British SIGINT and the Bear, 1919-1941.
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By contrast, the total staff at GC&CS was 3 293 by December 1942 and reached a wartime peak of almost 9 000 by January 1945.  These latter figures marked not only a quantitative change but also - as Marxist historians might say - a qualitative change in the very nature of GC&CS as an intelligence organization.
A fascination with the mysteries of codes and cyphers can too easily obscure the fact that cryptanalysis is only one stage in a sequence of interacting processes taking us from raw signal to distributed intelligence report.  Before the cryptanalysts can set to work, interception must have delivered the goods. As an old recipe for "Rabbit Stew" prudently began, "First catch your rabbit". GC&CS drew its particular rabbits from two main sources.
During the First World War, British censorship offices had been established at various critical nodes in the international telegraph network such as Hong Kong, Malta and Bermuda. As a result, copies of all foreign government telegrams passing through these points as well as all those passing in and out of Britain became automatically available to the British censorship authorities. The latter were then in a position to supply the codebreakers with copies. In peacetime, with the disappearance of censorship, this was no longer possible. However, clause 4 of the amended Official Secrets Act of 1920 together with other appropriate arrangements appear to have ensured that telegram scrutiny could be continued on a fairly broad front. 
The second source of raw material was the Y-service, that is to say the interception of radio traffic by domestic and foreign stations run by the Armed Services. The Admiralty disposed over landbased stations at Scarborough and Flowerdown and later ran a bureau for interception and cryptology in the Far East, initially at Hong-Kong but eventually moved to Singapore at the outbreak of the Second World war. The War Office had a station at Chatham as well as important stations in the Middle East and India. The Air Ministry joined the game around 1923 when they set up a radio monitoring station at Waddington.
In 1924, the so-called "Cryptography and Interception Committee" on which served representatives of GC&CS and the Armed Services, met for the first time to apportion the work and settle priorities in areas of common interest.  Subsequently called the Coordination of W/T Interception Committee, it met only infrequently in the 1920s and 1930s. 
GC&CS was entirely dependent on the Armed Services for its radio interception. But how did the latter look upon the Cypher School? In principle, their attitude was doubtless positive. After all there was no shortage of examples from the Great War of golden eggs being supplied by code-breaking geese. At the same time, they regarded the Cypher School not as an intelligence department in embryo but as a cryptological research bureau to which they could profitably second qualified personnel. In due course, specialized sections emerged within GC&CS - a Naval Section in 1924, a Military Section in 1930 and an Air Section in 1936, thus providing a suitable internal framework for this kind of collaboration. GC&CS became an interservice facility with the cooperative gains that this implied. Nonetheless, the Service departments were reluctant to concede too much of what they regarded as their own areas of competence. Traffic analysis (T/A) - traditionally described as that branch of SIGINT dealing with the external characteristics of signal communications and as such a natural offshoot of interception - was an obvious case in point but there were others. The British Army in India, for example, had its own group of cryptanalysts who worked on the traffic taken there. The attitude lingered on that GC&CS was mainly a peacetime store of competence which in wartime could be usefully dispersed and employed more efficiently in close proximity to operational headquarters. Lastly the service intelligence directorates had no intention of allowing the business of evaluating and distributing intelligence material to drift into the hands of civilians at GC&CS.
The subsequent achievements of GC&CS during the Second World War, should not blind the historian to its defects in the interwar period. Even allowing for the smallness of the organization and the tightness of its budget, it suffered from certain functional weaknesses. In one way, it was a victim of its own success. As "Josh" Cooper, who had joined the staff in October 1925, was to recall: "But most striking of all was the lack of interest in Radio itself. For twenty years 1919-1939 most of the work of GC&CS was diplomatic and the vast majority of raw material of all kinds came from commercial telegraphic links."  And he goes on to note in a revealing psychological insight that wireless "raw material was disliked and distrusted; it was messy pencil script, not neat typescript, and was often corrupt even when not obviously mutilated."  Cooper also makes the prima facie surprising observation that there was no direction finding (D/F) in Britain itself almost until the outbreak of the Second World War and when it was introduced no-one seemed to know what to do with it. 
Another weakness could be traced to the blinkers worn by some GC&CS cryptanalysts. However brilliant their minds, they evidently did not always fully grasp the advantages that modern technology offered to their activities and characteristically persisted instead with their own idiosyncratic "cardboard and bootlace" contraptions. 
