Doc/library/hmac.rst

:mod:`hmac` --- Keyed-Hashing for Message Authentication
========================================================

.. module:: hmac
   :synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation
              for Python.
.. moduleauthor:: Gerhard Häring <ghaering@users.sourceforge.net>
.. sectionauthor:: Gerhard Häring <ghaering@users.sourceforge.net>

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

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

This module implements the HMAC algorithm as described by :rfc:`2104`.


.. function:: new(key, msg=None, digestmod=None)

   Return a new hmac object.  *key* is a bytes object giving the secret key.  If
   *msg* is present, the method call ``update(msg)`` is made.  *digestmod* is
   the digest constructor or module for the HMAC object to use. It defaults to
   the :func:`hashlib.md5` constructor.


An HMAC object has the following methods:

.. method:: HMAC.update(msg)

   Update the hmac object with the bytes object *msg*.  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)``.


.. method:: HMAC.digest()

   Return the digest of the bytes passed to the :meth:`update` method so far.
   This bytes object will be the same length as the *digest_size* of the digest
   given to the constructor.  It may contain non-ASCII bytes, including NUL
   bytes.

   .. warning::

      When comparing the output of :meth:`digest` to an externally-supplied
      digest during a verification routine, it is recommended to use the
      :func:`compare_digest` function instead of the ``==`` operator
      to reduce the vulnerability to timing attacks.


.. method:: HMAC.hexdigest()

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

   .. warning::

      The output of :meth:`hexdigest` should not be compared directly to an
      externally-supplied digest during a verification routine. Instead, the
      externally supplied digest should be converted to a :class:`bytes`
      value and compared to the output of :meth:`digest` with
      :func:`compare_digest`.


.. method:: HMAC.copy()

   Return a copy ("clone") of the hmac object.  This can be used to efficiently
   compute the digests of strings that share a common initial substring.


This module also provides the following helper function:

.. function:: compare_digest(a, b)

   Return ``a == b``.  This function uses an approach designed to prevent timing
   analysis by avoiding content based short circuiting behaviour.  The inputs
   must either both support the buffer protocol (e.g. :class:`bytes` and
   :class:`bytearray` instances) or be ASCII-only :class:`str` instances as
   returned by :meth:`hexdigest`.  :class:`bytes` and :class:`str` instances
   can't be mixed.

   Using a short circuiting comparison (that is, one that terminates as soon as
   it finds any difference between the values) to check digests for correctness
   can be problematic, as it introduces a potential vulnerability when an
   attacker can control both the message to be checked *and* the purported
   signature value.  By keeping the plaintext consistent and supplying different
   signature values, an attacker may be able to use timing variations to search
   the signature space for the expected value in O(n) time rather than the
   desired O(2**n).

   .. note::

      While this function reduces the likelihood of leaking the contents of the
      expected digest via a timing attack, it still may leak some timing
      information when the input values differ in lengths as well as in error
      cases like unsupported types or non ASCII strings.  When the inputs have
      different length the timing depends solely on the length of ``b``.  It is
      assumed that the expected length of the digest is not a secret, as it is
      typically published as part of a file format, network protocol or API
      definition.

   .. versionadded:: 3.3


.. seealso::

   Module :mod:`hashlib`
      The Python module providing secure hash functions.