Exactly what part of SIDH is proven to be NP-hard? SIDH is quantum secure, why is it quantum secure? Exactly which part of the algorithm is proven to be NP-hard? Computing the isogeny function?
SIDH is believed to be quantum secure because the fastest known quantum attacks against it (which is based on claw finding algorithm) require exponential time with complexity $\mathcal{O}(p^{1/6})$.