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.NET Core Identity Server 4 Authentication VS Identity Authentication

I'm trying to understand the proper way to do authentication in ASP.NET Core. I've looked at several Resource (Most of which are out dated).
Simple-Implementation-Of-Microsoft-Identity
Introduction to Authentication with ASP.Core
MSDNs Introduction to Identity
Some people provide altenative solutions stating to use a cloud based solution such as Azure AD, or to Use IdentityServer4 and host my own Token Server.
In Older version Of .Net one of the simpler forms of authentication would be to create an Custom Iprinciple and store additional authentication user data inside.
public interface ICustomPrincipal : System.Security.Principal.IPrincipal
{
string FirstName { get; set; }
string LastName { get; set; }
}
public class CustomPrincipal : ICustomPrincipal
{
public IIdentity Identity { get; private set; }
public CustomPrincipal(string username)
{
this.Identity = new GenericIdentity(username);
}
public bool IsInRole(string role)
{
return Identity != null && Identity.IsAuthenticated &&
!string.IsNullOrWhiteSpace(role) && Roles.IsUserInRole(Identity.Name, role);
}
public string FirstName { get; set; }
public string LastName { get; set; }
public string FullName { get { return FirstName + " " + LastName; } }
}
public class CustomPrincipalSerializedModel
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
Then you would Serialize your data into a cookie and return it back to the client.
public void CreateAuthenticationTicket(string username) {
var authUser = Repository.Find(u => u.Username == username);
CustomPrincipalSerializedModel serializeModel = new CustomPrincipalSerializedModel();
serializeModel.FirstName = authUser.FirstName;
serializeModel.LastName = authUser.LastName;
JavaScriptSerializer serializer = new JavaScriptSerializer();
string userData = serializer.Serialize(serializeModel);
FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket(
1,username,DateTime.Now,DateTime.Now.AddHours(8),false,userData);
string encTicket = FormsAuthentication.Encrypt(authTicket);
HttpCookie faCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encTicket);
Response.Cookies.Add(faCookie);
}
My questions are:
How can I authenticate similar to the way done in previous version's of .Net does the old way still work or is there a newer version.
What are the pros and cons of using your own token server verses creating your own custom principle?
When using a cloud based solution or a separate Token server how would you Integrate that with your current application, would I would still need a users table in my application how would you associate the two?
Being that there are so many different solutions how can I create an enterprise application, to allow Login through Gmail/Facebook while still being able to expand to other SSO's
What are some simple implementations of these technologies?

