diff --git a/Makefile b/Makefile
index 793c6ca..6bbe262 100644
--- a/Makefile
+++ b/Makefile
@@ -12,6 +12,8 @@ OZ_COMPOSE  := infra/openzaak/docker-compose.yml
 OZ_BASE     := http://localhost:8000
 NRC_COMPOSE := infra/opennotificaties/docker-compose.yml
 NRC_BASE    := http://localhost:8001
+KC_COMPOSE  := infra/keycloak/docker-compose.yml
+KC_BASE     := http://localhost:8180
 STACK_FILES := -f $(OZ_COMPOSE) -f $(NRC_COMPOSE)
 
 # On a rootless Podman dev box, point Docker CLI/Compose at the Podman socket —
@@ -24,7 +26,7 @@ export DOCKER_HOST := unix://$(PODMAN_SOCK)
 endif
 endif
 
-.PHONY: ci lint build unit smoke down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down help
+.PHONY: ci lint build unit smoke down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down help
 
 ## ci: run the full pipeline — lint, build, unit, smoke (mirrors Gitea Actions)
 ci: lint build unit smoke
@@ -107,6 +109,21 @@ stack-smoke: stack-up
 stack-down:
 	docker compose $(STACK_FILES) down --volumes
 
+## keycloak-up: start Keycloak with the four imported realms
+keycloak-up:
+	docker compose -f $(KC_COMPOSE) up -d
+
+## keycloak-smoke: start Keycloak, then verify each realm logs in + returns its claim
+keycloak-smoke: keycloak-up
+	@bash -c 'for i in $$(seq 1 60); do \
+	    c=$$(curl -s -o /dev/null -w "%{http_code}" $(KC_BASE)/realms/digid/.well-known/openid-configuration || true); \
+	    [ "$$c" = "200" ] && break; sleep 3; done; echo "Keycloak ready ($$c)"'
+	python3 infra/keycloak/check_realms.py
+
+## keycloak-down: stop and remove Keycloak
+keycloak-down:
+	docker compose -f $(KC_COMPOSE) down --volumes
+
 ## help: list available targets
 help:
 	@grep -E '^## ' $(MAKEFILE_LIST) | sed 's/^## //'
diff --git a/docs/runbooks/keycloak.md b/docs/runbooks/keycloak.md
new file mode 100644
index 0000000..df43d93
--- /dev/null
+++ b/docs/runbooks/keycloak.md
@@ -0,0 +1,37 @@
+# Keycloak runbook
+
+Keycloak (`infra/keycloak/docker-compose.yml`) runs in dev mode with four realms
+imported at boot from `infra/keycloak/realms/`: **digid**, **eherkenning**, **eidas**,
+**medewerker**. It mocks the Dutch identity brokers so portals can do real OIDC logins
+locally. Host port **:8180**.
+
+## Quick test (`make`)
+
+```bash
+make keycloak-up      # start Keycloak + import realms (~30-60s first boot)
+make keycloak-smoke   # start + verify every realm logs in and returns its claim
+make keycloak-down    # stop + wipe
+```
+
+`make keycloak-smoke` runs `infra/keycloak/check_realms.py`, which does a password-grant
+login per realm and asserts the identifying claim:
+
+| Realm | User | Claim asserted |
+|---|---|---|
+| digid | jan-burger | `bsn` |
+| eherkenning | acme-ondernemer | `kvk` |
+| eidas | pierre-dupont | `eidas_id` |
+| medewerker | merel-behandelaar | role `behandelaar` |
+
+All test users / credentials are in [../synthetic-data.md](../synthetic-data.md).
+
+## Notes
+
+- **Admin console:** <http://localhost:8180/> — `admin` / `admin` (dev only).
+- **Client `big-portal`** is public with `standardFlowEnabled` (browser redirect login)
+  *and* `directAccessGrantsEnabled` (password grant, used by the smoke test).
+- **Dev store:** in-memory H2 via `start-dev`; realms re-import on each boot, so changes
+  made in the admin UI don't persist. Edit the realm JSONs to make durable changes.
+- **Image** pinned to `quay.io/keycloak/keycloak:26.1`.
