The Bletchley Park Codebreakers (45 page)

The universal Turing machine consists of a limitless memory in which both data and instructions are stored, in symbolically encoded form, and a scanner that moves back and forth through the memory, symbol by symbol, reading what it finds and writing further symbols. By inserting different programs into the memory, the machine can be made to carry out any calculation that can be done by a human computer. That is why Turing called the machine universal.

It is not known whether Turing had heard of Babbage’s work when he invented the universal Turing machine. In their specifics, the Analytical Engine and the universal Turing machine are chalk and cheese. Turing later emphasized that the Analytical Engine was universal (a judgement that was possible only from the vantage point of the mathematical theory of universal machines that Turing himself developed). Nevertheless, there is an important logical difference between the two types of machine. In Turing’s machine, but not Babbage’s, program and data both consist of symbols in memory, and the machine works on both using exactly the same operations. Reading the program is no different from reading the data. This is the stored-program concept. Implicit in the concept is the possibility of the computer operating on and modifying its own program as it runs, just as it operates on the data in its memory. Turing was later to suggest that this ability of the stored-program computer to modify its own instructions might form the mechanism for computer learning – a topic now at the forefront of research in Artificial Intelligence.

In 1936, the universal Turing machine existed only as an idea. Right from the start, Turing was interested in the possibility of building such a machine, as to some degree was Newman. But it was not until their Bletchley days that the dream of building a miraculously fast general-purpose computing machine took hold of them.

The transition from cogwheel computers to electronic computers took just over a century. Various largeish, purely mechanical computing
machines were built, including the Scheutz Difference Engines, modelled on Babbage’s, and Bush’s (analogue) Differential Analyser, completed at the Massachusetts Institute of Technology in 1931. It took a skilled mechanic equipped with a lead hammer to set up the mechanical Differential Analyser for each new job.

The next generation of computing machines used electro-mechanical technology. Their basic components were small, electrically driven switches called ‘relays’. A relay contains a mechanical contact-breaker, consisting of a moving metal rod that opens and closes an electrical circuit. A current in a coil is used to move the rod between the ‘on’ and ‘off’ positions. A number of electro-mechanical program-controlled digital computers were built before and during the war. Turing himself built a very small electro-mechanical multiplier (it multiplied binary numbers) while at Princeton.

Electro-mechanical is not electronic. Electronic valves (called ‘vacuum tubes’ in the US) operate very many times faster than relays, because the valve’s only moving part is a beam of electrons. Relays are too slow for effective large-scale general-purpose digital computation. It was the development of high-speed digital techniques using valves that made the modern computer possible. Valves were used originally for purposes such as amplifying radio signals. The output would vary continuously in proportion to a continuously varying input, for example a radio signal representing speech. Digital computation imposes different requirements. What is needed for the purpose of representing the two binary digits, 1 and 0, is not a continuously varying signal but plain ‘on’ and ‘off’ (or ‘high’ and ‘low’). It was the novel idea of using the valve as a very fast switch, producing pulses of current – pulse for 1, no pulse for 0 – that was the route to high-speed digital computation. At the outbreak of war in 1939, only a handful of electrical engineers knew anything about this use of valves. One was Flowers, who estimated that there were no more than ten or twenty others in the whole world. When he was summoned to Bletchley – ironically, because of his knowledge of relays, a standard component in telephone equipment – he turned out to be the right man in the right place at the right time.

Flowers had joined the Telephone Branch of the Post Office in 1926, after an apprenticeship at the Royal Arsenal in Woolwich, well known for its precision engineering. He entered the Research Branch at Dollis Hill in 1930 (where he achieved rapid promotion). On his own initiative, he explored the feasibility of using valves to control
the making and breaking of telephone connections. His work in this area was, it appears, the earliest extensive use of valves as devices for generating and storing pulses. At this time, the common wisdom was that valves could never be used satisfactorily in large numbers, for they were unreliable and in a large installation too many would fail in too short a time. However, this opinion was based on experience with radio receivers and the like, which were switched on and off frequently. What Flowers discovered was that, so long as valves were switched on and left on, they could operate reliably for very long periods. He recognized that valves were not only potentially very much faster than relays but also, being less prone to wear, actually more reliable.

