docs/hazmat/primitives/symmetric-encryption.rst

.. hazmat:: /fernet


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

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

.. testsetup::

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


Symmetric encryption is a way to `encrypt`_ or hide the contents of material
where the sender and receiver both use the same secret key. Note that symmetric
encryption is **not** sufficient for most applications because it only
provides secrecy but not authenticity. That means an attacker can't see the
message but 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 like
    :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
        >>> from cryptography.hazmat.backends import default_backend
        >>> backend = default_backend()
        >>> 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.backends.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.UnsupportedCipher`
        exception 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.UnsupportedCipher`
        exception will be raised.

.. _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. This must be kept secret. Either ``128``,
        ``192``, or ``256`` bits long.

.. 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. This must be kept secret. Either ``128``,
        ``192``, or ``256`` bits long.

.. 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. This must be kept secret. Either ``64``,
        ``128``, or ``192`` bits long. DES only uses ``56``, ``112``, or ``168``
        bits of the key as there is a parity byte in each component of the key.
        Some writing refers to there being up to three separate keys that are each
        ``56`` bits long, they can simply be concatenated to produce the full key.

.. class:: CAST5(key)

    .. versionadded:: 0.2

    CAST5 (also known as CAST-128) is a block cipher approved for use in the
    Canadian government by the `Communications Security Establishment`_. It is
    a variable key length cipher and supports keys from 40-128 bits in length.

    :param bytes key: The secret key, This must be kept secret. 40 to 128 bits
        in length in increments of 8 bits.

Weak Ciphers
------------

.. warning::

    These ciphers are considered weak for a variety of reasons. New
    applications should avoid their use and existing applications should
    strongly consider migrating away.

.. class:: Blowfish(key)

    Blowfish is a block cipher developed by Bruce Schneier. It is known to be
    susceptible to attacks when using weak keys. The author has recommended
    that users of Blowfish move to newer algorithms such as :class:`AES`.

    :param bytes key: The secret key. This must be kept secret. 32 to 448 bits
        in length in increments of 8 bits.

.. class:: ARC4(key)

    ARC4 (Alleged RC4) is a stream cipher with serious weaknesses in its
    initial stream output. Its use is strongly discouraged. ARC4 does not use
    mode constructions.

    :param bytes key: The secret key. This must be kept secret. Either ``40``,
        ``56``, ``64``, ``80``, ``128``, ``192``, or ``256`` bits in length.

    .. doctest::

        >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
        >>> from cryptography.hazmat.backends import default_backend
        >>> algorithm = algorithms.ARC4(key)
        >>> cipher = Cipher(algorithm, mode=None, backend=default_backend())
        >>> encryptor = cipher.encryptor()
        >>> ct = encryptor.update(b"a secret message")
        >>> decryptor = cipher.decryptor()
        >>> decryptor.update(ct)
        'a secret message'


.. _symmetric-encryption-modes:

