Playfair Cipher

Encrypt letter pairs through a 5×5 key square, the first cipher here that flattens single-letter frequency.

Digraphdemo

Objectives

Construct the key square from a keyword.
Apply the row, column, and rectangle rules to a digraph.
Explain what encrypting pairs buys, and what it still leaks.

25%

Encrypt letter pairs. 26 single-letter frequencies become 600+ digraph frequencies.

Key phrase

Key letters first (duplicates dropped), then the rest of the alphabet. I and J share a cell.

5 × 5 key matrix

I/J

Plaintext

The three rules

same row	each letter → the one to its right (wrapping)
same column	each letter → the one below (wrapping)
rectangle	each letter → same row, other letter’s column

prepared digraphs: click one to see the rule

Doubled letters inside a pair are split with X, and an odd tail is padded with one. A doubled X takes Q instead, because splitting XX with an X would only produce another XX.

ciphertext

UHQBU GFHVR HPRQD OQMMI HPYV

decrypted again (rules run backwards)

WEHAV EAMOL EABOR TTHED EALX

Exercise: encrypt BALLOON by hand. Check yourself above. Then answer: which rule fired for each pair, and why did LL not stay together?

Playfair hides single-letter frequency but we can still use the structure of the digraph to break it.

step 1/4

CPE 645 / CS 685

Playfair Cipher

General Monoalphabetic Substitution

Hill Cipher