Bifid cipher

Bifid cipher

In classical cryptography, the bifid cipher is a cipher which combines the Polybius square with transposition, and uses fractionation to achieve diffusion. It was invented around 1901 by Felix Delastelle.

Operation

First, a mixed alphabet Polybius square is drawn up: 1 2 3 4 5 1 B G W K Z 2 Q P N D S 3 I O A X E 4 F C L U M 5 T H Y V RThe message is converted to its coordinates in the usual manner, but they are written vertically beneath: F L E E A T O N C E 4 4 3 3 3 5 3 2 4 3 1 3 5 5 3 1 2 3 2 5They are then read out in rows: 4 4 3 3 3 5 3 2 4 3 1 3 5 5 3 1 2 3 2 5Then 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 SIn 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. Each block is then encrypted separately. Odd periods are slightly more secure than even periods.

See also

* Other ciphers by Delastelle:
** four-square cipher (related to Playfair)
** trifid cipher (similar to bifid)

External links

* [http://rumkin.com/tools/cipher/bifid.php Online Bifid Encipherer/Decipherer with polybius square generator]


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