title: Authenticate slack user in bot message posted event
body: <p>I have a Slack bot application that needs to authenticate messages received. Instead of receiving a token from Slack, my application will provide a token to Slack. Slack would then send the token to my application during each request. My application can then authenticate who the message came from.</p>
  
  <p>Is there a way on Slack's platform to be an Oauth provider to Slack or some way to authenticate messages?</p>
  
  <p>Thank you</p>