1.. hazmat:: 2 3Hash-based message authentication codes (HMAC) 4============================================== 5 6.. currentmodule:: cryptography.hazmat.primitives.hmac 7 8.. testsetup:: 9 10 import binascii 11 key = binascii.unhexlify(b"0" * 32) 12 13Hash-based message authentication codes (or HMACs) are a tool for calculating 14message authentication codes using a cryptographic hash function coupled with a 15secret key. You can use an HMAC to verify both the integrity and authenticity 16of a message. 17 18.. class:: HMAC(key, algorithm, backend=None) 19 20 HMAC objects take a ``key`` and a 21 :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance. 22 The ``key`` should be :doc:`randomly generated bytes </random-numbers>` and 23 is recommended to be equal in length to the ``digest_size`` of the hash 24 function chosen. You must keep the ``key`` secret. 25 26 This is an implementation of :rfc:`2104`. 27 28 .. doctest:: 29 30 >>> from cryptography.hazmat.primitives import hashes, hmac 31 >>> h = hmac.HMAC(key, hashes.SHA256()) 32 >>> h.update(b"message to hash") 33 >>> h.finalize() 34 b'#F\xdaI\x8b"e\xc4\xf1\xbb\x9a\x8fc\xff\xf5\xdex.\xbc\xcd/+\x8a\x86\x1d\x84\'\xc3\xa6\x1d\xd8J' 35 36 If the backend doesn't support the requested ``algorithm`` an 37 :class:`~cryptography.exceptions.UnsupportedAlgorithm` exception will be 38 raised. 39 40 If ``algorithm`` isn't a 41 :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance 42 then ``TypeError`` will be raised. 43 44 To check that a given signature is correct use the :meth:`verify` method. 45 You will receive an exception if the signature is wrong: 46 47 .. doctest:: 48 49 >>> h = hmac.HMAC(key, hashes.SHA256()) 50 >>> h.update(b"message to hash") 51 >>> h.verify(b"an incorrect signature") 52 Traceback (most recent call last): 53 ... 54 cryptography.exceptions.InvalidSignature: Signature did not match digest. 55 56 :param key: Secret key as ``bytes``. 57 :type key: :term:`bytes-like` 58 :param algorithm: An 59 :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` 60 instance such as those described in 61 :ref:`Cryptographic Hashes <cryptographic-hash-algorithms>`. 62 :param backend: An optional 63 :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` 64 instance. 65 66 :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the 67 provided ``backend`` does not implement 68 :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` 69 70 .. method:: update(msg) 71 72 :param msg: The bytes to hash and authenticate. 73 :type msg: :term:`bytes-like` 74 :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize` 75 :raises TypeError: This exception is raised if ``msg`` is not ``bytes``. 76 77 .. method:: copy() 78 79 Copy this :class:`HMAC` instance, usually so that we may call 80 :meth:`finalize` to get an intermediate digest value while we continue 81 to call :meth:`update` on the original instance. 82 83 :return: A new instance of :class:`HMAC` that can be updated 84 and finalized independently of the original instance. 85 :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize` 86 87 .. method:: verify(signature) 88 89 Finalize the current context and securely compare digest to 90 ``signature``. 91 92 :param bytes signature: The bytes to compare the current digest 93 against. 94 :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize` 95 :raises cryptography.exceptions.InvalidSignature: If signature does not 96 match digest 97 :raises TypeError: This exception is raised if ``signature`` is not 98 ``bytes``. 99 100 .. method:: finalize() 101 102 Finalize the current context and return the message digest as bytes. 103 104 After ``finalize`` has been called this object can no longer be used 105 and :meth:`update`, :meth:`copy`, :meth:`verify` and :meth:`finalize` 106 will raise an :class:`~cryptography.exceptions.AlreadyFinalized` 107 exception. 108 109 :return bytes: The message digest as bytes. 110 :raises cryptography.exceptions.AlreadyFinalized: 111