The Bletchley Park Codebreakers

by Michael Smith

  4. A system with doubly enciphered indicators was easy to diagnose because study of the indicator starting groups for a day’s traffic would quickly reveal eight consecutive places in which each letter in one of the first four places was always followed by the same letter in the place four places later (e.g. A in place 1 always followed by T in place 5 – sometimes the same letter would appear in both places, an occurrence known as a ‘female’). If several indicators were available, ‘chains’ could be constructed joining corresponding letters in places four apart: e.g. if the indicator pairings in places 1, 4 were AM, MN, NT … the partial chain AMNT … would arise. Given a large enough set of indicators – probably at least thirty, allowing for non-random choice of settings – more or less complete chains could be obtained for each pairing 1–4, 2–5 etc., and from these letter pairings (i.e. alphabets) deduced, especially if some message settings could be guessed. Dilly Knox called this process ‘boxing’.

  5. To recover the wiring of an unknown Enigma from intercepted traffic some kind of crib – i.e. a cipher text and its en clair equivalent – is necessary. No straight crib – e.g. the retransmission of a message deciphered from another system – for the networks using Abwehr multi-notched Enigma was discovered, and the best hope of breaking into the traffic was judged to be the attempt to decipher a day’s indicators. To do that with truly arbitrary message settings would require a large number of messages, but with slack operators using easily guessed four letters the task was managed with some 15–20 messages. This happened with the Berlin–Rome Sicherheitsdienst link, whose choice of indicators was so helpfully lewd as to produce a reprimand from Berlin to the station head, Kappler, reminding him that young girls had to decipher the messages in Berlin.

  6. A property of Enigma machines is that a letter will never be enciphered as itself. This meant that if, for example, it was suspected that messages were all starting in the same way, an analysis of the letters occurring in the first few places of thirty or more cipher texts would show that in each place one letter did not occur, so revealing the plain-text. This process was called a ‘boil’.

  7. The effect of the RH wheel is to join each disc on the end plate to a pin on the middle wheel, so that if the discs are labelled 1 to 26 clockwise (viewed from the LH side of the entry plate) and the pins on the middle wheel similarly labelled anti-clockwise, the connections made by the RH wheel may be entered in a table containing twenty-six rows and twenty-six columns, in which the entry in row r and column s means that in position s of the RH wheel the disc nrs of the end-plate is joined to pin r of the middle wheel. Each row of the square was known as a ‘rod’. If pins x and y of the middle wheel are connected by the wiring through that wheel, the LH wheel and the reflector, then placing the rods x and y together with matching columns will give the numbers of the end-plate discs connected through the machine at each position of the RH wheel for which the other parts of the machine stay the same.

  8. Points to note about rods are:

  a) if nrs appears in row r, column s, then nrs+1 will appear in row (r+1), column (s+1);

  b) the numbers in each diagonal of the rod square run from top left to bottom right in numerical order;

  c) if the keyboard keys and the light-bulbs are wired to the end-plate in the order QWERTZU … reading clockwise viewed from the keyboard – and hence anti-clockwise viewed from the RH wheel – letters can be substituted for the numbers on the rods using the substitution:

  Q L M N B …

  1 2 3 4 5 …

  The keyboard order is reversed because the end-plate discs are numbered anti-clockwise from the keyboard aspect. This troublesome business of clockwise and anti-clockwise led Dilly on occasion to tease new entrants with the question, ‘Which way does a clock go round?’

