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1# This file is dual licensed under the terms of the Apache License, Version
2# 2.0, and the BSD License. See the LICENSE file in the root of this repository
3# for complete details.
4
5from __future__ import absolute_import, division, print_function
6
7import six
8
9from cryptography import utils
10from cryptography.exceptions import (
11    AlreadyFinalized,
12    InvalidKey,
13    UnsupportedAlgorithm,
14    _Reasons,
15)
16from cryptography.hazmat.backends import _get_backend
17from cryptography.hazmat.backends.interfaces import HMACBackend
18from cryptography.hazmat.primitives import constant_time, hmac
19from cryptography.hazmat.primitives.kdf import KeyDerivationFunction
20
21
22@utils.register_interface(KeyDerivationFunction)
23class HKDF(object):
24    def __init__(self, algorithm, length, salt, info, backend=None):
25        backend = _get_backend(backend)
26        if not isinstance(backend, HMACBackend):
27            raise UnsupportedAlgorithm(
28                "Backend object does not implement HMACBackend.",
29                _Reasons.BACKEND_MISSING_INTERFACE,
30            )
31
32        self._algorithm = algorithm
33
34        if salt is None:
35            salt = b"\x00" * self._algorithm.digest_size
36        else:
37            utils._check_bytes("salt", salt)
38
39        self._salt = salt
40
41        self._backend = backend
42
43        self._hkdf_expand = HKDFExpand(self._algorithm, length, info, backend)
44
45    def _extract(self, key_material):
46        h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)
47        h.update(key_material)
48        return h.finalize()
49
50    def derive(self, key_material):
51        utils._check_byteslike("key_material", key_material)
52        return self._hkdf_expand.derive(self._extract(key_material))
53
54    def verify(self, key_material, expected_key):
55        if not constant_time.bytes_eq(self.derive(key_material), expected_key):
56            raise InvalidKey
57
58
59@utils.register_interface(KeyDerivationFunction)
60class HKDFExpand(object):
61    def __init__(self, algorithm, length, info, backend=None):
62        backend = _get_backend(backend)
63        if not isinstance(backend, HMACBackend):
64            raise UnsupportedAlgorithm(
65                "Backend object does not implement HMACBackend.",
66                _Reasons.BACKEND_MISSING_INTERFACE,
67            )
68
69        self._algorithm = algorithm
70
71        self._backend = backend
72
73        max_length = 255 * algorithm.digest_size
74
75        if length > max_length:
76            raise ValueError(
77                "Can not derive keys larger than {} octets.".format(max_length)
78            )
79
80        self._length = length
81
82        if info is None:
83            info = b""
84        else:
85            utils._check_bytes("info", info)
86
87        self._info = info
88
89        self._used = False
90
91    def _expand(self, key_material):
92        output = [b""]
93        counter = 1
94
95        while self._algorithm.digest_size * (len(output) - 1) < self._length:
96            h = hmac.HMAC(key_material, self._algorithm, backend=self._backend)
97            h.update(output[-1])
98            h.update(self._info)
99            h.update(six.int2byte(counter))
100            output.append(h.finalize())
101            counter += 1
102
103        return b"".join(output)[: self._length]
104
105    def derive(self, key_material):
106        utils._check_byteslike("key_material", key_material)
107        if self._used:
108            raise AlreadyFinalized
109
110        self._used = True
111        return self._expand(key_material)
112
113    def verify(self, key_material, expected_key):
114        if not constant_time.bytes_eq(self.derive(key_material), expected_key):
115            raise InvalidKey
116