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. In many contexts, a lack of authentication on encrypted messages can result in a loss of secrecy as well.

For this reason in nearly all contexts it is necessary 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. cryptography includes a recipe named :doc:`/fernet` that does this for you. To minimize the risk of security issues you should evaluate Fernet to see if it fits your needs before implementing anything using this module. If :doc:`/fernet` is not appropriate for your use-case then you may still benefit from :doc:`/hazmat/primitives/aead` which combines encryption and authentication securely.

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:

>>> import os
>>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
>>> key = os.urandom(32)
>>> iv = os.urandom(16)
>>> cipher = Cipher(algorithms.AES(key), modes.CBC(iv))
>>> encryptor = cipher.encryptor()
>>> ct = encryptor.update(b"a secret message") + encryptor.finalize()
>>> decryptor = cipher.decryptor()
>>> decryptor.update(ct) + decryptor.finalize()
b'a secret message'

param algorithm:	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>`.
raises cryptography.exceptions.UnsupportedAlgorithm:
	This is raised if the provided algorithm is unsupported.

.. method:: encryptor()

    :return: An encrypting
        :class:`~cryptography.hazmat.primitives.ciphers.CipherContext`
        instance.

    If the requested combination of ``algorithm`` and ``mode`` is
    unsupported an :class:`~cryptography.exceptions.UnsupportedAlgorithm`
    exception will be raised.

.. method:: decryptor()

    :return: A decrypting
        :class:`~cryptography.hazmat.primitives.ciphers.CipherContext`
        instance.

    If the requested combination of ``algorithm`` and ``mode`` is
    unsupported an :class:`~cryptography.exceptions.UnsupportedAlgorithm`
    exception will be raised.

Algorithms

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

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 key:	The secret key. This must be kept secret. Either 128, 192, or 256 :term:`bits` long.
type key:	:term:`bytes-like`

.. versionadded:: 38.0.0

An AES class that only accepts 128 bit keys. This is identical to the standard AES class except that it will only accept a single key length.

param key:	The secret key. This must be kept secret. 128 :term:`bits` long.
type key:	:term:`bytes-like`

.. versionadded:: 38.0.0

An AES class that only accepts 256 bit keys. This is identical to the standard AES class except that it will only accept a single key length.

param key:	The secret key. This must be kept secret. 256 :term:`bits` long.
type key:	:term:`bytes-like`

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 key:	The secret key. This must be kept secret. Either 128, 192, or 256 :term:`bits` long.
type key:	:term:`bytes-like`

.. versionadded:: 2.1

Note

In most cases users should use :class:`~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305` instead of this class. ChaCha20 alone does not provide integrity so it must be combined with a MAC to be secure. :class:`~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305` does this for you.

ChaCha20 is a stream cipher used in several IETF protocols. It is standardized in RFC 7539.

param key:	The secret key. This must be kept secret. 256 :term:`bits` (32 bytes) in length.
type key:	:term:`bytes-like`
param 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. The nonce does not need to be kept secret and may be included with the ciphertext. This must be 128 :term:`bits` in length. The 128-bit value is a concatenation of 4-byte little-endian counter and the 12-byte nonce (as described in RFC 7539).
type nonce:	:term:`bytes-like` Note In RFC 7539 the nonce is defined as a 96-bit value that is later concatenated with a block counter (encoded as a 32-bit little-endian). If you have a separate nonce and block counter you will need to concatenate it yourself before passing it. For example, if you have an initial block counter of 2 and a 96-bit nonce the concatenated nonce would be struct.pack("<i", 2) + nonce.

>>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
>>> nonce = os.urandom(16)
>>> algorithm = algorithms.ChaCha20(key, nonce)
>>> cipher = Cipher(algorithm, mode=None)
>>> encryptor = cipher.encryptor()
>>> ct = encryptor.update(b"a secret message")
>>> decryptor = cipher.decryptor()
>>> decryptor.update(ct)
b'a secret message'

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 key:	The secret key. This must be kept secret. Either 64, 128, or 192 :term:`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.
type key:	:term:`bytes-like`

.. 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 :term:`bits` in length.

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

.. 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 key:	The secret key. This must be kept secret. 128 :term:`bits` in length.
type key:	:term:`bytes-like`

.. versionadded:: 35.0.0

SM4 is a block cipher developed by the Chinese Government and standardized in the GB/T 32907-2016. It is used in the Chinese WAPI (Wired Authentication and Privacy Infrastructure) standard. (An English description is available at draft-ribose-cfrg-sm4-10.) This block cipher should be used for compatibility purposes where required and is not otherwise recommended for use.

param key:	The secret key. This must be kept secret. 128 :term:`bits` in length.
type key:	:term:`bytes-like`

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.

