Permalink
Find file Copy path
290 lines (185 sloc) 9.02 KB
.. hazmat::

Message digests (Hashing)

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

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.

>>> from cryptography.hazmat.backends import default_backend
>>> from cryptography.hazmat.primitives import hashes
>>> digest = hashes.Hash(hashes.SHA256(), backend=default_backend())
>>> 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:A :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.

SHA-2 family

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

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

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

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

.. 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.

.. 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.

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.

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.

.. 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.

.. 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.

.. 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.

.. 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.

.. 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.
.. 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.

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.

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

Interfaces

.. 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.
.. 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.