JezK
Edit File: hkdf.py
# 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, ) 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: typing.Any = None, ): self._algorithm = algorithm if salt is None: salt = b"\x00" * self._algorithm.digest_size else: utils._check_bytes("salt", salt) self._salt = salt self._hkdf_expand = HKDFExpand(self._algorithm, length, info) def _extract(self, key_material: bytes) -> bytes: h = hmac.HMAC(self._salt, self._algorithm) 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: typing.Any = None, ): self._algorithm = algorithm max_length = 255 * algorithm.digest_size if length > max_length: raise ValueError( "Cannot 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) 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