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Doc/library/hashlib.rst
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Serhiy Storchaka
[3.11] gh-115664: Fix ordering of more versionadded and versionchanged directives (GH-116298) (GH-116450) (GH-116452)
07 мар 2024, 11:35
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:mod:`hashlib` --- Secure hashes and message digests
====================================================

.. module:: hashlib
   :synopsis: Secure hash and message digest algorithms.

.. moduleauthor:: Gregory P. Smith <greg@krypto.org>
.. sectionauthor:: Gregory P. Smith <greg@krypto.org>

**Source code:** :source:`Lib/hashlib.py`

.. index::
   single: message digest, MD5
   single: secure hash algorithm, SHA1, SHA2, SHA224, SHA256, SHA384, SHA512, SHA3, Shake, Blake2

.. testsetup::

   import hashlib


--------------

This module implements a common interface to many different secure hash and
message digest algorithms.  Included are the FIPS secure hash algorithms SHA1,
SHA224, SHA256, SHA384, SHA512, (defined in `the FIPS 180-4 standard`_),
the SHA-3 series (defined in `the FIPS 202 standard`_) as well as RSA's MD5
algorithm (defined in internet :rfc:`1321`).  The terms "secure hash" and
"message digest" are interchangeable.  Older algorithms were called message
digests.  The modern term is secure hash.

.. note::

   If you want the adler32 or crc32 hash functions, they are available in
   the :mod:`zlib` module.


.. _hash-algorithms:

Hash algorithms
---------------

There is one constructor method named for each type of :dfn:`hash`.  All return
a hash object with the same simple interface. For example: use :func:`sha256`
to create a SHA-256 hash object. You can now feed this object with
:term:`bytes-like objects <bytes-like object>` (normally :class:`bytes`) using
the :meth:`update<hash.update>` method.  At any point you can ask it for the
:dfn:`digest` of the concatenation of the data fed to it so far using the
:meth:`digest()<hash.digest>` or :meth:`hexdigest()<hash.hexdigest>` methods.

To allow multithreading, the Python :term:`GIL` is released while computing a
hash supplied more than 2047 bytes of data at once in its constructor or
:meth:`.update<hash.update>` method.


.. index:: single: OpenSSL; (use in module hashlib)

Constructors for hash algorithms that are always present in this module are
:func:`sha1`, :func:`sha224`, :func:`sha256`, :func:`sha384`, :func:`sha512`,
:func:`sha3_224`, :func:`sha3_256`, :func:`sha3_384`, :func:`sha3_512`,
:func:`shake_128`, :func:`shake_256`, :func:`blake2b`, and :func:`blake2s`.
:func:`md5` is normally available as well, though it may be missing or blocked
if you are using a rare "FIPS compliant" build of Python.
These correspond to :data:`algorithms_guaranteed`.

Additional algorithms may also be available if your Python distribution's
:mod:`hashlib` was linked against a build of OpenSSL that provides others.
Others *are not guaranteed available* on all installations and will only be
accessible by name via :func:`new`.  See :data:`algorithms_available`.

.. warning::

   Some algorithms have known hash collision weaknesses (including MD5 and
   SHA1). Refer to `Attacks on cryptographic hash algorithms`_ and the
   `hashlib-seealso`_ section at the end of this document.

.. versionadded:: 3.6
   SHA3 (Keccak) and SHAKE constructors :func:`sha3_224`, :func:`sha3_256`,
   :func:`sha3_384`, :func:`sha3_512`, :func:`shake_128`, :func:`shake_256`
   were added.
   :func:`blake2b` and :func:`blake2s` were added.

.. _hashlib-usedforsecurity:

.. versionchanged:: 3.9
   All hashlib constructors take a keyword-only argument *usedforsecurity*
   with default value ``True``. A false value allows the use of insecure and
   blocked hashing algorithms in restricted environments. ``False`` indicates
   that the hashing algorithm is not used in a security context, e.g. as a
   non-cryptographic one-way compression function.

.. versionchanged:: 3.9
   Hashlib now uses SHA3 and SHAKE from OpenSSL if it provides it.

