namespace DNT.IDP
{
    public class Startup
    {
        public const string TwoFactorAuthenticationScheme = "idsrv.2FA";

        public void ConfigureServices(IServiceCollection services)
        {
            // ...

            services.AddAuthentication()
                .AddCookie(authenticationScheme: TwoFactorAuthenticationScheme)
                .AddGoogle(authenticationScheme: "Google", configureOptions: options =>
                {
                    options.SignInScheme = IdentityServerConstants.ExternalCookieAuthenticationScheme;
                    options.ClientId = Configuration["Authentication:Google:ClientId"];
                    options.ClientSecret = Configuration["Authentication:Google:ClientSecret"];
                });
        }

namespace DNT.IDP.Controllers.Account
{
    [SecurityHeaders]
    [AllowAnonymous]
    public class AccountController : Controller
    {
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Login(LoginInputModel model, string button)
        {
    // ... 

            if (ModelState.IsValid)
            {
                if (await _usersService.AreUserCredentialsValidAsync(model.Username, model.Password))
                {
                    var user = await _usersService.GetUserByUsernameAsync(model.Username);
                    
                    var id = new ClaimsIdentity();
                    id.AddClaim(new Claim(JwtClaimTypes.Subject, user.SubjectId));
                    await HttpContext.SignInAsync(scheme: Startup.TwoFactorAuthenticationScheme, principal: new ClaimsPrincipal(id));

                    await _twoFactorAuthenticationService.SendTemporaryCodeAsync(user.SubjectId);

                    var redirectToAdditionalFactorUrl =
                        Url.Action("AdditionalAuthenticationFactor",
                            new
                            {
                                returnUrl = model.ReturnUrl,
                                rememberLogin = model.RememberLogin
                            });                   

                    // request for a local page
                    if (Url.IsLocalUrl(model.ReturnUrl))
                    {
                        return Redirect(redirectToAdditionalFactorUrl);
                    }

                    if (string.IsNullOrEmpty(model.ReturnUrl))
                    {
                        return Redirect("~/");
                    }

                    // user might have clicked on a malicious link - should be logged
                    throw new Exception("invalid return URL");
                }

                await _events.RaiseAsync(new UserLoginFailureEvent(model.Username, "invalid credentials"));
                ModelState.AddModelError("", AccountOptions.InvalidCredentialsErrorMessage);
            }

            // something went wrong, show form with error
            var vm = await BuildLoginViewModelAsync(model);
            return View(vm);
        }

 await HttpContext.SignInAsync(scheme: Startup.TwoFactorAuthenticationScheme, principal: new ClaimsPrincipal(id));

    public interface ITwoFactorAuthenticationService
    {
        Task SendTemporaryCodeAsync(string subjectId);
        Task<bool> IsValidTemporaryCodeAsync(string subjectId, string code);
    }

using System.ComponentModel.DataAnnotations;

namespace DNT.IDP.Controllers.Account
{
    public class AdditionalAuthenticationFactorViewModel
    {
        [Required]
        public string Code { get; set; }

        public string ReturnUrl { get; set; }

        public bool RememberLogin { get; set; }
    }
}

namespace DNT.IDP.Controllers.Account
{
    public class AccountController : Controller
    {
        [HttpGet]
        public IActionResult AdditionalAuthenticationFactor(string returnUrl, bool rememberLogin)
        {
            // create VM
            var vm = new AdditionalAuthenticationFactorViewModel
            {
                RememberLogin = rememberLogin,
                ReturnUrl = returnUrl
            };

            return View(vm);
        }

        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> AdditionalAuthenticationFactor(
            AdditionalAuthenticationFactorViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return View(model);
            }

            // read identity from the temporary cookie
            var info = await HttpContext.AuthenticateAsync(Startup.TwoFactorAuthenticationScheme);
            var tempUser = info?.Principal;
            if (tempUser == null)
            {
                throw new Exception("2FA error");
            }

            // ... check code for user
            if (!await _twoFactorAuthenticationService.IsValidTemporaryCodeAsync(tempUser.GetSubjectId(), model.Code))
            {
                ModelState.AddModelError("code", "2FA code is invalid.");
                return View(model);
            }

            // login the user
            AuthenticationProperties props = null;
            if (AccountOptions.AllowRememberLogin && model.RememberLogin)
            {
                props = new AuthenticationProperties
                {
                    IsPersistent = true,
                    ExpiresUtc = DateTimeOffset.UtcNow.Add(AccountOptions.RememberMeLoginDuration)
                };
            }

            // issue authentication cookie for user
            var user = await _usersService.GetUserBySubjectIdAsync(tempUser.GetSubjectId());
            await _events.RaiseAsync(new UserLoginSuccessEvent(user.Username, user.SubjectId, user.Username));
            await HttpContext.SignInAsync(user.SubjectId, user.Username, props);

            // delete temporary cookie used for 2FA
            await HttpContext.SignOutAsync(Startup.TwoFactorAuthenticationScheme);

            if (_interaction.IsValidReturnUrl(model.ReturnUrl) || Url.IsLocalUrl(model.ReturnUrl))
            {
                return Redirect(model.ReturnUrl);
            }

            return Redirect("~/");
        }

@model AdditionalAuthenticationFactorViewModel

<div>
    <div class="page-header">
        <h1>2-Factor Authentication</h1>
    </div>

    @Html.Partial("_ValidationSummary")

    <div class="row">
        <div class="panel panel-default">
            <div class="panel-heading">
                <h3 class="panel-title">Input your 2FA code</h3>
            </div>
            <div class="panel-body">
                <form asp-route="Login">
                    <input type="hidden" asp-for="ReturnUrl" />
                    <input type="hidden" asp-for="RememberLogin" />

                    <fieldset>
                        <div class="form-group">
                            <label asp-for="Code"></label>
                            <input class="form-control" placeholder="Code" asp-for="Code" autofocus>
                        </div>

                        <div class="form-group">
                            <button class="btn btn-primary">Submit code</button>
                        </div>
                    </fieldset>
                </form>

            </div>
        </div>
    </div>
</div>