The Bletchley Park Codebreakers (10 page)

In 1942 all
Luftwaffe
Enigma key-lists except Brown were apparently prepared by one man, who often merged different
components of previous keys (for example,
Stecker)
when preparing new key-lists. The April Foxglove keys combined parts of the January Red and March Mustard keys, with the remainders being used to make up the April Red key-list. In June, Locust, Mosquito, Snowdrop, Hornet and Garlic also consisted of partial key repeats. Key repeats died out at the end of 1942, but were briefly revived between March and June 1943, when ciphers used by certain
Luftgau
(
Luftwaffe
administrative units) were allotted different discriminants, but some key components were identical, for example, Daffodil, Gorse, Lily and Speedwell formed one group.

Primrose, the cipher for
Luftgau
XXVIII, the
Luftwaffe
supply unit in Africa, yielded between 140 and 290 decrypts daily, mainly on logistics, on most days between June and November 1942 – almost as many decrypts as the prolific Red, whose total varied from 350 to 445 daily. Primrose was doubly useful since it also provided complete key repeats, although in random order, for Scorpion (employed by
Luftwaffe
close support units and
Luftwaffe
liaison officers with the Afrika Korps) for each month from June to November. Scorpion was very difficult to intercept in Britain, since it was transmitted with low power on medium frequencies by units near the fighting. Special arrangements were made to decipher it at Heliopolis, near Cairo, as well as at GC&CS, since it produced a great deal of intelligence about
Luftwaffe
plans and the ground fighting. The daily keys were available in advance so that messages in it could therefore be deciphered immediately they reached the Heliopolis centre.

In complete contrast to the
Luftwaffe
,
Heer
traffic in 1941 was divided into small, unrelated groups, with few re-encipherments, and light traffic. Hut 6 broke very little
Heer
Enigma before September 1941. Thus, although Vulture (used on the Russian front) was first read on 27 June 1941, shortly after the invasion of Russia, Hut 6 broke no Vulture keys for August, and only one for September. It solved only twelve
Heer
daily keys in August, all in Kestrel. By November Hut 6 was breaking both Vulture, which was the first
Heer
cipher to be broken regularly, and Chaffinch, but then lost them for four months after the Vulture traffic was reduced when the
Heer
began to rely more on land lines.

Heer
ciphers in North Africa were initially very difficult to solve. Hut 6 broke Chaffinch II (used between
Panzerarmee
Afrika and Rome and Berlin) for nine days in September 1941, and for part of 
October and from 2 November to 6 December, although often a week or more later. Traffic on Phoenix, which rose from twenty-five to over 200 messages daily during fighting in Libya, could only be taken by forward intercept sets with the 8th Army, since it was also transmitted at low power on medium frequencies. When it was broken, delays in sending the traffic back from 8th Army to Heliopolis for transmission to GC&CS led to considerable hold-ups, until arrangements were made for Hut 6 to send key-lists to Heliopolis. Hut 6’s solution of the
Heer
Mediterranean ciphers, such as Chaffinch, Phoenix and Thrush (which carried information about the air transport of supplies and reinforcements) increased considerably after April 1942. Even so, only 2,800
Heer
signals were decrypted in May 1942 out of 10,300
Heer
intercepts, compared with 19,400
Luftwaffe
decrypts from 31,000 intercepts, illustrating just how difficult it was to solve
Heer
Enigma.

Hut 6 found that many of the twenty-two
Luftwaffe
ciphers it was regularly breaking by the summer of 1942 were interrelated: the fifty
Heer
and
Luftwaffe
Enigma ciphers then being used presented it with a single, indivisible problem. It therefore had to try to break every single cipher, however unimportant for intelligence purposes, since there was no way of telling in advance which would prove the entry point to a different cipher. Even
Heer
Mediterranean ciphers, such as Mallard (a Rome administrative cipher), were sometimes broken by re-encipherments from Red and Scorpion. All the Mediterranean
Heer
keys solved in 1942 were linked by re-encipherments, but the only
Heer
cipher to produce cillies consistently in 1942 was Osprey which, although classified as a
Heer
cipher by Hut 6, was really the cipher of the Todt construction organization, which may explain its poor operating practices.

No repeats of key-lists were discovered in
Heer
traffic in 1941 or 1942. The success rate (decrypts as a percentage of the signals intercepted) against
Luftwaffe
traffic fell from 92 per cent in November 1941 to 50 per cent in November 1942, largely because Light Blue went out of service in January 1942, and the main
Luftwaffe
traffic started to use eight or more ciphers. During 1942, the success rate against
Heer
traffic was only 0.6 per cent (a mere thirty decrypts) in January but rose to 27 per cent in May. Intercepts of all Hut 6 traffic increased from 32,000 per month in September 1941 to 82,000 in November 1942. The overall percentage of the traffic (
Luftwaffe
,
Heer
and Railway) broken by Hut 6 remained around 50 per cent of the total intercepts in 1942. Unidentified traffic, where Hut 6 did not
know which cipher was being used, was reduced from 17 per cent of the total traffic in September 1941 to only 2.4 per cent in November 1942, illustrating Hut 6’s and the Y service’s increasing mastery of the
Luftwaffe
and
Heer
radio nets, and their skill in reconstructing the extensive lists of cipher discriminants used by them.

SS decrypts increased considerably in 1942 following the discovery in April that daily reports (codenamed HOR-HUG by Bletchley Park) of concentration camp numbers provided half the Orange
Stecker
. The HOR-HUG reports gave the numbers of people in certain concentration camps, which were encoded by a letter for digit substitution (so providing ten letters – the first five
Stecker
pairs), but ceased in early 1943, after which the reports were sent by land line. Orange keys could often be broken with an unused part of a bombe, while the other parts were being used for a different cipher.

