If we observe any two pairs of letters that encrypt to each other, like AX ⇆ BY, then we immediately know that: Generally, any time we observe that a letter X encrypts (or decrypts) to some other letter Y in any letter pair, we know that X and Y must belong either to the same row or to the same column. In fact, we can learn quite a lot about the structure of the key table just by applying some simple logic to the observed letter pairs: But it would be nice to do more than that. from the letter exchange symmetry of the Playfair cipher: if we know that XY encrypts to PQ, then we also know that YX encrypts to QP. Of course, we can infer some new pairs directly, e.g. The only problem is that, while common letter pairs like TH, HE or AT are likely to occur pretty often both in our known plaintext and in any unknown messages we might want to decode, other less common pairs of letters might not happen to appear in our known plaintext. Even if we never figure out the actual key table, having such a dictionary is basically as good as having the key. Basically, by doing this, we're just treating the Playfair cipher as a simple substitution cipher on letter pairs. In principle, with enough known plaintext / ciphertext pairs, we can just compile a (nearly) complete dictionary of letter pairs like this, and use it to decrypt unknown messages. In particular, the following key tables are all equivalent: Original: Row shift: Column shift: 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.First of all, you cannot uniquely determine the keyword of a Playfair cipher, or even the key table constructed from it, simply because there are multiple equivalent key tables that will produce the same ciphertext (and multiple keywords that will produce each table). 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. Once computers were invented and used to break codes, the Playfair cipher was no longer used. 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. Other countries–Australia, Germany, and New Zealand–would use the Playfair cipher in the 1940’s. The Playfair cipher was predominantly used by British forces during the Second Boer War (1899-1902) and World War I (1914-1918). 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 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. Instead of a twenty-six possible monograms, with a digraph there are six-hundred possibilities. This is important because it makes breaking messages much, much harder. 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. 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. I was always curious as to how this cipher worked–and of course the history behind it–so I looked it up. Throughout my upbringing, I often heard of detectives and spies using the Playfair cipher as a way to encode/decode messages’ meanings.
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