How to decode Firebase JWT token in Python

Question

I have added Firebase to allow clients to authenticate directly from the web app client (browser). I am using the firebase-web JS package and it works great. I can see in my browser that I receive a user object with information about the user, including an idToken.

I need to then authenticate this user on my server backend, which is python django. In the Firebase docs I found a how-to for exactly what I am trying to do, which is to verify the id token.

Since they don't have the supported Firebase sdk for python, I need to use a third party solution. I have come to the python-jose package after finding it listed on the jwt.io site. The example looks simple enough:

jwt.decode(token, 'secret', algorithms=['RS256'])

This is my first time using JWT. I don't know what to use for the 'secret'. I tried pasting my id token as token, and the web API key from the Firebase console for secret, but got this error:

jose.exceptions.JWKError: RSA key format is not supported

I also tried the JWT debugger, which seems to be reading most of my id token correctly, but the signature verification is looking for a public and/or a private keys, which like the 'secret' are escaping me.

I am really at a loss for how to find this secret, and how to verify the JWT id token in general. The information on the Firebase docs (third-party section) is:

Finally, ensure that the ID token was signed by the private key corresponding to the token's kid claim. Grab the public key from https://www.googleapis.com/robot/v1/metadata/x509/[email protected] and use a JWT library to verify the signature. Use the value of max-age in the Cache-Control header of the response from that endpoint to know when to refresh the public keys.

I have tried pasting the whole json blob from that googleapis url into the JWT debugger, but still getting an "invalid signature" alert. I don't understand how to use that public key.

Should python-jose work for this approach? If so, what should I use for the secret? If not, can someone point me in the right direction?

Thanks.

Answer 1

I finally found the answer I was looking for in this post: Migrating Python backend from Gitkit to to Firebase-Auth with python-jose for token verification

Since the time of the post there have been updates made to the python-jose package, which gives better support for firebase id tokens. Here is some working code ( jose version 1.3.1 ) on how to use python to decode the firebase id token:

import urllib, json
from jose import jwt

idtoken = "<id token passed to server from firebase auth>"

target_audience = "<firebase app id>"

certificate_url = 'https://www.googleapis.com/robot/v1/metadata/x509/[email protected]'

response = urllib.urlopen(certificate_url)
certs = response.read()
certs = json.loads(certs)

#will throw error if not valid
user = jwt.decode(idtoken, certs, algorithms='RS256', audience=target_audience)
print user