Add google-endpoints to third_party/.

third_party/google-endpoints/oauth2client/client.py

+# Copyright 2014 Google Inc. All rights reserved.
+#
+# 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.
+
+"""An OAuth 2.0 client.
+
+Tools for interacting with OAuth 2.0 protected resources.
+"""
+
+import base64
+import collections
+import copy
+import datetime
+import json
+import logging
+import os
+import socket
+import sys
+import tempfile
+import time
+import shutil
+import six
+from six.moves import urllib
+
+import httplib2
+from oauth2client import GOOGLE_AUTH_URI
+from oauth2client import GOOGLE_DEVICE_URI
+from oauth2client import GOOGLE_REVOKE_URI
+from oauth2client import GOOGLE_TOKEN_URI
+from oauth2client import GOOGLE_TOKEN_INFO_URI
+from oauth2client._helpers import _from_bytes
+from oauth2client._helpers import _to_bytes
+from oauth2client._helpers import _urlsafe_b64decode
+from oauth2client import clientsecrets
+from oauth2client import util
+
+
+__author__ = 'jcgregorio@google.com (Joe Gregorio)'
+
+HAS_OPENSSL = False
+HAS_CRYPTO = False
+try:
+    from oauth2client import crypt
+    HAS_CRYPTO = True
+    if crypt.OpenSSLVerifier is not None:
+        HAS_OPENSSL = True
+except ImportError:
+    pass
+
+
+logger = logging.getLogger(__name__)
+
+# Expiry is stored in RFC3339 UTC format
+EXPIRY_FORMAT = '%Y-%m-%dT%H:%M:%SZ'
+
+# Which certs to use to validate id_tokens received.
+ID_TOKEN_VERIFICATION_CERTS = 'https://www.googleapis.com/oauth2/v1/certs'
+# This symbol previously had a typo in the name; we keep the old name
+# around for now, but will remove it in the future.
+ID_TOKEN_VERIFICATON_CERTS = ID_TOKEN_VERIFICATION_CERTS
+
+# Constant to use for the out of band OAuth 2.0 flow.
+OOB_CALLBACK_URN = 'urn:ietf:wg:oauth:2.0:oob'
+
+# Google Data client libraries may need to set this to [401, 403].
+REFRESH_STATUS_CODES = [401]
+
+# The value representing user credentials.
+AUTHORIZED_USER = 'authorized_user'
+
+# The value representing service account credentials.
+SERVICE_ACCOUNT = 'service_account'
+
+# The environment variable pointing the file with local
+# Application Default Credentials.
+GOOGLE_APPLICATION_CREDENTIALS = 'GOOGLE_APPLICATION_CREDENTIALS'
+# The ~/.config subdirectory containing gcloud credentials. Intended
+# to be swapped out in tests.
+_CLOUDSDK_CONFIG_DIRECTORY = 'gcloud'
+# The environment variable name which can replace ~/.config if set.
+_CLOUDSDK_CONFIG_ENV_VAR = 'CLOUDSDK_CONFIG'
+
+# The error message we show users when we can't find the Application
+# Default Credentials.
+ADC_HELP_MSG = (
+    'The Application Default Credentials are not available. They are '
+    'available if running in Google Compute Engine. Otherwise, the '
+    'environment variable ' +
+    GOOGLE_APPLICATION_CREDENTIALS +
+    ' must be defined pointing to a file defining the credentials. See '
+    'https://developers.google.com/accounts/docs/'
+    'application-default-credentials for more information.')
+
+# The access token along with the seconds in which it expires.
+AccessTokenInfo = collections.namedtuple(
+    'AccessTokenInfo', ['access_token', 'expires_in'])
+
+DEFAULT_ENV_NAME = 'UNKNOWN'
+
+# If set to True _get_environment avoid GCE check (_detect_gce_environment)
+NO_GCE_CHECK = os.environ.setdefault('NO_GCE_CHECK', 'False')
+
+_SERVER_SOFTWARE = 'SERVER_SOFTWARE'
+_GCE_METADATA_HOST = '169.254.169.254'
+_METADATA_FLAVOR_HEADER = 'Metadata-Flavor'
+_DESIRED_METADATA_FLAVOR = 'Google'
+
+
+class SETTINGS(object):
+    """Settings namespace for globally defined values."""
+    env_name = None
+
+
+class Error(Exception):
+    """Base error for this module."""
+
+
+class FlowExchangeError(Error):
+    """Error trying to exchange an authorization grant for an access token."""
+
+
+class AccessTokenRefreshError(Error):
+    """Error trying to refresh an expired access token."""
+
+
+class HttpAccessTokenRefreshError(AccessTokenRefreshError):
+    """Error (with HTTP status) trying to refresh an expired access token."""
+    def __init__(self, *args, **kwargs):
+        super(HttpAccessTokenRefreshError, self).__init__(*args)
+        self.status = kwargs.get('status')
+
+
+class TokenRevokeError(Error):
+    """Error trying to revoke a token."""
+
+
+class UnknownClientSecretsFlowError(Error):
+    """The client secrets file called for an unknown type of OAuth 2.0 flow."""
+
+
+class AccessTokenCredentialsError(Error):
+    """Having only the access_token means no refresh is possible."""
+
+
+class VerifyJwtTokenError(Error):
+    """Could not retrieve certificates for validation."""
+
+
+class NonAsciiHeaderError(Error):
+    """Header names and values must be ASCII strings."""
+
+
+class ApplicationDefaultCredentialsError(Error):
+    """Error retrieving the Application Default Credentials."""
+
+
+class OAuth2DeviceCodeError(Error):
+    """Error trying to retrieve a device code."""
+
+
+class CryptoUnavailableError(Error, NotImplementedError):
+    """Raised when a crypto library is required, but none is available."""
+
+
+def _abstract():
+    raise NotImplementedError('You need to override this function')
+
+
+class MemoryCache(object):
+    """httplib2 Cache implementation which only caches locally."""
+
+    def __init__(self):
+        self.cache = {}
+
+    def get(self, key):
+        return self.cache.get(key)
+
+    def set(self, key, value):
+        self.cache[key] = value
+
+    def delete(self, key):
+        self.cache.pop(key, None)
+
+
+class Credentials(object):
+    """Base class for all Credentials objects.
+
+    Subclasses must define an authorize() method that applies the credentials
+    to an HTTP transport.
+
+    Subclasses must also specify a classmethod named 'from_json' that takes a
+    JSON string as input and returns an instantiated Credentials object.
+    """
+
+    NON_SERIALIZED_MEMBERS = ['store']
+
+    def authorize(self, http):
+        """Take an httplib2.Http instance (or equivalent) and authorizes it.
+
+        Authorizes it for the set of credentials, usually by replacing
+        http.request() with a method that adds in the appropriate headers and
+        then delegates to the original Http.request() method.
+
+        Args:
+            http: httplib2.Http, an http object to be used to make the refresh
+                  request.
+        """
+        _abstract()
+
+    def refresh(self, http):
+        """Forces a refresh of the access_token.
+
+        Args:
+            http: httplib2.Http, an http object to be used to make the refresh
+                  request.
+        """
+        _abstract()
+
+    def revoke(self, http):
+        """Revokes a refresh_token and makes the credentials void.
+
+        Args:
+            http: httplib2.Http, an http object to be used to make the revoke
+                  request.
+        """
+        _abstract()
+
+    def apply(self, headers):
+        """Add the authorization to the headers.
+
+        Args:
+            headers: dict, the headers to add the Authorization header to.
+        """
+        _abstract()
+
+    def _to_json(self, strip):
+        """Utility function that creates JSON repr. of a Credentials object.
+
+        Args:
+            strip: array, An array of names of members to not include in the
+                   JSON.
+
+        Returns:
+            string, a JSON representation of this instance, suitable to pass to
+            from_json().
+        """
+        t = type(self)
+        d = copy.copy(self.__dict__)
+        for member in strip:
+            if member in d:
+                del d[member]
+        if (d.get('token_expiry') and
+                isinstance(d['token_expiry'], datetime.datetime)):
+            d['token_expiry'] = d['token_expiry'].strftime(EXPIRY_FORMAT)
+        # Add in information we will need later to reconsistitue this instance.
+        d['_class'] = t.__name__
+        d['_module'] = t.__module__
+        for key, val in d.items():
+            if isinstance(val, bytes):
+                d[key] = val.decode('utf-8')
+            if isinstance(val, set):
+                d[key] = list(val)
+        return json.dumps(d)
+
+    def to_json(self):
+        """Creating a JSON representation of an instance of Credentials.
+
+        Returns:
+            string, a JSON representation of this instance, suitable to pass to
+            from_json().
+        """
+        return self._to_json(Credentials.NON_SERIALIZED_MEMBERS)
+
+    @classmethod
+    def new_from_json(cls, s):
+        """Utility class method to instantiate a Credentials subclass from JSON.
+
+        Expects the JSON string to have been produced by to_json().
+
+        Args:
+            s: string or bytes, JSON from to_json().
+
+        Returns:
+            An instance of the subclass of Credentials that was serialized with
+            to_json().
+        """
+        json_string_as_unicode = _from_bytes(s)
+        data = json.loads(json_string_as_unicode)
+        # Find and call the right classmethod from_json() to restore
+        # the object.
