alt Hacker NewsThis is a great book and touches on the subject you mention https://simonsingh.net/books/the-code-book/
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hermitcrab • last Tuesday at 9:01 PM
The author of this book also runs an excellent weekly maths newsletter/quiz for 11-16 year olds, and it's free:
Having read this book, I set some codes for my son to break. Each code, once broken, told him the location of the next coded message. And they got progressively harder. It was a fun challenge.