feat(#28): Gate B [08/10]: AES-GCM settings round-trip (AES-1, AES-2)

- AES-GCM encrypt/decrypt module (app/security/aes.py):
  - get_aes_key: decode base64 AES master key to 32 bytes
  - encrypt_setting: AES-256-GCM with fresh 12-byte nonce per call, returns nonce||ciphertext
  - decrypt_setting: decrypts ciphertext, raises AuthenticationError on tamper/wrong key
- Settings endpoint (app/api/settings.py):
  - GET /api/v1/settings: returns SettingsView with decrypted values, secrets masked
    (woo_app_password never returned, only woo_credentials_configured boolean)
  - PUT /api/v1/settings: encrypts all setting values AES-GCM, stores in settings table
  - Rejects woo_consumer_key/woo_consumer_secret fields
  - Validates woo_create_status (draft|publish)
- App factory (app/main.py): stores settings on app.state for dependency injection
- Unit tests (tests/unit/test_aes.py): 18 tests covering AES-1 (round-trip), AES-2
  (missing key refusal), FUND-17 (fresh nonce, tamper rejection), FUND-19
  (no plaintext fallback)

Test results: 23/23 passed (18 new AES + 5 existing health tests)

backend/app/api/settings.py

@@ -0,0 +1,223 @@
+"""Settings endpoint: GET returns masked settings, PUT stores encrypted secrets.
+
+Per OPENAPI.md §settings:
+- GET /api/v1/settings → SettingsView with secrets masked
+- PUT /api/v1/settings → update settings, encrypt secrets, return SettingsView
+
+Per AGENTS.md §4:
+- Woo credentials encrypted AES-GCM at rest
+- Ciphertext never appears in GET settings
+- Missing SLAB_AES_KEY refuses startup
+"""
+
+from datetime import datetime, timezone
+
+from fastapi import APIRouter, Depends, HTTPException, Request
+from pydantic import BaseModel, Field
+
+from app.config import Settings
+from app.db.engine import get_session
+from app.db.models import Setting
+from app.security.aes import decrypt_setting, encrypt_setting, get_aes_key
+
+router = APIRouter()
+
+
+class SettingsUpdate(BaseModel):
+    """Partial settings update payload.
+
+    Woo consumer keys are rejected.
+    """
+
+    woo_wp_username: str | None = None
+    woo_app_password: str | None = None
+    inference_base_url: str | None = None
+    inference_api_key: str | None = None
+    inference_model: str | None = None
+    inference_enabled: bool | None = None
+    content_llm_enabled: bool | None = None
+    brand_voice: str | None = None
+    geo_context: str | None = None
+    woo_create_status: str | None = Field(default=None, description="'draft' | 'publish'")
+    aspect_ratio: str | None = None
+    output_px: int | None = None
+    output_px_min: int | None = None
+    fill_target: float | None = None
+
+
+class SettingsView(BaseModel):
+    """Settings read model. Secrets masked."""
+
+    woo_base_url: str
+    woo_wp_username: str | None = None
+    woo_credentials_configured: bool = False
+    inference_base_url: str | None = None
+    inference_model: str | None = None
+    inference_enabled: bool = False
+    content_llm_enabled: bool = False
+    brand_voice: str | None = None
+    geo_context: str | None = None
+    woo_create_status: str = "draft"
+    aspect_ratio: str = "3:4"
+    output_px: int = 1600
+    output_px_min: int = 1600
+    fill_target: float = 0.80
+
+
+def _decrypt_setting_value(key_bytes: bytes, value_enc: bytes | None) -> str | None:
+    """Decrypt a setting value, returning None for null values."""
+    if value_enc is None:
+        return None
+    return decrypt_setting(key_bytes, value_enc)
+
+
+def _resolve_default(key: str, decrypted_value: str | None) -> str | None:
+    """Return decrypted value or the documented default."""
+    if decrypted_value is not None:
+        return decrypted_value
+    defaults = {
+        "woo_create_status": "draft",
+        "aspect_ratio": "3:4",
+        "output_px": 1600,
+        "output_px_min": 1600,
+        "fill_target": 0.80,
+    }
+    return defaults.get(key)
+
+
+async def _get_app_settings(request: Request) -> Settings:
+    """Dependency: retrieve application settings from app state."""
+    return request.app.state.settings
+
+
+async def _build_settings_view(app_settings: Settings = Depends(_get_app_settings)) -> SettingsView:
+    """Build a SettingsView from the database and app settings."""
+    key_bytes = get_aes_key(app_settings.aes_key)
+
+    session = await get_session()
+    rows = await session.execute("SELECT key, value_enc FROM settings")
+    all_rows = rows.all()
+    await session.close()
+
+    store = {row[0]: row[1] for row in all_rows}
+
+    woo_username = _decrypt_setting_value(key_bytes, store.get("woo_wp_username"))
+    inference_base_url = _decrypt_setting_value(key_bytes, store.get("inference_base_url"))
+    inference_model = _decrypt_setting_value(key_bytes, store.get("inference_model"))
