title: Oauth2 flow without redirect_uri
body: <p>I am creating an Android/iOS app which communicates with a Node.js server and would like to identify them securely on my server using Google (and/or Facebook) and OAuth2. I've looked at the following documentation: <a href="https://developers.google.com/+/web/signin/server-side-flow" rel="noreferrer">https://developers.google.com/+/web/signin/server-side-flow</a></p>
  
  <p>I do not need authorization, I only need authentication (I only want to make sure that the person calling my Node.js service is the person they say they are). To achieve this, if I understand properly, I have to let the user log in using Google on the client side, this will give them an authorization_code which they can then give to my server. My server can then exchange that code for an access_token, and therefore retrieve information about the user. I am then guaranteed that the user is the person they say they are.</p>
  
  <p>The Google documentations (link above) says: "In the Authorized redirect URI field, delete the default value. It is not used for this case.", however, for my server to exchange the authorization_code for an access_token, it <strong>needs</strong> to provide a <code>redirect_uri</code>, am I missing something?</p>
  
  <p>The <code>redirect_uri</code> is useless for Unity games, for instance (since logging in with Google simply opens a new "window", which is closed when logged in, no redirection involved).</p>
  
  <p>TL;DR
  How do you use OAuth2 to authenticate users between my client and my server without redirection?</p>