TL;DR
IdentityServer = token encryption and validation services via OAuth 2.0/OpenId-Connect
ASP.NET Identity = current Identity Management strategy in ASP.NET
How can I authenticate similar to the way done in previous version's of .Net does the old way still work or is there a newer version.
I see no reason why you couldn't achieve the old way in ASP.NET Core, but in general, that strategy was replaced with ASP.NET Identity, and ASP.NET Identity is alive and well in ASP.NET Core.
https://learn.microsoft.com/en-us/aspnet/core/security/authentication/identity
ASP.NET Identity uses a backing store like SQL Server to hold user information like username, password (hashed), email, phone and easily be extended to hold FirstName, LastName or whatever else. So, there really no reason to encrypt user information into a cookie and pass it back and forth from client to server. It supports notions like user claims, user tokens, user roles, and external logins. Here are the entities in ASP.NET Identity:
AspNetUsers
AspNetUserRoles
AspNetUserClaims
AspNetUserLogins (for linking external identity providers, like Google, AAD)
AspNetUserTokens (for storing things like access_tokens and refresh_tokens amassed by the user)
What are the pros and cons of using your own token server verses creating your own custom principle?
A token server would be a system that generates a simple data structure containing Authorization and/or Authentication information. Authorization usually takes the for of a token named access_token. This would be the "keys to the house", so to speak, letting you through the doorway and into the residence of a protected resource, usually a web api. For Authentication, the id_token contains a unique identifier for a user/person. While it is common to put such an identifier in the access_token, there is now a dedicated protocol for doing that: OpenID-Connect.
The reason to have your own Security Token Service (STS), would to be to safeguard your information assets, via cryptography, and control which clients (applications) can access those resources. Furthermore, the standards for identity controls now exist in OpenID-Connect specifications. IdentityServer is an example of a OAuth 2.0 Authorization Server combined with an OpenID-Connect Authentication server.
But none of this is necessary if you just want a user table in your application. You don't need a token server- just use ASP.NET Identity. ASP.NET Identity maps your User to a ClaimsIdentity object on the server- no need for a custom IPrincipal class.
When using a cloud based solution or a separate Token server how would you Integrate that with your current application, would I would still need a users table in my application how would you associate the two?
See these tutorials for integrating separate identity solutions with an application:
https://identityserver4.readthedocs.io/en/latest/quickstarts/0_overview.html
https://auth0.com/docs/quickstart/webapp/aspnet-core
At a minimum you would need a two column table mapping the username to the external provider's user identifier. This is what the AspNetUserLogins table does in ASP.NET Identity. The rows in that table however are dependent on the being a User record in AspNetUsers.
ASP.NET Identity supports external providers like Google, Microsoft, Facebook, any OpenID-Connect provider, Azure AD are already there. (Google and Microsoft have already implemented the OpenID-Connect protocol so you don't need their custom integration packages either, like this one, for example). Also, ADFS is not yet available on ASP.NET Core Identity.
See this doc to get started with external providers in ASP.NET Identity:
https://learn.microsoft.com/en-us/aspnet/core/security/authentication/social/
Being that there are so many different solutions how can I create an enterprise application, to allow Login through Gmail/Facebook while still being able to expand to other SSO's
As explained above, ASP.NET Identity already does this. It's fairly easy to create an "External Providers" table and data drive your external login process. So when a new "SSO" comes along, just add a new row with the properties like the provider's url, the client id and secret they give you. ASP.NET Identity already has the UI built in there Visual Studio templates, but see Social Login for cooler buttons.
Summary
If you just need a users table with password sign in capabilities and a user profile, then ASP.NET Identity is perfect. No need to involve external authorities. But, if have many applications needing to access many apis, then an independent authority to secure and validate identity and access tokens makes sense. IdentityServer is a good fit, or see openiddict-core, or Auth0 for a cloud solution.
My apologies is this isn't hitting the mark or if it is too introductory. Please feel free to interact to get to the bulls-eye you are looking for.
Addendum: Cookie Authentication
To do bare bones authentication with cookies, follow these steps. But, to my knowledge a custom claims principal is not supported. To achieve the same effect, utilize the Claims list of the ClaimPrincipal object.
Create a new ASP.NET Core 1.1 Web Application in Visual Studio 2015/2017 choosing "No Authentication" in the dialog. Then add package:
Microsoft.AspNetCore.Authentication.Cookies
Under the Configure method in Startup.cs place this (before app.UseMvc):
app.UseCookieAuthentication(new CookieAuthenticationOptions
{
AuthenticationScheme = "MyCookieMiddlewareInstance",
LoginPath = new PathString("/Controller/Login/"),
AutomaticAuthenticate = true,
AutomaticChallenge = true
});
Then build a login ui and post the html Form to an Action Method like this:
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Login(String username, String password, String returnUrl = null)
{
ViewData["ReturnUrl"] = returnUrl;
if (ModelState.IsValid)
{
// check user's password hash in database
// retrieve user info
var claims = new List<Claim>
{
new Claim(ClaimTypes.Name, username),
new Claim("FirstName", "Alice"),
new Claim("LastName", "Smith")
};
var identity = new ClaimsIdentity(claims, "Password");
var principal = new ClaimsPrincipal(identity);
await HttpContext.Authentication.SignInAsync("MyCookieMiddlewareInstance", principal);
return RedirectToLocal(returnUrl);
}
ModelState.AddModelError(String.Empty, "Invalid login attempt.");
return View();
}
The HttpContext.User object should have your custom claims and are easily retrievable the List collection of the ClaimPrincipal.
I hope this suffices, as a full Solution/Project seems a bit much for a StackOverflow post.

