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The Basics of Building a Security Token Service

by Steve Syfuhs / October 29, 2010 04:00 PM

Last week at TechDays in Toronto I ran into a fellow I worked with while I was at Woodbine.  He works with a consulting firm Woodbine uses, and he caught my session on Windows Identity Foundation.  His thoughts were (essentially—paraphrased) that the principle of Claims Authentication was sound and a good idea, however implementing it requires a major investment.  Yes.  Absolutely.  You will essentially be adding a new tier to the application.  Hmm.  I’m not sure if I can get away with that analogy.  It will certainly feel like you are adding a new tier anyway.

What strikes me as the main investment is the Security Token Service.  When you break it down, there are a lot of moving parts in an STS.  In a previous post I asked what it would take to create something similar to ADFS 2.  I said it would be fairly straightforward, and broke down the parts as well as what would be required of them.  I listed:

  • Token Services
  • A Windows Authentication end-point
  • An Attribute store-property-to-claim mapper (maps any LDAP properties to any claim types)
  • An application management tool (MMC snap-in and PowerShell cmdlets)
  • Proxy Services (Allows requests to pass NAT’ed zones)

These aren’t all that hard to develop.  With the exception of the proxy services and token service itself, there’s a good chance we have created something similar to each one if user authentication is part of an application.  We have the authentication endpoint: a login form to do SQL Authentication, or the Windows Authentication Provider for ASP.NET.  We have the attribute store and something like a claims mapper: Active Directory, SQL databases, etc.  We even have an application management tool: anything you used to manage users in the first place.  This certainly doesn’t get us all the way there, but they are good starting points.

Going back to my first point, the STS is probably the biggest investment.  However, it’s kind of trivial to create an STS using WIF.  I say that with a big warning though: an STS is a security system.  Securing such a system is NOT trivial.  Writing your own STS probably isn’t the best way to approach this.  You would probably be better off to use an STS like ADFS.  With that being said it’s good to know what goes into building an STS, and if you really do have the proper resources to develop one, as well as do proper security testing (you probably wouldn’t be reading this article on how to do it in that case…), go for it.

For the sake of simplicity I’ll be going through the Fabrikam Shipping demo code since they did a great job of creating a simple STS.  The fun bits are in the Fabrikam.IPSts project under the Identity folder.  The files we want to look at are CustomSecurityTokenService.cs, CustomSecurityTokenServiceConfiguration.cs, and the default.aspx code file.  I’m not sure I like the term “configuration”, as the way this is built strikes me as factory-ish.

image

The process is pretty simple.  A request is made to default.aspx which passes the request to FederatedPassiveSecurityTokenServiceOperations.ProcessRequest() as well as a newly instantiated CustomSecurityTokenService object by calling CustomSecurityTokenServiceConfiguration.Current.CreateSecurityTokenService().

The configuration class contains configuration data for the STS (hence the name) like the signing certificate, but it also instantiates an instance of the STS using the configuration.  The code for is simple:

namespace Microsoft.Samples.DPE.Fabrikam.IPSts
{
    using Microsoft.IdentityModel.Configuration;
    using Microsoft.IdentityModel.SecurityTokenService;

    internal class CustomSecurityTokenServiceConfiguration
: SecurityTokenServiceConfiguration
    {
        private static CustomSecurityTokenServiceConfiguration current;

        private CustomSecurityTokenServiceConfiguration()
        {
            this.SecurityTokenService = typeof(CustomSecurityTokenService);
            this.SigningCredentials =
new X509SigningCredentials(this.ServiceCertificate);
            this.TokenIssuerName = "https://ipsts.fabrikam.com/";
        }

        public static CustomSecurityTokenServiceConfiguration Current
        {
            get
            {
                if (current == null)
                {
                    current = new CustomSecurityTokenServiceConfiguration();
                }

                return current;
            }
        }
    }
}

It has a base type of SecurityTokenServiceConfiguration and all it does is set the custom type for the new STS, the certificate used for signing, and the issuer name.  It then lets the base class handle the rest.  Then there is the STS itself.  It’s dead simple.  The custom class has a base type of SecurityTokenService and overrides a couple methods.  The important method it overrides is GetOutputClaimsIdentity():

protected override IClaimsIdentity GetOutputClaimsIdentity(
IClaimsPrincipal principal, RequestSecurityToken request, Scope scope)
{
    var inputIdentity = (IClaimsIdentity)principal.Identity;

    Claim name = inputIdentity.Claims.Single(claim =>
claim.ClaimType == ClaimTypes.Name);
    Claim email = new Claim(ClaimTypes.Email,
Membership.Provider.GetUser(name.Value, false).Email);
    string[] roles = Roles.Provider.GetRolesForUser(name.Value);

    var issuedIdentity = new ClaimsIdentity();
    issuedIdentity.Claims.Add(name);
    issuedIdentity.Claims.Add(email);

    foreach (var role in roles)
    {
        var roleClaim = new Claim(ClaimTypes.Role, role);
        issuedIdentity.Claims.Add(roleClaim);
    }

    return issuedIdentity;
}

It gets the authenticated user, grabs all the roles from the RolesProvider, and generates a bunch of claims then returns the identity.  Pretty simple.

At this point you’ve just moved the authentication and Roles stuff away from the application.  Nothing has really changed data-wise.  If you only cared about roles, name, and email you are done.  If you needed something more you could easily add in the logic to grab the values you needed. 

By no means is this production ready, but it is a good basis for how the STS creates claims.

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Steve is a renaissance kid when it comes to technology. He spends his time in the security stack.