The Bletchley Park Codebreakers (28 page)
Read The Bletchley Park Codebreakers Online
Authors: Michael Smith
Anglo-American co-operation on naval Enigma proved extremely successful, due to the excellent relations between Hut 8 and OP-20-GM. Without the help of OP-20-G’s superb four-rotor bombes, Shark and the other M4 ciphers could not have been consistently broken after mid-1943. Hut 8 broke about 1,120,000 out of the 1,550,000
Kriegsmarine
Enigma signals intercepted during the war, although only 530,000 decrypts were sent to the Admiralty, since many dealt with weather forecasts and other matters which did not affect operations, even indirectly. It was a magnificent performance, given that Hut 8’s complement never exceeded about 150 staff. Naval Ultra was the most important of the many intelligence sources, including photo-reconnaissance and the invaluable HF-DF fixes, available to naval intelligence during the war. Without Ultra from Shark, in particular, the course of the war would have been very different, since it must be very doubtful whether the Allies could have established naval supremacy in the Atlantic until the second half of 1943. Ultra was only one of many factors, including shipbuilding capacity, modern sensors and weapons, trained seamen and airmen, which contributed to the Allied victory, but Hut 8’s crowning achievement was to save countless lives, on both sides of the conflict, by helping to shorten the war.
Chapter 12 gives an insider’s views on breaking naval Enigma. Rolf Noskwith was a young Cambridge mathematics graduate when he was recruited for Bletchley under a new system headed by C. P. Snow.
He joined Hut 8 in June 1941, when it was starting to get into its stride, and was assigned to its crib room because of his fluent German. Hut 4 (Naval Section) had attempted to provide cribs until Hut 8 set up its own crib room in 1941, even though there was very little plain-text in 1940 from past decrypts, and virtually no continuity in breaking the few Enigma daily keys that had been solved. Frank Birch had erroneously believed that if Alan Turing, in Hut 8, tried out cribs with the bombes ‘systematically they would work’. But Birch had misunderstood the problems involved in using cribs, largely because Turing was ‘a lamentable explainer’. Some of Birch’s suggestions would have involved testing tens of thousands of results for a single
crib, which would have been completely beyond Hut 8’s scant resources in 1940 and the capacity of the sole bombe that was available until towards the end of the year.
However, as Hut 4 gained experience and more decrypts became available, Hut 8 came to appreciate the very close co-operation that developed between its crib room and Naval Section, including the latter’s intelligence sub-sections, a unit breaking hand ciphers and the U-boat plotting room there. When a second black-out threatened to develop in March 1943 on Shark, the cipher used by the North Atlantic U-boats, sterling work in Naval Section helped to save the day, and to prevent the black-out from lasting for the several months originally forecast.
Hut 8 would probably not have solved the Porpoise naval cipher when it did if Hut 4 had not kept pressing it to investigate the traffic, which Hut 8 thought used a much more complex system than emerged. Fortunately, the liaison between Huts 4 and 8 on all topics became much closer and more successful as the war progressed, although surprisingly it was not until May 1943 that regular meetings began between Hut 8 and the relevant parts of Naval Section. Happily, at the end of the war in Europe, Birch was able to write to the head of Hut 8, Patrick Mahon, of ‘two independent entities so closely, continuously, and cordially united as our two Sections’. Co-operation, whether with other Huts at Bletchley, with OP-20-G, or with GC&CS’s outposts, was the key to the many superb Allied successes in the codebreaking war.
RE
At the outbreak of the Second World War I was an undergraduate at Cambridge, reading mathematics. As I also spoke German I thought that I had some qualifications for ‘decoding’ work. This view was accepted by the University Recruiting Board and included in a wider recommendation for my war service. There was then a setback when I failed two medicals. I therefore continued with my studies and in 1940 I was interviewed for work at what I later recognized to be Bletchley Park – but my appointment was vetoed because of my foreign birth. In 1941 I had a similar interview with C. P. Snow (the future novelist who was then a Civil Service Commissioner) and Hugh Alexander. The rules had since changed. 1 was appointed at a salary of £250 per
annum as a Junior Assistant (later Junior Administrative Officer) at the Foreign Office which had jurisdiction over GC&CS.
