The Two-square cipher, also called double Playfair, is a manual symmetric encryption technique. It was developed to ease the cumbersome nature of the large encryption/decryption matrix used in the four-square cipher while still being slightly stronger than the single-square Playfair cipher. The technique encrypts pairs of letters (digraphs), and thus falls into a category of ciphers known as polygraphic substitution ciphers. This adds significant strength to the encryption when compared with monographic substitution ciphers, which operate on single characters. The use of digraphs makes the two-square technique less susceptible to frequency analysis attacks, as the analysis must be done on 676 possible digraphs rather than just 26 for monographic substitution. The frequency analysis of digraphs is possible, but considerably more difficult, and it generally requires a much larger ciphertext in order to be useful. History Félix Delastelle described the cipher in his 1901 book Traité élémentaire de cryptographie under the name damiers bigrammatiques réduits (reduced digraphic checkerboard), with both horizontal and vertical types. The two-alphabet checkerboard was described by William F. Friedman in his book Advanced Military Cryptography (1931) and in the later Military Cryptanalysis and Military Cryptanalytics series. Friedman's co-author on Military Cryptanalytics, Lambros D. Callimahos described the cipher in Collier's Encyclopedia in the Cryptography article. The encyclopedia description was then adapted into an article in The Cryptogram of the American Cryptogram Association in 1972. After this, the cipher became a regular cipher type in ACA puzzles. In 1987, Noel Currer‐Briggs described the double Playfair cipher used by Germans in World War II. In this case, double Playfair refers to a method using two Polybius squares plus seriation. Even variants of Double Playfair that encipher each pair of letters twice are considered weaker than the double transposition cipher. Other slight variants, also incorporating seriation, are described in Schick (1987) and David (1996). The two-square cipher is not described in some other 20th century popular cryptography books e.g. by Helen Fouché Gaines (1939) or William Maxwell Bowers (1959), although both describe the Playfair cipher and four-square cipher. Using two-square The two-square cipher uses two 5x5 matrices and comes in two varieties, horizontal and vertical. The horizontal two-square has the two matrices side by side. The vertical two-square has one below the other. Each of the 5x5 matrices contains the letters of the alphabet (usually omitting "Q" or putting both "I" and "J" in the same location to reduce the alphabet