Well, all cryptography lies in practice, in the sense that there is no proof that P isn't = NP, with a small constant, theoretically. Indeed, public key cryptography might not be theoretically possible at all. Likewise, one-way functions (secure hashes) might simply not exist theoretically.

It is very important to remember that there is really next to no theoretical basis for the existence of cryptography at all. It's completely an applied field.

Likewise, if someone builds a physical architecture that solves prohibitively impractical cryptographic problems easily through a different means (e.g. quantum computing) then if it is cheap, nobody will care about the theoretical strength of the algorithm on classical computers.