HS7-42 3

To enable a signal network such as that employed by SOE to operate efficiently, one of the main essentials required ia a high grade cipher. This was possible over the main line and commercial networks because Book and machine ciphers could be used.

The work of the cipher department is undoubtedly recorded more fully elsewhere, therefore it is intended for purpose of this signal history, to show briefly the progress made, and record approximately the amount of operational cipher material supplied to all areas served by SOE Signals.

To equip field station with a safe code by sending printed codes into the field was, (in early stages), thought quite impossible. The enemy maintained strict "snap controls" and agents were adverse to carrying incriminating documents with them. They preferred a simple code which could be memorised from a key poem. These poems were chosen by the agents themselves and were in the main a nursery rhyme or folk song easily remembered by the individual concerned. The poems were duly recorded and tabulated at the Base Station before the agents departed for the field.

It soon became apparent that the enemy was able to effectively 'break" a high percentage of SOE signal traffic, and new methods were sought to counteract this danger. Various methods were tried out but without much success. The reason being that the basic principle was adhered to whilst improvements were concentrated on the idea of making the original method more complicated for the enemy to breakdown. This in turn imposed a greater hardship on the agent personally as all it meant was they had more to remember. The result was a higher percentage of error and indecipherable messages which produced serious delay in operations.

Late 1942 the newly formed London HQ Cipher Security Section developed an entirely new method. This was known as the WOK (Worked Out Key) method. This meant that the agents could now be armed with a cipher micro-photographically prepared in such a way that if they were concerned their codes could easily be destroyed. This gave the agents much more confidence, and at the same time reduced the percentage of error to a minimum. This method remained in operation for some time and was instrumental in preventing the enemy from reading operational messages.

Research and development continued and in late 1943 the OTP (One Time Pad) was produced. This was not only perfected in micro-photographic form but also invisibly (?) printed on silk handkerchiefs and other articles of clothing.

This new method also completely "foxed" the enemy cryptographer. A safe code had at last been found which could be carried without the slightest fear of detection even though a particular agent might be stopped and minutely searched. Furthermore, during paramilitary operations the One Time Pad was produced on waterproof paper capable of being saturated in water for a long time and still remain legible.

A method of passing messages over the BBC at dictation speed by secret code was also developed which proved most satisfactory.

Some idea of the volume of cipher material produced from 1943 onwards can be obtained by perusal of the over-all returns of major cipher items shown below:

WOK: 2283.
BBC (Strips): 196.
OTP: (Micro-photo): 3052.
OTP (Waterproof): 7942.

RADIO FINGERPRINTING.

Radio-Fingerprinting is a method used to establish the identity of radio operators. This done by comparing recordings of individual transmissions. These recordings are taken at regular intervals on a tape. By careful analysis, skilled "readers" can identify without question, any operator morse "handwriting".

In 1943, it was felt that some method of identification was essential to SOE Signals, and for this purpose Radio Fingerprinting was introduced.

This added safeguard was instrumental in proving the identity of SOE operators whose circuits were possibly in jeopardy or considered unsafe, and enabled operations to continue with complete confidence.

The "Readers" skill was developed to a high standard of efficiency, in fact so much so, that operators returning from occupied zones, on being shown the result attained, were amazed.

In one particular case, an operator had trained several WT operators in the field, and to use his own words, was confident that he had "tricked us" by allowing his pupils to work his signal plan from time to time. He was shown records proving that each change of operator had been detected. The findings were compared with details he was able to supply and on every occasion the change was correctly identified.

Other operators raised the question as to what happened when the transmissions varied, probably due to nervous tension and anxiety as to their personal security if in a tight corner etc.

Radio Fingerprinting records produced evidence of identification under all sorts of conditions. When this was proved, several operators expressed the view that, had it been known how effectively the system worked, they would have felt much safer on operations.

From its inception until the close of hostilities, more than 600 individual cases were examined. recordings and checking being done at regular monthly intervals.

EQUIPMENT.

At the outbreak of war suitable equipment for use by clandestine operators was not available, and circumstances had not yet arisen to form a basis upon which to state requirements.

In the early days ideas were centred on the provision of a wireless set for the clandestine operator, and the scope of use to which equipment was eventually put, i.e. para-military was not then foreseen. It was not until the final stages of the war when advances in the scientific field made it possible to provide sets suitable to meet all needs.

