Auth callout is post-NATS client authentication, so it would not solve the "auth web flow" for authentication. Instead, the resulting token from that would be set as a cookie that then would be passed into the nats.ws client connection. The auth callout service would use that token to map to the concrete NATS user. The mechanism of doing that is up to the implementation. One option is to manage NATS claims into the OIDC provider (for the user authenticating) and then the auth service would decode that source JWT and extract the NATS claims and generate the NATS user JWT in the response.

Thanks that observation is extremely helpful. If I have it down then the intended flow looks something like?

- Web client is directed to some token vending service. This service implement authn in a manner of its choosing (i.e. OAuth) then sets a NATS client JWT in the cookie per https://docs.nats.io/running-a-nats-service/configuration/se... - Nats.ws client connection provides cookie during connection to perform client auth - If further authz/fine-grained control is needed the auth callout mechanism can be used. This would have access to the provided cookie/token so any claims needed for access control could be stapled on during step one and used at this point?

For GPs original question -- I'm running a fairly old Keycloak version (v8) but it does appear to set a JWT in KEYCLOAK_IDENTITY and KEYCLOAK_IDENTITY_LEGACY.

Am I right in understanding that IFF the token is signed with Ed25519 and both sub and iss are an NKEY value this is sufficient for NATS to accept that cookie as a credential?

Yes that reads correct. The `sub` would a NATS user public nkey, the `iss` would be the NATS account public nkey (either the issuer nkey in config-mode or existing nkey in decentralized auth).

As long as it can verify the chain of trust for the user JWT that is returned, it should work.

The three schema types are shown here: https://docs.nats.io/running-a-nats-service/configuration/se...

auth request comes in -> generate user jwt, sign + encode -> respond with auth response.

As long as the necessary bits of the response and user JWT conform, it will work.