# Copyright 2021 Google LLC
#
# 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.

# [START python-jwt-signature-example]
"""A utility for verifying Json Web Tokens (JWT)."""

import datetime

from absl import app
from absl import flags
from absl import logging
import tink
from tink import jwt


FLAGS = flags.FLAGS

_PUBLIC_KEYSET_PATH = flags.DEFINE_string(
    'public_keyset_path', None, 'Path to public keyset in Tink JSON format.')
_PUBLIC_JWK_SET_PATH = flags.DEFINE_string(
    'public_jwk_set_path', None, 'Path to public keyset in JWK set format.')
_AUDIENCE = flags.DEFINE_string('audience', None,
                                'Audience to be used in the token')
_TOKEN_PATH = flags.DEFINE_string('token_path', None,
                                  'Path to the signature file.')


def main(argv):
  del argv  # Unused.

  # Initialise Tink
  try:
    jwt.register_jwt_signature()
  except tink.TinkError as e:
    logging.exception('Error initialising Tink: %s', e)
    return 1

  # Read the keyset into a KeysetHandle
  if _PUBLIC_KEYSET_PATH.present:
    with open(_PUBLIC_KEYSET_PATH.value, 'rt') as public_keyset_file:
      try:
        text = public_keyset_file.read()
        keyset_handle = tink.read_no_secret_keyset_handle(
            tink.JsonKeysetReader(text))
      except tink.TinkError as e:
        logging.exception('Error reading public keyset: %s', e)
        return 1
  elif _PUBLIC_JWK_SET_PATH.present:
    with open(_PUBLIC_JWK_SET_PATH.value, 'rt') as public_jwk_set_file:
      try:
        text = public_jwk_set_file.read()
        keyset_handle = jwt.jwk_set_to_public_keyset_handle(text)
      except tink.TinkError as e:
        logging.exception('Error reading public JWK set: %s', e)
        return 1
  else:
    logging.info(
        'Either --public_keyset_path or --public_jwk_set_path must be set')

  now = datetime.datetime.now(tz=datetime.timezone.utc)
  try:
    jwt_verify = keyset_handle.primitive(jwt.JwtPublicKeyVerify)
  except tink.TinkError as e:
    logging.exception('Error creating JwtPublicKeyVerify: %s', e)
    return 1

  # Verify token
  with open(_TOKEN_PATH.value, 'rt') as token_file:
    token = token_file.read()
  validator = jwt.new_validator(expected_audience=_AUDIENCE.value)
  try:
    verified_jwt = jwt_verify.verify_and_decode(token, validator)
    expires_in = verified_jwt.expiration() - now
    logging.info('Token is valid and expires in %s seconds', expires_in.seconds)
    return 0
  except tink.TinkError as e:
    logging.info('JWT verification failed: %s', e)
    return 1


if __name__ == '__main__':
  flags.mark_flags_as_required(['audience', 'token_path'])
  app.run(main)

# [END python-jwt-signature-example]
