Editing Four-square cipher (section)

{{short description|Symmetric encryption cipher}}
{{one source|date=June 2024}}
The '''four-square cipher''' is a manual [[symmetric key algorithm|symmetric]] [[encryption]] technique.<ref name="Bowers1959">{{cite book|author=William Maxwell Bowers|title=Digraphic substitution: the Playfair cipher, the four square cipher|url=https://books.google.com/books?id=vEs8AQAAIAAJ|year=1959|publisher=American Cryptogram Association|page=25}}</ref>  It was invented by the French cryptographer [[Felix Delastelle]].

The technique encrypts pairs of letters (''digraphs''), and falls into a category of ciphers known as [[Substitution cipher#Polygraphic substitution|polygraphic substitution cipher]]s. This adds significant strength to the encryption when compared with [[Substitution cipher#Simple substitution|monographic substitution cipher]]s which operate on single characters. The use of digraphs makes the four-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.