:mod:`!hmac` --- Keyed-Hashing for Message Authentication
=========================================================
.. module:: hmac
:synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation
.. moduleauthor:: Gerhard Häring <[email protected]>
.. sectionauthor:: Gerhard Häring <[email protected]>
**Source code:** :source:`Lib/hmac.py`
--------------
This module implements the HMAC algorithm as described by :rfc:`2104`.
.. function:: new(key, msg=None, digestmod)
Return a new hmac object. *key* is a bytes or bytearray object giving the
secret key. If *msg* is present, the method call ``update(msg)`` is made.
*digestmod* is the digest name, digest constructor or module for the HMAC
object to use. It may be any name suitable to :func:`hashlib.new`.
Despite its argument position, it is required.
.. versionchanged:: 3.4
Parameter *key* can be a bytes or bytearray object.
Parameter *msg* can be of any type supported by :mod:`hashlib`.
Parameter *digestmod* can be the name of a hash algorithm.
.. versionchanged:: 3.8
The *digestmod* argument is now required. Pass it as a keyword
argument to avoid awkwardness when you do not have an initial *msg*.
.. function:: digest(key, msg, digest)
Return digest of *msg* for given secret *key* and *digest*. The
function is equivalent to ``HMAC(key, msg, digest).digest()``, but
uses an optimized C or inline implementation, which is faster for messages
that fit into memory. The parameters *key*, *msg*, and *digest* have
the same meaning as in :func:`~hmac.new`.
CPython implementation detail, the optimized C implementation is only used
when *digest* is a string and name of a digest algorithm, which is
supported by OpenSSL.
.. versionadded:: 3.7
An HMAC object has the following methods:
.. method:: HMAC.update(msg)
Update the hmac object with *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)``.
.. versionchanged:: 3.4
Parameter *msg* can be of any type supported by :mod:`hashlib`.
.. 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.
A hash object has the following attributes:
.. attribute:: HMAC.digest_size
The size of the resulting HMAC digest in bytes.
.. attribute:: HMAC.block_size
The internal block size of the hash algorithm in bytes.
.. versionadded:: 3.4
.. attribute:: HMAC.name
The canonical name of this HMAC, always lowercase, e.g. ``hmac-md5``.
.. versionadded:: 3.4
.. versionchanged:: 3.10
Removed the undocumented attributes ``HMAC.digest_cons``, ``HMAC.inner``,
and ``HMAC.outer``.
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 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:: 3.3
.. versionchanged:: 3.10
The function uses OpenSSL's ``CRYPTO_memcmp()`` internally when
available.
.. seealso::
Module :mod:`hashlib`
The Python module providing secure hash functions.
Codebase Browser
cpython