Modes
~~~~~

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

.. class:: CBC(initialization_vector)

    CBC (Cipher Block Chaining) is a mode of operation for block ciphers. It is
    considered cryptographically strong.

    **Padding is required when using this mode.**

    :param bytes initialization_vector: Must be random bytes. They do not need
        to be kept secret and they can be included in a transmitted message.
        Must be the same number of bytes as the ``block_size`` of the cipher.
        Each time something is encrypted a new ``initialization_vector`` should
        be generated. Do not reuse an ``initialization_vector`` with a given
        ``key``, and particularly do not use a constant
        ``initialization_vector``.

    A good construction looks like:

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives.ciphers.modes import CBC
        >>> iv = os.urandom(16)
        >>> mode = CBC(iv)

    While the following is bad and will leak information:

    .. doctest::

        >>> from cryptography.hazmat.primitives.ciphers.modes import CBC
        >>> iv = "a" * 16
        >>> mode = CBC(iv)


.. class:: CTR(nonce)

    .. warning::

        Counter mode is not recommended for use with block ciphers that have a
        block size of less than 128-bits.

    CTR (Counter) is a mode of operation for block ciphers. It is considered
    cryptographically strong. It transforms a block cipher into a stream
    cipher.

    **This mode does not require padding.**

    :param bytes nonce: Should be random bytes. It is critical to never reuse a
        ``nonce`` with a given key.  Any reuse of a nonce with the same key
        compromises the security of every message encrypted with that key. Must
        be the same number of bytes as the ``block_size`` of the cipher with a
        given key. The nonce does not need to be kept secret and may be
        included with the ciphertext.

.. class:: OFB(initialization_vector)

    OFB (Output Feedback) is a mode of operation for block ciphers. It
    transforms a block cipher into a stream cipher.

    **This mode does not require padding.**

    :param bytes initialization_vector: Must be random bytes. They do not need
        to be kept secret and they can be included in a transmitted message.
        Must be the same number of bytes as the ``block_size`` of the cipher.
        Do not reuse an ``initialization_vector`` with a given ``key``.

.. class:: CFB(initialization_vector)

    CFB (Cipher Feedback) is a mode of operation for block ciphers. It
    transforms a block cipher into a stream cipher.

    **This mode does not require padding.**

    :param bytes initialization_vector: Must be random bytes. They do not need
        to be kept secret and they can be included in a transmitted message.
        Must be the same number of bytes as the ``block_size`` of the cipher.
        Do not reuse an ``initialization_vector`` with a given ``key``.

.. class:: GCM(initialization_vector, tag=None)

    .. danger::

        When using this mode you **must** not use the decrypted data until
        :meth:`~cryptography.hazmat.primitives.interfaces.CipherContext.finalize`
        has been called. GCM provides **no** guarantees of ciphertext integrity
        until decryption is complete.

    GCM (Galois Counter Mode) is a mode of operation for block ciphers. An
    AEAD (authenticated encryption with additional data) mode is a type of
    block cipher mode that simultaneously encrypts the message as well as
    authenticating it. Additional unencrypted data may also be authenticated.
    Additional means of verifying integrity such as
    :doc:`HMAC </hazmat/primitives/hmac>` are not necessary.

    **This mode does not require padding.**

    :param bytes initialization_vector: Must be random bytes. They do not need
        to be kept secret and they can be included in a transmitted message.
        NIST `recommends a 96-bit IV length`_ for performance critical
        situations but it can be up to 2\ :sup:`64` - 1 bits. Do not reuse an
        ``initialization_vector`` with a given ``key``.

    .. note::

        Cryptography will generate a 128-bit tag when finalizing encryption.
        You can shorten a tag by truncating it to the desired length but this
        is **not recommended** as it lowers the security margins of the
        authentication (`NIST SP-800-38D`_ recommends 96-bits or greater).
        If you must shorten the tag the minimum allowed length is 4 bytes
        (32-bits). Applications **must** verify the tag is the expected length
        to guarantee the expected security margin.

    :param bytes tag: The tag bytes to verify during decryption. When
        encrypting this must be ``None``.

    .. testcode::

        import os

        from cryptography.hazmat.primitives.ciphers import (
            Cipher, algorithms, modes
        )

        def encrypt(key, plaintext, associated_data):
            # Generate a random 96-bit IV.
            iv = os.urandom(12)

            # Construct a AES-GCM Cipher object with the given key and a
            # randomly generated IV.
            encryptor = Cipher(
