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.. hazmat::
Key derivation functions
========================
.. currentmodule:: cryptography.hazmat.primitives.kdf
Key derivation functions derive bytes suitable for cryptographic operations
from passwords or other data sources using a pseudo-random function (PRF).
Different KDFs are suitable for different tasks such as:
* Cryptographic key derivation
Deriving a key suitable for use as input to an encryption algorithm.
Typically this means taking a password and running it through an algorithm
such as :class:`~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC` or
:class:`~cryptography.hazmat.primitives.kdf.hkdf.HKDF`.
This process is typically known as `key stretching`_.
* Password storage
When storing passwords you want to use an algorithm that is computationally
intensive. Legitimate users will only need to compute it once (for example,
taking the user's password, running it through the KDF, then comparing it
to the stored value), while attackers will need to do it billions of times.
Ideal password storage KDFs will be demanding on both computational and
memory resources.
.. currentmodule:: cryptography.hazmat.primitives.kdf.pbkdf2
.. 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 an alternate key storage KDF such as `scrypt`_ is generally
considered a better solution.
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. This can be used to control the length of time the operation
takes. Higher numbers help mitigate brute force attacks against derived
keys. 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.PBKDF2HMACBackend`
provider.
:raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
provided ``backend`` does not implement
:class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`
.. method:: derive(key_material)
:param bytes key_material: The input key material. For PBKDF2 this
should be a password.
:return bytes: the derived 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 bytes key_material: The input key material. This is the same as
``key_material`` in :meth:`derive`.
:param bytes expected_key: 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.
.. currentmodule:: cryptography.hazmat.primitives.kdf.hkdf
.. class:: HKDF(algorithm, length, salt, info, backend)
.. versionadded:: 0.2
`HKDF`_ (HMAC-based Extract-and-Expand Key Derivation Function) is suitable
for deriving keys of a fixed size used for other cryptographic operations.
.. warning::
HKDF should not be used for password storage.
.. doctest::
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> salt = os.urandom(16)
>>> info = b"hkdf-example"
>>> hkdf = HKDF(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... info=info,
... backend=backend
... )
>>> key = hkdf.derive(b"input key")
>>> hkdf = HKDF(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... info=info,
... backend=backend
... )
>>> hkdf.verify(b"input key", 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
``255 * (algorithm.digest_size // 8)``.
:param bytes salt: A salt. Randomizes the KDF's output. Optional, but
highly recommended. Ideally as many bits of entropy as the security
level of the hash: often that means cryptographically random and as
long as the hash output. Worse (shorter, less entropy) salt values can
still meaningfully contribute to security. May be reused. Does not have
to be secret, but may cause stronger security guarantees if secret; see
`RFC 5869`_ and the `HKDF paper`_ for more details. If ``None`` is
explicitly passed a default salt of ``algorithm.digest_size // 8`` null
bytes will be used.
:param bytes info: Application specific context information. If ``None``
is explicitly passed an empty byte string will be used.
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
provider.
:raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the
provided ``backend`` does not implement
:class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
.. method:: derive(key_material)
:param bytes key_material: The input key material.
:retunr bytes: The derived key.
Derives a new key from the input key material by performing both the
extract and expand operations.
.. 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.
.. _`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
.. _`PBKDF2`: https://en.wikipedia.org/wiki/PBKDF2
.. _`scrypt`: https://en.wikipedia.org/wiki/Scrypt
.. _`key stretching`: https://en.wikipedia.org/wiki/Key_stretching
.. _`HKDF`:
.. _`RFC 5869`: https://tools.ietf.org/html/rfc5869
.. _`HKDF paper`: https://eprint.iacr.org/2010/264
|
