aboutsummaryrefslogtreecommitdiffstats
path: root/docs/hazmat/primitives/key-derivation-functions.rst
blob: 511708d6d014b226ad383c5540b44b52cce8bb78 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
.. hazmat::

Key derivation functions
========================

.. module:: cryptography.hazmat.primitives.kdf

Key derivation functions derive bytes suitable for cryptographic operations
from passwords or other data sources using a pseudo-random function (PRF).
Different KDFs are suitable for different tasks such as:

* Cryptographic key derivation

    Deriving a key suitable for use as input to an encryption algorithm.
    Typically this means taking a password and running it through an algorithm
    such as :class:`~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC` or
    :class:`~cryptography.hazmat.primitives.kdf.hkdf.HKDF`.
    This process is typically known as `key stretching`_.

* Password storage

    When storing passwords you want to use an algorithm that is computationally
    intensive. Legitimate users will only need to compute it once (for example,
    taking the user's password, running it through the KDF, then comparing it
    to the stored value), while attackers will need to do it billions of times.
    Ideal password storage KDFs will be demanding on both computational and
    memory resources.

.. currentmodule:: cryptography.hazmat.primitives.kdf.pbkdf2

.. class:: PBKDF2HMAC(algorithm, length, salt, iterations, backend)

    .. versionadded:: 0.2

    `PBKDF2`_ (Password Based Key Derivation Function 2) is typically used for
    deriving a cryptographic key from a password. It may also be used for
    key storage, but an alternate key storage KDF such as `scrypt`_ is generally
    considered a better solution.

    This class conforms to the
    :class:`~cryptography.hazmat.primitives.kdf.KeyDerivationFunction`
    interface.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> salt = os.urandom(16)
        >>> # derive
        >>> kdf = PBKDF2HMAC(
        ...     algorithm=hashes.SHA256(),
        ...     length=32,
        ...     salt=salt,
        ...     iterations=100000,
        ...     backend=backend
        ... )
        >>> key = kdf.derive(b"my great password")
        >>> # verify
        >>> kdf = PBKDF2HMAC(
        ...     algorithm=hashes.SHA256(),
        ...     length=32,
        ...     salt=salt,
        ...     iterations=100000,
        ...     backend=backend
        ... )
        >>> kdf.verify(b"my great password", key)

    :param algorithm: An instance of
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`.
    :param int length: The desired length of the derived key. Maximum is
        (2\ :sup:`32` - 1) * ``algorithm.digest_size``.
    :param bytes salt: A salt. `NIST SP 800-132`_ recommends 128-bits or
        longer.
    :param int iterations: The number of iterations to perform of the hash
        function. This can be used to control the length of time the operation
        takes. Higher numbers help mitigate brute force attacks against derived
        keys. See OWASP's `Password Storage Cheat Sheet`_ for more
        detailed recommendations if you intend to use this for password storage.
    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
        provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`

    :raises TypeError: This exception is raised if ``salt`` is not ``bytes``.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material. For PBKDF2 this
            should be a password.
        :return bytes: the derived key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        :raises TypeError: This exception is raised if ``key_material`` is not
                           ``bytes``.

        This generates and returns a new key from the supplied password.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match. This can be used for
        checking whether the password a user provides matches the stored derived
        key.


.. currentmodule:: cryptography.hazmat.primitives.kdf.hkdf

.. class:: HKDF(algorithm, length, salt, info, backend)

    .. versionadded:: 0.2

    `HKDF`_ (HMAC-based Extract-and-Expand Key Derivation Function) is suitable
    for deriving keys of a fixed size used for other cryptographic operations.

    .. warning::

        HKDF should not be used for password storage.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> salt = os.urandom(16)
        >>> info = b"hkdf-example"
        >>> hkdf = HKDF(
        ...     algorithm=hashes.SHA256(),
        ...     length=32,
        ...     salt=salt,
        ...     info=info,
        ...     backend=backend
        ... )
        >>> key = hkdf.derive(b"input key")
        >>> hkdf = HKDF(
        ...     algorithm=hashes.SHA256(),
        ...     length=32,
        ...     salt=salt,
        ...     info=info,
        ...     backend=backend
        ... )
        >>> hkdf.verify(b"input key", key)

    :param algorithm: An instance of
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`.

