cmac.rst

Cipher-based message authentication code (CMAC)

cryptography.hazmat.primitives.cmac

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 4493.

0.4

CMAC objects take a ~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm instance.

>>> 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'CTx1dxc8x0ex15xbe4exdbxb6x84xcaxd9Xk'

If the backend doesn't support the requested algorithm an ~cryptography.exceptions.UnsupportedAlgorithm exception will be raised.

If algorithm isn't a ~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm instance then TypeError will be raised.

To check that a given signature is correct use the verify method. You will receive an exception if the signature is wrong:

>>> 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 ~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm.

param backend

An instance of ~cryptography.hazmat.backends.interfaces.CMACBackend.

raises TypeError

This is raised if the provided algorithm is not an instance of ~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm

raises cryptography.exceptions.UnsupportedAlgorithm

This is raised if the provided backend does not implement ~cryptography.hazmat.backends.interfaces.CMACBackend

update(data)

param bytes data

The bytes to hash and authenticate.

raises cryptography.exceptions.AlreadyFinalized

See finalize

raises TypeError

This exception is raised if data is not bytes.

copy()

Copy this CMAC instance, usually so that we may call finalize to get an intermediate value while we continue to call update on the original instance.

return

A new instance of CMAC that can be updated and finalized independently of the original instance.

raises cryptography.exceptions.AlreadyFinalized

See finalize

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 finalize

raises cryptography.exceptions.InvalidSignature

If signature does not match digest

raises TypeError

This exception is raised if signature is not bytes.

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 update, copy, verify and finalize will raise an ~cryptography.exceptions.AlreadyFinalized exception.

return bytes

The message authentication code as bytes.

raises cryptography.exceptions.AlreadyFinalized