+        module_name = data['_module']
+        try:
+            module_obj = __import__(module_name)
+        except ImportError:
+            # In case there's an object from the old package structure,
+            # update it
+            module_name = module_name.replace('.googleapiclient', '')
+            module_obj = __import__(module_name)
+
+        module_obj = __import__(module_name,
+                                fromlist=module_name.split('.')[:-1])
+        kls = getattr(module_obj, data['_class'])
+        from_json = getattr(kls, 'from_json')
+        return from_json(json_string_as_unicode)
+
+    @classmethod
+    def from_json(cls, unused_data):
+        """Instantiate a Credentials object from a JSON description of it.
+
+        The JSON should have been produced by calling .to_json() on the object.
+
+        Args:
+            unused_data: dict, A deserialized JSON object.
+
+        Returns:
+            An instance of a Credentials subclass.
+        """
+        return Credentials()
+
+
+class Flow(object):
+    """Base class for all Flow objects."""
+    pass
+
+
+class Storage(object):
+    """Base class for all Storage objects.
+
+    Store and retrieve a single credential. This class supports locking
+    such that multiple processes and threads can operate on a single
+    store.
+    """
+
+    def acquire_lock(self):
+        """Acquires any lock necessary to access this Storage.
+
+        This lock is not reentrant.
+        """
+        pass
+
+    def release_lock(self):
+        """Release the Storage lock.
+
+        Trying to release a lock that isn't held will result in a
+        RuntimeError.
+        """
+        pass
+
+    def locked_get(self):
+        """Retrieve credential.
+
+        The Storage lock must be held when this is called.
+
+        Returns:
+            oauth2client.client.Credentials
+        """
+        _abstract()
+
+    def locked_put(self, credentials):
+        """Write a credential.
+
+        The Storage lock must be held when this is called.
+
+        Args:
+            credentials: Credentials, the credentials to store.
+        """
+        _abstract()
+
+    def locked_delete(self):
+        """Delete a credential.
+
+        The Storage lock must be held when this is called.
+        """
+        _abstract()
+
+    def get(self):
+        """Retrieve credential.
+
+        The Storage lock must *not* be held when this is called.
+
+        Returns:
+            oauth2client.client.Credentials
+        """
+        self.acquire_lock()
+        try:
+            return self.locked_get()
+        finally:
+            self.release_lock()
+
+    def put(self, credentials):
+        """Write a credential.
+
+        The Storage lock must be held when this is called.
+
+        Args:
+            credentials: Credentials, the credentials to store.
+        """
+        self.acquire_lock()
+        try:
+            self.locked_put(credentials)
+        finally:
+            self.release_lock()
+
+    def delete(self):
+        """Delete credential.
+
+        Frees any resources associated with storing the credential.
+        The Storage lock must *not* be held when this is called.
+
+        Returns:
+            None
+        """
+        self.acquire_lock()
+        try:
+            return self.locked_delete()
+        finally:
+            self.release_lock()
+
+
+def clean_headers(headers):
+    """Forces header keys and values to be strings, i.e not unicode.
+
+    The httplib module just concats the header keys and values in a way that
+    may make the message header a unicode string, which, if it then tries to
+    contatenate to a binary request body may result in a unicode decode error.
+
+    Args:
+        headers: dict, A dictionary of headers.
+
+    Returns:
+        The same dictionary but with all the keys converted to strings.
+    """
+    clean = {}
+    try:
+        for k, v in six.iteritems(headers):
+            if not isinstance(k, six.binary_type):
+                k = str(k)
+            if not isinstance(v, six.binary_type):
+                v = str(v)
+            clean[_to_bytes(k)] = _to_bytes(v)
+    except UnicodeEncodeError:
+        raise NonAsciiHeaderError(k, ': ', v)
+    return clean
+
+
+def _update_query_params(uri, params):
+    """Updates a URI with new query parameters.
+
+    Args:
+        uri: string, A valid URI, with potential existing query parameters.
+        params: dict, A dictionary of query parameters.
+
+    Returns:
+        The same URI but with the new query parameters added.
+    """
+    parts = urllib.parse.urlparse(uri)
+    query_params = dict(urllib.parse.parse_qsl(parts.query))
+    query_params.update(params)
+    new_parts = parts._replace(query=urllib.parse.urlencode(query_params))
+    return urllib.parse.urlunparse(new_parts)
+
+
+class OAuth2Credentials(Credentials):
+    """Credentials object for OAuth 2.0.
+
+    Credentials can be applied to an httplib2.Http object using the authorize()
+    method, which then adds the OAuth 2.0 access token to each request.
+
+    OAuth2Credentials objects may be safely pickled and unpickled.
+    """
+
+    @util.positional(8)
+    def __init__(self, access_token, client_id, client_secret, refresh_token,
+                 token_expiry, token_uri, user_agent, revoke_uri=None,
+                 id_token=None, token_response=None, scopes=None,
+                 token_info_uri=None):
+        """Create an instance of OAuth2Credentials.
+
+        This constructor is not usually called by the user, instead
+        OAuth2Credentials objects are instantiated by the OAuth2WebServerFlow.
+
+        Args:
+            access_token: string, access token.
+            client_id: string, client identifier.
+            client_secret: string, client secret.
+            refresh_token: string, refresh token.
+            token_expiry: datetime, when the access_token expires.
+            token_uri: string, URI of token endpoint.
+            user_agent: string, The HTTP User-Agent to provide for this
+                        application.
+            revoke_uri: string, URI for revoke endpoint. Defaults to None; a
+                        token can't be revoked if this is None.
+            id_token: object, The identity of the resource owner.
+            token_response: dict, the decoded response to the token request.
+                            None if a token hasn't been requested yet. Stored
+                            because some providers (e.g. wordpress.com) include
+                            extra fields that clients may want.
+            scopes: list, authorized scopes for these credentials.
+          token_info_uri: string, the URI for the token info endpoint. Defaults
+                          to None; scopes can not be refreshed if this is None.
+
+        Notes:
+            store: callable, A callable that when passed a Credential
+                   will store the credential back to where it came from.
+                   This is needed to store the latest access_token if it
+                   has expired and been refreshed.
+        """
+        self.access_token = access_token
+        self.client_id = client_id
+        self.client_secret = client_secret
+        self.refresh_token = refresh_token
+        self.store = None
+        self.token_expiry = token_expiry
+        self.token_uri = token_uri
+        self.user_agent = user_agent
+        self.revoke_uri = revoke_uri
+        self.id_token = id_token
+        self.token_response = token_response
+        self.scopes = set(util.string_to_scopes(scopes or []))
+        self.token_info_uri = token_info_uri
+
+        # True if the credentials have been revoked or expired and can't be
+        # refreshed.
+        self.invalid = False
+
+    def authorize(self, http):
+        """Authorize an httplib2.Http instance with these credentials.
+
+        The modified http.request method will add authentication headers to
+        each request and will refresh access_tokens when a 401 is received on a
+        request. In addition the http.request method has a credentials
+        property, http.request.credentials, which is the Credentials object
+        that authorized it.
+
+        Args:
+            http: An instance of ``httplib2.Http`` or something that acts
+                  like it.
+
+        Returns:
+            A modified instance of http that was passed in.
+
+        Example::
+
+            h = httplib2.Http()
+            h = credentials.authorize(h)
+
+        You can't create a new OAuth subclass of httplib2.Authentication
+        because it never gets passed the absolute URI, which is needed for
+        signing. So instead we have to overload 'request' with a closure
+        that adds in the Authorization header and then calls the original
+        version of 'request()'.
+        """
+        request_orig = http.request
+
+        # The closure that will replace 'httplib2.Http.request'.
+        def new_request(uri, method='GET', body=None, headers=None,
+                        redirections=httplib2.DEFAULT_MAX_REDIRECTS,
+                        connection_type=None):
+            if not self.access_token:
+                logger.info('Attempting refresh to obtain '
+                            'initial access_token')
+                self._refresh(request_orig)
+
+            # Clone and modify the request headers to add the appropriate
+            # Authorization header.
+            if headers is None:
+                headers = {}
+            else:
+                headers = dict(headers)
+            self.apply(headers)
+
+            if self.user_agent is not None:
+                if 'user-agent' in headers:
+                    headers['user-agent'] = (self.user_agent + ' ' +
+                                             headers['user-agent'])
+                else:
+                    headers['user-agent'] = self.user_agent
+
+            body_stream_position = None
+            if all(getattr(body, stream_prop, None) for stream_prop in
+                   ('read', 'seek', 'tell')):
+                body_stream_position = body.tell()
+
+            resp, content = request_orig(uri, method, body,
+                                         clean_headers(headers),
+                                         redirections, connection_type)
+
+            # A stored token may expire between the time it is retrieved and
+            # the time the request is made, so we may need to try twice.
+            max_refresh_attempts = 2
+            for refresh_attempt in range(max_refresh_attempts):
+                if resp.status not in REFRESH_STATUS_CODES:
+                    break
+                logger.info('Refreshing due to a %s (attempt %s/%s)',
+                            resp.status, refresh_attempt + 1,
+                            max_refresh_attempts)
+                self._refresh(request_orig)
+                self.apply(headers)
+                if body_stream_position is not None:
+                    body.seek(body_stream_position)
+
+                resp, content = request_orig(uri, method, body,
+                                             clean_headers(headers),
+                                             redirections, connection_type)
+
+            return (resp, content)
+
+        # Replace the request method with our own closure.
