The Bletchley Park Codebreakers (8 page)

Hut 6 solved Green for 25 October, the first wartime key to be broken in Britain, almost immediately after Turing’s return from France with the correct information about rotors IV and V. There was great relief when soon afterwards it broke Red (the main
Luftwaffe
cipher, which was widely employed for operational and administrative purposes) for 6 January, since there had been fears that the indicating system might have changed on 1 January. Hut 6 solved about fifty daily keys in Red, Green and Blue (a
Luftwaffe
practice cipher) between mid-January and late March 1940 using the Zygalski sheets.
A new cipher, Yellow, was first intercepted on 10 April, during the Norwegian campaign, and was then broken each day until it went out of service on 14 May. Yellow carried both
Luftwaffe
and
Heer
traffic, and also gave some information about ship movements and German intentions. However, little use could be made of the decrypts operationally. Hut 3, which was responsible for analysing and collating the intelligence contained in the decrypts, as well as translating them, was formed into three eight-hour watches. At this stage its staff were mostly ‘amateur soldiers and airmen, unversed in the ways of military intelligence’, and lacking the necessary experience to deal with the plethora of codenames, abbreviations, arcane technical terms and other mysteries in the decrypts. There were not even enough staff or teleprinters to handle the flood of intelligence from Yellow and Red. To compound matters, the service departments had been expecting a silent war where radio silence ruled, and were insufficiently organized to handle the copious intelligence produced by Huts 3 and 6.

On 1 May, the inevitable happened: all the ciphers except Yellow dropped doubly enciphered message keys. The Germans had at last realized that doubly enciphered message keys made Enigma vulnerable. All Enigma except Yellow became unreadable until 22 May, when Red for 20 May was solved using a highly ingenious method called ‘cillies’ (see Appendix IV), which had been discovered by Knox around late January 1940. Hut 6 abandoned work on other Enigma to solve about 1,000 messages on Red each day for the rest of May. The resulting flood of operational intelligence was teleprinted to Whitehall much more quickly than during the Norwegian campaign, and this time Whitehall was ready to forward it to the operational commands. Hut 6 depended completely upon cillies and related methods until the first bombe with a diagonal board entered service in August 1940, and continued to rely on them substantially for the remainder of 1940, and yet later whenever it had no other cribs. Red remained breakable with practically no gaps throughout the war, and was always the largest source by far of Hut 6 Ultra.

At the July meeting with the Poles, Knox had realized immediately that the Polish methods were completely vulnerable to a change in the indicating system, since they depended upon doubly enciphered message keys. Knox and Turing therefore decided to develop a British bombe which would be immune to any such change. The first British bombe was ordered before 1 November 1939. It was completely
different from the Polish model, since it used cribs to test certain assumptions about a key’s components, and did not merely check for ‘females’, which was the technique used by the Polish
bombas
. ‘Doc’ Harold Keen of the British Tabulating Machine Company at Letchworth was responsible for its detailed design, although the basic design concept, which defeated the all-important
Stecker
with their 150 million million key space, was the product of Turing’s genius. The bombes were not, despite some claims to the contrary, even remote forerunners of the computer, since their internal architecture bore not the slightest resemblance to that of a computer. Nor was Colossus (the Bletchley Park electronic computer) ever used against Enigma, as is sometimes suggested.

