The Bletchley Park Codebreakers (39 page)

17

HOW DILLY KNOX AND HIS GIRLS BROKE THE
ABWEHR
ENIGMA

KEITH BATEY

Intelligence from decrypts of the
Abwehr
(the German Secret Service) was largely responsible for enabling MI5 to control the entire German espionage network in the United Kingdom. It also enabled the organizers of the ‘Double Cross’ system of playing back German agents to be sure that they were successfully deceiving the Germans and, through strategic deception, played a major part in the success of the D-Day landings.

Radio traffic between various European capitals and Berlin, which had been intercepted – though spasmodically and in small amounts – from late 1939, was thought to be between outstations of the
Abwehr
and its headquarters, and had been diagnosed as having been enciphered on a four-wheel Enigma from its use of eight-letter indicators, such as GIWM XPEB, since that was a common practice when using the four-wheel commercial Enigma (see page 279, paragraph 3). The traffic was
referred to Gordon Welchman in Hut 6 early in 1941, but although the number of signals intercepted had increased, following breaks into
Abwehr
hand ciphers in 1940, it remained much too insignificant to offer any prospect of evaluating the indicators, and as no cribs were available Hut 6 made no progress with the problem. Welchman, no doubt not being serious, asked me if sufficient information could quickly be got to break the machine if someone could get into an embassy for the purpose; I was discouraging, and mercifully no such tactic was tried.

As Dilly Knox had handed over his work on the Italian naval Enigma, he was asked to tackle the unsolved traffic around mid-1941 and, remarkably, had made great headway with the solution by October 1941. His note, dated 28 October, reporting his success to Alastair Denniston, the operational head of GC&CS, is endearingly eccentric and typically obscure; sadly, it ends with a scribbled postscript saying that he would welcome a discussion but would be away on 30 October – in fact his stomach cancer was to keep him away for the rest of his life. Ralph Erskine sent me a copy of the note in 1999 – until then I had no reliable information about how Dilly had solved the problem.

Dilly saw, as had Hut 6, that the number of signals on any one day was too small to make it feasible to evaluate the indicators (i.e. decipher the message settings from them), but he saw that if he could find two days where the same wheel order was used and such that the
Grundstellung
of one day could be got from the other by rotating each wheel and the reflector through the same number of places, he would in effect double the number of indicators available on one setting, because the letter pairings at each position of the
Grundstellungen
would be related by a QWERTZU … substitution (see page 281, paragraph 9). He therefore organized a search for two such days with the help of the Bletchley Park card sorting and tabulation section – this must have involved Dilly in some very persuasive negotiations, because the section’s resources were usually overloaded. The search was unsuccessful, which Dilly thought suspicious, though why is puzzling: it is doubtful that there were as many as a hundred days of traffic worth working on, implying fewer than sixteen on each of the six possible wheel orders. The probability that two days would have the desired relationship was 1 in 17,576 – but Dilly was disinclined to be troubled by probability calculations (an attitude inculcated in his
lieutenants). Freeborn did, however, find a day on which the positions 1–5 and 2–6 fitted the bill – which Dilly reported as ‘finding what was wanted standing, Like the abomination of desolation, precisely where it should not – on a single setting’. He called the phenomenon a ‘crab’ on the basis that matters moved sideways. He also said that the discovery came as no surprise to him: presumably by this he meant that he was ready for something odd; if he had had the slightest suspicion that the machine had numerous wheel turnovers he would surely have had each day’s indicators carefully looked at, in which case crabs would have been found with little trouble, and Freeborn saved a tiresome task.

The crucial discovery of the crab was pure serendipity, but Dilly took full advantage of it: from the discovery he deduced that:

a) the Enigma had a QWERTZU… diagonal: the sequence from the top right to the bottom left of the rod square when the rod labels are in QWERTZU order (see Figure 17 and page 282, paragraph 8);

b) occasionally, between consecutive positions four places apart on the Grundstellung, all the wheels and reflector moved together;

c) hence the reflector moved during enciphering – a feature not previously encountered by GC&CS in an Enigma; d the wheels had numerous turnover positions, otherwise crabs would be very rare: this was also a feature not previously met with;

e) ‘as everything that has a middle also has a beginning and an end’ there would be positions at which all the wheels and reflector turned together without doing the same four places later – this he called a ‘lobster’, arguing that it was half a crab.

Dilly said that he condemned a crab as useless because, although it was a great help in finding the alphabets at the four positions of the
Grundstellung
, the alphabets could not lead to deductions about rods (see page 280, paragraph 8 (d)) because of the turnovers between each pair. Lobsters were useful because the QWERTZU … relation between the two alphabets at the lobster position greatly helped to discover them, so giving the alphabets four places away, which, with luck, would not be separated by a turnover.