I arrived at Bletchley on 20 June 1941, the day after my twenty-second birthday. Alexander met me at the station and took me through the Park entrance to one of the Nissen-type huts which had been erected in the grounds of the Victorian mansion. This was Hut 8, the unit of which Alexander was acting head and for which he had recruited me. It was only then that I was told that our task was to break the ciphers used by the German Navy. Despite my talk about decoding I had no conception of the work I would be doing and I entered a completely new world when I learned that the German Navy enciphered its signals using a machine called Enigma, which was also used by the German Army and Air Force.
I joined the crib section of Hut 8, headed by Shaun Wylie. We were known as cribsters and our function was to produce cribs, a crib being a guess of what a portion of a particular signal might be saying. A correct crib, tested on the bombes (the electro-mechanical machines developed by Alan Turing), would lead to the solution of a day’s keys.
Next door to us was a section, staffed mostly by mathematicians, which used a sophisticated application of probability theory, designed by Turing to reduce the number of variables which the bombes had to test. The process required the manipulation of big sheets of paper, each with punched holes representing an individual signal. These sheets were manufactured in Banbury: the process was therefore called Banburismus and the people doing the work were known as Banburists. There were ways in which cribsters and Banburists could help each other.
When I arrived in Hut 8, the keys for June and July 1941 had been captured by ‘pinches’, i.e. successful raids on German naval units, mostly small ships stationed in the remote North Atlantic to supply weather reports. Until 1 August we were therefore reading Enigma messages almost as quickly as the Germans for whom they were intended. While valuable intelligence was obtained from this material we studied it carefully in order to identify potential cribs.
A significant proportion of the messages turned out to be ‘dummies’, consisting of nonsense words like
DONAU DAMPFSCHIFF FAHRTS GESELLSCHAFTS KAPITAEN
(Danube steamship navigation company captain), further text such as ‘intended to deceive the enemy’ and finally a jumble of letters. Their purpose was to maintain
a fairly even flow of traffic so that no inferences would be drawn from an upsurge in the number of signals when some special activity was planned.
The most common cribs were derived from weather reports. The Germans sent out regular messages, usually twice a day, from weather stations at various ports, e.g. Boulogne, Hook of Holland, Royan etc. A typical message would read ‘from weather station Boulogne x weather forecast for’, followed by the area and period of time covered by the forecast. The operator sending out such a signal had strict orders to vary the text from message to message, but not uncommonly these orders were disregarded so that the same text appeared day after day. At other times it was possible to discern a shift system, three operators on a rota, each with his pet text which then occurred at predictable intervals.
Even then there could be numerous variations in the wording of a weather report. It was possible to narrow the choice by making use of an important characteristic of Enigma: a letter in the plain-text could be encrypted as any other letter but never as itself. For example, if a signal had ‘V as its first letter, the underlying message could not start with ‘von’, the German for ‘from’. If we had a crib for the message starting with VON, we would say that it ‘crashed’. Conversely, if we had a long crib, say twenty-five or more letters, which did not crash, there would be an increased chance that it might be correct.
The captured material ran out at the end of July so that 1 August 1941 brought a great challenge for Hut 8. By an apparent stroke of amazing good fortune the day produced two probable weather messages (identified by frequency, code sign and time) which had both been encrypted with the same starting position of the wheels in the machine. Their beginnings were:
1. MON.O 2. MVV.E
It seemed highly probable that the decrypts would begin as follows:
M O N . O .
V V V (W) E .
M V V . E .
V O N V O N
This was a perfect example of a ‘depth crib’, combining the probable plain-text of two or more signals, where the starting
positions were identical or in close proximity. The letters MV in the first column illustrated a ‘click’, where a common letter in both signals had to have a common decrypt. The subsequent parts OV/VO, NV/VN, OE/EO were known as ‘reciprocoggers’, i.e. paired letters which might be encrypts/decrypts of each other: such pairs increased the probability that the solution was correct. VVV and VONVON were alternative forms used in signals to mean ‘from’, and the messages were likely to continue with the German for weather station, i.e. WETTERWARTE or, more frequently, WEWA. This led to a small number of permutations which were almost certain to include a crib of sufficient length for the bombes. Despite this hopeful start we failed to achieve a break, and 1 and 2 August remained among the very few days which were never read.