  To complicate matters further, if the rod labels are arranged in the order QWERTZU… the rod square will have diagonals running from top right to bottom left!

  d) If a rod contains two or more letters of a text enciphered with its associated wheel in the RH position and there is no turnover between them, then the associated plain-text letters must also lie on one rod. This fact was especially useful if two adjacent cipher letters were on the same rod, because the deciphered letters must then also lie on one rod; as the bigrams in any two adjacent rod columns were usually unlikely to occur in plain-text (e.g. JT, VQ etc.), this reduced the number of possible decipherments; and with luck and persistence it was sometimes possible to ‘rod out’ a piece of plain-text and recover the day’s key.

  e) Because of the diagonal structure of the rod square (cf. (b) above), if a rod had a bigram AG in places 8, 9, say, then AH was part of column 8, SJ part of column 7, and so on reading down the diagonal. This fact enabled a wheel’s wiring to be discovered if five or six sets of letter pairings were available for each of four consecutive positions, because if one assumed, given a pairing FS in position 2 and HL in position 3, say, that FL was a rod bigram then SH would also be a bigram, and so column 1 would contain pairs GW and DK, leading to implications about bigrams at other positions and hence other pairings in column 1; wrong assumptions would rapidly lead to contradictory deductions, and a correct assumption would lead to no contradictions.

  9. If discs numbered x and y on the end-plate are connected through the machine, then (x+1) and (y+1) will be connected if the three wheels and reflector are each moved one place forward. Using the substitution of numbers by letters as described in paragraph 8(c), it follows that if, for example, letters T and V are connected at the setting ABCD, letters R and C – i.e. the preceding letters in QWERTZU … – will be connected at BCDE, and so on; and any setting obtained by rotating all three wheels and reflector by x positions will produce an alphabet got from that at ABCD by replacing each letter by that x positions before it in the QWERTZU… sequence. The chains obtained by ‘boxing’ the alphabets at 1–5, 2–6, 3–7, 4–8 at the second setting will also be obtained from those at ABCD by a QWERTZU … substitution.

  Figure 17 Rod square for the Green Wheel – 3 Beetles (in rows e, t and j) are emboldened and cirlced

  * The present author married Miss Mavis Lever on 5 November 1942

  18

  BREAKING TUNNY AND THE BIRTH OF COLOSSUS

  SHAUN WYLIE

  Introduction

  The breaking of the Enigma machine ciphers is invariably cited as the great achievement of the Bletchley Park codebreakers. But the breaking of the German enciphered teleprinter traffic, given the generic codename of ‘Fish’, was a far greater achievement. After an early break by John Tiltman, who yet again succeeded in making something extraordinarily difficult seem very easy, Bill Tutte, another of the unsung British codebreakers, broke the Lorenz SZ 40/42 teleprinter cipher attachment. The solution of this cipher machine and its traffic, codenamed ‘Tunny’ by GC&CS, must rank as one of the finest cryptanalytical achievements of all time. It also led to the development of Colossus, the world’s first semi-programmable electronic computer.

  This chapter by Shaun Wylie, who worked on Tunny at Bletchley, gives a real insight into the very considerable effort devoted by GC&CS to its solution. It was more than worthwhile, providing intelligence of the highest grade, including communications from Hitler direct to his frontline commanders during the Allied invasion of Europe. Tunny decrypts may even have helped the Russians to win the Battle of Kursk, the turning point on the Eastern Front. Stalin was distrustful of the sanitized intelligence being given to him by official British sources, but the raw Tunny decrypts passed to Moscow by John Cairncross, the ‘Fifth Man’ in the Cambridge spy ring and a member of Hut 3, gave him details of the German battle plans straight from the horse’s mouth. Not only did the Tunny decrypts provide high-grade intelligence, they provided it in unprecedented quantity. Walter Jacobs, a US Army codebreaker who worked at Bletchley Park, wrote in an official report on the operation to break Tunny that in March 1945 alone ‘upwards of five million letters of current transmission, containing intelligence of the high
est order, were deciphered’.

  Tunny was not the only German teleprinter cipher broken at Bletchley Park. The British codebreakers also worked on the Siemens and Halske T52 teleprinter cipher machines, codenamed ‘Sturgeon’. The T52 had at least four variants. In a little-known operation, GC&CS reconstructed them all, including the T52d, which was a significantly more complex machine than the earlier models. But for a number of reasons, including the fact that the machines were mainly used by the Luftwaffe - on which Enigma traffic already provided a great deal of information - Bletchley Park decided to concentrate on Tunny.

