"""AES-GCM secret encryption for Woo app password + inference API key (SETTINGS-20/25, QUALITY-125).

Master key = SLAB_AES_KEY env (32-byte base64). Stored form value_enc =
base64(nonce(12) || ciphertext+GCM tag). Decrypt failures raise DecryptError —
detectable, no crash, no secret leak (SETTINGS-75 masking degrades to
configured:false on a bad blob).
"""
from __future__ import annotations

import base64
import os

from cryptography.exceptions import InvalidTag
from cryptography.hazmat.primitives.ciphers.aead import AESGCM


class DecryptError(Exception):
    """Tampered, corrupt, or wrong-key ciphertext. Message never carries the blob."""


def _get_key() -> bytes:
    key_b64 = os.environ.get("SLAB_AES_KEY")
    if not key_b64:
        raise RuntimeError("SETTINGS-25: SLAB_AES_KEY is required (32-byte base64)")
    try:
        key = base64.b64decode(key_b64, validate=True)
    except Exception:
        raise RuntimeError("SETTINGS-25: SLAB_AES_KEY must be valid base64") from None
    if len(key) != 32:
        raise RuntimeError("SETTINGS-25: SLAB_AES_KEY must decode to exactly 32 bytes")
    return key


def encrypt(plaintext: str) -> str:
    """Encrypt a plaintext secret -> base64(nonce || ciphertext) for value_enc."""
    if not isinstance(plaintext, str):
        raise TypeError("plaintext must be str")
    nonce = os.urandom(12)
    ct = AESGCM(_get_key()).encrypt(nonce, plaintext.encode("utf-8"), None)
    return base64.b64encode(nonce + ct).decode("ascii")


def decrypt(ciphertext_b64: str) -> str:
    """Decrypt a value_enc blob; verifies the GCM tag. Raises DecryptError (no leak)."""
    if not isinstance(ciphertext_b64, str):
        raise TypeError("ciphertext_b64 must be str")
    try:
        blob = base64.b64decode(ciphertext_b64, validate=True)
    except Exception:
        raise DecryptError("corrupt base64") from None
    if len(blob) < 12:
        raise DecryptError("corrupt: too short for nonce")
    try:
        pt = AESGCM(_get_key()).decrypt(blob[:12], blob[12:], None)
    except InvalidTag:
        raise DecryptError("decrypt failed: invalid tag (tampered or wrong key)") from None
    except Exception:
        raise DecryptError("decrypt failed") from None
    return pt.decode("utf-8")