On Wed, Aug 14, 2024, at 9:21 AM, Felix Lechner wrote:
> The serving someone else's substitutes could also arise more innocently,
> for example via a technical misconfiguration or because of an incentive
> system that rewards the contribution of substitutes.

Yes, indeed. And you may very well want such an incentive system, because having many people distribute substitutes in a P2P system is a natural way for people to contribute their own bandwidth.

> Is it possible for someone to reliably attest that they individually
> built a reproducible work product?  I believe the needed variation in
> inputs, like a hash, is incompatible with the goal of reproducability.

I think it's possible if the signature is detached from the reproducible work product to be signed. For example, it's like the difference between an embedded and detached signature of a file signed by GPG. Distributing a detached signature alongside a file doesn't change the hash of the file that's been signed.

Of course, you may not have built the build inputs yourself either - but those can be authenticated separately. (Recursion!)