text: # This file is dual licensed under the terms of the Apache License, Version
  # 2.0, and the BSD License. See the LICENSE file in the root of this repository
  # for complete details.
  
  from __future__ import absolute_import, division, print_function
  
  from cryptography_patched import utils
  from cryptography_patched.exceptions import (
      AlreadyFinalized, InvalidKey, UnsupportedAlgorithm, _Reasons
  )
  from cryptography_patched.hazmat.backends.interfaces import PBKDF2HMACBackend
  from cryptography_patched.hazmat.primitives import constant_time
  from cryptography_patched.hazmat.primitives.kdf import KeyDerivationFunction
  
  
  @utils.register_interface(KeyDerivationFunction)
  class PBKDF2HMAC(object):
      def __init__(self, algorithm, length, salt, iterations, backend):
          if not isinstance(backend, PBKDF2HMACBackend):
              raise UnsupportedAlgorithm(
                  "Backend object does not implement PBKDF2HMACBackend.",
                  _Reasons.BACKEND_MISSING_INTERFACE
              )
  
          if not backend.pbkdf2_hmac_supported(algorithm):
              raise UnsupportedAlgorithm(
                  "{} is not supported for PBKDF2 by this backend.".format(
                      algorithm.name),
                  _Reasons.UNSUPPORTED_HASH
              )
          self._used = False
          self._algorithm = algorithm
          self._length = length
          utils._check_bytes("salt", salt)
          self._salt = salt
          self._iterations = iterations
          self._backend = backend
  
      def derive(self, key_material):
          if self._used:
              raise AlreadyFinalized("PBKDF2 instances can only be used once.")
          self._used = True
  
          utils._check_byteslike("key_material", key_material)
          return self._backend.derive_pbkdf2_hmac(
              self._algorithm,
              self._length,
              self._salt,
              self._iterations,
              key_material
          )
  
      def verify(self, key_material, expected_key):
          derived_key = self.derive(key_material)
          if not constant_time.bytes_eq(derived_key, expected_key):
              raise InvalidKey("Keys do not match.")
  