# 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_functionimport sixfrom cryptography import utilsfrom cryptography.exceptions import (    AlreadyFinalized, InvalidKey, UnsupportedAlgorithm, _Reasons)from cryptography.hazmat.backends.interfaces import HMACBackendfrom cryptography.hazmat.primitives import constant_time, hmacfrom cryptography.hazmat.primitives.kdf import KeyDerivationFunction@utils.register_interface(KeyDerivationFunction)class HKDF(object):    def __init__(self, algorithm, length, salt, info, backend):        if not isinstance(backend, HMACBackend):            raise UnsupportedAlgorithm(                "Backend object does not implement HMACBackend.",                _Reasons.BACKEND_MISSING_INTERFACE            )        self._algorithm = algorithm        if not (salt is None or isinstance(salt, bytes)):            raise TypeError("salt must be bytes.")        if salt is None:            salt = b"\x00" * (self._algorithm.digest_size // 8)        self._salt = salt        self._backend = backend        self._hkdf_expand = HKDFExpand(self._algorithm, length, info, backend)    def _extract(self, key_material):        h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)        h.update(key_material)        return h.finalize()    def derive(self, key_material):        if not isinstance(key_material, bytes):            raise TypeError("key_material must be bytes.")        return self._hkdf_expand.derive(self._extract(key_material))    def verify(self, key_material, expected_key):        if not constant_time.bytes_eq(self.derive(key_material), expected_key):            raise InvalidKey@utils.register_interface(KeyDerivationFunction)class HKDFExpand(object):    def __init__(self, algorithm, length, info, backend):        if not isinstance(backend, HMACBackend):            raise UnsupportedAlgorithm(                "Backend object does not implement HMACBackend.",                _Reasons.BACKEND_MISSING_INTERFACE            )        self._algorithm = algorithm        self._backend = backend        max_length = 255 * (algorithm.digest_size // 8)        if length > max_length:            raise ValueError(                "Can not derive keys larger than {0} octets.".format(                    max_length                ))        self._length = length        if not (info is None or isinstance(info, bytes)):            raise TypeError("info must be bytes.")        if info is None:            info = b""        self._info = info        self._used = False    def _expand(self, key_material):        output = [b""]        counter = 1        # BUG: CWE-20 Improper Input Validation        # while (self._algorithm.digest_size // 8) * len(output) < self._length:        # FIXED:         while self._algorithm.digest_size * (len(output) - 1) < self._length:            h = hmac.HMAC(key_material, self._algorithm, backend=self._backend)            h.update(output[-1])            h.update(self._info)            h.update(six.int2byte(counter))            output.append(h.finalize())            counter += 1        return b"".join(output)[:self._length]    def derive(self, key_material):        if not isinstance(key_material, bytes):            raise TypeError("key_material must be bytes.")        if self._used:            raise AlreadyFinalized        self._used = True        return self._expand(key_material)    def verify(self, key_material, expected_key):        if not constant_time.bytes_eq(self.derive(key_material), expected_key):            raise InvalidKey