backend-app-api: move limited token creation to UserTokenHandler

Co-authored-by: Camila Belo <camilaibs@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/backend-app-api/src/services/implementations/auth/UserTokenHandler.ts

@@ -19,6 +19,7 @@ import { DiscoveryService } from '@backstage/backend-plugin-api';
 import { AuthenticationError } from '@backstage/errors';
 import { TokenTypes } from '@backstage/plugin-auth-node';
 import {
+  base64url,
   createRemoteJWKSet,
   decodeJwt,
   decodeProtectedHeader,
@@ -102,6 +103,37 @@ export class UserTokenHandler {
     return { userEntityRef };
   }
 
+  createLimitedUserToken(backstageToken: string) {
+    const [headerRaw, payloadRaw] = backstageToken.split('.');
+    const header = JSON.parse(
+      new TextDecoder().decode(base64url.decode(headerRaw)),
+    );
+    const payload = JSON.parse(
+      new TextDecoder().decode(base64url.decode(payloadRaw)),
+    );
+
+    const limitedUserToken = [
+      base64url.encode(
+        JSON.stringify({
+          typ: 'vnd.backstage.limited-user',
+          alg: header.alg,
+          kid: header.kid,
+        }),
+      ),
+      base64url.encode(
+        JSON.stringify({
+          sub: payload.sub,
+          ent: payload.ent,
+          iat: payload.iat,
+          exp: payload.exp,
+        }),
+      ),
+      payload.uip,
+    ].join('.');
+
+    return { token: limitedUserToken, expiresAt: new Date(payload.exp * 1000) };
+  }
+
   /**
    * If the last keystore refresh is stale, update the keystore URL to the latest
    */

packages/backend-app-api/src/services/implementations/auth/authServiceFactory.ts

@@ -27,7 +27,7 @@ import {
 } from '@backstage/backend-plugin-api';
 import { AuthenticationError } from '@backstage/errors';
 import { TokenTypes } from '@backstage/plugin-auth-node';
-import { base64url, decodeJwt, decodeProtectedHeader } from 'jose';
+import { decodeJwt, decodeProtectedHeader } from 'jose';
 import { UserTokenHandler } from './UserTokenHandler';
 
 /** @internal */
@@ -231,34 +231,7 @@ class DefaultAuthService implements AuthService {
       );
     }
 
-    const [headerRaw, payloadRaw] = backstageToken.split('.');
-    const header = JSON.parse(
-      new TextDecoder().decode(base64url.decode(headerRaw)),
-    );
-    const payload = JSON.parse(
-      new TextDecoder().decode(base64url.decode(payloadRaw)),
-    );
-
-    const limitedUserToken = [
-      base64url.encode(
-        JSON.stringify({
-          typ: 'vnd.backstage.limited-user',
-          alg: header.alg,
-          kid: header.kid,
-        }),
-      ),
-      base64url.encode(
-        JSON.stringify({
-          sub: payload.sub,
-          ent: payload.ent,
-          iat: payload.iat,
-          exp: payload.exp,
-        }),
-      ),
-      payload.uip,
-    ].join('.');
-
-    return { token: limitedUserToken, expiresAt: new Date(payload.exp * 1000) };
+    return this.userTokenHandler.createLimitedUserToken(backstageToken);
   }
 
   #getJwtExpiration(token: string) {