feat: Redis-backed rate limiting with stricter auth limits

- Add rate_limit_auth_requests (5/min) and rate_limit_auth_window_seconds (60) settings
- Add rate_limit_redis_enabled flag for opt-in Redis usage
- Refactor _SlidingWindowCounter into InMemorySlidingWindow class
- Add RedisSlidingWindow using sorted sets with fallback to in-memory
- Add third _auth_strict_limiter for POST /auth/* paths (5 req/min)
- Add protocol-based backend selection at module load time
- Update tests for auth strict limiter and Redis fallback behavior

Co-Authored-By: Paperclip <noreply@paperclip.ing>

api/src/cartsnitch_api/cache.py

@@ -1,26 +1,51 @@
 """Redis/DragonflyDB caching helpers."""
 
+import redis.asyncio as redis
+
 from cartsnitch_api.config import settings
 
 
 class CacheClient:
-    """Stub for Redis/DragonflyDB caching.
+    """Redis/DragonflyDB caching with connection pooling.
 
     Will be used for expensive queries: price trends, product comparisons.
     Cache invalidation via Redis pub/sub events from other services.
     """
 
     def __init__(self) -> None:
-        self.url = settings.redis_url
+        self._pool: redis.ConnectionPool | None = None
+        self._client: redis.Redis | None = None
+
+    async def initialize(self) -> None:
+        """Initialize the Redis connection pool."""
+        self._pool = redis.ConnectionPool.from_url(
+            settings.redis_url,
+            max_connections=20,
+            decode_responses=True,
+        )
+        self._client = redis.Redis(connection_pool=self._pool)
+
+    async def close(self) -> None:
+        """Close the Redis connection pool."""
+        if self._client:
+            await self._client.aclose()
+        if self._pool:
+            await self._pool.aclose()
 
     async def get(self, key: str) -> str | None:
-        # TODO: implement with redis-py async
-        return None
+        if not self._client:
+            return None
+        return await self._client.get(key)
 
     async def set(self, key: str, value: str, ttl_seconds: int = 300) -> None:
-        # TODO: implement with redis-py async
-        pass
+        if not self._client:
+            return
+        await self._client.set(key, value, ex=ttl_seconds)
 
     async def delete(self, key: str) -> None:
-        # TODO: implement with redis-py async
-        pass
+        if not self._client:
+            return
+        await self._client.delete(key)
+
+
+cache_client = CacheClient()

api/src/cartsnitch_api/config.py

@@ -32,6 +32,9 @@ class Settings(BaseSettings):
 
     rate_limit_requests: int = 60
     rate_limit_window_seconds: int = 60
+    rate_limit_auth_requests: int = 5
+    rate_limit_auth_window_seconds: int = 60
+    rate_limit_redis_enabled: bool = True
     rate_limit_enabled: bool = True
 
     _PLACEHOLDER_VALUES = {"change-me-in-production"}
@@ -72,7 +75,9 @@ class Settings(BaseSettings):
     def normalize_database_url(self):
         """Normalize postgresql:// → postgresql+asyncpg:// for the asyncpg driver."""
         if self.database_url.startswith("postgresql://"):
-            self.database_url = self.database_url.replace("postgresql://", "postgresql+asyncpg://", 1)
+            self.database_url = self.database_url.replace(
+                "postgresql://", "postgresql+asyncpg://", 1
+            )
         return self
 
 

api/src/cartsnitch_api/database.py

@@ -6,7 +6,14 @@ from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_asyn
 
 from cartsnitch_api.config import settings
 
-engine = create_async_engine(settings.database_url, echo=False)
+engine = create_async_engine(
+    settings.database_url,
+    echo=False,
+    pool_size=10,
+    max_overflow=20,
+    pool_pre_ping=True,
+    pool_recycle=3600,
+)
 async_session_factory = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
 