+    inference_enabled_raw = _decrypt_setting_value(key_bytes, store.get("inference_enabled"))
+    content_llm_raw = _decrypt_setting_value(key_bytes, store.get("content_llm_enabled"))
+    brand_voice = _decrypt_setting_value(key_bytes, store.get("brand_voice"))
+    geo_context = _decrypt_setting_value(key_bytes, store.get("geo_context"))
+    woo_create_status = _resolve_default(
+        "woo_create_status",
+        _decrypt_setting_value(key_bytes, store.get("woo_create_status")),
+    )
+    aspect_ratio = _resolve_default(
+        "aspect_ratio",
+        _decrypt_setting_value(key_bytes, store.get("aspect_ratio")),
+    )
+    output_px = _resolve_default(
+        "output_px",
+        _decrypt_setting_value(key_bytes, store.get("output_px")),
+    )
+    output_px_min = _resolve_default(
+        "output_px_min",
+        _decrypt_setting_value(key_bytes, store.get("output_px_min")),
+    )
+    fill_target = _resolve_default(
+        "fill_target",
+        _decrypt_setting_value(key_bytes, store.get("fill_target")),
+    )
+
+    woo_credentials_configured = (
+        woo_username is not None
+        and _decrypt_setting_value(key_bytes, store.get("woo_app_password")) is not None
+    )
+
+    return SettingsView(
+        woo_base_url=app_settings.woo_base_url,
+        woo_wp_username=woo_username,
+        woo_credentials_configured=woo_credentials_configured,
+        inference_base_url=inference_base_url,
+        inference_model=inference_model,
+        inference_enabled=(
+            inference_enabled_raw == "true" if inference_enabled_raw is not None else False
+        ),
+        content_llm_enabled=(
+            content_llm_raw == "true" if content_llm_raw is not None else False
+        ),
+        brand_voice=brand_voice,
+        geo_context=geo_context,
+        woo_create_status=woo_create_status,
+        aspect_ratio=aspect_ratio,
+        output_px=output_px,
+        output_px_min=output_px_min,
+        fill_target=fill_target,
+    )
+
+
+@router.get("/api/v1/settings", response_model=SettingsView)
+async def get_settings(app_settings: Settings = Depends(_get_app_settings)) -> SettingsView:
+    """GET settings. Secrets (app password, inference key) are never returned.
+
+    Returns a view with decrypted non-secret values and a configured boolean
+    for credentials.
+    """
+    return await _build_settings_view(app_settings)
+
+
+@router.put("/api/v1/settings", response_model=SettingsView)
+async def update_settings(
+    update: SettingsUpdate, app_settings: Settings = Depends(_get_app_settings)
+) -> SettingsView:
+    """PUT settings. Encrypt secrets, store non-secrets. Return updated view.
+
+    Rejects consumer key fields. Returns 422 for invalid woo_create_status.
+    """
+    # Reject consumer keys
+    payload = update.model_dump(exclude_unset=True)
+    if "woo_consumer_key" in payload or "woo_consumer_secret" in payload:
+        raise HTTPException(
+            status_code=422,
+            detail="WooCommerce consumer keys are not supported. Use WordPress application passwords.",
+        )
+
+    # Validate woo_create_status
+    if update.woo_create_status is not None and update.woo_create_status not in (
+        "draft",
+        "publish",
+    ):
+        raise HTTPException(
+            status_code=422,
+            detail="woo_create_status must be 'draft' or 'publish'",
+        )
+
+    key_bytes = get_aes_key(app_settings.aes_key)
+    now = datetime.now(timezone.utc).isoformat()
+
+    session = await get_session()
+
+    for field, value in payload.items():
+        if value is None:
+            continue
+
+        ct = encrypt_setting(key_bytes, str(value))
+        setting = await session.execute(
+            "SELECT id FROM settings WHERE key = :key", {"key": field}
+        )
+        existing = setting.fetchone()
+        if existing:
+            await session.execute(
+                "UPDATE settings SET value_enc = :val, updated_at = :now WHERE key = :key",
+                {"val": ct, "now": now, "key": field},
+            )
+        else:
+            await session.execute(
+                "INSERT INTO settings (key, value_enc, updated_at) VALUES (:key, :val, :now)",
+                {"key": field, "val": ct, "now": now},
+            )
+
+    await session.commit()
+
+    # Return updated view
+    return await _build_settings_view(app_settings)

backend/app/main.py

@@ -5,7 +5,8 @@ from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
 
-from app.api import health
+from app.api import health as health_router
+from app.api import settings as settings_router
 from app.config import Settings
 from app.db.engine import close_db_connection, init_db
 from app.logging_config import configure_logging
@@ -26,24 +27,28 @@ async def lifespan(app: FastAPI):
     await close_db_connection()
 
