Generate (and cache) a token to use as the default value

Author: wcabus

src/Pages/Home/JwtDecoder/JwtDecoder.cshtml

@@ -43,7 +43,7 @@
                 <div class="h-100 d-flex flex-column">
                     <div class="form-group flex-grow-1">
                         <label for="jwt-input" class="sr-only">Paste your JWT here...</label>
-                        <div id="jwt-input" class="form-control bg-dark text-light p-2 h-100 jwt-input-editable" contenteditable="true" rows="8" style="min-height: 10em;" placeholder="Paste your JWT here..."></div>
+                        <div id="jwt-input" class="form-control bg-dark text-light p-2 h-100 jwt-input-editable" contenteditable="true" rows="8" style="min-height: 10em;" placeholder="Paste your JWT here...">@Model.View?.Token</div>
                     </div>
                     <div class="form-group">
                         <label for="jwks-url">Issuer, Discovery Document or JWKs URI</label>
@@ -327,7 +327,8 @@
                 }
             });
             
-            $('#jwt-input').on('input', async function() {
+            const jwtInput = $('#jwt-input');
+            jwtInput.on('input', async function() {
                 decodedJwt = {
                     header: null,
                     payload: null,
@@ -361,7 +362,14 @@
                 }
             });
 
-            await showDecodedJwt(null, null, null, ' ');
+            @if (Model.View?.Token != null)
+            {
+                @:jwtInput.trigger('input'); // Trigger input event to decode the JWT if it was provided
+            }
+            else
+            {
+                @:await showDecodedJwt(null, null, null, ' ');
+            }
         }
 
         function colorJwtInput(target, parts) {

src/Pages/Home/JwtDecoder/JwtDecoder.cshtml.cs

@@ -1,9 +1,62 @@
-using Microsoft.AspNetCore.Authorization;
+using IdentityModel.Client;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.Extensions.Caching.Memory;
 
 namespace IdentityServerHost.Pages.Home;
 
 [AllowAnonymous]
-public class JwtDecoder : PageModel
+public class JwtDecoder(IHttpClientFactory clientFactory, IMemoryCache cache) : PageModel
 {
+    private const string CacheKey = "jwt_decoder::access_token";
+    
+    public ViewModel View { get; set; } = default!;
+
+    public async Task<IActionResult> OnGet()
+    {
+        var token = await GetOrCreateToken();
+        View = new ViewModel { Token = token };
+        
+        return Page();
+    }
+
+    private async Task<string> GetOrCreateToken()
+    {
+        try
+        {
+            if (cache.TryGetValue(CacheKey, out string cachedToken))
+            {
+                return cachedToken;
+            }
+            
+            var request = HttpContext.Request;
+            var authority = request.Scheme + "://" + request.Host.ToUriComponent();
+            
+            var client = clientFactory.CreateClient();
+            var response = await client.RequestClientCredentialsTokenAsync(new ClientCredentialsTokenRequest
+            {
+                Address = $"{authority}/connect/token",
+
+                ClientId = "m2m",
+                ClientSecret = "secret",
+
+                Scope = "api"
+            });
+
+            if (!response.IsError)
+            {
+                var token = response.AccessToken;
+                cache.Set(CacheKey, token, TimeSpan.FromMinutes(20));
+                
+                return token;
+            }
+        }
+        catch
+        {
+            return null;
+        }
+        
+        return null;
+    }
 }

src/Pages/Home/JwtDecoder/ViewModel.cs

@@ -0,0 +1,6 @@
+namespace IdentityServerHost.Pages.Home;
+
+public class ViewModel
+{
+    public string Token { get; set; }
+}