Playfair Cipher | Lecture Note

University:
University of Alberta
Course:
CMPUT 333 | Security in a Networked World
Academic year:

2024
Views:

273

Pages:

2
Author:

Neena N.

Playfair cipher The Playfair cipher is a polygraphic substitution cipher that encrypts pairs of letters instead of individual letters. It was invented by Charles Wheatstone in 1854 and named after Lord Playfair, who promoted its use. How does it work? The Playfair cipher uses a 5x5 grid of letters to encrypt and decrypt messages. The grid is filled with the letters of the alphabet, with one letter omitted (usually J, but sometimes Q is omitted instead). To encrypt a message, the plaintext is divided into pairs of letters. Each pair of letters is then encrypted using the corresponding letters in the grid. Example: To encrypt the message "HELLO WORLD" using a Playfair cipher with the key "CAT", we first create a 5x5 grid with the key word: CAT EBFGH IJKLM NOPQR STUVW Then, we divide the plaintext into pairs of letters: ● ● ● ● ● HE LL OW OR LD For each pair of letters, we find the corresponding letters in the grid and encrypt them as follows: ● If the two letters are in the same row, we encrypt them with the letters to their right. For example, "HE" is encrypted as "BM". ● If the two letters are in the same column, we encrypt them with the letters below them. For example, "LL" is encrypted as "EI". ● If the two letters are in different rows and columns, we encrypt them with the letters at the corners of the rectangle formed by the two letters. For example, "OW" is encrypted as "FM". Using this method, we can encrypt the entire plaintext message: ● Plaintext: HELLO WORLD ● Ciphertext: BMELXIRLD Decryption To decrypt a message encrypted with the Playfair cipher, we simply reverse the encryption process. We find the corresponding letters in the grid for each pair of ciphertext letters and then read the plaintext row by row. Advantages of the Playfair Cipher ● Polygraphic substitution: The Playfair cipher is more resistant to frequency analysis than simple substitution ciphers, as it encrypts pairs of letters instead of individual letters. ● Relatively easy to use: The Playfair cipher can be easily implemented and used without any special equipment. Disadvantages of the Playfair Cipher ● Limited key space: The number of possible keys for the Playfair cipher is relatively small, making it vulnerable to brute-force attacks. ● Requires knowledge of the key: Both the sender and the recipient must know the key in order to encrypt and decrypt messages. Modern Applications While the Playfair cipher is no longer considered secure for modern cryptographic purposes, it can still be used for educational purposes or as a simple puzzle. It is also a good starting point for understanding the principles of polygraphic substitution ciphers.