This. While this API is not RESTful at all, that's just a nomenclature problem; it's totally fine as an API [1]. RPC is perfectly appropriate for a wide range of problems and there's no need to shoehorn every application into REST just because people say It's the Thing to Do. I could imagine a RESTful API for this, as people have above, and it might work well. But it isn't some sort of obvious best practice.
[1] Except that it really ought to use POST.