The first bombe, named Victory, entered service on 14 March but it lacked the diagonal board designed by Gordon Welchman, and therefore produced far too many ‘stops’, which had to be tested by hand – a very time-consuming process. It was therefore only used against naval Enigma in a somewhat basic way, and not in attacking
Luftwaffe
or
Heer
Enigma. The first improved bombe (named Agnus, later corrupted to Agnes or Aggie), with a diagonal board, came into service in mid-August 1940. Bombes comprised thirty-six banks of high-speed electrically driven Enigmas (except for a few early models, which had thirty banks), with three or more diagonal boards. The diagonal board made it much easier to devise bombe menus, and reduced the number of false stops by 99 per cent – from 676 to four with an eleven-letter menu giving two ‘closures’. Depending on the composition of a menu, up to three rotor orders could be run on a single bombe. Setting up a bombe menu took thirty-five to fifty minutes, and changing a wheel order, ten minutes. A complete bombe run lasted about fifteen minutes, after the bombe was set up. Each bombe had two Wrens assigned to it: an operator and a tester, who made a preliminary check on the stop data to see whether they should be sent to a ‘testing party’ in Hut 6 for further examination. The two bombes in service in 1940 broke about 180 daily keys, all but a few (probably under ten) being
Luftwaffe
Enigma – mostly Red, which comprised about 120 keys. The daily keys (including naval Enigma keys) solved by the British bombes rose to a peak of 9,064 in 1943, and declined slightly to 8,444 in 1944, when US Navy bombes also solved a substantial number of keys.

From about the end of 1940 onwards, the bombes were the essential
basis of Hut 6’s solutions of
Heer
and
Luftwaffe
Enigma. Bombes at Bletchley Park were an inter-service resource, and were not ‘owned’ by any one Hut. Demands for bombe time by Hut 8 therefore had to be fitted in with the much more complex situation on Hut 6 ciphers. When Hut 8 suddenly needed a large number of machines, it could badly disturb the Hut 6 programme. Fortunately, very few issues of bombe priority had to be referred to Whitehall for guidance, since it would have been far too time-consuming a process: one example was at the end of March 1942 when the Y Board (which was responsible for overall Sigint policy) considered a proposal to allot six bombes out of about twenty then available to work on Shark (the four-rotor cipher used by the Atlantic U-boats) for about twenty-five days. In the event, only 7 per cent of the total bombe time was allocated to Shark in April – about forty-two bombe days out of 600. It was just as well, since the original proposal would have severely disrupted Hut 6’s work. Another major decision on priorities was taken in July 1942, when the PQ 17 convoy was under way and the fighting in North Africa had also become critical. It was decided that the threat to PQ 17 justified priority being given to the naval work on Dolphin. There were remarkably few disputes about sharing the bombes, partly due to the close friendship between Stuart Milner-Barry in Hut 6 and Hugh Alexander in Hut 8, and the fact that Milner-Barry fully appreciated the critical importance of the Battle of the Atlantic. A panel of five ‘bombe controllers’ – Milner-Barry, John Manisty and John Monroe from Hut 6, and Shaun Wylie and Hugh Alexander from Hut 8 – was established in mid-1942. The controller on duty settled all questions of bombe priority without leading to any friction between the two Huts.

By May 1943, fifty-eight ‘standard’ three-rotor bombes, which did not print stop data, and fourteen ‘Jumbos’, which gave a printout, were being run by 900 Wrens and maintained by seventy-five RAF mechanics. By May 1945, 1,675 Wrens and about 265 men, virtually all from the RAF, were running and maintaining 211 bombes, including 66 four-rotor bombes for naval work. The bombe efficiency rate (the number of possible solutions, less those missed due to a bombe fault) hovered around 96 per cent throughout the war. Since each bombe contained about eleven miles of wire and 1,000,000 soldered contacts and received no preventative maintenance it was a minor miracle that they worked so well, twenty-four hours a day, seven days a week. A US Army contingent, the 6812th Signal Security Detachment, arrived
in February 1944 to operate a bank of about eight bombes in the Eastcote bombe outstation from October onwards. The 6812th achieved a high degree of efficiency, with a daily average of 71.5 runs in April 1945 (twenty minutes per run, including the set-up time, etc.). It was a considerable accomplishment, since the theoretical maximum was seventy-two – the highest recorded figure by the Wrens was fifty-seven (twenty-six minutes per run).

A good crib for the bombes consisted of between twenty and thirty letters, but about fifteen would do at a pinch (for example, ‘
Geraetklarmeldung
’ - equipment ready). Cribs comprising as many as seventy-five letters were not uncommon, while occasionally they were 200 or more letters long. Considering that for all practical purposes cribs had to be letter perfect, it is amazing that so many long cribs were found. The following crib was part of a series called ‘Sultan’s
Meldung
’, which broke Red on no fewer than six separate days in April 1943:

It was easily recognizable, since it was transmitted daily on specific frequencies at three set times by the
Luftwaffe’s Fliegerkorps
X (codenamed ‘Sultan’).

