Bifid cipher
Encyclopedia
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
.
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 R
The message is converted to its coordinate
s 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 5
They 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 5
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.
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.
Cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...
, the bifid cipher is a cipher which combines the Polybius square
Polybius square
In cryptography, the Polybius square, also known as the Polybius checkerboard, is a device invented by the Ancient Greek historian and scholar Polybius, described in , for fractionating plaintext characters so that they can be represented by a smaller set of symbols.-Basic form :The original square...
with transposition
Transposition cipher
In cryptography, a transposition cipher is a method of encryption by which the positions held by units of plaintext are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext. That is, the order of the units is changed...
, and uses fractionation to achieve diffusion
Confusion and diffusion
In cryptography, confusion and diffusion are two properties of the operation of a secure cipher which were identified by Claude Shannon in his paper Communication Theory of Secrecy Systems, published in 1949....
. It was invented around 1901 by Felix Delastelle
Felix Delastelle
Félix Marie Delastelle was a Frenchman most famous for his invention of several systems of polygraphic substitution ciphers including the bifid, trifid, and the four-square ciphers....
.
Operation
First, a mixed alphabetSubstitution cipher
In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system; the "units" may be single letters , pairs of letters, triplets of letters, mixtures of the above, and so forth...
Polybius square
Polybius square
In cryptography, the Polybius square, also known as the Polybius checkerboard, is a device invented by the Ancient Greek historian and scholar Polybius, described in , for fractionating plaintext characters so that they can be represented by a smaller set of symbols.-Basic form :The original 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 R
The message is converted to its coordinate
Cartesian coordinate system
A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...
s 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 5
They 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 5
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
Ciphertext
In cryptography, ciphertext 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 that is unreadable by a human or computer without the proper cipher...
character depends on two plaintext
Plaintext
In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties....
characters, so the bifid is a digraphic cipher, like the Playfair cipher
Playfair cipher
The Playfair cipher or Playfair square is a manual symmetric encryption technique and was the first literal digraph substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair who promoted the use of the cipher.The technique encrypts pairs of...
. 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 cipherFour-square cipherThe four-square cipher is a manual symmetric encryption technique. It was invented by famous French cryptographer Felix Delastelle.The technique encrypts pairs of letters , and thus falls into a category of ciphers known as polygraphic substitution ciphers...
(related to Playfair) - trifid cipherTrifid cipherIn classical cryptography, the trifid cipher is a cipher invented around 1901 by Felix Delastelle, which extends the concept of the bifid cipher to a third dimension, allowing each symbol to be fractionated into 3 elements instead of two...
(similar to bifid)
- four-square cipher