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.. hazmat::
Key Derivation Functions
========================
.. currentmodule:: cryptography.hazmat.primitives.kdf
Key derivation functions derive key material from passwords or other data
sources using a pseudo-random function (PRF). Each KDF is suitable for
different tasks (cryptographic key derivation, password storage,
key stretching) so match your needs to their capabilities.
.. class:: PBKDF2HMAC(algorithm, length, salt, iterations, backend):
.. versionadded:: 0.2
PBKDF2 (Password Based Key Derivation Function 2) is typically used for
deriving a cryptographic key from a password. It may also be used for
key storage, but other key storage KDFs such as `scrypt`_ or `bcrypt`_
are generally considered better solutions since they are designed to be
slow.
This class conforms to the
:class:`~cryptography.hazmat.primitives.interfaces.KeyDerivationFunction`
interface.
.. doctest::
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> salt = os.urandom(16)
>>> # derive
>>> kdf = PBKDF2HMAC(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... iterations=100000,
... backend=backend
... )
>>> key = kdf.derive(b"my great password")
>>> # verify
>>> kdf = PBKDF2HMAC(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... iterations=100000,
... backend=backend
... )
>>> kdf.verify(b"my great password", key)
:param algorithm: An instance of a
:class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
provider.
:param int length: The desired length of the derived key. Maximum is
(2\ :sup:`32` - 1) * ``algorithm.digest_size``.
:param bytes salt: A salt. `NIST SP 800-132`_ recommends 128-bits or
longer.
:param int iterations: The number of iterations to perform of the hash
function. See OWASP's `Password Storage Cheat Sheet`_ for more
detailed recommendations if you intend to use this for password storage.
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
provider.
.. method:: derive(key_material)
:param key_material bytes: The input key material. For PBKDF2 this
should be a password.
:return: The new key.
:raises cryptography.exceptions.AlreadyFinalized: This is raised when
:meth:`derive` or
:meth:`verify` is
called more than
once.
This generates and returns a new key from the supplied password.
.. method:: verify(key_material, expected_key)
:param key_material bytes: The input key material. This is the same as
``key_material`` in :meth:`derive`.
:param expected_key bytes: The expected result of deriving a new key,
this is the same as the return value of
:meth:`derive`.
:raises cryptography.exceptions.InvalidKey: This is raised when the
derived key does not match
the expected key.
:raises cryptography.exceptions.AlreadyFinalized: This is raised when
:meth:`derive` or
:meth:`verify` is
called more than
once.
This checks whether deriving a new key from the supplied
``key_material`` generates the same key as the ``expected_key``, and
raises an exception if they do not match. This can be used for
checking whether the password a user provides matches the stored derived
key.
.. _`NIST SP 800-132`: http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf
.. _`Password Storage Cheat Sheet`: https://www.owasp.org/index.php/Password_Storage_Cheat_Sheet
.. _`bcrypt`: http://en.wikipedia.org/wiki/Bcrypt
.. _`scrypt`: http://en.wikipedia.org/wiki/Scrypt
|
