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Four-square cipher

Enter the key squares below and select encryption or decryption. Enter the text to convert it.
Since we can only use 25 characters in each key square, there is a possibility to choose which letter to substitute by another letter (by default J is substituted by I) or to omit a letter.

Key squares:
Use standard squares (with key):
       Key top right:
       Key bottom left:
Use manual or random squares:
Replace a letter: Replace By
Skip a letter: Skip:
Method:
Background

The four-square cipher is a manual symmetric encryption technique. It was invented by the French cryptographer Felix Delastelle.
The technique encrypts pairs of letters (digraphs) and thus falls into a category of ciphers known as polygraphic substitution ciphers. This adds significant strength to the encryption when compared with monographic substitution ciphers which operate on single characters.

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