fix(auth): encrypt stored cookie keys; tighten the link and sign-out flows

- Data-protection keys are encrypted (AES-256-GCM, key derived from the
  server secret) before they reach the DataProtectionKeys table.
- /link only follows local return paths and keeps the form when a
  signed-in visitor has an identity waiting to be linked.
- A failed sign-out says so instead of reloading as if it worked.

DorkNet.Server/Services/AuthService.cs

@@ -19,13 +19,18 @@ public class AuthService(IConfiguration config, DorkNetDbContext db)
     // configs) and leave Jwt:Secret as the appsettings.json placeholder;
     // the mismatched-secret bug shows up only there.
     private string SecretKey =>
-        Environment.GetEnvironmentVariable("DORKNET_JWT_SECRET")
-        ?? Environment.GetEnvironmentVariable("RECNET_JWT_SECRET")
-        ?? config["Jwt:Secret"]
+        ConfiguredSecret(config)
         ?? throw new InvalidOperationException(
             "JWT secret not configured. Set DORKNET_JWT_SECRET env var or " +
             "Jwt:Secret in appsettings.Local.json.");
 
+    /// <summary>The server secret, in the one resolution order every user of
+    /// it shares (token signing, token validation, key encryption).</summary>
+    public static string? ConfiguredSecret(IConfiguration config) =>
+        Environment.GetEnvironmentVariable("DORKNET_JWT_SECRET")
+        ?? Environment.GetEnvironmentVariable("RECNET_JWT_SECRET")
+        ?? config["Jwt:Secret"];
+
     public string GenerateToken(long playerId) => CreateJwt(playerId, TimeSpan.FromHours(12), "access");
 
     /// <summary>

DorkNet.Server/Services/DataProtectionKeyEncryption.cs

@@ -0,0 +1,67 @@
+using System.Security.Cryptography;
+using System.Text;
+using System.Xml.Linq;
+using Microsoft.AspNetCore.DataProtection.XmlEncryption;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DorkNet.Server.Services;
+
+/// <summary>
+/// Encrypts the data-protection key ring (the keys behind the sign-in
+/// cookies) before it is stored in the <c>DataProtectionKeys</c> table:
+/// AES-256-GCM under a key derived (HKDF-SHA256) from the server secret.
+/// The secret comes from the environment, not the database, so a database
+/// backup alone holds no usable keys. Changing the secret makes the stored
+/// keys unreadable: the server then creates a new key and everyone signs in
+/// again.
+/// </summary>
+public sealed class DataProtectionKeyEncryption : IXmlEncryptor
+{
+    private const int NonceSize = 12;
+    private const int TagSize = 16;
+    private static readonly byte[] Info = Encoding.UTF8.GetBytes("DorkNet.DataProtectionKeys.v1");
+    private static readonly XName Root = "dorknetEncryptedKey";
+    private readonly byte[] _key;
+
+    public DataProtectionKeyEncryption(string secret)
+    {
+        if (string.IsNullOrEmpty(secret))
+            throw new InvalidOperationException("The server secret is required to encrypt the data-protection keys.");
+        _key = HKDF.DeriveKey(HashAlgorithmName.SHA256, Encoding.UTF8.GetBytes(secret), 32, salt: null, info: Info);
+    }
+
+    public EncryptedXmlInfo Encrypt(XElement plaintextElement)
+    {
+        var plaintext = Encoding.UTF8.GetBytes(plaintextElement.ToString(SaveOptions.DisableFormatting));
+        var payload = new byte[NonceSize + TagSize + plaintext.Length];
+        var nonce = payload.AsSpan(0, NonceSize);
+        RandomNumberGenerator.Fill(nonce);
+        using (var aes = new AesGcm(_key, TagSize))
+            aes.Encrypt(nonce, plaintext, payload.AsSpan(NonceSize + TagSize), payload.AsSpan(NonceSize, TagSize));
+        return new EncryptedXmlInfo(
+            new XElement(Root, new XAttribute("version", "1"), Convert.ToBase64String(payload)),
+            typeof(Decryptor));
+    }
+
+    public XElement DecryptElement(XElement encrypted)
+    {
+        if (encrypted.Name != Root || (string?)encrypted.Attribute("version") != "1")
+            throw new CryptographicException("Unrecognised encrypted data-protection key.");
+        var payload = Convert.FromBase64String(encrypted.Value);
+        if (payload.Length < NonceSize + TagSize)
+            throw new CryptographicException("Encrypted data-protection key is truncated.");
+        var plaintext = new byte[payload.Length - NonceSize - TagSize];
+        using (var aes = new AesGcm(_key, TagSize))
+            aes.Decrypt(payload.AsSpan(0, NonceSize), payload.AsSpan(NonceSize + TagSize),
+                payload.AsSpan(NonceSize, TagSize), plaintext);
+        return XElement.Parse(Encoding.UTF8.GetString(plaintext));
+    }
+
+    /// <summary>Created by the data-protection system from the stored type
+    /// name, with the app's service provider.</summary>
+    public sealed class Decryptor(IServiceProvider services) : IXmlDecryptor
+    {
+        public XElement Decrypt(XElement encryptedElement) =>
+            services.GetRequiredService<DataProtectionKeyEncryption>().DecryptElement(encryptedElement);
+    }
+}

