MVC WebAPI authentication from Windows Forms

Just create authentication token on server-side and store it in your database or even in cache. Then send this token with requests from your win forms application. WebApi should check this token all the time. It's good enough and you have full control over your auth process.

Let me share, how it works for me:

Object with Auth details:

public class TokenIdentity
{
    public int UserID { get; set; }

    public string AuthToken { get; set; }

    public ISocialUser SocialUser { get; set; }
}

Web API Auth Controller:

  public class AuthController : ApiController
    {
        public TokenIdentity Post(
            SocialNetwork socialNetwork,
            string socialUserID,
            [FromUri]string socialAuthToken,
            [FromUri]string deviceRegistrationID = null,
            [FromUri]DeviceType? deviceType = null)
        {
            var socialManager = new SocialManager();

            var user = socialManager.GetSocialUser(socialNetwork, socialUserID, socialAuthToken);

            var tokenIdentity = new AuthCacheManager()
                .Authenticate(
                    user,
                    deviceType,
                    deviceRegistrationID);

            return tokenIdentity;
        }
    }

Auth Cache Manager:

public class AuthCacheManager : AuthManager
    {
        public override TokenIdentity CurrentUser
        {
            get
            {
                var authToken = HttpContext.Current.Request.Headers["AuthToken"];
                if (authToken == null) return null;

                if (HttpRuntime.Cache[authToken] != null)
                {
                    return (TokenIdentity) HttpRuntime.Cache.Get(authToken);
                }

                return base.CurrentUser;
            }
        }

        public int? CurrentUserID
        {
            get
            {
                if (CurrentUser != null)
                {
                    return CurrentUser.UserID;
                }
                return null;
            }
        }

        public override TokenIdentity Authenticate(
            ISocialUser socialUser, 
            DeviceType? deviceType = null, 
            string deviceRegistrationID = null)
        {
            if (socialUser == null) throw new ArgumentNullException("socialUser");
            var identity = base.Authenticate(socialUser, deviceType, deviceRegistrationID);

            HttpRuntime.Cache.Add(
                identity.AuthToken,
                identity,
                null,
                DateTime.Now.AddDays(7),
                Cache.NoSlidingExpiration,
                CacheItemPriority.Default,
                null);

            return identity;
        }
    }

Auth Manager:

 public abstract class AuthManager
    {
        public virtual TokenIdentity CurrentUser
        {
            get
            {
                var authToken = HttpContext.Current.Request.Headers["AuthToken"];
                if (authToken == null) return null;

                using (var usersRepo = new UsersRepository())
                {
                    var user = usersRepo.GetUserByToken(authToken);

                    if (user == null) return null;

                    return new TokenIdentity
                    {
                        AuthToken = user.AuthToken,
                        SocialUser = user,
                        UserID = user.ID
                    };
                }
            }
        }

        public virtual TokenIdentity Authenticate(
            ISocialUser socialUser, 
            DeviceType? deviceType = null, 
            string deviceRegistrationID = null)
        {
            using (var usersRepo = new UsersRepository())
            {
                var user = usersRepo.GetUserBySocialID(socialUser.SocialUserID, socialUser.SocialNetwork);

                user = (user ?? new User()).CopyFrom(socialUser);

                user.AuthToken = System.Guid.NewGuid().ToString();

                if (user.ID == default(int))
                {
                    usersRepo.Add(user);
                }

                usersRepo.SaveChanges();

                return new TokenIdentity
                {
                    AuthToken = user.AuthToken,
                    SocialUser = user,
                    UserID = user.ID
                };
            }
        }
    }

Global Action Filter:

public class TokenAuthenticationAttribute : System.Web.Http.Filters.ActionFilterAttribute
{
    public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
    {
        if (actionContext.Request.RequestUri.AbsolutePath.Contains("api/auth"))
        {
            return;
        }

        var authManager = new AuthCacheManager();

        var user = authManager.CurrentUser;

        if (user == null)
        {
            throw new HttpResponseException(HttpStatusCode.Unauthorized);
        }

        //Updates the authentication
        authManager.Authenticate(user.SocialUser);
    }
}

Global.asax registration:

GlobalConfiguration.Configuration.Filters.Add(new AuthFilterAttribute());

The idea is that AuthCacheManager extends AuthManager and decorates it's methods and properties. If there is nothing inside cache then go check database.

You could use token based authentication. Here's a great article illustrating how you could write a custom action filter that uses RSA public/private cryptography.