Add Login page

Author: damienbod

aspnetcore/security/authentication/configure-oidc-web-authentication/sample/oidc-net8/RazorPageOidc/Pages/Login.cshtml

@@ -0,0 +1,4 @@
+@page
+@model RazorPageOidc.Pages.LoginModel
+@{
+}

aspnetcore/security/authentication/configure-oidc-web-authentication/sample/oidc-net8/RazorPageOidc/Pages/Login.cshtml.cs

@@ -0,0 +1,40 @@
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace RazorPageOidc.Pages;
+
+[AllowAnonymous]
+public class LoginModel : PageModel
+{
+    [BindProperty(SupportsGet = true)]
+    public string? ReturnUrl { get; set; }
+
+    public async Task OnGetAsync()
+    {
+        var properties = GetAuthProperties(ReturnUrl);
+        await HttpContext.ChallengeAsync(properties);
+    }
+
+    private static AuthenticationProperties GetAuthProperties(string? returnUrl)
+    {
+        const string pathBase = "/";
+
+        // Prevent open redirects.
+        if (string.IsNullOrEmpty(returnUrl))
+        {
+            returnUrl = pathBase;
+        }
+        else if (!Uri.IsWellFormedUriString(returnUrl, UriKind.Relative))
+        {
+            returnUrl = new Uri(returnUrl, UriKind.Absolute).PathAndQuery;
+        }
+        else if (returnUrl[0] != '/')
+        {
+            returnUrl = $"{pathBase}{returnUrl}";
+        }
+
+        return new AuthenticationProperties { RedirectUri = returnUrl };
+    }
+}