Add verified App Link handoff and mobile token exchange endpoint

Replaces the custom-scheme auto-redirect (which triggers Chrome's
confirmation prompt) with a verified Android App Link handoff:

- public/.well-known/assetlinks.json for space.einundzwanzig.mobile
  (debug cert fingerprint; add the release cert before store builds)
- GET /app/auth handoff: opens the app directly when the App Link is
  verified; renders a button-based fallback page otherwise
- POST /api/mobile/token: trades a NIP-55-signed login event for a
  Sanctum token — used when Amber's callback opens the app directly
- complete/confirm/signedCallback now redirect to the handoff URL

app/Http/Controllers/MobileAuthController.php

@@ -14,6 +14,7 @@ use Illuminate\Http\RedirectResponse;
 use Illuminate\Http\Request;
 use Illuminate\Support\Facades\Log;
 use Illuminate\Validation\ValidationException;
+use Laravel\Sanctum\NewAccessToken;
 
 /**
  * Auth flow for the Einundzwanzig mobile app.
@@ -76,6 +77,76 @@ final class MobileAuthController extends Controller
         return response()->json(['status' => 'OK']);
     }
 
+    /**
+     * Exchange a NIP-55-signed login event for a personal access token.
+     *
+     * Used by the mobile app when the signer callback opens the app
+     * directly via a verified Android App Link: the app receives the
+     * signed event in the deep-link path and trades it in here.
+     */
+    public function token(Request $request): JsonResponse
+    {
+        $validated = $request->validate([
+            'k1' => ['required', 'string', 'size:64'],
+            'event' => ['required', 'array'],
+            'device_name' => ['nullable', 'string', 'max:64'],
+        ]);
+
+        if (! ctype_xdigit($validated['k1'])) {
+            return response()->json(['status' => 'ERROR', 'reason' => 'Invalid k1'], 400);
+        }
+
+        try {
+            $npub = NostrLogin::verifyEvent($validated['event'], $validated['k1']);
+        } catch (ValidationException) {
+            Log::warning('Mobile token exchange verification failed', [
+                'k1' => $validated['k1'],
+                'ip' => $request->ip(),
+            ]);
+
+            return response()->json(['status' => 'ERROR', 'reason' => 'Signature was NOT VERIFIED'], 400);
+        }
+
+        $user = NostrLogin::findOrCreateUser($npub);
+        FetchNostrProfileJob::dispatch($user);
+
+        $token = $this->createDeviceToken($user, $validated['device_name'] ?? self::DEFAULT_DEVICE_NAME);
+
+        Log::info('Mobile app token issued via token exchange', [
+            'user_id' => $user->id,
+        ]);
+
+        return response()->json([
+            'token' => $token->plainTextToken,
+            'user' => [
+                'id' => $user->id,
+                'name' => $user->name,
+            ],
+        ]);
+    }
+
+    /**
+     * Browser fallback for the app handoff URL (/app/auth?token=…).
+     *
+     * When the Android App Link is verified, navigation to this URL opens
+     * the app directly and this route never renders. Without verification
+     * (e.g. sideloaded build) the page offers the einundzwanzig:// deep
+     * link behind a button — a user gesture, so Chrome opens the app
+     * without its confirmation prompt.
+     */
+    public function handoff(Request $request)
+    {
+        $token = (string) $request->query('token', '');
+
+        if ($token === '') {
+            return redirect()->route('auth.mobile');
+        }
+
+        return view('auth.app-handoff', [
+            'deepLink' => self::ALLOWED_REDIRECT_URIS[0].'?token='.urlencode($token),
+        ]);
+    }
+
     /**
      * Handle the NIP-55 signer callback in its path-based form.
      *
@@ -125,9 +196,7 @@ final class MobileAuthController extends Controller
             'ip' => $request->ip(),
         ]);
 
-        return view('auth.mobile-bridge', [
-            'deepLink' => $this->issueToken($user, $request),
-        ]);
+        return redirect()->to($this->issueToken($user, $request));
     }
 
     /**
@@ -171,22 +240,30 @@ final class MobileAuthController extends Controller
     }
 
     /**
-     * Issue a personal access token named after the device and build the
-     * whitelisted deep link that hands it to the app. Existing tokens with
-     * the same device name are replaced so repeated logins don't accumulate
-     * tokens.
+     * Issue a personal access token for the session's device and build the
+     * app handoff URL. The handoff URL is a verified Android App Link, so
+     * navigating to it opens the app directly; its browser fallback page
+     * offers the einundzwanzig:// deep link behind a button.
      */
     private function issueToken(User $user, Request $request): string
     {
         $mobileAuth = (array) $request->session()->pull('mobile_auth', []);
 