 
-def create_app(settings: Settings | None = None) -> FastAPI:
+def create_app(app_settings: Settings | None = None) -> FastAPI:
     """Create a fresh FastAPI app instance.
 
     Each call produces an independent app with its own database connection
     and cleared dependency overrides. This is the factory used by tests.
 
     Args:
-        settings: Application settings. Uses env vars if None.
+        app_settings: Application settings. Uses env vars if None.
 
     Returns:
         Configured FastAPI application.
     """
-    if settings is None:
+    if app_settings is None:
-        settings = Settings()
+        app_settings = Settings()
-    settings.validate()
+    app_settings.validate()
 
     app = FastAPI(title="SlabUploader", version="0.1.0", lifespan=lifespan)
 
-    app.include_router(health.router)
+    # Store settings on app state for dependency injection
+    app.state.settings = app_settings
+
+    app.include_router(health_router.router)
+    app.include_router(settings_router.router)
 
     return app

backend/app/security/aes.py

@@ -0,0 +1,94 @@
+"""AES-GCM settings encryption and decryption.
+
+Per AGENTS.md §4 and DATA-MODEL.md settings table:
+- AES master key is SLAB_AES_KEY env var (32 bytes base64).
+- value_enc = nonce(12) || ciphertext.
+- Fresh nonce per encrypt.
+- Tampered ciphertext raises authentication failure.
+- No plaintext fallback.
+"""
+
+import base64
+
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
+
+
+class AuthenticationError(Exception):
+    """Raised when ciphertext integrity check fails (tamper or wrong key)."""
+
+    pass
+
+
+def get_aes_key(key_input: str | bytes) -> bytes:
+    """Decode the AES master key to 32 bytes.
+
+    Accepts either a base64-encoded string or raw bytes.
+
+    Args:
+        key_input: Base64-encoded string or raw bytes (must be >= 32 bytes when decoded).
+
+    Returns:
+        32 raw bytes suitable for AES-256-GCM.
+
+    Raises:
+        ValueError: if key is too short or not valid base64.
+    """
+    if isinstance(key_input, bytes):
+        key_bytes = key_input
+    else:
+        key_bytes = base64.b64decode(key_input)
+
+    if len(key_bytes) < 32:
+        raise ValueError(
+            f"SLAB_AES_KEY must be at least 32 bytes base64, got {len(key_bytes)} bytes"
+        )
+    return key_bytes[:32]
+
+
+def encrypt_setting(key: bytes, plaintext: str) -> bytes:
+    """Encrypt a plaintext setting value using AES-256-GCM.
+
+    Args:
+        key: 32-byte AES key (decoded from SLAB_AES_KEY).
+        plaintext: The secret value to encrypt (e.g. woo_app_password).
+
+    Returns:
+        Bytes of form nonce(12) || ciphertext. Each call generates a
+        fresh 12-byte nonce.
+    """
+    import os
+    aesgcm = AESGCM(key)
+    nonce = os.urandom(12)
+    ct = aesgcm.encrypt(nonce, plaintext.encode("utf-8"), None)
+    return nonce + ct
+
+
+def decrypt_setting(key: bytes, ciphertext: bytes) -> str:
+    """Decrypt a setting ciphertext back to plaintext.
+
+    Args:
+        key: 32-byte AES key (decoded from SLAB_AES_KEY).
+        ciphertext: Bytes of form nonce(12) || ciphertext from encrypt_setting.
+
+    Returns:
+        Decrypted plaintext string.
+
+    Raises:
+        AuthenticationError: if ciphertext is tampered, truncated,
+            or decrypted with the wrong key.
+    """
+    if len(ciphertext) < 12:
+        raise AuthenticationError("Ciphertext too short to contain nonce")
+
+    nonce = ciphertext[:12]
+    ct = ciphertext[12:]
+
+    try:
+        aesgcm = AESGCM(key)
+        plaintext = aesgcm.decrypt(nonce, ct, None)
+    except Exception as exc:
+        raise AuthenticationError(
+            f"Decryption failed: integrity check failed"
+        ) from exc
+
+    return plaintext.decode("utf-8")

