add dotnet backend

backend-dotnet/JwtValidator.cs

@@ -0,0 +1,58 @@
+using System.IdentityModel.Tokens.Jwt;
+using Microsoft.IdentityModel.Protocols;
+using Microsoft.IdentityModel.Protocols.OpenIdConnect;
+using Microsoft.IdentityModel.Tokens;
+
+namespace DexDemoBackend;
+
+public class JwtValidator
+{
+    private readonly AppConfig _config;
+    private readonly HttpClient _httpClient;
+    private ConfigurationManager<OpenIdConnectConfiguration>? _configManager;
+
+    public JwtValidator(AppConfig config)
+    {
+        _config = config;
+        _httpClient = new HttpClient(new HttpClientHandler 
+        { 
+            ServerCertificateCustomValidationCallback = (_, _, _, _) => true 
+        });
+    }
+
+    public async Task<IDictionary<string, object>> ValidateToken(string token)
+    {
+        try
+        {
+            // Lazy init OIDC configuration manager
+            _configManager ??= new ConfigurationManager<OpenIdConnectConfiguration>(
+                $"{_config.DexIssuer}.well-known/openid-configuration",
+                new OpenIdConnectConfigurationRetriever(),
+                new HttpDocumentRetriever(_httpClient) { RequireHttps = false }
+            );
+
+            var oidcConfig = await _configManager.GetConfigurationAsync(CancellationToken.None);
+            
+            var validationParameters = new TokenValidationParameters
+            {
+                ValidateIssuer = true,
+                ValidIssuer = _config.DexIssuer,
+                ValidateAudience = false,
+                ValidateLifetime = true,
+                ValidateIssuerSigningKey = true,
+                IssuerSigningKeys = oidcConfig.SigningKeys
+            };
+
+            var handler = new JwtSecurityTokenHandler();
+            var principal = handler.ValidateToken(token, validationParameters, out _);
+            
+            var jwtToken = handler.ReadJwtToken(token);
+            return jwtToken.Payload;
+        }
+        catch (Exception ex)
+        {
+            throw new UnauthorizedAccessException($"Token validation error: {ex.Message}", ex);
+        }
+    }
+}
+