It's all the same ;) http://xkcd.com/435/

that line struck me as well. its kind of a philosophical statement, actually. mathematics is a formal logical system but nothing in computing is pure mathematics, even if the theory of computing and the theorems of cryptography are expressed in mathematics. ultimately a machine is actually DOING the cryptographic operations and that solidly brings it into the realm of physics.

I've been wrestling in my head about which parts are physics and which parts are mathematics.

Especially from an epistemological perspective: Can we know, for example, that encryption becomes easier / cheaper more quickly than brute-force decryption because, in some sense, the physics of the universe have it that way?

When you add hardware to cryptography, it becomes physics. Just think about various side-channel attacks.

Mathematical laws underpinning any system make its "physics".

He does it a couple times, I think. I noticed that as well.