#!/usr/bin/env python3
#
# Copyright 2012 Google Inc.
# Copyright 2020 Manos Pitsidianakis
#
# 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.

"""Performs client tasks for testing IMAP OAuth2 authentication.

To use this script, you'll need to have registered with Google as an OAuth
application and obtained an OAuth client ID and client secret.
See https://developers.google.com/identity/protocols/OAuth2 for instructions on
registering and for documentation of the APIs invoked by this code.

This script has 3 modes of operation.

1. The first mode is used to generate and authorize an OAuth2 token, the
first step in logging in via OAuth2.

  oauth2 --user=xxx@gmail.com \
    --client_id=1038[...].apps.googleusercontent.com \
    --client_secret=VWFn8LIKAMC-MsjBMhJeOplZ \
    --generate_oauth2_token

The script will converse with Google and generate an oauth request
token, then present you with a URL you should visit in your browser to
authorize the token. Once you get the verification code from the Google
website, enter it into the script to get your OAuth access token. The output
from this command will contain the access token, a refresh token, and some
metadata about the tokens. The access token can be used until it expires, and
the refresh token lasts indefinitely, so you should record these values for
reuse.

2. The script will generate new access tokens using a refresh token.

  oauth2 --user=xxx@gmail.com \
    --client_id=1038[...].apps.googleusercontent.com \
    --client_secret=VWFn8LIKAMC-MsjBMhJeOplZ \
    --refresh_token=1/Yzm6MRy4q1xi7Dx2DuWXNgT6s37OrP_DW_IoyTum4YA

3. The script will generate an OAuth2 string that can be fed
directly to IMAP or SMTP. This is triggered with the --generate_oauth2_string
option.

  oauth2 --generate_oauth2_string --user=xxx@gmail.com \
    --access_token=ya29.AGy[...]ezLg

The output of this mode will be a base64-encoded string. To use it, connect to a
IMAPFE and pass it as the second argument to the AUTHENTICATE command.

  a AUTHENTICATE XOAUTH2 a9sha9sfs[...]9dfja929dk==
"""

import base64
import imaplib
import json
from optparse import OptionParser
import smtplib
import sys
import urllib.request, urllib.parse, urllib.error


def SetupOptionParser():
  # Usage message is the module's docstring.
  parser = OptionParser(usage=__doc__)
  parser.add_option('--generate_oauth2_token',
                    action='store_true',
                    dest='generate_oauth2_token',
                    help='generates an OAuth2 token for testing')
  parser.add_option('--generate_oauth2_string',
                    action='store_true',
                    dest='generate_oauth2_string',
                    help='generates an initial client response string for '
                         'OAuth2')
  parser.add_option('--client_id',
                    default=None,
                    help='Client ID of the application that is authenticating. '
                         'See OAuth2 documentation for details.')
  parser.add_option('--client_secret',
                    default=None,
                    help='Client secret of the application that is '
                         'authenticating. See OAuth2 documentation for '
                         'details.')
  parser.add_option('--access_token',
                    default=None,
                    help='OAuth2 access token')
  parser.add_option('--refresh_token',
                    default=None,
                    help='OAuth2 refresh token')
  parser.add_option('--scope',
                    default='https://mail.google.com/',
                    help='scope for the access token. Multiple scopes can be '
                         'listed separated by spaces with the whole argument '
                         'quoted.')
  parser.add_option('--test_imap_authentication',
                    action='store_true',
                    dest='test_imap_authentication',
                    help='attempts to authenticate to IMAP')
  parser.add_option('--test_smtp_authentication',
                    action='store_true',
                    dest='test_smtp_authentication',
                    help='attempts to authenticate to SMTP')
  parser.add_option('--user',
                    default=None,
                    help='email address of user whose account is being '
                         'accessed')
  parser.add_option('--quiet',
                    action='store_true',
                    default=False,
                    dest='quiet',
                    help='Omit verbose descriptions and only print '
                         'machine-readable outputs.')
  return parser


# The URL root for accessing Google Accounts.
GOOGLE_ACCOUNTS_BASE_URL = 'https://accounts.google.com'


# Hardcoded dummy redirect URI for non-web apps.
REDIRECT_URI = 'urn:ietf:wg:oauth:2.0:oob'


def AccountsUrl(command):
  """Generates the Google Accounts URL.