The threat of war led to a rejuvenating injection of new blood. The combination of machine encryption and short-wave radio demanded an integrated attack which brought together the close study of signal traffic with the skills of the cryptanalyst. This was recognised in varying degrees but it remained for the right balance to be struck administratively. War itself would iron out these areas of friction, thus allowing GC&CS to emerge at last as an integrated SIGINT organization, built along recognisably modern lines.
The priorities of GC&CS were determined in accordance with perceived threats. For most of the 1920s, the new British cryptanalytical organization found comparatively little military traffic to interest it.  It was otherwise with diplomatic systems such as those adopted in the communications between the new Bolshevik state and its foreign representatives. Among those working on this problem was Fetterlein, who had served in the Tsar's cabinet noire. Initially making use of rather simple types of cryptosystem, the Soviet traffic was soon broken and remained on tap for some time despite spectacular leaks by the British authorities and corresponding Soviet countermeasures.  When in 1927, however, the Arcos Raid incident led to yet another revelation that their traffic was still being read, the Russians at last switched to One Time Pad. OTP was an awkward system for coping with a multi-node communications network where traffic volume risked being high but it offered - if properly implemented  - perfect security. Thereafter Soviet diplomatic communications defied cryptanalysis for some time. 
The British attack on Soviet diplomatic cyphers in the period 1919-1927 did not rely exclusively on the contributions of GC&CS. Important work was also done independently by the Army signal intelligence organization in India which disposed over a network of listening stations and maintained a small staff of cryptologists at their interception centre at Abbotabad in the North-West Frontier Province near Rawalpindi. Among them was John H Tiltman, an army officer who was to chalk up a number of notable cryptanalytical successes in the course of a distinguished career.  Prior to Abbotobad, he had served with GC&CS in London. Thereafter had followed a tour of duty in the Middle East. In March 1930 he was appointed to head the newly formed Military Section at GC&CS. His name will recur several times in the ensuing narrative.
Although popular treatments tend to concentrate on the mental brilliance of cryptanalysts, a major determinant in the practical breaking of cryptosystems is the volume of relevant traffic taken. One difficulty recorded by a member of the GC&CS staff who began working on Soviet diplomatic traffic at the end of 1925, was precisely the lack of intercepts of the desired kind. This, however, improved substantially due to the exertions of SIS who obtained a flow of material from the Persian post office. An even more voluminous amount of material was acquired in similar fashion from the post office in Peking which allowed the solution of whole additive tables  employed in the recyphering of Soviet messages. Some of these Peking decrypts were among those read aloud in the House of Commons after the Arcos raid.
While British access to Soviet diplomatic traffic vanished in 1927, another type of important non-military communication would turn out to be readable from 1930 onwards, due to the efforts of Tiltman.  This was the traffic carried by the Comintern network, a worldwide system of clandestine stations controlled by a station near Moscow. It is at present unknown how long the British authorities were able to follow this traffic and to what extent. The Comintern itself was formally dissolved by Stalin in 1943 as a token gesture to his Western allies. It is claimed in the official history of British intelligence during the Second World war that all British work on Soviet cryptosystems ceased in 1941. Nonetheless it is perhaps interesting to note that Robert Cecil, a wartime assistant to C, speaks of "SIS intercepting and decrypting the instructions that were being sent from the Kremlin to partisan groups and resistance movements under Communist control" after the dissolution of the Comintern. 
A small Naval Section at GC&CS was established in 1924.  It consisted of just two permanent members, assisted by Lambert, the GC&CS W/T expert. Interception of Naval signal traffic was initially carried out by the landbased station at Flowerdown which proved quite inadequate to the task. As a result, "procedure Y", as radio interception became known, was also assigned to HMS Ships. It proved far from popular with Fleet W/T operators. Moreover the task of examining intercepts and writing reports took up a great deal of valuable time. There were also technical problems to be overcome. A major drawback was the time it took for Fleet intercepts to reach GC&CS. The task of assigning particular traffic to different fleets was also by no means easy and in the early days any break in continuity of coverage was a handicap. Gradually, however, a scheme covering most of the world was built up. Initially the main traffic intercepted was French, Italian, Japanese, Soviet and US. 
