The Playfair cipher is an incredibLe code system that is both difficult to decrypt without a computer ... Once you know that Q, for example, equals E, then that’s true for every Q in the problem.

In 1854, Sir Charles Wheatstone invented the cipher ... Playfair, pick a keyword and write it into a five-by-five square, omitting repeated letters and combining I and J in one cell. In this ...