| Current File : //proc/self/root/usr/share/doc/python2-cryptography-1.7.2/docs/hazmat/primitives/asymmetric/dh.rst |
.. hazmat::
Diffie-Hellman key exchange
===========================
.. currentmodule:: cryptography.hazmat.primitives.asymmetric.dh
`Diffie-Hellman key exchange`_ (D–H) is a method that allows two parties
to jointly agree on a shared secret using an insecure channel.
Exchange Algorithm
~~~~~~~~~~~~~~~~~~
For most applications the ``shared_key`` should be passed to a key
derivation function.
.. code-block:: pycon
>>> from cryptography.hazmat.backends import default_backend
>>> from cryptography.hazmat.primitives.asymmetric import dh
>>> parameters = dh.generate_parameters(generator=2, key_size=2048,
... backend=default_backend())
>>> private_key = parameters.generate_private_key()
>>> peer_public_key = parameters.generate_private_key().public_key()
>>> shared_key = private_key.exchange(peer_public_key)
DHE (or EDH), the ephemeral form of this exchange, is **strongly
preferred** over simple DH and provides `forward secrecy`_ when used.
You must generate a new private key using :func:`~DHParameters.generate_private_key` for
each :meth:`~DHPrivateKeyWithSerialization.exchange` when performing an DHE key
exchange.
To assemble a :class:`~DHParameters` and a :class:`~DHPublicKey` from
primitive integers, you must first create the
:class:`~DHParameterNumbers` and :class:`~DHPublicNumbers` objects. For
example if **p**, **g**, and **y** are :class:`int` objects received from a
peer::
pn = dh.DHParameterNumbers(p, g)
parameters = pn.parameters(default_backend())
peer_public_numbers = dh.DHPublicNumbers(y, pn)
peer_public_key = peer_public_numbers.public_key(default_backend())
See also the :class:`~cryptography.hazmat.backends.interfaces.DHBackend`
API for additional functionality.
Group parameters
~~~~~~~~~~~~~~~~
.. function:: generate_parameters(generator, key_size, backend)
.. versionadded:: 0.9
Generate a new DH parameter group for use with ``backend``.
:param generator: The :class:`int` to use as a generator. Must be
2 or 5.
:param key_size: The bit length of the prime modulus to generate.
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.DHBackend`
instance.
:returns: DH parameters as a new instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
:raises ValueError: If ``key_size`` is not at least 512.
.. class:: DHParameters
.. versionadded:: 0.9
.. method:: generate_private_key()
.. versionadded:: 0.9
Generate a DH private key. This method can be used to generate many
new private keys from a single set of parameters.
:return: An instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPrivateKey`.
.. class:: DHParametersWithSerialization
.. versionadded:: 0.9
Inherits from :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
.. method:: parameter_numbers()
Return the numbers that make up this set of parameters.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameterNumbers`.
Key interfaces
~~~~~~~~~~~~~~
.. class:: DHPrivateKey
.. versionadded:: 0.9
.. attribute:: key_size
The bit length of the prime modulus.
.. method:: public_key()
Return the public key associated with this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicKey`.
.. method:: parameters()
Return the parameters associated with this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
.. class:: DHPrivateKeyWithSerialization
.. versionadded:: 0.9
Inherits from :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPrivateKey`.
.. method:: private_numbers()
Return the numbers that make up this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPrivateNumbers`.
.. method:: exchange(peer_public_key)
.. versionadded:: 1.7
:param DHPublicKeyWithSerialization peer_public_key: The public key for the
peer.
:return bytes: The agreed key. The bytes are ordered in 'big' endian.
.. class:: DHPublicKey
.. versionadded:: 0.9
.. attribute:: key_size
The bit length of the prime modulus.
.. method:: parameters()
Return the parameters associated with this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
.. class:: DHPublicKeyWithSerialization
.. versionadded:: 0.9
Inherits from :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicKey`.
.. method:: public_numbers()
Return the numbers that make up this public key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicNumbers`.
Numbers
~~~~~~~
.. class:: DHParameterNumbers(p, g)
.. versionadded:: 0.8
The collection of integers that define a Diffie-Hellman group.
.. attribute:: p
:type: int
The prime modulus value.
.. attribute:: g
:type: int
The generator value. Must be 2 or 5.
.. class:: DHPrivateNumbers(x, public_numbers)
.. versionadded:: 0.8
The collection of integers that make up a Diffie-Hellman private key.
.. attribute:: public_numbers
:type: :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicNumbers`
The :class:`DHPublicNumbers` which makes up the DH public
key associated with this DH private key.
.. attribute:: x
:type: int
The private value.
.. class:: DHPublicNumbers(y, parameter_numbers)
.. versionadded:: 0.8
The collection of integers that make up a Diffie-Hellman public key.
.. attribute:: parameter_numbers
:type: :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameterNumbers`
The parameters for this DH group.
.. attribute:: y
:type: int
The public value.
.. _`Diffie-Hellman key exchange`: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
.. _`forward secrecy`: https://en.wikipedia.org/wiki/Forward_secrecy