# -*- coding: utf-8 -*-
#
# 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.
"""Crypto-related routines for oauth2client."""

import imp
import json
import logging
import os
import time

from oauth2client._helpers import _json_encode
from oauth2client._helpers import _urlsafe_b64decode
from oauth2client._helpers import _urlsafe_b64encode


CLOCK_SKEW_SECS = 300  # 5 minutes in seconds
AUTH_TOKEN_LIFETIME_SECS = 300  # 5 minutes in seconds
MAX_TOKEN_LIFETIME_SECS = 86400  # 1 day in seconds


logger = logging.getLogger(__name__)


class AppIdentityError(Exception):
  pass


def _TryOpenSslImport():
  """Import OpenSSL, avoiding the explicit import where possible.

  Importing OpenSSL 0.14 can take up to 0.5s, which is a large price
  to pay at module import time. However, it's also possible for
  ``imp.find_module`` to fail to find the module, even when it's
  installed. (This is the case in various exotic environments,
  including some relevant for Google.) So we first try a fast-path,
  and fall back to the slow import as needed.

  Args:
    None
  Returns:
    None
  Raises:
    ImportError if OpenSSL is unavailable.

  """
  try:
    _, _package_dir, _ = imp.find_module('OpenSSL')
    if not (os.path.isfile(os.path.join(_package_dir, 'crypto.py')) or
            os.path.isfile(os.path.join(_package_dir, 'crypto.so')) or
            os.path.isdir(os.path.join(_package_dir, 'crypto'))):
      raise ImportError('No module named OpenSSL.crypto')
    return
  except ImportError:
    import OpenSSL.crypto


try:
  _TryOpenSslImport()
  from oauth2client._openssl_crypt import OpenSSLVerifier
  from oauth2client._openssl_crypt import OpenSSLSigner
  from oauth2client._openssl_crypt import pkcs12_key_as_pem
except ImportError:
  OpenSSLVerifier = None
  OpenSSLSigner = None
  def pkcs12_key_as_pem(*args, **kwargs):
    raise NotImplementedError('pkcs12_key_as_pem requires OpenSSL.')


try:
  from oauth2client._pycrypto_crypt import PyCryptoVerifier
  from oauth2client._pycrypto_crypt import PyCryptoSigner
except ImportError:
  PyCryptoVerifier = None
  PyCryptoSigner = None


if OpenSSLSigner:
  Signer = OpenSSLSigner
  Verifier = OpenSSLVerifier
elif PyCryptoSigner:
  Signer = PyCryptoSigner
  Verifier = PyCryptoVerifier
else:
  raise ImportError('No encryption library found. Please install either '
                    'PyOpenSSL, or PyCrypto 2.6 or later')


def make_signed_jwt(signer, payload):
  """Make a signed JWT.

  See http://self-issued.info/docs/draft-jones-json-web-token.html.

  Args:
    signer: crypt.Signer, Cryptographic signer.
    payload: dict, Dictionary of data to convert to JSON and then sign.

  Returns:
    string, The JWT for the payload.
  """
  header = {'typ': 'JWT', 'alg': 'RS256'}

  segments = [
      _urlsafe_b64encode(_json_encode(header)),
      _urlsafe_b64encode(_json_encode(payload)),
  ]
  signing_input = '.'.join(segments)

  signature = signer.sign(signing_input)
  segments.append(_urlsafe_b64encode(signature))

  logger.debug(str(segments))

  return '.'.join(segments)


def verify_signed_jwt_with_certs(jwt, certs, audience):
  """Verify a JWT against public certs.

  See http://self-issued.info/docs/draft-jones-json-web-token.html.

  Args:
    jwt: string, A JWT.
    certs: dict, Dictionary where values of public keys in PEM format.
    audience: string, The audience, 'aud', that this JWT should contain. If
      None then the JWT's 'aud' parameter is not verified.

  Returns:
    dict, The deserialized JSON payload in the JWT.

  Raises:
    AppIdentityError if any checks are failed.
  """
  segments = jwt.split('.')

  if len(segments) != 3:
    raise AppIdentityError('Wrong number of segments in token: %s' % jwt)
  signed = '%s.%s' % (segments[0], segments[1])

  signature = _urlsafe_b64decode(segments[2])

  # Parse token.
  json_body = _urlsafe_b64decode(segments[1])
  try:
    parsed = json.loads(json_body.decode('utf-8'))
  except:
    raise AppIdentityError('Can\'t parse token: %s' % json_body)

  # Check signature.
  verified = False
  for pem in certs.values():
    verifier = Verifier.from_string(pem, True)
    if verifier.verify(signed, signature):
      verified = True
      break
  if not verified:
    raise AppIdentityError('Invalid token signature: %s' % jwt)

  # Check creation timestamp.
  iat = parsed.get('iat')
  if iat is None:
    raise AppIdentityError('No iat field in token: %s' % json_body)
  earliest = iat - CLOCK_SKEW_SECS

  # Check expiration timestamp.
  now = int(time.time())
  exp = parsed.get('exp')
  if exp is None:
    raise AppIdentityError('No exp field in token: %s' % json_body)
  if exp >= now + MAX_TOKEN_LIFETIME_SECS:
    raise AppIdentityError('exp field too far in future: %s' % json_body)
  latest = exp + CLOCK_SKEW_SECS

  if now < earliest:
    raise AppIdentityError('Token used too early, %d < %d: %s' %
                           (now, earliest, json_body))
  if now > latest:
    raise AppIdentityError('Token used too late, %d > %d: %s' %
                           (now, latest, json_body))

  # Check audience.
  if audience is not None:
    aud = parsed.get('aud')
    if aud is None:
      raise AppIdentityError('No aud field in token: %s' % json_body)
    if aud != audience:
      raise AppIdentityError('Wrong recipient, %s != %s: %s' %
                             (aud, audience, json_body))

  return parsed