docs/hazmat/primitives/symmetric-encryption.rst - platform/external/python/cryptography - Gitiles

 .. 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 (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
         >>> 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.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.

 .. _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
                       ``168`` bits of the key, there is a parity byte in each
                       component of the key), in some materials these are
                       referred to as being up to three separate keys (each
                       ``56`` bits long), they can simply be concatenated to
                       produce the full key. This must be kept secret.

 .. 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, 40-128 bits in length (in increments of
                       8).  This must be kept secret.

 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, 32-448 bits in length (in increments of
                       8).  This must be kept secret.

 .. 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, ``40``, ``56``, ``64``, ``80``, ``128``,
                       ``192``, or ``256`` bits in length.  This must be kept
                       secret.

     .. 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 (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 alongside 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 (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 (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 encrypts the message as well as authenticating it
     (and optionally additional data that is not encrypted) simultaneously.
     Additional means of verifying integrity (like
     :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 (they can be included
                                         in a transmitted message). NIST
                                         `recommends 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 emit 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 and our 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, and thus result in information leakage

     **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 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
                             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 (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`` interface. ``AEADCipherContext`` contains an
     additional method ``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 (e.g.
     :class:`~cryptography.hazmat.primitives.ciphers.modes.GCM`) you will receive
     a return object conforming to the ``AEADEncryptionContext`` interface (as
     well as ``AEADCipherContext``).  This interface provides one additional
     attribute ``tag``. ``tag`` can only be obtained after ``finalize()``.

     .. 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 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
	.. 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 (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
	>>> 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.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.

	.. _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
	``168`` bits of the key, there is a parity byte in each
	component of the key), in some materials these are
	referred to as being up to three separate keys (each
	``56`` bits long), they can simply be concatenated to
	produce the full key. This must be kept secret.

	.. 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, 40-128 bits in length (in increments of
	8). This must be kept secret.

	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, 32-448 bits in length (in increments of
	8). This must be kept secret.

	.. 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, ``40``, ``56``, ``64``, ``80``, ``128``,
	``192``, or ``256`` bits in length. This must be kept
	secret.

	.. 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 (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 alongside 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 (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 (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 encrypts the message as well as authenticating it
	(and optionally additional data that is not encrypted) simultaneously.
	Additional means of verifying integrity (like
	: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 (they can be included
	in a transmitted message). NIST
	`recommends 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 emit 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 and our 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, and thus result in information leakage

	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 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
	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 (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`` interface. ``AEADCipherContext`` contains an
	additional method ``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 (e.g.
	:class:`~cryptography.hazmat.primitives.ciphers.modes.GCM`) you will receive
	a return object conforming to the ``AEADEncryptionContext`` interface (as
	well as ``AEADCipherContext``). This interface provides one additional
	attribute ``tag``. ``tag`` can only be obtained after ``finalize()``.

	.. 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 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