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: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::
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:`str` (ASCII only, as e.g. returned by
:meth:`HMAC.hexdigest`), or any type that supports the buffer protocol
(e.g. :class:`bytes`).
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.
|
