Four-square cipher

The four-square cipher is a manual symmetric encryption technique invented by the French cryptographer Felix Delastelle. It encrypts pairs of letters, called digraphs, and so belongs to the polygraphic substitution ciphers. Working on digraphs makes it harder to attack by frequency analysis than a monographic substitution cipher, because the analyst must count 676 possible digraphs rather than 26 single letters, and a much larger ciphertext is generally needed for the analysis to be useful. The cipher uses four 5 by 5 squares of letters arranged in a larger square, with the alphabet reduced to fit by leaving out Q or by placing I and J together. The upper-left and lower-right squares are plaintext squares holding a standard alphabet, while the upper-right and lower-left squares are ciphertext squares built from a keyword or phrase followed by the remaining letters, so the cipher allows two separate keys.