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Windows Azure platform AppFabric Access Control: Service Namespace

In the previous article, I provided an example scenario of how claims-based access control might map to the real-world event of a customer entering a bar.  This article will build upon that concept, eventually enabling a Customer application to obtain a token from our Bouncer (ACS), and then use that token to order a drink from the Bartender web service.

Now let’s walk through how to create a Service Namespace in Windows Azure, which is needed to permit your application and/or web service to utilize AppFabric Access Control.

Service Namespace

At this time, Windows Azure platform AppFabric provides two services for developers to use: Service Bus and Access Control.  Within a Project (i.e., the unit of billing for an Azure account), a “Service Namespace” is used to organize a set of these AppFabric service endpoints.  Services within a Service Namespace share the same DNS naming convention, geography (datacenter where the services are hosted) and owner key (security for managing the services). 

To create a new Service Namespace, start by logging into the Windows Azure portal (http://windows.azure.com).  Navigate to the AppFabric section, choose the Project in which the Service Namespace will be created, and then click the “Add Service Namespace” link.  The next screen will prompt for a Name, a Region, and the size of the ServiceBus Connection Packs for the new Service Namespace: 

create_service_namespace

  • The name must be unique among all Service Namespaces in existence because this name will be incorporated into DNS aliases.  The Windows Azure portal provides a “Validate Name” function to ensure that the name entered meets the required syntax criteria and that it has not already been used.
  • Region maps to the Microsoft data center where the services will run.  Ideally, this should be as close to the users as possible.
  • “ServiceBus Connection Packs” is to support the Windows Azure platform AppFabric Service Bus.  Since this article is only examining Access Control, the number of ServiceBus connections can be set to zero.

Once you click the “Create” button, Windows Azure will generate the services and DNS entries for your new Service Namespace.  However, looking at the information page on the portal for the Service Namespace reveals nothing that is noticeably useful for configuring these services:

service_namespace_info

Configuring Access Control

On the portal, the only actions that you can perform are “Generate a New [Management] Key” and “Delete the Service Namespace”.  How then do we configure Access Control?

Access Control, like most things in Windows Azure, is configured via a set of web services.  While robust tooling is something that is currently being developed (or something that you could develop on your own), there are a few demo tools included as source code in the Windows Azure platform AppFabric SDK V1.0, including a Windows (GUI) application called Access Control Service Management Browser (AcmBrowser).

By default, after installing the SDK, you can find the Visual Studio solution for the AcmBrowser application in:

C:\Program Files\Windows Azure platform AppFabric SDK\V1.0\Samples\AccessControl\ExploringFeatures\Management\AcmBrowser

To use this application, you will need the Service Namespace’s name (the one that you entered during the creation process) and Management Key for that namespace (this is “Current Management Key” string on the screenshot above).  After providing these values to AcmBrowser, you can then click the “Load from the Cloud” button to see what is already configured.  As you would expect, new namespaces will have nothing pre-configured, as indicated by the “Resources” tree on the left showing Issuers, Scopes, and Token Policies, but these nodes contain no children.

mgt_browser_1  

 

Token Policy

The first thing that we need to create is a Token Policy.  This item defines a length of time, in seconds, that a token will be valid until it expires, as well as a symmetric key that is used to sign tokens.  This symmetric key will be shared with our Bartender web service, but not the Customer application (else the customer would be able to generate tokens themselves, bypassing ACS and being capable of spoofing any claim).

To create a Token Policy using AcmBrowser, right click on the “Token Policy” node and choose “Create…”:

create_new_tokenpolicy

In this example, tokens created using the “BouncerPolicy” will expire 12 hours (60 * 60 * 12 = 43200 seconds) after they have been generated, and the Signing Key was created by clicking the “Generate” button.  Should this key ever be compromised, a new key can be generated in the same way.

 

Scope

ACS is not an identity provider.  You don’t maintain users in ACS, but rather, maintain a list of trusted identity providers (Issuers) that can supply claims on behalf of their users.  With this federated model, the claims presented may be named different things, but convey the same information.  For example, a claim containing a Birthdate may be named “DOB” by one Issuer and “Date of Birth” by another Issuer, but both sources supply the same type of information. 

