docs/hazmat/primitives/cryptographic-hashes.rst - platform/external/python/cryptography - Git at Google

 .. hazmat::

 Message digests (Hashing)
 =========================

 .. module:: cryptography.hazmat.primitives.hashes

 .. class:: Hash(algorithm, backend=None)

     A cryptographic hash function takes an arbitrary block of data and
     calculates a fixed-size bit string (a digest), such that different data
     results (with a high probability) in different digests.

     This is an implementation of
     :class:`~cryptography.hazmat.primitives.hashes.HashContext` meant to
     be used with
     :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`
     implementations to provide an incremental interface to calculating
     various message digests.

     .. doctest::

         >>> from cryptography.hazmat.primitives import hashes
         >>> digest = hashes.Hash(hashes.SHA256())
         >>> digest.update(b"abc")
         >>> digest.update(b"123")
         >>> digest.finalize()
         b'l\xa1=R\xcap\xc8\x83\xe0\xf0\xbb\x10\x1eBZ\x89\xe8bM\xe5\x1d\xb2\xd29%\x93\xafj\x84\x11\x80\x90'

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

     Keep in mind that attacks against cryptographic hashes only get stronger
     with time, and that often algorithms that were once thought to be strong,
     become broken. Because of this it's important to include a plan for
     upgrading the hash algorithm you use over time. For more information, see
     `Lifetimes of cryptographic hash functions`_.

     :param algorithm: A
         :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`
         instance such as those described in
         :ref:`below <cryptographic-hash-algorithms>`.
     :param backend: An optional
         :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
         instance.

     :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
         provided ``backend`` does not implement
         :class:`~cryptography.hazmat.backends.interfaces.HashBackend`

     .. method:: update(data)

         :param bytes data: The bytes to be hashed.
         :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`.
         :raises TypeError: This exception is raised if ``data`` is not ``bytes``.

     .. method:: copy()

         Copy this :class:`Hash` instance, usually so that you may call
         :meth:`finalize` to get an intermediate digest value while we continue
         to call :meth:`update` on the original instance.

         :return: A new instance of :class:`Hash` that can be updated
              and finalized independently of the original instance.
         :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`.

     .. method:: finalize()

         Finalize the current context and return the message digest as bytes.

         After ``finalize`` has been called this object can no longer be used
         and :meth:`update`, :meth:`copy`, and :meth:`finalize` will raise an
         :class:`~cryptography.exceptions.AlreadyFinalized` exception.

         :return bytes: The message digest as bytes.


 .. _cryptographic-hash-algorithms:

 SHA-2 family
 ~~~~~~~~~~~~

 .. class:: SHA224()

     SHA-224 is a cryptographic hash function from the SHA-2 family and is
     standardized by NIST. It produces a 224-bit message digest.

 .. class:: SHA256()

     SHA-256 is a cryptographic hash function from the SHA-2 family and is
     standardized by NIST. It produces a 256-bit message digest.

 .. class:: SHA384()

     SHA-384 is a cryptographic hash function from the SHA-2 family and is
     standardized by NIST. It produces a 384-bit message digest.

 .. class:: SHA512()

     SHA-512 is a cryptographic hash function from the SHA-2 family and is
     standardized by NIST. It produces a 512-bit message digest.

 .. class:: SHA512_224()

     .. versionadded:: 2.5

     SHA-512/224 is a cryptographic hash function from the SHA-2 family and is
     standardized by NIST. It produces a 224-bit message digest.

 .. class:: SHA512_256()

     .. versionadded:: 2.5

     SHA-512/256 is a cryptographic hash function from the SHA-2 family and is
     standardized by NIST. It produces a 256-bit message digest.

 BLAKE2
 ~~~~~~

 `BLAKE2`_ is a cryptographic hash function specified in :rfc:`7693`. BLAKE2's
 design makes it immune to `length-extension attacks`_, an advantage over the
 SHA-family of hashes.

 .. note::

     While the RFC specifies keying, personalization, and salting features,
     these are not supported at this time due to limitations in OpenSSL 1.1.0.

 .. class:: BLAKE2b(digest_size)

     BLAKE2b is optimized for 64-bit platforms and produces an 1 to 64-byte
     message digest.

     :param int digest_size: The desired size of the hash output in bytes. Only
         ``64`` is supported at this time.

