Four square (idea)
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|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.