Comment by jefc1111
This is a great book and touches on the subject you mention https://simonsingh.net/books/the-code-book/
This is a great book and touches on the subject you mention https://simonsingh.net/books/the-code-book/
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.