BEYOND LETTERS: THE STRADDLING CHECKERBOARD CIPHER
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BEYOND LETTERS: THE STRADDLING CHECKERBOARD CIPHER
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The straddling checkerboard takes the grid idea a step further. Instead of a uniform 5x5 table, it uses an irregular layout with a 10-column top row for the most frequent letters, and two additional rows underneath for the rest of the alphabet. This design was historically used by spies because it produces compact, low-redundancy number strings.
Straddling Checkerboard (English letter frequency)
0 1 2 3 4 5 6 7 8 9
E T A O N R I S
2 B C D F G H J K L M
8 P Q U V W X Y Z . /
Figure 1: The eight most common English letters (E T A O N R I S) sit in the top row and encode as single digits. Rows 2 and 8 are 'shift' rows -- any digit that starts with 2 or 8 signals a two-digit code follows.
The efficiency comes from letting the most common letters -- E, T, A, O, N, R, I, S -- use just one digit each, while rarer letters need two. This variable-length encoding shortens the overall message and makes traffic analysis harder, which is why the straddling checkerboard was a staple of mid-20th-century espionage, often paired with a substitution key for extra security.
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https://books.apple.com/us/audiobook/id1844651132
https://books.apple.com/us/audiobook/zephyrz/id1813172003
https://books.apple.com/us/audiobook/the-mnemosyne-protocol/id1891016838
