Playfair Cipher Encode and Decode

History of the Playfair Cipher

The Playfair Cipher, invented by the English engineer Charles Wheatstone in the mid-19th century, gained popularity during World War I. However, it was formalized and published in 1854 by his colleague Sir Lyon Playfair, from whom it gets its name. Primarily used in a military context, Playfair is a polyalphabetic substitution cipher that was employed to secure confidential communications.

Principle of the Playfair Cipher

Method for Creating the Matrix

The basic principle of Playfair relies on the use of a 5x5 matrix, also known as the Playfair grid, filled with a secret key. Once the grid is pre-filled with the secret key, it is completed with the letters of the alphabet in alphabetical order, excluding the letters already entered by the key. Since the grid has only 25 letters, one letter must be omitted. In the English variant, J and I are merged. In the French variant, W is omitted as it is rarely used. Once the grid is filled, the message to be encrypted is divided into pairs of two letters.

Here is an example of a grid in the English variant, pre-filled with the key "PASSWORD":

Encryption of Letter Pairs

Encryption of letter pairs follows some simple rules. If the two letters of a pair are in the same row of the grid, they are replaced by the next letters in that row. If the letters are in the same column, they are replaced by the next letters in that column. If the letters form a rectangle, they are replaced by the diagonally opposite letters of the rectangle. Finally, if the letters are in different rows and columns, they are replaced by the letters that form a rectangle with them in the grid. For example, in the following table:

The letter pair "GI" is in the same row, so it is encrypted as "HK"; the letter pair "PL" is encrypted as "RV" because it is in the same column; and the letter pair "PE" is encrypted as the pair "OR" because they are neither in the same row nor the same column but are opposite corners of a rectangle. The special case of two identical letters in a pair is handled by adding the letter "X" (just a convention, nothing formal) between them.

Principle of Playfair Cryptanalysis

Although the Playfair Cipher was effective for a time, it was not invulnerable. Several techniques were developed to break this cipher.

Frequency Analysis

Frequency analysis remains a powerful method. Even though Playfair alters the frequencies of letters, certain patterns can be identified, allowing cryptanalysts to determine the key.

Letter Pair Attacks

By deliberately manipulating letter pairs, attackers can create situations where Playfair's rules lead to specific results, helping to determine the used key.

Brute Force Attacks

Due to the relatively small size of the Playfair matrix, brute force attacks become practical. Attackers can try different keys until they find the correct one.
Although the Playfair Cipher, innovative in its time, has since been surpassed by more advanced techniques, its fascinating history and educational principles make it an interesting study in the field of cryptography.