.. 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, backend)

    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.backends import default_backend
        >>> from cryptography.hazmat.primitives import hashes, hmac
        >>> h = hmac.HMAC(key, hashes.SHA256(), backend=default_backend())
        >>> 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'

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

    .. code-block:: pycon

        >>> 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``.
    :param algorithm: A
        :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
        provider such as those described in
        :ref:`Cryptographic Hashes <cryptographic-hash-algorithms>`.
    :param backend: A
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
        provider.

    .. method:: update(msg)

        :param bytes msg: The bytes to hash and authenticate.
        :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`

    .. method:: copy()

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

        :return: A new instance of :class:`HMAC` which 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

    .. method:: finalize()

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

        Once ``finalize`` is called this object can no longer be used and
        :meth:`update`, :meth:`copy`, and :meth:`finalize` will raise
        :class:`~cryptography.exceptions.AlreadyFinalized`.

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