A frequent crib on Phoenix, which was used by
Panzerarmee
Afrika for its high-level operational communications between divisions and Corps, read:

To Welchman’s regret, the general responsible for the unit sending it was captured, with the result that the crib died out.

Hut 6 benefited greatly from the German reluctance to make the
Stecker
in
Heer
and
Luftwaffe
key-lists completely random.
Stecker
were not repeated on two successive days: if B and X were connected on Friday, that combination was not used on Saturday. The compilers of
Luftwaffe
key-lists also had a rule by which a letter was not connected to the next letter in the alphabet (B was not connected to C,
C to D, and so on). Bletchley added a ‘Consecutive
Stecker
Knockout’ (CSKO) modification to the bombes to take account of this restriction, by eliminating stops which included consecutively steckered letters. A different rule prevented any rotor being used on consecutive days in the same position in the machine. Thus in the day before and after rotor order II, V, I was used, rotor II could not be used in the left-hand (slow) position, rotor V in the middle position or I in the fast (right-hand) position, reducing the rotor orders to be tested to thirty-two, instead of sixty. In addition, a rule under which no rotor order was used more than once a month was very helpful, especially towards the end of a month. And some ciphers started to use only half the available rotor orders, which became known as ‘Nigelian’ wheel orders (possibly being named after Nigel Forward, a member of Hut 6).

Hut 6 also derived considerable help from the following basic mistakes by the German cipher operators:

In September 1940, Hut 6 had broken Brown, which was used by the
Luftwaffe’s
KGr 100, a pathfinder type unit, and by the Sixth Company of the
Luftwaffe’s
Signals Experimental Regiment. Until Brown ceased in late 1941, it gave indirect warnings of many KGr 100 raids, as well as information which later enabled an advanced ‘beam’ radio bombing system known as the
X-Gerät
to be jammed. The
Brown radio operators were so ‘peculiarly incautious in their radio chat’ that quite a lot of the intelligence attributed to Brown in fact came merely from ‘scarcely disguised plain language’ and not from decrypts. Daily keys could sometimes be solved by hand, especially on Brown, since it employed only six or seven pairs of
Stecker
instead of the usual ten. Moreover, Brown days were paired, with the letters linked by
Stecker
not being repeated on the second day of a pair.

Heer
Enigma was always much more difficult to break than
Luftwaffe
traffic, because
Heer
cipher discipline was always much higher than in the
Luftwaffe
, although the
Luftwaffe
improved in 1943.
Heer
Enigma included few cillies, or stereotyped beginnings or endings. Even in mid-1943, Dennis Babbage, the chief cryptanalyst in Hut 6, was worried that increased security precautions, such as the inclusion of ‘padding’ words at the start of messages and ‘burying’ addresses instead of leaving them at the start of signals, would soon prevent most
Heer
and
Luftwaffe
Enigma from being broken. Hut 6 wanted to find purely cryptanalytic or statistical methods to solve Enigma, but was never able to do so. Its worries and problems were to increase greatly as the war went on.

In January 1942, Hut 6 was receiving about 1,400 intercepts per day (43,600 per month), including Railway Enigma traffic, and producing 580 decrypts each day (18,000 per month). By May 1943, the number of daily intercepts varied between 3,300 and 6,000 – between 102,000 and 186,000 per month. No figure for decrypts is available for May 1943, but it would probably have been about half of the intercepts. To handle so much traffic clearly required a very efficient and flexible organization. By May 1943 (the period to which the description in the next four paragraphs relates), Hut 6 had evolved into five main sections, with a total of about 400 staff: a central party known as No. VI Intelligence School (later renamed Sixta), control party, operations group, operational watch and research party (see Figure 4.2).