How to properly verify the integrity of a Google token?

Question

I am trying to securely login a user via their Google account. I am at the stage that I retrieve the userID and oAuthToken using this cordova plugin.

Now I am at the point where I need to send these credentials to my server and then on the server side validate the integrity of the token. Essentially I am trying to perform this part of the process.

I am confused as to which token I should try to verify using the Google client api. Should I use the userID or the oAuthToken ?

The documentation mentions validating the userID but I find this strange. Wouldn't this mean that if anyone gets my user id they can essentially break into my server ? The user id never changes (correct me if I am wrong here) so it seems insecure to validate on this. Wouldn't it make more sense to validate the oAuthToken which is set to expire ? Or does the user ID also expire ?

Any advice would be welcome.

Thanks,

Fido

Edit:

For anyone interested. My confusion arose due to not understanding fully three tokens which the google api can return: userId, oAuthToken, and idToken. Briefly:The userId is returned with most api calls identifying the user. This appears to be constant. The oAuthToken in my case was returned when I accessed the google api as an Android client. The tokenId is returned when accessing the api as a web client. So those wishing to do server side validation using a mobile retrieved token should access the api as a web client. The token returned can then be validated server side using code similar to the accepted answer below.

Answer 1

you need to validate the IdToken, never send the userId on an open line. the IdToken expires quickly and it is virtually impregnable to brute force impersonation attacks.

this php snippet receives an HTTP request that starts with idtoken=, validates your token serverside and returns either the complete array or user email:

<?php
$inputRaw =  file_get_contents('php://input');
$idToken= substr($inputRaw,8);
$fp = fopen('twoStepOutput.txt', 'a');

$url = 'https://www.googleapis.com/oauth2/v3/tokeninfo?id_token='.$idToken;
$ch = curl_init($url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
$json = json_decode($response, true);
curl_close($ch);

$userEmail = $json["email"];
$clientId = $json["azp"];
//fwrite($fp, date("YmdHis")."\r\n$idToken\r\n");
fwrite($fp, date("YmdHis")."\r\n");
fwrite($fp, "email Confirmed by GOOGLE:[$userEmail]\r\n");
//print_r($json); // returns array console readable
print_r($clientId); // returns google client id for verification (without transfering user data)
fclose($fp);
?>

just in case you are in doubts, this is what an IdToken looks like:

eypZCI6OiJSUzI1JhbGciNiIsImtIjk4MzQxMzgyMWJmMzhiNTJlM4OTI2YTllMTc0YTc5MWMwNGMifQ.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.L4peW11TD0bDOlvYKNY60ieZ1sbZfW9gEImcuxVA5f9U_4N49Io1CFXoGKmEPR_ij4q38tF2drPMOKijQePwlrxDui37ubzAdVkuksCJUobzjD1_eccF_8GldP5Y1_XsU8xrZeEnfabfiYpr-VwoLzIeNNUdy9SUbUWjMHNcvf4dGFMzE_SONHr57igjHK3rGkbvLo-UduFngm3e-EL0YR2zOKOVj1Qs8g8_qpWgkn8XABTme1thmuU8OfC-HaF9_B2Zk2UCsnOu4ApiYZk3DPIKgeX6AF11kYnzgvciYheWeddly0foT4G00C7w_wgtd-LSRw0XZltec_MPMa2QSA