TL;DR
I would really like to Show A Full posting on how to properly implement IdentityServer4 but I tried to fit All of the Text in but it was beyond the limit of what StackOverflow Accepts so instead I will right some tips and things I've learned.
What are the Benefits of using a Token Server Vs ASP Identity?
A token server, has a lot of benefit's but it isn't right for everyone. If you are implementing an enterprise like solution, where you want multiple client to be able to login, Token server is your best bet, but if you just making a simple website that want to support External Logins, You can get Away With ASP Identity and some Middleware.
Identity Server 4 Tips
Identity server 4 is pretty well documented compared to a lot of other frameworks I've seen but it's hard to start from scratch and see the whole picture.
My first mistak was trying to use OAuth as authentication, Yes, there are ways to do so but OAuth is for Authorization not authentication, if you want to Authenticate use OpenIdConnect (OIDC)
In my case I wanted to create A javascript client, who connects to a web api.
I looked at a lot of the solutions, but initially I tried to use the the webapi to call the Authenticate against Identity Server and was just going to have that token persist because it was verified against the server. That flow potentially can work but It has a lot of flaws.
Finally the proper flow when I found the Javascript Client sample I got the right flow. You Client logs in, and sets a token. Then you have your web api consume the OIdc Client, which will verify your access token against IdentityServer.
Connecting to Stores and Migrations
I had a lot of a few misconceptions with migrations at first. I was under the impression that running a migration Generated the SQL from the dll internally, instead of using you're configured Context to figure out how to create the SQL.
There are two syntaxes for Migrations knowing which one your computer uses is important:
dotnet ef migrations add InitialIdentityServerMigration -c ApplicationDbContext
Add-Migration InitialIdentityServerDbMigration -c ApplicationDbContext
I think the parameter after the Migration is the name, why you need a name I'm not sure, the ApplicationDbContext is a Code-First DbContext in which you want to create.
Migrations use some auto-magic to find you're Connection string from how your start up is configured, I just assumed it used a connection from the Server Explorer.
If you have multiple projects make sure you have the project with the ApplicationDbContext set as your start up.
There is a lot of moving parts when Implementing Authorization and Authentication, Hopefully, this post helps someone. The easiest way to full understand authentications is to pick apart their examples to piece everything together and make sure your read the documentation

ASP.NET Identity - this is the build in a way to authenticate your application whether it is Bearer or Basic Authentication, It gives us the readymade code to perform User registration, login, change the password and all.
Now consider we have 10 different applications and it is not feasible to do the same thing in all 10 apps. that very fragile and very bad practice.
to resolve this issue what we can able to do is centralize our Authentication and authorization so whenever any change with this will not affect all our 10 apps.
The identity server provides you the capability to do the same. we can create one sample web app which just used as Identity service and it will validate your user and provide s some JWT access token.

I have always used the built in ASP.NET Identity (and previously Membership) authorisation/authentication, I have implemented Auth0 recently (https://auth0.com) and recommend this as something else to try.

Social logins are not hard to implement with Identity, but there is some initial setup involved and sometimes the steps you find online in the docs are not identical, usually you can find help for that under the developers section of the platform you are trying to setup the social logins for. Identity is the replacement of the old membership functionality found in legacy versions of the .net framework.What I have found surprising is that edge use cases, like passing a jwt token you already have to a web api are not covered anywhere in the examples online even on pluralsight, I am sure you don't need your own token authority to do this but I have not found a single example on how to pass data in a get or post that isn't dealing with a self-hosted server.

