Questions to Ask an IdP

The usual framing is a shootout: line up the self-hostable identity providers, compare features, pick a winner, deploy it. That framing quietly assumes the thing worth testing here — that one system should do the whole job.

The more useful move is to split the job first:

1. Who authenticates humans?
2. Who issues tokens for services?
3. Does the backend use what those tokens say?

The first two are different problems. The first is about authentication policy, credential strength, recovery flows, and how much you trust a password. The second is about token shapes, claims, exchange, and what a backend can prove about a request's provenance.

They are allowed to have different answers. Forcing both into one system means choosing a product that is inevitably stronger at one than the other, and then living with the weaker half. Once you accept two answers, each question gets to be decided on its own merits.

The third isn't a question about the identity provider at all — which is exactly why it gets skipped. You're shopping for an IdP, so you ask IdP questions. But an issuer that can express delegation buys nothing if the backend discards it on arrival, and that turned out to be the case here, measured rather than assumed: Acting on a Human's Behalf has the result. It doesn't change the choice below. It does thin out one of the arguments for it.

What actually forced the split

The concrete requirement was delegation — a service acting on a person's behalf, with both identities visible to the backend. In RFC 8693 terms: token exchange with an actor_token.

This turns out to be an unusually good discriminator, because "supports RFC 8693" is a claim many products make while implementing meaningfully different subsets of it.

The evidence

What follows was checked against each project's own documentation. Quotes are verbatim. Where something wasn't checked, that's stated rather than glossed.

Kanidm

RFC 8693 support landed in v1.9.0 (2026-02-17) — but not in the delegating form. From its service-account documentation:

"Service accounts can exchange their API bearer token for OAuth2/OIDC tokens (access/id/refresh) without user consent or interaction."

The subject_token must be the service account's own API token. And explicitly:

"actor_token is not supported for this flow."

Read carefully, this is effect-equivalent to client_credentials: a service obtains a token for itself. That's a genuinely useful feature. It is not delegation, and no configuration turns it into delegation, because the parameter that would express delegation isn't accepted.

authentik

Supports both forms:

"authentik supports both impersonation and delegation as defined by RFC 8693."

With a version floor:

"Delegation and on-behalf-of token exchange are available in authentik 2026.8 and later."

Two packaging facts, both checked directly: nixpkgs currently carries 2026.5.6 — below the release where delegation arrived — and there is no NixOS module for it. So adopting it means both waiting on (or driving) a package bump and writing the service definition yourself.

Operationally it's a Python application plus a separate worker process, with a stated minimum of 2 cores and 2 GB RAM. Its own documentation describes Compose as being "for test setups and small-scale production".

Zitadel

Supports delegation, and the documentation reads as more settled — no beta qualifier in the current docs.

"currently only a valid access token or ID token are allowed as actor token"

"The user represented by the actor token must have the impersonation permission set, or else the request will be rejected"

That second line matters: delegation isn't ambient, it's a permission you grant. There are four permission levels along two axes (IAM/Org × Admin/EndUser), so the authority to act on someone's behalf is itself scoped rather than binary.

Operationally: Go and PostgreSQL, roughly 512 MB RAM for the service, a NixOS module at services/web-apps/zitadel.nix, package 2.71.7. It federates to generic OIDC providers, and is explicit about what that means for the resulting user model:

"All user profiles are managed within ZITADEL, allowing for a unified view of user identities regardless of the IdP used for authentication."

It also does passkeys natively per FIDO2/WebAuthn.

Keycloak — not evaluated

This was not checked. Keycloak was excluded early on operational grounds — a JVM deployment, heavier than the rest of the estate — and its delegation capabilities were never investigated.

Stating that plainly matters. Keycloak is the most widely deployed option in this category and may well handle delegation perfectly. It isn't ranked here because no ranking was performed on it, and presenting an unexamined product alongside three examined ones would misrepresent how much work went into the comparison.

Kanidm's own axis

It would be easy to read the above as "Kanidm fell short." That misreads the situation, and its own comparison documentation makes the point: it offers stronger authentication policy, WebAuthn attestation (which the others don't provide), Unix authentication integration, and a deliberate choice to implement its own database rather than depend on an external SQL server.

Those are real strengths, and they all sit on the first axis — authenticating humans well. None of them are weakened by the absence of actor_token, because delegation is a question on the second axis.

Kanidm chose a design space on purpose. The requirement here simply grew outside it. That's a different finding from "built badly", and the distinction is the whole reason the two-question framing is worth adopting.

The plan

Not yet implemented — this is the decision, not a report on living with it:

Keep Kanidm as the identity source for humans. Add Zitadel alongside it for token issuance and delegation, federating to Kanidm for authentication.

Humans keep authenticating where they already do. Services get an issuer that can mint tokens naming two parties. Neither system is asked to do the thing it wasn't designed for.

Two reasons this is an addition rather than a migration:

1. Real services already authenticate against Kanidm in production — the git forge and the fediverse server.
2. The passkeys are domain-bound. Moving would ask every user to re-enrol their hardware. That constraint deserves its own note: WebAuthn Credentials Are Domain-Bound.

The second reason is the one that would still apply even if the first didn't.

Where this actually stands

Seedling. The decision is made; nothing is deployed. Most of the above is documentation-checking, which is exactly the kind of evidence that survives contact with reality least well — federation in particular tends to be where the interesting problems are, and none of them have been met yet.

One piece has since been measured rather than read: the exchange itself works end to end against the real backend, and that backend ignores the act claim. The plan below is unchanged by it — the reasons for it were never only about audit — but one of its arguments is thinner than it looked.

Two known gaps, kept visible rather than quietly dropped: Keycloak was never evaluated, and running two identity systems has an ongoing operational cost that hasn't been paid yet and therefore can't honestly be reported on. This note gets revisited once it has.