Doc/library/hashlib.rst

: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>


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

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, and SHA512 (defined in FIPS 180-2) 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.

.. warning::

   Some algorithms have known hash collision weaknesses, see the FAQ at the end.

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:`sha1` to
create a SHA1 hash object. You can now feed this object with objects conforming
to the buffer interface (normally :class:`bytes` objects) using the
:meth:`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` or
:meth:`hexdigest` methods.

.. note::

   For better multithreading performance, the Python GIL is released for
   strings of more than 2047 bytes at object creation or on update.

.. note::

   Feeding string objects is to :meth:`update` is not supported, as hashes work
   on bytes, not on characters.

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

Constructors for hash algorithms that are always present in this module are
:func:`md5`, :func:`sha1`, :func:`sha224`, :func:`sha256`, :func:`sha384`, and
:func:`sha512`.  Additional algorithms may also be available depending upon the
OpenSSL library that Python uses on your platform.

For example, to obtain the digest of the byte string ``b'Nobody inspects the
spammish repetition'``::

   >>> import hashlib
   >>> m = hashlib.md5()
   >>> m.update(b"Nobody inspects")
   >>> m.update(b" the spammish repetition")
   >>> m.digest()
   b'\xbbd\x9c\x83\xdd\x1e\xa5\xc9\xd9\xde\xc9\xa1\x8d\xf0\xff\xe9'
   >>> m.digest_size
   16
   >>> m.block_size
   64

More condensed:

   >>> hashlib.sha224(b"Nobody inspects the spammish repetition").hexdigest()
   'a4337bc45a8fc544c03f52dc550cd6e1e87021bc896588bd79e901e2'

A generic :func:`new` constructor that takes the string name of the desired
algorithm as its first parameter also exists to allow access to the above listed
hashes as well as any other algorithms that your OpenSSL library may offer.  The
named constructors are much faster than :func:`new` and should be preferred.

Using :func:`new` with an algorithm provided by OpenSSL:

   >>> h = hashlib.new('ripemd160')
   >>> h.update(b"Nobody inspects the spammish repetition")
   >>> h.hexdigest()
   'cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc'

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


.. data:: digest_size

   The size of the resulting hash in bytes.

.. data:: block_size

   The internal block size of the hash algorithm in bytes.

A hash object has the following methods:


.. method:: hash.update(arg)

   Update the hash object with the object *arg*, which must be interpretable as
   a buffer of bytes.  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 2048 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 array 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.


.. 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.

   http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf
      The FIPS 180-2 publication on Secure Hash Algorithms.

   http://www.cryptography.com/cnews/hash.html
      Hash Collision FAQ with information on which algorithms have known issues and
      what that means regarding their use.