>But if the unimaginable happens, that would leave us with cryptography based solely on information theory: one-time pads and their variants.

If I remember correctly Zones of Thought series by Vernor Vinge has aliens that can do that (I don't want to spoil out why, because it's one of the great ideas in the book). As a countermeasure, there are spaceships carrying massive amount of bits around to be used as one-time pads.