"""Key encryption at rest (spec sections 43-44).

Private keys that the platform must keep (CA and server keys) are stored
in PostgreSQL ONLY in encrypted form, using AES-256-GCM authenticated
encryption. The master key comes from the SECRET_ENCRYPTION_KEY
environment variable (Docker secret in production) and never touches
the database.

Wire format: "v1:<nonce-hex>:<ciphertext-hex>" — the version prefix
keeps the format evolvable without breaking existing rows.
"""

from __future__ import annotations

import os

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

from app.config import get_settings

_FORMAT = "v1"


def _master_key() -> bytes:
    """Derive the 32-byte AES key from the configured secret.

    The secret is hashed to exactly 32 bytes (AES-256) regardless of the
    length the operator chose for SECRET_ENCRYPTION_KEY.
    """
    import hashlib

    secret = get_settings().secret_encryption_key.encode("utf-8")
    return hashlib.sha256(secret).digest()


def encrypt_secret(plaintext: str) -> str:
    """Encrypt a PEM string into the storage format."""
    nonce = os.urandom(12)  # 96-bit nonce, GCM standard
    aesgcm = AESGCM(_master_key())
    ciphertext = aesgcm.encrypt(nonce, plaintext.encode("utf-8"), None)
    return f"{_FORMAT}:{nonce.hex()}:{ciphertext.hex()}"


def decrypt_secret(blob: str) -> str:
    """Decrypt a stored secret back to plaintext PEM."""
    try:
        version, nonce_hex, ciphertext_hex = blob.split(":", 2)
    except ValueError as exc:
        raise ValueError("corrupt secret blob: bad format") from exc
    if version != _FORMAT:
        raise ValueError(f"unsupported secret blob version: {version}")
    aesgcm = AESGCM(_master_key())
    try:
        plaintext = aesgcm.decrypt(
            bytes.fromhex(nonce_hex), bytes.fromhex(ciphertext_hex), None
        )
    except Exception as exc:
        raise ValueError("secret decryption failed (wrong master key?)") from exc
    return plaintext.decode("utf-8")