Edit File: symmetric-encryption.rst
.. hazmat:: /fernet Symmetric encryption ==================== .. module:: cryptography.hazmat.primitives.ciphers 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/mac/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:: >>> import os >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes >>> from cryptography.hazmat.backends import default_backend >>> backend = default_backend() >>> key = os.urandom(32) >>> iv = os.urandom(16) >>> 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.ciphers.CipherAlgorithm` instance such as those described :ref:`below <symmetric-encryption-algorithms>`. :param mode: A :class:`~cryptography.hazmat.primitives.ciphers.modes.Mode` instance such as those described :ref:`below <symmetric-encryption-modes>`. :param backend: A :class:`~cryptography.hazmat.backends.interfaces.CipherBackend` instance. :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if the provided ``backend`` does not implement :class:`~cryptography.hazmat.backends.interfaces.CipherBackend` .. method:: encryptor() :return: An encrypting :class:`~cryptography.hazmat.primitives.ciphers.CipherContext` instance. If the backend doesn't support the requested combination of ``cipher`` and ``mode`` an :class:`~cryptography.exceptions.UnsupportedAlgorithm` exception will be raised. .. method:: decryptor() :return: A decrypting :class:`~cryptography.hazmat.primitives.ciphers.CipherContext` instance. If the backend doesn't support the requested combination of ``cipher`` and ``mode`` an :class:`~cryptography.exceptions.UnsupportedAlgorithm` 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, Triple 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. .. class:: SEED(key) .. versionadded:: 0.4 SEED is a block cipher developed by the Korea Information Security Agency (KISA). It is defined in :rfc:`4269` and is used broadly throughout South Korean industry, but rarely found elsewhere. :param bytes key: The secret key. This must be kept secret. ``128`` bits in length. 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' .. class:: IDEA(key) IDEA (`International Data Encryption Algorithm`_) is a block cipher created in 1991. It is an optional component of the `OpenPGP`_ standard. This cipher is susceptible to attacks when using weak keys. It is recommended that you do not use this cipher for new applications. :param bytes key: The secret key. This must be kept secret. ``128`` bits in length. .. _symmetric-encryption-modes: Modes ~~~~~ .. module:: 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 :doc:`random bytes </random-numbers>`. 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 unique, a :term:`nonce`. 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 :doc:`random bytes </random-numbers>`. 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 :doc:`random bytes </random-numbers>`. 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:: CFB8(initialization_vector) CFB (Cipher Feedback) is a mode of operation for block ciphers. It transforms a block cipher into a stream cipher. The CFB8 variant uses an 8-bit shift register. **This mode does not require padding.** :param bytes initialization_vector: Must be :doc:`random bytes </random-numbers>`. 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, min_tag_length=16) .. danger:: When using this mode you **must** not use the decrypted data until :meth:`~cryptography.hazmat.primitives.ciphers.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/mac/hmac>` are not necessary. **This mode does not require padding.** :param bytes initialization_vector: Must be unique, a :term:`nonce`. 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). Applications wishing to allow truncation must pass the ``min_tag_length`` parameter. .. versionchanged:: 0.5 The ``min_tag_length`` parameter was added in ``0.5``, previously truncation down to ``4`` bytes was always allowed. :param bytes tag: The tag bytes to verify during decryption. When encrypting this must be ``None``. :param bytes min_tag_length: The minimum length ``tag`` must be. By default this is ``16``, meaning tag truncation is not allowed. Allowing tag truncation is strongly discouraged for most applications. :raises ValueError: This is raised if ``len(tag) < min_tag_length``. An example of securely encrypting and decrypting data with ``AES`` in the ``GCM`` mode looks like: .. 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 an 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): # 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 ---------- .. currentmodule:: cryptography.hazmat.primitives.ciphers .. 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.ciphers.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:: update_into(data, buf) .. versionadded:: 1.8 .. warning:: This method allows you to avoid a memory copy by passing a writable buffer and reading the resulting data. You are responsible for correctly sizing the buffer and properly handling the data. This method should only be used when extremely high performance is a requirement and you will be making many small calls to ``update_into``. :param bytes data: The data you wish to pass into the context. :param buf: A writable Python buffer that the data will be written into. This buffer should be ``len(data) + n - 1`` bytes where ``n`` is the block size (in bytes) of the cipher and mode combination. :return int: Number of bytes written. :raises NotImplementedError: This is raised if the version of ``cffi`` used is too old (this can happen on older PyPy releases). :raises ValueError: This is raised if the supplied buffer is too small. .. doctest:: >>> import os >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes >>> from cryptography.hazmat.backends import default_backend >>> backend = default_backend() >>> key = os.urandom(32) >>> iv = os.urandom(16) >>> cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=backend) >>> encryptor = cipher.encryptor() >>> # the buffer needs to be at least len(data) + n - 1 where n is cipher/mode block size in bytes >>> buf = bytearray(31) >>> len_encrypted = encryptor.update_into(b"a secret message", buf) >>> # get the ciphertext from the buffer reading only the bytes written to it (len_encrypted) >>> ct = bytes(buf[:len_encrypted]) + encryptor.finalize() >>> decryptor = cipher.decryptor() >>> len_decrypted = decryptor.update_into(ct, buf) >>> # get the plaintext from the buffer reading only the bytes written (len_decrypted) >>> bytes(buf[:len_decrypted]) + decryptor.finalize() 'a secret message' .. 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`` instance. ``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. .. class:: CipherAlgorithm A named symmetric encryption algorithm. .. attribute:: name :type: str The standard name for the mode, for example, "AES", "Camellia", or "Blowfish". .. attribute:: key_size :type: int The number of bits in the key being used. .. class:: BlockCipherAlgorithm A block cipher algorithm. .. attribute:: block_size :type: int The number of bits in a block. Interfaces used by the symmetric cipher modes described in :ref:`Symmetric Encryption Modes <symmetric-encryption-modes>`. .. currentmodule:: cryptography.hazmat.primitives.ciphers.modes .. class:: Mode A named cipher mode. .. attribute:: name :type: str This should be the standard shorthand name for the mode, for example Cipher-Block Chaining mode is "CBC". The name may be used by a backend to influence the operation of a cipher in conjunction with the algorithm's name. .. method:: validate_for_algorithm(algorithm) :param cryptography.hazmat.primitives.ciphers.CipherAlgorithm algorithm: Checks that the combination of this mode with the provided algorithm meets any necessary invariants. This should raise an exception if they are not met. For example, the :class:`~cryptography.hazmat.primitives.ciphers.modes.CBC` mode uses this method to check that the provided initialization vector's length matches the block size of the algorithm. .. class:: ModeWithInitializationVector A cipher mode with an initialization vector. .. attribute:: initialization_vector :type: bytes Exact requirements of the initialization are described by the documentation of individual modes. .. class:: ModeWithNonce A cipher mode with a nonce. .. attribute:: nonce :type: bytes Exact requirements of the nonce are described by the documentation of individual modes. .. class:: ModeWithAuthenticationTag A cipher mode with an authentication tag. .. attribute:: tag :type: bytes Exact requirements of the tag are described by the documentation of individual modes. .. _`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://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38D.pdf .. _`Communications Security Establishment`: https://www.cse-cst.gc.ca .. _`encrypt`: https://ssd.eff.org/en/module/what-encryption .. _`CRYPTREC`: https://www.cryptrec.go.jp/english/ .. _`significant patterns in the output`: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_Codebook_.28ECB.29 .. _`International Data Encryption Algorithm`: https://en.wikipedia.org/wiki/International_Data_Encryption_Algorithm .. _`OpenPGP`: http://openpgp.org