+        http.request = new_request
+
+        # Set credentials as a property of the request method.
+        setattr(http.request, 'credentials', self)
+
+        return http
+
+    def refresh(self, http):
+        """Forces a refresh of the access_token.
+
+        Args:
+            http: httplib2.Http, an http object to be used to make the refresh
+                  request.
+        """
+        self._refresh(http.request)
+
+    def revoke(self, http):
+        """Revokes a refresh_token and makes the credentials void.
+
+        Args:
+            http: httplib2.Http, an http object to be used to make the revoke
+                  request.
+        """
+        self._revoke(http.request)
+
+    def apply(self, headers):
+        """Add the authorization to the headers.
+
+        Args:
+            headers: dict, the headers to add the Authorization header to.
+        """
+        headers['Authorization'] = 'Bearer ' + self.access_token
+
+    def has_scopes(self, scopes):
+        """Verify that the credentials are authorized for the given scopes.
+
+        Returns True if the credentials authorized scopes contain all of the
+        scopes given.
+
+        Args:
+            scopes: list or string, the scopes to check.
+
+        Notes:
+            There are cases where the credentials are unaware of which scopes
+            are authorized. Notably, credentials obtained and stored before
+            this code was added will not have scopes, AccessTokenCredentials do
+            not have scopes. In both cases, you can use refresh_scopes() to
+            obtain the canonical set of scopes.
+        """
+        scopes = util.string_to_scopes(scopes)
+        return set(scopes).issubset(self.scopes)
+
+    def retrieve_scopes(self, http):
+        """Retrieves the canonical list of scopes for this access token.
+
+        Gets the scopes from the OAuth2 provider.
+
+        Args:
+            http: httplib2.Http, an http object to be used to make the refresh
+                  request.
+
+        Returns:
+            A set of strings containing the canonical list of scopes.
+        """
+        self._retrieve_scopes(http.request)
+        return self.scopes
+
+    def to_json(self):
+        return self._to_json(Credentials.NON_SERIALIZED_MEMBERS)
+
+    @classmethod
+    def from_json(cls, s):
+        """Instantiate a Credentials object from a JSON description of it.
+
+        The JSON should have been produced by calling .to_json() on the object.
+
+        Args:
+            data: dict, A deserialized JSON object.
+
+        Returns:
+            An instance of a Credentials subclass.
+        """
+        s = _from_bytes(s)
+        data = json.loads(s)
+        if (data.get('token_expiry') and
+                not isinstance(data['token_expiry'], datetime.datetime)):
+            try:
+                data['token_expiry'] = datetime.datetime.strptime(
+                    data['token_expiry'], EXPIRY_FORMAT)
+            except ValueError:
+                data['token_expiry'] = None
+        retval = cls(
+            data['access_token'],
+            data['client_id'],
+            data['client_secret'],
+            data['refresh_token'],
+            data['token_expiry'],
+            data['token_uri'],
+            data['user_agent'],
+            revoke_uri=data.get('revoke_uri', None),
+            id_token=data.get('id_token', None),
+            token_response=data.get('token_response', None),
+            scopes=data.get('scopes', None),
+            token_info_uri=data.get('token_info_uri', None))
+        retval.invalid = data['invalid']
+        return retval
+
+    @property
+    def access_token_expired(self):
+        """True if the credential is expired or invalid.
+
+        If the token_expiry isn't set, we assume the token doesn't expire.
+        """
+        if self.invalid:
+            return True
+
+        if not self.token_expiry:
+            return False
+
+        now = datetime.datetime.utcnow()
+        if now >= self.token_expiry:
+            logger.info('access_token is expired. Now: %s, token_expiry: %s',
+                        now, self.token_expiry)
+            return True
+        return False
+
+    def get_access_token(self, http=None):
+        """Return the access token and its expiration information.
+
+        If the token does not exist, get one.
+        If the token expired, refresh it.
+        """
+        if not self.access_token or self.access_token_expired:
+            if not http:
+                http = httplib2.Http()
+            self.refresh(http)
+        return AccessTokenInfo(access_token=self.access_token,
+                               expires_in=self._expires_in())
+
+    def set_store(self, store):
+        """Set the Storage for the credential.
+
+        Args:
+            store: Storage, an implementation of Storage object.
+                   This is needed to store the latest access_token if it
+                   has expired and been refreshed. This implementation uses
+                   locking to check for updates before updating the
+                   access_token.
+        """
+        self.store = store
+
+    def _expires_in(self):
+        """Return the number of seconds until this token expires.
+
+        If token_expiry is in the past, this method will return 0, meaning the
+        token has already expired.
+
+        If token_expiry is None, this method will return None. Note that
+        returning 0 in such a case would not be fair: the token may still be
+        valid; we just don't know anything about it.
+        """
+        if self.token_expiry:
+            now = datetime.datetime.utcnow()
+            if self.token_expiry > now:
+                time_delta = self.token_expiry - now
+                # TODO(orestica): return time_delta.total_seconds()
+                # once dropping support for Python 2.6
+                return time_delta.days * 86400 + time_delta.seconds
+            else:
+                return 0
+
+    def _updateFromCredential(self, other):
+        """Update this Credential from another instance."""
+        self.__dict__.update(other.__getstate__())
+
+    def __getstate__(self):
+        """Trim the state down to something that can be pickled."""
+        d = copy.copy(self.__dict__)
+        del d['store']
+        return d
+
+    def __setstate__(self, state):
+        """Reconstitute the state of the object from being pickled."""
+        self.__dict__.update(state)
+        self.store = None
+
+    def _generate_refresh_request_body(self):
+        """Generate the body that will be used in the refresh request."""
+        body = urllib.parse.urlencode({
+            'grant_type': 'refresh_token',
+            'client_id': self.client_id,
+            'client_secret': self.client_secret,
+            'refresh_token': self.refresh_token,
+        })
+        return body
+
+    def _generate_refresh_request_headers(self):
+        """Generate the headers that will be used in the refresh request."""
+        headers = {
+            'content-type': 'application/x-www-form-urlencoded',
+        }
+
+        if self.user_agent is not None:
+            headers['user-agent'] = self.user_agent
+
+        return headers
+
+    def _refresh(self, http_request):
+        """Refreshes the access_token.
+
+        This method first checks by reading the Storage object if available.
+        If a refresh is still needed, it holds the Storage lock until the
+        refresh is completed.
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          refresh request.
+
+        Raises:
+            HttpAccessTokenRefreshError: When the refresh fails.
+        """
+        if not self.store:
+            self._do_refresh_request(http_request)
+        else:
+            self.store.acquire_lock()
+            try:
+                new_cred = self.store.locked_get()
+
+                if (new_cred and not new_cred.invalid and
+                        new_cred.access_token != self.access_token and
+                        not new_cred.access_token_expired):
+                    logger.info('Updated access_token read from Storage')
+                    self._updateFromCredential(new_cred)
+                else:
+                    self._do_refresh_request(http_request)
+            finally:
+                self.store.release_lock()
+
+    def _do_refresh_request(self, http_request):
+        """Refresh the access_token using the refresh_token.
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          refresh request.
+
+        Raises:
+            HttpAccessTokenRefreshError: When the refresh fails.
+        """
+        body = self._generate_refresh_request_body()
+        headers = self._generate_refresh_request_headers()
+
+        logger.info('Refreshing access_token')
+        resp, content = http_request(
+            self.token_uri, method='POST', body=body, headers=headers)
+        content = _from_bytes(content)
+        if resp.status == 200:
+            d = json.loads(content)
+            self.token_response = d
+            self.access_token = d['access_token']
+            self.refresh_token = d.get('refresh_token', self.refresh_token)
+            if 'expires_in' in d:
+                self.token_expiry = datetime.timedelta(
+                    seconds=int(d['expires_in'])) + datetime.datetime.utcnow()
+            else:
+                self.token_expiry = None
+            # On temporary refresh errors, the user does not actually have to
+            # re-authorize, so we unflag here.
+            self.invalid = False
+            if self.store:
+                self.store.locked_put(self)
+        else:
+            # An {'error':...} response body means the token is expired or
+            # revoked, so we flag the credentials as such.
+            logger.info('Failed to retrieve access token: %s', content)
+            error_msg = 'Invalid response %s.' % resp['status']
+            try:
+                d = json.loads(content)
+                if 'error' in d:
+                    error_msg = d['error']
+                    if 'error_description' in d:
+                        error_msg += ': ' + d['error_description']
+                    self.invalid = True
+                    if self.store:
+                        self.store.locked_put(self)
+            except (TypeError, ValueError):
+                pass
+            raise HttpAccessTokenRefreshError(error_msg, status=resp.status)
+
+    def _revoke(self, http_request):
+        """Revokes this credential and deletes the stored copy (if it exists).
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          revoke request.
+        """
+        self._do_revoke(http_request, self.refresh_token or self.access_token)
+
+    def _do_revoke(self, http_request, token):
+        """Revokes this credential and deletes the stored copy (if it exists).
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          refresh request.