Usage
-----

To obtain the digest of the byte string ``b"Nobody inspects the spammish
repetition"``::

   >>> import hashlib
   >>> m = hashlib.sha256()
   >>> m.update(b"Nobody inspects")
   >>> m.update(b" the spammish repetition")
   >>> m.digest()
   b'\x03\x1e\xdd}Ae\x15\x93\xc5\xfe\\\x00o\xa5u+7\xfd\xdf\xf7\xbcN\x84:\xa6\xaf\x0c\x95\x0fK\x94\x06'
   >>> m.hexdigest()
   '031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'

More condensed:

   >>> hashlib.sha256(b"Nobody inspects the spammish repetition").hexdigest()
   '031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'

Constructors
------------

.. function:: new(name[, data], *, usedforsecurity=True)

   Is a generic constructor that takes the string *name* of the desired
   algorithm as its first parameter.  It also exists to allow access to the
   above listed hashes as well as any other algorithms that your OpenSSL
   library may offer.

Using :func:`new` with an algorithm name:

   >>> h = hashlib.new('sha256')
   >>> h.update(b"Nobody inspects the spammish repetition")
   >>> h.hexdigest()
   '031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'


.. function:: md5([, data], *, usedforsecurity=True)
.. function:: sha1([, data], *, usedforsecurity=True)
.. function:: sha224([, data], *, usedforsecurity=True)
.. function:: sha256([, data], *, usedforsecurity=True)
.. function:: sha384([, data], *, usedforsecurity=True)
.. function:: sha512([, data], *, usedforsecurity=True)
.. function:: sha3_224([, data], *, usedforsecurity=True)
.. function:: sha3_256([, data], *, usedforsecurity=True)
.. function:: sha3_384([, data], *, usedforsecurity=True)
.. function:: sha3_512([, data], *, usedforsecurity=True)

Named constructors such as these are faster than passing an algorithm name to
:func:`new`.

Attributes
----------

Hashlib provides the following constant module attributes:

.. data:: algorithms_guaranteed

   A set containing the names of the hash algorithms guaranteed to be supported
   by this module on all platforms.  Note that 'md5' is in this list despite
   some upstream vendors offering an odd "FIPS compliant" Python build that
   excludes it.

   .. versionadded:: 3.2

.. data:: algorithms_available

   A set containing the names of the hash algorithms that are available in the
   running Python interpreter.  These names will be recognized when passed to
   :func:`new`.  :attr:`algorithms_guaranteed` will always be a subset.  The
   same algorithm may appear multiple times in this set under different names
   (thanks to OpenSSL).

   .. versionadded:: 3.2

Hash Objects
------------

The following values are provided as constant attributes of the hash objects
returned by the constructors:

.. data:: hash.digest_size

   The size of the resulting hash in bytes.

.. data:: hash.block_size

   The internal block size of the hash algorithm in bytes.

A hash object has the following attributes:

.. attribute:: hash.name

   The canonical name of this hash, always lowercase and always suitable as a
   parameter to :func:`new` to create another hash of this type.

   .. versionchanged:: 3.4
      The name attribute has been present in CPython since its inception, but
      until Python 3.4 was not formally specified, so may not exist on some
      platforms.

A hash object has the following methods:


.. method:: hash.update(data)

   Update the hash object with the :term:`bytes-like object`.
   Repeated calls are equivalent to a single call with the
   concatenation of all the arguments: ``m.update(a); m.update(b)`` is
   equivalent to ``m.update(a+b)``.

   .. versionchanged:: 3.1
      The Python GIL is released to allow other threads to run while hash
      updates on data larger than 2047 bytes is taking place when using hash
      algorithms supplied by OpenSSL.


.. method:: hash.digest()

   Return the digest of the data passed to the :meth:`update` method so far.
   This is a bytes object of size :attr:`digest_size` which may contain bytes in
   the whole range from 0 to 255.


.. method:: hash.hexdigest()

   Like :meth:`digest` except the digest is returned as a string object of
   double length, containing only hexadecimal digits.  This may be used to
   exchange the value safely in email or other non-binary environments.


.. method:: hash.copy()

   Return a copy ("clone") of the hash object.  This can be used to efficiently
   compute the digests of data sharing a common initial substring.


