The playfair is a digraph substitution cypher using a key to encrypt messages. In order to encrypt, the letters of the key are placed on a 5x5
square, dropping any duplicate letters and padding with the letters of the alphabet to complete the square.
Encryption follows the rules of the plaintext:
Taking the key as good day
, the square becomes:
G O D A Y B C E F G H I J K L M N P Q R S T U V W
Encrypting the message: live long and prosper
( LI VE LO NG AN DP RO SP ER
):
LI
forms a row:H I J K L
so slide and wrap to HJ
:
H I -> J K L ->
VE
forms a rectangle:E F J K P Q U V
so take the opposite corners, first letter is V
so we obtain UF
:
E -> F J K P Q U <- V
LO
forms a rectangle:O -> Y C G I <- L
so we obtain IY
.
NG
forms a rectangle:C <- G I L N -> R
so we obtain RC
.
AN
forms a rectangle:O <- A C F I K N -> Q
so we obtain OQ
.
DP
forms a column:D | E J P | U
so we obtain EU
.
RO
forms a rectangle:O -> Y C G I L N <- R
so we obtain NY
.
SP
forms a rectangle:M <- P S -> U
so we obtain UM
.
ER
forms a rectangle:E -> G J L P <- R
so we obtain GP
.
Thus, the final cyphertext reads: HJ UF IY RC OQ EU NY UM GP
.
To decrypt, perform the inverse operation of the last three rules and drop any extra "X" or "Q" that do not make sense in the final message.
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