Straddling Checkerboard

Back in the ancient days before computers and the interweb, the transmitting of secrets was an art. There were many masters, and many more who claimed to be. Interestingly, most of the great ideas and techniques followed the same basic formulas, and masters' claimed innovations were often nothing more but a rehash of old techniques. Some of these techniques formed the basic ciphers. (The word “cipher” or “cypher” was the ancient term for zero. Many interesting theories exist on how it gained its modern meaning.)

The basic ciphers (e.g., substitution, transposition, modular rotation, polyalphabetic substitution) were developed early on, most before the fall of the Roman Empire. Little was changed until the renaissance, when advances in mathematics revealed the importance of letter frequency, letter combinations, and other tell-tale patterns in the art of codebreaking. The old substitution codes you’re used to seeing in the newspaper were suddenly easy to crack. Polyalphabetic substitutions helped some, but were far from impregnable.

In what may be the first in a series of posts on traditional cryptography (hand encoded/decoded), I would like to introduce you to the Straddling Checkerboard, also known as a monome-dinome cipher. This is one of the more elegant ciphers to come along, though not perfectly secure. In fact, it is one of several combined components of the infamous VIC cipher.

In this cipher, the characters are substituted by numbers using a grid. High frequency letters, those that appear most often in English, are placed in the first row and are thus written with single digits. This cuts down on the length of the overall code in what’s called “compression”. The rest of the characters fill in the next two lines, with two special characters used signals to say that the following character is a literal number. You can read all about how to encode and decode the cipher at the wiki page.

The above flash piece (yes, it’s flash… you can use it right in this page) generates a random straddling checkerboard key and handles the encryption automatically. Type something into the message box in plaintext and the code will be generated in ciphertext in the box below.

So why am I making this? So you can teach your kids how to use the checkerboard and leave each other messages on your fridge! No, not really, but that’d be cool. It’s part of a bigger project that’s even nerdier, which I will reveal in the next post. Until then, enjoy writing codes.

Source & Example

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