:mod:!hmac --- Keyed-Hashing for Message Authentication

.. module:: hmac :synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation

Source code: :source:Lib/hmac.py

This module implements the HMAC algorithm as described by :rfc:2104. The interface allows to use any hash function with a fixed digest size. In particular, extendable output functions such as SHAKE-128 or SHAKE-256 cannot be used with HMAC.

.. 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

.. class:: HMAC

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.