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# 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.
import typing
from cryptography import utils
from cryptography.exceptions import (
AlreadyFinalized,
InvalidKey,
UnsupportedAlgorithm,
_Reasons,
)
from cryptography.hazmat.backends import _get_backend
from cryptography.hazmat.backends.interfaces import HMACBackend
from cryptography.hazmat.primitives import constant_time, hashes, hmac
from cryptography.hazmat.primitives.kdf import KeyDerivationFunction
class HKDF(KeyDerivationFunction):
def __init__(
self,
algorithm: hashes.HashAlgorithm,
length: int,
salt: typing.Optional[bytes],
info: typing.Optional[bytes],
backend=None,
):
backend = _get_backend(backend)
if not isinstance(backend, HMACBackend):
raise UnsupportedAlgorithm(
"Backend object does not implement HMACBackend.",
_Reasons.BACKEND_MISSING_INTERFACE,
)
self._algorithm = algorithm
if salt is None:
salt = b"\x00" * self._algorithm.digest_size
else:
utils._check_bytes("salt", salt)
self._salt = salt
self._backend = backend
self._hkdf_expand = HKDFExpand(self._algorithm, length, info, backend)
def _extract(self, key_material: bytes) -> bytes:
h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)
h.update(key_material)
return h.finalize()
def derive(self, key_material: bytes) -> bytes:
utils._check_byteslike("key_material", key_material)
return self._hkdf_expand.derive(self._extract(key_material))
def verify(self, key_material: bytes, expected_key: bytes) -> None:
if not constant_time.bytes_eq(self.derive(key_material), expected_key):
raise InvalidKey
class HKDFExpand(KeyDerivationFunction):
def __init__(
self,
algorithm: hashes.HashAlgorithm,
length: int,
info: typing.Optional[bytes],
backend=None,
):
backend = _get_backend(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
if length > max_length:
raise ValueError(
"Can not derive keys larger than {} octets.".format(max_length)
)
self._length = length
if info is None:
info = b""
else:
utils._check_bytes("info", info)
self._info = info
self._used = False
def _expand(self, key_material: bytes) -> bytes:
output = [b""]
counter = 1
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(bytes([counter]))
output.append(h.finalize())
counter += 1
return b"".join(output)[: self._length]
def derive(self, key_material: bytes) -> bytes:
utils._check_byteslike("key_material", key_material)
if self._used:
raise AlreadyFinalized
self._used = True
return self._expand(key_material)
def verify(self, key_material: bytes, expected_key: bytes) -> None:
if not constant_time.bytes_eq(self.derive(key_material), expected_key):
raise InvalidKey
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