There was early work on using IPFS as a substitute distribution mechanism, but the API changed and nobody has picked up the existing work yet. But I agree that this would be a great feature and I hope someone will feel motivated to pick up and assemble the pieces.
What are the security implications of this?
The implementation requires that you have at least one authorized substitute server advertising the same hash.
In simplified terms, if ci.guix.gnu.org advertises a substitute for /gnu/store/abc123-foo, with the checksum "xyz789" (and the cryptographic signature of that advertisement checks out), your daemon can safely download that file over P2P.
Ah, I think I'm misunderstanding the intent here. Clearly P2P distribution of checksummed binaries can be safe, I was just wondering if there were a solution to the build farm being behind. It seems like you can't really trust the first build of any artifact unless it comes from a central source.
There have been discussions of an "N of P" distribution, i.e. if 80% of available peers (or substitute servers) advertise the same build result, then treat it as safe.
I expect that both will be implemented, and the choice left up to the user.
I'm not using Guix, but if I was I would be happy to do this. I have no data cap on my home internet, and my PC is usually on.