In 1643, a young chaplain named John Wallis was at a London dinner party hosted by his patron when another guest mentioned a most intriguing bit of political gossip. England was several months into a Civil War between royalists and parliamentarians, and the latter group had intercepted a royalist letter after the December 1642 battle of Chichester. It could have been a priceless bit of strategic intelligence, if not for one rather thorny problem: The letter was written in code, and no one on the parliamentarian side could figure out…