+            token: A string used as the token to be revoked. Can be either an
+                   access_token or refresh_token.
+
+        Raises:
+            TokenRevokeError: If the revoke request does not return with a
+                              200 OK.
+        """
+        logger.info('Revoking token')
+        query_params = {'token': token}
+        token_revoke_uri = _update_query_params(self.revoke_uri, query_params)
+        resp, content = http_request(token_revoke_uri)
+        if resp.status == 200:
+            self.invalid = True
+        else:
+            error_msg = 'Invalid response %s.' % resp.status
+            try:
+                d = json.loads(_from_bytes(content))
+                if 'error' in d:
+                    error_msg = d['error']
+            except (TypeError, ValueError):
+                pass
+            raise TokenRevokeError(error_msg)
+
+        if self.store:
+            self.store.delete()
+
+    def _retrieve_scopes(self, http_request):
+        """Retrieves the list of authorized scopes from the OAuth2 provider.
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          revoke request.
+        """
+        self._do_retrieve_scopes(http_request, self.access_token)
+
+    def _do_retrieve_scopes(self, http_request, token):
+        """Retrieves the list of authorized scopes from the OAuth2 provider.
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          refresh request.
+            token: A string used as the token to identify the credentials to
+                   the provider.
+
+        Raises:
+            Error: When refresh fails, indicating the the access token is
+                   invalid.
+        """
+        logger.info('Refreshing scopes')
+        query_params = {'access_token': token, 'fields': 'scope'}
+        token_info_uri = _update_query_params(self.token_info_uri,
+                                              query_params)
+        resp, content = http_request(token_info_uri)
+        content = _from_bytes(content)
+        if resp.status == 200:
+            d = json.loads(content)
+            self.scopes = set(util.string_to_scopes(d.get('scope', '')))
+        else:
+            error_msg = 'Invalid response %s.' % (resp.status,)
+            try:
+                d = json.loads(content)
+                if 'error_description' in d:
+                    error_msg = d['error_description']
+            except (TypeError, ValueError):
+                pass
+            raise Error(error_msg)
+
+
+class AccessTokenCredentials(OAuth2Credentials):
+    """Credentials object for OAuth 2.0.
+
+    Credentials can be applied to an httplib2.Http object using the
+    authorize() method, which then signs each request from that object
+    with the OAuth 2.0 access token. This set of credentials is for the
+    use case where you have acquired an OAuth 2.0 access_token from
+    another place such as a JavaScript client or another web
+    application, and wish to use it from Python. Because only the
+    access_token is present it can not be refreshed and will in time
+    expire.
+
+    AccessTokenCredentials objects may be safely pickled and unpickled.
+
+    Usage::
+
+        credentials = AccessTokenCredentials('<an access token>',
+            'my-user-agent/1.0')
+        http = httplib2.Http()
+        http = credentials.authorize(http)
+
+    Raises:
+        AccessTokenCredentialsExpired: raised when the access_token expires or
+                                       is revoked.
+    """
+
+    def __init__(self, access_token, user_agent, revoke_uri=None):
+        """Create an instance of OAuth2Credentials
+
+        This is one of the few types if Credentials that you should contrust,
+        Credentials objects are usually instantiated by a Flow.
+
+        Args:
+            access_token: string, access token.
+            user_agent: string, The HTTP User-Agent to provide for this
+                        application.
+            revoke_uri: string, URI for revoke endpoint. Defaults to None; a
+                        token can't be revoked if this is None.
+        """
+        super(AccessTokenCredentials, self).__init__(
+            access_token,
+            None,
+            None,
+            None,
+            None,
+            None,
+            user_agent,
+            revoke_uri=revoke_uri)
+
+    @classmethod
+    def from_json(cls, s):
+        data = json.loads(_from_bytes(s))
+        retval = AccessTokenCredentials(
+            data['access_token'],
+            data['user_agent'])
+        return retval
+
+    def _refresh(self, http_request):
+        raise AccessTokenCredentialsError(
+            'The access_token is expired or invalid and can\'t be refreshed.')
+
+    def _revoke(self, http_request):
+        """Revokes the access_token and deletes the store if available.
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          revoke request.
+        """
+        self._do_revoke(http_request, self.access_token)
+
+
+def _detect_gce_environment():
+    """Determine if the current environment is Compute Engine.
+
+    Returns:
+        Boolean indicating whether or not the current environment is Google
+        Compute Engine.
+    """
+    # NOTE: The explicit ``timeout`` is a workaround. The underlying
+    #       issue is that resolving an unknown host on some networks will take
+    #       20-30 seconds; making this timeout short fixes the issue, but
+    #       could lead to false negatives in the event that we are on GCE, but
+    #       the metadata resolution was particularly slow. The latter case is
+    #       "unlikely".
+    connection = six.moves.http_client.HTTPConnection(
+        _GCE_METADATA_HOST, timeout=1)
+
+    try:
+        headers = {_METADATA_FLAVOR_HEADER: _DESIRED_METADATA_FLAVOR}
+        connection.request('GET', '/', headers=headers)
+        response = connection.getresponse()
+        if response.status == 200:
+            return (response.getheader(_METADATA_FLAVOR_HEADER) ==
+                    _DESIRED_METADATA_FLAVOR)
+    except socket.error:  # socket.timeout or socket.error(64, 'Host is down')
+        logger.info('Timeout attempting to reach GCE metadata service.')
+        return False
+    finally:
+        connection.close()
+
+
+def _in_gae_environment():
+    """Detects if the code is running in the App Engine environment.
+
+    Returns:
+        True if running in the GAE environment, False otherwise.
+    """
+    if SETTINGS.env_name is not None:
+        return SETTINGS.env_name in ('GAE_PRODUCTION', 'GAE_LOCAL')
+
+    try:
+        import google.appengine  # noqa: unused import
+    except ImportError:
+        pass
+    else:
+        server_software = os.environ.get(_SERVER_SOFTWARE, '')
+        if server_software.startswith('Google App Engine/'):
+            SETTINGS.env_name = 'GAE_PRODUCTION'
+            return True
+        elif server_software.startswith('Development/'):
+            SETTINGS.env_name = 'GAE_LOCAL'
+            return True
+
+    return False
+
+
+def _in_gce_environment():
+    """Detect if the code is running in the Compute Engine environment.
+
+    Returns:
+        True if running in the GCE environment, False otherwise.
+    """
+    if SETTINGS.env_name is not None:
+        return SETTINGS.env_name == 'GCE_PRODUCTION'
+
+    if NO_GCE_CHECK != 'True' and _detect_gce_environment():
+        SETTINGS.env_name = 'GCE_PRODUCTION'
+        return True
+    return False
+
+
+class GoogleCredentials(OAuth2Credentials):
+    """Application Default Credentials for use in calling Google APIs.
+
+    The Application Default Credentials are being constructed as a function of
+    the environment where the code is being run.
+    More details can be found on this page:
+    https://developers.google.com/accounts/docs/application-default-credentials
+
+    Here is an example of how to use the Application Default Credentials for a
+    service that requires authentication::
+
+        from googleapiclient.discovery import build
+        from oauth2client.client import GoogleCredentials
+
+        credentials = GoogleCredentials.get_application_default()
+        service = build('compute', 'v1', credentials=credentials)
+
+        PROJECT = 'bamboo-machine-422'
+        ZONE = 'us-central1-a'
+        request = service.instances().list(project=PROJECT, zone=ZONE)
+        response = request.execute()
+
+        print(response)
+    """
+
+    def __init__(self, access_token, client_id, client_secret, refresh_token,
+                 token_expiry, token_uri, user_agent,
+                 revoke_uri=GOOGLE_REVOKE_URI):
+        """Create an instance of GoogleCredentials.
+
+        This constructor is not usually called by the user, instead
+        GoogleCredentials objects are instantiated by
+        GoogleCredentials.from_stream() or
+        GoogleCredentials.get_application_default().
+
+        Args:
+            access_token: string, access token.
+            client_id: string, client identifier.
+            client_secret: string, client secret.
+            refresh_token: string, refresh token.
+            token_expiry: datetime, when the access_token expires.
+            token_uri: string, URI of token endpoint.
+            user_agent: string, The HTTP User-Agent to provide for this
+                        application.
+            revoke_uri: string, URI for revoke endpoint. Defaults to
+                        GOOGLE_REVOKE_URI; a token can't be revoked if this
+                        is None.
+        """
+        super(GoogleCredentials, self).__init__(
+            access_token, client_id, client_secret, refresh_token,
+            token_expiry, token_uri, user_agent, revoke_uri=revoke_uri)
+
+    def create_scoped_required(self):
+        """Whether this Credentials object is scopeless.
+
+        create_scoped(scopes) method needs to be called in order to create
+        a Credentials object for API calls.
+        """
+        return False
+
+    def create_scoped(self, scopes):
+        """Create a Credentials object for the given scopes.
+
+        The Credentials type is preserved.
+        """
+        return self
+
+    @property
+    def serialization_data(self):
+        """Get the fields and values identifying the current credentials."""
+        return {
+            'type': 'authorized_user',
+            'client_id': self.client_id,
+            'client_secret': self.client_secret,
+            'refresh_token': self.refresh_token
+        }
+
+    @staticmethod
+    def _implicit_credentials_from_gae():
+        """Attempts to get implicit credentials in Google App Engine env.
