Bifid cipher

How it works

Félix Delastelle described Bifid in 1895. Under each letter of the message write its row and column in a keyed 5 by 5 square, giving two lines of digits. Read the row line and then the column line as one sequence, cut it into pairs again and turn each pair back into a letter. With a period, this is done block by block.

Decryption writes the coordinates of the cipher letters in one line; its first half is the row line of the plaintext and its second half the column line.

Splitting letters into coordinates and mixing them (fractionation) means that each cipher letter depends on two different plain letters, so plain frequencies disappear. But within a block the structure is regular, and with the period known, statistical attacks on coordinate pairs recover the square from a few hundred letters.