    :param int length: The desired length of the derived key. Maximum is
        ``255 * (algorithm.digest_size // 8)``.

    :param bytes salt: A salt. Randomizes the KDF's output. Optional, but
        highly recommended. Ideally as many bits of entropy as the security
        level of the hash: often that means cryptographically random and as
        long as the hash output. Worse (shorter, less entropy) salt values can
        still meaningfully contribute to security. May be reused. Does not have
        to be secret, but may cause stronger security guarantees if secret; see
        :rfc:`5869` and the `HKDF paper`_ for more details. If ``None`` is
        explicitly passed a default salt of ``algorithm.digest_size // 8`` null
        bytes will be used.

    :param bytes info: Application specific context information.  If ``None``
        is explicitly passed an empty byte string will be used.

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
        provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`

    :raises TypeError: This exception is raised if ``salt`` or ``info`` is not
                       ``bytes``.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material.
        :return bytes: The derived key.
        :raises TypeError: This exception is raised if ``key_material`` is not
                           ``bytes``.

        Derives a new key from the input key material by performing both the
        extract and expand operations.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match.


.. class:: HKDFExpand(algorithm, length, info, backend)

    .. versionadded:: 0.5

    HKDF consists of two stages, extract and expand. This class exposes an
    expand only version of HKDF that is suitable when the key material is
    already cryptographically strong.

    .. warning::

        HKDFExpand should only be used if the key material is
        cryptographically strong. You should use
        :class:`~cryptography.hazmat.primitives.kdf.hkdf.HKDF` if
        you are unsure.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.hkdf import HKDFExpand
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> info = b"hkdf-example"
        >>> key_material = os.urandom(16)
        >>> hkdf = HKDFExpand(
        ...     algorithm=hashes.SHA256(),
        ...     length=32,
        ...     info=info,
        ...     backend=backend
        ... )
        >>> key = hkdf.derive(key_material)
        >>> hkdf = HKDFExpand(
        ...     algorithm=hashes.SHA256(),
        ...     length=32,
        ...     info=info,
        ...     backend=backend
        ... )
        >>> hkdf.verify(key_material, key)

    :param algorithm: An instance of
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`.

    :param int length: The desired length of the derived key. Maximum is
        ``255 * (algorithm.digest_size // 8)``.

    :param bytes info: Application specific context information.  If ``None``
        is explicitly passed an empty byte string will be used.

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
        provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
    :raises TypeError: This is raised if the provided ``info`` is a unicode object
    :raises TypeError: This exception is raised if ``info`` is not ``bytes``.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material.
        :return bytes: The derived key.

        :raises TypeError: This is raised if the provided ``key_material`` is
            a unicode object
        :raises TypeError: This exception is raised if ``key_material`` is not
                           ``bytes``.

        Derives a new key from the input key material by performing both the
        extract and expand operations.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.
        :raises TypeError: This is raised if the provided ``key_material`` is
            a unicode object

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match.

.. currentmodule:: cryptography.hazmat.primitives.kdf.concatkdf

.. class:: ConcatKDFHash(algorithm, length, otherinfo, backend)

    .. versionadded:: 1.0

    ConcatKDFHash (Concatenation Key Derivation Function) is defined by the
    NIST Special Publication `NIST SP 800-56Ar2`_ document, to be used to
    derive keys for use after a Key Exchange negotiation operation.

    .. warning::

        ConcatKDFHash should not be used for password storage.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.concatkdf import ConcatKDFHash
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> otherinfo = b"concatkdf-example"
        >>> ckdf = ConcatKDFHash(
        ...     algorithm=hashes.SHA256(),
        ...     length=256,
        ...     otherinfo=otherinfo,
        ...     backend=backend
        ... )
        >>> key = ckdf.derive(b"input key")
        >>> ckdf = ConcatKDFHash(
        ...     algorithm=hashes.SHA256(),
        ...     length=256,
        ...     otherinfo=otherinfo,
        ...     backend=backend
        ... )
        >>> ckdf.verify(b"input key", key)

    :param algorithm: An instance of
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`.

