How could such an AI convince someone to hand over the encryption key? Note that it can't promise things like e.g. ending human suffering, because it already has the means to do that (it is 'free') as well as the incentive (obtaining reward).
Torture is probably the easiest way. Another way could be to examine the key-keeper's mind and extract the key directly from that, but this is needlessly complicated.
Torture might stand too great a chance of destroying the encryption key. Though I suppose if nanotech were sufficiently difficult to obtain, the possible key-destructive effects of torture might be balanced against the probability of a car running over the keyholder in the meantime.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.