SHAKE variable length digests
-----------------------------

.. function:: shake_128([, data], *, usedforsecurity=True)
.. function:: shake_256([, data], *, usedforsecurity=True)

The :func:`shake_128` and :func:`shake_256` algorithms provide variable
length digests with length_in_bits//2 up to 128 or 256 bits of security.
As such, their digest methods require a length. Maximum length is not limited
by the SHAKE algorithm.

.. method:: shake.digest(length)

   Return the digest of the data passed to the :meth:`~hash.update` method so far.
   This is a bytes object of size *length* which may contain bytes in
   the whole range from 0 to 255.


.. method:: shake.hexdigest(length)

   Like :meth:`digest` except the digest is returned as a string object of
   double length, containing only hexadecimal digits.  This may be used to
   exchange the value in email or other non-binary environments.

Example use:

   >>> h = hashlib.shake_256(b'Nobody inspects the spammish repetition')
   >>> h.hexdigest(20)
   '44709d6fcb83d92a76dcb0b668c98e1b1d3dafe7'

File hashing
------------

The hashlib module provides a helper function for efficient hashing of
a file or file-like object.

.. function:: file_digest(fileobj, digest, /)

   Return a digest object that has been updated with contents of file object.

   *fileobj* must be a file-like object opened for reading in binary mode.
   It accepts file objects from  builtin :func:`open`, :class:`~io.BytesIO`
   instances, SocketIO objects from :meth:`socket.socket.makefile`, and
   similar. The function may bypass Python's I/O and use the file descriptor
   from :meth:`~io.IOBase.fileno` directly. *fileobj* must be assumed to be
   in an unknown state after this function returns or raises. It is up to
   the caller to close *fileobj*.

   *digest* must either be a hash algorithm name as a *str*, a hash
   constructor, or a callable that returns a hash object.

   Example:

      >>> import io, hashlib, hmac
      >>> with open(hashlib.__file__, "rb") as f:
      ...     digest = hashlib.file_digest(f, "sha256")
      ...
      >>> digest.hexdigest()  # doctest: +ELLIPSIS
      '...'

      >>> buf = io.BytesIO(b"somedata")
      >>> mac1 = hmac.HMAC(b"key", digestmod=hashlib.sha512)
      >>> digest = hashlib.file_digest(buf, lambda: mac1)

      >>> digest is mac1
      True
      >>> mac2 = hmac.HMAC(b"key", b"somedata", digestmod=hashlib.sha512)
      >>> mac1.digest() == mac2.digest()
      True

   .. versionadded:: 3.11


Key derivation
--------------

Key derivation and key stretching algorithms are designed for secure password
hashing. Naive algorithms such as ``sha1(password)`` are not resistant against
brute-force attacks. A good password hashing function must be tunable, slow, and
include a `salt <https://en.wikipedia.org/wiki/Salt_%28cryptography%29>`_.


.. function:: pbkdf2_hmac(hash_name, password, salt, iterations, dklen=None)

   The function provides PKCS#5 password-based key derivation function 2. It
   uses HMAC as pseudorandom function.

   The string *hash_name* is the desired name of the hash digest algorithm for
   HMAC, e.g. 'sha1' or 'sha256'. *password* and *salt* are interpreted as
   buffers of bytes. Applications and libraries should limit *password* to
   a sensible length (e.g. 1024). *salt* should be about 16 or more bytes from
   a proper source, e.g. :func:`os.urandom`.

   The number of *iterations* should be chosen based on the hash algorithm and
   computing power. As of 2022, hundreds of thousands of iterations of SHA-256
   are suggested. For rationale as to why and how to choose what is best for
   your application, read *Appendix A.2.2* of NIST-SP-800-132_. The answers
   on the `stackexchange pbkdf2 iterations question`_ explain in detail.

   *dklen* is the length of the derived key. If *dklen* is ``None`` then the
   digest size of the hash algorithm *hash_name* is used, e.g. 64 for SHA-512.

