I am tasked with implementing single sign-on for our customer's as part of our next release. The flow exists as follows:
There are multiple options to implement SSO for a .NET application.
Check out the following tutorials online:
Basics of Single Sign on, July 2012
GaryMcAllisterOnline: ASP.NET MVC 4, ADFS 2.0 and 3rd party STS integration (IdentityServer2), Jan 2013
The first one uses ASP.NET Web Forms, while the second one uses ASP.NET MVC4.
If your requirements allow you to use a third-party solution, also consider OpenID. There's an open source library called DotNetOpenAuth.
For further information, read MSDN blog post Integrate OpenAuth/OpenID with your existing ASP.NET application using Universal Providers.
Hope this helps!