How to authenticate programmatically to a Cloud Identity-Aware Proxy (Cloud IAP)-secured resource using user default credentials?

Question

I would like to be able to programmatically generate an id token for iap using the user default credential on a dev environment (i.e. my own laptop with google cloud sdk installed and logged in).

When following the documentation, I managed to generate an authorization token using a service account file.

When using google.auth.default on my personal computer, I can see the credentials of type google.oauth2.credentials.Credentials have a refresh_token. I wanted to use it to generate the token as it is done with curl in the documentation under Authenticating from a desktop app -> Accessing the application but I could not make it work. Does someone know if there is a way to authenticate this way ?

Answer 1

As Matthew stated, project for Client ID used to obtain refresh token should match project for IAP Client ID. Gcloud uses Client ID and secret defined in path_to/google-cloud-sdk/lib/googlecloudsdk/api_lib/auth/util.py for default credentials (DEFAULT_CREDENTIALS_DEFAULT_CLIENT_ID and DEFAULT_CREDENTIALS_DEFAULT_CLIENT_SECRET). Because of that, you can't use refresh token from google.auth.default() without util.py change, as an attempt to obtain ID token will fail with:

{
 "error": "invalid_audience",
 "error_description": "The audience client and the client need to be in the same project."
}

Your options are:

1. Obtain refresh token (cache it to avoid the need to do user grant every time) and ID token as per Matthew's reply / documentation.
2. Patch Client ID and secret present in the gcloud util.py (might be changed with gcloud updates).

Sample code for both options:

import google.auth
import requests
import json

from webbrowser import open_new_tab
from time import sleep

# use gcloud app default credentials if gcloud's util.py is patched
def id_token_from_default_creds(audience): 
    cred, proj = google.auth.default()
    # data necessary for ID token
    client_id = cred.client_id
    client_secret= cred.client_secret
    refresh_token = str(cred.refresh_token)
    return id_token_from_refresh_token(client_id, client_secret, refresh_token, audience)

def id_token_from_refresh_token(client_id, client_secret, refresh_token, audience):
    oauth_token_base_URL = "https://www.googleapis.com/oauth2/v4/token"
    payload = {"client_id": client_id, "client_secret": client_secret,
                "refresh_token": refresh_token, "grant_type": "refresh_token",
                "audience": audience}
    res = requests.post(oauth_token_base_URL, data=payload)
    return (str(json.loads(res.text)[u"id_token"]))

# obtain ID token for provided Client ID: get authorization code -> exchange for refresh token -> obtain and return ID token
def id_token_from_client_id(client_id, client_secret, audience):
    auth_code = get_auth_code(client_id)
    refresh_token = get_refresh_token_from_code(auth_code, client_id, client_secret)
    return id_token_from_refresh_token(client_id, client_secret, refresh_token, audience)

def get_auth_code(client_id):
    auth_url = "https://accounts.google.com/o/oauth2/v2/auth?client_id=%s&response_type=code&scope=openid%%20email&access_type=offline&redirect_uri=urn:ietf:wg:oauth:2.0:oob"%client_id
    open_new_tab(auth_url)
    sleep(1)
    return raw_input("Authorization code: ")

def get_refresh_token_from_code(auth_code, client_id, client_secret):
    oauth_token_base_URL = 'https://www.googleapis.com/oauth2/v4/token'
    payload = {"code": auth_code, "client_id": client_id, "client_secret": client_secret,
                "redirect_uri": "urn:ietf:wg:oauth:2.0:oob", "grant_type": "authorization_code"}
    res = requests.post(oauth_token_base_URL, data=payload)
    return (str(json.loads(res.text)[u"refresh_token"]))

print("ID token from client ID: %s" % id_token_from_client_id("<Other client ID>", "<Other client secret>", "<IAP Client ID>")) # other client ID should be from the same project as IAP Client ID 
print("ID token from \"default\" credentials: %s" % id_token_from_default_creds("<IAP Client ID>"))