  MS

  In the autumn of 1943, I was moved from Hut 8 of Bletchley Park, where I had been working on naval Enigma, to Hut 11, where I worked on Tunny, the Lorenz SZ 40/42 teleprinter cipher machine. For me, Hut 8 had been fascinating and immediate. I was in a section, part of whose job it was to study the deciphered signals and know as much as possible about them. Suddenly what we did produced the Enigma settings and a day’s traffic tumbled out. The satisfaction to be had from the work I was involved in on Tunny was different. We formed part of a process, improving our methods of using machines and devising new approaches; actual plain-text was none of our business, only its statistical features. We were told of the importance of the intelligence derived from Tunny, and we prided ourselves on the efficiency of our part of the operation; but the results weren’t immediate. If, however. Tunny was less exciting for me than Enigma, there was one personal bonus that more than compensated: I met and married Odette Murray, who was one of the first batch of Wrens in the section.

  Fish

  When the Germans invaded Russia, they started to use a new type of enciphered transmission between central headquarters and headquarters in the field. The signal was a string of teleprinter characters: each character consisted of five impulses (or ‘bits’), each impulse being either positive (a dot) or negative (a cross). These signals were taken at a station in Knockholt, near Sevenoaks in Kent, where they were painstakingly transcribed. They reached Bletchley Park on teleprinter tape and handwritten on the Red intercept forms.

  All such systems were known at Bletchley Park as Fish. There were three kinds: Thrasher, Sturgeon and Tunny. Nothing positive was ever found out about Thrasher. Sturgeon (the Siemens and Halske T52 series of machines) was diagnosed after a remarkable feat of analysis by Michael Crum, an Oxford research mathematician; the system, however, was too complex for this heroic diagnosis to be followed by exploitation. Tunny, on the other hand, was both diagnosed and exploited, with a valuable harvest of intelligence. This chapter is therefore about Tunny.

  The Tunny Machine

  In Tunny, the cipher stream (Z) is obtained from the stream of plain-text (P) by adding a stream of key (K), character by character. Symbolically Z = P + K. Two characters are added together by adding their corresponding bits level by level; two different bits add to a cross, two equal bits to a dot. So for instance (using the international teleprinter code):

  Subtraction is the same as addition (making decipherment the same process as encipherment), so that also

  H = N + O and N = H + O

  The key-stream in its turn was the sum of two streams:

  K= χ + ψ'

  The χ-stream (chi-stream) and the ψ-stream (psi-stream) were generated by twelve wheels in the Tunny machine, each with a different number of pins on its circumference, carrying its pattern of dots and crosses. The operator set the pattern by making each pin active (cross) or inactive (dot). The total number of pins on a wheel determined the length of its pseudo-random pattern.

  i) Five of these, called by us the χ-wheels (chi-wheels), moved regularly, producing the chi-stream. The lengths of their patterns were 41, 31, 29, 26 and 23, the wheel of length 41 giving the χ1-stream for level 1 in the teleprinter code, the next for level 2, and so on up to level 5.

  ii) Another five, the ψ-wheels (psi-wheels), had patterns of lengths 43, 47, 51, 53 and 59. Had they also moved regularly they would have produced a psi-stream. In fact they moved under instruction from a ‘total motor’ stream of bits; when the bit was a dot, they all stood still; when it was a cross they all moved. That produced the ‘extended’ stream ψ' (extended psi).

  iii) The other two wheels were motor wheels of lengths 61 and 37; the longer μ61 moved regularly; when it was at a cross, the shorter μ37 moved on one place; when at a dot, μ37 stood still. The resulting extended stream μ37' provided the basic motor stream of dots and crosses. In the original Tunny machine it was the basic motor that constituted the total motor. In later versions, a ‘limitation’ stream was generated from other streams, which combined with the basic motor to give the total motor stream.

