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In cryptography, a straddling checkerboard is a device for converting an alphabetic plaintext into digits whilst simultaneously achieving fractionation (a simple form of information diffusion) and data compression relative to other schemes using digits[1]. It also is known as a monome-dinome cipher.

A straddling checkerboard is set up something like this:

 01234 56789

The first row is populated with the ten digits, 0-9. They can be presented in order, as in the above table, or scrambled for additional security. The second row is typically set up with high-frequency letters (mnemonic ESTONIA-R), leaving two blank spots. It has no row label. The remaining rows are labeled with each digit that was not assigned a letter in the second row, and then filled out with the rest of the alphabet.

Much like the ordering of the digits in the top row, the alphabet can be presented in order (as it is here), or scrambled with a keyword or other technique. Since there are 30 slots in our grid, and we skipped two letters in the first row, there will be two spare cells in the other rows. We have filled these cells with a period '.', and a slash '/' to be used as a numeric escape character (indicating that a numeral follows). It doesn't matter where these spares go, so long as the sender and receiver use the same system.

To encipher, a letter in the second row is simply replaced by the number labeling its column. Letters in the third and fourth rows are replaced by a two-digit number representing their row and column numbers. Mapping one-digit numbers to common letters reduces the length of the ciphertext, while also concealing the identities of the two-digit numbers by reducing the frequency of their first digits. Here is an example:

212722 65

The resulting message, 3113212731223655, may be sent directly (if the table is scrambled), but is usually processed through a second cipher stage, such as transposition or substitution. As a simple example, we will add a secret key number (say, 0452) using modular (non-carrying) arithmetic:

  3113 2127 3122 3655
+ 0452 0452 0452 0452
= 3565 2579 3574 3007

Optionally, we could then use the same straddling checkerboard to convert the ciphertext back into letters:

3565257 93574 3007

Deciphering is simply the reverse of these processes. Although the size of groups can vary, deciphering is unambiguous because whenever the next element to be deciphered starts with a 2 or a 6, it is a pair; otherwise, it is a singleton.

Ciphers that use a straddling checkerboard include the VIC cipher and SECOM

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