.. hazmat::

Hash-based Message Authentication Codes
=======================================

.. 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 integrity as well as authenticate a
message.

.. class:: HMAC(key, algorithm)

    HMAC objects take a ``key`` and a provider of
    :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`.
    The ``key`` should be randomly generated bytes 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`.

    .. doctest::

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

    .. method:: update(msg)

        :param bytes msg: The bytes to hash and authenticate.

    .. method:: copy()

        :return: a new instance of this object with a copied internal state.

    .. method:: finalize()

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

        Once ``finalize`` is called this object can no longer be used.

        :return bytes: The message digest as bytes.