It's utterly trivial to match by itself. Adding length to a real hash is a mild difficulty increase. Adding a second hash is a massive difficulty increase.
> Isn't it an additional constraint that works orthogonal to the hash?
Yes. But so is a second hash.
> It serves to restrict the space of collisions and hence directly reduces the exploit surface.
Very inefficiently.
> Moreover, the length can be independently verified of the hash function, and its space & time complexity is negligible.
A second hash is independent of the first hash too. Hashing a second time compared to downloading and hashing the first time is pretty close to negligible.
It's utterly trivial to match by itself. Adding length to a real hash is a mild difficulty increase. Adding a second hash is a massive difficulty increase.
> Isn't it an additional constraint that works orthogonal to the hash?
Yes. But so is a second hash.
> It serves to restrict the space of collisions and hence directly reduces the exploit surface.
Very inefficiently.
> Moreover, the length can be independently verified of the hash function, and its space & time complexity is negligible.
A second hash is independent of the first hash too. Hashing a second time compared to downloading and hashing the first time is pretty close to negligible.