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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. .. class:: HMAC(key, algorithm) 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`. .. doctest:: >>> from cryptography.hazmat.primitives import hashes, hmac >>> key = b'test key. Beware! A real key should use os.urandom or TRNG to generate' >>> h = hmac.HMAC(key, hashes.SHA256()) >>> h.update(b"message to hash") >>> signature = h.finalize() >>> signature b'k\xd9\xb29\xefS\xf8\xcf\xec\xed\xbf\x95\xe6\x97X\x18\x9e%\x11DU1\x9fq}\x9a\x9c\xe0)y`=' 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()) >>> h.update(b"message to hash") >>> h_copy = h.copy() # get a copy of `h' to be reused >>> h.verify(signature) >>> >>> h_copy.verify(b"an incorrect signature") Traceback (most recent call last): ... cryptography.exceptions.InvalidSignature: Signature did not match digest. :param key: The secret key. :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>`. :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the provided ``algorithm`` isn't supported. .. 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:
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