docs/raw/_explicit_https.graffle/data.plist

.. hazmat::


Symmetric Encryption
====================

.. currentmodule:: cryptography.hazmat.primitives.ciphers

.. testsetup::

    import binascii
    key = binascii.unhexlify(b"0" * 32)
    iv = binascii.unhexlify(b"0" * 32)

    from cryptography.hazmat.bindings import default_backend
    backend = default_backend()


Symmetric encryption is a way to encrypt (hide the plaintext value) material
where the sender and receiver both use the same key. Note that symmetric
encryption is **not** sufficient for most applications, because it only
provides secrecy (an attacker can't see the message) but not authenticity (an
attacker can create bogus messages and force the application to decrypt them).
For this reason it is *strongly* recommended to combine encryption with a
message authentication code, such as :doc:`HMAC </hazmat/primitives/hmac>`, in
an "encrypt-then-MAC" formulation as `described by Colin Percival`_.

.. class:: Cipher(algorithm, mode, backend)

    Cipher objects combine an algorithm (such as
    :class:`~cryptography.hazmat.primitives.ciphers.algorithms.AES`) with a
    mode (such as
    :class:`~cryptography.hazmat.primitives.ciphers.modes.CBC` or
    :class:`~cryptography.hazmat.primitives.ciphers.modes.CTR`). A simple
    example of encrypting (and then decrypting) content with AES is:

    .. doctest::

        >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
        >>> cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=backend)
        >>> encryptor = cipher.encryptor()
        >>> ct = encryptor.update(b"a secret message") + encryptor.finalize()
        >>> decryptor = cipher.decryptor()
        >>> decryptor.update(ct) + decryptor.finalize()
        'a secret message'

    :param algorithms: A
        :class:`~cryptography.hazmat.primitives.interfaces.CipherAlgorithm`
        provider such as those described
        :ref:`below <symmetric-encryption-algorithms>`.
    :param mode: A :class:`~cryptography.hazmat.primitives.interfaces.Mode`
        provider such as those described
        :ref:`below <symmetric-encryption-modes>`.
    :param backend: A
        :class:`~cryptography.hazmat.bindings.interfaces.CipherBackend`
        provider.

    .. method:: encryptor()

        :return: An encrypting
            :class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
            provider.

        If the backend doesn't support the requested combination of ``cipher``
        and ``mode`` an :class:`cryptography.exceptions.UnsupportedAlgorithm`
        will be raised.

    .. method:: decryptor()

        :return: A decrypting
            :class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
            provider.

        If the backend doesn't support the requested combination of ``cipher``
        and ``mode`` an :class:`cryptography.exceptions.UnsupportedAlgorithm`
        will be raised.


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

.. class:: CipherContext

    When calling ``encryptor()`` or ``decryptor()`` on a ``Cipher`` object
    you will receive a return object conforming to the ``CipherContext``
    interface. You can then call ``update(data)`` with data until you have fed
    everything into the context. Once that is done call ``finalize()`` to
    finish the operation and obtain the remainder of the data.

    Block ciphers require that plaintext or ciphertext always be a multiple of
    their block size, because of that **padding** is often required to make a
    message the correct size. ``CipherContext`` will not automatically apply
    any padding; you'll need to add your own. For block ciphers the recommended
    padding is :class:`cryptography.hazmat.primitives.padding.PKCS7`. If you
    are using a stream cipher mode (such as
    :class:`cryptography.hazmat.primitives.modes.CTR`) you don't have to worry
    about this.

    .. method:: update(data)

        :param bytes data: The data you wish to pass into the context.
        :return bytes: Returns the data that was encrypted or decrypted.
        :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`

        When the ``Cipher`` was constructed in a mode that turns it into a
        stream cipher (e.g.
        :class:`cryptography.hazmat.primitives.ciphers.modes.CTR`), this will
        return bytes immediately, however in other modes it will return chunks,
        whose size is determined by the cipher's block size.

    .. method:: finalize()

        :return bytes: Returns the remainder of the data.
        :raises ValueError: This is raised when the data provided isn't
                            correctly padded to be a multiple of the
                            algorithm's block size.

        Once ``finalize`` is called this object can no longer be used and
        :meth:`update` and :meth:`finalize` will raise
        :class:`~cryptography.exceptions.AlreadyFinalized`.

.. class:: AEADCipherContext

    When calling ``encryptor()`` or ``decryptor()`` on a ``Cipher`` object
    with an AEAD mode you will receive a return object conforming to the
    ``AEADCipherContext`` interface, in addition to the ``CipherContext``
    interface. ``AEADCipherContext`` contains an additional method
    ``authenticate_additional_data`` for adding additional authenticated but
    unencrypted data. You should call this before calls to ``update``. When you
    are done call ``finalize()`` to finish the operation. Once this is complete
    you can obtain the tag value from the ``tag`` property.

    .. method:: authenticate_additional_data(data)

        :param bytes data: The data you wish to authenticate but not encrypt.
        :raises: :class:`~cryptography.exceptions.AlreadyFinalized`

    .. attribute:: tag

        :return bytes: Returns the tag value as bytes.
        :raises: :class:`~cryptography.exceptions.NotYetFinalized` if called
                 before the context is finalized.

.. _symmetric-encryption-algorithms:

Algorithms
~~~~~~~~~~

.. currentmodule:: cryptography.hazmat.primitives.ciphers.algorithms

.. class:: AES(key)

    AES (Advanced Encryption Standard) is a block cipher standardized by NIST.
    AES is both fast, and cryptographically strong. It is a good default
    choice for encryption.

    :param bytes key: The secret key, either ``128``, ``192``, or ``256`` bits.
                      This must be kept secret.

.. class:: Camellia(key)

    Camellia is a block cipher approved for use by CRYPTREC and ISO/IEC.
    It is considered to have comparable security and performance to AES, but
    is not as widely studied or deployed.

    :param bytes key: The secret key, either ``128``, ``192``, or ``256`` bits.
                      This must be kept secret.


.. class:: TripleDES(key)

    Triple DES (Data Encryption Standard), sometimes referred to as 3DES, is a
    block cipher standardized by NIST. Triple DES has known crypto-analytic
    flaws, however none of them currently enable a practical attack.
    Nonetheless, Triples DES is not recommended for new applications because it
    is incredibly slow; old applications should consider moving away from it.

    :param bytes key: The secret key, either ``64``, ``128``, or ``192`` bits
                      (note that DES functionally uses ``56``, ``112``, or