  The whole contraption, remarkably, was diagnosed from the transmissions themselves.

  The Transmissions

  Initially there was a single experimental communications link; later links were put into use covering continental Europe and North Africa. The German operator (assumed masculine in what follows), sitting at his teleprinter console, sent information in clear about the initial wheel settings and then switched over to the cipher mode. Whatever he then put on line was automatically enciphered by the Tunny machine and automatically deciphered at the other end. He could input messages that had already been punched onto teleprinter tape (recognized at Knockholt as such and noted as ‘Auto’ on the red forms), or he could tap out messages himself; in either case he would intersperse the messages with operator chat, doodling and corrections (all noted by Knockholt as ‘Hand’). One transmission could include many messages.

  To start a new transmission, he could choose new initial wheel positions or (apparently) he could press some gadget that returned the wheels to the starting positions just used. Fortunately for us, he quite often took the easy option, sending two transmissions from the same start. Then (except when the link was using one of two particular limitations) the two transmissions had the same key-stream throughout:

  Za = K + Pa and

  Zb = K + Pb

  So, subtracting, Za – Zb = Pa – Pb.

  The observed stream Za – Zb could often be analysed by linguists as Pa – Pb, and the plain-texts of the two transmissions recovered in whole or in part. From that the key-stream can be formed as (for instance) Za – Pa. Such transmissions were said to provide a ‘depth’.

  The Diagnosis

  The diagnosis of Tunny was a triumph of the research section at Bletchley Park. No doubt other members of the section contributed, but the two feats are ascribed to John Tiltman and Bill Tutte. Tiltman was an already revered cryptanalyst, oozing confidence; Tutte was a recently recruited Cambridge research student, on the face of it (but only on the face of it) oozing diffidence.

  Exploitation of Tunny traffic depended on three processes:

  i) Diagnosis of the machine, once and for all.

  ii) Recovery of wheel patterns, once for each link and pattern-period.

  iii) Setting of known patterns, once for each transmission.

  In early Tunny, the preamble to a transmission would include an indicator, twelve letters sent as Anton, Bertha … (A, B, etc., which were spelt out in full phonetically, to reduce garbles and errors). On 30 August 1941, two long transmissions had exactly the same indicator. A previous depth had been partly read on the correct assumption that Z = P + K. Tiltman could therefore attack this new depth on that assumption. After breaking into the beginning by reading likely starts he discovered that both transmissions were hand-sendings of the same message, but with different spacings, misspellings and corrections. This fortunate feature enabled Tiltman to battle his way through the entire depth. So different were the two texts that at the 3,976th character, when the shorter one ended, the longer still had 100 more characters.

  The upshot of this was that the research section had (by subtracting the recovered plain-text from its cipher) almost 4,000 characters of key. Their only problem was what to do with it. It was months before they had a smell of a feature, and the final diagnosis did
not emerge until January 1942. In what follows, the analysis is ascribed to its chief architect Tutte, although others may have made occasional contributions.

  Among the multitude of things tried on this key-stream, Tutte looked at long repeats within levels. He noticed that the distances apart of these long repeats in the first level tended to be multiples of 41. Accordingly, he wrote that level out on a width of 41 so as to place many of the long repeats in the same columns. For each set of five consecutive columns, he made a count of its five-bit characters. The distributions were significantly non-random; moreover, the distribution for one set could be matched well with that of another by adding an appropriate five-bit character throughout. These five-bit addends then turned out to fit together to give a 41-bit pattern.

  This cyclic pattern was called χ1. He subtracted it from the first level of the key-stream to yield a stream (that would later be called ψ'1). It seemed to be approximately periodic and was correctly diagnosed as an extended pattern of length 43. Calling this pattern ψ1, the observed stream was the extended ψ'1.