Details of Encryption process.

The cipher clerk was handed the message to be enciphered. He began by writing the time and the number of letters in the message on a transmission sheet. He laid a transparent ‘flimsy’ over the stencil and selected a cell to start the message. This cell was identified by its row and column coordinates, let us say ea ac, which were then encoded with a bigram table such as:

a b c d e
l c g q j
o y e v z
n b s k x
i a p r u
d w m f h

So coordinates 'eaac' could be encoded to a choice of letters, including 'zios' or 'jodg' and so on. The transmission sheet would then look something like this:

1307 – 54 – zios

Next he looks at the message text. Following his Instruction Manual, he replaces numerals with text making abbreviations where appropriate – for example ‘aqt’ for ‘acht’ – and he enciphers place names with a given substitution cipher.

Now he writes the message onto the flimsy, from the starting cell from left to right, row by row, using the white cells only. If the end of the stencil is reached, continuation proceeds in the top row. He puts an arrow to mark the starting column and rules of the top of the message (see below).

Finally he identifies the offset for the take-out column by adding together the digits of the minutes and the number of letters. In our example this is 0+7+5+4=16. He counts 16 columns to the right of the start column and puts a cross there. And he rules off the bottom of the text.

Now he is ready to take out the ciphertext. He starts in the column marked 19, then follows the transposition key 20,21...25,1,2...18. The ciphertext is thus

NITOT SNILM AISMT OOISA HENEE AIRTP SDTPP HMSSS YATHO AWIOO TSOS

Messages may be enciphered and deciphered using the Windows program written by Geoff Sullivan to accompany my article in Cryptologia "RS44 - the epitome of hand field ciphers". The next section gives details on how to use this program.