secret_loader/secret_loader/__init__.py

"""# Secret Loader System.

System for loading secrets from a variety of sources.

Usage:

```python
import secret_loader
secrets = secret_loader.SecretLoader(env_key_prefix = 'MYAPP')

db_username = secrets.load_or_fail('DATABASE_USERNAME')
db_password = secrets.load_or_fail('DATABASE_PASSWORD')
```


Secret loading order:

0. Hardcoded values. **This is purely for debugging, prototyping, and for
   configuring below options.**
1. Files pointed to by environment variables. Docker friendly.
2. Secrets folder. Also Docker friendly.
3. [Pass: the standard unix password
   manager](https://www.passwordstore.org/). Most suited for personal
   usage; very unsuited for server environments. Requires `pass` installed
   locally, and configuration of the `PASS_STORE_SUBFOLDER` through one of the above
   methods.
4. Vault instance if configured. Suited for production environments. **NOTE:
   This is barely supported.** Requires `hvac` python package.

## Future extensions

- [ ] Key casing should be more consistent
    * Case-insensitive for hardcoded and `load`.
    * Upper case for environment variables.
    * Lower case for files and others.
- [ ] New special configuration value for switching the `secrets` directory.
- [ ] Wrap secrets in intelligent strings:
    * [ ] Instead of returning `None` on unloaded, return `UnknownSecret`, that produce error when formatted.
    * [ ] `repr(secret)` should not include contents, but only the secret and how it was loaded.
    * [ ] Methods on `Secret` should be kept minimal.
- [ ] Avoid leakage to swap files.
    * Possibly Mlock? [Does not seem to work](https://stackoverflow.com/questions/29524020/prevent-ram-from-paging-to-swap-area-mlock)
    * Alternatively use [mmap](https://docs.python.org/3/library/mmap.html) and [memoryview](https://stackoverflow.com/questions/18655648/what-exactly-is-the-point-of-memoryview-in-python)?§
- [ ] Vault:
  * [ ] Ensure vault code path works.
  * [ ] Document usage and requirements.
"""

import logging
import os
import subprocess
from pathlib import Path

try:
    import hvac
except ImportError:
    hvac = None

logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)

ENV_KEY_PREFIX = 'ENV_KEY_PREFIX'

ENV_KEY_VAULT_URL = 'VAULT_URL'
ENV_KEY_VAULT_TOKEN = 'VAULT_TOKEN'
ENV_KEY_VAULT_MOUNT_POINT = 'VAULT_MOUNT_POINT'

ENV_KEY_PASS_FOLDER = 'PASS_STORE_SUBFOLDER'

DEFAULT_SECRETS_DIRECTORY = Path('./secrets/')

class SecretLoader:
    """Main entry point for loading secrets.

    See module documentation for usage.
    """

    def __init__(self, **hardcoded: str):
        """Initialize `SecretLoader`.

        Hardcoded values are stored directly, and can be used to configure the
        other subsystems.
        """
        self.hardcoded: dict[str, str] = hardcoded
        self.pass_folder = None
        self.vault_client = None
        self.env_key_prefix = None

        # Setup environment
        self.env_key_prefix = self._load_or_none(ENV_KEY_PREFIX)
        if self.env_key_prefix is not None:
            assert not self.env_key_prefix.endswith('_'), 'Prefix must not end with _ (this will be added automatically)'

        # Setup pass
        self.pass_folder = self._load_or_none(ENV_KEY_PASS_FOLDER)

        # Setup vault
        if hvac:
            self.vault_client = hvac.Client(
                url=self._load_or_none(ENV_KEY_VAULT_URL),
                token=self._load_or_none(ENV_KEY_VAULT_TOKEN),
                vault_mount_point=self._load_or_none(ENV_KEY_VAULT_MOUNT_POINT),
            )

    def load_or_fail(self, env_key: str) -> str:
        """Load secret with the given key, from one of the backends or
        throw an exception if not found.
        """
        value = self._load_or_none(env_key)
        if value is None:
            msg = f'Failed to load secret with key: {env_key}'
            raise Exception(msg)
        logger.info('Loaded secret with key: %s', env_key)
        return value

    def load(self, env_key: str) -> str | None:
        """Load secret with the given key, from one of the backends or
        return `None` if not found.

        A warning log line is emitted.
        """
        value = self._load_or_none(env_key)
        if value is None:
            logger.warning('Failed to load secret with key: %s', env_key)
        else:
            logger.info('Loaded secret with key: %s', env_key)
        return value

    def _load_or_none(self, env_key: str) -> str | None:
        """Load secret with the given key, from one of the backends or
        return `None` if not found.
        """
        return (
            self.hardcoded.get(env_key)
            or self._load_or_none_path_or_file(env_key)
            or self._load_or_none_local_password_store(env_key)
            or self._load_or_none_vault(env_key)
        )

    def _load_or_none_path_or_file(self, env_key: str) -> str | None:
        """Load secret for given key, from either an environment defined key,
        or from the `secrets` directory.

        Returns `None` if the secret is not present in either the environment
        or the directory.
        """
        filepath: Path | str | None = os.environ.get(f'{self.env_key_prefix}_{env_key}')

        if filepath is None:
            filepath = DEFAULT_SECRETS_DIRECTORY / env_key.lower()

        try:
            with open(filepath) as f:
                return f.read().strip()
        except FileNotFoundError:
            return None

    def _load_or_none_local_password_store(self, env_key: str) -> str | None:
        """Load secret from the `pass` password manager.

        Returns `None` if the `pass` password manager is not configured, or if
        the secret is not present in the `pass` password manager.
        """
        if self.pass_folder is None:
            return None

        cmd = ['pass', 'show', f'{self.pass_folder}/{env_key.lower()}']
        process = subprocess.run(cmd, capture_output =  True)
        if process.returncode:
            return None

        password_file = process.stdout.decode('utf8')
        for line in password_file.split('\n'):
            return line
        return None

    def _load_or_none_vault(self, env_key: str) -> str | None:
        """Load secret from the configured vault instance.

        Returns `None` if vault instance is not configured, or if the value
        instance does not know about the secret.
        """
        if self.vault_client is None:
            return None

        read_secret_result = self.vault_client.secrets.kv.v1.read_secret(
            path=env_key.lower(),
            mount_point=self.vault_mount_point,
        )
        return read_secret_result['data']['value']