BIFID CIPHER:
The bifid cipher is a cipher which combines the Polybius square with transposition and uses fractionation to achieve diffusion. It was invented by Felix Delastelle around 1901.
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 R
The 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 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 worked example:
44 33 35 32 43 13 55 31 23 25
U A E O L W R I N S
In this way, Ciphertext character depends on two plaintext characters, so the bifid cipher, like the playfair cipher.
*To decrypt, the procedure is simply reversed.*
To decrypt:
U A E O L W R I N S
Get the coordinates of each letters:
44 33 35 32 43 13 55 31 23 25
Or
44333532431355312325
Then,
4433353243
1355312325
So the coordinates of the plain text are:
41-43-35-35-33-51-32-23-42-35
Use the Mixed Alphabet Polybius Square to find the plain text:
41 43 35 35 33 51 32 23 42 35
F L E E A T O N C E
