security: encrypt synchronized unfiled drafts (Closes #1098)

src/main.py

@@ -60,7 +60,12 @@ from src.push_endpoint_policy import UnsafePushEndpoint, validate_public_push_en
 from src.push_subscription_store import PushSubscriptionStore
 from src.request_boundary import RequestBodyLimitMiddleware, request_body_limit
 from src.saved_search_store import SavedSearchConflict, SavedSearchStore
-from src.unfiled_draft_store import UnfiledDraftConflict, UnfiledDraftStore
+from src.unfiled_draft_store import (
+    UnfiledDraftConflict,
+    UnfiledDraftEncryptionError,
+    UnfiledDraftStore,
+    decode_unfiled_draft_encryption_key,
+)
 from src.security_event_store import SecurityEventStore, SecurityEventStoreError
 from src.suggestion_engine import compute
 from src.later_store import LaterStore
@@ -2285,7 +2290,10 @@ def _completed_filed_review_store() -> CompletedFiledReviewStore:
 
 def _unfiled_draft_store() -> UnfiledDraftStore:
     return UnfiledDraftStore(
-        os.getenv("STACKCHAIN_UNFILED_DRAFT_DB", str(_state_dir / "unfiled-drafts.sqlite3"))
+        os.getenv("STACKCHAIN_UNFILED_DRAFT_DB", str(_state_dir / "unfiled-drafts.sqlite3")),
+        encryption_key=decode_unfiled_draft_encryption_key(
+            os.getenv("STACKCHAIN_UNFILED_DRAFT_ENCRYPTION_KEY", "")
+        ),
     )
 
 
@@ -2386,7 +2394,7 @@ async def get_unfiled_drafts(response: Response):
     login = await _confirmed_login()
     try:
         snapshot = await asyncio.to_thread(_unfiled_draft_store().get, login)
-    except (OSError, sqlite3.Error):
+    except (OSError, sqlite3.Error, UnfiledDraftEncryptionError):
         raise HTTPException(
             status_code=503,
             detail="Draft synchronization is unavailable",
@@ -2416,7 +2424,7 @@ async def replace_unfiled_drafts(payload: UnfiledDraftCollection):
         )
     except ValueError as exc:
         raise HTTPException(status_code=422, detail=str(exc))
-    except (OSError, sqlite3.Error):
+    except (OSError, sqlite3.Error, UnfiledDraftEncryptionError):
         raise HTTPException(
             status_code=503,
             detail="Draft synchronization is unavailable",

src/unfiled_draft_store.py

@@ -3,11 +3,15 @@
 import base64
 import binascii
 import json
+import os
 import re
 import sqlite3
 from datetime import datetime
 from pathlib import Path
 
+from cryptography.exceptions import InvalidTag
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
+
 from src.private_state import connect_private_sqlite
 
 
@@ -24,16 +28,41 @@ class UnfiledDraftConflict(ValueError):
         self.snapshot = snapshot
 
 
+class UnfiledDraftEncryptionError(RuntimeError):
+    """Raised when private draft state cannot be authenticated and decrypted."""
+
+
+def decode_unfiled_draft_encryption_key(encoded: str) -> bytes:
+    """Decode the independently injected 256-bit draft-encryption key."""
+    try:
+        key = base64.b64decode(encoded, validate=True)
+    except (binascii.Error, ValueError) as error:
+        raise UnfiledDraftEncryptionError(
+            "unfiled draft encryption key is invalid"
+        ) from error
+    if len(key) != 32:
+        raise UnfiledDraftEncryptionError(
+            "unfiled draft encryption key must decode to exactly 32 bytes"
+        )
+    return key
+
+
 class UnfiledDraftStore:
     def __init__(
         self,
         path: str | Path,
         *,
+        encryption_key: bytes,
         limit: int = 20,
         max_total_bytes: int = 12 * 1024 * 1024,
         timeout: float = 1.0,
     ):
         self.path = Path(path)
+        if not isinstance(encryption_key, bytes) or len(encryption_key) != 32:
+            raise UnfiledDraftEncryptionError(
+                "unfiled draft encryption requires exactly 32 key bytes"
+            )
+        self._cipher = AESGCM(encryption_key)
         self.limit = limit
         self.max_total_bytes = max_total_bytes
         self.timeout = timeout
@@ -61,18 +90,71 @@ class UnfiledDraftStore:
         return normalized
 
