Playfair Cipher 1.not even the large number of keys in a monoalphabetic cipher provides security 2.one approach to improving security was to encrypt multiple letters 3.the Playfair Cipher is an example 4.invented by Charles Wheatstone in 1854,but named after his friend Baron Playfair Playfair Key Matrix 1.a 5X5 matrix of letters based on a keyword 2.fill in letters of keyword (sans duplicates) 3.fill rest of matrix with other letters Encrypting and Decrypting -plaintext is encrypted two letters at a time 1. if a pair is a repeated letter, insert filler like X’ 2. if both letters fall in the same row, replace each with letter to right (wrapping back to start from end) 3. if both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom) 4. otherwise each letter is replaced by the letter in the same row and in the column of the other letter of the pair
