The decrypted German messages had, as mentioned earlier, been the most valuable sources during the weeks before the German attack on the Soviet Union on 22 June 1941. This situation would remain the position for a few years. With the attack, the teleprinter traffic to the German commands in Oslo and Rovaniemi, as well as the diplomatic traffic Stockholm - Berlin increased. The information received by the Swedish authorities became more detailed than before.
During the first year, from summer 1941 to summer 1942, when the German campaign against the Soviet Union took place, the decryption of the German teleprinter traffic provided extraordinary intelligence. German military plans and German politics towards Sweden could be clarified with the utmost certainty. However, no reliable knowledge was obtained about Adolf Hitler's political and strategic intentions. Intelligence throwing any light on the innermost reasoning of the people close to Hitler rarely or never existed.
The supreme army commands in Oslo and Rovaniemi did not command any of the decisive operations of German warfare. Hitler therefore seldom interfered in what went on in these theatres of operations. Nevertheless, even if the embassy in Stockholm and the commands in Norway and northern Finland were on the periphery of German interests, the intercepted internal German briefings and compilations had a great intelligence value for Sweden.
Adlercreutz's restrictions on distributing the decrypts to external recipients, except for the senior officers of the Defence Staff and the Intelligence Department, were mainly aimed at not exposing this exclusive source. Special instructions about other aspects of handling the material were issued in September 1941 by Samuel Åkerhielm in his capacity as deputy chief of the Defence Staff. The purpose was, of course, not to reveal the source. The decrypted messages had to be communicated and handled in secure ways. It was not permitted to refer to this material in conversations, and even less so on the telephone or in writing. When the messages were no longer needed, they had to be burned in controlled conditions.
The German confidence in the Geheimschreiber's security was, as explained earlier, an illusion. However, even the Swedish belief that the Geheimschreiber's secret still was a secret, except in Sweden, soon also became an illusion.
Some time in August 1941 the Soviet Union obtained access to the decrypted material. The courier Allan Emanuel Nyblad had the task of transporting the decrypted messages from Karlaplan 4 to the Staff building ``Grå Huset'' on Östermalmsgatan 87. He was a rather quiet man. He had been recruited as an agent, on ideological grounds, by the Soviet Union. The espionage was carried out in the following manner. On his way to the Grå Huset, Nyblad went to a rented flat situated along his usual route and photographed the messages he carried. The photos were given to the Soviet representatives, who had promised Nyblad a prominent position in a future communistic Sweden. He is not likely either to have received or demanded any money worth mentioning.
It is a reasonable assumption that the Soviet intelligence services (the NKVD and GRU) followed the Swedish success with interest. Moscow must have welcomed the likelihood that Sweden, with the aid of the intelligence, would take as strong as possible a position against Germany. But at the same time, the decrypted texts could also create doubts about Swedish power and will to withstand the German pressure.
Here we may reflect that the Soviet Union was taken by total surprise in 1941. During spring 1941 information about the coming German attack was leaked from Sweden to Great Britain (via the naval attaché) who passed on the information to the Soviet Union. However, General Golikov, then the chief of the GRU (the military intelligence service), actively contributed to the surprise by playing down the warnings from western sources, especially British, for reasons of political expediency. (The British intelligence service, SIS or MI 6, had until the Second World War primarily been working against the Soviet Union. Distrust can therefore be considered to be the explanation.)
Nyblad's spying was exposed in January 1942. It could not be exactly established which messages came into Russian hands through Nyblad, as he could not remember clearly on which days he had copied the material (T. Thorén)[11]. It is not known whether the Soviet Union derived any benefit from the information.
Less than six months after Nyblad's spying stopped, the Defence Staff discovered a new leak about the successful Swedish codebreaking activities. This time the leak soon had devastating consequences.
On 22 June 1942, Colonel Carl Björnstjerna, chief of the newly created foreign affairs section, which was directly subordinated to the chief of the Defence Staff, wrote to Major-General von Stedingk, the military attaché in Helsingfors, ``A serious mishap has taken place. The Germans have been warned by the Finns that we have succeeded in breaking their G-schreiber. For this reason they are changing keys, message channels and everything ...''.
