Applications of number theory

I mentioned RSA encryption a number of times. What is exceptionally notable about it is that you can tell everyone "Send me messages in this secure way", and you can read the messages, but nobody else can read one another's messages. So it doesn't require you, as your first step, to smuggle a code book.

Lots more in Schroeder's Number theory in science and communication, which the library seems to have four copies of (in various editions). This is where I learned about the golden ratio's use in designing speakers.