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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.
The Two-square cipher, also called double Playfair, is a manual symmetric encryption technique. [1] It was developed to ease the cumbersome nature of the large encryption/decryption matrix used in the four-square cipher while still being slightly stronger than the single-square Playfair cipher .
In this cipher, a 5 x 5 grid is filled with the letters of a mixed alphabet (two letters, usually I and J, are combined). A digraphic substitution is then simulated by taking pairs of letters as two corners of a rectangle, and using the other two corners as the ciphertext (see the Playfair cipher main article for a diagram). Special rules ...
A monoalphabetic type substitution using a keyword mixed alphabet in a 5 x 5 square; A comb-transposition (German: Kammwürfel) A book key (German: Heftschlüssel) A double Transposition cipher (German: Doppelwürfel) 4-S-40 This system was used until 1926 or 1927; The Playfair cipher in Single Playfair (German: Kastenschlüssel) (TS-42)
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Delastelle's other polygraphic substitution ciphers included the trifid [3] and four-square ciphers. [4] The last of these is a variant on the earlier Playfair cipher: Delastelle may have been unaware of Playfair, but he had certainly read of the fractionating cipher described by Pliny Chase in 1859. There are few biographical details.
Then divided up into pairs again, and the pairs turned back into letters using the square: 44 33 35 32 43 13 55 31 23 25 U A E O L W R I N S In this way, each ciphertext character depends on two plaintext characters, so the bifid is a digraphic cipher, like the Playfair cipher. To decrypt, the procedure is simply reversed.
Again, following Sacco's observation, this method disrupts a fractionating cipher such as Seriated Playfair. Crosswords are also a possible source of keywords. A grid of the size illustrated has a word for each day of the month, the squares being numbered.