The OAuth is the new buzz in the world of SharePoint 2013 App development.  Just to remember, OAuth is not the protocol for authenticating users to access SharePoint. It would still be done by Claims Authentication. The OAuth comes into picture when we want to authenticate and authorize SharePoint 2013 Apps.

I’ll start with some briefing on OAuth and the key concepts that we need to understand about OAuth. OAuth is the internet protocol for creating and managing app identity. It is also a cross-platform mechanism for authentication and authorizing apps. The OAuth is also the emerging internet standard which is used by Facebook, Twitter and Google.

OAuth gives the power and flexibility of having app identity in addition to the user identity. Here are the some pointers about App Identity

• App should be granted permissions independently of user permission
• App can request specific permission from the user during installation
• App can be granted more permission than the user (Elevation)
• App is constrained to what it can do during and after installation

Here are some important concepts around OAuth

1. Content Owner – User who grants permission to content in a site

2. Client App – This is the remote App (running on a Cloud or Hosted environment) that needs permission to Site Content . In our case it is SharePoint 2013 App

3. Content Server – The web server that serves the content to be accessed by App. In our case it is SharePoint 2013 Server (Cloud or On-Premise)

4. Authentication Server – Trusted server that authenticates apps and creates oAuth tokens. In our case it is Azure ACS server or oAuth compatible authentication server

Let’s see what is happening in each step in the above picture.

Step 1 –> The user accesses the SharePoint 2013 portal and SharePoint 2013 authenticates the user using Claims Authentication

Step 2 –>  SharePoint 2013 requests for the Context Token for the user, from Windows Azure ACS (Access Control Services)

Step 3 –> ACS returns Context Token

Step 4 –> SharePoint 2013 passes the Context Token to the user

Step 5 –> User accesses App using Context Token

Step 6 –> Client App pulls Refresh Token from the Context Token and requests ACS for oAuthToken

Step 7 –> ACS server returns OAuth token to the client app

Step 8 –> Client App makes CSOM/REST calls to SharePoint site by passing OAuth Token

Step 9 –> SharePoint 2013 returns site content to App based on the App Permission Manifests

Step 10 –> Client App returns the App Content to the user

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