Using Windows Domain accounts AND application-managed accounts

It's easy to create an ASP.NET MVC application that authenticates based on windows domain user. It's also easy to create one that uses individual accounts stored using Entity Framework. In fact, there are project templates for both.
But I want to utilize BOTH kinds of authentication in the same application. I tried to combine the code from both project templates. I have a problem in Startup.Auth.cs.
// from "Individual Accounts" template
app.UseCookieAuthentication(new CookieAuthenticationOptions
{
AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
LoginPath = new PathString("/Account/Login"),
Provider = new CookieAuthenticationProvider
{
OnValidateIdentity = SecurityStampValidator.OnValidateIdentity<ApplicationUserManager, ApplicationUser>(
validateInterval: TimeSpan.FromMinutes(30),
regenerateIdentity: (manager, user) => user.GenerateUserIdentityAsync(manager))
}
});
The existence of cookie authentication owin middleware seems to cause domain identities to become un-authenticated. If I take this line out, the domain authentication works. But without it, I can't seem to support individual user accounts.
I've downloaded the katana project source code and examined CookieAuthenticationHandler.cs, but I don't quite understand how it works in the context of an OWIN pipeline.
How can I use the ASP.net identity framework to allow my application to authenticate users from the windows domain OR an application-specific user store?

The simplest approach is to have 2 different presentation Projects only for Authentication/Authorization.
This has the advantage of leaning on existing framework and standard configuration.
From there, you decide to either
create an AD user for every internet user, or
create a DB/Internet user for every AD user.
Creating an Identity user for each AD user is easier to implement further. Then the same cookies and filters can exist in the entire app.
In that case you can either
use subdomain(s) for your app
AD Authentiction Project can have the singular purpose of Authentication / Authorization, then the Web App can represent the rest of your app.
Alternatively, If you want a truly Unified Solution, use MohammadYounes/Owin-MixedAuth
MohammadYounes/Owin-MixedAuth
Install-Package OWIN-MixedAuth
In Web.config
<location path="MixedAuth">
<system.webServer>
<security>
<authentication>
<windowsAuthentication enabled="true" />
</authentication>
</security>
</system.webServer>
</location>
In in Startup.Auth.cs
app.UseMixedAuth(cookieOptions);
:
:
How it works:
The handler uses ApplyResponseChallengeAsync to confirm the request is a 401 challenge. If so, it redirects to the callback path to request authentication from IIS which is configured to query the AD.
AuthenticationResponseChallenge challenge = Helper.LookupChallenge(
Options.AuthenticationType, Options.AuthenticationMode);
A 401 challenge is caused by an unauthorized users attempting to use a resource that requires Authentication
The handler uses InvokeAsync to check if a request is coming from a callback path (IIS) and then calls AuthenticateCoreAsync
protected async override System.Threading.Tasks.Task<AuthenticationTicket>
AuthenticateCoreAsync()
{
AuthenticationProperties properties = UnpackStateParameter(Request.Query);
if (properties != null)
{
var logonUserIdentity = Options.Provider.GetLogonUserIdentity(Context);
if (logonUserIdentity.AuthenticationType != Options.CookieOptions.AuthenticationType
&& logonUserIdentity.IsAuthenticated)
{
AddCookieBackIfExists();
ClaimsIdentity claimsIdentity = new ClaimsIdentity(
logonUserIdentity.Claims, Options.SignInAsAuthenticationType);
// ExternalLoginInfo GetExternalLoginInfo(AuthenticateResult result)
claimsIdentity.AddClaim(new Claim(ClaimTypes.NameIdentifier,
logonUserIdentity.User.Value, null, Options.AuthenticationType));
//could grab email from AD and add it to the claims list.
var ticket = new AuthenticationTicket(claimsIdentity, properties);
var context = new MixedAuthAuthenticatedContext(
Context,
claimsIdentity,
properties,
Options.AccessTokenFormat.Protect(ticket));
await Options.Provider.Authenticated(context);
return ticket;
}
}
return new AuthenticationTicket(null, properties);
}
AuthenticateCoreAsync uses AddCookieBackIfExists to read the claims cookie created by AD and creates it's own Claims based.
AD users are provided a Claims based Cookie identical to Web Users. AD is now like any other 3rd party authenticator (Google, FB, LinkedIN)