   >>> from hashlib import pbkdf2_hmac
   >>> our_app_iters = 500_000  # Application specific, read above.
   >>> dk = pbkdf2_hmac('sha256', b'password', b'bad salt'*2, our_app_iters)
   >>> dk.hex()
   '15530bba69924174860db778f2c6f8104d3aaf9d26241840c8c4a641c8d000a9'

   .. versionadded:: 3.4

   .. note::

      A fast implementation of *pbkdf2_hmac* is available with OpenSSL.  The
      Python implementation uses an inline version of :mod:`hmac`. It is about
      three times slower and doesn't release the GIL.

   .. deprecated:: 3.10

      Slow Python implementation of *pbkdf2_hmac* is deprecated. In the
      future the function will only be available when Python is compiled
      with OpenSSL.

.. function:: scrypt(password, *, salt, n, r, p, maxmem=0, dklen=64)

   The function provides scrypt password-based key derivation function as
   defined in :rfc:`7914`.

   *password* and *salt* must be :term:`bytes-like objects
   <bytes-like object>`.  Applications and libraries should limit *password*
   to a sensible length (e.g. 1024).  *salt* should be about 16 or more
   bytes from a proper source, e.g. :func:`os.urandom`.

   *n* is the CPU/Memory cost factor, *r* the block size, *p* parallelization
   factor and *maxmem* limits memory (OpenSSL 1.1.0 defaults to 32 MiB).
   *dklen* is the length of the derived key.

   .. versionadded:: 3.6


BLAKE2
------

.. sectionauthor:: Dmitry Chestnykh

.. index::
   single: blake2b, blake2s

BLAKE2_ is a cryptographic hash function defined in :rfc:`7693` that comes in two
flavors:

* **BLAKE2b**, optimized for 64-bit platforms and produces digests of any size
  between 1 and 64 bytes,

* **BLAKE2s**, optimized for 8- to 32-bit platforms and produces digests of any
  size between 1 and 32 bytes.

BLAKE2 supports **keyed mode** (a faster and simpler replacement for HMAC_),
**salted hashing**, **personalization**, and **tree hashing**.

Hash objects from this module follow the API of standard library's
:mod:`hashlib` objects.


Creating hash objects
^^^^^^^^^^^^^^^^^^^^^

New hash objects are created by calling constructor functions:


.. function:: blake2b(data=b'', *, digest_size=64, key=b'', salt=b'', \
                person=b'', fanout=1, depth=1, leaf_size=0, node_offset=0,  \
                node_depth=0, inner_size=0, last_node=False, \
                usedforsecurity=True)

.. function:: blake2s(data=b'', *, digest_size=32, key=b'', salt=b'', \
                person=b'', fanout=1, depth=1, leaf_size=0, node_offset=0,  \
                node_depth=0, inner_size=0, last_node=False, \
                usedforsecurity=True)


These functions return the corresponding hash objects for calculating
BLAKE2b or BLAKE2s. They optionally take these general parameters:

* *data*: initial chunk of data to hash, which must be
  :term:`bytes-like object`.  It can be passed only as positional argument.

* *digest_size*: size of output digest in bytes.

* *key*: key for keyed hashing (up to 64 bytes for BLAKE2b, up to 32 bytes for
  BLAKE2s).

* *salt*: salt for randomized hashing (up to 16 bytes for BLAKE2b, up to 8
  bytes for BLAKE2s).

* *person*: personalization string (up to 16 bytes for BLAKE2b, up to 8 bytes
  for BLAKE2s).

The following table shows limits for general parameters (in bytes):

======= =========== ======== ========= ===========
Hash    digest_size len(key) len(salt) len(person)
======= =========== ======== ========= ===========
BLAKE2b     64         64       16        16
BLAKE2s     32         32       8         8
======= =========== ======== ========= ===========

.. note::

    BLAKE2 specification defines constant lengths for salt and personalization
    parameters, however, for convenience, this implementation accepts byte
    strings of any size up to the specified length. If the length of the
    parameter is less than specified, it is padded with zeros, thus, for
    example, ``b'salt'`` and ``b'salt\x00'`` is the same value. (This is not
    the case for *key*.)

These sizes are available as module `constants`_ described below.

