"""Fernet-based encryption for credential blobs.

The key is a single 32-byte value (urlsafe-base64-encoded; what
Fernet.generate_key produces) stored at a fixed path inside the
images/data root. Created on first boot if absent; mode 0600. No KDF
needed — the file contents are already maximum-entropy random bytes.

Operator backup procedure must include this file alongside the rest
of /images/ — losing it makes existing encrypted_blob rows
undecryptable (recovery = delete the rows and re-upload).
"""

import os
from pathlib import Path

from cryptography.fernet import Fernet, InvalidToken


class InvalidCredentialBlob(Exception):
    """Raised when decryption fails (wrong key, tampered blob, …)."""


class CredentialCrypto:
    """Fernet encrypt/decrypt with an on-disk key file.

    Instantiate with a path; the file is created on first access and
    reused thereafter. Tests pass a tmp_path; production calls with
    `IMAGES_ROOT / "secrets" / "credential_key.b64"`.
    """

    def __init__(self, key_path: Path):
        self._key_path = Path(key_path)
        self._fernet = Fernet(self._load_or_create_key())

    def _load_or_create_key(self) -> bytes:
        if self._key_path.exists():
            return self._key_path.read_bytes()
        parent = self._key_path.parent
        parent.mkdir(parents=True, exist_ok=True)
        os.chmod(parent, 0o700)
        key = Fernet.generate_key()
        self._key_path.write_bytes(key)
        os.chmod(self._key_path, 0o600)
        return key

    def encrypt(self, plaintext: str) -> bytes:
        return self._fernet.encrypt(plaintext.encode("utf-8"))

    def decrypt(self, ciphertext: bytes) -> str:
        try:
            return self._fernet.decrypt(ciphertext).decode("utf-8")
        except InvalidToken as exc:
            raise InvalidCredentialBlob(
                "credential blob is corrupt or encrypted with a different key"
            ) from exc