kmac256.rst

KMAC256

KMAC256 is a variable-length Message Authenticated Code (MAC) derived from SHA-3 and standardized in NIST SP 800-185.

KMAC256 provides a security strength of 256 bits. It must be keyed with a secret of 32 bytes or more.

This is an example showing how to generate a KMAC256 tag:

>>> from Crypto.Hash import KMAC256
>>>
>>> secret = b'Protect this thirty-two byte key'
>>> mac = KMAC256.new(key=secret, mac_len=16)
>>> mac.update(b'Hello')
>>> print(mac.hexdigest())
4ba8c9808f10b3bf5621f393363f4e1a

And this is an example showing how to validate the KMAC256 tag:

>>> from Crypto.Hash import KMAC256
>>>
>>> # We have received a message 'msg' together
>>> # with its MAC 'mac_tag'
>>>
>>> secret = b'Protect this thirty-two byte key'
>>> mac = KMAC256.new(key=secret, mac_len=16)
>>> mac.update(msg)
>>> try:
>>>   mac.verify(mac_tag)
>>>   print("The message '%s' is authentic" % msg)
>>> except ValueError:
>>>   print("The message or the key is wrong")

An application can select the length of the MAC tag by means of the initialization parameter mac_len. For instance, while the traditional HMAC-SHA256 can only produce 32-byte tags, with KMAC256 you can produce 16-byte tags (see the examples above) but also a 33-byte tag:

>>> from Crypto.Hash import KMAC256
>>>
>>> secret = b'Protect this thirty-two byte key'
>>> mac = KMAC256.new(key=secret, mac_len=33)
>>> mac.update(b'Hello')
>>> print(mac.hexdigest())
518938a66f4ce8f50a35cf77d16f002d5734da495eb6dea1e41191e657890ba4ad

Note how the 16-byte tag is NOT just the truncated version of the 33-byte tag: they are cryptographically uncorrelated.

Crypto.Hash.KMAC256