It's for this reason that I haven't been able to use pre-baked solutions for authentication. In our project, the passing years (and agile approach) have left us with 4 different ways to authenticate which is annoying, but we support all legacy versions of apps in the field so we have to preserve it all (at least for now).
I ended up creating a factory that figures out the authentication mechanism (through any of several means such as token format, presence of some other thing) and then returns a wrapper that carries the logic for validating that authentication method and setting the principal.
This gets kicked off in a custom HTTP module so that the principal is built and authenticated before the request gets to the controller. In your case, windows Auth would be the final fallback, I think. In our Web API application, we took the same approach but through a delegating handler instead of HTTP module. It's a type of local token federation, you could say. The current implementation allows us to add or modify any validation procedure, or add any other token format; in the end, the user ends up with a proper identity or gets denied. Only took a few days to implement.

It seems to me the best answer to this question is to use an authentication and authorization framework. There are plenty to choose from (both commercial and free). You could, of course, write your own but I would discourage it. Lots of very smart people get this wrong.
I would take a look at IdentityServer3. It's certainly not the only solution but its a pretty good authentication and authorization framework. It's open source and pretty easy to get up and running in a hurry. Your use case is a common one and you will find some very useful information at the link above. Clean separation between authorization and authentication, social authentication options, easy to work with json web tokens that encapsulate user claims, etc.
How it can help you
IdentityServer3 allows you to configure Identity Providers to handle authentication and there are plenty of extension points that will allow you to implement a chain of responsibility that can handle both of your scenarios. From the docs:
IdentityServer supports authentication using external identity providers. The external authentication mechanism must be encapsulated in a Katana authentication middleware.
Katana itself ships with middleware for Google, Facebook, Twitter, Microsoft Accounts, WS-Federation and OpenID Connect - but there are also community developed middlewares (including Yahoo, LinkedIn, and SAML2p).
To configure the middleware for the external providers, add a method to your project that accepts an IAppBuilder and a string as parameters.
IdentityServer3 supports AD as an identity providor via a browser login window and will support a more programmatic implementation via a custom grant. You can also take a look here for some more information on IdentityServer3 and AD authentication.
It will support windows authentication as well and you can take a look at here for information and examples on implementing that.
There is a pretty good getting started example here as well.
With the right configuration IdentityServer3 can handle your requirements. You will need to implement your own authentication providers and plug them into the framework but there isn't much more to it than that. As for authorization goes, there are plenty of options there as well.

Using OAuth to connect to a Windows Azure Active Directory

In the scaffolding for an ASP.NET MVC project, the StartUp.Auth.cs file currently contains this code:
public partial class Startup
{
// For more information on configuring authentication, please visit http://go.microsoft.com/fwlink/?LinkId=301864
public void ConfigureAuth(IAppBuilder app)
{
// Enable the application to use a cookie to store information for the signed in user
app.UseCookieAuthentication(new CookieAuthenticationOptions
{
AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
LoginPath = new PathString("/Account/Login")
});
// Use a cookie to temporarily store information about a user logging in with a third party login provider
app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);
// Uncomment the following lines to enable logging in with third party login providers
app.UseMicrosoftAccountAuthentication(
clientId: "0000000000000000",
clientSecret: "xxxx-xxxxxxxxxxxxxxxxxxx-xxxxxxx");
//app.UseTwitterAuthentication(
// consumerKey: "",
// consumerSecret: "");
//app.UseFacebookAuthentication(
// appId: "",
// appSecret: "");
//app.UseGoogleAuthentication();
}
}
Uncommenting the app.UseXxxAuthentication() lines and adding in your provider's key and secret gives you the ability to use the respective providers to perform OAuth logins. Under the covers, these methods use classes derived from the Owin class AuthenticationMiddleware.
I have looked on the web, but I cannot find a custom implementation of AuthenticationMiddleware that links directly to a Windows Azure Active Directory instance. Are there any such implementations?
Is this the right way to use OAuth to connect to my Windows Azure Active Directory instance?