+
+        If the current environment is not detected as App Engine, returns None,
+        indicating no Google App Engine credentials can be detected from the
+        current environment.
+
+        Returns:
+            None, if not in GAE, else an appengine.AppAssertionCredentials
+            object.
+        """
+        if not _in_gae_environment():
+            return None
+
+        return _get_application_default_credential_GAE()
+
+    @staticmethod
+    def _implicit_credentials_from_gce():
+        """Attempts to get implicit credentials in Google Compute Engine env.
+
+        If the current environment is not detected as Compute Engine, returns
+        None, indicating no Google Compute Engine credentials can be detected
+        from the current environment.
+
+        Returns:
+            None, if not in GCE, else a gce.AppAssertionCredentials object.
+        """
+        if not _in_gce_environment():
+            return None
+
+        return _get_application_default_credential_GCE()
+
+    @staticmethod
+    def _implicit_credentials_from_files():
+        """Attempts to get implicit credentials from local credential files.
+
+        First checks if the environment variable GOOGLE_APPLICATION_CREDENTIALS
+        is set with a filename and then falls back to a configuration file (the
+        "well known" file) associated with the 'gcloud' command line tool.
+
+        Returns:
+            Credentials object associated with the
+            GOOGLE_APPLICATION_CREDENTIALS file or the "well known" file if
+            either exist. If neither file is define, returns None, indicating
+            no credentials from a file can detected from the current
+            environment.
+        """
+        credentials_filename = _get_environment_variable_file()
+        if not credentials_filename:
+            credentials_filename = _get_well_known_file()
+            if os.path.isfile(credentials_filename):
+                extra_help = (' (produced automatically when running'
+                              ' "gcloud auth login" command)')
+            else:
+                credentials_filename = None
+        else:
+            extra_help = (' (pointed to by ' + GOOGLE_APPLICATION_CREDENTIALS +
+                          ' environment variable)')
+
+        if not credentials_filename:
+            return
+
+        # If we can read the credentials from a file, we don't need to know
+        # what environment we are in.
+        SETTINGS.env_name = DEFAULT_ENV_NAME
+
+        try:
+            return _get_application_default_credential_from_file(
+                credentials_filename)
+        except (ApplicationDefaultCredentialsError, ValueError) as error:
+            _raise_exception_for_reading_json(credentials_filename,
+                                              extra_help, error)
+
+    @classmethod
+    def _get_implicit_credentials(cls):
+        """Gets credentials implicitly from the environment.
+
+        Checks environment in order of precedence:
+        - Google App Engine (production and testing)
+        - Environment variable GOOGLE_APPLICATION_CREDENTIALS pointing to
+          a file with stored credentials information.
+        - Stored "well known" file associated with `gcloud` command line tool.
+        - Google Compute Engine production environment.
+
+        Raises:
+            ApplicationDefaultCredentialsError: raised when the credentials
+                                                fail to be retrieved.
+        """
+        # Environ checks (in order).
+        environ_checkers = [
+            cls._implicit_credentials_from_gae,
+            cls._implicit_credentials_from_files,
+            cls._implicit_credentials_from_gce,
+        ]
+
+        for checker in environ_checkers:
+            credentials = checker()
+            if credentials is not None:
+                return credentials
+
+        # If no credentials, fail.
+        raise ApplicationDefaultCredentialsError(ADC_HELP_MSG)
+
+    @staticmethod
+    def get_application_default():
+        """Get the Application Default Credentials for the current environment.
+
+        Raises:
+            ApplicationDefaultCredentialsError: raised when the credentials
+                                                fail to be retrieved.
+        """
+        return GoogleCredentials._get_implicit_credentials()
+
+    @staticmethod
+    def from_stream(credential_filename):
+        """Create a Credentials object by reading information from a file.
+
+        It returns an object of type GoogleCredentials.
+
+        Args:
+            credential_filename: the path to the file from where the
+                                 credentials are to be read
+
+        Raises:
+            ApplicationDefaultCredentialsError: raised when the credentials
+                                                fail to be retrieved.
+        """
+        if credential_filename and os.path.isfile(credential_filename):
+            try:
+                return _get_application_default_credential_from_file(
+                    credential_filename)
+            except (ApplicationDefaultCredentialsError, ValueError) as error:
+                extra_help = (' (provided as parameter to the '
+                              'from_stream() method)')
+                _raise_exception_for_reading_json(credential_filename,
+                                                  extra_help,
+                                                  error)
+        else:
+            raise ApplicationDefaultCredentialsError(
+                'The parameter passed to the from_stream() '
+                'method should point to a file.')
+
+
+def _save_private_file(filename, json_contents):
+    """Saves a file with read-write permissions on for the owner.
+
+    Args:
+        filename: String. Absolute path to file.
+        json_contents: JSON serializable object to be saved.
+    """
+    temp_filename = tempfile.mktemp()
+    file_desc = os.open(temp_filename, os.O_WRONLY | os.O_CREAT, 0o600)
+    with os.fdopen(file_desc, 'w') as file_handle:
+        json.dump(json_contents, file_handle, sort_keys=True,
+                  indent=2, separators=(',', ': '))
+    shutil.move(temp_filename, filename)
+
+
+def save_to_well_known_file(credentials, well_known_file=None):
+    """Save the provided GoogleCredentials to the well known file.
+
+    Args:
+        credentials: the credentials to be saved to the well known file;
+                     it should be an instance of GoogleCredentials
+        well_known_file: the name of the file where the credentials are to be
+                         saved; this parameter is supposed to be used for
+                         testing only
+    """
+    # TODO(orestica): move this method to tools.py
+    # once the argparse import gets fixed (it is not present in Python 2.6)
+
+    if well_known_file is None:
+        well_known_file = _get_well_known_file()
+
+    config_dir = os.path.dirname(well_known_file)
+    if not os.path.isdir(config_dir):
+        raise OSError('Config directory does not exist: %s' % config_dir)
+
+    credentials_data = credentials.serialization_data
+    _save_private_file(well_known_file, credentials_data)
+
+
+def _get_environment_variable_file():
+    application_default_credential_filename = (
+      os.environ.get(GOOGLE_APPLICATION_CREDENTIALS,
+                     None))
+
+    if application_default_credential_filename:
+        if os.path.isfile(application_default_credential_filename):
+            return application_default_credential_filename
+        else:
+            raise ApplicationDefaultCredentialsError(
+                'File ' + application_default_credential_filename +
+                ' (pointed by ' +
+                GOOGLE_APPLICATION_CREDENTIALS +
+                ' environment variable) does not exist!')
+
+
+def _get_well_known_file():
+    """Get the well known file produced by command 'gcloud auth login'."""
+    # TODO(orestica): Revisit this method once gcloud provides a better way
+    # of pinpointing the exact location of the file.
+
+    WELL_KNOWN_CREDENTIALS_FILE = 'application_default_credentials.json'
+
+    default_config_dir = os.getenv(_CLOUDSDK_CONFIG_ENV_VAR)
+    if default_config_dir is None:
+        if os.name == 'nt':
+            try:
+                default_config_dir = os.path.join(os.environ['APPDATA'],
+                                                  _CLOUDSDK_CONFIG_DIRECTORY)
+            except KeyError:
+                # This should never happen unless someone is really
+                # messing with things.
+                drive = os.environ.get('SystemDrive', 'C:')
+                default_config_dir = os.path.join(drive, '\\',
+                                                  _CLOUDSDK_CONFIG_DIRECTORY)
+        else:
+            default_config_dir = os.path.join(os.path.expanduser('~'),
+                                              '.config',
+                                              _CLOUDSDK_CONFIG_DIRECTORY)
+
+    return os.path.join(default_config_dir, WELL_KNOWN_CREDENTIALS_FILE)
+
+
+def _get_application_default_credential_from_file(filename):
+    """Build the Application Default Credentials from file."""
+
+    from oauth2client import service_account
+
+    # read the credentials from the file
+    with open(filename) as file_obj:
+        client_credentials = json.load(file_obj)
+
+    credentials_type = client_credentials.get('type')
+    if credentials_type == AUTHORIZED_USER:
+        required_fields = set(['client_id', 'client_secret', 'refresh_token'])
+    elif credentials_type == SERVICE_ACCOUNT:
+        required_fields = set(['client_id', 'client_email', 'private_key_id',
+                               'private_key'])
+    else:
+        raise ApplicationDefaultCredentialsError(
+            "'type' field should be defined (and have one of the '" +
+            AUTHORIZED_USER + "' or '" + SERVICE_ACCOUNT + "' values)")
+
+    missing_fields = required_fields.difference(client_credentials.keys())
+
+    if missing_fields:
+        _raise_exception_for_missing_fields(missing_fields)
+
+    if client_credentials['type'] == AUTHORIZED_USER:
+        return GoogleCredentials(
+            access_token=None,
+            client_id=client_credentials['client_id'],
+            client_secret=client_credentials['client_secret'],
+            refresh_token=client_credentials['refresh_token'],
+            token_expiry=None,
+            token_uri=GOOGLE_TOKEN_URI,
+            user_agent='Python client library')
+    else:  # client_credentials['type'] == SERVICE_ACCOUNT
+        return service_account._ServiceAccountCredentials(
+            service_account_id=client_credentials['client_id'],
+            service_account_email=client_credentials['client_email'],
+            private_key_id=client_credentials['private_key_id'],
+            private_key_pkcs8_text=client_credentials['private_key'],
+            scopes=[])
+
+
+def _raise_exception_for_missing_fields(missing_fields):
+    raise ApplicationDefaultCredentialsError(
+        'The following field(s) must be defined: ' + ', '.join(missing_fields))
+
+
+def _raise_exception_for_reading_json(credential_file,
+                                      extra_help,
+                                      error):
+    raise ApplicationDefaultCredentialsError(
+      'An error was encountered while reading json file: ' +
+      credential_file + extra_help + ': ' + str(error))
+
+
+def _get_application_default_credential_GAE():
+    from oauth2client.appengine import AppAssertionCredentials
+
+    return AppAssertionCredentials([])
+
+
+def _get_application_default_credential_GCE():
+    from oauth2client.gce import AppAssertionCredentials
+
+    return AppAssertionCredentials([])
+
+
+class AssertionCredentials(GoogleCredentials):
+    """Abstract Credentials object used for OAuth 2.0 assertion grants.
