This.

And the complex math and engineering to get the internal surface area of the burn going for the acceleration profile you want isn't exactly something you can "steal". Because it's a lot of process and institutional knowledge. It's not just like that's one secret document you can steal for part "XYZ" like the turbopump of a liquid fueled rocket, not that there isn't a bunch of process and institutional knowledge there too, but the pieces are at least somewhat more discreet and self-contained.

So you might have the math to figure out the surface area, say like a hollow tube up the inside of the solid fuel, vs. a star shape that gives you more surface area, which would burn faster and give more thrust, but there's then the issue of how to make the molds, how to position them, how to take them back out. Is it one solid casting, or segments like the Space Shuttle SRB's?  You can spy/steal or buy tech info, but it's much much harder to get the whole process end-to-end to make it work.

here's just some of the shapes solid fuel cores can use, and a rocket might have a few different kinds at different places along it's length.

https://i.stack.imgur.com/PzDnY.gif