halkyon/docs/adr/0023-openid-connect-module.md

# ADR 0023: Signing in with OpenID Connect, a module others can use

- **Status:** decided 23 Oct 2026, by the developer (D1–D4); the design below is built
  as ADR 0020's step 2, whose D12 and D13 it carries.
- **Builds on:** ADR 0014 (halkyon_base, the first Serverpod module), ADR 0020 (signing in
  with SimpleLogin only).

## Context

ADR 0020 has learn.halkyon.eu sign people in with SimpleLogin, through OpenID Connect's
code flow. Asked on 23 Oct whether that could be "a separate module that others may use
as well at their convenience with Serverpod", what Serverpod v4.0.4 offers:

- **A contract for providers of one's own**: an `IdentityProvider`, built by an
  `IdentityProviderBuilder` the host passes to `AuthServices` beside the stock ones, an
  endpoint on `IdpBaseEndpoint`, an account model linking the provider's ID to
  Serverpod's `AuthUser`, and a controller on the device ("Custom providers" in
  Serverpod's documentation, still being written on 23 Oct). `OAuth2PkceUtil` exchanges
  a code, PKCE optional, and is exported.
- **No OpenID Connect**: no provider for it, no discovery, and no exported check of ID
  tokens (Google's and Firebase's `IdTokenVerifier` is internal and refuses tokens
  without `typ`, ADR 0020's D12).

## Decisions (23 Oct 2026, the developer's)

- **D1 OpenID Connect, with a SimpleLogin preset** (recommended, over SimpleLogin alone):
  any provider that follows OpenID Connect's code flow (Keycloak, Authentik, Zitadel,
  Authelia, Pocket ID, …), which Serverpod lacks; SimpleLogin a preset that knows its
  ways (no PKCE, no `typ`). Only SimpleLogin is tried live; the rest is tested to the
  specification. The packages' names stay generic.
- **D2 BSD-3-Clause** (recommended, over Apache-2.0, MIT, and AGPL v3 as Halkyon's
  other packages): Serverpod's own licence, so whoever builds on Serverpod can take it,
  closed apps too.
- **D3 In this repository, published later** (recommended, over a repository of its own
  now, or offering it to Serverpod): under `packages/` with `publish_to: none`, so a
  change to it and to Halkyon is one commit; to pub.dev once it runs on
  learn.halkyon.eu, asked again before, as publishing is public.
- **D4 Now, as ADR 0020's step 2** (recommended, over moving it out after v0.20.0): the
  boundary drawn first costs less than pulling Halkyon's code apart later, a table
  moved between modules included; v0.20.0 comes about a day later.

## Design

1. **Packages:** `halkyon_oidc_server`, a Serverpod module (nickname `oidc`), with its
   generated client `halkyon_oidc_client`; `halkyon_oidc_flutter`, the device's half,
   with ADR 0020's step 4. halkyon_base builds on the module; nothing of Halkyon's is in
   it.
2. **Providers:** a host names one or more, each under a key of its own that never
   changes (`simplelogin`): the issuer, the endpoints (from the provider's discovery
   document, checked to name the same issuer, or given), the client's ID and secret,
   the scopes, PKCE or not, the signing algorithms its tokens may use (RS256 unless
   said), and the callback address the provider has registered. The SimpleLogin preset
   fills in all but the client and the callback.
3. **What it keeps:** a person's identity at a provider, the provider's key and the
   `sub` (`halkyon_oidc_identity`): each `sub` one person's at a provider, one per
   person and provider. Nothing of what the token says besides; the claims go to the
   host's code, which decides (Halkyon drops them, ADR 0020's 4).
4. **Attempts** (ADR 0020's D13): `halkyon_oidc_attempt`, the ID PostgreSQL's
   `gen_random_uuid()` (122 random bits), the secret only the device holds; the state
   and the nonce, 32 random bytes each, and PKCE's verifier where used; who began it,
   if signed in; ten minutes, taken once. The host may begin the state with a marker of
   its own (ADR 0020's forwarding page reads one).
5. **Checking a sign-in** (ADR 0020's D12): the code exchanged with the client's secret
   (`OAuth2PkceUtil`); the ID token's header without `typ` asked for, its algorithm one
   of the provider's, its key the one its `kid` names in the provider's keys (kept an
   hour, fetched again for an unknown `kid` at most once a minute, every fetch with a
   timeout); the issuer, the audience (and `azp` with more than one), the expiry and the
   time issued, a minute of clocks' difference allowed, the nonce the attempt's.
6. **Two ways to use it:**
   - **The stock way**, as Serverpod's providers: the host's endpoint on
     `OidcIdpBaseEndpoint`, `begin` and `login`: a known `sub` gets a session, an
     unknown one a new person; linking to the signed-in caller where the attempt was
     begun so.
   - **The parts**, for a host with rules of its own (Halkyon: the register, a handle
     before an account, ADR 0016 and 0020): `begin`, `verify` (the identity proven, and
     its claims, touching no account), `link`, `accountOf`, `unlink`, called from its
     own endpoints.
7. **The forwarding page** goes into the module (`addOidcCallbackForwarding`), with the
   targets the host fixes: what ADR 0020's 3 has Halkyon need, a provider that takes
   https callbacks alone, each registered exactly, others need alike.

## Consequences

- Halkyon's sign-in rests on a module of its own making, with its own tests and
  README, kept usable by others: its interface changes with care once published.
- Serverpod hosts get OpenID Connect; a provider other than SimpleLogin may still need
  a fix the specification's tests didn't foresee.
- The module's tables are part of every host's migrations, as halkyon_base's are.