A Playfair Cipher $($occasionally also called a Playfair Square or a Wheatstone$-$Playfair Cipher$)$ is an encryption technique, the first to perform bigram substitution. This system was developed by Charles Wheatstone, but it ended up being named for its biggest supporter, Lord Playfair $($Wheatstone$\text{'}$s friend, actually$).$ The first known use of the system was in $1854.$ It is trivial for modern computers to break, but was in common use through World War I.

Using a $5$x$5$ table that is constructed from a key or passphrase, each pair of characters in the plaintext is searched for, manipulated, and occasionally rotated to generate a pair of ciphertext characters. This encryption technique was revolutionary, building on both substitution and Vigenere ciphers $($you can read about these in more detail than our lecture in Chapter $3$ of the textbook$)$ to create $600$ encryption pairs rather than the $26$ provided with individual characters. If you get stuck with some of the descriptions below, you can also consult the Playfair Cipher Wikipedia entry Links to an external site.; however, some of the rules for this project have been slightly modified from those listed by Wikipedia.

Note: Because this project is a bit bigger than previous projects, you have most of two weeks to complete it instead of the usual one full week. But still, don't delay in getting it started!