The process of enciphering messages for the Centre was divided into two parts. The first stage is comparatively simple and can easily be carried in the head, and, as I have stated, I frequently made notes in this First Stage Cipher for ease and convenience. The second stage involves the "closing" of the first simple encipherment against the text of a code book. (In this case, a "code book" is any ordinary published book that may be selected.)
The first stage of encipherment is based on a key word, which must have six letters and which is changed at intervals by the Centre. In the example to follow the key word is PLAYER. This is written out horizontally, and below it- in line- the remainder of the alphabet is written out (in the case of PLAYER in groups of six) together with a symbol for "signal" (@) and Full Stop. "Signal" means that the message changes over from letters to numbers.
At this stage the "pattern" looks like this:
It is now necessary to assign a number to each letter, and for this purpose the numbers 1-9, 00-09, and 40-49 are used. (In the first set of numbers the figure 4 is omitted as it would lead to confusion with the double numbers beginning with 40.)
Those letters which are changed into single figures (1-9) are ASINTOER (arranged in this order for mnemonic purposes), the numbers being assigned to the relevant letters as they occur reading vertically and starting in the top left-hand corner of the diagram. With these numbers in place, the diagram now looks as follows:
To this pattern are now added the double numbers (00-09, 40-49), starting from the top left corner and going down the columns consecutively filling in the blanks.
With all the numbers inserted, the diagram now looks like this:
(This pattern is, of course, changed occasionally by the Centre by allocating a different "mnemonic" or by changing the code word, or both.)
A message enciphered in the first stage would look like this. (After a change from words to numerals, indicated by the symbol "signal," the numerals are each repeated three times.)
These figures are then split up into five-figure groups, the third and antepenultimate groups being left blank to take "recognition groups." At this stage the message begins to look like a ciphered message, but is still by no means secure and would present no difficulties to a cryptographer. Here it is with the blanks left for the recognition groups and the last group made up with noughts to complete a five-figure group.
At this point the first stage of encipherment is complete. It is now necessary to "close" the message by re-enciphering it against the selected portion of the "code book." In practice, I used a Swiss book of trade statistics, but not having that by me now, I think it might be appropriate to take a passage at random from the report of the Royal Commission on the Canadian spy case.
On page 77, line 14, is the following phrase: "that even if the adherent or member should refuse to engage in activities so clearly illegal and which constitute so clear a betrayal of his or her country."
The enciphering phrase can be taken from anywhere in the book, starting at any word in any line. In this case it starts at the fourth word of the line, i.e., "that."
From here on the key phrase is enciphered in exactly the same way as was the original message, and written out in five-figure groups, leaving here also the third and antepenultimate groups blank, as below:
These groups are then added to the groups of the original ciphered message, the tens being ignored, and the final result is this:
Now the message is complete save for the two recognition groups which enable the recipient to decipher the message. These are made up- in different ways - from the "code book group," the "fixed group," and the "message group."
The code book group is composed of the page number, line number, and word number of the key phrase. In this case, page 77, line 14, and word (from left) 4-77144.
The fixed group – 69696 - is given by the Centre and changed only on orders from the Centre.
The message group is, for the first recognition group, the fifth group of the message as enciphered so far, i.e., 43290; for the second recognition group, the message group is the fifth group from the end of the message as enciphered so far, i.e., 69482.
Now in each case the code book group, the fixed group, and the message group are added together. Thus:
These two totals are now inserted in the appropriate spaces in the already enciphered message and the final result reads as follows:
DECIPHERMENT
In decipherment, naturally, the first essential is to unravel the components of the two recognition groups. To illustrate this process clearly, let us recapitulate how the two recognition groups were arrived at. The first recognition group was composed of the code book group (C), the fixed group (F), and the fifth group of the enciphered message (not counting the blank) (Ml); the second recognition group was composed of the code book group (C), the fixed recognition group (F), and the fifth group from the end of the enciphered message (not counting the blank) (M2). Therefore the first recognition group is made up of C+F+Ml, and the second recognition group is made up of C+F+M2.
To break down the first recognition group, subtract from it the fixed group (69696- already known by agreement) (F), and the sixth group of the completed message (Ml), thus leaving C. The second recognition group is really only a cross-check, for on subtracting from it F and M2 the result should also be C.
Once C is known, the corresponding passage in the key book can be turned up, and the decipherment follows by using the reverse procedure of the encipherment.