Return to Four square (idea)

The four square [cipher] is similar to the [playfair cipher] in its construction. Two '5x5' [grid]s are created with [mixed alphabet]s. These are placed at [diagonal] opposites of four squares. For the following example the [key]s of 'key number one' and 'number two key' shall be used.

               (1st cipher text)
(1st clear text)
  A B C D E       K E Y N U
  F G H I K       M B R O A
  L M N O P       C D F G H
  Q R S T U       I L P Q S
  V W X Y Z       T V W X Z

  N U M B E       A B C D E
  R T W O K       F G H I K
  Y A C D F       L M N O P
  G H I L P       Q R S T U
  Q S V X Z       V W X Y Z
(2nd cipher text)
                (2nd clear text)
The plain text is then split into groups of two letters. [Nulls] or some particular character may be added to make the pairings work. The message "how about dinner tonight" becomes:
HO WA BO UT DI NN ER TO NI GH TX

Next, the two letters are located in the clear text boxes. The original characters are in bold, the cypher in italic.

  . . . . . | . . . . .
  . . H ----------- O .
  . . | . . | . . . | .
  . . | . . | . . . | .
  . . | . . | . . . | .
------|-----+-------|------
  . . | . . | . . . | .
  . . | . . | . . . | .
  . . C ----------- O .
  . . . . . | . . . . .
  . . . . . | . . . . .
The first plain text becomes the [character] in the first cipher text, and the second plain text becomes the character in the second cipher text. The fully enciphered stream is:
OC TU NA QL NO FC EL QL GW RT PX

Of course, this can also be extended to a 6x6 block with A-Z and 0-9 and 10x10 block to encode numeric data.

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