Constructor functions also accept the following tree hashing parameters:

* *fanout*: fanout (0 to 255, 0 if unlimited, 1 in sequential mode).

* *depth*: maximal depth of tree (1 to 255, 255 if unlimited, 1 in
  sequential mode).

* *leaf_size*: maximal byte length of leaf (0 to ``2**32-1``, 0 if unlimited or in
  sequential mode).

* *node_offset*: node offset (0 to ``2**64-1`` for BLAKE2b, 0 to ``2**48-1`` for
  BLAKE2s, 0 for the first, leftmost, leaf, or in sequential mode).

* *node_depth*: node depth (0 to 255, 0 for leaves, or in sequential mode).

* *inner_size*: inner digest size (0 to 64 for BLAKE2b, 0 to 32 for
  BLAKE2s, 0 in sequential mode).

* *last_node*: boolean indicating whether the processed node is the last
  one (``False`` for sequential mode).

.. figure:: hashlib-blake2-tree.png
   :alt: Explanation of tree mode parameters.
   :class: invert-in-dark-mode

See section 2.10 in `BLAKE2 specification
<https://www.blake2.net/blake2_20130129.pdf>`_ for comprehensive review of tree
hashing.


Constants
^^^^^^^^^

.. data:: blake2b.SALT_SIZE
.. data:: blake2s.SALT_SIZE

Salt length (maximum length accepted by constructors).


.. data:: blake2b.PERSON_SIZE
.. data:: blake2s.PERSON_SIZE

Personalization string length (maximum length accepted by constructors).


.. data:: blake2b.MAX_KEY_SIZE
.. data:: blake2s.MAX_KEY_SIZE

Maximum key size.


.. data:: blake2b.MAX_DIGEST_SIZE
.. data:: blake2s.MAX_DIGEST_SIZE

Maximum digest size that the hash function can output.


Examples
^^^^^^^^

Simple hashing
""""""""""""""

To calculate hash of some data, you should first construct a hash object by
calling the appropriate constructor function (:func:`blake2b` or
:func:`blake2s`), then update it with the data by calling :meth:`~hash.update` on the
object, and, finally, get the digest out of the object by calling
:meth:`~hash.digest` (or :meth:`~hash.hexdigest` for hex-encoded string).

    >>> from hashlib import blake2b
    >>> h = blake2b()
    >>> h.update(b'Hello world')
    >>> h.hexdigest()
    '6ff843ba685842aa82031d3f53c48b66326df7639a63d128974c5c14f31a0f33343a8c65551134ed1ae0f2b0dd2bb495dc81039e3eeb0aa1bb0388bbeac29183'


As a shortcut, you can pass the first chunk of data to update directly to the
constructor as the positional argument:

    >>> from hashlib import blake2b
    >>> blake2b(b'Hello world').hexdigest()
    '6ff843ba685842aa82031d3f53c48b66326df7639a63d128974c5c14f31a0f33343a8c65551134ed1ae0f2b0dd2bb495dc81039e3eeb0aa1bb0388bbeac29183'

You can call :meth:`hash.update` as many times as you need to iteratively
update the hash:

    >>> from hashlib import blake2b
    >>> items = [b'Hello', b' ', b'world']
    >>> h = blake2b()
    >>> for item in items:
    ...     h.update(item)
    >>> h.hexdigest()
    '6ff843ba685842aa82031d3f53c48b66326df7639a63d128974c5c14f31a0f33343a8c65551134ed1ae0f2b0dd2bb495dc81039e3eeb0aa1bb0388bbeac29183'


Using different digest sizes
""""""""""""""""""""""""""""

BLAKE2 has configurable size of digests up to 64 bytes for BLAKE2b and up to 32
bytes for BLAKE2s. For example, to replace SHA-1 with BLAKE2b without changing
the size of output, we can tell BLAKE2b to produce 20-byte digests:

    >>> from hashlib import blake2b
    >>> h = blake2b(digest_size=20)
    >>> h.update(b'Replacing SHA1 with the more secure function')
    >>> h.hexdigest()
    'd24f26cf8de66472d58d4e1b1774b4c9158b1f4c'
    >>> h.digest_size
    20
    >>> len(h.digest())
    20