 
@@ -14,3 +21,8 @@ async def get_db() -> AsyncGenerator[AsyncSession, None]:
     """FastAPI dependency that yields an async DB session."""
     async with async_session_factory() as session:
         yield session
+
+
+async def dispose_engine() -> None:
+    """Dispose the database engine, closing all pooled connections."""
+    await engine.dispose()

api/src/cartsnitch_api/main.py

@@ -5,6 +5,8 @@ from contextlib import asynccontextmanager
 from fastapi import APIRouter, FastAPI
 
 from cartsnitch_api.auth.routes import router as auth_router
+from cartsnitch_api.cache import cache_client
+from cartsnitch_api.database import dispose_engine
 from cartsnitch_api.middleware.cors import add_cors_middleware
 from cartsnitch_api.middleware.error_handler import add_error_handlers, add_error_monitor_middleware
 from cartsnitch_api.middleware.rate_limit import add_rate_limit_middleware
@@ -23,9 +25,10 @@ from cartsnitch_api.routes.user import router as user_router
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    # TODO: initialize DB session pool, Redis connection, service clients
+    await cache_client.initialize()
     yield
-    # TODO: cleanup connections
+    await cache_client.close()
+    await dispose_engine()
 
 
 def create_app() -> FastAPI:

api/src/cartsnitch_api/middleware/cors.py

@@ -11,6 +11,6 @@ def add_cors_middleware(app: FastAPI) -> None:
         CORSMiddleware,
         allow_origins=settings.cors_origins,
         allow_credentials=True,
-        allow_methods=["*"],
-        allow_headers=["*"],
+        allow_methods=["GET", "POST", "PUT", "DELETE", "PATCH", "OPTIONS"],
+        allow_headers=["Content-Type", "Authorization", "Accept", "Origin", "X-Requested-With"],
     )

api/src/cartsnitch_api/middleware/rate_limit.py

@@ -4,18 +4,32 @@ Uses in-memory sliding window as fallback, Redis/DragonflyDB when available.
 Per-IP limiting on public endpoints, per-token limiting on authenticated endpoints.
 """
 
+import hashlib
+import logging
 import time
+import uuid
 from collections import defaultdict
 from threading import Lock
+from typing import Protocol
 
 from fastapi import FastAPI, Request, status
 from fastapi.responses import JSONResponse
+from redis.asyncio import Redis, RedisError
 from starlette.middleware.base import BaseHTTPMiddleware
 
 from cartsnitch_api.config import settings
 
+logger = logging.getLogger(__name__)
 
-class _SlidingWindowCounter:
+
+class RateLimitBackend(Protocol):
+    """Protocol for rate limit backends."""
+
+    async def is_allowed(self, key: str) -> tuple[bool, int, int]:
+        """Check if request is allowed. Returns (allowed, remaining, retry_after)."""
+
+
+class InMemorySlidingWindow:
     """Thread-safe in-memory sliding window rate limiter."""
 
     def __init__(self, max_requests: int, window_seconds: int) -> None:
@@ -24,13 +38,12 @@ class _SlidingWindowCounter:
         self._hits: dict[str, list[float]] = defaultdict(list)
         self._lock = Lock()
 
-    def is_allowed(self, key: str) -> tuple[bool, int, int]:
+    async def is_allowed(self, key: str) -> tuple[bool, int, int]:
         """Check if request is allowed. Returns (allowed, remaining, retry_after)."""
         now = time.monotonic()
         cutoff = now - self.window_seconds
 
         with self._lock:
-            # Prune expired entries
             self._hits[key] = [t for t in self._hits[key] if t > cutoff]
 
             current_count = len(self._hits[key])
@@ -43,15 +56,84 @@ class _SlidingWindowCounter:
             return True, remaining, 0
 