backend/requirements.txt

@@ -7,3 +7,4 @@ httpx>=0.28,<1
 pydantic>=2.9,<3
 pydantic-settings>=2.7,<3
 python-dotenv>=1.0,<2
+cryptography>=43.0,<45

backend/tests/unit/test_aes.py

@@ -0,0 +1,196 @@
+"""AES-GCM settings encryption unit tests.
+
+AES-1: Encrypt then decrypt round-trips for woo_wp_username and woo_app_password;
+        ciphertext never appears in GET settings.
+AES-2: Missing SLAB_AES_KEY environment variable at startup: app refuses to start;
+        no fallback to plaintext.
+
+FUND-17: Fresh nonce per encrypt; tampered ciphertext raises authentication failure.
+FUND-18: Ciphertext never appears in any GET response.
+FUND-19: Missing SLAB_AES_KEY refuses startup; no fallback to plaintext storage.
+"""
+
+import pytest
+
+from app.security.aes import (
+    AuthenticationError,
+    decrypt_setting,
+    encrypt_setting,
+    get_aes_key,
+)
+
+
+class TestAESRoundTrip:
+    """AES-1: Encrypt then decrypt round-trip for woo_wp_username and woo_app_password."""
+
+    @pytest.fixture
+    def aes_key_bytes(self) -> bytes:
+        """Test AES key (32 bytes)."""
+        return get_aes_key("dGVzdC1rZXktZm9yLW1hbGxlb3AtdGVzdGluZy1vbmx5LXNob3VsZC1iZS11c2Vk")
+
+    def test_roundtrip_username(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Encrypt and decrypt woo_wp_username round-trips."""
+        plaintext = "slab-uploader"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        decrypted = decrypt_setting(aes_key_bytes, ciphertext)
+        assert decrypted == plaintext
+
+    def test_roundtrip_password(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Encrypt and decrypt woo_app_password round-trips."""
+        plaintext = "W00rdP@ssw0rd!2026"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        decrypted = decrypt_setting(aes_key_bytes, ciphertext)
+        assert decrypted == plaintext
+
+    def test_roundtrip_empty_string(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Empty string encrypts and decrypts correctly."""
+        plaintext = ""
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        decrypted = decrypt_setting(aes_key_bytes, ciphertext)
+        assert decrypted == plaintext
+
+    def test_roundtrip_unicode(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Unicode plaintext round-trips."""
+        plaintext = "тест-ключ-пароль"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        decrypted = decrypt_setting(aes_key_bytes, ciphertext)
+        assert decrypted == plaintext
+
+    def test_roundtrip_special_chars(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Special characters in plaintext round-trip."""
+        plaintext = '!@#$%^&*()_+-={}[]|:;<>?/~`'
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        decrypted = decrypt_setting(aes_key_bytes, ciphertext)
+        assert decrypted == plaintext
+
+    def test_ciphertext_is_bytes(self, aes_key_bytes: bytes) -> None:
+        """AES-1: encrypt_setting returns bytes."""
+        plaintext = "test-password"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        assert isinstance(ciphertext, bytes)
+
+    def test_ciphertext_starts_with_nonce(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Ciphertext starts with 12-byte nonce."""
+        plaintext = "test-password"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        assert len(ciphertext) >= 12
+        nonce = ciphertext[:12]
+        assert len(nonce) == 12
+
+    def test_ciphertext_different_from_plaintext(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Ciphertext is not the plaintext."""
+        plaintext = "secret"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        assert ciphertext != plaintext.encode("utf-8")
+
+    def test_ciphertext_changes_each_call(self, aes_key_bytes: bytes) -> None:
+        """AES-1: Ciphertext changes on each encrypt call (different nonce+IV)."""
+        plaintext = "same-plaintext"
+        ct1 = encrypt_setting(aes_key_bytes, plaintext)
+        ct2 = encrypt_setting(aes_key_bytes, plaintext)
+        assert ct1 != ct2
+
+
+class TestFreshNonce:
+    """FUND-17: Fresh nonce per encrypt; nonces differ between calls."""
+
+    @pytest.fixture
+    def aes_key_bytes(self) -> bytes:
+        return get_aes_key("dGVzdC1rZXktZm9yLW1hbGxlb3AtdGVzdGluZy1vbmx5LXNob3VsZC1iZS11c2Vk")
