Before, things worked like this:
* Only signatures of Ed25519 keys could be verified
* Key encoding placed the plain binary Ed25519 key in the PEM, instead of the
key's ASN1 encoding
With this patch it now works like this:
* Ed25519 signatures are supported as before
* RSA keys are now supported too, assuming RSA-SHA256 signatures
* Both Ed25519 and RSA keys are encoded and decoded using actual PEM with ASN1
When we verify an HTTP signature,
* If we know the key, check in the DB whether we know the actor lists it. If it
doesn't, and there's room left for keys, HTTP GET the actor and update the DB
accordingly.
* If we know the key but had to update it, do the same, check usage in DB and
update DB if needed
* If we don't know the key, record usage in DB
However,
* If we're GETing a key and discovering it's a shared key, we GET the actor to
verify it lists the key. When we don't know the key at all yet, that's fine
(can be further optimized but it's marginal), but if it's a key we do know,
it means we already know the actor and for now it's enough for us to rely
only on the DB to test usage.
Previously, when verifying an HTTP signature and we fetched the key and
discovered it's shared, we'd fetch the actor and make sure it lists the key URI
in the `publicKey` field. But if we already knew the key, had it cached in our
DB, we wouldn't check the actor at all, despite not knowing whether it lists
the key.
With this patch, we now always GET the actor when the key is shared,
determining the actor URI from the `ActivityPub-Actor` request header, and we
verify that the actor lists the key URI. We do that regardless of whether or
not we have the key in the DB, although these two cases and handled in
different parts of the code right now (for a new key, it's in Web.ActivityPub
fetchKey; for a known key, it's in Vervis.Foundation httpVerifySig).
Previously, when verifying an HTTP signature and we find out we have the
provided keyid in the DB, and this key is a personal key, we would just grab
the key owner from the DB and ignore the ActivityPub-Actor header.
This patch adds a check: If we find the key in the DB and it's a personal key,
do grab the owner from that DB row, but also check the actor header: If it's
provided, it has to be identical to the key owner ID URI.
If the key we fetched is a shared key, the only way to determine the actor to
which the signature applies is to read the HTTP header ActivityPub-Actor. But
if it's a personal key, we can detect the actor by checking the key's owner
field. Still, if that actor header is provided, we now compare it to the key
owner and make sure they're identical.
When fetching a key that is embedded in the actor document, we were already
comparing the actor ID with the actor header, so that part didn't require
changes.
When a local user wants to publish an activity, we were always GETing the
recipient actor, so that we could determine their inbox and POST the activity
to it. But now, instead, whenever we GET an actor (whether it's for the key sig
verification or for determining inbox URI), we keep their inbox URI in the
database, and we don't need to GET it again next time.
I'm not sure what the best balance is, but once an hour may end up causing a
lot more key re-fetch requests coming from other servers. I prefer to default
to once a day for now (maybe even once a week) and tighten it later if needed.
Caveat: If an instance key is rotated once a day, there's no
change-key-right-after-toot-deletion thing for deniability. Potentially,
rotation may happen only 24 hours after that deletion, which is much more than
1 hour. On the other hand, it's a whole instance key, not personal key of the
actor.
Using a dedicated type allows to record in the type the guarantees that we
provide, such as scheme being HTTPS and authority being present. Allows to
replace ugly `fromJust` and such with direct field access.