As far as cryptanalysis on the Russian traffic was concerned, progress was extremely slow until the setting up of a W/T station at Sarafand in Palestine greatly increased the volume of traffic taken. The history of this particular station is of some interest.  During the Great War, the Army had set up a number of Wireless Observation Groups (W.O.G.s) in France and the Middle East. At the end of hostilities, these were redeployed. No. 3 W.O.G was sent to Constantinople while No. 4 W.O.G. at Baghdad was in 1920 engaged in monitoring Bolshevik traffic between the HQ in Baku and their forces in Northern Persia. In 1923, No. 3 W.O.G. was withdrawn to Sarafand along with part of No. 2 W.O.G. at Baghdad to form No. 2 Wireless Company. The Company had its H.Q. and No. 1 Section in Sarafand and its No. 2 Section - formed from the remnants of No. 4 W.O.G - in Baghdad. In 1929, the Baghdad Section was finally moved to Sarafand. The station was a complete organization in the sense that apart from its signal interception duties, it possessed a staff of cryptologists who provided Middle East Command with intelligence. Raw material and decrypts were also sent to the War Office in London. Just as Sarafand had been involved in the monitoring of Soviet diplomatic traffic in the period 1923-1925 so its assistance was later sought in monitoring Soviet naval traffic.
By 1927 headway had been made in discovering the Russian system of recyphering but further progress was hindered by lack of raw material. The following year the Head of Naval Section joined H.M.S. Curacao for the Baltic cruise of the 2nd light Cruiser Squadron, in the hope of learning more about Soviet naval cyphers but once again lack of intercepts proved a stumbling block. Nevertheless the trip was useful in allowing types of traffic to be identified and useful information about W/T procedures, call signs, wavelengths etc. was collected.
In 1931, an attempt was made to summarise what had hitherto been learned about Soviet naval cyphers. This resulted among other things in a report on Black Sea radio traffic by Cooper who had been sent to Sarafand to work on Soviet intercepts.  His analysis was based on some three thousand messages taken during 1930. While a good deal had been learned about Soviet operating procedures, the concrete cryptological results were still rather meagre. The main low-grade system based on a book of between 100 and 200 groups and recyphered by substitution seems to have been well understood but the highgrade cyphers proved refractory. Among the latter, one of the so-called common cyphers - i. e. a cypher held by all stations - failed to yield to attack because of scarcity of material. Cooper also examined special cyphers - i. e. those held by two stations only where the code was recyphered by means of short addition tables consisting of 30 to 40 five-figure groups. The tables were issued in a series of 50 or 100 with a fresh table being used for every message sent. Every station had a separate set of tables for every other station to which it sent messages and another separate set for every station from which it received them, thus ensuring that no table was ever used for more than one message. To solve the tables from single messages would have required a number of messages to be sent, each of some 200 groups. Since the average length of a message was 30 groups and the maximum 100, Cooper concluded that there was very little chance of making progress on this type of cypher. The Head of the Naval Section summed up the situation: "The results up to now have not been crowned with success and Russian thoroughness and efficiency in cyphers seems to have spread to its naval communications".  Incidentally this was not simply a British observation. In a well-documented essay on German cryptanalysis in the interwar period, J W M Chapman notes that German specialists found that "after 1931, even those simpler Soviet wireless codes for low-level army and airforce traffic that had previously presented no difficulties in the past began to be tightened up and the standard achieved reached that of the Tsarist army at its best in 1916." 
The rest of the thirties brought no conspicuous improvement. A small amount of raw Soviet naval traffic seems to have been filed but no cryptological work of any significance was undertaken on it. In any case, there were other far more pressing threats to consider. During the Abyssinian crisis of 1935, the main effort at the Naval Section was switched to Italian cyphers. The involvement of German and Italian forces on the side of General Franco in the Spanish Civil War led to a burst of activity directed at German, Italian and Spanish systems which resulted among other things in the recovering of the wiring of the 3-wheel "unsteckered" Enigma used for inter-Axis naval traffic. For Britain, it was Hitler's military machine not the Red Army which constituted the primary military threat on the horizon.
Information about the coverage of Soviet military traffic other than naval during the 1930s is sparse but fascinating. Apparently an RAF officer (Swanborough) visited Estonia in 1933 and made an agreement whereby in return for radio interception and D/F equipment, the Estonians would provide the British with copies of their intercepts along with any cryptanalytical results obtained.  This scheme ran into technical difficulties arising from the fact that the Estonian D/F baseline was far too short for their giant neighbour. The Russians also employed a sophisticated system of changing callsigns and frequencies. As a result, the traffic taken covered a heterogeneous collection of stations - some just over the border in the Leningrad Military District, others as far away as the Ukraine - with little or no continuity. Among the material sent to England was traffic making use of low grade systems which had usually been broken by the Estonians. This was deemed to have very little intelligence facevalue. A variety of high grade system material was also included but there was never enough of any one type to make a cryptanalytical attack feasible. A retrospective assessment held that controlled interception of selected lines of traffic with good T/A backup might at this time have produced very interesting results. The problem was that the British were unable to exercise the necessary control over the direction of Estonian sigint efforts.
