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The four-square cipher uses four 5 by 5 (5x5) matrices arranged in a square. Each of the 5 by 5 matrices contains the letters of the alphabet (usually omitting "Q" or putting both "I" and "J" in the same location to reduce the alphabet to fit).
All polyalphabetic ciphers based on the Caesar cipher can be described in terms of the tabula recta. The tabula recta uses a letter square with the 26 letters of the alphabet followed by 26 rows of additional letters, each shifted once to the left from the one above it. This, in essence, creates 26 different Caesar ciphers. [1]
A key could be used to reorder the alphabet in the square, with the letters (without duplicates) of the key being placed at the beginning and the remaining letters following it in alphabetical order. [2] For example, the key phrase "polybius cipher" would lead to the reordered square below.
The Playfair cipher or Playfair square or Wheatstone–Playfair cipher is a manual symmetric encryption technique and was the first literal digram substitution cipher. The scheme was invented in 1854 by Charles Wheatstone , but bears the name of Lord Playfair for promoting its use.
This is the reverse of the Vigenère cipher, which here enables decryption rather than encryption. Jackie Fisher, Captain R.N. 1883, later First Sea Lord 1904–1910, 1914–1915. Assigning each letter its place in the alphabet, the keyword corresponds to 1, 1, 25, 3, 5, 8, 13, 21.
The tap code is based on a Polybius square using a 5×5 grid of letters representing all the letters of the Latin alphabet, except for K, which is represented by C. . Each letter is communicated by tapping two numbers, the first designating the row and the second (after a pause) designating the column.
In 1925 Luigi Sacco of the Italian Signals Corps began writing a book on ciphers which included reflections on the codes of the Great War, Nozzioni di crittografia. He observed that Fleissner's method could be applied to a fractionating cipher, such as a Delastelle Bifid or Four-Square, with considerable increase in security.
The poem code was ultimately replaced with the one-time pad, specifically the letter one-time pad (LOP). In LOP, the agent was provided with a string of letters and a substitution square. The plaintext was written under the string on the pad. The pairs of letters in each column (such as P and L) indicated a unique letter on the square (Q).