At this stage Dilly organized a lobster hunt: he probably restricted the hunt to days with at least ten indicators intercepted; for each day ‘chains’ were made for places 1–5, 2–6 and so on; if a letter pairing was assumed in, say, place 1, the chains gave deductions about other
pairings in 1 and 5, and if a lobster existed 1–2, there would be several pairings implied for 6; it was then easy to see whether these were consistent with the implications of the chain 2–6. Clearly the hunt involved a great deal of careful, tedious work, but Dilly announced with evident pride that ‘after two days Miss Lever,
*
by very good and careful work, succeeded in an evaluation which contained sufficient non-carry units to ascertain the green wheel’.

The evaluation of the first day showed that the operators (no doubt contrary to strict instructions) nearly always used Christian names, swear words, or obvious keyboard sequences as message settings, and as this greatly eased the evaluation for other days a fair number of solutions soon accumulated, from which followed recovery of the wiring of two other wheels, one (designated Blue) with 15 turnovers and the other with 17 (Red). With so many turnovers these recoveries were difficult, the Red especially so – it was only recovered by David Rees after a complex operation. Each evaluation produced seven consecutive places at which it was shown whether the wheel had a turnover or not: if a turnover the position was marked by +, if not by – so that the turnovers produced by an evaluation would be shown by a sequence such as – +++ – + –. With evaluations for several days these sequences could be fitted together in a process akin to dendrochronology (though much simpler!) to give the complete sequence of turnovers for the wheel, known as the ‘wheel-track’.

Having discovered the wheel wiring and wheel-tracks, it remained to discover how the letters on the wheel tyres related to the turnovers. This necessitated decipherment of a message, a formidable problem which Dilly tackled at home because of his illness. His reports do not say how he accomplished the break, and I was unable to ask him as I only followed David Rees on secondment from Hut 6 (permanent, as it turned out) at the beginning of November 1941. Dilly was helped by Margaret Rock, who stayed with him for some time at his home in Courns Wood, but I never asked her how the solution came about, possibly because she too was off sick for some months from December 1941. Undoubtedly Dilly used rods, since other possible attacks would have involved statistics, which he disliked. Analysis of the first letters in the intercepted messages established that most, if not all, began ‘NRX’ (‘Nr’ followed by ‘x’ as a separator), since N very rarely appeared in the first position, R in the second, or X in the third. Dilly,
or Margaret, would have noticed that the Green wheel rod square contained adjacent occurrences of RX on two rods (two letters on the same rod were called beetles by Dilly) and a N*X beetle. On a day with twenty or more messages – according to my recollection quite common in November, probably because more resources had been allocated to interception following Dilly’s break – it would be unlucky not to have messages starting at two of the beetle positions, and the message settings would confirm that their relative rod positions were correct; the beetles would be easily found by noting the indicators in which the bigrams associated with the beetles on the rod square appeared as the second two, or the first and third, letters. This process, repeated if necessary on two or three days with the Green wheel in the right-hand (RH) position, would fix the tyre in relation to the rod positions, which would be related to the wheel-track’s eleven turnovers by the evaluation of the indicators.

The Blue rod square had NRX on one rod, but no other relevant beetles; finding at the start of a message one of the trigrams in the same column of the rod square as the NRX beetle would make it highly probable that it gave the rod position at the start of the message. There were just three places on the Blue wheel-track which had no turnover in three consecutive positions of the wheel, so that study of the indicator evaluations for several Blue days would, by the process described, fix the tyre in relation to the wheel-track. The same procedure would, though with more trouble, fix the tyre in relation to the Red wheel-track, the Red wheel having a N*X and a RX beetle; Dilly, having sorted out the other two wheels, possibly used rods to read more text in one or two messages.

However Dilly went about the problem, Denniston informed ‘C, who was also the director of GC&CS, that ‘Knox has again justified his reputation as our most original investigator of Enigma problems … He read one message on December 8th. He attributes the success to two young girl members of his staff. Miss Rock and Miss Lever, and he gives them all the credit. He is of course the leader, but no doubt has selected and trained his staff to assist him in his somewhat unusual methods.’ Denniston implied that Dilly was being kind, but in fact he greatly valued the work of his two lieutenants, saying ‘Give me a Lever and a Rock and I will move the Universe.’ This reading of a message completed Dilly’s triumph and gave him the satisfaction of being brought into the mainstream again, having been distanced from
the work of Hut 6 and Hut 8 (rightly so, because the crib/machine production line methods involved were not his style). Dilly’s note of 28 October 1941 said that he expected the recovery of keys to be laborious and he asked for the gradual return of the Cottage staff who had been seconded to Hut 8 during the slack period.