This traffic continued to arrive at irregular intervals until Estonia was finally occupied by the Soviet Union in 1940. At GC&CS, P K Fetterlein was given the task of dealing with it, first in the Military Section and later in Air Section to which he was transferred. The traffic was not, however, specifically "airforce" in character.
As we shall see, Estonia was far from being the only "Border State" from which GC&CS obtained Soviet traffic for analysis in the runup to the Second World War. Sometimes this was arranged directly; sometimes it came about indirectly as a byproduct to other agreements. An illustration of the latter is Franco-British SIGINT cooperation. As the clouds began to loom in Europe in 1938, the liaison between British and French SIGINT which had existed in the years 1914-1918, was resumed and expanded to include the Poles with whom the French enjoyed good relations in the intelligence field. Although the main focus of interest was Germany, work on Soviet systems was also shared. As a result, a consignment of miscellaneous Soviet traffic taken in Latvia and Lithuania now became available to GC&CS via the "Scarlet Pimpernels" received from their French colleagues. 
The Ribbentrop-Molotov Pact naturally gave the study of Soviet traffic a new actuality. However, when the coverage of naval traffic was reviewed in October 1939, the results were by no means encouraging. The only Admiralty station in Britain covering Soviet traffic was Flowerdown which, it was held, was not ideally suited for the task.  It was therefore imperative to increase coverage as quickly as possible and the decision was made to expand facilities at Scarborough to allow for the monitoring of Soviet material. At the same time, steps were taken to improve collection facilities abroad.
In February 1940, in conjunction with the setting up of a cryptological bureau in Cairo, the Y Committee ruled (1) that the station at Sarafand should intercept all Russian traffic, West of the Caspian Sea and (2) that the take should be sent to India for investigation partly because the Indian Y-organization was considered to have a long-established and particular expertise in dealing with Russian traffic and partly because Sarafand was overloaded with work on Italian military traffic. This did not however prevent it from producing useful intelligence on Soviet merchant shipping movements in the Black Sea during the summer of 1940. Other W/T stations in the Middle East would also be involved at various times in monitoring Soviet communications while in the Far East, the Far East Combined Bureau (F.E.C.B) kept a watch on traffic in the Vladivostok area.
Various complementary initiatives to increase British coverage of Soviet traffic had meanwhile been taken in northern Europe, among them an arrangement with the British Minister at Stockholm for a receiver and operators to be installed at the legation, assuming that this could be done without Swedish objection.  This scheme apparently first surfaced around 21 October 1939 although it is not clear from the British documentation when the operators were finally in place and able to begin their work. On November 30, the Red Army began its onslaught on Finland. An outbreak of hostilities invariably leads to an explosion of military traffic of all kinds. Among those listening in were the two operators at the British Legation in Stockholm. By 26 December 1939, GC&CS was noting for its internal record that 4 and 5-figure Soviet Baltic naval traffic was being intercepted "by special arrangement" in Stockholm.
This particular British SIGINT operation in Sweden was destined to be short-lived for reasons that are unexplained. On 17 February 1940 the Y Committee was informed by Lieutenant Westhead that the two men in Stockholm were being withdrawn.  According to Commander Denniston at GC&CS, this move was not anticipated to materially affect work on Russian cyphers since Flowerdown was now producing large quantities of Russian naval material and it was hoped to get more from the Finns. At the same time, it was noted that no cryptographic assistance could as yet be given to the Finns because the relevant Russian codebooks had not been broken. Evidently preliminary Anglo-Finnish discussions about possible intelligence collaboration had taken place behind the scenes.
