Throughout my upbringing, I often heard of detectives and spies using the Playfair cipher as a way to encode/decode messages’ meanings. I was always curious as to how this cipher worked–and of course the history behind it–so I looked it up. The Playfair cipher–which if you think about it, you REALLY are not “playing fair” if you are using a cipher, now are you?–was created by Charles Wheatstone, an English scientist and inventor, in 1854.
The Playfair cipher is notable because it is one of the first ciphers that paired letters (also known as a digraph) instead of using a single letter cipher. This is important because it makes breaking messages much, much harder. Instead of a twenty-six possible monograms, with a digraph; there are six-hundred possibilities.
The Playfair cipher was originally thought by the British Foreign Office as too complex; they feared that using this cipher would take too much time and would be ineffective in the field. Once Wheatstone established that with proper training even school children could properly use the Playfair cipher, Britain would make the Playfair cipher their prominent tool to encrypt secret, but non-critical information in the battlefields. The Playfair cipher was predominantly used by British forces during the Second Boer War (1899-1902) and World War I (1914-1918). Other countries–Australia, Germany, and New Zealand–would use the Playfair cipher in the 1940’s. The Playfair cipher was popular because it was complex enough to throw off cryptographers, but didn’t require any special tools or equipment to solve.
Once computers were invented and used to break codes, the Playfair cipher was no longer used. With the sophistication of digital encryption, the Playfair cipher was not an acceptable form of encoding messages, because a computer could solve Playfair ciphers in a matter of seconds.
If you are interested in learning how the Playfair cipher works, Berkeley has a great explanation on their website. Below is a video from National Treasure 2: Book of Secrets, where they use a computer with a Playfair Decoder to break a message. Easy as pie with the right code phrase-key!