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.. hazmat::

Hash-based message authentication codes (HMAC)

.. currentmodule:: cryptography.hazmat.primitives.hmac


.. testsetup::

    import binascii
    key = binascii.unhexlify(b"0" * 32)

Hash-based message authentication codes (or HMACs) are a tool for calculating message authentication codes using a cryptographic hash function coupled with a secret key. You can use an HMAC to verify both the integrity and authenticity of a message.

HMAC objects take a key and a :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance. The key should be :doc:`randomly generated bytes </random-numbers>` and is recommended to be equal in length to the digest_size of the hash function chosen. You must keep the key secret.

This is an implementation of RFC 2104.

>>> from cryptography.hazmat.primitives import hashes, hmac
>>> h = hmac.HMAC(key, hashes.SHA256())
>>> h.update(b"message to hash")
>>> h.finalize()
b'#F\xdaI\x8b"e\xc4\xf1\xbb\x9a\x8fc\xff\xf5\xdex.\xbc\xcd/+\x8a\x86\x1d\x84\'\xc3\xa6\x1d\xd8J'

If the backend doesn't support the requested algorithm an :class:`~cryptography.exceptions.UnsupportedAlgorithm` exception will be raised.

If algorithm isn't a :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance then TypeError will be raised.

To check that a given signature is correct use the :meth:`verify` method. You will receive an exception if the signature is wrong:

>>> h = hmac.HMAC(key, hashes.SHA256())
>>> h.update(b"message to hash")
>>> h.verify(b"an incorrect signature")
Traceback (most recent call last):
...
cryptography.exceptions.InvalidSignature: Signature did not match digest.
param key:Secret key as bytes.
type key::term:`bytes-like`
param algorithm:An :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance such as those described in :ref:`Cryptographic Hashes <cryptographic-hash-algorithms>`.
param backend:An optional :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` instance.
raises cryptography.exceptions.UnsupportedAlgorithm:
 This is raised if the provided backend does not implement :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` 
.. method:: update(msg)

    :param msg: The bytes to hash and authenticate.
    :type msg: :term:`bytes-like`
    :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
    :raises TypeError: This exception is raised if ``msg`` is not ``bytes``.


.. method:: copy()

    Copy this :class:`HMAC` instance, usually so that we may call
    :meth:`finalize` to get an intermediate digest value while we continue
    to call :meth:`update` on the original instance.

    :return: A new instance of :class:`HMAC` that can be updated
        and finalized independently of the original instance.
    :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`


.. method:: verify(signature)

    Finalize the current context and securely compare digest to
    ``signature``.

    :param bytes signature: The bytes to compare the current digest
                            against.
    :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
    :raises cryptography.exceptions.InvalidSignature: If signature does not
                                                      match digest
    :raises TypeError: This exception is raised if ``signature`` is not
                       ``bytes``.


.. method:: finalize()

    Finalize the current context and return the message digest as bytes.

    After ``finalize`` has been called this object can no longer be used
    and :meth:`update`, :meth:`copy`, :meth:`verify` and :meth:`finalize`
    will raise an :class:`~cryptography.exceptions.AlreadyFinalized`
    exception.

    :return bytes: The message digest as bytes.
    :raises cryptography.exceptions.AlreadyFinalized: