110 lines
4.3 KiB
ReStructuredText
110 lines
4.3 KiB
ReStructuredText
.. hazmat::
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Hash-based message authentication codes
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=======================================
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.. currentmodule:: cryptography.hazmat.primitives.hmac
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.. testsetup::
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import binascii
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key = binascii.unhexlify(b"0" * 32)
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Hash-based message authentication codes (or HMACs) are a tool for calculating
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message authentication codes using a cryptographic hash function coupled with a
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secret key. You can use an HMAC to verify both the integrity and authenticity
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of a message.
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.. class:: HMAC(key, algorithm, backend)
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HMAC objects take a ``key`` and a
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:class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance.
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The ``key`` should be :doc:`randomly generated bytes </random-numbers>` and
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is recommended to be equal in length to the ``digest_size`` of the hash
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function chosen. You must keep the ``key`` secret.
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This is an implementation of :rfc:`2104`.
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.. doctest::
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>>> from cryptography.hazmat.backends import default_backend
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>>> from cryptography.hazmat.primitives import hashes, hmac
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>>> h = hmac.HMAC(key, hashes.SHA256(), backend=default_backend())
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>>> h.update(b"message to hash")
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>>> h.finalize()
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'#F\xdaI\x8b"e\xc4\xf1\xbb\x9a\x8fc\xff\xf5\xdex.\xbc\xcd/+\x8a\x86\x1d\x84\'\xc3\xa6\x1d\xd8J'
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If the backend doesn't support the requested ``algorithm`` an
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:class:`~cryptography.exceptions.UnsupportedAlgorithm` exception will be
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raised.
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If ``algorithm`` isn't a
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:class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` instance
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then ``TypeError`` will be raised.
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To check that a given signature is correct use the :meth:`verify` method.
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You will receive an exception if the signature is wrong:
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.. doctest::
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>>> h = hmac.HMAC(key, hashes.SHA256(), backend=default_backend())
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>>> h.update(b"message to hash")
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>>> h.verify(b"an incorrect signature")
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Traceback (most recent call last):
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...
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cryptography.exceptions.InvalidSignature: Signature did not match digest.
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:param bytes key: Secret key as ``bytes``.
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:param algorithm: An
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:class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`
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instance such as those described in
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:ref:`Cryptographic Hashes <cryptographic-hash-algorithms>`.
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:param backend: An
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:class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
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instance.
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:raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
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provided ``backend`` does not implement
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:class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
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.. method:: update(msg)
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:param bytes msg: The bytes to hash and authenticate.
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:raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
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:raises TypeError: This exception is raised if ``msg`` is not ``bytes``.
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.. method:: copy()
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Copy this :class:`HMAC` instance, usually so that we may call
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:meth:`finalize` to get an intermediate digest value while we continue
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to call :meth:`update` on the original instance.
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:return: A new instance of :class:`HMAC` that can be updated
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and finalized independently of the original instance.
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:raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
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.. method:: verify(signature)
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Finalize the current context and securely compare digest to
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``signature``.
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:param bytes signature: The bytes to compare the current digest
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against.
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:raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
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:raises cryptography.exceptions.InvalidSignature: If signature does not
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match digest
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:raises TypeError: This exception is raised if ``signature`` is not
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``bytes``.
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.. method:: finalize()
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Finalize the current context and return the message digest as bytes.
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After ``finalize`` has been called this object can no longer be used
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and :meth:`update`, :meth:`copy`, :meth:`verify` and :meth:`finalize`
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will raise an :class:`~cryptography.exceptions.AlreadyFinalized`
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exception.
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:return bytes: The message digest as bytes.
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:raises cryptography.exceptions.AlreadyFinalized:
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