                algorithms.AES(key),
                modes.GCM(iv),
                backend=default_backend()
            ).encryptor()

            # associated_data will be authenticated but not encrypted,
            # it must also be passed in on decryption.
            encryptor.authenticate_additional_data(associated_data)

            # Encrypt the plaintext and get the associated ciphertext.
            # GCM does not require padding.
            ciphertext = encryptor.update(plaintext) + encryptor.finalize()

            return (iv, ciphertext, encryptor.tag)

        def decrypt(key, associated_data, iv, ciphertext, tag):
            if len(tag) != 16:
                raise ValueError(
                    "tag must be 16 bytes -- truncation not supported"
                )

            # Construct a Cipher object, with the key, iv, and additionally the
            # GCM tag used for authenticating the message.
            decryptor = Cipher(
                algorithms.AES(key),
                modes.GCM(iv, tag),
                backend=default_backend()
            ).decryptor()

            # We put associated_data back in or the tag will fail to verify
            # when we finalize the decryptor.
            decryptor.authenticate_additional_data(associated_data)

            # Decryption gets us the authenticated plaintext.
            # If the tag does not match an InvalidTag exception will be raised.
            return decryptor.update(ciphertext) + decryptor.finalize()

        iv, ciphertext, tag = encrypt(
            key,
            b"a secret message!",
            b"authenticated but not encrypted payload"
        )

        print(decrypt(
            key,
            b"authenticated but not encrypted payload",
            iv,
            ciphertext,
            tag
        ))

    .. testoutput::

        a secret message!


Insecure Modes
--------------

.. warning::

    These modes are insecure. New applications should never make use of them,
    and existing applications should strongly consider migrating away.


.. class:: ECB()

    ECB (Electronic Code Book) is the simplest mode of operation for block
    ciphers. Each block of data is encrypted in the same way. This means
    identical plaintext blocks will always result in identical ciphertext
    blocks, which can leave `significant patterns in the output`_.

    **Padding is required when using this mode.**

Interfaces
----------

.. class:: CipherContext

    When calling ``encryptor()`` or ``decryptor()`` on a ``Cipher`` object
    the result will conform 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 the plaintext or ciphertext always be a multiple
    of their block size. Because of that **padding** is sometimes 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
            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 an
        :class:`~cryptography.exceptions.AlreadyFinalized` exception.

.. class:: AEADCipherContext

    When calling ``encryptor`` or ``decryptor`` on a ``Cipher`` object
    with an AEAD mode (e.g.
    :class:`~cryptography.hazmat.primitives.ciphers.modes.GCM`) the result will
    conform to the ``AEADCipherContext`` and ``CipherContext`` interfaces. If
    it is an encryption context it will additionally be an
    ``AEADEncryptionContext`` provider. ``AEADCipherContext`` contains an
    additional method :meth:`authenticate_additional_data` for adding
    additional authenticated but unencrypted data (see note below). You should
    call this before calls to ``update``. When you are done call `finalize``
    to finish the operation.

    .. note::

        In AEAD modes all data passed to ``update()`` will be both encrypted
        and authenticated. Do not pass encrypted data to the
        ``authenticate_additional_data()`` method. It is meant solely for
        additional data you may want to authenticate but leave unencrypted.

    .. method:: authenticate_additional_data(data)

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

.. class:: AEADEncryptionContext

    When creating an encryption context using ``encryptor`` on a ``Cipher``
    object with an AEAD mode such as
    :class:`~cryptography.hazmat.primitives.ciphers.modes.GCM` an object
    conforming to both the ``AEADEncryptionContext`` and ``AEADCipherContext``
    interfaces will be returned.  This interface provides one
    additional attribute ``tag``. ``tag`` can only be obtained after
    ``finalize`` has been called.

    .. attribute:: tag

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


.. _`described by Colin Percival`: http://www.daemonology.net/blog/2009-06-11-cryptographic-right-answers.html
.. _`recommends a 96-bit IV length`: http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-spec.pdf
.. _`NIST SP-800-38D`: http://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf
.. _`Communications Security Establishment`: http://www.cse-cst.gc.ca
.. _`encrypt`: https://ssd.eff.org/tech/encryption
.. _`CRYPTREC`: http://www.cryptrec.go.jp/english/
.. _`significant patterns in the output`: http://en.wikipedia.org/wiki/Cipher_block_chaining#Electronic_codebook_.28ECB.29