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.

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Four-square cipher tool

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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.