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Authors: Michael Smith
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espionage links should be monitored. This was subsequently refined to interception and decryption of the links between Moscow and communist parties across Europe. A small Russian team was secretly set up at a GC&CS outpost overlooking London’s Park Lane to break the cipher. The keys were taken from an English edition of Shakespeare’s plays, in order to avoid the risk of the operator being caught with a codebook. By the late summer of 1943, less than two years after they had closed their Russian section, ostensibly for the duration of the war, the British code-breakers were again reading Soviet traffic. The end of the Second World War was almost two years away, but the preparations for the second Cold War had already begun.
3
REMINISCENCES ON THE ENIGMA
HUGH FOSS
Introduction
Although Bolshevik codes and ciphers had been the main target of the interwar years, by the time the codebreakers moved to Bletchley Park in late August 1939, these had been replaced by the attempts to break Enigma. It has been suggested that it was not until shortly before the war that the GC&CS codebreakers began to make any effort to break an Enigma machine. Although it is certainly true that Enigma had not enjoyed the highest priority during the interwar years, it was far from ignored. As the late Hugh Foss, who joined GC&CS in 1924, explains in the following chapter, he first looked into the possibility of breaking the machine in 1927, paradoxically with a view to seeing whether the British might want to use it themselves.
The workings of the machine will be explained in later chapters. But for those as yet unfamiliar with the subject, it looked essentially like a small typewriter in a wooden box. On most models, there was a standard continental QWERTZU keyboard,
as opposed to the British QWERTY, and above that a lampboard with a series of lights, one for each letter of the alphabet. Inside the machine were a series of three, or sometimes four, rotors, which were the main elements of the encipherment system. The operator typed in the letters of the plain-text message. The action of depressing the key sent an electrical impulse through the machine and the enciphered letters lit up on the lampboard. On the later
Wehrmacht
models, there was a plugboard, or
Stecker
system, which increased the variations of encipherment to around 159 million million million possible settings. Put like that, it seemed impossible to break. But the reality was that there were still only twenty-six letters in the German alphabet and that gave the codebreakers a chance. Examining the commercial C Model he was given, where the number of different possible settings was merely several million, Foss decided that it had a high degree of security but, given certain conditions, it could be broken. If you knew a piece of original plain-text, a ‘crib’, that was at least 180 letters long, the wiring of the first two wheels could be worked out; and if the wiring was known, a crib of just fifteen letters would be sufficient to break the machine settings.
GC&CS did a very small amount of work on the machine during the early 1930s. But it was not until the Spanish Civil War in 1936 that there was any real attempt to break live Enigma traffic. After some initial work by Josh Cooper, an Enigma machine, given by the Germans to the Italians and Spanish (the K Model), was broken by Dilly Knox on 24 April 1937, using an improved version of the system recommended by Foss ten years earlier.
Here, in a paper written in September 1949, Foss describes the early British work on the commercial ‘C Model’ Enigma, and the beginning of the contacts with the French and Poles that were to become so important to the subsequent British breaks. The mention of the QWERTZU, or diagonal, in this paper is a reference to the order of the wiring between the keyboard and the first set of contacts inside the machine. The British had been unable to work out the order in which the keys were connected to
3
REMINISCENCES ON THE ENIGMA
HUGH FOSS
Introduction
Although Bolshevik codes and ciphers had been the main target of the interwar years, by the time the codebreakers moved to Bletchley Park in late August 1939, these had been replaced by the attempts to break Enigma. It has been suggested that it was not until shortly before the war that the GC&CS codebreakers began to make any effort to break an Enigma machine. Although it is certainly true that Enigma had not enjoyed the highest priority during the interwar years, it was far from ignored. As the late Hugh Foss, who joined GC&CS in 1924, explains in the following chapter, he first looked into the possibility of breaking the machine in 1927, paradoxically with a view to seeing whether the British might want to use it themselves.
The workings of the machine will be explained in later chapters. But for those as yet unfamiliar with the subject, it looked essentially like a small typewriter in a wooden box. On most models, there was a standard continental QWERTZU keyboard,
as opposed to the British QWERTY, and above that a lampboard with a series of lights, one for each letter of the alphabet. Inside the machine were a series of three, or sometimes four, rotors, which were the main elements of the encipherment system. The operator typed in the letters of the plain-text message. The action of depressing the key sent an electrical impulse through the machine and the enciphered letters lit up on the lampboard. On the later
Wehrmacht
models, there was a plugboard, or
Stecker
system, which increased the variations of encipherment to around 159 million million million possible settings. Put like that, it seemed impossible to break. But the reality was that there were still only twenty-six letters in the German alphabet and that gave the codebreakers a chance. Examining the commercial C Model he was given, where the number of different possible settings was merely several million, Foss decided that it had a high degree of security but, given certain conditions, it could be broken. If you knew a piece of original plain-text, a ‘crib’, that was at least 180 letters long, the wiring of the first two wheels could be worked out; and if the wiring was known, a crib of just fifteen letters would be sufficient to break the machine settings.
GC&CS did a very small amount of work on the machine during the early 1930s. But it was not until the Spanish Civil War in 1936 that there was any real attempt to break live Enigma traffic. After some initial work by Josh Cooper, an Enigma machine, given by the Germans to the Italians and Spanish (the K Model), was broken by Dilly Knox on 24 April 1937, using an improved version of the system recommended by Foss ten years earlier.
Here, in a paper written in September 1949, Foss describes the early British work on the commercial ‘C Model’ Enigma, and the beginning of the contacts with the French and Poles that were to become so important to the subsequent British breaks. The mention of the QWERTZU, or diagonal, in this paper is a reference to the order of the wiring between the keyboard and the first set of contacts inside the machine. The British had been unable to work out the order in which the keys were connected to
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