by counting the number of holes showing through both Banburies.’
The Banbury sheets were about ten inches wide and several feet long. They had columns of alphabets printed vertically on them, giving horizontal lines of As, Bs, Cs, etc. Clerks punched holes into the paper to correspond with the
encyphered
messages, thus with a message beginning JKFTU, the J of the first column would be punched out, the K in the second column, the F in the third column and so on. They then aligned the sheets of paper over each other on top of a dark-coloured table and moved them to the left or right looking for repeats where the holes coincided and the table showed through.
The aim was to find points in a number of messages where a sequence of the machine coincided. If there were two messages with the indicators equating, for example, to the starting points XYK and XYM, then the second message would start two spaces on. So if the initial letter of the second message was moved to a position over the third letter of the first message then the letters in each column would be encoded in the same position. This would show up in an unusual number of repeats. The more repeats there were the more likelihood there was of the two sequences having been encyphered in the same position.
‘If you’re lucky and you’re lucky pretty frequently, you might come across a four-or five-letter repeat,’ said Peter Twinn.
You would say to yourself, ‘A five-letter repeat, it’s greatly against the odds, there must be a reason for it, what is it?’ and the answer is that it represents the re-encodement of the same German word in both messages and you might be able to make a reasonable guess at what it was, having seen some German
messages encyphered in the past. So that would give you a little start and then you would try and fit a third message on and you might find with a bit of luck that, when you staggered it off with both of them, this third message had two trigrams. One clicked with one of your messages and another trigram clicked with three in a quite different place on the first message. I’m leaving out a lot of the difficulties, but you gradually build up a selection of twelve or fifteen messages out of the day’s traffic which if you make some other guesses and, if you’re very, very lucky, you can do one of a number of three things. You can, for a start, cut down the number of wheel orders the Bombes need to check. But you can also either find out the wiring of a brand new wheel or you can work out with a reasonable degree of accuracy what these messages might be saying.
The Hollerith tabulating machines, mechanical digital data processors, which made the initial searches for the Banburismus ‘fits’, were provided by the British Tabulating Machine Company (BTM) based at Letchworth. They were controlled by a BTM expert called Frederic Freeborn, who ran the central index in Hut 7, which initially housed the automated Hollerith punch-card sorters. Under Freeborn’s direction, female clerks not only carried out searches of the Enigma traffic for features that might assist the codebreakers, they cross-referenced every piece of information passing through Bletchley onto punch cards. A request for details of a radio station, unit, codeword, covername or indeed any type of activity would swiftly produce every card containing any previous mention or occurrence.
Marjorie Halcrow, a 22-year-old graduate from Aberdeen University, was one of those recruited by Freeborn to work in Hut 7. ‘The cards were actually punched up on a machine about the size of a typewriter,’ she said.
There was a room containing about twenty or thirty of them called the punch room where girls copied the coded messages
onto these punch cards. The main room contained much larger machines, about the size of a small piano, called the sorting machines which could read the cards and sort the hundreds of thousands of messages into different categories. There were loads of sorters and there were collating machines that were even larger. The whole department was filled with machinery. It was a very noisy place, all banging on all night and day long.
Eventually there were so many Hollerith machines in use that they were operated at four separate outstations clustered around Bletchley. This frequently forgotten part of Britain’s wartime codebreaking operations was not always used as efficiently as it might have been. Welchman recalled:
The cryptanalytical sections would have had a better service if they had simply discussed their needs with Freeborn instead of dictating to him. This would have given him the chance to programme the overall use of his equipment and staff in a way that would have been advantageous not only for each individual problem solution, but for the overall service he was providing.
Hut 8 was now divided into four sections. The registration room, where the traffic arrived and was sorted; the ‘Banburismus Room’, where codebreakers tried to break the keys using Banburismus; the ‘Crib Room’, where the codebreakers tried out cribs; and ‘the Big Room’ where female clerks punched up the messages on the Banbury sheets and decyphered messages on Type-X machines once the wheel order, settings and keys had been recovered.
The codebreakers themselves rarely read the decrypts. Peter Twinn recalled having very little interest in what they were actually saying. ‘I would have to confess I don’t think I really understood the full significance of it,’ he said.
I think I’d have to excuse myself by saying that we lived at that
time in a very narrow little world. Remember that I was an inexperienced lad of twenty-four or twenty-five and I’d come into it straight from university, I don’t think I had a real grasp of what a major war was all about and we did work very much in a rather monastical way. I don’t recall ever having decoded a message from start to finish to see what it said. I was much more interested in the methodology for getting German out of a coded message.
The decyphered messages were passed via Z Watch, the Naval Section’s equivalent of Hut 3, over a newly installed teleprinter link that allowed the Hut 8 decrypts to be sent direct to the OIC, giving prior warning of the wolf-pack patrol lines and allowing the convoys to be routed away from danger. The OIC was fully indoctrinated into Enigma so there was no need for the agent’s disguise required for Hut 6 German Army and
Luftwaffe
decrypts. The result was truly dramatic. Between March and June 1941, the U-Boats had sunk 282,000 tons of shipping a month. From July, the figure dropped to 120,000 tons a month and by November, when the wolf packs were temporarily
withdrawn
from the Atlantic, to 62,000 tons.
The breaking of the Naval Enigma was one of the main reasons for this drop in the fortunes of the U-Boats,
providing
the British with a welcome respite during which the vital supplies had a much greater chance of getting through, Harry Hinsley said.
It has been calculated that, allowing for the increased number of U-Boats at sea, about one-and-a-half million tons of
shipping
(350) ships were saved. This intermission was invaluable for the level of British supplies, the building of new shipping and the development of anti-submarine defences.
Despite the brilliance of men like Turing and Alexander, the Naval Enigma could not have been broken without the
Bombes. Larger improved versions known as Jumbos because of their size had been introduced and in order to protect them from German air raids they were dispersed to new outstations at nearby Wavendon and Adstock. At the same time, the Bletchley Bombes were moved out of the back room in Hut 1 and into Hut 11, acquiring a number of new operators.
The eight Wrens who arrived on 24 March 1941 were a trial measure. Previously the Bombes had been operated by soldiers, airmen and sailors who before being called up had worked for BTM, which built the Bombes. But male servicemen were at a premium and the number of Bombes was being constantly increased to cope with the need to keep the breaks of keys going, so it was decided to try the Wrens out as ‘an experiment’.