 
-# Module-level counters — one for public (per-IP), one for auth (per-token)
-_public_limiter = _SlidingWindowCounter(
-    max_requests=settings.rate_limit_requests,
-    window_seconds=settings.rate_limit_window_seconds,
-)
-_auth_limiter = _SlidingWindowCounter(
-    max_requests=settings.rate_limit_requests * 5,  # 300/min for authenticated users
-    window_seconds=settings.rate_limit_window_seconds,
-)
+class RedisSlidingWindow:
+    """Redis-backed sliding window rate limiter using sorted sets."""
+
+    def __init__(self, redis: Redis, max_requests: int, window_seconds: int) -> None:
+        self.redis = redis
+        self.max_requests = max_requests
+        self.window_seconds = window_seconds
+
+    async def is_allowed(self, key: str) -> tuple[bool, int, int]:
+        """Check if request is allowed. Returns (allowed, remaining, retry_after)."""
+        try:
+            now = time.monotonic()
+            cutoff = now - self.window_seconds
+            now_ms = int(now * 1000)
+            cutoff_ms = int(cutoff * 1000)
+
+            pipe = self.redis.pipeline()
+            pipe.zremrangebyscore(key, 0, cutoff_ms)
+            pipe.zcard(key)
+            results = await pipe.execute()
+
+            current_count = results[1]
+
+            if current_count >= self.max_requests:
+                oldest = await self.redis.zrange(key, 0, 0, withscores=True)
+                if oldest:
+                    retry_after = int((oldest[0][1] - cutoff) / 1000) + 1
+                else:
+                    retry_after = self.window_seconds
+                return False, 0, retry_after
+
+            member = f"{now_ms}:{uuid.uuid4().hex[:8]}"
+            pipe = self.redis.pipeline()
+            pipe.zadd(key, {member: now_ms})
+            pipe.expire(key, self.window_seconds)
+            await pipe.execute()
+
+            remaining = self.max_requests - current_count - 1
+            return True, remaining, 0
+
+        except RedisError as e:
+            logger.warning("Redis rate limit error, falling back to in-memory: %s", e)
+            in_memory = InMemorySlidingWindow(self.max_requests, self.window_seconds)
+            return await in_memory.is_allowed(key)
+
+
+_redis_client: Redis | None = None
+_use_redis = False
+
+if settings.rate_limit_redis_enabled:
+    try:
+        _redis_client = Redis.from_url(settings.redis_url)
+        _use_redis = True
+        logger.info("Rate limiting will use Redis at %s", settings.redis_url)
+    except Exception as e:
+        logger.warning("Failed to connect to Redis for rate limiting, using in-memory: %s", e)
+        _use_redis = False
+
+if _use_redis and _redis_client:
+    _public_limiter = RedisSlidingWindow(
+        _redis_client, settings.rate_limit_requests, settings.rate_limit_window_seconds
+    )
+    _auth_limiter = RedisSlidingWindow(
+        _redis_client, settings.rate_limit_requests * 5, settings.rate_limit_window_seconds
+    )
+    _auth_strict_limiter = RedisSlidingWindow(
+        _redis_client, settings.rate_limit_auth_requests, settings.rate_limit_auth_window_seconds
+    )
+else:
+    _public_limiter = InMemorySlidingWindow(
+        settings.rate_limit_requests, settings.rate_limit_window_seconds
+    )
+    _auth_limiter = InMemorySlidingWindow(
+        settings.rate_limit_requests * 5, settings.rate_limit_window_seconds
+    )
+    _auth_strict_limiter = InMemorySlidingWindow(
+        settings.rate_limit_auth_requests, settings.rate_limit_auth_window_seconds
+    )
 
 
 def _get_client_ip(request: Request) -> str:
@@ -62,30 +144,30 @@ def _get_client_ip(request: Request) -> str:
     return request.client.host if request.client else "unknown"
 
 
-def _get_rate_limit_key(request: Request) -> tuple[str, _SlidingWindowCounter]:
+def _get_rate_limit_key(request: Request) -> tuple[str, RateLimitBackend]:
     """Determine rate limit key and which limiter to use."""
     if request.url.path.startswith("/public"):
         return f"ip:{_get_client_ip(request)}", _public_limiter
 
-    # For authenticated endpoints, use Bearer token as key if present
+    if request.url.path.startswith("/auth/") and request.method == "POST":
+        return f"ip:{_get_client_ip(request)}", _auth_strict_limiter
+
     auth_header = request.headers.get("authorization", "")
     if auth_header.startswith("Bearer "):
         token = auth_header[7:]
-        # Use last 16 chars of token as key to avoid storing full tokens
-        return f"token:{token[-16:]}", _auth_limiter
+        token_hash = hashlib.sha256(token.encode()).hexdigest()
+        return f"token:{token_hash}", _auth_limiter
 
-    # Fallback to IP for unauthenticated non-public endpoints
     return f"ip:{_get_client_ip(request)}", _public_limiter
 
 
 class RateLimitMiddleware(BaseHTTPMiddleware):
     async def dispatch(self, request: Request, call_next):
-        # Skip rate limiting when disabled (e.g. in tests) or for health checks
         if not settings.rate_limit_enabled or request.url.path == "/health":
             return await call_next(request)
 
         key, limiter = _get_rate_limit_key(request)
-        allowed, remaining, retry_after = limiter.is_allowed(key)
+        allowed, remaining, retry_after = await limiter.is_allowed(key)
 
         if not allowed:
             return JSONResponse(

api/tests/test_middleware/test_rate_limit.py

@@ -1,47 +1,184 @@
 """Tests for rate limiting middleware."""
 
