Notes/Controllers/LoginController.cs

using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Mvc;
using System.ComponentModel.DataAnnotations;
using System.Security.Claims;

namespace BinaryDad.Notes.Controllers
{
    public class LoginController : Controller
    {
        private readonly IConfiguration configuration;

        public LoginController(IConfiguration configuration)
        {
            this.configuration = configuration;
        }

        [Route("login")]
        public IActionResult Login()
        {
            return View();
        }

        [ActionName(nameof(Login))]
        [Route("login")]
        [HttpPost]
        public async Task<IActionResult> LoginPost([Required] string passphrase, string returnUrl)
        {
            if (ModelState.IsValid)
            {
                var appPassphrase = configuration["APP_PASSPHRASE"];

                if (passphrase == appPassphrase)
                {
                    var claims = new List<Claim>
                    {
                        new Claim(ClaimTypes.GivenName, "Ryan")
                    };

                    var claimsIdentity = new ClaimsIdentity(claims, CookieAuthenticationDefaults.AuthenticationScheme);

                    await HttpContext.SignInAsync(new ClaimsPrincipal(claimsIdentity));

                    return Redirect(returnUrl);
                }

                ModelState.AddModelError("", "Invalid login");
            }

            return View();
        }

        [Route("logout")]
        public async Task<IActionResult> Logout()
        {
            await HttpContext.SignOutAsync();

            return Redirect("/");
        }
    }
}