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Four-square cipher - Wikipedia, the free encyclopedia (990 words) |
 | The four-square cipher uses four 5 by 5 matrices arranged in a square. |
| To generate the ciphertext squares, one would first fill in the spaces in the matrix with the letters of a keyword or phrase (dropping any duplicate letters), then fill the remaining spaces with the rest of the letters of the alphabet in order (again omitting "Q" to reduce the alphabet to fit). |
| When only the ciphertext is known, brute force cryptanalysis of the cipher involves searching through the key space for matches between the frequency of occurrence of digrams (pairs of letters) and the known frequency of occurrence of digrams in the assumed language of the original message. |
| Chapter 8 -- The Cipher Exchange (5798 words) |
| Squares 1 and 3 are plain unkeyed (I/J in same cell); squares 2 and 4 are keyed. |
| Cipher substitutes are found at the other corners of that rectangle, first in square 2, the second in square 4. |
| The cipher equivalents are those letters forming the opposite corners of a rectangle determined by the pt pair. |
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