     @staticmethod
-    def _snapshot(row) -> dict:
-        return {"revision": 0, "drafts": []} if row is None else {
-            "revision": int(row[0]), "drafts": json.loads(row[1])
-        }
+    def _aad(login: str, revision: int) -> bytes:
+        return f"stackchain:unfiled-drafts:v1\0{login}\0{revision}".encode()
+
+    def _encrypt(self, login: str, revision: int, drafts: list[dict]) -> str:
+        plaintext = json.dumps(drafts, separators=(",", ":")).encode()
+        nonce = os.urandom(12)
+        sealed = nonce + self._cipher.encrypt(
+            nonce, plaintext, self._aad(login, revision)
+        )
+        return "v1:" + base64.urlsafe_b64encode(sealed).decode()
+
+    def _decrypt(self, login: str, revision: int, payload: str) -> list[dict]:
+        try:
+            sealed = base64.b64decode(payload[3:], altchars=b"-_", validate=True)
+            if len(sealed) < 12 + 16:
+                raise ValueError("encrypted payload is too short")
+            plaintext = self._cipher.decrypt(
+                sealed[:12], sealed[12:], self._aad(login, revision)
+            )
+            drafts = json.loads(plaintext)
+            if not isinstance(drafts, list):
+                raise ValueError("decrypted payload is not a collection")
+            return drafts
+        except (binascii.Error, InvalidTag, UnicodeDecodeError, ValueError, json.JSONDecodeError) as error:
+            raise UnfiledDraftEncryptionError(
+                "unfiled drafts could not be decrypted"
+            ) from error
+
+    def _snapshot(self, login: str, row) -> tuple[dict, bool]:
+        if row is None:
+            return {"revision": 0, "drafts": []}, False
+        revision, payload = int(row[0]), row[1]
+        if payload.startswith("v1:"):
+            drafts = self._decrypt(login, revision, payload)
+            return {"revision": revision, "drafts": drafts}, False
+        try:
+            drafts = json.loads(payload)
+        except (TypeError, json.JSONDecodeError) as error:
+            raise UnfiledDraftEncryptionError(
+                "legacy unfiled drafts could not be decoded"
+            ) from error
+        if not isinstance(drafts, list):
+            raise UnfiledDraftEncryptionError(
+                "legacy unfiled drafts could not be decoded"
+            )
+        return {"revision": revision, "drafts": drafts}, True
 
     def get(self, login: str) -> dict:
+        login = self._login(login)
         with self._connect() as connection:
+            connection.execute("BEGIN IMMEDIATE")
             row = connection.execute(
                 "SELECT revision, drafts FROM unfiled_drafts WHERE login = ?",
-                (self._login(login),),
+                (login,),
             ).fetchone()
-        return self._snapshot(row)
+            snapshot, legacy = self._snapshot(login, row)
+            if legacy:
+                connection.execute(
+                    "UPDATE unfiled_drafts SET drafts = ? WHERE login = ?",
+                    (
+                        self._encrypt(login, snapshot["revision"], snapshot["drafts"]),
+                        login,
+                    ),
+                )
+        return snapshot
 
     @staticmethod
     def _filing_plan(raw: object) -> dict | None:
@@ -233,16 +315,16 @@ class UnfiledDraftStore:
         if not isinstance(expected_revision, int) or isinstance(expected_revision, bool) or expected_revision < 0:
             raise ValueError("revision is invalid")
         normalized = self._normalize(drafts)
-        serialized = json.dumps(normalized, separators=(",", ":"))
         with self._connect() as connection:
             connection.execute("BEGIN IMMEDIATE")
             row = connection.execute(
                 "SELECT revision, drafts FROM unfiled_drafts WHERE login = ?", (login,)
             ).fetchone()
-            current = self._snapshot(row)
+            current, _legacy = self._snapshot(login, row)
             if current["revision"] != expected_revision:
                 raise UnfiledDraftConflict(current)
             revision = expected_revision + 1
+            serialized = self._encrypt(login, revision, normalized)
             connection.execute(
                 "INSERT INTO unfiled_drafts(login, revision, drafts) VALUES (?, ?, ?) "
                 "ON CONFLICT(login) DO UPDATE SET revision=excluded.revision, drafts=excluded.drafts",