Playfair Cipher

The Playfair cipher was invented by Charles Wheatstone in 1854 and promoted by Lord Playfair. It encrypts pairs of letters (digraphs) using a 5×5 key square, making it much harder to crack with simple frequency analysis than a single-letter substitution. It was used by British forces during the Boer War and both World Wars.

Keyword

J→I Q dropped

Padding:

Plain text

Playfair cipher

letters 0 digraphs 0 padding inserted 0

Key Square

Built from the keyword with duplicates removed, then filled with the remaining alphabet.

Digraph Breakdown

Rules

Same row: each letter is replaced by the one to its right (wrapping).
Same column: each letter is replaced by the one below (wrapping).
Rectangle: each letter is replaced by the one in its own row in the column of the other letter.
Pairs of identical letters are split by inserting a padding letter (default X).
Most implementations merge J into I to fit the 25-cell grid.