.. hazmat::

Backend Interfaces
==================

.. currentmodule:: cryptography.hazmat.backends.interfaces


Backend implementations may provide a number of interfaces to support operations
such as :doc:`/hazmat/primitives/symmetric-encryption`,
:doc:`/hazmat/primitives/cryptographic-hashes`, and
:doc:`/hazmat/primitives/hmac`.

A specific ``backend`` may provide one or more of these interfaces.


.. class:: CipherBackend

    A backend which provides methods for using ciphers for encryption
    and decryption.

    .. method:: cipher_supported(cipher, mode)

        Check if a ``cipher`` and ``mode`` combination is supported by
        this backend.

        :param cipher: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.CipherAlgorithm`
            provider.
        :param mode: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.Mode` provider.

        :returns: ``True`` if the specified ``cipher`` and ``mode`` combination
            is supported by this backend, otherwise ``False``


    .. method:: create_symmetric_encryption_ctx(cipher, mode)

        Create a
        :class:`~cryptography.hazmat.primitives.interfaces.CipherContext` that
        can be used for encrypting data with the symmetric ``cipher`` using
        the given ``mode``.

        :param cipher: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.CipherAlgorithm`
            provider.
        :param mode: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.Mode` provider.

        :returns:
            :class:`~cryptography.hazmat.primitives.interfaces.CipherContext`

        :raises ValueError: When tag is not None in an AEAD mode


    .. method:: create_symmetric_decryption_ctx(cipher, mode)

        Create a
        :class:`~cryptography.hazmat.primitives.interfaces.CipherContext` that
        can be used for decrypting data with the symmetric ``cipher`` using
        the given ``mode``.

        :param cipher: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.CipherAlgorithm`
            provider.
        :param mode: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.Mode` provider.

        :returns:
            :class:`~cryptography.hazmat.primitives.interfaces.CipherContext`

        :raises ValueError: When tag is None in an AEAD mode


.. class:: HashBackend

    A backend with methods for using cryptographic hash functions.

    .. method:: hash_supported(algorithm)

        Check if the specified ``algorithm`` is supported by this backend.

        :param algorithm: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
            provider.

        :returns: ``True`` if the specified ``algorithm`` is supported by this
            backend, otherwise ``False``.


    .. method:: create_hash_ctx(algorithm)

        Create a
        :class:`~cryptography.hazmat.primitives.interfaces.HashContext` that
        uses the specified ``algorithm`` to calculate a message digest.

        :param algorithm: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
            provider.

        :returns:
            :class:`~cryptography.hazmat.primitives.interfaces.HashContext`


.. class:: HMACBackend

    A backend with methods for using cryptographic hash functions as message
    authentication codes.

    .. method:: hmac_supported(algorithm)

        Check if the specified ``algorithm`` is supported by this backend.

        :param algorithm: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
            provider.

        :returns: ``True`` if the specified ``algorithm`` is supported for HMAC
            by this backend, otherwise ``False``.

    .. method:: create_hmac_ctx(algorithm)

        Create a
        :class:`~cryptography.hazmat.primitives.interfaces.HashContext` that
        uses the specified ``algorithm`` to calculate a hash-based message
        authentication code.

        :param algorithm: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
            provider.

        :returns:
            :class:`~cryptography.hazmat.primitives.interfaces.HashContext`


.. class:: PBKDF2HMACBackend

    .. versionadded:: 0.2

    A backend with methods for using PBKDF2 using HMAC as a PRF.

    .. method:: pbkdf2_hmac_supported(algorithm)

        Check if the specified ``algorithm`` is supported by this backend.

        :param algorithm: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
            provider.

        :returns: ``True`` if the specified ``algorithm`` is supported for
            PBKDF2 HMAC by this backend, otherwise ``False``.

    .. method:: derive_pbkdf2_hmac(self, algorithm, length, salt, iterations,
                                   key_material)

        :param algorithm: An instance of a
            :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
            provider.

        :param int length: The desired length of the derived key. Maximum is
            (2\ :sup:`32` - 1) * ``algorithm.digest_size``

        :param bytes salt: A salt.

        :param int iterations: The number of iterations to perform of the hash
            function. This can be used to control the length of time the
            operation takes. Higher numbers help mitigate brute force attacks
            against derived keys.

        :param bytes key_material: The key material to use as a basis for
            the derived key. This is typically a password.

        :return bytes: Derived key.