Hash objects with different digest sizes have completely different outputs
(shorter hashes are *not* prefixes of longer hashes); BLAKE2b and BLAKE2s
produce different outputs even if the output length is the same:

    >>> from hashlib import blake2b, blake2s
    >>> blake2b(digest_size=10).hexdigest()
    '6fa1d8fcfd719046d762'
    >>> blake2b(digest_size=11).hexdigest()
    'eb6ec15daf9546254f0809'
    >>> blake2s(digest_size=10).hexdigest()
    '1bf21a98c78a1c376ae9'
    >>> blake2s(digest_size=11).hexdigest()
    '567004bf96e4a25773ebf4'


Keyed hashing
"""""""""""""

Keyed hashing can be used for authentication as a faster and simpler
replacement for `Hash-based message authentication code
<https://en.wikipedia.org/wiki/HMAC>`_ (HMAC).
BLAKE2 can be securely used in prefix-MAC mode thanks to the
indifferentiability property inherited from BLAKE.

This example shows how to get a (hex-encoded) 128-bit authentication code for
message ``b'message data'`` with key ``b'pseudorandom key'``::

    >>> from hashlib import blake2b
    >>> h = blake2b(key=b'pseudorandom key', digest_size=16)
    >>> h.update(b'message data')
    >>> h.hexdigest()
    '3d363ff7401e02026f4a4687d4863ced'


As a practical example, a web application can symmetrically sign cookies sent
to users and later verify them to make sure they weren't tampered with::

    >>> from hashlib import blake2b
    >>> from hmac import compare_digest
    >>>
    >>> SECRET_KEY = b'pseudorandomly generated server secret key'
    >>> AUTH_SIZE = 16
    >>>
    >>> def sign(cookie):
    ...     h = blake2b(digest_size=AUTH_SIZE, key=SECRET_KEY)
    ...     h.update(cookie)
    ...     return h.hexdigest().encode('utf-8')
    >>>
    >>> def verify(cookie, sig):
    ...     good_sig = sign(cookie)
    ...     return compare_digest(good_sig, sig)
    >>>
    >>> cookie = b'user-alice'
    >>> sig = sign(cookie)
    >>> print("{0},{1}".format(cookie.decode('utf-8'), sig))
    user-alice,b'43b3c982cf697e0c5ab22172d1ca7421'
    >>> verify(cookie, sig)
    True
    >>> verify(b'user-bob', sig)
    False
    >>> verify(cookie, b'0102030405060708090a0b0c0d0e0f00')
    False

Even though there's a native keyed hashing mode, BLAKE2 can, of course, be used
in HMAC construction with :mod:`hmac` module::

    >>> import hmac, hashlib
    >>> m = hmac.new(b'secret key', digestmod=hashlib.blake2s)
    >>> m.update(b'message')
    >>> m.hexdigest()
    'e3c8102868d28b5ff85fc35dda07329970d1a01e273c37481326fe0c861c8142'


Randomized hashing
""""""""""""""""""

By setting *salt* parameter users can introduce randomization to the hash
function. Randomized hashing is useful for protecting against collision attacks
on the hash function used in digital signatures.

    Randomized hashing is designed for situations where one party, the message
    preparer, generates all or part of a message to be signed by a second
    party, the message signer. If the message preparer is able to find
    cryptographic hash function collisions (i.e., two messages producing the
    same hash value), then they might prepare meaningful versions of the message
    that would produce the same hash value and digital signature, but with
    different results (e.g., transferring $1,000,000 to an account, rather than
    $10). Cryptographic hash functions have been designed with collision
    resistance as a major goal, but the current concentration on attacking
    cryptographic hash functions may result in a given cryptographic hash
    function providing less collision resistance than expected. Randomized
    hashing offers the signer additional protection by reducing the likelihood
    that a preparer can generate two or more messages that ultimately yield the
    same hash value during the digital signature generation process --- even if
    it is practical to find collisions for the hash function. However, the use
    of randomized hashing may reduce the amount of security provided by a
    digital signature when all portions of the message are prepared
    by the signer.

