docs/hazmat/primitives/interfaces.rst

.. hazmat::

Interfaces
==========


``cryptography`` uses `Abstract Base Classes`_ as interfaces to describe the
properties and methods of most primitive constructs. Backends may also use
this information to influence their operation. Interfaces should also be used
to document argument and return types.

.. _`Abstract Base Classes`: https://docs.python.org/3/library/abc.html


Symmetric ciphers
-----------------

.. currentmodule:: cryptography.hazmat.primitives.interfaces


.. class:: CipherAlgorithm

    A named symmetric encryption algorithm.

    .. attribute:: name

        :type: str

        The standard name for the mode, for example, "AES", "Camellia", or
        "Blowfish".

    .. attribute:: key_size

        :type: int

        The number of bits in the key being used.


.. class:: BlockCipherAlgorithm

    A block cipher algorithm.

    .. attribute:: block_size

        :type: int

        The number of bits in a block.


Cipher modes
~~~~~~~~~~~~

Interfaces used by the symmetric cipher modes described in
:ref:`Symmetric Encryption Modes <symmetric-encryption-modes>`.

.. class:: Mode

    A named cipher mode.

    .. attribute:: name

        :type: str

        This should be the standard shorthand name for the mode, for example
        Cipher-Block Chaining mode is "CBC".

        The name may be used by a backend to influence the operation of a
        cipher in conjunction with the algorithm's name.

    .. method:: validate_for_algorithm(algorithm)

        :param CipherAlgorithm algorithm:

        Checks that the combination of this mode with the provided algorithm
        meets any necessary invariants. This should raise an exception if they
        are not met.

        For example, the :class:`~cryptography.hazmat.primitives.modes.CBC`
        mode uses this method to check that the provided initialization
        vector's length matches the block size of the algorithm.


.. class:: ModeWithInitializationVector

    A cipher mode with an initialization vector.

    .. attribute:: initialization_vector

        :type: bytes

        Exact requirements of the initialization are described by the
        documentation of individual modes.


.. class:: ModeWithNonce

    A cipher mode with a nonce.

    .. attribute:: nonce

        :type: bytes

        Exact requirements of the nonce are described by the documentation of
        individual modes.

Asymmetric interfaces
---------------------

.. class:: AsymmetricSignatureContext

    .. versionadded:: 0.2

    .. method:: update(data)

        :param bytes data: The data you want to sign.

    .. method:: finalize()

        :return bytes signature: The signature.


.. class:: AsymmetricVerificationContext

    .. versionadded:: 0.2

    .. method:: update(data)

        :param bytes data: The data you wish to verify using the signature.

    .. method:: verify()

        :raises cryptography.exceptions.InvalidSignature: If the signature does
            not validate.


.. class:: AsymmetricPadding

    .. versionadded:: 0.2

    .. attribute:: name

DSA
~~~

In 0.8 the DSA key interfaces were moved to the
:mod:`cryptography.hazmat.primitives.asymmetric.dsa` module.


RSA
~~~

In 0.8 the RSA key interfaces were moved to the
:mod:`cryptography.hazmat.primitives.asymmetric.rsa` module.


Elliptic Curve
~~~~~~~~~~~~~~

In 0.8 the EC key interfaces were moved to the
:mod:`cryptography.hazmat.primitives.asymmetric.ec` module.


Hash algorithms
---------------

.. class:: HashAlgorithm

    .. attribute:: name

        :type: str

        The standard name for the hash algorithm, for example: ``"sha256"`` or
        ``"whirlpool"``.

    .. attribute:: digest_size

        :type: int

        The size of the resulting digest in bytes.

    .. attribute:: block_size

        :type: int

        The internal block size of the hash algorithm in bytes.


.. class:: HashContext

    .. attribute:: algorithm

        A :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm` that
        will be used by this context.

    .. method:: update(data)

        :param bytes data: The data you want to hash.

    .. method:: finalize()

        :return: The final digest as bytes.

    .. method:: copy()

        :return: A :class:`~cryptography.hazmat.primitives.interfaces.HashContext`
             that is a copy of the current context.


Key derivation functions
------------------------

.. class:: KeyDerivationFunction

    .. versionadded:: 0.2

    .. method:: derive(key_material)

        :param bytes key_material: The input key material. Depending on what
                                   key derivation function you are using this
                                   could be either random bytes, or a user
                                   supplied password.
        :return: The new key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This generates and returns a new key from the supplied key material.

    .. method:: verify(key_material, expected_key)

        :param bytes key_material: The input key material. This is the same as
                                   ``key_material`` in :meth:`derive`.
        :param bytes expected_key: The expected result of deriving a new key,
                                   this is the same as the return value of
                                   :meth:`derive`.
        :raises cryptography.exceptions.InvalidKey: This is raised when the
                                                    derived key does not match
                                                    the expected key.
        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
                                                          :meth:`derive` or
                                                          :meth:`verify` is
                                                          called more than
                                                          once.

        This checks whether deriving a new key from the supplied
        ``key_material`` generates the same key as the ``expected_key``, and
        raises an exception if they do not match. This can be used for
        something like checking whether a user's password attempt matches the
        stored derived key.


`Message Authentication Code`_
------------------------------

.. class:: CMACContext

    :class:`CMACContext` has been deprecated in favor of :class:`MACContext`.

    .. versionadded:: 0.4

    .. method:: update(data)

        :param bytes data: The data you want to authenticate.

    .. method:: finalize()

        :return: The message authentication code.

    .. method:: copy()

        :return: A :class:`~cryptography.hazmat.primitives.interfaces.CMACContext`
            that is a copy of the current context.

.. class:: MACContext

    .. versionadded:: 0.7

    .. method:: update(data)

        :param bytes data: The data you want to authenticate.

    .. method:: finalize()

        :return: The message authentication code.

    .. method:: copy()

        :return: A
            :class:`~cryptography.hazmat.primitives.interfaces.MACContext` that
            is a copy of the current context.

    .. method:: verify(signature)

        :param bytes signature: The signature to verify.

        :raises cryptography.exceptions.InvalidSignature: This is raised when
            the provided signature does not match the expected signature.


.. _`CMAC`: https://en.wikipedia.org/wiki/CMAC