Allow selecting root public key by ID

[seh]

In order to more easily accommodate rotating of root private keys when issuing biscuits, allow consumers to choose which root public key to use when verifying the biscuit based on the key ID embedded within it at composition time, if any. Consumers can then accept biscuits signed with several root keys, learning to accept signatures from a rolling set of both older and newer keys.

Introduce the (*Biscuit).AuthorizerFor method as an eventual replacement for the longstanding (*Biscuit).Authorizer method, along with with two new options for supplying either a single public key or a mapping from ID to public key (together with an optional default public key to use when the biscuit in question embeds no root key ID). Alternately, callers may supply a projection function that consumes an optional root key ID.

Fixes #150.

[seh]

I started using this patch in my project by way of a replace directive in my go.mod file, and find that it works well so far.

[seh]

Something that's a little odd about this approach: Given that we already merged #151 that exposed the Biscuit.RootKeyID method, a caller already has the information he needs to consider an embedded key ID and choose a root public key before calling on the (*Biscuit).AuthorizerFor method proposed in this patch. We wind up with two different ways to accomplish the same outcome.

[divarvel]

In other libraries (biscuit-rust, biscuit-haskell at least), parsing and signature verification are done at the same time. This makes the library harder to misuse, and in the case of biscuit-haskell, this allows to abort parsing as soon as possible. (It is still possible to parse a biscuit into an UnverifiedBiscuit type for inspection purposes)

I think the go library would benefit from this approach.

[seh]

I think the go library would benefit from this approach.

If I follow you correctly, in the Go library, rather that having the existing Unmarshal function consume just a byte vector and return a *Biscuit, it would also consume a projection function to select a root public key—like I've proposed in this patch—so that it could do the work that produces an *Authorizer at the same time. Right now, Unmarshal returns an unverified *Biscuit, which is interesting to inspect in some cases, but not yet trustworthy.

[seh]

I think the go library would benefit from this approach.

I agree, but I'm not sure if you're proposing that as work to follow merging this proposed patch, or something we should pursue now instead of this patch? In other words, do we have any hope of this patch getting merged soon?

[divarvel]

i’m sorry i was not clear, that’s something i think should be done at some point, not instead of this

[divarvel]

patch looks good to me, i can merge it if you don’t have anything to add to it.

thanks!

[seh]

i can merge it if you don’t have anything to add to it.

Thank you. It's settled for me. I've been using it to good effect.

After we merge it, I'll then start asking about a point or minor release of the module so that I can rely on a "real" version.

[seh]

I'll then start asking about a point or minor release of the module so that I can rely on a "real" version.

See #155 for that request.