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The tool of the ordinary SOE wireless operator was a short-wave morse transmitter, or rather a transceiver - that is, a transmitter and receive combined. The type generally used by operators in France was called the B mark II; it weighed thirty pounds, and fitted into an ordinary smallish suitcase some two feet long. Its frequency range was quite wide - 3.5 to 16 megacycles a second; but its signal was weak, for a set so small could produce more than 20 watts at the best. It needed moreover seventy feet of aerial, well spread out, and likely to catch the eye of a policeman looking for it. Exactly what frequency the set worked on was determined by removable crystals; every operator needed two crystals at least, one for day and one for night work, and might have several more. Crystals are delicate, and can be easily be broken, in transit or if they are dropped; and they share with transceivers another disadvantage: they are practically impossible to disguise as anything else. They are at least small enough to lie on the palm of the hand, which makes them easy to conceal, by wrapping them up in something like a pair of pyjamas. But event the smallest transceiver ever available to SOE in France, the A mark III, which measured about ten inches by seven by five, was too bulky to be so simply hidden; and if a transceiver was discovered in a search, only luck would enable the holder to bluff his way out of imminent trouble. A few fortunate operators were able to persuade stupid policemen that they were carrying dictaphones or some kind of cinema aparatus; one, otherwise undistinguished, correctly judged the characters of the two types who had made him open his suitcase and bought their silence for a thousand francs each. Another, more flamboyant, is said to have looked the German who stopped him straight in the eye: `Je suis un officier britannique, voici mon poste de radio'; he received a cheerful `Va t'en donc' in reply. A woman operator, stopped by a couple of Feldgendarmes at a snap road control in the country on the only occasion she ever had to travel with her set - it was on the carrier of her bicycle - laid herself to be charming to both of them; each made an assignation with her; neither remembered to look at her suitcase; and she never saw them again. But an expert could spot a normal transmitting set the moment the case it was in was opened. To meet this difficulty, the camouflage section produced a few sets disguised as ordinary household receivers; but this disguise would hardly deceive for a moment anyone who took the back off the set and kew what he as about.

Security troubles for the operator might arise also if he ran his set off the mains. The German intelligence service's wireless direction-finding (D/F) teams were numerous and efficient, probably better than the British, for whom Langelaan claimed that if ever an unidentified transmitter was heard `in a matter of minutes a first, rough direction-finding operation had been accomplished. If the transmitter was anywhere in the United Kingdom, in less than an hour experts equipped with mobile listening and measuring instruments were converging on the region where it had been located'. French operators in the field early discovered that a long transmission in a large town would probably bring a detection van to the door within thirty minutes. The Germans soon worked out a technique for establishing what part of a town a clandestine operator was working in, by cutting off the current sub-district by sub-district and noting when the clandestine transmission was interrupted; then they could concentrate their efforts on the sub-disctrict affected, and hope to track down quickly at least the block, if not the building, the set was working from. There were several counters to this: posting a protection team, who would warn the operator to hide at the approach of a D/F van, or even of a man sauntering down the street with his collar turned up, in case he was a Gestapo agent with a miniature listening set held to his ear; transmitting from an isolated spot in the country, instead of a town; or using a accumulator instead of the mains, though this raised a problem of its own: how to keep the accumulator charged. The best protection of all, better even than constant changes of crystal during a transmission to confuse the enemy, or constant changes of the place of working, was constant attention to brevity by the writers of messages. The less a set was used the less the chance there was that D/F teams would pick it up. In the heroic early days an operator might spend several hours a day at his set: almost all the early operators, as a direct result, were caught. People grew more wily later. Yet as they had to spend less time hammering away at their morse keys, operators began to take more interest in the other affairs of their circuits; this brought different dangers. In retrospect, the security section laid down that `The ideal is for the W/T operator to do nothing but W/T work, to see his organizer as little as possible, if at all, and to have contact with the fewest possible number of the circuit'. But this was bound to lead anybody who was not exceptionally self-reliant to the verge of distraction from boredom; which in turn might drive him to do things that would expose him unecessarily, to the enemy.

This was a lasting difficulty. Another, graver one was only temporary: it was that wireless communications with the field were not at first under SOE's own control. Of course, SOE like any other user, however secret, had to secure through an interservice frequency board a wavelength allotment; a merely technical point. Much worse trouble arose from what various high authorities supposed to be the case when SOE's work began: that the obstacles in the way of clandestine wireless communication with the continent were so complex that they all needed to be handled through a single group of staff officers who were outside SOE altogether, in the body whose responsibility it was to secure intelligence from enemy and enemy-occupied countries. Relations between these officers and their superiors on the one hand, and the staff of SOE on the other, were lucid sketch of the dangers and difficulties that arose for another secret service in a comparable situation has been published by Dewavrin; who brings out also the deadly combination of intertia with incompetence that he found he had to fight. In the end, it was agreed in the light of experience that SOE could and should run its own wireless affairs; work, indeed make, its own sets, train its own operators, invent its own ciphers, and do its own diciphering. A signals directorate assumed full powers from 1 June 1942.

Having secured full control of wireless on the home front, SOE's high command was disinclined to part with it to agents in the field. The visionary scheme for creating a French secret army over a quarter of a million strong that André Girard (Carte) dangled before F section in 1942 included a proposal to equip that army with wireless sets for internal communication. Like the rest of his ideas, this was a grandiose conception insufficiently worked out in detail; and though he engaged Bodington's enthusiasm for the plan nothing came of it. For it ran clean against every principle of sound clandestine organization; and it raised special complications about cipher of its own. F did send a number of sets out to the Riviera by felucca that autumn, to be used in this scheme if it rippened; Cammaerts, the only man to get much work out of any part of Girard's following, later found himself with thirty of them to look after, but they were already almost useless with damp and neglect, and he left them severely alone.