+- Claims are injected by OIDC protocol mappers on `big-portal` (user attribute → token
+  claim); `medewerker` roles come through `realm_access.roles`.
diff --git a/docs/synthetic-data.md b/docs/synthetic-data.md
new file mode 100644
index 0000000..c145825
--- /dev/null
+++ b/docs/synthetic-data.md
@@ -0,0 +1,35 @@
+# Synthetic data
+
+All credentials here are **dev-only** synthetic test data — never real personal data,
+never used outside local development.
+
+## Keycloak realms (S-02)
+
+Keycloak runs at <http://localhost:8180> (admin console: **admin / admin**). Four realms
+are imported at boot from `infra/keycloak/realms/`. Each has a public OIDC client
+**`big-portal`** (standard flow + direct access grants enabled, redirect URIs `*` for dev).
+
+All test users share the password **`test123`**.
+
+| Realm | Mimics | User | Identifying claim |
+|---|---|---|---|
+| `digid` | DigiD (burgers) | `jan-burger` | `bsn` = `123456782` |
+| `eherkenning` | eHerkenning (bedrijven) | `acme-ondernemer` | `kvk` = `12345678` |
+| `eidas` | eIDAS (EU) | `pierre-dupont` | `eidas_id` = `FR/NL/AB-1234-5678` |
+| `medewerker` | Internal staff | `merel-behandelaar` | role `behandelaar` |
+| `medewerker` | Internal staff | `tom-teamlead` | roles `behandelaar`, `teamlead` |
+
+The identifying claims are injected via OIDC protocol mappers on `big-portal`
+(user-attribute → token claim); `medewerker` roles appear in `realm_access.roles`.
+
+## Get a token (for testing)
+
+```bash
+curl -s -X POST \
+  http://localhost:8180/realms/digid/protocol/openid-connect/token \
+  -d grant_type=password -d client_id=big-portal \
+  -d username=jan-burger -d password=test123 -d scope=openid | jq -r .access_token
+```
+
+Decode the JWT payload to see the `bsn` claim. `make keycloak-smoke` checks every realm
+automatically.
diff --git a/infra/keycloak/check_realms.py b/infra/keycloak/check_realms.py
new file mode 100644
index 0000000..e338d29
--- /dev/null
+++ b/infra/keycloak/check_realms.py
@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+"""Smoke-check the Keycloak realms: each realm's OIDC login works (password grant)
+and returns its expected identifying claim. Stdlib only. Exits non-zero on failure.
+"""
+import base64, json, sys, urllib.error, urllib.parse, urllib.request
+
+BASE = "http://localhost:8180"
+CLIENT = "big-portal"
+PWD = "test123"
+
+# realm, user, claim ("__roles__" => check realm_access.roles), expected-contains
+CHECKS = [
+    ("digid", "jan-burger", "bsn", "123456782"),
+    ("eherkenning", "acme-ondernemer", "kvk", "12345678"),
+    ("eidas", "pierre-dupont", "eidas_id", "FR/NL"),
+    ("medewerker", "merel-behandelaar", "__roles__", "behandelaar"),
+]
+
+
+def decode(jwt):
+    p = jwt.split(".")[1]
+    p += "=" * (-len(p) % 4)
+    return json.loads(base64.urlsafe_b64decode(p))
+
+
+def grant(realm, user):
+    data = urllib.parse.urlencode({
+        "grant_type": "password", "client_id": CLIENT,
+        "username": user, "password": PWD, "scope": "openid",
+    }).encode()
+    req = urllib.request.Request(
+        f"{BASE}/realms/{realm}/protocol/openid-connect/token", data=data,
+        headers={"Content-Type": "application/x-www-form-urlencoded"})
+    with urllib.request.urlopen(req, timeout=20) as r:
+        return json.loads(r.read())
+
+
+def main():
+    ok = True
+    for realm, user, claim, expect in CHECKS:
+        try:
+            at = decode(grant(realm, user)["access_token"])
+            if claim == "__roles__":
+                val = at.get("realm_access", {}).get("roles", [])
+                good = expect in val
+            else:
+                val = at.get(claim)
+                good = val is not None and expect in str(val)
+            print(f"{realm:12} {user:18} login OK | {claim} = {val} "
+                  f"[{'OK' if good else 'UNEXPECTED'}]")
+            ok = ok and good
+        except urllib.error.HTTPError as e:
+            ok = False
+            print(f"{realm:12} {user:18} LOGIN FAILED {e.code}: {e.read()[:200]!r}")
+    print("keycloak smoke OK" if ok else "keycloak smoke FAILED")
+    sys.exit(0 if ok else 1)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/infra/keycloak/docker-compose.yml b/infra/keycloak/docker-compose.yml
new file mode 100644
index 0000000..b47dcc9
--- /dev/null
+++ b/infra/keycloak/docker-compose.yml
@@ -0,0 +1,29 @@
+# Keycloak (S-02) with four pre-seeded realms imported at boot:
+#   digid · eherkenning · eidas · medewerker
+# Dev mode, H2 in-memory store, realm JSONs imported from ./realms.
