oidc: one verifier per issuer, because IdPs mint one per application

Authentik derives the issuer from the application slug, so two applications mean
two issuers - and a token's `iss` must match whoever signed it. A single pinned
issuer could therefore only ever serve one of the two clients.

So there is a verifier per issuer, and each accepts only the client belonging to
it. That is tighter than the previous arrangement as well as more general: a
token minted for the phone cannot be replayed at the admin login, and vice
versa, because they arrive at different verifiers with different audiences.

ECHOLOT_OIDC_APP_ISSUER is optional - empty means both clients share
ECHOLOT_OIDC_ISSUER, which is what IdPs with one global issuer do.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/internal/config/config.go

@@ -74,7 +74,13 @@ type Config struct {
 	OIDCIssuer      string // ECHOLOT_OIDC_ISSUER / --oidc-issuer
 	OIDCClientID    string // ECHOLOT_OIDC_CLIENT_ID / --oidc-client-id (confidential, admin UI)
 	OIDCAppClientID string // ECHOLOT_OIDC_APP_CLIENT_ID / --oidc-app-client-id (public, the phone app)
-	OIDCAdminGroup  string // ECHOLOT_OIDC_ADMIN_GROUP / --oidc-admin-group
+	// Issuer for the app's client, when the IdP gives each application its own.
+	//
+	// Authentik derives the issuer from the application slug, so two applications mean two
+	// issuers — and a token's `iss` must match the one that minted it. Empty means both clients
+	// share ECHOLOT_OIDC_ISSUER, which is what IdPs with a single global issuer do.
+	OIDCAppIssuer  string // ECHOLOT_OIDC_APP_ISSUER / --oidc-app-issuer
+	OIDCAdminGroup string // ECHOLOT_OIDC_ADMIN_GROUP / --oidc-admin-group
 
 	// Break-glass admin username; the password lives hashed in the state store.
 	AdminUser string // ECHOLOT_ADMIN_USER / --admin-user
@@ -167,6 +173,7 @@ func Load(args []string) (*Config, *Actions, error) {
 	fs.StringVar(&c.OIDCIssuer, "oidc-issuer", envOr("OIDC_ISSUER", ""), "OpenID Connect issuer URL; empty disables sign-in")
 	fs.StringVar(&c.OIDCClientID, "oidc-client-id", envOr("OIDC_CLIENT_ID", ""), "confidential OIDC client id for the admin UI")
 	fs.StringVar(&c.OIDCAppClientID, "oidc-app-client-id", envOr("OIDC_APP_CLIENT_ID", ""), "public OIDC client id used by the Android app (PKCE)")
+	fs.StringVar(&c.OIDCAppIssuer, "oidc-app-issuer", envOr("OIDC_APP_ISSUER", ""), "issuer for the app client when the IdP uses per-application issuers; empty = same as --oidc-issuer")
 	fs.StringVar(&c.OIDCAdminGroup, "oidc-admin-group", envOr("OIDC_ADMIN_GROUP", ""), "group claim required for admin access; empty means nobody is an admin via OIDC")
 	fs.StringVar(&c.OIDCClientSecret, "oidc-client-secret", secretOr("OIDC_CLIENT_SECRET", ""), "secret for the confidential admin client; prefer ECHOLOT_OIDC_CLIENT_SECRET_FILE")
 	fs.StringVar(&c.AdminBaseURL, "admin-base-url", envOr("ADMIN_BASE_URL", ""), "public URL of the admin UI, for the OIDC redirect (e.g. https://admin.example.net)")

server/internal/control/control.go

@@ -59,8 +59,12 @@ type Server struct {
 	// Granted server->client sends (spec §5). Both consume an asymmetric grant.
 	DownTrain func(sess *session.Session, g *session.Grant, count, sizeBytes, intervalUs int) (int, error)
 	BigSend   func(sess *session.Session, g *session.Grant, sizes []int, df bool) ([]dataplane.BigSendResult, error)
-	// OIDC verifies ID tokens when the operator has configured an issuer (may be nil).
+	// OIDC verifies ID tokens presented by the *app* (may be nil).
 	OIDC *oidc.Verifier
+	// AdminOIDC verifies tokens from the admin UI's own client. Separate because an IdP may
+	// give each application its own issuer — Authentik derives it from the application slug —
+	// and a verifier pins exactly one issuer and the clients belonging to it.
+	AdminOIDC *oidc.Verifier
 	// Runs stores uploaded measurement documents (may be nil: uploads unsupported).
 	Runs *runs.Store
 	// FragSend emits one datagram as hand-built IP fragments in a chosen order (may be nil: