Example 18

EXAMPLE 18 Decrypting a Playfair Ciphertext

To decode the ciphertext TB TZ PN OB AC HC EU WB that was encoded using the keyword BEATLES and the matrix

$\begin{matrix} B & E & A & T & L \\ S & C & D & F & G \\ H & I & J & K & M \\ N & O & P & R & U \\ V & W & X & Y & Z \end{matrix}$

we use the same matrix but reverse the encryption rules 1 and 2.

Playfair Decryption Rules

$1^{'}$ . If both letters are in the same row, each of the two letters is replaced by the letter to the immediate left of it: ET becomes BA. If a letter is at the beginning of a row, it is replaced by the letter at the end: HM becomes MK.
$2^{'}$ . If both letters are in the same column, each of the two letters is replaced by the letter immediately above it: WO becomes OI. If a letter is at the top of a column, it is replaced by the letter at the bottom: LU becomes ZM.

Applying these rules gives us

Plaintext	TB	TZ	PN	OB	AC	HC	EU	WB
Ciphertext	AL	LY	OU	NE	ED	IS	LO	VE

Regrouping the letters to make sense, we get the message ALL YOU NEED IS LOVE.