     :raises ValueError: If the ``digest_size`` is invalid.

 .. class:: BLAKE2s(digest_size)

     BLAKE2s is optimized for 8 to 32-bit platforms and produces a
     1 to 32-byte message digest.

     :param int digest_size: The desired size of the hash output in bytes. Only
         ``32`` is supported at this time.

     :raises ValueError: If the ``digest_size`` is invalid.

 SHA-3 family
 ~~~~~~~~~~~~

 SHA-3 is the most recent NIST secure hash algorithm standard. Despite the
 larger number SHA-3 is not considered to be better than SHA-2. Instead, it uses
 a significantly different internal structure so that **if** an attack appears
 against SHA-2 it is unlikely to apply to SHA-3. SHA-3 is significantly slower
 than SHA-2 so at this time most users should choose SHA-2.

 .. class:: SHA3_224()

     .. versionadded:: 2.5

     SHA3/224 is a cryptographic hash function from the SHA-3 family and is
     standardized by NIST. It produces a 224-bit message digest.

 .. class:: SHA3_256()

     .. versionadded:: 2.5

     SHA3/256 is a cryptographic hash function from the SHA-3 family and is
     standardized by NIST. It produces a 256-bit message digest.

 .. class:: SHA3_384()

     .. versionadded:: 2.5

     SHA3/384 is a cryptographic hash function from the SHA-3 family and is
     standardized by NIST. It produces a 384-bit message digest.

 .. class:: SHA3_512()

     .. versionadded:: 2.5

     SHA3/512 is a cryptographic hash function from the SHA-3 family and is
     standardized by NIST. It produces a 512-bit message digest.

 .. class:: SHAKE128(digest_size)

     .. versionadded:: 2.5

     SHAKE128 is an extendable output function (XOF) based on the same core
     permutations as SHA3. It allows the caller to obtain an arbitrarily long
     digest length. Longer lengths, however, do not increase security or
     collision resistance and lengths shorter than 128 bit (16 bytes) will
     decrease it.

     :param int digest_size: The length of output desired. Must be greater than
         zero.

     :raises ValueError: If the ``digest_size`` is invalid.

 .. class:: SHAKE256(digest_size)

     .. versionadded:: 2.5

     SHAKE256 is an extendable output function (XOF) based on the same core
     permutations as SHA3. It allows the caller to obtain an arbitrarily long
     digest length. Longer lengths, however, do not increase security or
     collision resistance and lengths shorter than 256 bit (32 bytes) will
     decrease it.

     :param int digest_size: The length of output desired. Must be greater than
         zero.

     :raises ValueError: If the ``digest_size`` is invalid.

 SHA-1
 ~~~~~

 .. warning::

     SHA-1 is a deprecated hash algorithm that has practical known collision
     attacks. You are strongly discouraged from using it. Existing applications
     should strongly consider moving away.

 .. class:: SHA1()

     SHA-1 is a cryptographic hash function standardized by NIST. It produces an
     160-bit message digest. Cryptanalysis of SHA-1 has demonstrated that it is
     vulnerable to practical collision attacks, and collisions have been
     demonstrated.

 MD5
 ~~~

 .. warning::

     MD5 is a deprecated hash algorithm that has practical known collision
     attacks. You are strongly discouraged from using it. Existing applications
     should strongly consider moving away.

 .. class:: MD5()

     MD5 is a deprecated cryptographic hash function. It produces a 128-bit
     message digest and has practical known collision attacks.


 Interfaces
 ~~~~~~~~~~

 .. class:: HashAlgorithm

     .. attribute:: name

         :type: str

         The standard name for the hash algorithm, for example: ``"sha256"`` or
         ``"blake2b"``.

     .. attribute:: digest_size

         :type: int

         The size of the resulting digest in bytes.


 .. class:: HashContext

     .. attribute:: algorithm

         A :class:`HashAlgorithm` that will be used by this context.

     .. method:: update(data)

         :param bytes data: The data you want to hash.

     .. method:: finalize()

         :return: The final digest as bytes.

     .. method:: copy()

         :return: A :class:`HashContext` that is a copy of the current context.