+#
+#   docker compose -f infra/keycloak/docker-compose.yml up -d
+#   curl -s -o /dev/null -w '%{http_code}\n' \
+#     http://localhost:8180/realms/digid/.well-known/openid-configuration   # -> 200
+#
+# Admin console: http://localhost:8180/  (admin / admin — dev only)
+services:
+  keycloak:
+    image: quay.io/keycloak/keycloak:26.1
+    command: ["start-dev", "--import-realm"]
+    environment:
+      KC_BOOTSTRAP_ADMIN_USERNAME: admin
+      KC_BOOTSTRAP_ADMIN_PASSWORD: admin
+      # Older var names too, harmless on 26.x:
+      KEYCLOAK_ADMIN: admin
+      KEYCLOAK_ADMIN_PASSWORD: admin
+      KC_HEALTH_ENABLED: "true"
+      KC_HTTP_ENABLED: "true"
+    ports:
+      - "8180:8080"
+    volumes:
+      - ./realms:/opt/keycloak/data/import:ro,z
+    networks: [cg]
+
+networks:
+  cg:
diff --git a/infra/keycloak/realms/digid-realm.json b/infra/keycloak/realms/digid-realm.json
new file mode 100644
index 0000000..28f1c51
--- /dev/null
+++ b/infra/keycloak/realms/digid-realm.json
@@ -0,0 +1,43 @@
+{
+  "realm": "digid",
+  "enabled": true,
+  "displayName": "Mock DigiD",
+  "clients": [
+    {
+      "clientId": "big-portal",
+      "enabled": true,
+      "publicClient": true,
+      "standardFlowEnabled": true,
+      "directAccessGrantsEnabled": true,
+      "redirectUris": ["*"],
+      "webOrigins": ["*"],
+      "protocolMappers": [
+        {
+          "name": "bsn",
+          "protocol": "openid-connect",
+          "protocolMapper": "oidc-usermodel-attribute-mapper",
+          "config": {
+            "user.attribute": "bsn",
+            "claim.name": "bsn",
+            "jsonType.label": "String",
+            "id.token.claim": "true",
+            "access.token.claim": "true",
+            "userinfo.token.claim": "true"
+          }
+        }
+      ]
+    }
+  ],
+  "users": [
+    {
+      "username": "jan-burger",
+      "enabled": true,
+      "firstName": "Jan",
+      "lastName": "Burger",
+      "email": "jan.burger@example.nl",
+      "emailVerified": true,
+      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "attributes": { "bsn": ["123456782"] }
+    }
+  ]
+}
diff --git a/infra/keycloak/realms/eherkenning-realm.json b/infra/keycloak/realms/eherkenning-realm.json
new file mode 100644
index 0000000..256d600
--- /dev/null
+++ b/infra/keycloak/realms/eherkenning-realm.json
@@ -0,0 +1,43 @@
+{
+  "realm": "eherkenning",
+  "enabled": true,
+  "displayName": "Mock eHerkenning",
+  "clients": [
+    {
+      "clientId": "big-portal",
+      "enabled": true,
+      "publicClient": true,
+      "standardFlowEnabled": true,
+      "directAccessGrantsEnabled": true,
+      "redirectUris": ["*"],
+      "webOrigins": ["*"],
+      "protocolMappers": [
+        {
+          "name": "kvk",
+          "protocol": "openid-connect",
+          "protocolMapper": "oidc-usermodel-attribute-mapper",
+          "config": {
+            "user.attribute": "kvk",
+            "claim.name": "kvk",
+            "jsonType.label": "String",
+            "id.token.claim": "true",
+            "access.token.claim": "true",
+            "userinfo.token.claim": "true"
+          }
+        }
+      ]
+    }
+  ],
+  "users": [
+    {
+      "username": "acme-ondernemer",
+      "enabled": true,
+      "firstName": "Anita",
+      "lastName": "Ondernemer",
+      "email": "anita@acme.nl",
+      "emailVerified": true,
+      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "attributes": { "kvk": ["12345678"] }
+    }
+  ]
+}
diff --git a/infra/keycloak/realms/eidas-realm.json b/infra/keycloak/realms/eidas-realm.json
new file mode 100644
index 0000000..2a2e066
--- /dev/null
+++ b/infra/keycloak/realms/eidas-realm.json
@@ -0,0 +1,43 @@
+{
+  "realm": "eidas",
+  "enabled": true,
+  "displayName": "Mock eIDAS",
+  "clients": [
+    {
+      "clientId": "big-portal",
+      "enabled": true,
+      "publicClient": true,
+      "standardFlowEnabled": true,
+      "directAccessGrantsEnabled": true,
+      "redirectUris": ["*"],
+      "webOrigins": ["*"],
+      "protocolMappers": [
+        {
+          "name": "eidas_id",
+          "protocol": "openid-connect",
+          "protocolMapper": "oidc-usermodel-attribute-mapper",
+          "config": {
+            "user.attribute": "eidas_id",
+            "claim.name": "eidas_id",
+            "jsonType.label": "String",
+            "id.token.claim": "true",
+            "access.token.claim": "true",
+            "userinfo.token.claim": "true"
+          }
+        }
+      ]
+    }
+  ],
+  "users": [
+    {
+      "username": "pierre-dupont",
+      "enabled": true,
+      "firstName": "Pierre",
+      "lastName": "Dupont",
+      "email": "pierre.dupont@example.fr",
+      "emailVerified": true,
+      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "attributes": { "eidas_id": ["FR/NL/AB-1234-5678"] }
+    }
+  ]
+}
diff --git a/infra/keycloak/realms/medewerker-realm.json b/infra/keycloak/realms/medewerker-realm.json
new file mode 100644
index 0000000..40ab79a
--- /dev/null
+++ b/infra/keycloak/realms/medewerker-realm.json
@@ -0,0 +1,44 @@
+{
+  "realm": "medewerker",
+  "enabled": true,
+  "displayName": "Medewerkers",
+  "roles": {
+    "realm": [
+      { "name": "behandelaar", "description": "Behandelt registratieaanvragen" },
+      { "name": "teamlead", "description": "Teamleider behandeling" }
+    ]
+  },
+  "clients": [
+    {
+      "clientId": "big-portal",
+      "enabled": true,
+      "publicClient": true,
+      "standardFlowEnabled": true,
+      "directAccessGrantsEnabled": true,
+      "redirectUris": ["*"],
+      "webOrigins": ["*"]
+    }
+  ],
+  "users": [
+    {
+      "username": "merel-behandelaar",
+      "enabled": true,
+      "firstName": "Merel",
+      "lastName": "Behandelaar",
+      "email": "merel@big.example.nl",
+      "emailVerified": true,
+      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "realmRoles": ["behandelaar"]
+    },
+    {
+      "username": "tom-teamlead",
+      "enabled": true,
+      "firstName": "Tom",
+      "lastName": "Teamlead",
+      "email": "tom@big.example.nl",
+      "emailVerified": true,
+      "credentials": [{ "type": "password", "value": "test123", "temporary": false }],
+      "realmRoles": ["behandelaar", "teamlead"]
+    }
+  ]
+}