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.
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.