+
+    This credential does not require a flow to instantiate because it
+    represents a two legged flow, and therefore has all of the required
+    information to generate and refresh its own access tokens. It must
+    be subclassed to generate the appropriate assertion string.
+
+    AssertionCredentials objects may be safely pickled and unpickled.
+    """
+
+    @util.positional(2)
+    def __init__(self, assertion_type, user_agent=None,
+                 token_uri=GOOGLE_TOKEN_URI,
+                 revoke_uri=GOOGLE_REVOKE_URI,
+                 **unused_kwargs):
+        """Constructor for AssertionFlowCredentials.
+
+        Args:
+            assertion_type: string, assertion type that will be declared to the
+                            auth server
+            user_agent: string, The HTTP User-Agent to provide for this
+                        application.
+            token_uri: string, URI for token endpoint. For convenience defaults
+                       to Google's endpoints but any OAuth 2.0 provider can be
+                       used.
+            revoke_uri: string, URI for revoke endpoint.
+        """
+        super(AssertionCredentials, self).__init__(
+            None,
+            None,
+            None,
+            None,
+            None,
+            token_uri,
+            user_agent,
+            revoke_uri=revoke_uri)
+        self.assertion_type = assertion_type
+
+    def _generate_refresh_request_body(self):
+        assertion = self._generate_assertion()
+
+        body = urllib.parse.urlencode({
+            'assertion': assertion,
+            'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+        })
+
+        return body
+
+    def _generate_assertion(self):
+        """Generate assertion string to be used in the access token request."""
+        _abstract()
+
+    def _revoke(self, http_request):
+        """Revokes the access_token and deletes the store if available.
+
+        Args:
+            http_request: callable, a callable that matches the method
+                          signature of httplib2.Http.request, used to make the
+                          revoke request.
+        """
+        self._do_revoke(http_request, self.access_token)
+
+
+def _RequireCryptoOrDie():
+    """Ensure we have a crypto library, or throw CryptoUnavailableError.
+
+    The oauth2client.crypt module requires either PyCrypto or PyOpenSSL
+    to be available in order to function, but these are optional
+    dependencies.
+    """
+    if not HAS_CRYPTO:
+        raise CryptoUnavailableError('No crypto library available')
+
+
+class SignedJwtAssertionCredentials(AssertionCredentials):
+    """Credentials object used for OAuth 2.0 Signed JWT assertion grants.
+
+    This credential does not require a flow to instantiate because it
+    represents a two legged flow, and therefore has all of the required
+    information to generate and refresh its own access tokens.
+
+    SignedJwtAssertionCredentials requires either PyOpenSSL, or PyCrypto
+    2.6 or later. For App Engine you may also consider using
+    AppAssertionCredentials.
+    """
+
+    MAX_TOKEN_LIFETIME_SECS = 3600  # 1 hour in seconds
+
+    @util.positional(4)
+    def __init__(self,
+                 service_account_name,
+                 private_key,
+                 scope,
+                 private_key_password='notasecret',
+                 user_agent=None,
+                 token_uri=GOOGLE_TOKEN_URI,
+                 revoke_uri=GOOGLE_REVOKE_URI,
+                 **kwargs):
+        """Constructor for SignedJwtAssertionCredentials.
+
+        Args:
+            service_account_name: string, id for account, usually an email
+                                  address.
+            private_key: string or bytes, private key in PKCS12 or PEM format.
+            scope: string or iterable of strings, scope(s) of the credentials
+                   being requested.
+            private_key_password: string, password for private_key, unused if
+                                  private_key is in PEM format.
+            user_agent: string, HTTP User-Agent to provide for this
+                        application.
+            token_uri: string, URI for token endpoint. For convenience defaults
+                       to Google's endpoints but any OAuth 2.0 provider can be
+                       used.
+            revoke_uri: string, URI for revoke endpoint.
+            kwargs: kwargs, Additional parameters to add to the JWT token, for
+                    example sub=joe@xample.org.
+
+        Raises:
+            CryptoUnavailableError if no crypto library is available.
+        """
+        _RequireCryptoOrDie()
+        super(SignedJwtAssertionCredentials, self).__init__(
+            None,
+            user_agent=user_agent,
+            token_uri=token_uri,
+            revoke_uri=revoke_uri,
+        )
+
+        self.scope = util.scopes_to_string(scope)
+
+        # Keep base64 encoded so it can be stored in JSON.
+        self.private_key = base64.b64encode(_to_bytes(private_key))
+        self.private_key_password = private_key_password
+        self.service_account_name = service_account_name
+        self.kwargs = kwargs
+
+    @classmethod
+    def from_json(cls, s):
+        data = json.loads(_from_bytes(s))
+        retval = SignedJwtAssertionCredentials(
+            data['service_account_name'],
+            base64.b64decode(data['private_key']),
+            data['scope'],
+            private_key_password=data['private_key_password'],
+            user_agent=data['user_agent'],
+            token_uri=data['token_uri'],
+            **data['kwargs']
+        )
+        retval.invalid = data['invalid']
+        retval.access_token = data['access_token']
+        return retval
+
+    def _generate_assertion(self):
+        """Generate the assertion that will be used in the request."""
+        now = int(time.time())
+        payload = {
+            'aud': self.token_uri,
+            'scope': self.scope,
+            'iat': now,
+            'exp': now + SignedJwtAssertionCredentials.MAX_TOKEN_LIFETIME_SECS,
+            'iss': self.service_account_name
+        }
+        payload.update(self.kwargs)
+        logger.debug(str(payload))
+
+        private_key = base64.b64decode(self.private_key)
+        return crypt.make_signed_jwt(crypt.Signer.from_string(
+            private_key, self.private_key_password), payload)
+
+# Only used in verify_id_token(), which is always calling to the same URI
+# for the certs.
+_cached_http = httplib2.Http(MemoryCache())
+
+
+@util.positional(2)
+def verify_id_token(id_token, audience, http=None,
+                    cert_uri=ID_TOKEN_VERIFICATION_CERTS):
+    """Verifies a signed JWT id_token.
+
+    This function requires PyOpenSSL and because of that it does not work on
+    App Engine.
+
+    Args:
+        id_token: string, A Signed JWT.
+        audience: string, The audience 'aud' that the token should be for.
+        http: httplib2.Http, instance to use to make the HTTP request. Callers
+              should supply an instance that has caching enabled.
+        cert_uri: string, URI of the certificates in JSON format to
+                  verify the JWT against.
+
+    Returns:
+        The deserialized JSON in the JWT.
+
+    Raises:
+        oauth2client.crypt.AppIdentityError: if the JWT fails to verify.
+        CryptoUnavailableError: if no crypto library is available.
+    """
+    _RequireCryptoOrDie()
+    if http is None:
+        http = _cached_http
+
+    resp, content = http.request(cert_uri)
+    if resp.status == 200:
+        certs = json.loads(_from_bytes(content))
+        return crypt.verify_signed_jwt_with_certs(id_token, certs, audience)
+    else:
+        raise VerifyJwtTokenError('Status code: %d' % resp.status)
+
+
+def _extract_id_token(id_token):
+    """Extract the JSON payload from a JWT.
+
+    Does the extraction w/o checking the signature.
+
+    Args:
+        id_token: string or bytestring, OAuth 2.0 id_token.
+
+    Returns:
+        object, The deserialized JSON payload.
+    """
+    if type(id_token) == bytes:
+        segments = id_token.split(b'.')
+    else:
+        segments = id_token.split(u'.')
+
+    if len(segments) != 3:
+        raise VerifyJwtTokenError(
+            'Wrong number of segments in token: %s' % id_token)
+
+    return json.loads(_from_bytes(_urlsafe_b64decode(segments[1])))
+
+
+def _parse_exchange_token_response(content):
+    """Parses response of an exchange token request.
+
+    Most providers return JSON but some (e.g. Facebook) return a
+    url-encoded string.
+
+    Args:
+        content: The body of a response
+
+    Returns:
+        Content as a dictionary object. Note that the dict could be empty,
+        i.e. {}. That basically indicates a failure.
