The straddling checkerboard cipher was originally invented by the Argenti, the Pope’s secretaries, in the 16th century. It was later also used by the Communists in the Spanish Civil War and by Soviet spies in WW2.
Another example is Che Guevara. When he was captured, the following cipher was found on him:
How does it work?
For the Spraddling Checkerboard, the 26 capital letters are written in three rows and ten columns of a table. Usually you start with a password that contains the most common letters, such as “ERNSTL”, and fill up the rest of the table with the remaining letters, in the simplest case in alphabetical order. It is important to leave two fields blank in the first row of the table (not necessarily the last two as here). In the last line, after all letters have been entered, two fields remain free, which can be filled with special characters (here “.” and “/”). These can be assigned special functions if required, such as letter-digit switching, but can also simply be blenders.
The above table (literally: “straddling checkerboard”) now allows the monoalphabetic monographic substitution of letters by numbers, where the letters in the first line are replaced by one-digit numbers and the letters in the other two lines by two-digit numbers. This feature, namely that the plaintext characters are replaced by ciphertext characters of different lengths, is called “spreading”. The resulting cipher is called “spread”.
Dealing with digits
In the classic variant, digits are marked by the preceding character / and are taken over directly.
Alternatively, a 4×10 square with the digits 0-9 is used.
- An additional keyword that defines the rest of the alphabet.
- An additional key that defines the order of the columns different from 0-9.