Comments by geo_m (Top 2 by date)

That would need deeper knowledge of the situation. As you probably need only authentication, while using TLS, you may just use basic authentication (due to .net4 tag assuming you're using IIS). Or if the situation allows, you might consider using NTLM (Windows Authentication). Both of them will be handled by IIS for you automatically.
Also for advanced stuff you may look at http://msdn.microsoft.com/en-us/library/ff359110.aspx (but I personally never did something with that as it is not suitable to our needs :( )

Some other stuff would probably need code changes.