  Args:
    command: The command to execute.

  Returns:
    A URL for the given command.
  """
  return '%s/%s' % (GOOGLE_ACCOUNTS_BASE_URL, command)


def UrlEscape(text):
  # See OAUTH 5.1 for a definition of which characters need to be escaped.
  return urllib.parse.quote(text, safe='~-._')


def UrlUnescape(text):
  # See OAUTH 5.1 for a definition of which characters need to be escaped.
  return urllib.parse.unquote(text)


def FormatUrlParams(params):
  """Formats parameters into a URL query string.

  Args:
    params: A key-value map.

  Returns:
    A URL query string version of the given parameters.
  """
  param_fragments = []
  for param in sorted(iter(params.items()), key=lambda x: x[0]):
    param_fragments.append('%s=%s' % (param[0], UrlEscape(param[1])))
  return '&'.join(param_fragments)


def GeneratePermissionUrl(client_id, scope='https://mail.google.com/'):
  """Generates the URL for authorizing access.

  This uses the "OAuth2 for Installed Applications" flow described at
  https://developers.google.com/accounts/docs/OAuth2InstalledApp

  Args:
    client_id: Client ID obtained by registering your app.
    scope: scope for access token, e.g. 'https://mail.google.com'
  Returns:
    A URL that the user should visit in their browser.
  """
  params = {}
  params['client_id'] = client_id
  params['redirect_uri'] = REDIRECT_URI
  params['scope'] = scope
  params['response_type'] = 'code'
  return '%s?%s' % (AccountsUrl('o/oauth2/auth'),
                    FormatUrlParams(params))


def AuthorizeTokens(client_id, client_secret, authorization_code):
  """Obtains OAuth access token and refresh token.

  This uses the application portion of the "OAuth2 for Installed Applications"
  flow at https://developers.google.com/accounts/docs/OAuth2InstalledApp#handlingtheresponse

  Args:
    client_id: Client ID obtained by registering your app.
    client_secret: Client secret obtained by registering your app.
    authorization_code: code generated by Google Accounts after user grants
        permission.
  Returns:
    The decoded response from the Google Accounts server, as a dict. Expected
    fields include 'access_token', 'expires_in', and 'refresh_token'.
  """
  params = {}
  params['client_id'] = client_id
  params['client_secret'] = client_secret
  params['code'] = authorization_code
  params['redirect_uri'] = REDIRECT_URI
  params['grant_type'] = 'authorization_code'
  request_url = AccountsUrl('o/oauth2/token')

  response = urllib.request.urlopen(request_url, urllib.parse.urlencode(params).encode()).read()
  return json.loads(response)


def RefreshToken(client_id, client_secret, refresh_token):
  """Obtains a new token given a refresh token.

  See https://developers.google.com/accounts/docs/OAuth2InstalledApp#refresh

  Args:
    client_id: Client ID obtained by registering your app.
    client_secret: Client secret obtained by registering your app.
    refresh_token: A previously-obtained refresh token.
  Returns:
    The decoded response from the Google Accounts server, as a dict. Expected
    fields include 'access_token', 'expires_in', and 'refresh_token'.
  """
  params = {}
  params['client_id'] = client_id
  params['client_secret'] = client_secret
  params['refresh_token'] = refresh_token
  params['grant_type'] = 'refresh_token'
  request_url = AccountsUrl('o/oauth2/token')

  response = urllib.request.urlopen(request_url, urllib.parse.urlencode(params).encode()).read()
  return json.loads(response)


def GenerateOAuth2String(username, access_token, base64_encode=True):
  """Generates an IMAP OAuth2 authentication string.