Liaison with what was called "the Finnish Cryptographic Unit" - presumably Major Hallamaa's organization - had been established early in 1940 - during a visit of Tiltman to Finland.  No hint is given of the name of his principal talking partner but it should be remembered that Tiltman was not simply a run-of-the-mill intelligence officer: he was a leading technical expert in the field of cryptology and SIGINT. It transpired that the Finns were prepared to supply Russian intelligence provided the British paid for their equipment. Tiltman's trip was followed up by a visit of Admiral Godfrey, the Director of British Naval Intelligence, who visited Finland in September 1940. This was judged to yield "very satisfactory" results.  The Finns had apparently agreed to supply copies of all their intercepts and cryptanalytical successes regarding Soviet traffic on condition that the British did the same. The main stumbling block in the exchange of material proved to be communications between Finland and Britain via Stockholm. Otherwise the arrangement seems to have functioned satisfactorily until the launching of Barbarossa.
This Anglo-Finnish intelligence arrangement must be seen as one of a number of similar and more or less contemporaneous joint schemes - all naturally highly secret involving the sharing of intelligence on Russia which the Finns had reached with foreign intelligence services including those of Sweden,  Germany  and Japan. 
During the Winter War, Germany had been extremely circumspect in its relations with Finland for fear of antagonising Russia to which it was bound by the terms of the Ribbentrop-Molotov Pact.  In August 1940, this policy changed decisively when the Führer gave the go-ahead for the supply of armaments to the Finns.  As the months passed, Germany step-by-step consolidated its position on Russia's northern flank. As it observed the march of events, GC&CS became increasingly apprehensive that the Finnish Y-organization might disclose its cooperation with Britain as well as British knowledge of Russian cyphers to the Germans. A telegram sent through the SIS in Helsinki on 20 June - i. e. virtually on the eve of Barbarossa - apparently helped to calm doubts on this score.  It was stated (1) that liaison between the Finns and GC&CS would cease in the event of a war between Russia and Germany and (2) that the Finnish Director of Military Intelligence made full assurances that their cooperation with GC&CS would not be disclosed to the Germans provided the British disclosed nothing to Russia. On 22 June, a further telegram arrived to say that the W/T station in Finland, intercepting material on behalf of the British, was closing down. An intriguing episode in Anglo-Finnish intelligence collaboration had suddenly ended.
A variety of questions arise: how much money was paid by the British? What equipment was purchased by the Finns for the money received? What SIGINT material was given by the Finns to the British? Conversely what cryptological assistance was given by GC&CS to the Finns? Lastly how far was the agreement to preserve the confidentiality of Anglo-Finnish intelligence collaboration faithfully honoured? One naturally wonders if there were any Finnish documents relevant to these matters among the secret papers transferred to Sweden in operation Stella Polaris.  As always with the history of secret intelligence, it may be some time before all the surviving pieces of the puzzle emerge.
Due to the various arrangements to expand the coverage of Soviet traffic which have been described above, 1940 witnessed an appreciable increase in the flow of signals available for study by GC&CS analysts and others. A special Russian Section at Wavendon, with a staff of 13 and run on an interservice basis, tackled Naval, Military, Air and Diplomatic traffic.  Two people in section 8G of Naval Intelligence were engaged in Russian T/A.  Work on Russian systems was also carried out elsewhere in GC&CS's nooks and crannies. French Naval officers attached to the Naval Section, were set to work on 5 figure Baltic and Black Sea traffic. Decodes from the 4-figure Baltic books had apparently begun to produce results.  Work was also done on Russian systems in the Meteorological Section. Before the Second World War, all countries had exchanged weather observations via radio. After the outbreak of hostilities, this cooperation ceased. The British went over to landlines while the Germans and later the Russians went over to encrypted radio traffic for the communication of their weather reports. Soviet "meteo", as it was called, was broken due to the efforts of Tiltman and McVittie. The special German-Soviet liaison cypher, RUSMET 3, was broken in principle in November 1940.
Another important group engaged on work on Russian cyphers in England was the Polish party at Stanmore where the Polish Wireless Research Unit was based. After the fall of France, the Polish General Staff moved to London where its intelligence chiefs of Section II were soon in touch with their British opposite numbers about the deployment of their various resources to best advantage. In August 1940, they drew the attention of their British hosts to the fact that they had their own W/T operators in the country who were skilled in Russian interception as well as two specialists on the subject of Russian codes and cyphers. At the same time, a report was handed over summarizing the results which had hitherto been obtained by one of these specialists working under difficult circumstances.  This began with a survey of radio transmitters in Soviet service and ended with an enumeration of various Soviet codes identified as being in use at the beginning of 1940: OKK 5 (Obshchi Komandirski Kod 5), AK 39 (Aviokod 39), ChN (Chernomorski Kod), the NKVD code and so on. OKK5 and AK 39 were said to be employed in all kinds of unit, from battalion level upwards. The codes were subsequently recyphered. The author claimed that he had succeeded in breaking OKK5 and AK 39 "to a considerable extent", falling short of complete success only because of scarcity of material due to the poor audibility of Soviet field W/T stations in Paris. The work had been continued on a freetime basis after the transfer to England. Reception was said to be somewhat better in London than in Paris.
