Blood price of the missi.., p.21
Blood-Price of the Missionary's Gold,
p.21
4. The Playfair square, named for Lord Playfair who first promoted it as a means of sending coded messages, encrypts pairs of letters to make frequency analysis much harder. It was first used seriously by the British in the Second Boer War and extensively in the First World War.
Pro Se Press, Blood-Price of the Missionary's Gold