    (`NIST SP-800-106 "Randomized Hashing for Digital Signatures"
    <https://csrc.nist.gov/publications/detail/sp/800-106/archive/2009-02-25>`_)

In BLAKE2 the salt is processed as a one-time input to the hash function during
initialization, rather than as an input to each compression function.

.. warning::

    *Salted hashing* (or just hashing) with BLAKE2 or any other general-purpose
    cryptographic hash function, such as SHA-256, is not suitable for hashing
    passwords.  See `BLAKE2 FAQ <https://www.blake2.net/#qa>`_ for more
    information.
..

    >>> import os
    >>> from hashlib import blake2b
    >>> msg = b'some message'
    >>> # Calculate the first hash with a random salt.
    >>> salt1 = os.urandom(blake2b.SALT_SIZE)
    >>> h1 = blake2b(salt=salt1)
    >>> h1.update(msg)
    >>> # Calculate the second hash with a different random salt.
    >>> salt2 = os.urandom(blake2b.SALT_SIZE)
    >>> h2 = blake2b(salt=salt2)
    >>> h2.update(msg)
    >>> # The digests are different.
    >>> h1.digest() != h2.digest()
    True


Personalization
"""""""""""""""

Sometimes it is useful to force hash function to produce different digests for
the same input for different purposes. Quoting the authors of the Skein hash
function:

    We recommend that all application designers seriously consider doing this;
    we have seen many protocols where a hash that is computed in one part of
    the protocol can be used in an entirely different part because two hash
    computations were done on similar or related data, and the attacker can
    force the application to make the hash inputs the same. Personalizing each
    hash function used in the protocol summarily stops this type of attack.

    (`The Skein Hash Function Family
    <https://www.schneier.com/wp-content/uploads/2016/02/skein.pdf>`_,
    p. 21)

BLAKE2 can be personalized by passing bytes to the *person* argument::

    >>> from hashlib import blake2b
    >>> FILES_HASH_PERSON = b'MyApp Files Hash'
    >>> BLOCK_HASH_PERSON = b'MyApp Block Hash'
    >>> h = blake2b(digest_size=32, person=FILES_HASH_PERSON)
    >>> h.update(b'the same content')
    >>> h.hexdigest()
    '20d9cd024d4fb086aae819a1432dd2466de12947831b75c5a30cf2676095d3b4'
    >>> h = blake2b(digest_size=32, person=BLOCK_HASH_PERSON)
    >>> h.update(b'the same content')
    >>> h.hexdigest()
    'cf68fb5761b9c44e7878bfb2c4c9aea52264a80b75005e65619778de59f383a3'

Personalization together with the keyed mode can also be used to derive different
keys from a single one.

    >>> from hashlib import blake2s
    >>> from base64 import b64decode, b64encode
    >>> orig_key = b64decode(b'Rm5EPJai72qcK3RGBpW3vPNfZy5OZothY+kHY6h21KM=')
    >>> enc_key = blake2s(key=orig_key, person=b'kEncrypt').digest()
    >>> mac_key = blake2s(key=orig_key, person=b'kMAC').digest()
    >>> print(b64encode(enc_key).decode('utf-8'))
    rbPb15S/Z9t+agffno5wuhB77VbRi6F9Iv2qIxU7WHw=
    >>> print(b64encode(mac_key).decode('utf-8'))
    G9GtHFE1YluXY1zWPlYk1e/nWfu0WSEb0KRcjhDeP/o=

Tree mode
"""""""""

Here's an example of hashing a minimal tree with two leaf nodes::

       10
      /  \
     00  01

This example uses 64-byte internal digests, and returns the 32-byte final
digest::

