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" !lae Ju@plJbѿ i !.. 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 both the integrity and authenticity of a message. .. class:: HMAC(key, algorithm, backend) HMAC objects take a ``key`` and a :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance. The ``key`` should be :doc:`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` 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: .. doctest:: >>> h = hmac.HMAC(key, hashes.SHA256(), backend=default_backend()) >>> 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 bytes key: Secret key as ``bytes``. :param algorithm: An :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance such as those described in :ref:`Cryptographic Hashes