How does the Four-square cipher work?

Four 5×5 squares are arranged in a 2×2 block: plain alphabets top-left and bottom-right, and two keyword squares top-right and bottom-left. Each plaintext pair is located in the two plain squares, and the cipher letters are read from the opposite corners in the keyed squares.

How is it different from Playfair?

Playfair uses one square and shifts letters within rows, columns or rectangles. Four-square uses two keyed squares and two plain squares, which removes Playfair's reciprocal quirks and lets a pair of identical letters encrypt without forced padding between them.

Why are I and J merged?

Each 5×5 square has only 25 cells, so I and J share a cell to fit the 26-letter alphabet. A decrypted I may stand for a J depending on context.

Why is the text padded with X?

The cipher works on pairs of letters, so if the message has an odd number of letters a single X is appended to complete the final pair.

Is the Four-square cipher secure?

It is harder to break than Playfair because it uses two independent keys and breaks up letter-frequency patterns, but it is still a classical hand cipher and is not secure against modern cryptanalysis.

Four-Square Cipher Encrypt & Decrypt

The Four-square cipher, invented by Félix Delastelle in the late 1800s, is a digraph substitution cipher that encrypts pairs of letters using four 5×5 grids. Two of the grids hold keyword alphabets and two hold the plain alphabet, arranged in a 2×2 block. Using two separate keys makes it noticeably stronger than the single-square Playfair cipher. This tool builds the squares and applies the real rules in your browser.

How it works

Arrange four 5×5 squares in a block. The top-left and bottom-right squares hold the plain alphabet (I and J merged). The top-right square is keyed with keyword 1 and the bottom-left square is keyed with keyword 2.

Split the plaintext into pairs, padding a final lone letter with X. For each pair, find the first letter in the top-left plain square and the second letter in the bottom-right plain square. The two ciphertext letters are taken from the opposite corners of the rectangle they form: the first comes from the top-right keyed square at (row of letter 1, column of letter 2), and the second from the bottom-left keyed square at (row of letter 2, column of letter 1).

Decryption simply reverses the lookup: locate the ciphertext letters in the two keyed squares and read the plaintext pair from the corresponding cells of the two plain squares.

Example

With keyword 1 EXAMPLE and keyword 2 KEYWORD, the plaintext pair HE encrypts to FY: H sits at row 1, column 2 of the plain square and E at row 0, column 4, so the cipher letters are the top-right square’s cell (1, 4) and the bottom-left square’s cell (0, 2).

Notes

Because each square can be keyed independently, the Four-square cipher does not suffer Playfair’s reversible-pair weakness, and identical letters in a pair encrypt cleanly without an inserted X. Decrypted text may still contain a trailing padding X or an I that should be read as J.