Editing Playfair cipher (section)

{{short description|Early block substitution cipher}}
{{Distinguish|Wadsworth's cipher}}
[[File:Playfair Cipher 01 HI to BM (cropped).png|thumb|The Playfair cipher uses a 5×5 grid of letters, and encrypts a message by breaking the text into pairs of letters and swapping them according to their positions in a rectangle within that grid: "HI" becomes "BM".]]

The '''Playfair cipher''' or '''Playfair square''' or '''Wheatstone–Playfair cipher''' is a manual [[symmetric key algorithm|symmetric]] [[encryption]] technique and was the first literal [[polygraphic substitution|digram substitution]] cipher. The scheme was invented in 1854 by [[Charles Wheatstone]], but bears the name of [[Lord Playfair]] for promoting its use.

The technique encrypts pairs of letters (''[[bigram]]s'' or ''digrams''), instead of single letters as in the simple [[substitution cipher]] and rather more complex [[Vigenère cipher]] systems then in use. The Playfair cipher is thus significantly harder to break since the [[frequency analysis]] used for simple substitution ciphers does not work with it.  The frequency analysis of bigrams is possible, but considerably more difficult. With 600<ref>No duplicate letters are allowed, and one letter is omitted (Q) or combined (I/J), so the calculation is 600 = 25×24.</ref> possible bigrams rather than the 26 possible monograms (single symbols, usually letters in this context), a considerably larger cipher text is required in order to be useful.