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File Content: dsa.rst

.. hazmat::

DSA
===

.. module:: 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 :term:`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: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.DSABackend`.

:return: An instance of
        :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateKey`.

: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 :attr:`~DSAParameterNumbers.q`. 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: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.DSABackend`.

:return: An instance of
        :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAParameters`.

: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.asymmetric.dsa.DSAPrivateKey`
instance.

.. 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()
    ... )
    >>> 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, default_backend())
    >>> 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, default_backend())
    >>> 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(backend)

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

: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(backend)

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

: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(backend)

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

: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`.

.. class:: DSAParametersWithNumbers

.. versionadded:: 0.5

Extends :class:`DSAParameters`.

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

.. 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.

.. class:: DSAPrivateKeyWithSerialization

.. versionadded:: 0.8

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

.. 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`
        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:: 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)

: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.

: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`: http://www.ecrypt.eu.org/ecrypt2/documents/D.SPA.20.pdf

Name	Size	Permission
dh.rst	11613 bytes	0644
dsa.rst	13810 bytes	0644
ec.rst	22500 bytes	0644
index.rst	1254 bytes	0644
rsa.rst	23897 bytes	0644
serialization.rst	21721 bytes	0644
utils.rst	2968 bytes	0644
x25519.rst	3352 bytes	0644