Used during WWII, this was a letter addition
square that was used to add OTP 5-letter groups to the plaintext
groups in order to obtain 5-letter cipher groups. Frode Weierud
expands on this.
"The table could have been made like a simple square with one
vertical standard alphabet as the first left-hand column, however to
minimize the risk of taking off the wrong ciphertext letter the
standard alphabet has been copied in all of the columns. The
substitution alphabets in each column are symmetric in the sense
that if the entry for A is Ae then the entry for E is Ea. (see
column B). The addition square is used to encipher by finding the
corresponding OTP key letter in the top row such as to select the
column to use and the cipher letter is the lower case letter in red
standing next to the plaintext letter to be enciphered. Deciphering
works the same way as the substitution alphabets are symmetrical as
mentioned above. The silk square pictured below looks exactly the
same as those issued by SOE to its radio operators".
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One complete "page" of silk
code chart. Click to enlarge. (Photo by Michael Graham) |