  See https://developers.google.com/google-apps/gmail/oauth2_overview

  Args:
    username: the username (email address) of the account to authenticate
    access_token: An OAuth2 access token.
    base64_encode: Whether to base64-encode the output.

  Returns:
    The SASL argument for the OAuth2 mechanism.
  """
  auth_string = 'user=%s\1auth=Bearer %s\1\1' % (username, access_token)
  if base64_encode:
    auth_string = base64.b64encode(bytes(auth_string, 'utf-8'))
  return auth_string


def TestImapAuthentication(user, auth_string):
  """Authenticates to IMAP with the given auth_string.

  Prints a debug trace of the attempted IMAP connection.

  Args:
    user: The Gmail username (full email address)
    auth_string: A valid OAuth2 string, as returned by GenerateOAuth2String.
        Must not be base64-encoded, since imaplib does its own base64-encoding.
  """
  print()
  imap_conn = imaplib.IMAP4_SSL('imap.gmail.com')
  imap_conn.debug = 4
  imap_conn.authenticate('XOAUTH2', lambda x: auth_string)
  imap_conn.select('INBOX')


def TestSmtpAuthentication(user, auth_string):
  """Authenticates to SMTP with the given auth_string.

  Args:
    user: The Gmail username (full email address)
    auth_string: A valid OAuth2 string, not base64-encoded, as returned by
        GenerateOAuth2String.
  """
  print()
  smtp_conn = smtplib.SMTP('smtp.gmail.com', 587)
  smtp_conn.set_debuglevel(True)
  smtp_conn.ehlo('test')
  smtp_conn.starttls()
  smtp_conn.docmd('AUTH', 'XOAUTH2 ' + base64.b64encode(auth_string))


def RequireOptions(options, *args):
  missing = [arg for arg in args if getattr(options, arg) is None]
  if missing:
    print('Missing options: %s' % ' '.join(missing), file=sys.stderr)
    sys.exit(-1)


def main(argv):
  options_parser = SetupOptionParser()
  (options, args) = options_parser.parse_args()
  if options.refresh_token:
    RequireOptions(options, 'client_id', 'client_secret')
    response = RefreshToken(options.client_id, options.client_secret,
                            options.refresh_token)
    if options.quiet:
      print(response['access_token'])
    else:
      print('Access Token: %s' % response['access_token'])
      print('Access Token Expiration Seconds: %s' % response['expires_in'])
  elif options.generate_oauth2_string:
    RequireOptions(options, 'user', 'access_token')
    oauth2_string = GenerateOAuth2String(options.user, options.access_token)
    if options.quiet:
      print(oauth2_string.decode('utf-8'))
    else:
      print('OAuth2 argument:\n' + oauth2_string.decode('utf-8'))
  elif options.generate_oauth2_token:
    RequireOptions(options, 'client_id', 'client_secret')
    print('To authorize token, visit this url and follow the directions:')
    print('  %s' % GeneratePermissionUrl(options.client_id, options.scope))
    authorization_code = input('Enter verification code: ')
    response = AuthorizeTokens(options.client_id, options.client_secret,
                                authorization_code)
    print('Refresh Token: %s' % response['refresh_token'])
    print('Access Token: %s' % response['access_token'])
    print('Access Token Expiration Seconds: %s' % response['expires_in'])
  elif options.test_imap_authentication:
    RequireOptions(options, 'user', 'access_token')
    TestImapAuthentication(options.user,
        GenerateOAuth2String(options.user, options.access_token,
                             base64_encode=False))
  elif options.test_smtp_authentication:
    RequireOptions(options, 'user', 'access_token')
    TestSmtpAuthentication(options.user,
        GenerateOAuth2String(options.user, options.access_token,
                             base64_encode=False))
  else:
    options_parser.print_help()
    print('Nothing to do, exiting.')
    return


if __name__ == '__main__':
  main(sys.argv)