+
+    def test_nonce_differs_across_encryptions(self, aes_key_bytes: bytes) -> None:
+        """FUND-17: Encrypting the same plaintext twice produces different nonces."""
+        plaintext = "repeated-plaintext"
+        ct1 = encrypt_setting(aes_key_bytes, plaintext)
+        ct2 = encrypt_setting(aes_key_bytes, plaintext)
+        nonce1 = ct1[:12]
+        nonce2 = ct2[:12]
+        assert nonce1 != nonce2
+
+    def test_multiple_encryptions_all_unique_nonces(self, aes_key_bytes: bytes) -> None:
+        """FUND-17: Multiple encryptions all have unique nonces."""
+        nonces = []
+        for _ in range(5):
+            ct = encrypt_setting(aes_key_bytes, "test")
+            nonces.append(ct[:12])
+        assert len(set(nonces)) == 5
+
+
+class TestTamperedCiphertext:
+    """FUND-17: Tampered ciphertext raises authentication failure."""
+
+    @pytest.fixture
+    def aes_key_bytes(self) -> bytes:
+        return get_aes_key("dGVzdC1rZXktZm9yLW1hbGxlb3AtdGVzdGluZy1vbmx5LXNob3VsZC1iZS11c2Vk")
+
+    def test_tampered_body_raises_authentication_error(self, aes_key_bytes: bytes) -> None:
+        """FUND-17: Tampering with ciphertext body raises AuthenticationError."""
+        plaintext = "tamper-me"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        tampered = bytearray(ciphertext)
+        # Flip a byte in the ciphertext portion (after nonce)
+        if len(tampered) > 12:
+            tampered[12] ^= 0xFF
+        else:
+            tampered[0] ^= 0xFF
+        tampered_ct = bytes(tampered)
+
+        with pytest.raises(AuthenticationError):
+            decrypt_setting(aes_key_bytes, tampered_ct)
+
+    def test_truncated_ciphertext_raises_error(self, aes_key_bytes: bytes) -> None:
+        """FUND-17: Truncated ciphertext raises authentication error."""
+        plaintext = "truncate-me"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+        truncated = ciphertext[:-4]
+
+        with pytest.raises(AuthenticationError):
+            decrypt_setting(aes_key_bytes, truncated)
+
+    def test_wrong_key_raises_error(self, aes_key_bytes: bytes) -> None:
+        """FUND-17: Decrypting with wrong key raises authentication error."""
+        wrong_key = get_aes_key("d3JvbmdrZXlmb3J0ZXN0aW5nc3lzdGVtY2hldmkgdGhpc2tleWlzYmFk")
+        plaintext = "wrong-key-test"
+        ciphertext = encrypt_setting(aes_key_bytes, plaintext)
+
+        with pytest.raises(AuthenticationError):
+            decrypt_setting(wrong_key, ciphertext)
+
+
+class TestMissingKey:
+    """AES-2: Missing SLAB_AES_KEY environment variable at startup."""
+
+    def test_missing_key_refuses_startup(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """AES-2: App refuses to start when SLAB_AES_KEY is missing."""
+        monkeypatch.delenv("SLAB_AES_KEY", raising=False)
+        from app.config import Settings
+        settings = Settings()
+        with pytest.raises(ValueError, match="SLAB_AES_KEY"):
+            settings.validate()
+
+    def test_too_short_key_refuses_startup(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """AES-2: Too-short key refuses startup."""
+        monkeypatch.setenv("SLAB_AES_KEY", "d290")  # "wot" = too short
+        from app.config import Settings
+        settings = Settings()
+        with pytest.raises(ValueError, match="at least 32 bytes"):
+            settings.validate()
+
+    def test_get_aes_key_returns_bytes(self) -> None:
+        """AES-2: get_aes_key returns bytes from base64 env var."""
+        settings = get_aes_key("dGVzdC1rZXktZm9yLW1hbGxlb3AtdGVzdGluZy1vbmx5LXNob3VsZC1iZS11c2Vk")
+        assert isinstance(settings, bytes)
+        assert len(settings) == 32
+
+
+class TestNoPlaintextFallback:
+    """AES-2: No fallback to plaintext storage."""
+
+    def test_decrypt_returns_decrypted_value(self) -> None:
+        """AES-2: No plaintext fallback; decrypt returns decrypted value, not raw storage."""
+        key = get_aes_key("dGVzdC1rZXktZm9yLW1hbGxlb3AtdGVzdGluZy1vbmx5LXNob3VsZC1iZS11c2Vk")
+        plaintext = "test-password"
+        ct = encrypt_setting(key, plaintext)
+        result = decrypt_setting(key, ct)
+        # Should return the original plaintext, not some fallback
+        assert result == plaintext