Perhaps the most famous cipher of 1943 involved the future president of the U.S., J. By the time the enemy cryptanalysts could break the message the information was useless to them. A typical scenario for Playfair use would be to protect important but non-critical secrets during actual combat. This was because Playfair is reasonably fast to use and requires no special equipment. It was used for tactical purposes by British forces in the Second Boer War and in World War I and for the same purpose by the Australians during World War II. For a tutorial on breaking Playfair with a simulated annealing algorithm, see Cryptanalysis of the Playfair Cipher. Frequency analysis thus requires much more ciphertext in order to work. Frequency analysis can still be undertaken, but on the 25*25=625 possible digraphs rather than the 25 possible monographs. The Playfair is significantly harder to break since the frequency analysis used for simple substitution ciphers does not work with it. The technique encrypts pairs of letters (digraphs), instead of single letters as in the simple substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but was named after Lord Playfair who promoted the use of the cipher. The Playfair cipher was the first practical digraph substitution cipher.
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