DorkNet.Server/Startup/ServiceCollectionExtensions.cs

@@ -3,6 +3,7 @@ using Microsoft.AspNetCore.Authentication.Cookies;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
 using Microsoft.AspNetCore.Authentication.OpenIdConnect;
 using Microsoft.AspNetCore.DataProtection;
+using Microsoft.AspNetCore.DataProtection.KeyManagement;
 using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.IdentityModel.Tokens;
@@ -345,9 +346,7 @@ public static class ServiceCollectionExtensions
     // older Coolify configs.
     private static void AddAuthentication(WebApplicationBuilder builder)
     {
-        var jwtSecret = Environment.GetEnvironmentVariable("DORKNET_JWT_SECRET")
-            ?? Environment.GetEnvironmentVariable("RECNET_JWT_SECRET")
-            ?? builder.Configuration["Jwt:Secret"]
+        var jwtSecret = AuthService.ConfiguredSecret(builder.Configuration)
             ?? throw new InvalidOperationException(
                 "JWT secret not configured. Set the DORKNET_JWT_SECRET env var or " +
                 "add `Jwt:Secret` to appsettings.Local.json (the install-plugin.ps1 " +
@@ -422,10 +421,17 @@ public static class ServiceCollectionExtensions
         services.AddSingleton<LinkAttemptLimiter>();
 
         // Cookie encryption keys live in the database, so a redeploy or a
-        // second replica doesn't invalidate everyone's session.
+        // second replica doesn't invalidate everyone's session. They're
+        // stored encrypted with a key derived from the server secret, which
+        // lives outside the database, so a database backup alone can't forge
+        // sessions.
+        services.AddSingleton(new DataProtectionKeyEncryption(
+            AuthService.ConfiguredSecret(builder.Configuration) ?? string.Empty));
         services.AddDataProtection()
             .SetApplicationName("DorkNet")
             .PersistKeysToDbContext<DorkNetDbContext>();
+        services.AddOptions<KeyManagementOptions>()
+            .Configure<DataProtectionKeyEncryption>((o, enc) => o.XmlEncryptor = enc);
 
         var auth = services.AddAuthentication()
             .AddCookie(ExternalSignIn.WebScheme, o =>

DorkNet.Server/site/src/components/Header.tsx

@@ -34,9 +34,15 @@ export function Header() {
     getMe().then(setMe).catch(() => { /* site works without sign-in */ });
   }, []);
 
+  const [signOutError, setSignOutError] = useState<string | null>(null);
   const doSignOut = async () => {
-    await signOut().catch(() => {});
-    window.location.assign('/');
+    setSignOutError(null);
+    try {
+      await signOut();
+      window.location.assign('/');
+    } catch (e) {
+      setSignOutError((e as Error).message);
+    }
   };
 
   // Sign in / account, shown only when the server has a sign-in provider.
@@ -136,6 +142,15 @@ export function Header() {
         </div>
       )}
 
+      {signOutError && (
+        <div className="border-t border-danger/30 bg-danger/10">
+          <div className="mx-auto max-w-6xl px-4 sm:px-6 py-2 flex items-center justify-between gap-3 text-sm text-danger">
+            <span>Couldn't sign out ({signOutError}). You're still signed in; try again.</span>
+            <button type="button" className="btn-ghost !px-2 !py-1 text-xs" onClick={() => setSignOutError(null)}>Dismiss</button>
+          </div>
+        </div>
+      )}
+
       {signinError && (
         <div className="border-t border-danger/30 bg-danger/10">
           <div className="mx-auto max-w-6xl px-4 sm:px-6 py-2 flex items-center justify-between gap-3 text-sm text-danger">

DorkNet.Server/site/src/lib/account.ts

@@ -19,6 +19,13 @@ export function getMe(fresh = false): Promise<AccountMe> {
   return cached;
 }
 
+/** `url` if it is a path on this site, else `/` (the server applies the same
+ * rule): never navigate to an address taken from the query string otherwise. */
+export function localPath(url: string | null | undefined): string {
+  if (!url || !url.startsWith('/') || url.startsWith('//') || url.startsWith('/\\')) return '/';
+  return url;
+}
+
 /** Starts sign-in at the provider; comes back to `returnUrl` on this site. */
 export function signInUrl(returnUrl = window.location.pathname + window.location.search): string {
   return `/api/auth/external/login?returnUrl=${encodeURIComponent(returnUrl)}`;

DorkNet.Server/site/src/pages/Link.tsx

@@ -1,12 +1,12 @@
 import { useEffect, useState } from 'react';
 import { Link as RouterLink, useNavigate, useSearchParams } from 'react-router';
-import { getMe, link, LINK_ERRORS, signInUrl, type AccountMe } from '../lib/account';
+import { getMe, link, LINK_ERRORS, localPath, signInUrl, type AccountMe } from '../lib/account';
 
 // After a first sign-in with the provider: prove which DorkNet account this
 // identity belongs to, once, with that account's password.
 export function Link() {
   const [params] = useSearchParams();
-  const returnUrl = params.get('returnUrl') || '/';
+  const returnUrl = localPath(params.get('returnUrl'));
   const navigate = useNavigate();
   const [me, setMe] = useState<AccountMe | null>(null);
   const [username, setUsername] = useState('');
@@ -16,7 +16,10 @@ export function Link() {
 
   useEffect(() => {
     getMe(true).then((m) => {
-      if (m.signedIn) navigate(returnUrl, { replace: true });
+      // Signed in with nothing waiting to be linked: nothing to do here. (A
+      // signed-in visitor can still link a new identity, e.g. a second
+      // account's.)
+      if (m.signedIn && !m.pendingLink) navigate(returnUrl, { replace: true });
       else setMe(m);
     }).catch((e) => setErr((e as Error).message));
     // eslint-disable-next-line react-hooks/exhaustive-deps