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For_More_Tools:
.. hazmat::
DSA
===
.. module:: cryptography.hazmat.primitives.asymmetric.dsa
.. note::
DSA is a **legacy algorithm** and should generally be avoided in favor of
choices like
:doc:`EdDSA using curve25519</hazmat/primitives/asymmetric/ed25519>` or
:doc:`ECDSA</hazmat/primitives/asymmetric/ec>`.
`DSA`_ is a `public-key`_ algorithm for signing messages.
Generation
~~~~~~~~~~
.. function:: generate_private_key(key_size)
.. versionadded:: 0.5
.. versionchanged:: 3.0
Added support for 4096-bit keys for some legacy applications that
continue to use DSA despite the wider cryptographic community's
`ongoing protestations`_.
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 :term:`bits`. It should
be either 1024, 2048, 3072, or 4096. For keys generated in 2015 this
should be `at least 2048`_ (See page 41).
:return: An instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateKey`.
.. function:: generate_parameters(key_size)
.. versionadded:: 0.5
.. versionchanged:: 3.0
Added support for 4096-bit keys for some legacy applications that
continue to use DSA despite the wider cryptographic community's
`ongoing protestations`_.
Generate DSA parameters.
:param int key_size: The length of :attr:`~DSAParameterNumbers.q`. It
should be either 1024, 2048, 3072, or 4096. For keys generated in 2015
this should be `at least 2048`_ (See page 41).
:return: An instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameters`.
Signing
~~~~~~~
Using a :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateKey`
instance.
.. doctest::
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.asymmetric import dsa
>>> private_key = dsa.generate_private_key(
... key_size=1024,
... )
>>> data = b"this is some data I'd like to sign"
>>> signature = private_key.sign(
... data,
... hashes.SHA256()
... )
The ``signature`` is a ``bytes`` object, whose contents is DER encoded as
described in :rfc:`3279`. This can be decoded using
:func:`~cryptography.hazmat.primitives.asymmetric.utils.decode_dss_signature`.
If your data is too large to be passed in a single call, you can hash it
separately and pass that value using
:class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`.
.. doctest::
>>> from cryptography.hazmat.primitives.asymmetric import utils
>>> chosen_hash = hashes.SHA256()
>>> hasher = hashes.Hash(chosen_hash)
>>> hasher.update(b"data & ")
>>> hasher.update(b"more data")
>>> digest = hasher.finalize()
>>> sig = private_key.sign(
... digest,
... utils.Prehashed(chosen_hash)
... )
Verification
~~~~~~~~~~~~
Verification is performed using a
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicKey` instance.
You can get a public key object with
:func:`~cryptography.hazmat.primitives.serialization.load_pem_public_key`,
:func:`~cryptography.hazmat.primitives.serialization.load_der_public_key`,
:meth:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicNumbers.public_key`
, or
:meth:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateKey.public_key`.
.. doctest::
>>> public_key = private_key.public_key()
>>> public_key.verify(
... signature,
... data,
... hashes.SHA256()
... )
``verify()`` takes the signature in the same format as is returned by
``sign()``.
``verify()`` will raise an :class:`~cryptography.exceptions.InvalidSignature`
exception if the signature isn't valid.
If your data is too large to be passed in a single call, you can hash it
separately and pass that value using
:class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`.
.. doctest::
>>> chosen_hash = hashes.SHA256()
>>> hasher = hashes.Hash(chosen_hash)
>>> hasher.update(b"data & ")
>>> hasher.update(b"more data")
>>> digest = hasher.finalize()
>>> public_key.verify(
... sig,
... digest,
... utils.Prehashed(chosen_hash)
... )
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()
:returns: A new instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameters`.
.. 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.asymmetric.dsa.DSAParameterNumbers`
The :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameterNumbers`
associated with the public key.
.. method:: public_key()
:returns: A new instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicKey`.
.. 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.asymmetric.dsa.DSAPublicNumbers`
The :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicNumbers`
associated with the private key.
.. method:: private_key()
:returns: A new instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateKey`.
Key interfaces
~~~~~~~~~~~~~~
.. class:: DSAParameters
.. versionadded:: 0.3
`DSA`_ parameters.
.. method:: generate_private_key()
.. versionadded:: 0.5
Generate a DSA 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.dsa.DSAPrivateKey`.
.. method:: parameter_numbers()
Create a
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameterNumbers`
object.
:returns: A
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameterNumbers`
instance.
.. class:: DSAPrivateKey
.. versionadded:: 0.3
A `DSA`_ private key.
.. method:: public_key()
:return: :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicKey`
An DSA public key object corresponding to the values of the private key.
.. method:: parameters()
:return: :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameters`
The DSAParameters object associated with this private key.
.. attribute:: key_size
:type: int
The bit length of :attr:`~DSAParameterNumbers.q`.
.. method:: sign(data, algorithm)
.. versionadded:: 1.5
.. versionchanged:: 1.6
:class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`
can now be used as an ``algorithm``.
Sign one block of data which can be verified later by others using the
public key.
:param bytes data: The message string to sign.
:param algorithm: An instance of
:class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` or
:class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`
if the ``data`` you want to sign has already been hashed.
:return bytes: Signature.
.. method:: private_numbers()
Create a
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateNumbers`
object.
:returns: A
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateNumbers`
instance.
.. method:: private_bytes(encoding, format, encryption_algorithm)
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.TraditionalOpenSSL`,
:attr:`~cryptography.hazmat.primitives.serialization.PrivateFormat.OpenSSH`
or
: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.
.. class:: DSAPrivateKeyWithSerialization
.. versionadded:: 0.8
Alias for :class:`DSAPrivateKey`.
.. class:: DSAPublicKey
.. versionadded:: 0.3
A `DSA`_ public key.
.. attribute:: key_size
:type: int
The bit length of :attr:`~DSAParameterNumbers.q`.
.. method:: parameters()
:return: :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameters`
The DSAParameters object associated with this public key.
.. method:: public_numbers()
Create a
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicNumbers`
object.
:returns: A
:class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicNumbers`
instance.
.. method:: public_bytes(encoding, format)
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.
.. method:: verify(signature, data, algorithm)
.. versionadded:: 1.5
.. versionchanged:: 1.6
:class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`
can now be used as an ``algorithm``.
Verify one block of data was signed by the private key
associated with this public key.
:param bytes signature: The signature to verify.
:param bytes data: The message string that was signed.
:param algorithm: An instance of
:class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` or
:class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`
if the ``data`` you want to sign has already been hashed.
:raises cryptography.exceptions.InvalidSignature: If the signature does
not validate.
.. class:: DSAPublicKeyWithSerialization
.. versionadded:: 0.8
Alias for :class:`DSAPublicKey`.
.. _`DSA`: https://en.wikipedia.org/wiki/Digital_Signature_Algorithm
.. _`public-key`: https://en.wikipedia.org/wiki/Public-key_cryptography
.. _`FIPS 186-4`: https://csrc.nist.gov/publications/detail/fips/186/4/final
.. _`at least 2048`: https://www.cosic.esat.kuleuven.be/ecrypt/ecrypt2/documents/D.SPA.20.pdf
.. _`ongoing protestations`: https://buttondown.email/cryptography-dispatches/archive/cryptography-dispatches-dsa-is-past-its-prime/