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

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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.
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  • It is slightly stronger than for example the Playfair cipher, but also it is a little more manual work to perform.
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  • 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.
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  • The larger number of symbols than monoalphabetic ciphers produce makes it much more resistant to frequency analysis.
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  • Longer messages are often broken into smaller groups and then each group is encrypted by itself.
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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
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                 | Enigma machine
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