The Bletchley Park Codebreakers (40 page)

The correspondence that followed with Denniston harked back to the days of the first breaks of wartime Enigma when Dilly wanted to be involved with the intelligence side of codebreaking. ‘A scholar is bound to see his research through from the raw material to the final text,’ Dilly complained. Denniston’s reply on 11 November 1941 reveals their relationship with each other – also Denniston’s diplomatic forbearance!

Dilly was in fact terminally ill and never did take charge of the section which became known as ISK (Illicit (or Intelligence) Services Knox) to his great satisfaction; the name was highly appropriate as the
Abwehr
hand-cipher traffic had been broken in 1940 by Oliver Strachey, Dilly’s colleague from the early days of GC&CS, and his section was named ISOS (Intelligence (or Intelligence) Services Oliver Strachey). It was Peter Twinn, Dilly’s first colleague in the Cottage in 1939, who took over the running of the new ISK section early in 1942. Dilly remained at home in Courns Wood, with Margaret Rock liaising between him and Bletchley Park. He worked from his bed until he died on 27 February 1943, only getting up in order to receive the CMG ‘for services to his country’ from the Palace emissary.

ISK became operational early in 1942 and finally expanded into a staff of over one hundred in one of the new blocks. Four European
Abwehr
networks were attacked by ISK: two in the West and two in the East. As they all used different daily settings a considerable amount of work was involved. In spite of Dilly’s initial misgivings, evaluation of settings from doubly enciphered indicators proved fairly easy, and after straightforward rodding it was determined which was
the RH wheel, the rod position at the start of the
Grundstellung
, and the wheel-track, which together fixed the RH wheel
Ringstellung
. The rod couplings for the RH wheel at the
Grundstellung
positions in effect gave parts of the alphabets produced by the other two wheels and reflector at those positions, and so enabled the middle wheel and its rod position to be determined, but probably not its wheel-track because only two or three turnover positions would be involved. This meant that determination of the
Ringstellung
depended on rodding out a few letters at the start of a message, which could prove troublesome.

Priority was given to the production of catalogues (on the lines developed by Gordon Welchman for the Railway Enigma), which provided a convenient and quick way of finding which settings of the reflector, LH and middle wheels gave prescribed pairings required by the rod position discovered for the RH wheel. When these catalogues became available, after three or four weeks, the day’s key was easily settled once the RH wheel and its rod position at the
Grundstellung
were found. Priority was also given to the conversion of Typex machines to deal with the work of deciphering messages after the day’s key was found, because the multiple turnovers of the
Abwehr
machine made deciphering by hand exceedingly slow and barely practicable without a vast staff. This was an inconvenient result from the multiple turnovers which, as just shown, were actually immensely helpful in working out daily keys because the wheel-track fixed the RH wheel
Ringstellung
in relation to the rods.

Very occasionally a day’s indicators proved obdurate. To deal with such cases Mavis Lever, in spite of my strong scepticism (long regretted), had charts made for NRXEINS etc., arguing intuitively that the correct rod position of the RH wheel would probably have a chart ‘click’. This scheme worked and proved invaluable when the double encipherment of message settings was stopped late in 1942, leaving the NRXEINS cribs as the only entry to the traffic. In 1943 ISK acquired a special four-wheel bombe (known as
Fünf
) to handle days when rodding failed. Charts and bombe together enabled nearly all
Abwehr
Enigma traffic to be deciphered until the end of 1944 when an entirely new machine, the
Schlüsselgerät
41 (SG 41), was introduced.

Dilly had to start his attack on the unsolved
Abwehr
cipher machine from the initial assumption that a form of commercial Enigma was involved, since at that stage no other kind of four-wheel Enigma was
known to GC&CS. The following notes, which relate mainly to the commercial machine, give some of the theory of the machine which he could use in testing that assumption.

1. The Enigma connected a typewriter keyboard to a bank of
light-bulbs
through a variable electric circuit, so that if a letter – A, say – was pressed a bulb – X, say – would light; the circuit was reciprocal, so that if X was pressed at the same setting A would light.

2. The variable circuit was provided by three wheels coaxially mounted with a reflector on the left and a fixed circular plate (the entry rotor –
Eintrittwalze
– or end-plate) on the right. Each wheel had twenty-six spring-loaded pins equally spaced around one face, each wired through the wheel to one of twenty-six small discs similarly placed around the other; the reflector had twenty-six pins placed like those on the wheels; and the end-plate had twenty-six discs, each wired to one keyboard letter and one bulb. Each wheel and the reflector carried a movable tyre or ring around its rim, lettered A–Z spaced to match the pins and discs; when mounted in the machine each could be set in one of twenty-six positions, each identified by the letter on the tyre which showed through a window in the machine’s cover. The tyre could be fixed relative to the body of the wheel or reflector by a clip fixed to the wheel’s body at one end and having a small pin at the other, which could be fitted into a hole in the edge of the tyre, identified by the adjacent tyre letter. The wheels could be mounted in the machine in any order, with the pins to the right, and when mounted were pressed together by a lever-operated spring, ensuring effective electrical contact between adjacent sets of pins and discs. When a keyboard letter was pressed, the right-hand (RH – i.e. that next to the end plate) wheel rotated anti-clockwise (viewed from the front right) one place; in each complete rotation of the RH wheel through twenty-six places the middle wheel would move one place, at a position determined by a notch on the RH tyre (called the turnover position); and the left-hand (LH) wheel moved one place for each complete rotation of the middle wheel. The reflector remained stationary during encipherment, but could be set manually.

3. To decipher a message the recipient had to know the order in which the wheels were mounted, the clip positions (
Ringstellungen
(ring settings) to the Germans), and the position of each wheel at the start of encipherment (the message setting). The message setting could not safely be sent in clear: one way of hiding it, often used by the Germans, was to encipher it twice at a specified basic setting (the
Grundstellung
) and transmit the eight resulting letters as the indicator. Cipher instructions for an
Abwehr
network gave a key for a period, usually a day, specifying the wheel order,
Ringstellung
and
Grundstellung
to be used. Each operator was usually allowed to decide the setting he used for each message; inevitably, instead of using meaningless sets of four letters as intended, many operators chose names, keyboard sequences, or swear words.

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:

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