Straddling Checkerboard

History

The Straddling Checkerboard cipher first appeared during the Spanish Civil War (1936-1939) when it was used by the Russians.

Description

It produces a numeric cipher and is similar to the Monome-Dinome cipher but uses a 3x10 grid instead of a 3x8 grid. Above the grid the digits 0-9 are randomly placed, or this can be created from a 10-letter keyword using the alphabetical order of the letters. The bottom two rows of the grid are assigned separate numbers. The grid is filled with a 26-letter keyed alphabet created from a keyword, with repeated letters being omitted, followed by the unused letters of the alphabet in alphabetic order.

Finally a period (.) and forward slash (/), to indicate a number follows, are used to fill the last two cells of the grid. The two of the cells in the top row matching the same two numbers used as the row indicators are left empty, as are the last two cells in the bottom row. For example using the key word HOUSEPLANT and appending unused alphabet letters produces the keyed alphabet:

HOUSEPLANTBCDFGIJKMQRVWXYZ./

A 10-letter keyword, PLAYWRIGHT, is used to create the numerical key, 6510974238, from the order of its letters. This is then written into the grid and the bottom two rows assigned numbers to produce the following table:

6 5 1 0 9 7 4 2 3 8

H O U S E P L A

3 N T B C D F G I J K

7 M Q R V W X Y Z . /

The plaintext is converted to the cipher by locating the letters in the table and, if the letter is in the top row, the cipher number is the single digit in the same column. Letters in the other rows are replaced by the row digit followed by the column digit.

Example

Keyword #1: HOUSEPLANT

Keyword #2: PLAYWRIGHT (6510974238)

Row labels: 3 7

Plaintext: Always yield to temptation, because it may not pass your way again.

Plaintext: a l w a y s y i e l d t o t e m p t a t i o n b e c a

Ciphertext: 8 2 79 8 74 0 74 32 9 2 39 35 5 35 9 76 4 35 8 35 32 5 36 31 9 30 8

Plaintext: u s e i t m a y n o t p a s s y o u r w a y a g a i n

Ciphertext: 1 0 9 32 35 76 8 74 36 5 35 4 8 0 0 74 5 1 71 79 8 74 8 34 8 32 36

Ciphertext: 82798 74074 32923 93553 59764 35835 32536 31930 81093 23576 87436 53548 00745 17179 87483 48323 6

Solving

Solving method: Hill Climbing search.