This was an exceptional disappointment: for the next six months we demonstrated the efficiency of Banburismus, cribs and bombes by breaking almost every day, sometimes very quickly, occasionally with a few days’ delay. We had the satisfaction of learning that the resulting intelligence, used with great discretion in order not to compromise the source, led to a dramatic reduction of sinkings of merchant ships by U-boats in the Atlantic.
It was during this period that I was lucky to find a successful crib for an
Offizier
message. Such messages, thought to require a higher degree of secrecy, were encrypted by an officer using different pluggings
(Stecker)
and one of twenty-six starting positions of the wheels. At that time, the pluggings changed every day and the twenty-six positions every month. The encrypted result was then encrypted a second time by the regular operator with the normal key for the day, with a preamble featuring the word
Offizier
and a name (Anton for A, Berta for B, etc.) indicating the starting position.
I was not expecting to be successful and went home on leave before the testing on a bombe was complete. Wylie promised to confirm a positive result by sending me a telegram containing the name of a fish. When a telegram arrived with the word ‘pompano’ I had to look it up in a dictionary to make sure that ‘pompano’
was
a fish. The breakthrough resulted in more knowledge about the content of
Offizier
messages which subsequently helped us to read most of these signals without great difficulty.
We dealt successfully with a change in October 1941, when the Germans split the traffic into two keys, one for U-boats in the Atlantic
and one for the rest. We called the U-boat key Shark and the other Dolphin. For some reason the U-boats in the Arctic, north of Norway, remained on Dolphin.
We were forewarned by some messages that a more serious change would occur on 1 February 1942. On that day, the Enigma machines used for Shark started to use a fourth wheel. The change multiplied by twenty-six the number of combinations of variables to be tested by the bombes – it also made Banburismus impossible. Except on rare occasions we did not, at that time, have enough bombe capacity to try possible cribs. We therefore broke only three Shark keys between February and mid-December 1942. The consequence was an upsurge in the number of sinkings, which reached dangerous levels for the rest of that year.
I have been asked whether our prolonged inability to break Shark gave us a sense of guilt. While we knew the seriousness of the situation I cannot say that we felt guilty. First, we genuinely felt that, without more captured material, there was no short-term solution. Secondly we knew that there was a long-term solution because of plans, in collaboration with the Americans, to build more powerful bombes capable of coping with the four-wheel machines. Thirdly we were still regularly breaking Dolphin (the other main key) as well as, from the summer of 1942, a separate key called Porpoise used for traffic in the Mediterranean. We did have a sad time in July when we were late in breaking a crucial day while the Arctic convoy PQ17 was being slaughtered by U-boats and aircraft.
In his chapter on ‘Breaking Naval Enigma’, Ralph Erskine has given an account of the steps which enabled Hut 8 to resume its breaking of Shark from mid-December 1942 to June 1943 with the aid of ‘short signals’ used by U-boats for weather reports (WWs) and operational reports (B-bars). This success could not have been achieved without the men from HMS
Petard
, who gave their lives in capturing the necessary codebooks from U-559.
The construction of successful cribs for WWs was then dependent on the breaking of a meteorological cipher in Hut 10 under Philip Archer; the result became known in Hut 8 as Archery. In the later stages essential information relating to B-bars was provided by Edgar Jackson, head of one of the Naval Intelligence sections in Hut 4 (afterwards Block B). Within Hut 8 a major contribution to this work was made by Michael Ashcroft whose promising post-war career at the Treasury was tragically cut short by his death from cancer in 1949.
June 1943 marked the beginning of a new era for Hut 8. The first of the new bombes had arrived, capable of dealing with the variables of a four-wheel Enigma machine. Quite soon we had a growing arsenal of three-wheel and four-wheel bombes. This meant that we could do all the key breaking by cribs, without the help of Banburismus, even on Dolphin, which stuck to three wheels until 1944. In the following months there was a gradual exodus of talent from Hut 8: Wylie, Ashcroft and others left to join the Tunny section (Block F) under Max Newman, where they contributed to another of the great Bletchley successes. Finally, in autumn 1944, Alexander was transferred to work on Japanese naval codes and Patrick Mahon became head of Hut 8.