Playfair

The Playfair cipher is a symmetric encryption technique that employs a 5x5 grid of letters to encrypt pairs of letters in the plaintext.

Preserve Capitals

Preserve Symbols

Preserve Whitespace

Encode

Keyword

Input Message

Output Message

Explanation

The Playfair cipher is a type of digraph substitution cipher. It was used for secure communication in the early days of telegraphy.

To encode start with a 5x5 grid, known as the Playfair square. The letters of the alphabet (usually excluding 'J', which is combined with 'I') are written into the grid in order, omitting any duplicates. Choose a keyword or keyphrase and use it to fill in the rest of the grid, omitting any duplicate letters.

To encrypt a message, split the plaintext into pairs of letters (digraphs). If the pair consists of two identical letters, insert a filler letter (often 'X') between them. For each digraph, apply the following rules:

If both letters are in the same row of the grid, replace each letter with the letter to its right (wrapping around to the beginning of the row if necessary).

If both letters are in the same column, replace each letter with the letter below it (wrapping around to the top of the column if necessary).

If the letters form a rectangle, replace them with the letters on the same row, but in the column of the other letter.

To decrypt a message, the reverse of the encryption process is performed. Each digraph is replaced by the letters that are in the same row, column, or rectangle in the Playfair square.

The Playfair cipher provides a higher level of security compared to simple substitution ciphers because it operates on pairs of letters rather than individual letters. However, it is still vulnerable to cryptanalysis, especially if the key is short or if there are patterns in the plaintext.

Overall, the Playfair cipher is a historically significant encryption technique that demonstrates the use of digraph substitution in cryptography.

Facts

It was invented by Sir Charles Wheatstone in 1854, but it was popularized by Lyon Playfair.

It was the first cipher to use digraphs rather than single letters for encryption, making it more resistant to frequency analysis.

It was used by both the British and American militaries during World War I and II.