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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.

from __future__ import absolute_import, division, print_function

import six

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, hmac
from cryptography.hazmat.primitives.kdf import KeyDerivationFunction

@utils.register_interface(KeyDerivationFunction)
class HKDF(object):
    def __init__(self, algorithm, length, salt, info, 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):
        h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)
        h.update(key_material)
        return h.finalize()

def derive(self, key_material):
        utils._check_byteslike("key_material", key_material)
        return self._hkdf_expand.derive(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

@utils.register_interface(KeyDerivationFunction)
class HKDFExpand(object):
    def __init__(self, algorithm, length, info, 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):
        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(six.int2byte(counter))
            output.append(h.finalize())
            counter += 1

return b"".join(output)[: self._length]

def derive(self, key_material):
        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, expected_key):
        if not constant_time.bytes_eq(self.derive(key_material), expected_key):
            raise InvalidKey

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