import base64
import json
import os

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

# The obvious placeholder shipped in .env.example — must be replaced with a real key.
PLACEHOLDER_SECRET_KEY = "CHANGE_ME_generate_with_openssl_rand_base64_32"
# The valid-but-PUBLIC key git used to ship in .env.example. Anyone can decrypt creds
# encrypted with it, so warn — but don't hard-fail, since an existing install may have
# encrypted its credentials under it and hard-failing would brick it mid-flight.
_LEGACY_PUBLIC_KEY = "MDEyMzQ1Njc4OWFiY2RlZjAxMjM0NTY3ODlhYmNkZWY="


def secret_key_problem() -> tuple[str, bool] | None:
    """Return (message, fatal) if LYRA_SECRET_KEY is unusable or insecure, else None.
    fatal=True ⇒ the app should refuse to start; fatal=False ⇒ start but warn loudly."""
    b64 = os.environ.get("LYRA_SECRET_KEY")
    if not b64:
        return ("LYRA_SECRET_KEY is not set", True)
    if b64 == PLACEHOLDER_SECRET_KEY:
        return ("LYRA_SECRET_KEY is still the .env.example placeholder", True)
    try:
        k = base64.b64decode(b64)
    except Exception:
        return ("LYRA_SECRET_KEY is not valid base64", True)
    if len(k) != 32:
        return ("LYRA_SECRET_KEY must decode to 32 bytes", True)
    if b64 == _LEGACY_PUBLIC_KEY:
        return (
            "LYRA_SECRET_KEY is the old PUBLIC example key that shipped in git — anyone "
            "can decrypt your stored credentials; generate a fresh key and re-enter them",
            False,
        )
    return None


def _key() -> bytes:
    b64 = os.environ.get("LYRA_SECRET_KEY")
    if not b64:
        raise RuntimeError("LYRA_SECRET_KEY is not set")
    k = base64.b64decode(b64)
    if len(k) != 32:
        raise RuntimeError("LYRA_SECRET_KEY must decode to 32 bytes")
    return k


def encrypt_secret(plaintext: str) -> str:
    iv = os.urandom(12)
    full = AESGCM(_key()).encrypt(iv, plaintext.encode("utf-8"), None)  # ct || tag(16)
    ct, tag = full[:-16], full[-16:]
    return json.dumps(
        {
            "iv": base64.b64encode(iv).decode(),
            "ct": base64.b64encode(ct).decode(),
            "tag": base64.b64encode(tag).decode(),
        }
    )


def decrypt_secret(envelope: str) -> str:
    e = json.loads(envelope)
    iv = base64.b64decode(e["iv"])
    ct = base64.b64decode(e["ct"])
    tag = base64.b64decode(e["tag"])
    pt = AESGCM(_key()).decrypt(iv, ct + tag, None)  # AESGCM expects ct || tag
    return pt.decode("utf-8")