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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.
from __future__ import annotations
import typing
from cryptography import utils
from cryptography.exceptions import AlreadyFinalized, InvalidKey
from cryptography.hazmat.primitives import constant_time, hashes, hmac
from cryptography.hazmat.primitives.kdf import KeyDerivationFunction
def _int_to_u32be(n: int) -> bytes:
return n.to_bytes(length=4, byteorder="big")
def _common_args_checks(
algorithm: hashes.HashAlgorithm,
length: int,
otherinfo: typing.Optional[bytes],
) -> None:
max_length = algorithm.digest_size * (2**32 - 1)
if length > max_length:
raise ValueError(f"Cannot derive keys larger than {max_length} bits.")
if otherinfo is not None:
utils._check_bytes("otherinfo", otherinfo)
def _concatkdf_derive(
key_material: bytes,
length: int,
auxfn: typing.Callable[[], hashes.HashContext],
otherinfo: bytes,
) -> bytes:
utils._check_byteslike("key_material", key_material)
output = [b""]
outlen = 0
counter = 1
while length > outlen:
h = auxfn()
h.update(_int_to_u32be(counter))
h.update(key_material)
h.update(otherinfo)
output.append(h.finalize())
outlen += len(output[-1])
counter += 1
return b"".join(output)[:length]
class ConcatKDFHash(KeyDerivationFunction):
def __init__(
self,
algorithm: hashes.HashAlgorithm,
length: int,
otherinfo: typing.Optional[bytes],
backend: typing.Any = None,
):
_common_args_checks(algorithm, length, otherinfo)
self._algorithm = algorithm
self._length = length
self._otherinfo: bytes = otherinfo if otherinfo is not None else b""
self._used = False
def _hash(self) -> hashes.Hash:
return hashes.Hash(self._algorithm)
def derive(self, key_material: bytes) -> bytes:
if self._used:
raise AlreadyFinalized
self._used = True
return _concatkdf_derive(
key_material, self._length, self._hash, self._otherinfo
)
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 ConcatKDFHMAC(KeyDerivationFunction):
def __init__(
self,
algorithm: hashes.HashAlgorithm,
length: int,
salt: typing.Optional[bytes],
otherinfo: typing.Optional[bytes],
backend: typing.Any = None,
):
_common_args_checks(algorithm, length, otherinfo)
self._algorithm = algorithm
self._length = length
self._otherinfo: bytes = otherinfo if otherinfo is not None else b""
if algorithm.block_size is None:
raise TypeError(f"{algorithm.name} is unsupported for ConcatKDF")
if salt is None:
salt = b"\x00" * algorithm.block_size
else:
utils._check_bytes("salt", salt)
self._salt = salt
self._used = False
def _hmac(self) -> hmac.HMAC:
return hmac.HMAC(self._salt, self._algorithm)
def derive(self, key_material: bytes) -> bytes:
if self._used:
raise AlreadyFinalized
self._used = True
return _concatkdf_derive(
key_material, self._length, self._hmac, self._otherinfo
)
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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