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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
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