To protect enterprises API, the user credential, such as
user account, should be identified; therefore the IDM and
LADP are adopted for authenticating users outside of
enterprises and internal users, respectively.
OAuth2 is open authorization protocol and becomes a
RFC standard rfc6749 [12], which creates a consistent,
flexible, and policy-based authorization framework for web
applications, RESTful web services, mobile devices, and
browsers attempting to communicate with REST API,
specifically the cloud API. Figure 3 shows that the OAuth2
clearly separates the role of authorization from access control
by two different participants: authorization server and
resource server. Therefore, users can use a central
authorization server for accessing multiple resource servers. It
is similar to traditional Single Sign-On (SSO) architecture.
Moreover, the functionality of an OAuth2 authorization
server is same as an API Security Token Service (STS).