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# 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 six
from cryptography import utils
from cryptography.exceptions import AlreadyFinalized, InvalidKey
from cryptography.hazmat.primitives import constant_time, hmac, interfaces
@utils.register_interface(interfaces.KeyDerivationFunction)
class HKDF(object):
def __init__(self, algorithm, length, salt, info, backend):
self._algorithm = algorithm
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 isinstance(salt, six.text_type):
raise TypeError(
"Unicode-objects must be encoded before using them as a salt.")
if salt is None:
salt = b"\x00" * (self._algorithm.digest_size // 8)
self._salt = salt
if isinstance(info, six.text_type):
raise TypeError(
"Unicode-objects must be encoded before using them as info.")
if info is None:
info = b""
self._info = info
self._backend = backend
self._used = False
def _extract(self, key_material):
h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)
h.update(key_material)
return h.finalize()
def _expand(self, key_material):
output = [b""]
counter = 1
while (self._algorithm.digest_size // 8) * len(output) < 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 isinstance(key_material, six.text_type):
raise TypeError(
"Unicode-objects must be encoded before using them as key "
"material."
)
if self._used:
raise AlreadyFinalized
self._used = True
return self._expand(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
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