+import time
+from unittest.mock import AsyncMock, MagicMock, patch
+
 import pytest
 
-from cartsnitch_api.middleware.rate_limit import _SlidingWindowCounter
+from cartsnitch_api.config import settings
+from cartsnitch_api.middleware.rate_limit import (
+    InMemorySlidingWindow,
+    RedisSlidingWindow,
+    _get_client_ip,
+    _get_rate_limit_key,
+)
 
 
-class TestSlidingWindowCounter:
+class TestInMemorySlidingWindow:
     def test_allows_within_limit(self):
-        counter = _SlidingWindowCounter(max_requests=5, window_seconds=60)
+        limiter = InMemorySlidingWindow(max_requests=5, window_seconds=60)
         for i in range(5):
-            allowed, remaining, retry = counter.is_allowed("test-key")
+            allowed, remaining, retry = limiter.is_allowed("test-key")
             assert allowed is True
             assert remaining == 4 - i
 
     def test_blocks_over_limit(self):
-        counter = _SlidingWindowCounter(max_requests=3, window_seconds=60)
+        limiter = InMemorySlidingWindow(max_requests=3, window_seconds=60)
         for _ in range(3):
-            counter.is_allowed("test-key")
+            limiter.is_allowed("test-key")
 
-        allowed, remaining, retry = counter.is_allowed("test-key")
+        allowed, remaining, retry = limiter.is_allowed("test-key")
         assert allowed is False
         assert remaining == 0
         assert retry > 0
 
     def test_separate_keys(self):
-        counter = _SlidingWindowCounter(max_requests=2, window_seconds=60)
-        # Fill key-a
-        counter.is_allowed("key-a")
-        counter.is_allowed("key-a")
-        allowed_a, _, _ = counter.is_allowed("key-a")
+        limiter = InMemorySlidingWindow(max_requests=2, window_seconds=60)
+        limiter.is_allowed("key-a")
+        limiter.is_allowed("key-a")
+        allowed_a, _, _ = limiter.is_allowed("key-a")
         assert allowed_a is False
 
-        # key-b should still be allowed
-        allowed_b, remaining, _ = counter.is_allowed("key-b")
+        allowed_b, remaining, _ = limiter.is_allowed("key-b")
         assert allowed_b is True
         assert remaining == 1
 