-        $redirectUri = $mobileAuth['redirect_uri'] ?? self::ALLOWED_REDIRECT_URIS[0];
+        $deviceName = (string) ($mobileAuth['device_name'] ?? self::DEFAULT_DEVICE_NAME);
 
-        if (! in_array($redirectUri, self::ALLOWED_REDIRECT_URIS, true)) {
-            $redirectUri = self::ALLOWED_REDIRECT_URIS[0];
-        }
+        $token = $this->createDeviceToken($user, $deviceName);
 
-        $deviceName = str((string) ($mobileAuth['device_name'] ?? self::DEFAULT_DEVICE_NAME))
+        return route('auth.mobile.handoff').'?token='.urlencode($token->plainTextToken);
+    }
+
+    /**
+     * Create a personal access token named after the device. Existing
+     * tokens with the same device name are replaced so repeated logins
+     * don't accumulate tokens.
+     */
+    private function createDeviceToken(User $user, string $deviceName): NewAccessToken
+    {
+        $deviceName = str($deviceName)
             ->limit(64, '')
             ->whenEmpty(fn () => str(self::DEFAULT_DEVICE_NAME))
             ->value();
@@ -200,6 +277,6 @@ final class MobileAuthController extends Controller
             'device_name' => $deviceName,
         ]);
 
-        return $redirectUri.'?token='.urlencode($token->plainTextToken);
+        return $token;
     }
 }

public/.well-known/assetlinks.json

@@ -0,0 +1,12 @@
+[
+    {
+        "relation": ["delegate_permission/common.handle_all_urls"],
+        "target": {
+            "namespace": "android_app",
+            "package_name": "space.einundzwanzig.mobile",
+            "sha256_cert_fingerprints": [
+                "74:25:57:3B:24:69:97:97:45:8E:27:CC:1E:26:D7:A2:82:73:EC:BB:0D:B9:47:78:2A:18:B5:94:54:B0:79:ED"
+            ]
+        }
+    }
+]

resources/views/auth/app-handoff.blade.php

@@ -16,14 +16,15 @@
     </style>
 </head>
 <body>
+    {{-- No auto-redirect here: a JS navigation to a custom scheme triggers
+         Chrome's confirmation prompt. The button tap is a user gesture and
+         opens the app directly. With verified App Links this page never
+         renders — Android opens the app before the request is made. --}}
     <div class="card">
         <div class="check">✅</div>
         <h1>{{ __('Login bestätigt') }}</h1>
-        <p>{{ __('Du wirst jetzt zurück in die Einundzwanzig-App geleitet. Falls nichts passiert, tippe auf den Button.') }}</p>
+        <p>{{ __('Tippe auf den Button, um zurück zur Einundzwanzig-App zu gelangen.') }}</p>
         <a class="button" href="{{ $deepLink }}">{{ __('Zurück zur App') }}</a>
     </div>
-    <script>
-        setTimeout(function () { window.location.href = @js($deepLink); }, 400);
-    </script>
 </body>
 </html>

routes/api.php

@@ -89,5 +89,12 @@ Route::get('/nostr-login-callback', [MobileAuthController::class, 'nostrCallback
     ->middleware('throttle:30,1')
     ->name('auth.nostr.callback');
 
+// Token exchange for the mobile app: trades a NIP-55-signed login event
+// for a Sanctum personal access token (used when the signer callback
+// opens the app directly via a verified App Link).
+Route::post('/mobile/token', [MobileAuthController::class, 'token'])
+    ->middleware('throttle:30,1')
+    ->name('auth.mobile.token');
+
 Route::post('/check-auth-error', [LnurlAuthController::class, 'checkError'])
     ->name('auth.check-error');

routes/auth.php

@@ -50,12 +50,19 @@ Route::get('/auth/mobile/complete/{k1}', [MobileAuthController::class, 'complete
     ->name('auth.mobile.complete');
 
 // NIP-55 signer callback (Amber): k1 in the path, the signer appends the
-// URL-encoded signed event after the trailing slash.
+// URL-encoded signed event after the trailing slash. With verified App
+// Links this URL opens the app directly; this web route is the fallback.
 Route::get('/auth/mobile/signed/{payload}', [MobileAuthController::class, 'signedCallback'])
     ->where('payload', '.*')
     ->middleware('throttle:30,1')
     ->name('auth.mobile.signed');
 
