Have a set of keys instead of just one

[jakipatryk]

Background

To allow more UX-friendly keys rotation, having a set of keys for signing JWTs is needed. Once a set of keys is used, the "oldest" one can stop signing keys for time equal to expiration of JWT set in config. After that time, it can be replaced with new one.

Example (more smooth rotation):

1. Current keys: [A].
2. New key B is generated (for example by rotation of A). Current keys [B, A].
3. For some time (let's say 30 min) B is not used yet to sign JWT to allow backends based on LS to fetch this new set of keys.
4. After that time, B starts signing JWTs. A becomes "idle" for some time.
5. A gets removed. Current keys: [B].

A set of keys is also useful if we for some reason will have to revoke some JWTs, as we would only have to revoke one key from a set, leaving more users unaffected.

Feature

Have a set of keys for signing JWTs, each key having its own unique ID, and add this ID to header of JWT.

[benedeki]

Why point 3? Why the B cannot be be used after generation?
Imho it's "enough" if A is accepted for some time even after it's life oeriod has passed, and it's not being sent out anymore.

[jakipatryk]

Why point 3? Why the B cannot be be used after generation? Imho it's "enough" if A is accepted for some time even after it's life oeriod has passed, and it's not being sent out anymore.

Point 3 gives backends ("resource servers") time to fetch this new public key B, so that they are aware that this is a valid key. As it is not used yet to sign JWTs, the window for backends to perform the fetch is exactly that time. Without point 3, LS would be signing JWTs with new key, which backends will not be aware of until they refetch keys.

Though backends can do always refetch keys from LS when verification of JWT fails (particularly if key ID kid in the JWT is unknown to the backend). With that, the "waiting period" is not needed. But this assumes backends do that. It might be safer to not assume that.

This ticket is just a proposal for now, nothing that is decided yet, we'll need to check what the industry standard is.