Square (cipher)

The Square cipher, more commonly known as the "Playfair cipher," is a type of digraph substitution cipher that encrypts pairs of letters (bigrams) instead of single letters, which makes it more secure than simple substitution ciphers. It was invented by Charles Wheatstone in 1854 but is named after Lord Playfair, who promoted its use. To use the Playfair cipher, a square matrix of letters is constructed based on a keyword or phrase.