The web service that will eventually consume these claims, however, shouldn’t be concerned with knowing all of the variations.  Instead, ACS provides Claim Mapping capabilities that can rename claims, or express a new claim based on the presence of a known claim in the token request.  Before we can configure the claim mappings, though, we need to define a Scope and Issuers.

During the token request, one piece of information that is required is the URL where the token will eventually be used (known as the “Applies To”).  ACS uses this in order to know what set of rules to apply when generating that token. 

A Scope is the mechanism that ties a URL to a Token Policy, and is created within AcmBrowser by right clicking on the “Scopes” node and choosing “Create…”.  In this example, our Scope is defined with the Applies To URL of the Bartender web service and associated to the BouncerPolicy Token Policy.  Note that this “Applies To” URL is actually a substring of the real web service URL; any URL that is provided during the token request that starts with this same “Applies To” string will match to this scope, and it’s possible that an actual URL will match multiple Scopes (allowing for chaining of rules from the longest substring match to the shortest).

create_new_scope

 

Issuers

In a Claims-Based Identity and Access Control system, the destination application or web service does not concern itself with end-user authentication.  Instead, the user authenticates itself with its own identity management system (known as an Issuer), and this system then becomes a trusted source of claims about that user.  This Issuer might be a standalone application, a custom web service running on an intranet server, another AppFabric Service Namespace, or even an Active Directory Federation Services 2.0 system.  As a result, there are different ways to prove that information presented during a token request came from a trusted Issuer.

In the nightclub example, the customer presents a driver’s license that was issued by their state of residence.  Keeping with this concept, let’s define a few Issuers in ACS named after states (Ohio, Michigan, Indiana, etc) by first right clicking on the “Issuers” node in AcmBrowser and selecting “Create…”:

create_new_issuer

ACS is able to recognize and validate a token request originating from an Issuer in many different ways.  In this example, the “Ohio” Issuer is defined using a Symmetric 256-bit Key (other choices that could have been used include X509 and FedMetadata).  After sharing this secret with the Issuer, the “Issuer Name” and “Current Key” can be provided as claims during a token request, much like a username and password, or the key can be used by the Issuer to sign a token request.  But, more on that in another article.

 

[Scope] Rules

During the token request, an arbitrary number of claims will be presented to ACS.  The application that will finally consume the token generated by ACS may only require a subset of those claims, if any.  To decide what claims should be included in the output token, we need to create a set of Rules (one per output claim).

To create rules for a Scope, you first expand the Scope’s node in AcmBrowser, right-click on the “Rules” node, and then choose “Create…”:

Scope_Rules_Node

A Rule can be defined as either PassThrough or Simple.  A PassThrough rule is used to rename a claim from whatever the Issuer called it to whatever the final application will expect it to be called (the value will be preserved).  A Simple rule, on the other hand, will generate a claim (name and value) in the output if a specific claim (name and value) was present in the input.  Note that the input and output claim values in a Simple rule must be explicitly defined – you cannot use pattern matching, etc.

For the example scenario, we’ll assume that the Ohio Issuer uses “DOB” as the name of the “Birthdate” claim that the Bartender web service will be expecting.  A PassThrough rule is used to rename that claim so that the resulting token will include a claim called “Birthdate” instead (the provided value will be preserved).

create_new_rule

 

Wrapping Up

At this point, everything has been defined in memory on your machine, and nothing has actually been saved to the cloud.  To commit your changes, you will need to click on the “Save to Cloud” button in AcmBrowser.  Note that AcmBrowser is not sophisticated enough to merge with an existing set of configuration, so if your Service Namespace is not already empty, then you will need to click the “Clear Service Namespace in Cloud” button prior to saving to the cloud.  You can also load and save local copies of the configuration using the tool, which is useful for backup purposes as well as working on a particular configuration over the course of multiple days without disturbing what is already running in the cloud.

In the next part of this series, I’ll show code for how everything finally comes together for this contrived example.

(Continued in Part 3)

Further Reading: