fix: encrypt OIDC client secret at rest using AES-256-GCM

The oidc_client_secret was written to app_settings as plaintext,
unlike Maps and OpenWeather API keys which are protected with
apiKeyCrypto. An attacker with read access to the SQLite file
(e.g. via a backup download) could obtain the secret and
impersonate the application with the identity provider.

- Encrypt on write in PUT /api/admin/oidc via maybe_encrypt_api_key
- Decrypt on read in GET /api/admin/oidc and in getOidcConfig()
  (oidc.ts) before passing the secret to the OIDC client library
- Add a startup migration that encrypts any existing plaintext value
  already present in the database

server/src/db/migrations.ts

@@ -1,4 +1,5 @@
 import Database from 'better-sqlite3';
+import { encrypt_api_key } from '../services/apiKeyCrypto';
 
 function runMigrations(db: Database.Database): void {
   db.exec('CREATE TABLE IF NOT EXISTS schema_version (version INTEGER NOT NULL)');
@@ -448,6 +449,13 @@ function runMigrations(db: Database.Database): void {
     () => {
       try { db.exec('ALTER TABLE trips ADD COLUMN reminder_days INTEGER DEFAULT 3'); } catch (err: any) { if (!err.message?.includes('duplicate column name')) throw err; }
     },
+    // Encrypt any plaintext oidc_client_secret left in app_settings
+    () => {
+      const row = db.prepare("SELECT value FROM app_settings WHERE key = 'oidc_client_secret'").get() as { value: string } | undefined;
+      if (row?.value && !row.value.startsWith('enc:v1:')) {
+        db.prepare("UPDATE app_settings SET value = ? WHERE key = 'oidc_client_secret'").run(encrypt_api_key(row.value));
+      }
+    },
   ];
 
   if (currentVersion < migrations.length) {

server/src/routes/admin.ts

@@ -9,6 +9,7 @@ import { AuthRequest, User, Addon } from '../types';
 import { writeAudit, getClientIp, logInfo } from '../services/auditLog';
 import { getAllPermissions, savePermissions, PERMISSION_ACTIONS } from '../services/permissions';
 import { revokeUserSessions } from '../mcp';
+import { maybe_encrypt_api_key, decrypt_api_key } from '../services/apiKeyCrypto';
 
 const router = express.Router();
 
@@ -232,7 +233,7 @@ router.get('/audit-log', (req: Request, res: Response) => {
 
 router.get('/oidc', (_req: Request, res: Response) => {
   const get = (key: string) => (db.prepare("SELECT value FROM app_settings WHERE key = ?").get(key) as { value: string } | undefined)?.value || '';
-  const secret = get('oidc_client_secret');
+  const secret = decrypt_api_key(get('oidc_client_secret'));
   res.json({
     issuer: get('oidc_issuer'),
     client_id: get('oidc_client_id'),
@@ -247,7 +248,7 @@ router.put('/oidc', (req: Request, res: Response) => {
   const set = (key: string, val: string) => db.prepare("INSERT OR REPLACE INTO app_settings (key, value) VALUES (?, ?)").run(key, val || '');
   set('oidc_issuer', issuer);
   set('oidc_client_id', client_id);
-  if (client_secret !== undefined) set('oidc_client_secret', client_secret);
+  if (client_secret !== undefined) set('oidc_client_secret', maybe_encrypt_api_key(client_secret) ?? '');
   set('oidc_display_name', display_name);
   set('oidc_only', oidc_only ? 'true' : 'false');
   const authReq = req as AuthRequest;

server/src/routes/oidc.ts

@@ -5,6 +5,7 @@ import jwt from 'jsonwebtoken';
 import { db } from '../db/database';
 import { JWT_SECRET } from '../config';
 import { User } from '../types';
+import { decrypt_api_key } from '../services/apiKeyCrypto';
 
 interface OidcDiscoveryDoc {
   authorization_endpoint: string;
@@ -57,7 +58,7 @@ function getOidcConfig() {
   const get = (key: string) => (db.prepare("SELECT value FROM app_settings WHERE key = ?").get(key) as { value: string } | undefined)?.value || null;
   const issuer = process.env.OIDC_ISSUER || get('oidc_issuer');
   const clientId = process.env.OIDC_CLIENT_ID || get('oidc_client_id');
-  const clientSecret = process.env.OIDC_CLIENT_SECRET || get('oidc_client_secret');
+  const clientSecret = process.env.OIDC_CLIENT_SECRET || decrypt_api_key(get('oidc_client_secret'));
   const displayName = process.env.OIDC_DISPLAY_NAME || get('oidc_display_name') || 'SSO';
   if (!issuer || !clientId || !clientSecret) return null;
   return { issuer: issuer.replace(/\/+$/, ''), clientId, clientSecret, displayName };