In classical
cryptography
Cryptography, or cryptology (from "hidden, secret"; and ''graphein'', "to write", or ''-logy, -logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of Adversary (cryptography), ...
, the bifid cipher is a cipher which combines the
Polybius square with
transposition, and uses
fractionation
Fractionation is a separation process in which a certain quantity of a mixture (of gasses, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which t ...
to achieve
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
. It was invented around 1901 by
Felix Delastelle.
Operation
First, a
mixed alphabet Polybius square is drawn up, where the I and the J share their position:
The message is converted to its
coordinate
In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine and standardize the position of the points or other geometric elements on a manifold such as Euclidean space. The coordinates are ...
s in the usual manner, but they are written vertically beneath:
They are then read out in rows:
Then divided up into pairs again, and the pairs turned back into letters using the square:
In this way, each
ciphertext
In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext ...
character depends on two
plaintext
In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted.
Overview
With the advent of comp ...
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. One way to detect the period uses bigram statistics on ciphertext letters separated by half the period. For even periods, ''p'', as in the example above (p=10), ciphertext letters at a distance of ''p/2'' are influenced by ''two'' plaintext letters (e. g., U and W are influenced by F and L), but for odd periods, ''p'', ciphertext letters at distances of ''p/2'' (rounded either up or down) are influenced by ''three'' plaintext letters. Thus, odd periods are more secure than even against this form of cryptanalysis, because it would require more text to find a statistical anomaly in trigram plaintext statistics than bigram plaintext statistics.
See also
* Other ciphers by Delastelle:
**
four-square cipher
The four-square cipher is a manual symmetric encryption technique. 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 ...
(related to Playfair)
**
trifid cipher (similar to bifid)
References
External links
Online Bifid Encipherer/Decipherer with polybius square generator
{{Cryptography navbox , classical
Classical ciphers