+    """
+    resp = {}
+    content = _from_bytes(content)
+    try:
+        resp = json.loads(content)
+    except Exception:
+        # different JSON libs raise different exceptions,
+        # so we just do a catch-all here
+        resp = dict(urllib.parse.parse_qsl(content))
+
+    # some providers respond with 'expires', others with 'expires_in'
+    if resp and 'expires' in resp:
+        resp['expires_in'] = resp.pop('expires')
+
+    return resp
+
+
+@util.positional(4)
+def credentials_from_code(client_id, client_secret, scope, code,
+                          redirect_uri='postmessage', http=None,
+                          user_agent=None, token_uri=GOOGLE_TOKEN_URI,
+                          auth_uri=GOOGLE_AUTH_URI,
+                          revoke_uri=GOOGLE_REVOKE_URI,
+                          device_uri=GOOGLE_DEVICE_URI,
+                          token_info_uri=GOOGLE_TOKEN_INFO_URI):
+    """Exchanges an authorization code for an OAuth2Credentials object.
+
+    Args:
+        client_id: string, client identifier.
+        client_secret: string, client secret.
+        scope: string or iterable of strings, scope(s) to request.
+        code: string, An authorization code, most likely passed down from
+              the client
+        redirect_uri: string, this is generally set to 'postmessage' to match
+                      the redirect_uri that the client specified
+        http: httplib2.Http, optional http instance to use to do the fetch
+        token_uri: string, URI for token endpoint. For convenience defaults
+                   to Google's endpoints but any OAuth 2.0 provider can be
+                   used.
+        auth_uri: string, URI for authorization endpoint. For convenience
+                  defaults to Google's endpoints but any OAuth 2.0 provider
+                  can be used.
+        revoke_uri: string, URI for revoke endpoint. For convenience
+                    defaults to Google's endpoints but any OAuth 2.0 provider
+                    can be used.
+        device_uri: string, URI for device authorization endpoint. For
+                    convenience defaults to Google's endpoints but any OAuth
+                    2.0 provider can be used.
+
+    Returns:
+        An OAuth2Credentials object.
+
+    Raises:
+        FlowExchangeError if the authorization code cannot be exchanged for an
+        access token
+    """
+    flow = OAuth2WebServerFlow(client_id, client_secret, scope,
+                               redirect_uri=redirect_uri,
+                               user_agent=user_agent, auth_uri=auth_uri,
+                               token_uri=token_uri, revoke_uri=revoke_uri,
+                               device_uri=device_uri,
+                               token_info_uri=token_info_uri)
+
+    credentials = flow.step2_exchange(code, http=http)
+    return credentials
+
+
+@util.positional(3)
+def credentials_from_clientsecrets_and_code(filename, scope, code,
+                                            message=None,
+                                            redirect_uri='postmessage',
+                                            http=None,
+                                            cache=None,
+                                            device_uri=None):
+    """Returns OAuth2Credentials from a clientsecrets file and an auth code.
+
+    Will create the right kind of Flow based on the contents of the
+    clientsecrets file or will raise InvalidClientSecretsError for unknown
+    types of Flows.
+
+    Args:
+        filename: string, File name of clientsecrets.
+        scope: string or iterable of strings, scope(s) to request.
+        code: string, An authorization code, most likely passed down from
+              the client
+        message: string, A friendly string to display to the user if the
+                 clientsecrets file is missing or invalid. If message is
+                 provided then sys.exit will be called in the case of an error.
+                 If message in not provided then
+                 clientsecrets.InvalidClientSecretsError will be raised.
+        redirect_uri: string, this is generally set to 'postmessage' to match
+                      the redirect_uri that the client specified
+        http: httplib2.Http, optional http instance to use to do the fetch
+        cache: An optional cache service client that implements get() and set()
+               methods. See clientsecrets.loadfile() for details.
+        device_uri: string, OAuth 2.0 device authorization endpoint
+
+    Returns:
+        An OAuth2Credentials object.
+
+    Raises:
+        FlowExchangeError: if the authorization code cannot be exchanged for an
+                           access token
+        UnknownClientSecretsFlowError: if the file describes an unknown kind
+                                       of Flow.
+        clientsecrets.InvalidClientSecretsError: if the clientsecrets file is
+                                                 invalid.
+    """
+    flow = flow_from_clientsecrets(filename, scope, message=message,
+                                   cache=cache, redirect_uri=redirect_uri,
+                                   device_uri=device_uri)
+    credentials = flow.step2_exchange(code, http=http)
+    return credentials
+
+
+class DeviceFlowInfo(collections.namedtuple('DeviceFlowInfo', (
+        'device_code', 'user_code', 'interval', 'verification_url',
+        'user_code_expiry'))):
+    """Intermediate information the OAuth2 for devices flow."""
+
+    @classmethod
+    def FromResponse(cls, response):
+        """Create a DeviceFlowInfo from a server response.
+
+        The response should be a dict containing entries as described here:
+
+        http://tools.ietf.org/html/draft-ietf-oauth-v2-05#section-3.7.1
+        """
+        # device_code, user_code, and verification_url are required.
+        kwargs = {
+            'device_code': response['device_code'],
+            'user_code': response['user_code'],
+        }
+        # The response may list the verification address as either
+        # verification_url or verification_uri, so we check for both.
+        verification_url = response.get(
+            'verification_url', response.get('verification_uri'))
+        if verification_url is None:
+            raise OAuth2DeviceCodeError(
+                'No verification_url provided in server response')
+        kwargs['verification_url'] = verification_url
+        # expires_in and interval are optional.
+        kwargs.update({
+            'interval': response.get('interval'),
+            'user_code_expiry': None,
+        })
+        if 'expires_in' in response:
+            kwargs['user_code_expiry'] = (
+                datetime.datetime.now() +
+                datetime.timedelta(seconds=int(response['expires_in'])))
+        return cls(**kwargs)
+
+
+class OAuth2WebServerFlow(Flow):
+    """Does the Web Server Flow for OAuth 2.0.
+
+    OAuth2WebServerFlow objects may be safely pickled and unpickled.
+    """
+
+    @util.positional(4)
+    def __init__(self, client_id,
+                 client_secret=None,
+                 scope=None,
+                 redirect_uri=None,
+                 user_agent=None,
+                 auth_uri=GOOGLE_AUTH_URI,
+                 token_uri=GOOGLE_TOKEN_URI,
+                 revoke_uri=GOOGLE_REVOKE_URI,
+                 login_hint=None,
+                 device_uri=GOOGLE_DEVICE_URI,
+                 token_info_uri=GOOGLE_TOKEN_INFO_URI,
+                 authorization_header=None,
+                 **kwargs):
+        """Constructor for OAuth2WebServerFlow.
+
+        The kwargs argument is used to set extra query parameters on the
+        auth_uri. For example, the access_type and approval_prompt
+        query parameters can be set via kwargs.
+
+        Args:
+            client_id: string, client identifier.
+            client_secret: string client secret.
+            scope: string or iterable of strings, scope(s) of the credentials
+                   being requested.
+            redirect_uri: string, Either the string 'urn:ietf:wg:oauth:2.0:oob'
+                          for a non-web-based application, or a URI that
+                          handles the callback from the authorization server.
+            user_agent: string, HTTP User-Agent to provide for this
+                        application.
+            auth_uri: string, URI for authorization endpoint. For convenience
+                      defaults to Google's endpoints but any OAuth 2.0 provider
+                      can be used.
+            token_uri: string, URI for token endpoint. For convenience
+                       defaults to Google's endpoints but any OAuth 2.0
+                       provider can be used.
+            revoke_uri: string, URI for revoke endpoint. For convenience
+                        defaults to Google's endpoints but any OAuth 2.0
+                        provider can be used.
+            login_hint: string, Either an email address or domain. Passing this
+                        hint will either pre-fill the email box on the sign-in
+                        form or select the proper multi-login session, thereby
+                        simplifying the login flow.
+            device_uri: string, URI for device authorization endpoint. For
+                        convenience defaults to Google's endpoints but any
+                        OAuth 2.0 provider can be used.
+            authorization_header: string, For use with OAuth 2.0 providers that
+                                  require a client to authenticate using a
+                                  header value instead of passing client_secret
+                                  in the POST body.
+            **kwargs: dict, The keyword arguments are all optional and required
+                      parameters for the OAuth calls.
+        """
+        # scope is a required argument, but to preserve backwards-compatibility
+        # we don't want to rearrange the positional arguments
+        if scope is None:
+            raise TypeError("The value of scope must not be None")
+        self.client_id = client_id
+        self.client_secret = client_secret
+        self.scope = util.scopes_to_string(scope)
+        self.redirect_uri = redirect_uri
+        self.login_hint = login_hint
+        self.user_agent = user_agent
+        self.auth_uri = auth_uri
+        self.token_uri = token_uri
+        self.revoke_uri = revoke_uri
+        self.device_uri = device_uri
+        self.token_info_uri = token_info_uri
+        self.authorization_header = authorization_header
+        self.params = {
+            'access_type': 'offline',
+            'response_type': 'code',
+        }
+        self.params.update(kwargs)
+
+    @util.positional(1)
+    def step1_get_authorize_url(self, redirect_uri=None, state=None):
+        """Returns a URI to redirect to the provider.
