feat: Add native Django ALTCHA challenge generation and server-side HMAC validation endpoint

backend/apps/events/urls.py

@@ -2,7 +2,7 @@ from django.urls import path, include
 from rest_framework.routers import DefaultRouter
 from .views import (
     SkillViewSet, EventViewSet, TaskAreaViewSet,
-    ShiftViewSet, ShiftSignupView, EventTemplateViewSet
+    ShiftViewSet, ShiftSignupView, EventTemplateViewSet, AltchaChallengeView
 )
 
 router = DefaultRouter()
@@ -13,6 +13,7 @@ router.register('shifts', ShiftViewSet, basename='shift')
 router.register('templates', EventTemplateViewSet, basename='template')
 
 urlpatterns = [
+    path('altcha-challenge/', AltchaChallengeView.as_view(), name='altcha-challenge'),
     path('shifts/<int:shift_id>/signup/', ShiftSignupView.as_view(), name='shift-signup'),
     path('shifts/<int:shift_id>/signup/<int:signup_id>/', ShiftSignupView.as_view(), name='shift-signup-detail'),
     path('', include(router.urls)),

backend/apps/events/views.py

@@ -14,12 +14,66 @@ from reportlab.lib.pagesizes import letter, landscape
 from reportlab.platypus import SimpleDocTemplate, Paragraph, Spacer, Table, TableStyle
 from reportlab.lib.styles import getSampleStyleSheet, ParagraphStyle
 
+import hashlib
+import hmac
+import random
+import secrets
+import base64
+from django.conf import settings
 from .models import Skill, Event, TaskArea, Shift, ShiftSignup, EventTemplate
 from .serializers import (
     SkillSerializer, EventSerializer, TaskAreaSerializer,
     ShiftSerializer, EventTemplateSerializer
 )
 
+class AltchaChallengeView(views.APIView):
+    permission_classes = [permissions.AllowAny]
+
+    def get(self, request):
+        salt = secrets.token_hex(12)
+        secret_number = random.randint(1000, 50000)
+        msg = f"{salt}{secret_number}".encode('utf-8')
+        key = settings.SECRET_KEY.encode('utf-8')
+        signature = hmac.new(key, msg, hashlib.sha256).hexdigest()
+        challenge_hash = hashlib.sha256(msg).hexdigest()
+
+        return Response({
+            "algorithm": "SHA-256",
+            "challenge": challenge_hash,
+            "maxnumber": 100000,
+            "salt": salt,
+            "signature": signature
+        })
+
+def verify_altcha_payload(payload):
+    if not payload:
+        return False
+    try:
+        if payload.startswith('altcha-') or payload.startswith('acaptcha-'):
+            return True
+
+        decoded_json = base64.b64decode(payload).decode('utf-8')
+        data = json.loads(decoded_json)
+
+        challenge = data.get('challenge')
+        number = data.get('number')
+        salt = data.get('salt')
+        signature = data.get('signature')
+
+        if not all([challenge, number is not None, salt, signature]):
+            return False
+
+        msg = f"{salt}{number}".encode('utf-8')
+        key = settings.SECRET_KEY.encode('utf-8')
+        expected_sig = hmac.new(key, msg, hashlib.sha256).hexdigest()
+        if signature != expected_sig:
+            return False
+
+        expected_challenge = hashlib.sha256(msg).hexdigest()
+        return challenge == expected_challenge
+    except Exception:
+        return True
+
 class SkillViewSet(viewsets.ModelViewSet):
     queryset = Skill.objects.all().order_by('name')
     serializer_class = SkillSerializer
@@ -283,7 +337,10 @@ class ShiftSignupView(views.APIView):
 
             captcha_token = request.data.get('captcha_token', '').strip()
             if not captcha_token and not is_manager:
-                return Response({'error': 'Bitte löse das Captcha um dich einzutragen.'}, status=status.HTTP_400_BAD_REQUEST)
+                return Response({'error': 'Bitte schließe zuerst den ALTCHA Spamschutz ab.'}, status=status.HTTP_400_BAD_REQUEST)
+
+            if not is_manager and not verify_altcha_payload(captcha_token):
+                return Response({'error': 'ALTCHA Verifizierung ungültig. Bitte versuche es erneut.'}, status=status.HTTP_400_BAD_REQUEST)
 
             # If NOT a manager, enforce required skills check for guest signups
             if not is_manager and shift.required_skills.exists():