Yes! This is exactly what I was going to post. The article's recommendation to read Applied Cryptography and the HAC to "learn the theoretical background" left

Yes! This is exactly what I was going to post. The article's recommendation to read Applied Cryptography and the HAC to "learn the theoretical background" left me dejected, since neither is particularly that great in the area of theoretical underpinnings. (The HAC is a reference book, for Chrissake!) Both are great books in their own right, but they're not what I'd recommend for the theoretical background. Katz and Lindell's Introduction, on the other hand, is absolutely fantastic for the task (this was its design goal...). It introduces theoretical cryptography from the bottom-up and uses it to motivate the various primitives and constructions from the applied realm. It's really a great mix. The book has become my go-to recommendation for those who are serious about cryptography but have had relatively little exposure to it. It also doesn't assume the reader is an expert in all things computer science, which is nice. Goldreich's Foundations of Cryptography is more of a treatise on theoretical cryptography... it goes much deeper and starts out assuming the reader is pretty familiar with concepts from theoretical computer science and probability theory. The optional sections of Katz and Lindell's work end up being the opening chapters of the first volume --- and they're not optional. Block ciphers aren't even treated until the second book. It's a seriously theoretical series, which makes it great in its own right, but I would postpone reading it until well-after Katz and Lindell's book. (And a book on computational complexity, at minimum, for those not familiar with it.)