Playfair Cipher Step by Step

Invented by Charles Wheatstone in 1854 and promoted by Lord Playfair, this was the first digram cipher genuinely used in the field — the British employed it in the Boer War and the First World War. Its advantage at the time was practical: encipher in your head, with paper and pencil, no equipment at all, and still escape simple frequency analysis.

Enter the key and the text and the page shows the five-by-five grid assembled and the route of each pair. The three rules: letters sharing a row are replaced by their right-hand neighbours; sharing a column, by the ones below; and in different rows and columns, each swaps to the other's column, forming a rectangle. In every case the movement wraps around when it reaches the edge of the grid.

Preparing the text has rules of its own that often confuse: repeated letters within a pair are separated by an X, because the rectangle rule breaks down when both are the same, and a trailing X closes the pair when the length is odd. That is why deciphering turns up X letters in the middle of words — they are usually padding, and reading discards them.