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The Playfair cipher is a written code or symmetric encryption technique that was the first substitution cipher used for the encryption of data. Introduced in 1854, it involved the use of keys that arrange alphabetical letters in geometric patterns in order to encode messages.
The Playfair cipher is also known as the Playfair square.
Created by Sir Charles Wheatstone, the Playfair cipher is named for Lord Playfair, who promoted its use. It was later used by the British and Australians during World War I and World War II.
The Playfair cipher's essential method involves creating key tables that arrange the letters of the alphabet into a square grid. With these key tables, the user separates the text of a message into two-letter bits. To encode the message, each two-letter bit is transposed on the key table. Thus, to decode the message, the receiver requires the key table itself.
As an example of the Playfair cipher, begin with the following message text: HELLO WORLD. After splitting this message into two-letter bits, a user starts with the letters HE and LL. If the letter H is in the top left corner and the letter E is in the bottom right corner of the key table, the encoded two-letter bit, which consists of the letters in the top right corner and bottom left corner, is transposed on the key table. This "mirror" technique enables effective text encoding without any equipment or infrastructure, except for printed text on paper.
These types of ciphers constituted the most advanced encryption for the time period before modern computing emerged. By contrast, the most advanced encryption techniques of the digital era involve computer algorithms, rather than this type of labor-intensive manual encryption.