web services - How can I verify a Google authentication API access token?

Question

How can I verify a Google authentication access token?

I need to somehow query Google and ask: Is [given access token] valid for the [example@example.com] Google account?

Short version:
It's clear how an access token supplied through the Google Authentication Api :: OAuth Authentication for Web Applications can be used to then request data from a range of Google services. It is not clear how to check if a given access token is valid for a given Google account. I'd like to know how.

Long version:
I'm developing an API that uses token-based authentication. A token will be returned upon provision of a valid username+password or upon provision of a third-party token from any one of N verifiable services.

One of the third-party services will be Google, allowing a user to authenticate against my service using their Google account. This will later be extended to include Yahoo accounts, trusted OpenID providers and so on.

Schematic example of Google-based access:

alt text http://webignition.net/images/figures/auth_figure002.png

The 'API' entity is under my full control. The 'public interface' entity is any web- or desktop-based app. Some public interfaces are under my control, others will not be and others still I may never even know about.

Therefore I cannot trust the token supplied to the API in step 3. This will be supplied along with the corresponding Google account email address.

I need to somehow query Google and ask: Is this access token valid for example@example.com?

In this case, example@example.com is the Google account unique identifier - the email address someone uses to log in to their Google account. This cannot be assumed to be a Gmail address - someone can have a Google account without having a Gmail account.

The Google documentation clearly states how, with an access token, data can be retrieved from a number of Google services. Nothing seems to state how you can check if a given access token is valid in the first place.

Update The token is valid for N Google services. I can't try a token against a Google service as means of verifying it as I won't know which subset of all Google's services a given user actually uses.

Furthermore, I'll never be using the Google authentication access token to access any Google services, merely as a means of verifying a supposed Google user actually is who they say they are. If there is another way of doing this I'm happy to try.

Answer 1

For user check, just post get the access token as accessToken and post it and get the response

https://www.googleapis.com/oauth2/v1/tokeninfo?access_token=accessToken

you can try in address bar in browsers too, use httppost and response in java also

response will be like

{
     "issued_to": "xxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com",
     "audience": "xxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com",
     "user_id": "xxxxxxxxxxxxxxxxxxxxxxx",
     "scope": "https://www.googleapis.com/auth/userinfo.profile https://gdata.youtube.com",
     "expires_in": 3340,
     "access_type": "offline"
    }

The scope is the given permission of the accessToken. you can check the scope ids in this link

Update: New API post as below

https://oauth2.googleapis.com/tokeninfo?id_token=XYZ123

Response will be as

 {
 // These six fields are included in all Google ID Tokens.
 "iss": "https://accounts.google.com",
 "sub": "110169484474386276334",
 "azp": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "aud": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "iat": "1433978353",
 "exp": "1433981953",

 // These seven fields are only included when the user has granted the "profile" and
 // "email" OAuth scopes to the application.
 "email": "testuser@gmail.com",
 "email_verified": "true",
 "name" : "Test User",
 "picture": "https://lh4.googleusercontent.com/-kYgzyAWpZzJ/ABCDEFGHI/AAAJKLMNOP/tIXL9Ir44LE/s99-c/photo.jpg",
 "given_name": "Test",
 "family_name": "User",
 "locale": "en"
}

For more info, https://developers.google.com/identity/sign-in/android/backend-auth