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The four-square cipher is a manual symmetric encryption technique. [1] It was invented by the French cryptographer Felix Delastelle . The technique encrypts pairs of letters ( digraphs ), and falls into a category of ciphers known as polygraphic substitution ciphers .
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.
Other slight variants, also incorporating seriation, are described in Schick (1987) [10] and David (1996). [11]The two-square cipher is not described in some other 20th century popular cryptography books e.g. by Helen Fouché Gaines (1939) or William Maxwell Bowers (1959), although both describe the Playfair cipher and four-square cipher.
To solve the puzzle, one must recover the original lettering. Though once used in more serious applications, they are now mainly printed for entertainment in newspapers and magazines. Other types of classical ciphers are sometimes used to create cryptograms. An example is the book cipher, where a book or article is used to encrypt a message.
The shrinking generator uses two linear-feedback shift registers. One, called the A sequence, generates output bits, while the other, called the S sequence, controls their output. Both A and S are clocked; if the S bit is 1, then the A bit is output; if the S bit is 0, the A bit is discarded, nothing is output, and the registers are clocked again.
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.
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. Longer messages are first broken up into blocks of fixed length, called the period, and the above encryption procedure is applied to each block.
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