Decentralized Authorization Flow
In this section, we're going to examine the decentralized authorization flow.
Before we go into the details of the authorization flow, it's important to understand the subsystems.
ZMS (AuthZ Management System)
ZMS is where domains, roles, and policies are defined. This is Athenz's centralized authorization system and is likely part of a larger management system. In addition to allowing CRUD operations on the basic entities, ZMS provides an API to replicate the entities, per domain, to ZTS. It also can directly support the access check, both for internal management system checks, as well as a simple centralized deployment.
ZMS is the source of truth for domains, roles, and policies for centralized authorization. ZMS supports a centralized call to check if a principal has access to a resource. Because ZMS supports service identities, ZMS can authenticate services.
SIA (Service Identity Agent)
SIA is required for authenticating existing services. Any service should integrate SIA (or an equivalent system). To confirm a service's identity, SIA communicates with ZTS.
ZTS (AuthZ Token System)
ZTS, the authentication token service, is only needed to support decentralized or data plane functionality. In many ways, ZTS is like a local replica of ZMS's data to check a principal's authentication and confirm membership in roles within a domain. The authentication is in the form of a signed Access Token that can be presented to any decentralized service that wants to authorize access efficiently. If needed, multiple ZTS instances will be distributed to different data centers as needed to scale for issuing tokens.
ZPE (AuthZ Policy Engine)
Like ZTS, ZPE, the authorization policy engine is only needed to support the decentralized authorization. ZPE is the subsystem of Athenz that evaluates policies for a set of roles to yield an allowed or a denied response.
ZPE is a library that your service calls and only refers to a local policy cache for your services domain (a small amount of data).
ZPU (AuthZ PolicyEngine Updater)
Like ZTS and ZPE, ZPU is only needed to support the decentralized authorization. The policy updater is the utility that retrieves from ZTS the policy files for provisioned domains on a host, which ZPE uses to evaluate access requests.
Decentralized Access Control
A more interesting scenario introduces the local policy engine (ZPE), and a few supporting changes. Rather than directly asking for an access check with a principal identity, the identity is instead used to get an Access Token, and that is presented to the target service until it expires. This mechanism allows a service to make a completely local access check against ZPE, given an Access Token and locally cached policies.
Specifically, as shown in the diagram below, the client service/user uses its identity x.509 certificate along with its private key that it got from local SIA (1) to get an authorization token (Access Token) from ZTS (2), and then presents the access token to a target service to access its resources (3).
That service can make use of a local ZPE to validate the role assertions in the access token, and then correlate them to the asynchronously updated Policies for the domain involved. The ZPE can make the decision locally, without going off box. It has no knowledge about specific users, only roles, and because roles and policies are always in the domain, the amount of data to distribute to the machine is small.
The model works exactly the same when using X.509 certificates instead of tokens. The service would use its identity X.509 certificate to request a X.509 Role Certificate from ZTS and then would make the call to the target service using the X.509 Role Certificate. The target service would first validate the X.509 Certificate and call ZPE to extract the role certificate to carry out the authorization check.