Google's OpenIDConnect return a Base64 token that cannot be parsed

Question

As exercise to understand OpenIDConnect, I am trying to authenticate in my web app with Google following this guide.

The problem is I cannot read the token that Google s

Answer 1

Use base64url decoding (instead of plain base64) after deserialization of the compact representation of the token as in:

var http = new HttpClient(handler);
var result = await http.PostAsync("https://www.googleapis.com/oauth2/v3/token", postData);
var json = JObject.Parse(await result.Content.ReadAsStringAsync());
var payload = json.id_token.split('.')[1];
payload = payload.Replace('-', '+').Replace('_', '/');
var base64 = payload.PadRight(payload.Length + (4 - payload.Length % 4) % 4, '=');
var token = Convert.FromBase64String(base64);

Answer 2

From this post:

“id_token” is encoded in a format called JSON Web Token (JWT). JWT is the concatenation of “header”, “body”, “signature” by periods (.).

So you need to split id_token on . and decode just the 2nd segment:

var http = new HttpClient(handler);
var result = await http.PostAsync("https://www.googleapis.com/oauth2/v3/token", postData);
var json = JObject.Parse(await result.Content.ReadAsStringAsync());
var token = Convert.FromBase64String(json.id_token.split('.')[1]);