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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.
Block ciphers
-------------
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.aes import AES
>>> from cryptography.primitives.block import BlockCipher, CBC
>>> cipher = BlockCipher(AES(key), CBC(iv))
>>> cipher.encrypt("my secret message") + cipher.finalize()
# The ciphertext
[...]
Here ``key`` is the encryption key (which must be kept secret), and ``iv`` is
the initialization vector (which must be random). Exactly what form these
values should take is described for each of the ciphers and modes.
``encrypt()`` should be called repeatedly with additional plaintext, and it
will return the encrypted bytes, if there isn't enough data, it will buffer it
internally. ``finalize()`` should be called at the end, and will return
whatever data is left.
Ciphers
~~~~~~~
.. class:: cryptography.primitives.aes.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.CBC(initialization_vector)
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). Should be
the same number of bytes as the ``key``
for the cipher.
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