The situation prior to the inauguration of Independent SOE Communications is reflected in the following extracts from correspondence.


                                             Extracts from correspondence WT/S to M 9-4-1942.

1. The whole system of training WT agents for Western Europe is already in jeopardy and will suffer a severe set-back
    unless steps are taken at once with regard to operational equipment.

2. Any agents who leave after June 1^st must therefore take SOE sets with them. This means that training on those sets
    (other factors being equal) would have to begin on April 13^th.

The arrangements for design and manufacturing of equipment were put in hand. This decision was to influence technical design in all the services since it led to the manufacture of an immense range of midget components. Pending production, use was necessarily made of such equipment as was available.

In the early part of 1942 equipment known as A Mark I, a modified version of the Army 18 set was used.

On the many factors which influenced design was conditions in use called for the greatest consideration. The circumstances under which the operator worked varied from one great danger to one of good security and answers to queries sent to the field in the quest for guidance on the form of equipment were not infrequently diametricallly opposed. Always the sets were too heavy, and sometimes replies indicated they were too big or too small. The capture of a small set (capable of good all-round performance) led to a closer search.

In August 1942 the transceiver (transmitter and receiver combined) was known as the A mark II with a frequency range of 3 - 4,5 mcs (MHz) and 6 - 8 m/cs, over-all weight about 20 lbs (9 kg), size 15"x10"x4½", was produced and after satisfactory trials adopted for use in the field.

General Staff plans for the employment of SOE teams at greater distance from Base Stations at home and overseas made necessary the provision of equipment with a wider frequency range and increased power. New equipment known as the B mark I was designed to meet this requirement with a frequency range of 3,5 to 16 m/cs, over-all weight about 52 lbs (23,6 kg), size 26x18"x7½".
The performance of this set was most satisfactory but field operators raised objection to it on security grounds due to its seize and weight.

Advances in the scientific field coupled with the experience gained in design and availability of midget components, led to the production of the B mark II whereby technical performance was maintained and size and weight considerably reduced. The weight was actually reduced from 52 lbs to some 30 lbs (13,6 kg). This equipment, tropicalised where necessary, was adopted for use in all theatres for all purposes (clandestine and para0military).

Apart from the provision of a transceiver as such, a most difficult problem with which designers were faced was the means of power supply. The nature of public electricity supply in various countries and districts varied over a wide range, and the necessary provision to meet local circumstances had to be made. Further the constant availability of such supplies could not be depended upon, and means whereby the power required could be derived from batteries had necessarily to be incorporated. In the field, the problem of the reduction of weight of the essential components was not solved.

In certain areas such as the fastnesses of Yugoslavia, the jungles of Birma, New Guinea and Borneo, civil electrical supplies were non-existing. In these areas Signals were dependent upon the power derived from accumulators and the use of an extensive range of equipment wherewith to charge them.

The over-all weight and the obvious difficulties in obtaining transportation were factors which weighted heavily against the mobility and security of the field groups.

Field Stations which could be operated without the impedimenta of electrical mains and battery equipment were in demand.

The technical operational case was explained to the design branch by the Signals Directorate. Concurrent with this demand from overseas groups the need for such equipment arose in connection with a special operation planned in support of "Overlord".

In June 1943, the Nicholls set was designed. This set depended for its power supply upon a generator driven by hand. In use this equipment was successful, and from the appreciative comments received from the field (see evaluation reports) it was realised that the future design of clandestine equipment would be on these lines. However, while on the one hand the inate advantages of this equipment were manifest for general use, its operation did involve the availability of an assistant to the operator, and thus the set was not acceptable to the "lone" agent on grounds of security. The equipment was adopted for use by the Special Air Service (SAS) for their operation in North Western Europe and the Middle East.

Plans for employment of the Resistance groups in support of "Overlord" visualised the emergency of the groups from their clandestine retreats to formed bodies of the para-military type. The retention of high mobility by such bodies was essential to the performance of their tasks. The Signals technical situation therefore had to be reviewed in this light. Although plans and provision had already been made for the strategic siting of a very large number of sets to obviate transportation during enforced movement, these arrangements obviously would not meet all cases.

A WT set of suitable weight and bulk for use during the mobile operations was required. Equipment known as the
A mark III which derived its power from civil electrical supply was already in production. Its proportions and weight (9½"x4½"x7½" - 14 lbs (6,3 kg)) was such that when modified also to work from battery supplies, it was suitable for use in the mobile operations envisaged.