The whole business of encoding and enciphering wireless traffic with the field was naturally both complicated and highly secret; even at this interval of time it is useless to expect any authoritative public statement on the subject, and I have made no attempt to pursue it in detail. In the early years every operator took with him to France a personal code which he had memorised; this might be as simple as a Playfair code based on a single word, or consist of a string of numbers to guide him in transposition. A rather more elaborate system, called the `worked-out key', followed; this was based on a phrase, usually a line from a poem, chosen by the agent because it could be easily remembered. This was less easy to break than the previous ciphers, and no eveidence I have seen suggests that the Germans did in fact deciphere SOE's messsages of this kind before they had discovered from prisoners what the phrase for a particular message or set of messages was. However, the Paris SD, without the advantages of Beaulieu training, was able to manipulate worked-out key ciphers with complete desterity, while working some captured sets back to London. And the French continued touse, for trafficbetween the BCRA and French agents in France, codes which on one celebrated occasion the British were able to break practically at sight; it was thought in Baker Street that `every message' the French sent in their own codes `can be read by the Germans' as late as the end of March 1944. By this time the British were using a much safer, in fact a practically impenetrable, cipher: one-time pad. As this is now public knowledge, a summary account of it can turn no one's hair grey: the agent held a pad of silk slips, each printed with columns of random letters or figures from which any message could be enciphered or deciphered; he used the slips in the order he found them on the pad, and was supposed to tear each slip off and burn it after use. Home station held the only duplicate. The only snags SOE's operators found in this arrangement were that the silk was hard to burn and that home station sometimes referred to messages a fortnight old.

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And in the field the difficulties of coding in clandestine conditions were such that any addition to them only made things more complicated than ever; in seeking to introduce a mistake or a run of mistakes on purpose, an operator might easily introduce several more by accident and thus render what he was trying to say unintelligible to London. Besides, the checks were useless as safeguard in the most dangerous case, the case of an operator who changed sides, and went over altogether to the enemy; for of course he would betray his true checks as well as everything else. The Germans in any case soon did find out the double nature of the security check; and on many captured wireless operators they put such stern pressure to reveal what the true check was that only the most stoical and heroic could remain quite silent. yet one recourse remained for the operator under moral or physical torture; say what the true checks were, but mis-state how they were to be used.

This threw the responsibility of noticing that something was amiss back across the English Channel; and here the security check system ran into its worst difficulty of all - F section thought it unreliable. In some other country sectins, especially in the small ones, elaborate pains were taken to check over messages; the slightest slip was caustiously examined, and operator whose choice of words or frequency of error was at all eccentric became suspect at once; not so with F. The reasons for this were personal. Temperamentally, Buckmaster and many of his assistants were adventurous rather than methodical. They always inclined to resent suggestions bearing on their work that came from others. And the section signals officer for the last two years of operations in France, Georges Bégué, had worked a transmitter from the ZNO for six months with distinction without bothering about his security check at all. This was not a safe line to take, and trouble arose from it. F was not alone in doubting the efficacy of the checks; `Many members of the security directorate', Boyle was told late in the war, `have never been happy about the bland way Country Sections and Signals dismissed the identity checks and certain mutilations.

Yet another and rather more reliable method existed by which the identity of operators could be controlled; and they were encouraged to rely on it while under training. Before they left England, they each made a dummy transmission whch was graphically recorded on a special machine that illustrated each agent's personal style of sending; these styles or `fists' vary as widely as handwritings do, and are as readily reconisable - both by ear, and by this recording system loosely known as `fingerprinting'. Unfortunately, it turned out that the styles are also as readily imitable: this was the undoing of many of SOE's agents in Holland and of several in France. Fourteen SOE transmitters in Holland were successfully worked back to England by half-a-dozen German operators; the figures for France were not as large, but the imitations were almost as effective. In one important case the signals staff reported a German operator's first attempt to imitatean English one. Otherwise the files show numerous occasionswhen home station spotted that a new operator, an agent's local trainee, had taken over the set; but few when it was noticed at once that the set was in wrong hands. Moreover Buckmaster himself had doubts about fingerprinting for an intelligible reason: `we knew that agents often had to send their messages in the most difficult condition - it might be a central-heating plant in a loft or in the freezing cold of an outbuilding - and cramp or other afflictions could radically alter their methods of keying'. He added at once that `we at home had to be flexible and one cannot hide the fact that at times this flexibility led us to give the benefit of the doubt, at least for a while, to an operator who later turned out to be false. That was the fortune of war'.

In any case fingerprinting, like security checks, did not provide protection against the worst case, the agent who went over to the enemy hear and soul; for he would naturally work his own set back in his own style. No section working into France seems to have used operators ready to go that far.

Some arrangements for checking operators' reliability form an inevitable part of the running of any clandestine organisation; it is a fair criticism of SOE - which its security section made for itself, just after the war - that its arrangements were sometimes dangerously faulty. The signals directorate did include, from 1942, a section of its own devoted to signals security. But this body was staffed by signals and not security officers, and it was out of touch with country sections at the day-to-day working levels. Its time proved to be so much taken up with technical points that it did not provide an adequate safeguard, for some circuits in France at least. An operational security section whose principal task would be to conduct a daily review of the whole of the incoming wireless traffic from the field was perceived, too late, as amin requirement. But SOE introduced alaos a startling enovation: wireless communication to the field through the ordinary tranmissions of the BBC.