Historical Ciphers

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Historical Ciphers
Topic Started: Apr 22 2007, 05:22 PM (232 Views)
jdege	Apr 22 2007, 05:22 PM Post #1
Elite member Posts: 263 Group: Admin Member #246 Joined: December 7, 2006	I recently read Iain Pears' "An Instance of the Fingerpost". It's a historical murder mystery, set in the 1660's, in the form of four memoirs, written decades later, by four different witnesses to the event. One of the "writers" is John Wallis. From wikipedia: "John Wallis (1616-11-23 - 1703-10-28) was an English mathematician who is given partial credit for the development of modern calculus. Between 1643 and 1689 he served as chief cryptographer for Parliament and, later, the royal court. He is also credited with introducing the symbol ∞ for infinity." His work as royal cryptographer figures greatly in the story. There is some discussion of ciphers, and cryptanalysis. But there's one cipher that is most prominent in the story. It's described as a multi-alphabet cipher, changing from one alphabet to the next every 25 characters. Each alphabet is a plain caesar-shift. The indicator for which alphabet to use next is based on a book. And Wallis (the fictional Wallis) expresses the impossibility of solving this without the book. Now admittedly Pears is writing for a non-technical audience, and he's assuming no knowledge of cryptography on the part of his readers. He may have chosen this simply because it's simple to describe. But the idea that it can't be broken without the book is absurd, and I find it impossible to believe that Wallis could not have broken it, without the book. In fact, I think it'd be easier to break without the book, than to try to figure out how the book was used in determining the key stream. I mean - 25 characters is far more than is needed to solve a caesar shift. It only takes a moment. And to solve a bunch of successive caesar shifts isn't difficult, it's just tedious. So all of the effort in the novel in trying to identify and then to obtain the book that the cipher was based on seemed rather pointless. Which distracted me from enjoying it as much as I otherwise would have.
	When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.


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