A principle in SOE signal arrangements, the observance from which up in this time had never been departed from, was a complete ban on lateral inter-communication between parties behind the lines. In the clandestine stage this was vital to cipher and signal security. The co-ordinated employment of the groups in the para-military stage made the provision of lateral communication a necessary and the inclusion of means of radio telephony in equipment was demanded.

At the earlier stage in SOE Signals history a number of sets of different types would have been required to meet the widely different requirements of the various theaters of war. Something akin tp perfection was now being achieved in the design and provision of midget equipment and it was now possible to design a set of acceptable bulk, weight and performance which would fulfil all requirements.

This set was known as the B mark III measuring 27"x17"x8", overall weight approximately 70 lbs (31,8 kg).

The growth of the resistance movement in the various countries overseas made necessary the provision of means by which personnel single-handed, reception committees, and operational groups could receive information or action messages as circumstances required. A receiver known as the MCR, was designed and made available in very large numbers for this purpose. It enabled an agent operator to receive messages at places other than at the site where his transceiver was situated. When used by reception committees it was the means by which they were kept informed of the progress of air operations. It was also distributed in very large numbers to guerilla and operational groups and it was the means by which the action messages broadcast on and subsequent to D-Day were received.

The General Staff frequently required communication from locations to which equipment could only be transported by hand. A WT set of minimum proportions was therefore required. The provision of means by which the signalling conditions at any given moment could be ascertained was evolved and with this aid reliability is signalling became the possibility with equipment of very low power and consequently of small weight and bulk. The set designed and provided was known as the "Vest Pocket", overall weight about 5 lbs (2,3 kg), measuring 7½" x 4½" x 1½".

As soon as plans were made for the supply of material to the resistance groups in the field, it was clear that the provision of means of wireless telephone communications between aircraft and ground would be of great assistance to pilots and to the Reception Committees. A wireless telephone instrument known as the "S Phone" was provided for this purpose, an extremely low power employing ultra-high frequencies and of very small dimensions. Also, being directional in type, it could be used by the ground operator with good security. In use, this instrument proved itself most satisfactory.

The sitting of the headquarters at home and overseas was such that, in order to keep contact with their groups and agencies, communication were required by the General Staff to all points of the compass. In Europe, groups and agencies were distributed an an arc of 180 degrees since in the general interest and to obtain high efficiency from the use of low power transmitters, directional aerial systems were always employed for the transmission and reception. The technical planning of the Base Stations required great consideration and care.

The unavoidable problem in connection with the use of directional aerial systems for transmission on spot frequencies, was the multiplicity of aerials required for day and night signalling. In the United Kingdom it was necessary to erect three Baste Stations each with a definite geographical commitment. It was not until 1943 that the internal equipment of these stations could be standardised. Prior to this time use had to be made of equipment obtained from services and other sources and modified to meet our exacting operating requirements, which among other things, called for rapid changes of frequency.

By late 1943 the number of groups with whom communication was required increased considerably and SOE were again faced with the problem of providing adequate aerial systems at the Base Stations.

The development by the Post Office of a transmitter known as the Wide Band Amplifier, which enabled 12 simultaneous transmissions on one directional aerial, provided the solution of the aerial problem. These Wide Band Amplifiers replaced the individual transmitters since, apart from saving space, flexibility in the matter of frequency change made them ideal for our purpose.

In practice, the sitting of the reception station at a distance of 3 miles from the transmitter station proved satisfactory.

Directional aerial systems of the Rombic type fed to groups of receivers through WBA panels were standard technical arrangements in all Base Stations. The reception equipment was kept up-to-date by the installation of the latest type of receivers as they became available.

Provision for the reception of Signals by automatic means was also made, initially to assist in the elimination of corruptions, and on security grounds, but later, particularly in the case of Denmark, was used for the reception of messages transmitted at high speed from the field. Of the various means of recording signals, the gramophone record was best suited to requirements.

The improvements and progress made in design and performance of SOE Signals equipment was extensive. The figures shown overleaf give an approximate return of the vast supplies used throughout the period of hostilities. Examination of the close relationship between orders placed and deliveries accepted will demonstrate the high priority given to SOE production. Contacts placed with Ministry of Supply being in excess of 2½ millions sterling.

                                                                         w.mugge@home.nl

                                                                            11-06-2019