old_contents: # 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.
  
  from __future__ import absolute_import, division, print_function
  
  import struct
  
  import six
  
  from cryptography.exceptions import InvalidToken, UnsupportedAlgorithm
  from cryptography.hazmat.primitives import constant_time, hmac
  from cryptography.hazmat.primitives.hashes import SHA1, SHA256, SHA512
  
  
  class HOTP(object):
      def __init__(self, key, length, algorithm, backend):
          if len(key) < 16:
              raise ValueError("Key length has to be at least 128 bits.")
  
          if length < 6 or length > 8:
              raise ValueError("Length of HOTP has to be between 6 to 8.")
  
          if not isinstance(algorithm, (SHA1, SHA256, SHA512)):
              raise UnsupportedAlgorithm(
                  "Algorithm must be SHA1, SHA256 or SHA512")
  
          self._key = key
          self._length = length
          self._algorithm = algorithm
          self._backend = backend
  
      def generate(self, counter):
          truncated_value = self._dynamic_truncate(counter)
          hotp = truncated_value % (10 ** self._length)
          return "{0:0{1}}".format(hotp, self._length).encode()
  
      def verify(self, hotp, counter):
          if not constant_time.bytes_eq(self.generate(counter), hotp):
              raise InvalidToken("Supplied HOTP value does not match")
  
      def _dynamic_truncate(self, counter):
          ctx = hmac.HMAC(self._key, self._algorithm, self._backend)
          ctx.update(struct.pack(">Q", counter))
          hmac_value = ctx.finalize()
  
          offset_bits = six.indexbytes(hmac_value, len(hmac_value) - 1) & 0b1111
  
          offset = int(offset_bits)
          p = hmac_value[offset:offset + 4]
          return struct.unpack(">I", p)[0] & 0x7fffffff
  
new_contents: # 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.
  
  from __future__ import absolute_import, division, print_function
  
  import struct
  
  import six
  
  from cryptography.exceptions import InvalidToken
  from cryptography.hazmat.primitives import constant_time, hmac
  from cryptography.hazmat.primitives.hashes import SHA1, SHA256, SHA512
  
  
  class HOTP(object):
      def __init__(self, key, length, algorithm, backend):
          if len(key) < 16:
              raise ValueError("Key length has to be at least 128 bits.")
  
          if length < 6 or length > 8:
              raise ValueError("Length of HOTP has to be between 6 to 8.")
  
          if not isinstance(algorithm, (SHA1, SHA256, SHA512)):
              raise ValueError("Algorithm must be SHA1, SHA256 or SHA512")
  
          self._key = key
          self._length = length
          self._algorithm = algorithm
          self._backend = backend
  
      def generate(self, counter):
          truncated_value = self._dynamic_truncate(counter)
          hotp = truncated_value % (10 ** self._length)
          return "{0:0{1}}".format(hotp, self._length).encode()
  
      def verify(self, hotp, counter):
          if not constant_time.bytes_eq(self.generate(counter), hotp):
              raise InvalidToken("Supplied HOTP value does not match")
  
      def _dynamic_truncate(self, counter):
          ctx = hmac.HMAC(self._key, self._algorithm, self._backend)
          ctx.update(struct.pack(">Q", counter))
          hmac_value = ctx.finalize()
  
          offset_bits = six.indexbytes(hmac_value, len(hmac_value) - 1) & 0b1111
  
          offset = int(offset_bits)
          p = hmac_value[offset:offset + 4]
          return struct.unpack(">I", p)[0] & 0x7fffffff
  
subject: Change exception in use