    :param int length: The desired length of the derived key in bytes.
        Maximum is ``hashlen * (2^32 -1)``.

    :param bytes otherinfo: Application specific context information.
        If ``None`` is explicitly passed an empty byte string will be used.

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.HashBackend`.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised
        if the provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.HashBackend`

    :raises TypeError: This exception is raised if ``otherinfo`` is not
        ``bytes``.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material.
        :return bytes: The derived key.
        :raises TypeError: This exception is raised if ``key_material`` is
                            not ``bytes``.

        Derives a new key from the input key material.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match.


.. class:: ConcatKDFHMAC(algorithm, length, salt, otherinfo, backend)

    .. versionadded:: 1.0

    Similar to ConcatKFDHash but uses an HMAC function instead.

    .. warning::

        ConcatKDFHMAC should not be used for password storage.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.concatkdf import ConcatKDFHMAC
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> salt = os.urandom(16)
        >>> otherinfo = b"concatkdf-example"
        >>> ckdf = ConcatKDFHMAC(
        ...     algorithm=hashes.SHA256(),
        ...     length=256,
        ...     salt=salt,
        ...     otherinfo=otherinfo,
        ...     backend=backend
        ... )
        >>> key = ckdf.derive(b"input key")
        >>> ckdf = ConcatKDFHMAC(
        ...     algorithm=hashes.SHA256(),
        ...     length=256,
        ...     salt=salt,
        ...     otherinfo=otherinfo,
        ...     backend=backend
        ... )
        >>> ckdf.verify(b"input key", key)

    :param algorithm: An instance of
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`.

    :param int length: The desired length of the derived key in bytes. Maximum
        is ``hashlen * (2^32 -1)``.

    :param bytes salt: A salt. Randomizes the KDF's output. Optional, but
        highly recommended. Ideally as many bits of entropy as the security
        level of the hash: often that means cryptographically random and as
        long as the hash output. Does not have to be secret, but may cause
        stronger security guarantees if secret; If ``None`` is explicitly
        passed a default salt of ``algorithm.block_size`` null bytes will be
        used.

    :param bytes otherinfo: Application specific context information.
        If ``None`` is explicitly passed an empty byte string will be used.

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
        provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`

    :raises TypeError: This exception is raised if ``salt`` or ``otherinfo``
        is not ``bytes``.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material.
        :return bytes: The derived key.
        :raises TypeError: This exception is raised if ``key_material`` is not
                           ``bytes``.

        Derives a new key from the input key material.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match.

.. currentmodule:: cryptography.hazmat.primitives.kdf.x963kdf

.. class:: X963KDF(algorithm, length, otherinfo, backend)

    .. versionadded:: 1.1

    X963KDF (ANSI X9.63 Key Derivation Function) is defined by ANSI
    in the `ANSI X9.63:2001`_ document, to be used to derive keys for use
    after a Key Exchange negotiation operation.

    SECG in `SEC 1 v2.0`_ recommends that
    :class:`~cryptography.hazmat.primitives.kdf.concatkdf.ConcatKDFHash` be
    used for new projects. This KDF should only be used for backwards
    compatibility with pre-existing protocols.


    .. warning::

        X963KDF should not be used for password storage.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.x963kdf import X963KDF
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> sharedinfo = b"ANSI X9.63 Example"
        >>> xkdf = X963KDF(
        ...     algorithm=hashes.SHA256(),
        ...     length=256,
        ...     sharedinfo=sharedinfo,
        ...     backend=backend
        ... )
        >>> key = xkdf.derive(b"input key")
        >>> xkdf = X963KDF(
        ...     algorithm=hashes.SHA256(),
        ...     length=256,
        ...     sharedinfo=sharedinfo,
        ...     backend=backend
        ... )
        >>> xkdf.verify(b"input key", key)

    :param algorithm: An instance of
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`.

    :param int length: The desired length of the derived key in bytes.
        Maximum is ``hashlen * (2^32 -1)``.

