# Four-square cipher decoder and encoder

The four-square cipher was invented by Félix Delastelle. It uses four 5x5 squares (where I and J share position, or Q is omitted). Generally, the upper left square and the lower-right square contain the standard alphabet, while the other two act as the key. It translates letter two-by-two (digraphs) by matching them with letters in the key squares. Because is using digraphs, it is much less susceptible to frequency analysis than monographic substitution ciphers.

## Four-square cipher tool

Translate this letter into

## Options

### Squares

## Features

- The Four-square cipher is an example of a digraphic cipher. Each crypto letter depends on two letters in the plaintext.
- It is slightly stronger than for example the Playfair cipher, but also it is a little more manual work to perform.
- It can encrypt 25 characters. Usually the letter J is replaced by I, but also for example the letter Q could be replaced by C.
- The larger number of symbols than monoalphabetic ciphers produce makes it much more resistant to frequency analysis.
- Longer messages are often broken into smaller groups and then each group is encrypted by itself.

## Sample text

ZEALZDXELNIFRLHYEUISAHXSIOMMXL

The ciphertext above represents "WE HAVE BEEN DISCOVERED FLEE AT ONCE" encrypted using the keys FELIX and DELASTELLE.

See also: Code-Breaking overview | Adfgvx cipher | Adfgx cipher | Affine cipher | Atbash cipher | Baconian cipher | Beaufort cipher | Bifid cipher | Caesar cipher | Columnar transposition | Cryptogram | Double transposition | Enigma machine | Gronsfeld cipher | Keyed caesar cipher | One-time pad | Pigpen cipher | Playfair cipher | Rail fence cipher | Rot13 | Trifid cipher | Variant beaufort cipher | Vigenere cipher