UID -> ID Token

Gets a Firebase ID token for the selected user in one click. The app signs a custom token locally, exchanges it with Google through signInWithCustomToken, and shows the resulting ID token.

Needs: a service account, an app with an API key, and a selected user. The exchange uses the selected app’s key, so you can check that an Android or iOS app’s restricted key works.

Generating a token

1. Pick a user in the Users panel.
2. Optionally, enter claims in Custom claims (optional JSON). They are added to this token only.
3. Click Generate.

Under the token you get:

• Refresh token: a shortened view of the refresh token Google returned. It is shown for reference only; there is no button to copy it.
• Expires in: the token’s lifetime in seconds, normally 3600s.
• Decoded JWT claims: everything the ID token asserts. Claims stored on the user with User Custom Claims appear here alongside the standard ones.

Using the token

Send it as a bearer token to anything that verifies Firebase ID tokens, such as your backend’s verifyIdToken call or a Firebase security rule test:

curl -H "Authorization: Bearer $ID_TOKEN" https://localhost:8080/api/me

The token is a real credential for that user until it expires. Treat it like a password.

Signing in as the user

Exchanging a custom token is a real sign-in. The user’s Signed in date in the Firebase console updates, auth_time in the token is set to now, and the firebase claim records "sign_in_provider": "custom". If your backend treats the sign-in provider specially, or you track sign-in activity, bear that in mind.