The Bletchley Park Codebreakers by Michael Smith

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 completelydifferent 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,