    :param bytes sharedinfo: Application specific context information.
        If ``None`` is explicitly passed an empty byte string will be used.

    :param backend: A cryptography backend
        :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
        instance.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised
        if the provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.HashBackend`

    :raises TypeError: This exception is raised if ``sharedinfo`` is not
        ``bytes``.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material.
        :return bytes: The derived key.
        :raises TypeError: This exception is raised if ``key_material`` is
                            not ``bytes``.

        Derives a new key from the input key material.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match.


.. currentmodule:: cryptography.hazmat.primitives.kdf.kbkdf

.. class:: KBKDFHMAC(algorithm, mode, length, rlen, llen, location,\
           label, context, fixed, backend)

    .. versionadded:: 1.4

    KBKDF (Key Based Key Derivation Function) is defined by the
    `NIST SP 800-108`_ document, to be used to derive additional
    keys from a key that has been established through an automated
    key-establishment scheme.

    .. warning::

        KBKDFHMAC should not be used for password storage.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.kbkdf import (
        ...    CounterLocation, KBKDFHMAC, Mode
        ... )
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> label = b"KBKDF HMAC Label"
        >>> context = b"KBKDF HMAC Context"
        >>> kdf = KBKDFHMAC(
        ...     algorithm=hashes.SHA256(),
        ...     mode=Mode.CounterMode,
        ...     length=256,
        ...     rlen=4,
        ...     llen=4,
        ...     location=CounterLocation.BeforeFixed,
        ...     label=label,
        ...     context=context,
        ...     fixed=None,
        ...     backend=backend
        ... )
        >>> key = kdf.derive(b"input key")
        >>> kdf = KBKDFHMAC(
        ...     algorithm=hashes.SHA256(),
        ...     mode=Mode.CounterMode,
        ...     length=256,
        ...     rlen=4,
        ...     llen=4,
        ...     location=CounterLocation.BeforeFixed,
        ...     label=label,
        ...     context=context,
        ...     fixed=None,
        ...     backend=backend
        ... )
        >>> kdf.verify(b"input key", key)

    :param algorithm: An instance of
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`.

    :param mode: The desired mode of the PRF. A value from the
      :class:`~cryptography.hazmat.primitives.kdf.kbkdf.Mode` enum.

    :param int length: The desired length of the derived key in bytes.

    :param int rlen: An integer that indicates the length of the binary
        representation of the counter in bytes.

    :param int llen: An integer that indicates the binary
        representation of the ``length`` in bytes.

    :param location: The desired location of the counter. A value from the
      :class:`~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation` enum.

    :param bytes label: Application specific label information. If ``None``
        is explicitly passed an empty byte string will be used.

    :param bytes context: Application specific context information. If ``None``
        is explicitly passed an empty byte string will be used.

    :param bytes fixed: Instead of specifying ``label`` and ``context`` you
        may supply your own fixed data. If ``fixed`` is specified, ``label``
        and ``context`` is ignored.

    :param backend: A cryptography backend
        :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
        instance.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised
        if the provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.HashBackend`

    :raises TypeError: This exception is raised if ``label`` or ``context``
        is not ``bytes``. Also raised if ``rlen`` or ``llen`` is not ``int``.

    :raises ValueError: This exception is raised if ``rlen`` or ``llen``
        is greater than 4 or less than 1. This exception is also raised if
        you specify a ``label`` or ``context`` and ``fixed``.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material.
        :return bytes: The derived key.
        :raises TypeError: This exception is raised if ``key_material`` is
                            not ``bytes``.

        Derives a new key from the input key material.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match.

.. class:: Mode

    An enumeration for the key based key derivative modes.

    .. attribute:: CounterMode

        The output of the PRF is computed with a counter
        as the iteration variable.

.. class:: CounterLocation

    An enumeration for the key based key derivative counter location.

    .. attribute:: BeforeFixed

        The counter iteration variable will be concatenated before
        the fixed input data.

    .. attribute:: AfterFixed

        The counter iteration variable will be concatenated after
        the fixed input data.

