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 sends to my application> ``` var bytes = Convert.FromBase64String(codeEx.Id_token); var token = Encoding.ASCII.GetString(bytes); ``` It fails in the first line saying: "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters." The doc states: "An ID token is a cryptographically signed JSON object encoded in base 64. " For obvious reasons I cannot put the token here. I have tried: - The input is not a valid Base-64 string as it contains a non-base 64 character - Add '=' until the length is multiple of 4. - All together. I get the code exchange response, and deserialize it with the NewtonSoft.Json library: ``` var http = new HttpClient(handler); HttpResponseMessage result = await http.PostAsync("https://www.googl…

Accepted Answer

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);
```

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

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