Bifid cipher

The bifid cipher is a classical cipher that combines a Polybius square with transposition and uses fractionation to spread the influence of each plaintext letter. It is usually credited to the French cryptographer Felix Delastelle, who invented it around 1901. To encrypt, the sender writes each letter of the message as its row and column coordinates in a mixed-alphabet Polybius square in which I and J share a cell, writes those coordinates in two rows, reads them out as one long row, and turns the digits back into letters in pairs. Because each ciphertext letter depends on two plaintext letters, the bifid cipher is a digraphic cipher, like the Playfair cipher. Long messages are split into blocks of a fixed length called the period. Odd periods resist the statistical period-detection method better than even ones. Delastelle also devised the related four-square and trifid ciphers.