+
+        Args:
+            redirect_uri: string, Either the string 'urn:ietf:wg:oauth:2.0:oob'
+                          for a non-web-based application, or a URI that
+                          handles the callback from the authorization server.
+                          This parameter is deprecated, please move to passing
+                          the redirect_uri in via the constructor.
+            state: string, Opaque state string which is passed through the
+                   OAuth2 flow and returned to the client as a query parameter
+                   in the callback.
+
+        Returns:
+            A URI as a string to redirect the user to begin the authorization
+            flow.
+        """
+        if redirect_uri is not None:
+            logger.warning((
+                'The redirect_uri parameter for '
+                'OAuth2WebServerFlow.step1_get_authorize_url is deprecated. '
+                'Please move to passing the redirect_uri in via the '
+                'constructor.'))
+            self.redirect_uri = redirect_uri
+
+        if self.redirect_uri is None:
+            raise ValueError('The value of redirect_uri must not be None.')
+
+        query_params = {
+            'client_id': self.client_id,
+            'redirect_uri': self.redirect_uri,
+            'scope': self.scope,
+        }
+        if state is not None:
+            query_params['state'] = state
+        if self.login_hint is not None:
+            query_params['login_hint'] = self.login_hint
+        query_params.update(self.params)
+        return _update_query_params(self.auth_uri, query_params)
+
+    @util.positional(1)
+    def step1_get_device_and_user_codes(self, http=None):
+        """Returns a user code and the verification URL where to enter it
+
+        Returns:
+            A user code as a string for the user to authorize the application
+            An URL as a string where the user has to enter the code
+        """
+        if self.device_uri is None:
+            raise ValueError('The value of device_uri must not be None.')
+
+        body = urllib.parse.urlencode({
+            'client_id': self.client_id,
+            'scope': self.scope,
+        })
+        headers = {
+            'content-type': 'application/x-www-form-urlencoded',
+        }
+
+        if self.user_agent is not None:
+            headers['user-agent'] = self.user_agent
+
+        if http is None:
+            http = httplib2.Http()
+
+        resp, content = http.request(self.device_uri, method='POST', body=body,
+                                     headers=headers)
+        content = _from_bytes(content)
+        if resp.status == 200:
+            try:
+                flow_info = json.loads(content)
+            except ValueError as e:
+                raise OAuth2DeviceCodeError(
+                    'Could not parse server response as JSON: "%s", '
+                    'error: "%s"' % (content, e))
+            return DeviceFlowInfo.FromResponse(flow_info)
+        else:
+            error_msg = 'Invalid response %s.' % resp.status
+            try:
+                d = json.loads(content)
+                if 'error' in d:
+                    error_msg += ' Error: %s' % d['error']
+            except ValueError:
+                # Couldn't decode a JSON response, stick with the
+                # default message.
+                pass
+            raise OAuth2DeviceCodeError(error_msg)
+
+    @util.positional(2)
+    def step2_exchange(self, code=None, http=None, device_flow_info=None):
+        """Exchanges a code for OAuth2Credentials.
+
+        Args:
+            code: string, a dict-like object, or None. For a non-device
+                  flow, this is either the response code as a string, or a
+                  dictionary of query parameters to the redirect_uri. For a
+                  device flow, this should be None.
+            http: httplib2.Http, optional http instance to use when fetching
+                  credentials.
+            device_flow_info: DeviceFlowInfo, return value from step1 in the
+                              case of a device flow.
+
+        Returns:
+            An OAuth2Credentials object that can be used to authorize requests.
+
+        Raises:
+            FlowExchangeError: if a problem occurred exchanging the code for a
+                               refresh_token.
+            ValueError: if code and device_flow_info are both provided or both
+                        missing.
+        """
+        if code is None and device_flow_info is None:
+            raise ValueError('No code or device_flow_info provided.')
+        if code is not None and device_flow_info is not None:
+            raise ValueError('Cannot provide both code and device_flow_info.')
+
+        if code is None:
+            code = device_flow_info.device_code
+        elif not isinstance(code, six.string_types):
+            if 'code' not in code:
+                raise FlowExchangeError(code.get(
+                    'error', 'No code was supplied in the query parameters.'))
+            code = code['code']
+
+        post_data = {
+            'client_id': self.client_id,
+            'code': code,
+            'scope': self.scope,
+        }
+        if self.client_secret is not None:
+            post_data['client_secret'] = self.client_secret
+        if device_flow_info is not None:
+            post_data['grant_type'] = 'http://oauth.net/grant_type/device/1.0'
+        else:
+            post_data['grant_type'] = 'authorization_code'
+            post_data['redirect_uri'] = self.redirect_uri
+        body = urllib.parse.urlencode(post_data)
+        headers = {
+            'content-type': 'application/x-www-form-urlencoded',
+        }
+        if self.authorization_header is not None:
+            headers['Authorization'] = self.authorization_header
+        if self.user_agent is not None:
+            headers['user-agent'] = self.user_agent
+
+        if http is None:
+            http = httplib2.Http()
+
+        resp, content = http.request(self.token_uri, method='POST', body=body,
+                                     headers=headers)
+        d = _parse_exchange_token_response(content)
+        if resp.status == 200 and 'access_token' in d:
+            access_token = d['access_token']
+            refresh_token = d.get('refresh_token', None)
+            if not refresh_token:
+                logger.info(
+                    'Received token response with no refresh_token. Consider '
+                    "reauthenticating with approval_prompt='force'.")
+            token_expiry = None
+            if 'expires_in' in d:
+                token_expiry = (
+                    datetime.datetime.utcnow() +
+                    datetime.timedelta(seconds=int(d['expires_in'])))
+
+            extracted_id_token = None
+            if 'id_token' in d:
+                extracted_id_token = _extract_id_token(d['id_token'])
+
+            logger.info('Successfully retrieved access token')
+            return OAuth2Credentials(
+                access_token, self.client_id, self.client_secret,
+                refresh_token, token_expiry, self.token_uri, self.user_agent,
+                revoke_uri=self.revoke_uri, id_token=extracted_id_token,
+                token_response=d, scopes=self.scope,
+                token_info_uri=self.token_info_uri)
+        else:
+            logger.info('Failed to retrieve access token: %s', content)
+            if 'error' in d:
+                # you never know what those providers got to say
+                error_msg = (str(d['error']) +
+                             str(d.get('error_description', '')))
+            else:
+                error_msg = 'Invalid response: %s.' % str(resp.status)
+            raise FlowExchangeError(error_msg)
+
+
+@util.positional(2)
+def flow_from_clientsecrets(filename, scope, redirect_uri=None,
+                            message=None, cache=None, login_hint=None,
+                            device_uri=None):
+    """Create a Flow from a clientsecrets file.
+
+    Will create the right kind of Flow based on the contents of the
+    clientsecrets file or will raise InvalidClientSecretsError for unknown
+    types of Flows.
+
+    Args:
+        filename: string, File name of client secrets.
+        scope: string or iterable of strings, scope(s) to request.
+        redirect_uri: string, Either the string 'urn:ietf:wg:oauth:2.0:oob' for
+                      a non-web-based application, or a URI that handles the
+                      callback from the authorization server.
+        message: string, A friendly string to display to the user if the
+                 clientsecrets file is missing or invalid. If message is
+                 provided then sys.exit will be called in the case of an error.
+                 If message in not provided then
+                 clientsecrets.InvalidClientSecretsError will be raised.
+        cache: An optional cache service client that implements get() and set()
+               methods. See clientsecrets.loadfile() for details.
+        login_hint: string, Either an email address or domain. Passing this
+                    hint will either pre-fill the email box on the sign-in form
+                    or select the proper multi-login session, thereby
+                    simplifying the login flow.
+        device_uri: string, URI for device authorization endpoint. For
+                    convenience defaults to Google's endpoints but any
+                    OAuth 2.0 provider can be used.
+
+    Returns:
+        A Flow object.
+
+    Raises:
+        UnknownClientSecretsFlowError: if the file describes an unknown kind of
+                                       Flow.
+        clientsecrets.InvalidClientSecretsError: if the clientsecrets file is
+                                                 invalid.
+    """
+    try:
+        client_type, client_info = clientsecrets.loadfile(filename,
+                                                          cache=cache)
+        if client_type in (clientsecrets.TYPE_WEB,
+                           clientsecrets.TYPE_INSTALLED):
+            constructor_kwargs = {
+                'redirect_uri': redirect_uri,
+                'auth_uri': client_info['auth_uri'],
+                'token_uri': client_info['token_uri'],
+                'login_hint': login_hint,
+            }
+            revoke_uri = client_info.get('revoke_uri')
+            if revoke_uri is not None:
+                constructor_kwargs['revoke_uri'] = revoke_uri
+            if device_uri is not None:
+                constructor_kwargs['device_uri'] = device_uri
+            return OAuth2WebServerFlow(
+                client_info['client_id'], client_info['client_secret'],
+                scope, **constructor_kwargs)
+
+    except clientsecrets.InvalidClientSecretsError:
+        if message:
+            sys.exit(message)
+        else:
+            raise
+    else:
+        raise UnknownClientSecretsFlowError(
+            'This OAuth 2.0 flow is unsupported: %r' % client_type)