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:mod:`hmac` --- Keyed-Hashing for Message Authentication
========================================================
.. module:: hmac
:synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation
.. moduleauthor:: Gerhard Häring <ghaering@users.sourceforge.net>
.. sectionauthor:: Gerhard Häring <ghaering@users.sourceforge.net>
.. versionadded:: 2.2
**Source code:** :source:`Lib/hmac.py`
--------------
This module implements the HMAC algorithm as described by :rfc:`2104`.
.. function:: new(key[, msg[, digestmod]])
Return a new hmac object. 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 :data:`hashlib.md5` constructor.
An HMAC object has the following methods:
.. method:: HMAC.update(msg)
Update the hmac object with the string *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 strings passed to the :meth:`update` method so far.
This string will be the same length as the *digest_size* of the digest given to
the constructor. It may contain non-ASCII characters, 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::
When comparing the output of :meth:`hexdigest` 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.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,
making it appropriate for cryptography. *a* and *b* must both be of the
same type: either :class:`unicode` or a :term:`bytes-like object`.
.. note::
If *a* and *b* are of different lengths, or if an error occurs,
a timing attack could theoretically reveal information about the
types and lengths of *a* and *b*--but not their values.
.. versionadded:: 2.7.7
.. seealso::
Module :mod:`hashlib`
The Python module providing secure hash functions.
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