Doorkeeper is a gem (Rails engine) that makes it easy to introduce OAuth 2
provider functionality to your Ruby on Rails or Grape application.

* The OAuth 2.0 Authorization Framework
  - Authorization Code Flow
  - Access Token Scopes
  - Refresh token
  - Implicit grant
  - Resource Owner Password Credentials
  - Client Credentials
* OAuth 2.0 Token Revocation
* OAuth 2.0 Token Introspection
* OAuth 2.0 Threat Model and Security Considerations
* OAuth 2.0 for Native Apps
* Proof Key for Code Exchange by OAuth Public Clients