 .. _`Lifetimes of cryptographic hash functions`: https://valerieaurora.org/hash.html
 .. _`BLAKE2`: https://blake2.net
 .. _`length-extension attacks`: https://en.wikipedia.org/wiki/Length_extension_attack
	.. hazmat::

	Message digests (Hashing)
	=========================

	.. module:: cryptography.hazmat.primitives.hashes

	.. class:: Hash(algorithm, backend=None)

	A cryptographic hash function takes an arbitrary block of data and
	calculates a fixed-size bit string (a digest), such that different data
	results (with a high probability) in different digests.

	This is an implementation of
	:class:`~cryptography.hazmat.primitives.hashes.HashContext` meant to
	be used with
	:class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`
	implementations to provide an incremental interface to calculating
	various message digests.

	.. doctest::

	>>> from cryptography.hazmat.primitives import hashes
	>>> digest = hashes.Hash(hashes.SHA256())
	>>> digest.update(b"abc")
	>>> digest.update(b"123")
	>>> digest.finalize()
	b'l\xa1=R\xcap\xc8\x83\xe0\xf0\xbb\x10\x1eBZ\x89\xe8bM\xe5\x1d\xb2\xd29%\x93\xafj\x84\x11\x80\x90'

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

	Keep in mind that attacks against cryptographic hashes only get stronger
	with time, and that often algorithms that were once thought to be strong,
	become broken. Because of this it's important to include a plan for
	upgrading the hash algorithm you use over time. For more information, see
	`Lifetimes of cryptographic hash functions`_.

	:param algorithm: A
	:class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`
	instance such as those described in
	:ref:`below <cryptographic-hash-algorithms>`.
	:param backend: An optional
	:class:`~cryptography.hazmat.backends.interfaces.HashBackend`
	instance.

	:raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
	provided ``backend`` does not implement
	:class:`~cryptography.hazmat.backends.interfaces.HashBackend`

	.. method:: update(data)

	:param bytes data: The bytes to be hashed.
	:raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`.
	:raises TypeError: This exception is raised if ``data`` is not ``bytes``.

	.. method:: copy()

	Copy this :class:`Hash` instance, usually so that you may call
	:meth:`finalize` to get an intermediate digest value while we continue
	to call :meth:`update` on the original instance.

	:return: A new instance of :class:`Hash` that can be updated
	and finalized independently of the original instance.
	:raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`.

	.. method:: finalize()

	Finalize the current context and return the message digest as bytes.

	After ``finalize`` has been called this object can no longer be used
	and :meth:`update`, :meth:`copy`, and :meth:`finalize` will raise an
	:class:`~cryptography.exceptions.AlreadyFinalized` exception.

	:return bytes: The message digest as bytes.


	.. _cryptographic-hash-algorithms:

	SHA-2 family
	~~~~~~~~~~~~

	.. class:: SHA224()

	SHA-224 is a cryptographic hash function from the SHA-2 family and is
	standardized by NIST. It produces a 224-bit message digest.

	.. class:: SHA256()

	SHA-256 is a cryptographic hash function from the SHA-2 family and is
	standardized by NIST. It produces a 256-bit message digest.

	.. class:: SHA384()

	SHA-384 is a cryptographic hash function from the SHA-2 family and is
	standardized by NIST. It produces a 384-bit message digest.

	.. class:: SHA512()

	SHA-512 is a cryptographic hash function from the SHA-2 family and is
	standardized by NIST. It produces a 512-bit message digest.

	.. class:: SHA512_224()

	.. versionadded:: 2.5

	SHA-512/224 is a cryptographic hash function from the SHA-2 family and is
	standardized by NIST. It produces a 224-bit message digest.

	.. class:: SHA512_256()

	.. versionadded:: 2.5

	SHA-512/256 is a cryptographic hash function from the SHA-2 family and is
	standardized by NIST. It produces a 256-bit message digest.

	BLAKE2
	~~~~~~

	`BLAKE2`_ is a cryptographic hash function specified in :rfc:`7693`. BLAKE2's
	design makes it immune to `length-extension attacks`_, an advantage over the
	SHA-family of hashes.

	.. note::

	While the RFC specifies keying, personalization, and salting features,
	these are not supported at this time due to limitations in OpenSSL 1.1.0.

	.. class:: BLAKE2b(digest_size)

	BLAKE2b is optimized for 64-bit platforms and produces an 1 to 64-byte
	message digest.

	:param int digest_size: The desired size of the hash output in bytes. Only
	``64`` is supported at this time.

