Unable to decode credential from JWT

Question

I am trying to implement the new "Sign in with Google" button as described in https://developers.google.com/identity/gsi/web/guides/display-button .

Everything is fine, and I am able to get a response from the button with "credential" and "g_csrf_token" elements, which I can send to my server. However, using the Google API Client to decode the credential doesn't work. I'm trying to follow the instructions .

Here's my code:

    $id_token = filter_input(INPUT_POST, 'credential');
    $csrfToken = filter_input(INPUT_POST, 'g_csrf_token'); //??? Do we need this?
    
    $client = new Google_Client(['client_id' => $clientid]);
    $client->addScope("email"); // Recommended in another StackOverflow answer but makes no difference
    try {
        $payload = $client->verifyIdToken($id_token);
    } catch(Exception $ex) {
        $errorMessage = "Error in verifyIdToken():" . $ex->getMessage();
        // ...do stuff with the error message
    }
    // ...do stuff with the returned payload

The result is the error message id_token must be passed in or set as part of setAccessToken .

I've updated my Google API Client to v2.11.

I assume that I've missed a step somewhere - can someone help?

Answer 1

Have found a solution, by trial and error! Turns out that $id_token needs to be passed to the client twice, once in setAccessToken() and then again in verifyIdToken() . Omitting setAccessToken fails (like the error message says), but if you pass it in setAccessToken but NOT in verifyIdToken , that doesn't work either.

$id_token = filter_input(INPUT_POST, 'credential');
$client = new Google_Client(['client_id' => $clientid]);
try {
    $client->setAccessToken($id_token);
    $payload = $client->verifyIdToken($id_token);
} catch(Exception $ex) {
    $errorMessage = "Error in verifyIdToken():" . $ex->getMessage();
    // ...do stuff with the error message
}
// ...do stuff with the returned payload

It would nice, if you're at Google and picking this up, if you updated the documentation.