Policy for verifying with key

[haydentherapper]

Description

Currently, when verifying with a key, you must specify WithoutIdentitiesUnsafe() when creating the policy verifier because no identity is provided. It would be more accurate to have a method such as WithKeyHint or WithKeyID, since identities are not needed when verifying with a key. Either this method could take a key fingerprint, or it could take no fingerprint given the bundle should contain a key hint.

If verifying with a Sigstore bundle, a key hint should be provided as a public key identifier. As long as the trust root contains the public key, we can match based on this key hint (currently implemented here. If this is not populated, either a) we should err out, or b) we should use a hint provided via the policy method.

Related: sigstore/protobuf-specs#236 to simplify providing trusted keys.

cc @kommendorkapten @codysoyland @rdimitrov

[kommendorkapten]

Having either WithKeyHint or WithKey as the identity for a policy is what I would expect, as that's how the verification options in the protobuf specs repo are described: https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_verification.proto#L92

For a specific verification, I would prefer WithKey as it minimizes the risk of confusion. If WithKeyHint is used, I think it's preferable if that selection happens in a client controlled function, as an implementation may only have a single key, and always return that, regardless what the key hint is (the key hint not authenticated in the bundle).

That said, providing a matcher based on key hint (as @haydentherapper linked to) is a good utility option to provide, as it may be enough for certain scenarios.

[kommendorkapten]

See my comment in sigstore/protobuf-specs#236 (comment) for more reasoning.
The short story is that public keys are only weakly references via unauthenticated hints, certificate identities are strongly as the matching happens on authenticated data in the certificate.

[steiza]

This was added in #235