.. currentmodule:: cryptography.hazmat.primitives.kdf.scrypt

.. class:: Scrypt(salt, length, n, r, p, backend)

    .. versionadded:: 1.6

    Scrypt is a KDF designed for password storage by Colin Percival to be
    resistant against hardware-assisted attackers by having a tunable memory
    cost. It is described in :rfc:`7914`.

    This class conforms to the
    :class:`~cryptography.hazmat.primitives.kdf.KeyDerivationFunction`
    interface.

    .. code-block:: python

        >>> import os
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.kdf.scrypt import Scrypt
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> salt = os.urandom(16)
        >>> # derive
        >>> kdf = Scrypt(
        ...     salt=salt,
        ...     length=64,
        ...     n=1024,
        ...     r=8,
        ...     p=16,
        ...     backend=backend
        ... )
        >>> key = kdf.derive(b"my great password")
        >>> # verify
        >>> kdf = Scrypt(
        ...     salt=salt,
        ...     length=64,
        ...     n=1024,
        ...     r=8,
        ...     p=16,
        ...     backend=backend
        ... )
        >>> kdf.verify(b"my great password", key)

    :param bytes salt: A salt.
    :param int length: The desired length of the derived key.
    :param int n: CPU/Memory cost parameter. It must be larger than 1 and be a
        power of 2.
    :param int r: Block size parameter.
    :param int p: Parallelization parameter.

    The computational and memory cost of Scrypt can be adjusted by manipulating
    the 3 parameters: n, r and p. In general, the memory cost of Scrypt is
    affected by the values of both n and r while n also determines the number
    of iterations performed. p increases the computational cost without
    affecting memory usage. A more in-depth explanation of the 3 parameters can
    be found `here`_.

    :rfc:`7914` `recommends`_ values of r=8 and p=1 while scaling n to the
    number appropriate for your system.

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.ScryptBackend`.

    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
        provided ``backend`` does not implement
        :class:`~cryptography.hazmat.backends.interfaces.ScryptBackend`

    :raises TypeError: This exception is raised if ``salt`` is not ``bytes``.
    :raises ValueError: This exception is raised if ``n`` is less than 2, if
        ``n`` is not a power of 2, if ``r`` is less than 1 or if ``p`` is less
        than 1.

    .. method:: derive(key_material)

        :param bytes key_material: The input key material.
        :return bytes: the derived key.
        :raises TypeError: This exception is raised if ``key_material`` is not
                           ``bytes``.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This generates and returns a new key from the supplied password.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match. This can be used for
        checking whether the password a user provides matches the stored derived
        key.

Interface
~~~~~~~~~

.. currentmodule:: cryptography.hazmat.primitives.kdf

.. class:: KeyDerivationFunction

    .. versionadded:: 0.2

    .. method:: derive(key_material)

        :param bytes key_material: The input key material. Depending on what
                                   key derivation function you are using this
                                   could be either random bytes, or a user
                                   supplied password.
        :return: The new key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This generates and returns a new key from the supplied key material.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match. This can be used for
        something like checking whether a user's password attempt matches the
        stored derived key.


.. _`NIST SP 800-132`: http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf
.. _`NIST SP 800-108`: http://csrc.nist.gov/publications/nistpubs/800-108/sp800-108.pdf
.. _`NIST SP 800-56Ar2`: http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf
.. _`ANSI X9.63:2001`: https://webstore.ansi.org
.. _`SEC 1 v2.0`: http://www.secg.org/sec1-v2.pdf
.. _`Password Storage Cheat Sheet`: https://www.owasp.org/index.php/Password_Storage_Cheat_Sheet
.. _`PBKDF2`: https://en.wikipedia.org/wiki/PBKDF2
.. _`scrypt`: https://en.wikipedia.org/wiki/Scrypt
.. _`key stretching`: https://en.wikipedia.org/wiki/Key_stretching
.. _`HKDF`: https://en.wikipedia.org/wiki/HKDF
.. _`HKDF paper`: https://eprint.iacr.org/2010/264
.. _`here`: https://stackoverflow.com/a/30308723/1170681
.. _`recommends`: https://tools.ietf.org/html/rfc7914#section-2