+// App handoff: verified Android App Link — opens the app with the token.
+// In the browser (unverified install) it renders a button-based fallback.
+Route::get('/app/auth', [MobileAuthController::class, 'handoff'])
+    ->middleware('throttle:30,1')
+    ->name('auth.mobile.handoff');
+
 Route::post('/auth/mobile/confirm', [MobileAuthController::class, 'confirm'])
     ->middleware(['auth', 'throttle:30,1'])
     ->name('auth.mobile.confirm');

tests/Feature/Auth/MobileAuthTest.php

@@ -86,7 +86,7 @@ it('rejects a nostr signer callback whose event is bound to a different challeng
     expect(LoginKey::query()->where('k1', $k1)->exists())->toBeFalse();
 });
 
-it('completes the login via the path-based signer callback and shows the bridge page', function () {
+it('completes the login via the path-based signer callback and redirects to the app handoff', function () {
     Queue::fake();
 
     $k1 = bin2hex(random_bytes(32));
@@ -98,7 +98,8 @@ it('completes the login via the path-based signer callback and shows the bridge
         ->withSession(['mobile_auth' => ['redirect_uri' => 'einundzwanzig://auth', 'device_name' => 'Pixel 10']])
         ->get('/auth/mobile/signed/'.$k1.'/'.rawurlencode(json_encode($signedEvent)));
 
-    $response->assertOk()->assertSee('einundzwanzig://auth?token=', false);
+    $location = $response->headers->get('Location');
+    expect($location)->toStartWith(route('auth.mobile.handoff').'?token=');
 
     $user = User::query()->where('nostr', $npub)->first();
     expect($user)->not->toBeNull();
@@ -109,6 +110,44 @@ it('completes the login via the path-based signer callback and shows the bridge
 
     expect(LoginKey::query()->where('k1', $k1)->exists())->toBeTrue();
     Queue::assertPushed(FetchNostrProfileJob::class);
+
+    // The handoff fallback page offers the deep link behind a button.
+    $this->get($location)
+        ->assertOk()
+        ->assertSee('einundzwanzig://auth?token=', false);
+});
+
+it('exchanges a signed event for a token via the mobile token endpoint', function () {
+    Queue::fake();
+
+    $k1 = bin2hex(random_bytes(32));
+    [$signedEvent, $npub] = makeSignedMobileNostrEvent($k1);
+
+    $response = $this->postJson('/api/mobile/token', [
+        'k1' => $k1,
+        'event' => $signedEvent,
+        'device_name' => 'Pixel 10',
+    ]);
+
+    $response->assertOk()->assertJsonStructure(['token', 'user' => ['id', 'name']]);
+
+    $user = User::query()->where('nostr', $npub)->first();
+    expect($user)->not->toBeNull()
+        ->and($user->tokens()->first()->name)->toBe('Pixel 10');
+
+    $this->getJson('/api/user', ['Authorization' => 'Bearer '.$response->json('token')])
+        ->assertOk()
+        ->assertJsonPath('id', $user->id);
+});
+
+it('rejects a token exchange with a mismatched challenge', function () {
+    $k1 = bin2hex(random_bytes(32));
+    [$signedEvent] = makeSignedMobileNostrEvent(bin2hex(random_bytes(32)));
+
+    $this->postJson('/api/mobile/token', [
+        'k1' => $k1,
+        'event' => $signedEvent,
+    ])->assertBadRequest();
 });
 
 it('rejects a path-based signer callback with a tampered challenge', function () {
@@ -131,7 +170,7 @@ it('issues a token and redirects into the app when completing a verified login',
         ->get('/auth/mobile/complete/'.$k1);
 
     $location = $response->headers->get('Location');
-    expect($location)->toStartWith('einundzwanzig://auth?token=');
+    expect($location)->toStartWith(route('auth.mobile.handoff').'?token=');
 
     $token = $user->tokens()->first();
     expect($token)->not->toBeNull()
@@ -158,7 +197,7 @@ it('lets an already authenticated user connect the app and replaces tokens of th
         ->withSession(['mobile_auth' => ['redirect_uri' => 'einundzwanzig://auth', 'device_name' => 'Pixel 10']])
         ->post('/auth/mobile/confirm');
 
-    expect($response->headers->get('Location'))->toStartWith('einundzwanzig://auth?token=');
+    expect($response->headers->get('Location'))->toStartWith(route('auth.mobile.handoff').'?token=');
     expect($user->tokens()->where('name', 'Pixel 10')->count())->toBe(1);
 });