I second Simon Singh's "The Code Book" and David Kahn's "The Codebreakers". If you're interested a good story about a one-man struggle to get British secret ag

I second Simon Singh's "The Code Book" and David Kahn's "The Codebreakers". If you're interested a good story about a one-man struggle to get British secret agents in WWII to start using reasonably secure codes (especially unbreakable one-time-pads), I recommend "Between Silk and Cyanide" by Leo Marks. A stronger version of the British double-transposition code that Leo Marks hated is the Soviet VIC cipher[0], which only has about 38 bit key strength, but is surprisingly strong for a pencil-and-paper method. For pencil-and-paper plus a deck of playing cards, there's Danial Shiu's improvements on Bruce Schneier's Solitaire[1]. [0] https://en.wikipedia.org/wiki/VIC_cipher https://en.wikipedia.org/wiki/VIC_cipher [1] https://arxiv.org/abs/1909.06300 https://arxiv.org/abs/1909.06300