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Although UMA's primary use cases have centered on individual people (that is, the the "users" who managed manage access to their own online resources), the UMA notion of authorization as a service also has relevance to modern enterprises that must secure APIs and other web resources in a developer-friendly way.
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APIs by their nature are subdivisions of functionality exposed at a single domain, and would map well to arbitrarily fine-grained policy, for example, at the call method or even parameter level. However, outside the use of XACML, authorization policy granularity is coarse in traditional solutions: group filtering or URL regular expression matching at most. Further, it is often impractical to act according to policies from multiple authoritative sources.
Proposed Improvements
UMA makes the following solutions possible.
As a profile of OAuth 2.0 (IETF RFCs 6749 and 6750) that is complementary to OpenID Connect, UMA defines RESTful, JSON-based, standardized flows and constructs for coordinating the protection of any API or web resource in a way that will be familiar to any developer already acquainted with OAuth. Mobile developers accept technologies that use HTTP and JSON at their core.
UMA's notion of machine-readable resource set and scope descriptions creates an access control mechanism that enables control over specific API scopes (completely customizable buckets of API functionality), not just domains. With UMA, client app developers can handle authorization tasks by calling simple REST/JSON endpoints; administrators don't have to deploy a web server agent or reverse proxy to enable centralization.
UMA defines interfaces between authorization servers and resource servers that, by default, enable centralized policy decision-making for improved service delivery, auditing, policy administration, and accountability, even in a very loosely coupled "public API" environment. Custom profiles enable flexibility to move the decision-making line outward to distributed applications, to account for local preferences in API ecosystems. UMA does not standardize a policy expression language, enabling the enabling flexibility in policy expression and evaluation through XACML, other declarative policy languages, or procedural code as warranted by conditions. It also has a fluid way to handle federated authorization policy.
UMA inherits authentication agnosticism from OAuth. It concentrates on authorization, not on authentication. It has been profiled to work with OpenID Connect to gather identity claims from whoever is attempting access, and enables true claims-based authorization (with simple group- or role-based policies a natural subset).
Solution Scenario
In UMA trust model terminology, this scenario is in the category non-person entity (NPE) to person sharing. An organization – say, BusinessCo -- is the resource owner (technical term) and the Authorizing Party (contractual term), acting on its own behalf, protecting its own resources. A human "resource owner agent" acts in a IT administrator role.BusinessCo would run a service that does whatever elements of the authorization job it has chosen to centralize; this is the UMA authorization server. Think of this publishes numerous APIs. BusinessCo authorizes which people, clients, and networks can access a subset of the APIs. BusinessCo, as a subsidiary of ParentCo, relies on ParentCo to provide some of the required authorization policies.
When BusinessCo publishes and UMA-protects an API, its resource server acts as a policy enforcement point (PEP). The RS relies on an UMA authorization server to as a policy decision point (PDP) , though UMA's default profile gives less than 100% of the decision-making responsibility to it (the authorization server may in turn outsource actual decision-making to an XACML PDP or some other web service). This service would also expose policy administration point (PAP) functions to the IT administrator in some fashion. The service for it. The PEP can query multiple authorization servers to ensure that all required authorizations have been granted, for example the policies of both BusinessCo and ParentCo. In UMA trust model terminology, this scenario is in the category non-person entity (NPE) sharing because the resource owner is a corporate entity that also operates its own authorization server.
Enterprise resource users such as employees and partners operate client applications, such as web and mobile apps, in order to request access. How the respective PDPs make authorization decisions about these users and clients is up to the implementation. Each authorization server may itself be a policy information point (PIP) at which policies reside, or may call out to it may be a client of one or more PIPs.
BusinessCo's protected resources would sit behind the enterprise apps and APIs it has chosen to expose; these are the UMA resource servers. Think of these as policy enforcement points (PEPs), though UMA's default profile gives a bit of decision-making responsibility to them.
Client web or mobile applications, wielded by enterprise resource users such as employees and partners, are UMA clientsPIP services. To understand the nature of the requesting party, a PDP might be sent SAML- or OpenID Connect-based tokens by the requesting party's client, or may itself need to call out to an identity provider.
The PDP service would expose policy administration point (PAP) functions to IT administrators in some fashion. BusinessCo would need the capability to express their access policies; for example, BusinessCo or ParentCo might interface a standard enterprise entitlements management system that expresses polices in XACML or some other standard language.
Solution Flow
This scenario uses ordinary UMA flows, noting that it is a human resource server agent"Authorizing Party Agent", not BusinessCo, that sets policy and possibly performs any policy-dictated manual intervention to enable access requests to go through. The company Gluu has produced a swimlane diagram to illustrate.
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Gluu gave a demo of an enterprise UMA scenario at an UMA WG meeting on Jan 10, 2013; see the meeting notes. Also see their series of demo videos on YouTube and information on their open-source implementation.