This is significantly harder to break since the frequency analysis used for simple substitution ciphers is considerably more difficult.

To generate the table, one would first fill in the spaces of the table with the letters of the keyword (dropping any duplicate letters), then fill the remaining spaces with the rest of the letters of the alphabet in order (to reduce the alphabet to fit you can either omit Q or replace J with I).

Noticing the location of the two letters in the table, we apply the following rules, in order.

Encrypt the new pair, re-pair the remining letters and continue.

For example, using the table above, the letter pair MD would be encoded as UG.

The order is important - the first letter of the pair should be replaced first.

For example, using the table above, the letter pair EB would be encoded as WD.

In rules 2 and 3 shift up and left instead of down and right.

Once you are done, drop any extra Xs that dont make sense in the final message and locate any missing Qs or any Is that should be Js.

In Playfair Cipher, unlike traditional cipher, we encrypt a pair of alphabets (digraphs) instead of a single alphabet.

The key table is a 55 matrix consisting of alphabets that acts as the key for encryption of the plaintext.

Each of the 25 alphabets must be unique and one letter of the alphabet (usually j) is omitted from the table, as we need only 25 alphabets instead of 26.

If the plaintext has an odd number of characters, append z to the end to make the message of even length.

The receiver has the same key and can create the same key table, and he uses it to decrypt the ciphertext messages generated using that key.

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Playfair Cipher Program In Php