In classical cryptography, the bifid cipher is a cipher which combines the Polybius square with transposition, and uses fractionation to achieve diffusion.
First, a mixed alphabet 5x5 Polybius square is drawn up, where the I and the J share their position:
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | A | B | C | D | E |
| 2 | F | G | H | I/J | K |
| 3 | L | M | N | O | P |
| 4 | Q | R | S | T | U |
| 5 | V | W | X | Y | Z |
The message "SECRETMESSAGE" is converted to its coordinates in the usual manner (Row-Column), but instead of writting them horizontally, they are written vertically beneath:
| S | E | C | R | E | T | M | E | S | S | A | G | E |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 | 1 | 1 | 4 | 1 | 4 | 3 | 1 | 4 | 4 | 1 | 2 | 1 |
| 3 | 5 | 3 | 2 | 5 | 4 | 2 | 5 | 3 | 3 | 1 | 2 | 5 |
Then they are then written out in pairs horizontally:
41 14 14 31 44 12 13 53 25 42 53 31 25
Then those pairs are turned back into letters using the same Polybius square:
QDDLTBCXKRXLK
In this way, each ciphertext character depends on two plaintext characters. To decrypt, the procedure is simply reversed.