Double Cross in Cairo (34 page)

T
he cipher may best be described as an extension of the familiar ‘Playfair’ system.

The keyword is written down (omitting any repeated letters) as the beginning of a square of five letters by five. This square is then filled in by writing down the rest of the alphabet omitting all letters already used. In order to reduce the alphabet from twenty-six letters to twenty-five it is also necessary to omit the ‘K’. Thus, if the keyword is ELEMENTS the square will be:

Each letter of the ‘clear’ is represented by a pair of cipher letters. The first letter of the pair may be any letter in the same vertical line as the
letter to be enciphered: the second is any letter in the same horizontal line. Thus LD represents A, UM represents T, etc.

It will be noted that the cipher differs from (and has one great advantage over) Playfair in that there are sixteen alternative ways of encyphering any given letter. E can be enciphered as SL, SM, SN, ST, FL, FM, FN, FT, OL, OM, ON, OT, VL, VM, VN or VT.

‘K’ being omitted from the square, is the signal for numerals. The first two lines of the square, following K, become the figures 1 (E), 2 (L) etc., up to 10 (D). The signal for ‘numbers off’ is the letter O encoded, i.e. AH, WJ or the equivalent. The letters standing for numerals must also be encoded: 1 is not represented by E but by SL, OT or its equivalent.

It was arranged that the third word of each message should be the keyword for the next. Thus if a message (say on Monday) began with the words ‘Argent pas encore arrive…’ the square for Thursday’s message would be as follows:

In case of emergency or doubt, a standard keyword is arranged. If it was not known whether the other side had or had not received the last message, or likely to make any mistake about it, the square was to be constructed on the keyword EQUINOX. To indicate that this was being done, the first group of the message was to be SCOOI. This precaution proved a wise one. Owing partly to the incompetence of
the enemy, partly due to technical troubles, the emergency codeword has had to be used over and over again.

It will be clear to the expert that, in spite of the alternatives, the cipher does not present any very grave difficulties to the ‘cracker’. This does not matter, so far as we were concerned, though it should have caused the enemy some anxiety, had he been alive to our wireless security precautions. Meanwhile, it was easy and quick to work, and free from possible ambiguities.

B
etween 1942 and 1944 ‘A’ Force created a series of false military units with the intention of greatly exaggerating the strength of the Allied forces deployed in the Middle East. These fabricated units, which include battalions, regiments, divisions, crops and entire armies, supposedly possessed their own individual identifying insignia which aided specific observations and reports. Having been established earlier in the war, these fabricated components remained available for deception purposes throughout 1944.