# 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 annotations
import typing
from cryptography import utils
from cryptography.exceptions import AlreadyFinalized, InvalidKey
from cryptography.hazmat.primitives import constant_time, hashes
from cryptography.hazmat.primitives.kdf import KeyDerivationFunction
def _int_to_u32be(n: int) -> bytes:
return n.to_bytes(length=4, byteorder="big")
class X963KDF(KeyDerivationFunction):
def __init__(
self,
algorithm: hashes.HashAlgorithm,
length: int,
sharedinfo: bytes | None,
backend: typing.Any = None,
):
max_len = algorithm.digest_size * (2**32 - 1)
if length > max_len:
raise ValueError(f"Cannot derive keys larger than {max_len} bits.")
if sharedinfo is not None:
utils._check_bytes("sharedinfo", sharedinfo)
self._algorithm = algorithm
self._length = length
self._sharedinfo = sharedinfo
self._used = False
def derive(self, key_material: bytes) -> bytes:
if self._used:
raise AlreadyFinalized
self._used = True
utils._check_byteslike("key_material", key_material)
output = [b""]
outlen = 0
counter = 1
while self._length > outlen:
h = hashes.Hash(self._algorithm)
h.update(key_material)
h.update(_int_to_u32be(counter))
if self._sharedinfo is not None:
h.update(self._sharedinfo)
output.append(h.finalize())
outlen += len(output[-1])
counter += 1
return b"".join(output)[: self._length]
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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