# Playfair: the three digraph rules on a LANTERN square

The Playfair cipher encrypts pairs of letters using a 5 by 5 square built from a keyword. I and J share a cell, so the square holds 25 letters. Three rules decide what each pair becomes, and a short example shows all of them.

# Build the square

Use the keyword LANTERN. Remove the repeated N, then add the remaining letters in order, treating I and J as one:

• L A N T E
• R B C D F
• G H I K M
• O P Q S U
• V W X Y Z

# A worked example

Encrypt the pairs HE, LO, BU, NT, ER:

• Rectangle rule. HE sits at row 3, column 2 and row 1, column 5. The letters are on different rows and columns, so each is replaced by the letter in its own row and the other letter’s column: H becomes M and E becomes A, giving MA. BU works the same way and gives FP, and ER gives LF.
• Same column. L and O are both in column 1. Move each one down a row, wrapping at the bottom: L becomes R and O becomes V, giving RV.
• Same row. N and T are in the top row. Move each one right, wrapping at the end: N becomes T and T becomes E, giving TE.

To decrypt, reverse the movements: left instead of right, up instead of down, and the rectangle rule unchanged.

The full procedure, including how to split plaintext into pairs, is explained on the Playfair cipher page.

# Common mistakes

1. Moving the wrong way. Encryption goes right and down; decryption goes left and up.
2. Wrapping incorrectly. A letter at the end of a row wraps to the start of the same row, not the next one.
3. Reading the rectangle by rows instead of corners. Each letter takes the column of its partner, staying in its own row.
4. Forgetting the I/J merge. Decide in advance how J is written and how the result is read back.

Published by the caesarcipher.org team; drafted with AI assistance.