Hut 6 also solved the Railway traffic in eastern Europe (later codenamed Rocket) in 1941 and 1942. The traffic used a rewired version of commercial Enigma, without
Stecker
, and was therefore relatively easy to solve using hand methods. Lt. Col. John Tiltman first broke the traffic around the end of July 1940, after which the wiring of the machine was solved by Hut 8, which became responsible for reading the traffic until the work was later transferred to Hut 6. All 2,300 messages received in July and August 1940 were solved. The traffic ceased at the end of August, but began to be intercepted again in February 1941. The 90 per cent success rate against it in 1942 was the highest for all Hut 6 Enigma.

Hut 6 largely consolidated its position during 1943. However, it faced a significant challenge when the
Heer
stopped using discriminants in Enigma traffic on 1 September, and the
Luftwaffe
followed suit on 1 November. The small change represented a major improvement in security, since Hut 6 was now faced with 3,000 or so signals daily, all of which apparently used the same cipher but which in fact employed up to ninety different ciphers. The new procedure slowed Hut 6 quite considerably, and required it to increase its registration and decoding room staff substantially, from about 220 to 300. Hut 6 and the Y service were able to meet the challenge only because they had become highly experienced and very flexible organizations, capable of responding to any change quickly and efficiently.

In addition, Hut 6 had to prepare for the second front in late 1943 and early 1944. It identified many of the
Heer
’
s
Enigma ciphers and
their related frequencies and call signs with the help of the Y service, although it had little success in solving them before the Normandy landings in June, and thought that most of its intelligence on the
Heer
might have to be derived from Red and Flivo
(Heer-Luftwaffe
liaison) ciphers. Fortunately, some of the
Heer
ciphers being used in France were solved after the landings, although initially there were not many decrypts. However, following the Allied break-out in Normandy at the start of August, there was a massive increase in
Heer
traffic, which led to about five
Heer
ciphers being read daily and a huge rise in
Heer
decrypts, although the decrypts declined in September 1944 when the land war became static. So did
Luftwaffe
decrypts, when
Luftwaffe
operations were restricted, largely due to a shortage of fuel and trained pilots.

Throughout 1944 Hut 6 was very apprehensive that a new rewirable reflector,
Umkehrwalze
D (Dora – UKD), which had first appeared in some
Luftwaffe
ciphers in January, would be brought into general use, and blind it almost completely. Providentially, despite warnings that UKD would be widely used on 1 August, that did not happen. UKD was employed with about twenty-five
Luftwaffe
ciphers by March 1945, but seldom to the total exclusion of the standard reflector in the same cipher, which was a classic
Luftwaffe
blunder of the first order. But for the
Luftwaffe
bungles UKD would have had a devastating effect on Hut 6 and the production of Ultra.

The
Heer
brought an unbreakable system of enciphered call signs into service on 1 November 1944. This was a much-feared step, but it led to an only temporary drop in solutions of
Heer
Enigma, such were the combined capabilities of the Y service and Hut 6, who were greatly helped by the fact that major nets retained their fixed frequencies, confining the problem to about half of the
Heer
traffic. Almost four times as much bombe time was required to break
Heer
Enigma in November 1944, compared with November 1942, and more than twice as much for
Luftwaffe
Enigma (see Figure 4.3). Hut 6’s production of decrypts in 1944 would almost certainly have declined significantly if US Navy bombes had not been available, since they more than doubled the three-rotor bombe capacity available to Hut 6.

Fig 4.3 Enigma Breaks (Hut 6): 1942 and 1944

 

By early 1945 the
Heer
was employing only a few Enigma ciphers, which greatly expanded the traffic they carried. In turn, this resulted in many more re-encipherments and mistakes by operators – and in a considerable increase in the
Heer
decrypts. However, Hut 6 suffered what was perhaps its worst blow of the war on 1 February 1945, when the
Luftwaffe
implemented a system under which call signs were enciphered daily, and frequencies changed every third day. Although these precautions had been anticipated, they threatened the interception and solution of all
Luftwaffe
traffic, both Enigma and low-grade:
Luftwaffe
Enigma decrypts fell from 1,800 to 1,000 daily. A post-war history concluded that ‘if this internal German reform … had been a little more thorough, far reaching, and rigidly enforced … German messages henceforward would have become largely unbreakable’. Since it had been known in advance that the frequencies would usually be more easily identified by reference to the attributes of the low-grade codes being used than those of Enigma, a special ‘traffic watch’ was established by the Air Section in Hut 10 to sort and identify the traffic and pass its details to Hut 6. A combined Sixta and Air Section research party built up a number of frequency continuities by mid-February, but Hut 6 did not recover fully until around mid-March, when documents with details of the new systems were captured.
Heer
and
Luftwaffe
Ultra began to dry up in April, as Germany disintegrated.

From very small beginnings, Hut 6 evolved into a large, well-managed and highly adaptable unit. Only by doing so was it able to counter German measures that would have defeated a less flexible organization. The Y service operators also reached a very high degree of efficiency as the war progressed. Without their help, Hut 6 would have been greatly handicapped, especially when the
Heer
and
Luftwaffe
made significant changes to their radio procedures, as with enciphered call signs. Few
Heer
Enigma ciphers were easy to break at any time. Hut 6 gave considerable attention to Green, but solved it on only thirteen occasions throughout the war – and even then some of the breaks required the help of prisoners of war. If the
Luftwaffe’s
Enigma operators had been as well trained as their
Heer
counterparts, or if some German security procedures had been introduced early in the war, there would have been much less Ultra from Enigma. And while it is unwise to claim too much for Ultra, it was undoubtedly the prime source of intelligence for the Allies, and helped significantly to alter the course of the war in their favour.