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.. hazmat::
DSA
===
.. currentmodule:: cryptography.hazmat.primitives.asymmetric.dsa
`DSA`_ is a `public-key`_ algorithm for signing messages.
Generation
~~~~~~~~~~
.. function:: generate_private_key(key_size, backend)
.. versionadded:: 0.5
Generate a DSA private key from the given key size. This function will
generate a new set of parameters and key in one step.
:param int key_size: The length of the modulus in bits. It should be
either 1024, 2048 or 3072. For keys generated in 2015 this should
be `at least 2048`_ (See page 41). Note that some applications
(such as SSH) have not yet gained support for larger key sizes
specified in FIPS 186-3 and are still restricted to only the
1024-bit keys specified in FIPS 186-2.
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.DSABackend`
provider.
:return: A :class:`~cryptography.hazmat.primitives.interfaces.DSAPrivateKey`
provider.
:raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if
the provided ``backend`` does not implement
:class:`~cryptography.hazmat.backends.interfaces.DSABackend`
.. function:: generate_parameters(key_size, backend)
.. versionadded:: 0.5
Generate DSA parameters using the provided ``backend``.
:param int key_size: The length of the modulus in bits. It should be
either 1024, 2048 or 3072. For keys generated in 2015 this should
be `at least 2048`_ (See page 41). Note that some applications
(such as SSH) have not yet gained support for larger key sizes
specified in FIPS 186-3 and are still restricted to only the
1024-bit keys specified in FIPS 186-2.
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.DSABackend`
provider.
:return: A :class:`~cryptography.hazmat.primitives.interfaces.DSAParameters`
provider.
:raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if
the provided ``backend`` does not implement
:class:`~cryptography.hazmat.backends.interfaces.DSABackend`
Signing
~~~~~~~
Using a :class:`~cryptography.hazmat.primitives.interfaces.DSAPrivateKey`
provider.
.. doctest::
>>> from cryptography.hazmat.backends import default_backend
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.asymmetric import dsa
>>> private_key = dsa.generate_private_key(
... key_size=1024,
... backend=default_backend()
... )
>>> signer = private_key.signer(hashes.SHA256())
>>> data = b"this is some data I'd like to sign"
>>> signer.update(data)
>>> signature = signer.finalize()
The ``signature`` is a ``bytes`` object, whose contents is DER encoded as
described in :rfc:`6979`. This can be decoded using
:func:`~cryptography.hazmat.primitives.asymmetric.utils.decode_rfc6979_signature`.
Verification
~~~~~~~~~~~~
Using a :class:`~cryptography.hazmat.primitives.interfaces.DSAPublicKey`
provider.
.. doctest::
>>> public_key = private_key.public_key()
>>> verifier = public_key.verifier(signature, hashes.SHA256())
>>> verifier.update(data)
>>> verifier.verify()
``verifier()`` takes the signature in the same format as is returned by
``signer.finalize()``.
``verify()`` will raise an :class:`~cryptography.exceptions.InvalidSignature`
exception if the signature isn't valid.
Numbers
~~~~~~~
.. class:: DSAParameterNumbers(p, q, g)
.. versionadded:: 0.5
The collection of integers that make up a set of DSA parameters.
.. attribute:: p
:type: int
The public modulus.
.. attribute:: q
:type: int
The sub-group order.
.. attribute:: g
:type: int
The generator.
.. method:: parameters(backend)
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.DSABackend`
provider.
:returns: A new instance of a
:class:`~cryptography.hazmat.primitives.interfaces.DSAParameters`
provider.
.. class:: DSAPublicNumbers(y, parameter_numbers)
.. versionadded:: 0.5
The collection of integers that make up a DSA public key.
.. attribute:: y
:type: int
The public value ``y``.
.. attribute:: parameter_numbers
:type: :class:`~cryptography.hazmat.primitives.dsa.DSAParameterNumbers`
The :class:`~cryptography.hazmat.primitives.dsa.DSAParameterNumbers`
associated with the public key.
.. method:: public_key(backend)
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.DSABackend`
provider.
:returns: A new instance of a
:class:`~cryptography.hazmat.primitives.interfaces.DSAPublicKey`
provider.
.. class:: DSAPrivateNumbers(x, public_numbers)
.. versionadded:: 0.5
The collection of integers that make up a DSA private key.
.. warning::
Revealing the value of ``x`` will compromise the security of any
cryptographic operations performed.
.. attribute:: x
:type: int
The private value ``x``.
.. attribute:: public_numbers
:type: :class:`~cryptography.hazmat.primitives.dsa.DSAPublicNumbers`
The :class:`~cryptography.hazmat.primitives.dsa.DSAPublicNumbers`
associated with the private key.
.. method:: private_key(backend)
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.DSABackend`
provider.
:returns: A new instance of a
:class:`~cryptography.hazmat.primitives.interfaces.DSAPrivateKey`
provider.
.. _`DSA`: https://en.wikipedia.org/wiki/Digital_Signature_Algorithm
.. _`public-key`: https://en.wikipedia.org/wiki/Public-key_cryptography
.. _`FIPS 186-4`: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
.. _`at least 2048`: http://www.ecrypt.eu.org/documents/D.SPA.20.pdf
|