This report was submitted to Tiltman at GC&CS for his expert opinion which was blunt and to the point:  the information it contained while accurate was completely out of date and no mention was made in it of Russian higher grade cyphers. Tiltman doubted if any use could be made of the decoding expert mentioned in the report. On the other hand, given the lack of trained interception people, it was of considerable interest to know that there were Polish W/T operators with experience of monitoring Russian traffic in the country: indeed Tiltman could not understand why this fact had not been brought to his attention before. In general, he saw no point of establishing a separate station on the small scale suggested in the report.
Although the subsequent course of events is not entirely clear from the available British documentation, by October 1940 a decision had been made to make use of Poles at Stanmore for intercepting Russian traffic with Captain Grant of the Russian Section exercising a general supervision of the operators. 
Reviewing British efforts against Soviet cryptosystems in the period between the Molotov-Ribbentrop Pact and Barbarossa, Hinsley summarizes the results as follows:
[GC&CS] had broken the Russian meteorological cypher, read a considerable number of naval signals and decoded about a quarter of some 4 000 army and police messages; but like that which had long been exploited in India and at Sarafand, this was local traffic and, though useful for tactical information, it yielded nothing of strategic importance. 
After the launching of Barbarossa, Churchill - "the arch-instigator of intervention during the Civil War, the arch-enemy of Communism"  - was quick to assure Stalin of British support for Russia in her struggle against the German insurgents. Yet the Anglo-Soviet alliance which emerged, remained at best an uneasy one, quite different in character from its Anglo-American counterpart. What effect did the new turn of events have on British work on Soviet cryptosystems?
In an often cited footnote, the official history of British Intelligence in the Second World War states simply: "All work on Russian codes and cyphers was stopped from 22 June 1941, the day on which Germany attacked Russia, except that, to meet the need for daily appreciations of the weather on the eastern front, the Russian meteorological cypher was read again for a period beginning in October 1942."  But is this perhaps a truth with hidden reservations? As Bradley F Smith has acutely pointed out some care may be needed in its interpretation: it does not necessarily imply, for example, that the British Government ceased to intercept Soviet traffic. 
The newly released papers in the PRO underline the wisdom of this cautionary remark. Certainly they put paid to the idea that all British monitoring of Soviet signals ceased instantaneously on 22 June 1941. What we find instead, are references to discussions beginning in mid-September about reducing work on Russian traffic. By the eighteenth of that month, the Admiralty had agreed that Russian crypto at Sarafand should cease although it was noted simultaneously that it was still undecided whether or not "a watch on Black Sea W/T links" was to continue there.  Russian interception in the UK was to be reduced to 2 sets at Flowerdown and 2 sets at Cheadle. The Poles at Stanmore were to continue their interception and study of Russian traffic. Finally, the Russian Section at GC&CS was to be reduced to 3. The latter were to "keep an eye" on Russian Naval traffic and were expected to issue periodic reports about their work. Interception of Russian traffic other than naval was to cease.
In short, it appears that interception of some Russian traffic was going on as late as the end of September 1941. The exact nature of the "study" of Russian traffic being carried out by the Poles at Stanmore remains unclear although Appendix G of the 1989 German edition of Kozaczuk's book on Enigma speaks of the Poles at Stanmore working on the interception and solution of Soviet Army signals in the period 1942-1943. 
Despite these obscurities, it seems reasonable to conclude that British analytical work on Russian cryptosystems - but not necessarily British interception or certain work farmed out to the Poles - had been more or less discontinued by September 1941. Important lines of research would not apparently be revived until after the end of hostilities. While it is now well known that the US Army Signal Intelligence Service began the so-called VENONA project to examine and possibly exploit encrypted Soviet diplomatic communications on 1 February 1943, there is no evidence to suggest that analogous work was undertaken by their British counterpart during the war.  The divergence between British and American efforts in this area was probably due more to a question of resources than to any substantial difference in perceptions.
The new wartime alliance which emerged between Britain and the Soviet Union after Barbarossa would also have its SIGINT component. The details of Anglo-Soviet collaboration in this field belong, however, to another story.