+    def test_resets_after_window_expires(self):
+        limiter = InMemorySlidingWindow(max_requests=2, window_seconds=1)
+        for _ in range(2):
+            limiter.is_allowed("test-key")
+        allowed, remaining, _ = limiter.is_allowed("test-key")
+        assert allowed is False
+
+        time.sleep(1.1)
+        allowed, remaining, _ = limiter.is_allowed("test-key")
+        assert allowed is True
+        assert remaining == 1
+
+
+class TestGetClientIp:
+    def test_x_forwarded_for_single(self):
+        req = MagicMock()
+        req.headers = {"x-forwarded-for": "192.168.1.1"}
+        req.client = None
+        assert _get_client_ip(req) == "192.168.1.1"
+
+    def test_x_forwarded_for_multiple(self):
+        req = MagicMock()
+        req.headers = {"x-forwarded-for": "192.168.1.1, 10.0.0.1, 172.16.0.1"}
+        req.client = None
+        assert _get_client_ip(req) == "192.168.1.1"
+
+    def test_x_forwarded_for_with_port(self):
+        req = MagicMock()
+        req.headers = {"x-forwarded-for": "192.168.1.1:8080"}
+        req.client = None
+        assert _get_client_ip(req) == "192.168.1.1"
+
+    def test_no_forwarded_header(self):
+        req = MagicMock()
+        req.headers = {}
+        req.client.host = "127.0.0.1"
+        assert _get_client_ip(req) == "127.0.0.1"
+
+    def test_no_client(self):
+        req = MagicMock()
+        req.headers = {}
+        req.client = None
+        assert _get_client_ip(req) == "unknown"
+
+
+class TestGetRateLimitKey:
+    def _make_request(
+        self,
+        path: str = "/purchases",
+        method: str = "GET",
+        auth_header: str = "",
+        headers: dict | None = None,
+    ) -> MagicMock:
+        req = MagicMock()
+        req.url.path = path
+        req.method = method
+        req.headers = dict(headers) if headers else {}
+        if auth_header:
+            req.headers["authorization"] = auth_header
+        return req
+
+    def test_public_path_uses_public_limiter(self):
+        req = self._make_request("/public/inflation")
+        key, limiter = _get_rate_limit_key(req)
+        assert key.startswith("ip:")
+        assert limiter.max_requests == settings.rate_limit_requests
+
+    def test_auth_post_path_uses_strict_limiter(self):
+        req = self._make_request("/auth/login", method="POST")
+        key, limiter = _get_rate_limit_key(req)
+        assert key.startswith("ip:")
+        assert limiter.max_requests == settings.rate_limit_auth_requests
+        assert limiter.window_seconds == settings.rate_limit_auth_window_seconds
+
+    def test_auth_get_path_uses_auth_limiter(self):
+        req = self._make_request("/auth/me", method="GET")
+        key, limiter = _get_rate_limit_key(req)
+        assert key.startswith("ip:")
+        assert limiter.max_requests == settings.rate_limit_requests * 5
+
+    def test_authenticated_token_uses_auth_limiter(self):
+        req = self._make_request("/purchases", auth_header="Bearer token123")
+        key, limiter = _get_rate_limit_key(req)
+        assert key.startswith("token:")
+        assert limiter.max_requests == settings.rate_limit_requests * 5
+
+    def test_distinct_tokens_produce_distinct_keys(self):
+        req1 = self._make_request("/purchases", auth_header="Bearer token_alpha_12345")
+        req2 = self._make_request("/purchases", auth_header="Bearer token_beta_67890")
+        key1, _ = _get_rate_limit_key(req1)
+        key2, _ = _get_rate_limit_key(req2)
+        assert key1 != key2
+
+    def test_same_token_produces_same_key(self):
+        req1 = self._make_request("/purchases", auth_header="Bearer same_token_value_abc")
+        req2 = self._make_request("/purchases", auth_header="Bearer same_token_value_abc")
+        key1, _ = _get_rate_limit_key(req1)
+        key2, _ = _get_rate_limit_key(req2)
+        assert key1 == key2
+
+    def test_key_does_not_contain_raw_token_suffix(self):
+        raw_token = "my_secret_jwt_token_xyz"
+        req = self._make_request("/purchases", auth_header=f"Bearer {raw_token}")
+        key, _ = _get_rate_limit_key(req)
+        assert raw_token[-16:] not in key
+        assert raw_token not in key
+
+
+class TestRedisSlidingWindowFallback:
+    @pytest.mark.asyncio
+    async def test_fallback_on_redis_connection_error(self):
+        mock_redis = AsyncMock()
+        mock_redis.pipeline.return_value = AsyncMock()
+        pipe_mock = AsyncMock()
+        pipe_mock.execute.side_effect = Exception("Connection refused")
+        mock_redis.pipeline.return_value = pipe_mock
+
+        limiter = RedisSlidingWindow(mock_redis, max_requests=5, window_seconds=60)
+        allowed, remaining, retry = await limiter.is_allowed("test-key")
+        assert allowed is True
+        assert remaining == 4
+
+    @pytest.mark.asyncio
+    async def test_fallback_on_redis_error_during_pipeline(self):
+        mock_redis = AsyncMock()
+        pipe_mock = AsyncMock()
+        pipe_mock.execute.side_effect = Exception("Redis error")
+        mock_redis.pipeline.return_value = pipe_mock
+
+        limiter = RedisSlidingWindow(mock_redis, max_requests=3, window_seconds=60)
+        allowed, remaining, retry = await limiter.is_allowed("test-key")
+        assert allowed is True
+
 
 @pytest.mark.asyncio
 async def test_rate_limit_returns_429(client):
-    """Public endpoint should return 429 after limit exceeded."""
-    # The default limit is 60/min — we won't hit it in normal tests,
-    # but we verify the middleware adds rate limit headers.
     resp = await client.get("/public/inflation")
     assert "x-ratelimit-limit" in resp.headers
     assert "x-ratelimit-remaining" in resp.headers
@@ -49,7 +186,6 @@ async def test_rate_limit_returns_429(client):
 
 @pytest.mark.asyncio
 async def test_health_skips_rate_limit(client):
-    """Health endpoint should not have rate limit headers."""
     resp = await client.get("/health")
     assert resp.status_code == 200
     assert "x-ratelimit-limit" not in resp.headers

nginx.conf

@@ -9,6 +9,12 @@ server {
     gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript image/svg+xml;
     gzip_min_length 256;
 
+    # Security headers
+    add_header X-Frame-Options "SAMEORIGIN" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+    add_header Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self'; connect-src 'self' https://*.cartsnitch.com https://*.farh.net; frame-ancestors 'self'" always;
+
     # Health endpoint for K8s probes
     location /health {
         access_log off;

receiptwitness/src/receiptwitness/config.py

@@ -1,8 +1,12 @@
 """Service-specific configuration for ReceiptWitness."""
 
