dh.rst

.. hazmat::

Diffie-Hellman key exchange

.. currentmodule:: cryptography.hazmat.primitives.asymmetric.dh

Note

For security and performance reasons we suggest using :class:`~cryptography.hazmat.primitives.asymmetric.ec.ECDH` instead of DH where possible.

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. This allows mixing of additional information into the key, derivation of multiple keys, and destroys any structure that may be present.

Warning

This example does not give forward secrecy and is only provided as a demonstration of the basic Diffie-Hellman construction. For real world applications always use the ephemeral form described after this example.

>>> from cryptography.hazmat.backends import default_backend
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.asymmetric import dh
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
>>> # Generate some parameters. These can be reused.
>>> parameters = dh.generate_parameters(generator=2, key_size=2048,
...                                     backend=default_backend())
>>> # Generate a private key for use in the exchange.
>>> server_private_key = parameters.generate_private_key()
>>> # In a real handshake the peer is a remote client. For this
>>> # example we'll generate another local private key though. Note that in
>>> # a DH handshake both peers must agree on a common set of parameters.
>>> peer_private_key = parameters.generate_private_key()
>>> shared_key = server_private_key.exchange(peer_private_key.public_key())
>>> # Perform key derivation.
>>> derived_key = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
...     backend=default_backend()
... ).derive(shared_key)
>>> # And now we can demonstrate that the handshake performed in the
>>> # opposite direction gives the same final value
>>> same_shared_key = peer_private_key.exchange(
...     server_private_key.public_key()
... )
>>> same_derived_key = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
...     backend=default_backend()
... ).derive(same_shared_key)
>>> derived_key == same_derived_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:`~DHPrivateKey.exchange` when performing an DHE key exchange. An example of the ephemeral form:

>>> from cryptography.hazmat.backends import default_backend
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.asymmetric import dh
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
>>> # Generate some parameters. These can be reused.
>>> parameters = dh.generate_parameters(generator=2, key_size=2048,
...                                     backend=default_backend())
>>> # Generate a private key for use in the exchange.
>>> private_key = parameters.generate_private_key()
>>> # In a real handshake the peer_public_key will be received from the
>>> # other party. For this example we'll generate another private key and
>>> # get a public key from that. Note that in a DH handshake both peers
>>> # must agree on a common set of parameters.
>>> peer_public_key = parameters.generate_private_key().public_key()
>>> shared_key = private_key.exchange(peer_public_key)
>>> # Perform key derivation.
>>> derived_key = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
...     backend=default_backend()
... ).derive(shared_key)
>>> # For the next handshake we MUST generate another private key, but
>>> # we can reuse the parameters.
>>> private_key_2 = parameters.generate_private_key()
>>> peer_public_key_2 = parameters.generate_private_key().public_key()
>>> shared_key_2 = private_key_2.exchange(peer_public_key_2)
>>> derived_key_2 = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
...     backend=default_backend()
... ).derive(shared_key_2)

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:: 1.7

    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.

.. versionadded:: 1.7

.. method:: generate_private_key()

    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`.

.. method:: parameter_numbers()

    Return the numbers that make up this set of parameters.

    :return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameterNumbers`.

.. method:: parameter_bytes(encoding, format)

    .. versionadded:: 2.0

    Allows serialization of the parameters to bytes. Encoding (
    :attr:`~cryptography.hazmat.primitives.serialization.Encoding.PEM` or
    :attr:`~cryptography.hazmat.primitives.serialization.Encoding.DER`) and
    format (
    :attr:`~cryptography.hazmat.primitives.serialization.ParameterFormat.PKCS3`)
    are chosen to define the exact serialization.

    :param encoding: A value from the
        :class:`~cryptography.hazmat.primitives.serialization.Encoding` enum.

    :param format: A value from the
        :class:`~cryptography.hazmat.primitives.serialization.ParameterFormat`
        enum. At the moment only ``PKCS3`` is supported.

    :return bytes: Serialized parameters.

.. versionadded:: 1.7

Alias for :class:`DHParameters`.

Key interfaces

.. versionadded:: 1.7

A DH private key that is not an :term:`opaque key` also implements :class:`DHPrivateKeyWithSerialization` to provide serialization methods.

.. 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`.

.. method:: exchange(peer_public_key)

    .. versionadded:: 1.7

    :param DHPublicKey peer_public_key: The public key for
        the peer.

    :return bytes: The agreed key. The bytes are ordered in 'big' endian.

.. versionadded:: 1.7

This interface contains additional methods relating to serialization. Any object with this interface also has all the methods from :class:`DHPrivateKey`.

.. method:: private_numbers()

    Return the numbers that make up this private key.

    :return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPrivateNumbers`.

.. method:: private_bytes(encoding, format, encryption_algorithm)

    .. versionadded:: 1.8

    Allows serialization of the key to bytes. Encoding (
    :attr:`~cryptography.hazmat.primitives.serialization.Encoding.PEM` or
    :attr:`~cryptography.hazmat.primitives.serialization.Encoding.DER`),
    format (
    :attr:`~cryptography.hazmat.primitives.serialization.PrivateFormat.PKCS8`)
    and encryption algorithm (such as
    :class:`~cryptography.hazmat.primitives.serialization.BestAvailableEncryption`
    or :class:`~cryptography.hazmat.primitives.serialization.NoEncryption`)
    are chosen to define the exact serialization.

    :param encoding: A value from the
        :class:`~cryptography.hazmat.primitives.serialization.Encoding` enum.

    :param format: A value from the
        :class:`~cryptography.hazmat.primitives.serialization.PrivateFormat`
        enum.

    :param encryption_algorithm: An instance of an object conforming to the
        :class:`~cryptography.hazmat.primitives.serialization.KeySerializationEncryption`
        interface.

    :return bytes: Serialized key.

.. versionadded:: 1.7

.. 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`.

.. method:: public_numbers()

    Return the numbers that make up this public key.

    :return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicNumbers`.

.. method:: public_bytes(encoding, format)

    .. versionadded:: 1.8

    Allows serialization of the key to bytes. Encoding (
    :attr:`~cryptography.hazmat.primitives.serialization.Encoding.PEM` or
    :attr:`~cryptography.hazmat.primitives.serialization.Encoding.DER`) and
    format (
    :attr:`~cryptography.hazmat.primitives.serialization.PublicFormat.SubjectPublicKeyInfo`)
    are chosen to define the exact serialization.

    :param encoding: A value from the
        :class:`~cryptography.hazmat.primitives.serialization.Encoding` enum.

    :param format: A value from the
        :class:`~cryptography.hazmat.primitives.serialization.PublicFormat` enum.

    :return bytes: Serialized key.

.. versionadded:: 1.7

Alias for :class:`DHPublicKey`.

Numbers

.. 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 greater.

.. attribute:: q

    .. versionadded:: 1.8

    :type: int

    p subgroup order value.

.. method:: parameters(backend)

    .. versionadded:: 1.7

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.DHBackend`.

    :returns: A new instance of :class:`DHParameters`.

.. 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.

.. method:: private_key(backend)

    .. versionadded:: 1.7

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.DHBackend`.

    :returns: A new instance of :class:`DHPrivateKey`.

.. 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.

.. method:: public_key(backend)

    .. versionadded:: 1.7

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.DHBackend`.

    :returns: A new instance of :class:`DHPublicKey`.