Shane Duffy

Creating Single sign on using .NET

Posted on: August 2, 2007

We have a basic single sign-on requirement. We need to be able to allow users to login and pass that login to other distributed systems.

Here are the basic requirements:

1. We want to control the master user profile so that we can have access to the data and integrate it into our offline CRM systems.

2. We have a number of partners who provide hosted applications for us that require authentication. We need to provide a mechanism to have a single sign-on for users that can then be passed to to these partners.

3. The partners may have their own profile requirements that are independent of our needs. For example, let’s assume we have a partner that provides community forums and saves the user’s favourite forums in their profile. Only the partner cares about this information and centralizing it doesn’t add any real value.

4. The solution has to work in a distributed, multi-platform environment. The only access to and from the partners and us is through port 80 using either REST or Web Services calls.

5. The domain may not be the same and therefore you cannot share cookies (see http://www.codeproject.com/aspnet/aspnetsinglesignon.asp and www.411asp.net/func/goto?id=5998410 for a solution based on the assumption that you can simply poke the cookie created by ASP.NET)

6. There needs to be a way to transfer state from one server to another in cases where the user is logged in already. If user logs in Site A and then goes to Site B, they should appear to be still logged in.

So here is the basic design:

Single Sign On Diagram 1

1. User logs in by calling a login web service method (see this example for a web services method implementation of login). User is now authenticated on the main site.
2. User links to partner site which passes a token representation of the state (we could use the SessionID itself in .NET as a token).
3. The partner site receives the token and calls a ValidateToken web service method. This validates the passed in token and provides back the membership information for the current user.
4. If the partner site collects user information, the site can call the Update web service method to update the master profile.
5. If the user wants to link back from the partner site to the main site, the partner site provides a link back to the main site with the token embedded in the query string.
6. The main site receives the token and calls a ValidateToken web service method. This validates the passed in token and provides back the membership information for the current user.

This basic architecture should work with a couple basic caveats:

1. State needs to be managed centrally with the usual challenges of managing session state in a load balanced environment.
2. There needs to be some security around the token validation service to prevent malicious sites from getting access to the service. There are lots of solutions for this including putting the web services under VPN, including a site authentication mechanism in the web service method, filtering by IP, etc.
3. Each link needs to be dynamically re-written to include the token. If there are lots of existing cross-over points then this could be a challenge.
4. The parsing of the token needs to happen on every page request. In .NET this could be done using an HTTP Module as a central parsing code. Alternatively, you could have a redirect page that all pages link to first with the original target url embedded in a query parameter (this is how typical login pages work for example).
5. There needs to be some mechanism for keeping the state alive so that it doesn’t time out. This could again done using an HTTP Module that on every page request does a web service request to validate token to simply keep the state alive.
6. The solution assumes that there is a single backing master profile that distributes profile information. A “peer to peer” type model could be developed but it would be more complicated.

From a look and feel perspective, the solution doesn’t require a single login or registration experience but it probably would make sense. Keep the login and registration forms on the main web site and have them redirect back to the partner site if the request is coming from the partner.

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