You should be able to go to your Package Manager, and NuGet import the Katana Owin implementations for Windows Azure AD, which will be listed as Microsoft.Owin.Security.ActiveDirectory This is the middleware that enables an application to use Microsoft's technology for authentication. The current version as of this post is 2.0.2
Once you have that, you should be able to leverage the middleware for AD and ADFS 2.1 oAuth tokens like so:
WindowsAzureActiveDirectoryBearerAuthenticationOptions myoptions = new WindowsAzureActiveDirectoryBearerAuthenticationOptions();
myoptions.Audience = "https://login.windows.net/myendpoint";
myoptions.Tenant = "mydirectory.onmicrosoft.com";
myoptions.AuthenticationMode = Microsoft.Owin.Security.AuthenticationMode.Passive;
app.UseWindowsAzureActiveDirectoryBearerAuthentication(myoptions);
That should give you the ability to have the Owin middleware use Windows Azure AD Bearer Authentication in this scenario.
Happy coding!

I don't believe you can use WAAD in this way. Microsoft Account is for what used to be Windows Live ID (More information here), and this is different from WAAD. And the OAuth implementation in WAAD is not complete yet and in preview (more details here). The best way to use WAAD today is via WS-Federation / WIF.
The pain point in VS 2013 is that you can't do it easily manually, nor you can change the selected authentication once you created the project.
The easiest way to get the required configuration is to go and create new web app, and change the authentication. Chose Change Authentication at the very first step of the wizard (where you select the type of App - MVC, WebAPI, etc.). Then choose Organizational Account. It has only one option - Cloud single organization - enter your tenant domain name (may be the xxxx.onmicrosoft.com). And chose access level (Single Sign On, SSO + read directory data, SSO + read + write directory data). Next you will be asked to sign in with account which is Global Administrator in this Active Directory. The wizard will create necessary web.confg changes and Identity configuration.
There still no support in OWIN for WAAD, and it will create a new IdentityConfig.cs instead Startup.Auth.cs file. You can then copy generated files and web.config changes into your project. You can still combine WAAD with other providers and OWIN, but this still requires more advanced skills.
It is a little more complicated that it should be. But things may change for good in the future.

There is a new Owin middleware that adds Ws Federation authentication to your site with a few simple lines of code much like the individual account examples in the new MVC project template. It's currently in alpha but here is a link to an article explaining how to create your app in Windows Azure Active Directory and configure the OWIN middleware.
However this uses cookie authentication rather than OAuth tokens but this should be sufficient for a pure ASP MVC site.

WCF same Identity as on client?

For an experimenting project I'am struggling with a service. The client is an ASP.NET MVC 4 and the service will be build with WCF. For now all the systems are in a trusted subsystem so SSL/certificates is not necessarily.
The problem I'am currently dealing with is: Is het possible when I create a new GenericIdentity like:
IIdentity newIdentity = new GenericIdentity("Test", "Custom authentication");
string[] newRoles = { "TestRole" };
IPrincipal testPrincipal = new GenericPrincipal(newIdentity, newRoles);
Thread.CurrentPrincipal = testPrincipal;
It is possible to have the created user when I'am on the WCF service calling the following code:
ServiceSecurityContext.Current.WindowsIdentity;
Or
ServiceSecurityContext.Current.PrimaryIdentity;
Or
Thread.CurrentPrinicpal;
I get the user which I created in the client? Or do I have to write a WCF extensibility for this?
I'm currently using a WsHttpBinding and security mode Transport and clientCredentialType: Windows. Maybe something wrong about the configuration?

On server side (WCF), the security context will have the username value you have used to authenticate to the service.
This means, that if you have defined on client side windows authentication, WCF will find the security context filled with the username of the windows identity you used.
In order for you to authenticate to the servive with windows account, you can use the following code
on client side :
channelFactory.Credentials.Windows.ClientCredential =
new NetworkCredential(username, password, domain);
Having used this code, you can access the identity on server side with the below code :
OperationContext.Current.ServiceSecurityContext.PrimaryIdentity
In general, try to be explicit on client side authentication info. Currently, you seem to
be simply using the default behavior of windows authentication schema, by setting identity
on current thread.
I hope this helps.