+from pydantic import model_validator
 from pydantic_settings import BaseSettings
 
 
+_PLACEHOLDER_VALUES = {"change-me-in-production"}
+
+
 class ReceiptWitnessSettings(BaseSettings):
     model_config = {"env_prefix": "RW_"}
 
@@ -30,5 +34,34 @@ class ReceiptWitnessSettings(BaseSettings):
     # Mailgun inbound email webhook
     mailgun_webhook_signing_key: str = ""
 
+    @model_validator(mode="after")
+    def validate_required_vars(self):
+        errors = []
+        if not self.session_encryption_key or self.session_encryption_key in _PLACEHOLDER_VALUES:
+            errors.append(
+                "RW_SESSION_ENCRYPTION_KEY must be set to a secure value. "
+                'Generate one with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"'
+            )
+        if self.notifications_enabled and not self.resend_api_key:
+            errors.append(
+                "RW_RESEND_API_KEY must be set when RW_NOTIFICATIONS_ENABLED=true. "
+                "Get an API key from https://resend.com/api-keys"
+            )
+        if errors:
+            raise ValueError(
+                "ReceiptWitness startup failed — missing required config:\n"
+                + "\n".join(f"  - {e}" for e in errors)
+            )
+        return self
 
-settings = ReceiptWitnessSettings()
+
+class _LazySettings:
+    _instance: ReceiptWitnessSettings | None = None
+
+    def __getattr__(self, name: str):
+        if _LazySettings._instance is None:
+            _LazySettings._instance = ReceiptWitnessSettings()
+        return getattr(_LazySettings._instance, name)
+
+
+settings = _LazySettings()

receiptwitness/tests/conftest.py

@@ -1,12 +1,16 @@
 """Shared test fixtures."""
 
 import json
+import os
 from pathlib import Path
 
 import pytest
 
 FIXTURES_DIR = Path(__file__).parent / "fixtures"
 
+os.environ.setdefault("RW_SESSION_ENCRYPTION_KEY", "test-secret-key-for-unit-tests-only-32bytes!")
+os.environ.setdefault("RW_MAILGUN_WEBHOOK_SIGNING_KEY", "test-mailgun-signing-key")
+
 
 @pytest.fixture
 def meijer_receipt_data() -> dict:

receiptwitness/tests/test_config.py

@@ -0,0 +1,46 @@
+import pytest
+from receiptwitness.config import ReceiptWitnessSettings
+
+
+def test_valid_config():
+    s = ReceiptWitnessSettings(
+        session_encryption_key="7reF42nmTwbdN21PBoubGp7h_FU8qSimstmlaMLoRK8="
+    )
+    assert s.session_encryption_key
+
+
+def test_missing_session_encryption_key_raises():
+    with pytest.raises(ValueError, match="RW_SESSION_ENCRYPTION_KEY"):
+        ReceiptWitnessSettings(session_encryption_key="")
+
+
+def test_placeholder_session_encryption_key_raises():
+    with pytest.raises(ValueError, match="RW_SESSION_ENCRYPTION_KEY"):
+        ReceiptWitnessSettings(session_encryption_key="change-me-in-production")
+
+
+def test_notifications_enabled_without_resend_key_raises():
+    with pytest.raises(ValueError, match="RW_RESEND_API_KEY"):
+        ReceiptWitnessSettings(
+            session_encryption_key="7reF42nmTwbdN21PBoubGp7h_FU8qSimstmlaMLoRK8=",
+            notifications_enabled=True,
+            resend_api_key="",
+        )
+
+
+def test_notifications_disabled_without_resend_key_ok():
+    s = ReceiptWitnessSettings(
+        session_encryption_key="7reF42nmTwbdN21PBoubGp7h_FU8qSimstmlaMLoRK8=",
+        notifications_enabled=False,
+        resend_api_key="",
+    )
+    assert s.notifications_enabled is False
+
+
+def test_notifications_enabled_with_resend_key_ok():
+    s = ReceiptWitnessSettings(
+        session_encryption_key="7reF42nmTwbdN21PBoubGp7h_FU8qSimstmlaMLoRK8=",
+        notifications_enabled=True,
+        resend_api_key="re_test_1234567890",
+    )
+    assert s.resend_api_key == "re_test_1234567890"