No contemporary information exists about how ``the serious mishap'' happened. However, one need not be surprised. Swedish service personnel had been so open towards their Finnish colleagues that a leak was made possible. The openness shown during the winter of 1941, when it was conceivable that both Sweden and Finland could have common defence interests, continued after the summer of 1941. The Finnish military attaché, Colonel Stewen, was even treated like an insider in the higher Swedish staffs, and the Germans suspected him of communicating sensitive information to Sweden, who they presumed then passed it on to the USA and Great Britain. However, it was equally possible that both of these great powers were capable of acquiring the alleged ``leaked information'' themselves. When the Germans criticized their Finnish colleagues for gossiping, the latter defended themselves by revealing that the Swedes had intercepted the teleprinter connections and were breaking the messages. It is even possible that Colonel Stewen had seen decrypted messages; at least he knew about them. The Finnish intelligence about the Swedish codebreaking activity probably reached the Germans some days before 17 June 1942, when the big alert hit the German communications. In the beginning, the counter-measures were quite incoherent, but they were soon concentrated in two directions. One consisted of introducing new cipher machines or attachments to them.
On 21 July 1942 a new machine appeared in the traffic, T52C, ``Caesar''. In the beginning it appeared on only a few lines, while on the others the old machine remained in use. As time went, more and more C-machines were put into operation.
At first inspection the C-machine appeared to be completely normal. When the crypto department received parallel texts it could attack these as before, but the texts no longer fitted the previously known patterns. The sequences were no longer periodic or they had at least no short periods. Was this a new encryption method?
At last the crypto department hit upon the solution. Two texts had been solved and it appeared that two ``pin series'' were identical in the long sequences. They had already earlier worked on the hypothesis that the seemingly infinite sequences were generated by addition, modulo-two, of the results of two wheels and hence obtained very long periods. The identical sequences allowed this hypothesis to be tested. This had failed earlier. Was it true that the old A/B-machines' QEK-settings also were used in the C-machine? It was known that the C-machine could be used like the A/B-machine. For the A/B-machine they knew which five code-wheels were QEK-wheels, and also the settings. They then combined the wheels two by two in the ten combinations, but that did not work. However, when this procedure was repeated by leaving out four of the wheels it turned out to be correct. In this way it was possible to reveal the functioning of the C-machine.
The Germans had in a hurry made the mistake of keeping an element from an older, simpler system in a new cipher system. In the C-machine they used the same wheel-lengths and pin-patterns as in the A/B-machine and the keying principle was the same. They should have made the new machine completely independent of the old one, but it is likely that production difficulties created obstacles. The A/B-machine could be connected to the C-machine. Therefore the quick change of machines did not have any appreciable effect.
The other counter-measure taken by the Germans consisted in avoiding transmitting particularly important messages over Swedish telecommunication lines. It was therefore no longer possible to maintain the excellent intelligence about the German armed forces in Norway and Finland. A further deterioration occurred in October 1942 when all teleprinter traffic to and from Oslo and Rovaniemi was sent over Danish-Norwegian, or Finnish-Baltic cables. In certain cases cables were laid specially for this purpose. However, the teleprinter traffic to and from the German Embassy in Stockholm could still be intercepted and broken.
In October 1942 the Germans ordered the introduction of so-called ``Wahlwörter'' (randomly chosen words). The idea was actually sound. It is a good cryptological practice to avoid stereotype beginnings, which are usually where the codebreaker starts to look for an entry. The ``QET-texts'' were of course necessarily monotonous. Now the text would begin with a ``Wahlwort'' and in this way move the stereotype, fixed text further on to an undefined place in the message. However, the good intention failed. Many people follow instructions to the letter. Most people used the word given as an example in the instruction, which probably was SONNENSCHEIN (sunshine), as it occurred very often at first. Some managed to produce the word MONDSCHEIN (moonlight). The record was the word DONAUDAMPFSCHIFFSFARTSGESELLSCHAFTSKAPITÄN (Danube-steam-ship-company-captain). When the procedure with Wahlwörter was used correctly it became more difficult, but not impossible, to decrypt the incoming messages.
In May 1943 such radical changes in the keying procedures were introduced that codebreaking became almost impossible. The breaking of the Geheimschreiber's teleprinter messages therefore diminished considerably. A smaller group was left on the task to try, if possible, to do something with the newly received material. Otherwise they were engaged in decrypting older material that had not been dealt with earlier, and in following the traffic.
During 1944 the Geheimschreiber appeared in versions D and E and a mysterious machine called Y. The crypto department never succeeded in breaking these machines. Models D and E were further developments of the machines that carried the designations A, B and C. The Y-machine[12] could perhaps have been a further development of the Z-(Zusatz)gerät (Z-(additional)device = Z-attachment). But, as just mentioned, the traffic transmitted with these machines could never be decrypted, only followed.
Other leaks also occurred, but were never of any significance. It was the Finnish military attaché in Stockholm, Colonel Stewen, who exposed the secret.