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 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, 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. As shown above, the period is 5 so solve for 5 letters at a time. Each block is then encrypted separately. Statistical analysis to detect the period uses ciphertext letters. Since each letter corresponds to two numbers, it infers half the period, not the period directly. Thus, odd periods are more secure than even, because the statistical anomalies register both on half the period rounded down and rounded up.