I was going to recommend Singh's book as well. I have an older edition of it, and some of it is certainly out of date (I think it has a whole chapter on Freene

I was going to recommend Singh's book as well. I have an older edition of it, and some of it is certainly out of date (I think it has a whole chapter on Freenet, which may still be around but these days has been eclipsed by Tor), but the core principles are quite good. It's among the best explanations of asymmetric cryptography that I've read, anyway.