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Symmetric Encryption
====================
Symmetric encryption is a way to encrypt (hide the plaintext value) material
where the encrypter and decrypter both use the same key.
.. class:: cryptography.primitives.block.BlockCipher(cipher, mode)
Block ciphers work by encrypting content in chunks, often 64- or 128-bits.
They combine an underlying algorithm (such as AES), with a mode (such as
CBC, CTR, or GCM). A simple example of encrypting content with AES is:
.. code-block:: pycon
>>> from cryptography.primitives.block import BlockCipher, cipher, mode, padding
>>> cipher = BlockCipher(cipher.AES(key), mode.CBC(iv, padding.PKCS7()))
>>> cipher.encrypt("my secret message") + cipher.finalize()
# The ciphertext
[...]
:param cipher: One of the ciphers described below.
:param mode: One of the modes described below.
``encrypt()`` should be called repeatedly with new plaintext, and once the
full plaintext is fed in, ``finalize()`` should be called.
.. method:: encrypt(plaintext)
:param bytes plaintext: The text you wish to encrypt.
:return bytes: Returns the ciphertext that was added.
.. method:: finalize()
:return bytes: Returns the remainder of the ciphertext.
Ciphers
~~~~~~~
.. class:: cryptography.primitives.block.cipher.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.
Modes
~~~~~
.. class:: cryptography.primitives.block.mode.CBC(initialization_vector, padding)
CBC (Cipher block chaining) is a mode of operation for block ciphers. It is
considered cryptographically strong.
: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``.
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