I discovered to achieve this automatically in WCF that you must specify a certificate...
In the messages that are sent to the WCF service I created a few properties with the information in the datacontracts that is necessary to create a GenericPrinicpal (in this the username en the roles). Because I have an trusted subsystem I don't want to authenticate the user on each WCF-service (to much overhead for my scenario). See also Trusted subsystem
With this I created a WCF extension that implemented a parameter inspector and with reflection I set the credentials from the CurrentPrincipal in the specified properties of the contract. For more information about parameter inspectors and how to apply it

Adding a WIF STS service to existing projects

We have several websites that are set up in the following fashion:
Site1.Web - ASP.NET Web Project (.NET 4.0, WebForms)
Common.Core - Class Library Project (all db interaction)
The web project appears once for each site while the Common.Core project is shared among all sites. We have a login form in the web project that, in order to authenticate, calls into the class library. It would call off a code similar to below:
Common.Core.Authenticate auth = new Common.Core.Authenticate(conStr);
bool validLogin = auth.ValidateUser(userName, password);
if(validLogin)
{
Common.Core.User = auth.GetCurrentUser();
}
The higher ups are pushing for a middle layer service/app tier and want to use a more elegant solution to handle single sign on. Therefore, the decision has been made to use a WIF service to handle the login and validation. Furthermore, we want to try to minimize the code that has to change in each web project, i.e. try to keep as many changes as possible in Common.Core.
I have seen a few samples that show how to add an STS reference to a web project. This would work well in a scenario where the user validation isn't factored into another project like Core.Common. However, in our scenario, how could we handle validation while still going through the common class library?
Ideally, I would like to add an STS reference to the Core.Common class library and replace the direct db logic (auth.ValidateUser above) with a call to an STS service. However, is it even possible to do that? Does the request have to initiate in the web project? If so, is the STS reference required in both places?
Any tutorials or resources which follow the same web project -> class library -> STS service path would be greatly appreciated.

I would also recommend using WIF :-)
In a claims based scenario the authentication process is "reversed". Your app will not call anyone, it will receive the needed information from a trusted source (the STS).
The "STS Reference" is not a library reference. It's a logical connection between your app and the trusted source of security tokens. A token is the artifact your app will use to decide what to do with the user request.
I'd agree with #nzpcmad that it is likely you could entirely remove the calls to you Common.Core library. It might be useful to see what else can you do with it. What does the Common.Core.User object give you?
If it is just properties of a user (e.g. name, e-mail, roles, etc) it is very likely you could just create a new version that simply wraps the IPrincipal supplied byt WIF (a ClaimsPrincipal).
For example (approx. no error handling, pseudo-code):
public User CurrentUser()
{
var user = new User();
var cu = HttpContext.Current.User as IClaimsPrincipal;
user.Name = cu.Name;
user.eMail = (cu.Identity as IClaimsIdentity).Claims.First( c=> c.ClaimType = "eMail" ).Value;
return user;
}
As #nzpcmad says, you can use ADFS or some other STS. Your apps would not care.

One way to achieve this is to FedUtil the ASP.NET project with an instance of ADFS. Essentially, authentication is now "outsourced" and you can simply remove the call to the core library from your app. ADFS is then setup to return whatever attributes the program needs for authorisation as claims. You may need to transform these claims attributes to whatever attributes are passed back to the common core in subsequent calls.
Or you could make the common core "claims aware" in the sense that it now recognizes "claims attributes" as opposed to "common core" attributes. This involves using different .NET classes - no hookup to ADFS is required.
Word of warning - your authentication seems to be all DB related. ADFS cannot authenticate against a DB. It can only authenticate against an instance of AD in the domain that ADFS is installed in (or other AD if trust relationship between AD).
If you want to authenticate against a DB you need a custom STS which is then federated with ADFS. See here: Identity Server.