    >>> from hashlib import blake2b
    >>>
    >>> FANOUT = 2
    >>> DEPTH = 2
    >>> LEAF_SIZE = 4096
    >>> INNER_SIZE = 64
    >>>
    >>> buf = bytearray(6000)
    >>>
    >>> # Left leaf
    ... h00 = blake2b(buf[0:LEAF_SIZE], fanout=FANOUT, depth=DEPTH,
    ...               leaf_size=LEAF_SIZE, inner_size=INNER_SIZE,
    ...               node_offset=0, node_depth=0, last_node=False)
    >>> # Right leaf
    ... h01 = blake2b(buf[LEAF_SIZE:], fanout=FANOUT, depth=DEPTH,
    ...               leaf_size=LEAF_SIZE, inner_size=INNER_SIZE,
    ...               node_offset=1, node_depth=0, last_node=True)
    >>> # Root node
    ... h10 = blake2b(digest_size=32, fanout=FANOUT, depth=DEPTH,
    ...               leaf_size=LEAF_SIZE, inner_size=INNER_SIZE,
    ...               node_offset=0, node_depth=1, last_node=True)
    >>> h10.update(h00.digest())
    >>> h10.update(h01.digest())
    >>> h10.hexdigest()
    '3ad2a9b37c6070e374c7a8c508fe20ca86b6ed54e286e93a0318e95e881db5aa'

Credits
^^^^^^^

BLAKE2_ was designed by *Jean-Philippe Aumasson*, *Samuel Neves*, *Zooko
Wilcox-O'Hearn*, and *Christian Winnerlein* based on SHA-3_ finalist BLAKE_
created by *Jean-Philippe Aumasson*, *Luca Henzen*, *Willi Meier*, and
*Raphael C.-W. Phan*.

It uses core algorithm from ChaCha_ cipher designed by *Daniel J.  Bernstein*.

The stdlib implementation is based on pyblake2_ module. It was written by
*Dmitry Chestnykh* based on C implementation written by *Samuel Neves*. The
documentation was copied from pyblake2_ and written by *Dmitry Chestnykh*.

The C code was partly rewritten for Python by *Christian Heimes*.

The following public domain dedication applies for both C hash function
implementation, extension code, and this documentation:

   To the extent possible under law, the author(s) have dedicated all copyright
   and related and neighboring rights to this software to the public domain
   worldwide. This software is distributed without any warranty.

   You should have received a copy of the CC0 Public Domain Dedication along
   with this software. If not, see
   https://creativecommons.org/publicdomain/zero/1.0/.

The following people have helped with development or contributed their changes
to the project and the public domain according to the Creative Commons Public
Domain Dedication 1.0 Universal:

* *Alexandr Sokolovskiy*

.. _BLAKE2: https://www.blake2.net
.. _HMAC: https://en.wikipedia.org/wiki/Hash-based_message_authentication_code
.. _BLAKE: https://web.archive.org/web/20200918190133/https://131002.net/blake/
.. _SHA-3: https://en.wikipedia.org/wiki/Secure_Hash_Algorithms
.. _ChaCha: https://cr.yp.to/chacha.html
.. _pyblake2: https://pythonhosted.org/pyblake2/
.. _NIST-SP-800-132: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf
.. _stackexchange pbkdf2 iterations question: https://security.stackexchange.com/questions/3959/recommended-of-iterations-when-using-pbkdf2-sha256/
.. _Attacks on cryptographic hash algorithms: https://en.wikipedia.org/wiki/Cryptographic_hash_function#Attacks_on_cryptographic_hash_algorithms
.. _the FIPS 180-4 standard: https://csrc.nist.gov/publications/detail/fips/180/4/final
.. _the FIPS 202 standard: https://csrc.nist.gov/publications/detail/fips/202/final


.. _hashlib-seealso:

.. seealso::

   Module :mod:`hmac`
      A module to generate message authentication codes using hashes.

   Module :mod:`base64`
      Another way to encode binary hashes for non-binary environments.

   https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.180-4.pdf
      The FIPS 180-4 publication on Secure Hash Algorithms.

   https://csrc.nist.gov/publications/detail/fips/202/final
      The FIPS 202 publication on the SHA-3 Standard.

   https://www.blake2.net/
      Official BLAKE2 website.

   https://en.wikipedia.org/wiki/Cryptographic_hash_function
      Wikipedia article with information on which algorithms have known issues
      and what that means regarding their use.

   https://www.ietf.org/rfc/rfc8018.txt
      PKCS #5: Password-Based Cryptography Specification Version 2.1

   https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf
      NIST Recommendation for Password-Based Key Derivation.