feat(v2): complete v2 reimplementation

backend/src/backup_tool/security/repository_crypto.py

@@ -0,0 +1,102 @@
+from __future__ import annotations
+
+import os
+import stat
+from pathlib import Path
+
+from cryptography.exceptions import InvalidTag
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
+
+from backup_tool.config import Settings
+from backup_tool.ids import new_uuid7
+
+
+class RepositoryKeyError(ValueError):
+    pass
+
+
+def _directory(settings: Settings) -> Path:
+    directory = settings.data_dir / "repository-data-keys"
+    directory.mkdir(mode=0o700, exist_ok=True)
+    if (
+        directory.is_symlink()
+        or not directory.is_dir()
+        or stat.S_IMODE(directory.stat().st_mode) != 0o700
+    ):
+        raise RepositoryKeyError("repository data key directory is unsafe")
+    return directory
+
+
+def create_data_key(settings: Settings, repository_id: str) -> tuple[str, Path]:
+    key_id = str(new_uuid7())
+    directory = _directory(settings)
+    path = directory / f"{repository_id}.{key_id}.key"
+    try:
+        descriptor = os.open(path, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o600)
+        with os.fdopen(descriptor, "wb") as handle:
+            handle.write(os.urandom(32))
+            handle.flush()
+            os.fsync(handle.fileno())
+    except OSError as error:
+        raise RepositoryKeyError("repository data key cannot be created") from error
+    if stat.S_IMODE(path.stat().st_mode) != 0o600 or path.is_symlink():
+        path.unlink(missing_ok=True)
+        raise RepositoryKeyError("repository data key is unsafe")
+    return key_id, path
+
+
+def install_data_key(settings: Settings, repository_id: str, key_id: str, key: bytes) -> Path:
+    """Install recovered key material once; never replace an existing key file."""
+    if len(key) != 32 or "/" in repository_id or "/" in key_id:
+        raise RepositoryKeyError("repository data key is invalid")
+    path = _directory(settings) / f"{repository_id}.{key_id}.key"
+    flags = os.O_WRONLY | os.O_CREAT | os.O_EXCL
+    if hasattr(os, "O_NOFOLLOW"):
+        flags |= os.O_NOFOLLOW
+    try:
+        descriptor = os.open(path, flags, 0o600)
+        with os.fdopen(descriptor, "wb") as handle:
+            handle.write(key)
+            handle.flush()
+            os.fsync(handle.fileno())
+        if path.is_symlink() or stat.S_IMODE(path.stat().st_mode) != 0o600:
+            path.unlink(missing_ok=True)
+            raise RepositoryKeyError("repository data key is unsafe")
+    except OSError as error:
+        raise RepositoryKeyError("repository data key cannot be installed") from error
+    return path
+
+
+def object_aad(repository_id: str, key_id: str, kind: str, identity: str) -> bytes:
+    if kind not in {"blob", "manifest"} or not all((repository_id, key_id, identity)):
+        raise RepositoryKeyError("encrypted object metadata is invalid")
+    return f"BTENC:1:{repository_id}:{key_id}:{kind}:{identity}".encode()
+
+
+def encrypt_object(key: bytes, aad: bytes, plaintext: bytes) -> bytes:
+    if len(key) != 32:
+        raise RepositoryKeyError("repository data key is unavailable")
+    nonce = os.urandom(12)
+    return b"BTENC\x01" + nonce + AESGCM(key).encrypt(nonce, plaintext, aad)
+
+
+def decrypt_object(key: bytes, aad: bytes, stored: bytes) -> bytes:
+    if len(key) != 32 or not stored.startswith(b"BTENC\x01") or len(stored) < 35:
+        raise RepositoryKeyError("encrypted object is invalid")
+    try:
+        return AESGCM(key).decrypt(stored[6:18], stored[18:], aad)
+    except InvalidTag as error:
+        raise RepositoryKeyError("encrypted object is invalid") from error
+
+
+def load_data_key(settings: Settings, repository_id: str, key_id: str) -> bytes:
+    path = _directory(settings) / f"{repository_id}.{key_id}.key"
+    try:
+        if path.is_symlink() or stat.S_IMODE(path.stat().st_mode) != 0o600:
+            raise RepositoryKeyError("repository data key is unsafe")
+        key = path.read_bytes()
+    except OSError as error:
+        raise RepositoryKeyError("repository data key is unavailable") from error
+    if len(key) != 32:
+        raise RepositoryKeyError("repository data key is unavailable")
+    return key