title: What is the use of client_id and client_secret in 2-legged Authentication in OAuth 2.0
body: <p>Out of curiosity I would like to know why <code>client_id</code> and <code>client_secret</code> are required in 2-legged Authentication in OAuth 2.0?</p>
  
  <p>I am using grant_type = password and sending username &amp; password in the post method but in this I am getting error that Client credentials are not found in request body or header. When I pass <code>client_id</code> and <code>client_secret</code> I am able to get the access_token.</p>
  
  <p>I am trying to understand that why <code>client_id</code> and <code>client_secret</code> are required for 2 Logged Authentication. I think this is the best place I can get answer to my confusion.</p>
  
  <p>To be more specific I have a mobile application which communicates with a server over a web service API, and we are not having any 3rd party usage and that is the reason we are using 2-legged authentication but I am confused about the use of <code>client_id</code> and <code>client_secret</code>. Please excuse me if I am asking stupid question, but for me its a big confusion.</p>