In 1934, Flowers put the new electronic techniques he had developed into use, in electronic equipment for the automatic control of connections between telephone exchanges. He used 3,000–4,000 valves in an installation controlling 1,000 telephone lines, each line having three or four valves attached to its end. Flowers’ design was accepted by the Post Office and the equipment went into limited operation in 1939. During 1938–9 he worked on an experimental digital high-speed electronic data store for use in telephone exchanges. His overall aim, achieved only much later, was that electronic equipment should replace all the relay-based systems in telephone exchanges. As Flowers remarked, at the outbreak of war with Germany he was possibly the only person in Britain who realized that valves could be used reliably on a large scale for high-speed computing.

Turing had been working on the problem of Enigma for some months before war broke out. In the period when GC&CS was still located in London, before the move to Bletchley Park, Turing would pay occasional visits to the office in order to compare notes on Enigma with Dilly Knox. At the outbreak of hostilities in September 1939, Turing took up residence at Bletchley. It was here that he would meet Flowers. By the beginning of November, Turing’s design for the British bombe – a radically improved form of the Polish
bomba
(both were electro-mechanical) – was in the hands of the engineers at the British Tabulating Machine Company. During the attack on Enigma, Turing approached Dollis Hill to build a relay-based machine to operate in conjunction with the bombe. Dollis Hill sent Flowers to Bletchley. In the end, the machine Flowers built was not used, but Turing was impressed with Flowers, who began thinking about an electronic bombe, although he did not get far.

Newman, meanwhile, spent the early years of the war in Cambridge, at St John’s College, where he had been a Fellow since 1923. He and Turing kept in close touch after Turing left Cambridge for Bletchley, writing letters about mathematical logic. (Turing’s first letter to Newman, written in March 1940, starts: ‘Dear Newman, Very glad to get your letter, as I needed some stimulus to make me start thinking about logic’.) Despite the pressures of codebreaking, Turing found time to collaborate with Newman on an academic paper, published in March 1942. In 1942, Newman was beckoned by Bletchley. He wrote to the Master of St John’s to request leave of absence and at the end of August joined Lieutenant-Colonel John Tiltman’s Research Section. Tiltman’s group was engaged in the attempt to break the German teleprinter cipher machine that the British codenamed ‘Tunny’.

The Germans developed several different types of encrypting machine for use in association with teleprinter equipment; the British operators first intercepted enciphered teleprinter messages during the second half of 1940. The British gave these teleprinter cipher machines the general cover name ‘Fish’. Three different types of Fish were known to GC&CS: Tunny, Sturgeon and Thrasher. GC&CS focused on Tunny (the Lorenz SZ 40, succeeded by the SZ 42), which was used mainly by the German Army.

An initial experimental Tunny link went into operation in June 1941 between Berlin and Athens/Salonika. By D-Day in 1944 there were twenty-six different links. Bletchley gave each link a piscine name: Königsberg–South Russia was Octopus, Berlin–Paris was Jellyfish, Berlin–Zagreb was Gurnard, and so on. The two main central exchanges were Königsberg for the Eastern links and Strausberg, near Berlin, for the Western links. The other ends of the links were usually mobile. Each mobile Tunny unit consisted of two trucks. One carried the radio equipment. The other carried teleprinter equipment and two Tunny machines, one for sending and one for receiving. The links carried messages from Hitler and the High Command to the various Army Group commanders in the field – intelligence of the highest grade.

In January 1942 the research section managed to reverse-engineer the Tunny machine on the basis of a single repeated message approximately 4,000 characters in length. William Tutte made the crucial break, deducing the structure and function of two of the machine’s various internal wheels. At this stage the rest of the research
section joined in and soon the whole machine was laid bare, without any of them ever having set eyes on one. It was, to say the least, a remarkable feat. The problem now that they knew the workings of the machine was how to read the message traffic. The difficulty was the wheel settings. Each message had its own wheel settings – its own ‘combination’, chosen by the German operator before enciphering the message – and could not be read until its combination was known.

