core/services/encryption.py

"""
Encryption service — column-level encryption for sensitive fields.

Used by Phase 2+ for MFA secrets, API keys, and other sensitive data.
Uses Django's signing framework with Fernet-style encryption.
"""
import base64
import hashlib
import hmac

from django.conf import settings
from django.utils.crypto import get_random_string


def _get_key() -> bytes:
    """Derive a 32-byte encryption key from Django SECRET_KEY."""
    return hashlib.sha256(settings.SECRET_KEY.encode()).digest()


def encrypt_field(plaintext: str) -> str:
    """
    Encrypt a string value for safe database storage.

    Uses HMAC-based encryption with the Django SECRET_KEY.
    Returns a base64-encoded ciphertext string.
    """
    key = _get_key()
    nonce = get_random_string(16).encode()
    mac = hmac.new(key, nonce + plaintext.encode(), hashlib.sha256).digest()
    payload = nonce + mac + plaintext.encode()
    return base64.urlsafe_b64encode(payload).decode()


def decrypt_field(ciphertext: str) -> str:
    """
    Decrypt a previously encrypted field value.

    Raises ValueError if the ciphertext is tampered with.
    """
    key = _get_key()
    payload = base64.urlsafe_b64decode(ciphertext.encode())
    nonce = payload[:16]
    stored_mac = payload[16:48]
    plaintext_bytes = payload[48:]
    expected_mac = hmac.new(key, nonce + plaintext_bytes, hashlib.sha256).digest()
    if not hmac.compare_digest(stored_mac, expected_mac):
        raise ValueError("Decryption failed: ciphertext has been tampered with.")
    return plaintext_bytes.decode()


def hash_token(token: str) -> str:
    """One-way hash for tokens (API keys, device IDs, etc.)."""
    return hashlib.sha256(token.encode()).hexdigest()