How does the Playfair cipher work?

A 5×5 square is built from a keyword plus the rest of the alphabet, with I and J sharing a cell. The message is split into letter pairs and each pair is replaced according to its position in the square.

What are the three substitution rules?

If both letters share a row, each is replaced by the letter to its right. If they share a column, each is replaced by the letter below. Otherwise they form a rectangle and each letter is replaced by the one in its own row but the other letter's column.

Why are X letters inserted?

Playfair works on pairs, so a repeated letter in a pair (like the two letters in EGG) and any leftover single letter at the end are padded with X to keep every pair valid.

Why are I and J combined?

A 5×5 grid holds only 25 cells but the alphabet has 26 letters, so I and J are merged into one cell. During decryption an I may therefore need to be read as a J from context.

Is the Playfair cipher secure?

No. Although it resists simple single-letter frequency analysis better than a Caesar cipher, it was broken long ago and offers no protection against modern cryptanalysis.

Playfair Cipher Encrypt & Decrypt

The Playfair cipher was the first practical digraph substitution cipher, encrypting pairs of letters at a time instead of single letters. Invented in 1854 and named after Lord Playfair, it was used for tactical military messages because it could be performed by hand yet resisted casual frequency analysis. This tool builds the keyword square and applies the real rules in your browser.

How it works

First build a 5×5 square: write the keyword (dropping repeats) into the grid, then fill the remaining cells with the rest of the alphabet. Because the grid has 25 cells, I and J share one cell.

Split the plaintext into pairs. If a pair would contain two identical letters, insert an X between them; if the message has an odd length, pad the final letter with X. Then apply three rules to each pair:

Same row: replace each letter with the one immediately to its right, wrapping around.
Same column: replace each letter with the one immediately below it, wrapping around.
Rectangle: replace each letter with the one in its own row at the other letter’s column.

Decryption uses the same rules but shifts left and up instead of right and down.

Example

With the keyword MONARCHY, the digraph IN encrypts to a new pair determined by their rectangle in the square. Repeated letters such as the pair in BALLOON are split with X (BA LX LO ON) before encryption.

Notes

Decrypted output may contain padding X letters that you can usually remove by reading in context. Since I and J share a cell, a decrypted I might represent a J — again resolved by context.