A question I've been meaning to ask for a while is if we could use HPKP to bootstrap a TOFU (trust on first use) model.
i.e. you get your cert signed by both a traditional CA and a CA you set up yourself. the traditional CA would work for the first visit. However after that it gets pinned to your own CA.
You can then sign your own certs at your leisure e.g. offline or with private key on a yubikey.
But... What if a new user arrives and doesn't have your pinned CA? Your model would require to always have a trusted CA and if you always have them why trouble oneself with pinning custom one?
The idea is that the cert must be signed by my own CA. But may also be signed by another CA.
Meaning that after the first visit, a user is protected from a rogue CA issuing a cert for my site.
Unfortunately X 509 doesn't support multiple signatures. But your model is very similar to HPKP, just the details on what exactly is signed are different.
Thanks: that was the piece of the puzzle I was missing. I was confused by how people describe cross-signing as an intermediate cert being signed by two CAs.
On further research, it's done via having two intermediate certs.
But effectively: they accomplish cross-signing by having two different roots for the same leaf cert: we should be able to do that with 1 CA root, and 1 site-admin root (specified via some HPKP-like mechanism)
On the other hand, perhaps the limitation of one cert should be removed somehow? e.g. via an extension: old user-agents would just see the first signing cert, but others would see all of them?
