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.. hazmat:: Cipher-based message authentication code (CMAC) =============================================== .. currentmodule:: cryptography.hazmat.primitives.cmac .. testsetup:: import binascii key = binascii.unhexlify(b"0" * 32) `Cipher-based message authentication codes`_ (or CMACs) are a tool for calculating message authentication codes using a block cipher coupled with a secret key. You can use an CMAC to verify both the integrity and authenticity of a message. A subset of CMAC with the AES-128 algorithm is described in :rfc:`4493`. .. class:: CMAC(algorithm, backend) .. versionadded:: 0.4 CMAC objects take a :class:`~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm` instance. .. doctest:: >>> from cryptography.hazmat.backends import default_backend >>> from cryptography.hazmat.primitives import cmac >>> from cryptography.hazmat.primitives.ciphers import algorithms >>> c = cmac.CMAC(algorithms.AES(key), backend=default_backend()) >>> c.update(b"message to authenticate") >>> c.finalize() b'CT\x1d\xc8\x0e\x15\xbe4e\xdb\xb6\x84\xca\xd9Xk' 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.ciphers.BlockCipherAlgorithm` 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:: >>> c = cmac.CMAC(algorithms.AES(key), backend=default_backend()) >>> c.update(b"message to authenticate") >>> c.verify(b"an incorrect signature") Traceback (most recent call last): ... cryptography.exceptions.InvalidSignature: Signature did not match digest. :param algorithm: An instance of :class:`~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm`. :param backend: An instance of :class:`~cryptography.hazmat.backends.interfaces.CMACBackend`. :raises TypeError: This is raised if the provided ``algorithm`` is not an instance of :class:`~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm` :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the provided ``backend`` does not implement :class:`~cryptography.hazmat.backends.interfaces.CMACBackend` .. method:: update(data) :param bytes data: The bytes to hash and authenticate. :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize` :raises TypeError: This exception is raised if ``data`` is not ``bytes``. .. method:: copy() Copy this :class:`CMAC` instance, usually so that we may call :meth:`finalize` to get an intermediate value while we continue to call :meth:`update` on the original instance. :return: A new instance of :class:`CMAC` 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 the MAC to ``signature``. :param bytes signature: The bytes to compare the current CMAC 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 authentication code 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 authentication code as bytes. :raises cryptography.exceptions.AlreadyFinalized: .. _`Cipher-based message authentication codes`: https://en.wikipedia.org/wiki/CMAC