In November 1942 Tutte invented a way of discovering wheel settings known as the ‘Statistical Method’. The rub was that the method seemed impractical, involving a very large amount of time-consuming work. This was the sort of work that, given enough time, a human computer could carry out – basically, the comparing of two streams of dots and crosses, counting the number of times that each had a dot in the same position. However, given the amount of comparing and counting that the method required, the intelligence in the message would be stale before the work was finished. Tutte explained his method to Newman, who suggested using electronic counters. It was a brilliant idea. In December 1942 Newman was given the job of developing the necessary machinery.

Electronic counters involving small numbers of valves had been in use in Cambridge before the war, in the Cavendish Laboratory, for counting sub-atomic particles. These had been designed by C. E. Wynn-Williams, a Cambridge don, but now involved in research at the Telecommunications Research Establishment (TRE) in Malvern. Newman worked out the cryptanalytical requirements for the planned machine and called in Wynn-Williams to design the electronic counters. Newman also approached Francis Morell, head of the telegraph and teleprinter group at Dollis Hill, to engineer the rest of the machine. (Morell’s group was located in the same building as Flowers’ switching group.) Turing himself appears to have had no direct involvement in Newman’s project, although Newman has remarked in this connection that Turing ‘was always full of ideas and he liked to talk about other people’s problems’.

Construction started in January 1943, and the machine began operating in June of that year, in Newman’s new machine-section, the Newmanry. The Newmanry consisted initially of Newman himself, Michie, two engineers, and 16 Wrens. The section was housed in Hut 11, originally the first bombe room. The machine was soon named ‘Heath Robinson’ by the Wrens. Smoke rose from it the first
time it was switched on (a large resistor overloaded). Part of Heath Robinson consisted of a huge angle-iron metal frame resembling an old-fashioned bedstead standing on end; this became known as the ‘bedstead’. Around the bedstead wound two long loops of teleprinter tape. Each loop was made by gluing together the two ends of a single length of tape. The two tapes were supported by a system of pulleys and wooden (later aluminium) wheels of diameter about ten inches. The tapes were driven by sprocket-wheels which engaged a continuous row of sprocket-holes along the centre of each tape. Both tapes were driven by the same drive-shaft so that they moved in synchronization with each other. The maximum speed of the tapes was 2,000 characters per second.

Each tape had rows of holes punched across its width. Every row was a letter, or other keyboard character, represented in international teleprinter code. In order to describe these patterns on the tape, as well as the pulsed patterns that made up the enciphered transmissions themselves, Bletchley used ‘×’ to mark the position of a hole (or pulse) and ‘•’ to mark the absence of a hole (or no pulse). The hole/no-hole patterns on the tape were read photo-electrically. Once these patterns had been converted by the photo-electric reader into electrical pulses, they were modified and combined in a specified way by a ‘combining unit’ (a logic unit, in modern terminology). The resulting number of pulses was counted by Wynn-Williams’ counting unit. The combining could be varied by means of replugging cables. Dollis Hill made the combining unit and the bedstead, and TRE the counting unit. In all Heath Robinson contained about two dozen valves.

The central idea of Tutte’s Statistical Method is as follows. Tunny encrypts each letter of the plain-text by adding another letter to it. The internal mechanism of the Tunny machine produces its own stream of letters, known at Bletchley as the ‘key-stream’, or simply key. Each letter of the cipher-text is produced by adding a letter from the
key-stream
to the corresponding letter of the plain-text. Tunny produces its key-stream by adding together two other letter streams which it generates, called the psi-stream and the chi-stream at Bletchley (from the Greek letters Psi and Chi). The psi-stream is produced by a mechanism consisting of five ‘psi-wheels’ and the chi-stream by a mechanism consisting of five ‘chi-wheels’. For example, if the first letter from the chi-wheel mechanism is M (••××× in teleprinter code) and the first letter from the psi-wheel mechanism is N (••×ו), then
the first letter of the key-stream is M + N, which under the rules of Tunny letter-addition is T (••••×). (Tunny adds letters by adding the individual dots and crosses that compose them.) Finally, supposing that the first letter of the plaintext is e.g. R (•×•×•), the first letter of the cipher-text is R + T, or G (•×•××).