	:raises ValueError: If the ``digest_size`` is invalid.

	.. class:: BLAKE2s(digest_size)

	BLAKE2s is optimized for 8 to 32-bit platforms and produces a
	1 to 32-byte message digest.

	:param int digest_size: The desired size of the hash output in bytes. Only
	``32`` is supported at this time.

	:raises ValueError: If the ``digest_size`` is invalid.

	SHA-3 family
	~~~~~~~~~~~~

	SHA-3 is the most recent NIST secure hash algorithm standard. Despite the
	larger number SHA-3 is not considered to be better than SHA-2. Instead, it uses
	a significantly different internal structure so that if an attack appears
	against SHA-2 it is unlikely to apply to SHA-3. SHA-3 is significantly slower
	than SHA-2 so at this time most users should choose SHA-2.

	.. class:: SHA3_224()

	.. versionadded:: 2.5

	SHA3/224 is a cryptographic hash function from the SHA-3 family and is
	standardized by NIST. It produces a 224-bit message digest.

	.. class:: SHA3_256()

	.. versionadded:: 2.5

	SHA3/256 is a cryptographic hash function from the SHA-3 family and is
	standardized by NIST. It produces a 256-bit message digest.

	.. class:: SHA3_384()

	.. versionadded:: 2.5

	SHA3/384 is a cryptographic hash function from the SHA-3 family and is
	standardized by NIST. It produces a 384-bit message digest.

	.. class:: SHA3_512()

	.. versionadded:: 2.5

	SHA3/512 is a cryptographic hash function from the SHA-3 family and is
	standardized by NIST. It produces a 512-bit message digest.

	.. class:: SHAKE128(digest_size)

	.. versionadded:: 2.5

	SHAKE128 is an extendable output function (XOF) based on the same core
	permutations as SHA3. It allows the caller to obtain an arbitrarily long
	digest length. Longer lengths, however, do not increase security or
	collision resistance and lengths shorter than 128 bit (16 bytes) will
	decrease it.

	:param int digest_size: The length of output desired. Must be greater than
	zero.

	:raises ValueError: If the ``digest_size`` is invalid.

	.. class:: SHAKE256(digest_size)

	.. versionadded:: 2.5

	SHAKE256 is an extendable output function (XOF) based on the same core
	permutations as SHA3. It allows the caller to obtain an arbitrarily long
	digest length. Longer lengths, however, do not increase security or
	collision resistance and lengths shorter than 256 bit (32 bytes) will
	decrease it.

	:param int digest_size: The length of output desired. Must be greater than
	zero.

	:raises ValueError: If the ``digest_size`` is invalid.

	SHA-1
	~~~~~

	.. warning::

	SHA-1 is a deprecated hash algorithm that has practical known collision
	attacks. You are strongly discouraged from using it. Existing applications
	should strongly consider moving away.

	.. class:: SHA1()

	SHA-1 is a cryptographic hash function standardized by NIST. It produces an
	160-bit message digest. Cryptanalysis of SHA-1 has demonstrated that it is
	vulnerable to practical collision attacks, and collisions have been
	demonstrated.

	MD5
	~~~

	.. warning::

	MD5 is a deprecated hash algorithm that has practical known collision
	attacks. You are strongly discouraged from using it. Existing applications
	should strongly consider moving away.

	.. class:: MD5()

	MD5 is a deprecated cryptographic hash function. It produces a 128-bit
	message digest and has practical known collision attacks.


	Interfaces
	~~~~~~~~~~

	.. class:: HashAlgorithm

	.. attribute:: name

	:type: str

	The standard name for the hash algorithm, for example: ``"sha256"`` or
	``"blake2b"``.

	.. attribute:: digest_size

	:type: int

	The size of the resulting digest in bytes.


	.. class:: HashContext

	.. attribute:: algorithm

	A :class:`HashAlgorithm` that will be used by this context.

	.. method:: update(data)

	:param bytes data: The data you want to hash.

	.. method:: finalize()

	:return: The final digest as bytes.

	.. method:: copy()

	:return: A :class:`HashContext` that is a copy of the current context.


	.. _`Lifetimes of cryptographic hash functions`: https://valerieaurora.org/hash.html
	.. _`BLAKE2`: https://blake2.net
	.. _`length-extension attacks`: https://en.wikipedia.org/wiki/Length_extension_attack