# Copyright (c) 2015 OpenStack Foundation
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import hashlib
import hmac
import os

from swift.common.middleware.crypto.crypto_utils import CRYPTO_KEY_CALLBACK
from swift.common.swob import Request, HTTPException
from swift.common.utils import readconf, strict_b64decode
from swift.common.wsgi import WSGIContext


class KeyMasterContext(WSGIContext):
    """
    The simple scheme for key derivation is as follows: every path is
    associated with a key, where the key is derived from the path itself in a
    deterministic fashion such that the key does not need to be stored.
    Specifically, the key for any path is an HMAC of a root key and the path
    itself, calculated using an SHA256 hash function::

      <path_key> = HMAC_SHA256(<root_secret>, <path>)
    """
    def __init__(self, keymaster, account, container, obj):
        """
        :param keymaster: a Keymaster instance
        :param account: account name
        :param container: container name
        :param obj: object name
        """
        super(KeyMasterContext, self).__init__(keymaster.app)
        self.keymaster = keymaster
        self.account = account
        self.container = container
        self.obj = obj
        self._keys = None

    def fetch_crypto_keys(self, *args, **kwargs):
        """
        Setup container and object keys based on the request path.

        Keys are derived from request path. The 'id' entry in the results dict
        includes the part of the path used to derive keys. Other keymaster
        implementations may use a different strategy to generate keys and may
        include a different type of 'id', so callers should treat the 'id' as
        opaque keymaster-specific data.

        :returns: A dict containing encryption keys for 'object' and
                  'container' and a key 'id'.
        """
        if self._keys:
            return self._keys

        self._keys = {}
        account_path = os.path.join(os.sep, self.account)

        if self.container:
            path = os.path.join(account_path, self.container)
            self._keys['container'] = self.keymaster.create_key(path)

            if self.obj:
                path = os.path.join(path, self.obj)
                self._keys['object'] = self.keymaster.create_key(path)

            # For future-proofing include a keymaster version number and the
            # path used to derive keys in the 'id' entry of the results. The
            # encrypter will persist this as part of the crypto-meta for
            # encrypted data and metadata. If we ever change the way keys are
            # generated then the decrypter could pass the persisted 'id' value
            # when it calls fetch_crypto_keys to inform the keymaster as to how
            # that particular data or metadata had its keys generated.
            # Currently we have no need to do that, so we are simply persisting
            # this information for future use.
            self._keys['id'] = {'v': '1', 'path': path}

        return self._keys

    def handle_request(self, req, start_response):
        req.environ[CRYPTO_KEY_CALLBACK] = self.fetch_crypto_keys
        resp = self._app_call(req.environ)
        start_response(self._response_status, self._response_headers,
                       self._response_exc_info)
        return resp


class KeyMaster(object):
    """Middleware for providing encryption keys.

    The middleware requires its encryption root secret to be set. This is the
    root secret from which encryption keys are derived. This must be set before
    first use to a value that is at least 256 bits. The security of all
    encrypted data critically depends on this key, therefore it should be set
    to a high-entropy value. For example, a suitable value may be obtained by
    generating a 32 byte (or longer) value using a cryptographically secure
    random number generator. Changing the root secret is likely to result in
    data loss.
    """

    def __init__(self, app, conf):
        self.app = app
        self.keymaster_config_path = conf.get('keymaster_config_path')
        # The _get_root_secret() function is overridden by other keymasters
        self.root_secret = self._get_root_secret(conf)

    def _get_root_secret(self, conf):
        """
        This keymaster requires its ``encryption_root_secret`` option to be
        set. This must be set before first use to a value that is a base64
        encoding of at least 32 bytes. The encryption root secret is stored
        in either proxy-server.conf, or in an external file referenced from
        proxy-server.conf using ``keymaster_config_path``.

        :param conf: the keymaster config section from proxy-server.conf
        :type conf: dict

        :return: the encryption root secret binary bytes
        :rtype: bytearray
        """
        if self.keymaster_config_path:
            keymaster_opts = ['encryption_root_secret']
            if any(opt in conf for opt in keymaster_opts):
                raise ValueError('keymaster_config_path is set, but there '
                                 'are other config options specified: %s' %
                                 ", ".join(list(
                                     set(keymaster_opts).intersection(conf))))
            conf = readconf(self.keymaster_config_path, 'keymaster')
        b64_root_secret = conf.get('encryption_root_secret')
        try:
            binary_root_secret = strict_b64decode(b64_root_secret,
                                                  allow_line_breaks=True)
            if len(binary_root_secret) < 32:
                raise ValueError
            return binary_root_secret
        except ValueError:
            raise ValueError(
                'encryption_root_secret option in %s must be a base64 '
                'encoding of at least 32 raw bytes' % (
                    self.keymaster_config_path or 'proxy-server.conf'))

    def __call__(self, env, start_response):
        req = Request(env)

        try:
            parts = req.split_path(2, 4, True)
        except ValueError:
            return self.app(env, start_response)

        if req.method in ('PUT', 'POST', 'GET', 'HEAD'):
            # handle only those request methods that may require keys
            km_context = KeyMasterContext(self, *parts[1:])
            try:
                return km_context.handle_request(req, start_response)
            except HTTPException as err_resp:
                return err_resp(env, start_response)

        # anything else
        return self.app(env, start_response)

    def create_key(self, key_id):
        return hmac.new(self.root_secret, key_id,
                        digestmod=hashlib.sha256).digest()


def filter_factory(global_conf, **local_conf):
    conf = global_conf.copy()
    conf.update(local_conf)

    def keymaster_filter(app):
        return KeyMaster(app, conf)

    return keymaster_filter