The Playfair cipher or Playfair square, is a manual symmetric encryption technique and was the first literal digraph substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair who promoted the use of the cipher. The Playfair cipher uses a 5 by 5 table containing a keyword or phrase. Memorization of the keyword and 4 simple rules are all that’s required to create the 5 by 5 table and use the cipher. For this mission, we will do one better and use a 6 by 6 table.

For the key table, we should use ASCII letters in lowercase ("abcdefghijklmnopqrstuvwxyz") and digits ("0123456789"). They are have the following order:

"abcdefghijklmnopqrstuvwxyz0123456789"

To generate the key table, the spaces in the table must be filled with the letters contained in the keyword (dropping any duplicate letters and digits), then the remaining spaces are filled with the rest of the letters and digits of the alphabet in order. The key is written in the top rows of the table, from left to right. The keyword together with the conventions for filling in the 6 by 6 table constitute the cipher key.

To encrypt a message, we will need to prepare a block of text. Upper case letters get transposed into lower case of letters, we’d break the message into digraphs (groups of 2 letters) and skip white spaces and punctuation symbols. The result would turn a message like "Hello World!" into "he ll ow or ld", and would get mapped out in the key table. The two letters of the digraph are considered to be the opposite corners of a rectangle in the key table. Note the relative position of the corners of this...