Two-square cipher

The two-square cipher, also called the double Playfair, is a manual symmetric encryption technique. It was developed to ease the cumbersome large matrix of the four-square cipher while remaining slightly stronger than the single-square Playfair cipher. It encrypts pairs of letters, so it is a polygraphic substitution cipher, and that makes it less open to frequency analysis than a monographic cipher, since an analyst must work with 676 possible digraphs rather than 26 letters. The cipher uses two five by five matrices, placed side by side in the horizontal form or one below the other in the vertical form, each built from a keyword or phrase with the remaining letters filled in alphabetical order. Felix Delastelle described it in his 1901 book under the name reduced digraphic checkerboard, William F. Friedman described it in 1931, and it became a regular cipher type in American Cryptogram Association puzzles after 1972.