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 key:	The secret key. This must be kept secret. 32 to 448 :term:`bits` in length in increments of 8 bits.
type key:	:term:`bytes-like`

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

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

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 key:	The secret key. This must be kept secret. 128 :term:`bits` in length.
type key:	:term:`bytes-like`

Modes

.. module:: cryptography.hazmat.primitives.ciphers.modes

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 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.
type initialization_vector:	:term:`bytes-like`

A good construction looks like:

>>> 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:

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

Warning

Counter mode is not recommended for use with block ciphers that have a block size of less than 128-:term:`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 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.
type nonce:	:term:`bytes-like`

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 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.
type initialization_vector:	:term:`bytes-like`

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 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.
type initialization_vector:	:term:`bytes-like`

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 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.
type initialization_vector:	:term:`bytes-like`

!DANGER!

If you are encrypting data that can fit into memory you should strongly consider using :class:`~cryptography.hazmat.primitives.ciphers.aead.AESGCM` instead of this.

When using this mode you must not use the decrypted data until the appropriate finalization method (:meth:`~cryptography.hazmat.primitives.ciphers.CipherContext.finalize` or :meth:`~cryptography.hazmat.primitives.ciphers.AEADDecryptionContext.finalize_with_tag`) 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 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 264 - 1 :term:`bits`. Do not reuse an initialization_vector with a given key.
type initialization_vector:	:term:`bytes-like`

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 makes it easier to forge messages, and also potentially leaks the key (NIST SP-800-38D recommends 96-:term:`bits` or greater). Applications wishing to allow truncation can 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. When decrypting, it may be None if the tag is supplied on finalization using :meth:`~cryptography.hazmat.primitives.ciphers.AEADDecryptionContext.finalize_with_tag`. Otherwise, the tag is mandatory.
param int 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 or the initialization_vector is too short.

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),
        ).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),
        ).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::

    b'a secret message!'

.. versionadded:: 2.1

Warning

XTS mode is meant for disk encryption and should not be used in other contexts. cryptography only supports XTS mode with :class:`~cryptography.hazmat.primitives.ciphers.algorithms.AES`.

Note

AES XTS keys are double length. This means that to do AES-128 encryption in XTS mode you need a 256-bit key. Similarly, AES-256 requires passing a 512-bit key. AES 192 is not supported in XTS mode.

XTS (XEX-based tweaked-codebook mode with ciphertext stealing) is a mode of operation for the AES block cipher that is used for disk encryption.

This mode does not require padding.

param tweak:	The tweak is a 16 byte value typically derived from something like the disk sector number. A given (tweak, key) pair should not be reused, although doing so is less catastrophic than in CTR mode.
type tweak:	:term:`bytes-like`

Insecure modes

Warning

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

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

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 data: The data you wish to pass into the context.
    :type data: :term:`bytes-like`
    :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 data: The data you wish to pass into the context.
    :type data: :term:`bytes-like`
    :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 ValueError: This is raised if the supplied buffer is too small.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
        >>> key = os.urandom(32)
        >>> iv = os.urandom(16)
        >>> cipher = Cipher(algorithms.AES(key), modes.CBC(iv))
        >>> 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()
        b'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.

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 or decryption context it will additionally be an AEADEncryptionContext or AEADDecryptionContext instance, respectively. 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 data: Any data you wish to authenticate but not encrypt.
    :type data: :term:`bytes-like`
    :raises: :class:`~cryptography.exceptions.AlreadyFinalized`

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.

.. versionadded:: 1.9

When creating an encryption context using decryptor on a Cipher object with an AEAD mode such as :class:`~cryptography.hazmat.primitives.ciphers.modes.GCM` an object conforming to both the AEADDecryptionContext and AEADCipherContext interfaces will be returned. This interface provides one additional method :meth:`finalize_with_tag` that allows passing the authentication tag for validation after the ciphertext has been decrypted.

.. method:: finalize_with_tag(tag)

    :param bytes tag: The tag bytes to verify after decryption.
    :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, if ``min_tag_length`` is
        less than 4, or if ``len(tag) < min_tag_length``.
        ``min_tag_length`` is an argument to the ``GCM`` constructor.

    If the authentication tag was not already supplied to the constructor
    of the :class:`~cryptography.hazmat.primitives.ciphers.modes.GCM` mode
    object, this method must be used instead of
    :meth:`~cryptography.hazmat.primitives.ciphers.CipherContext.finalize`.

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 :term:`bits` in the key being used.

A block cipher algorithm.

.. attribute:: block_size

    :type: int

    The number of :term:`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

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".

.. 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.

A cipher mode with an initialization vector.

.. attribute:: initialization_vector

    :type: :term:`bytes-like`

    Exact requirements of the initialization are described by the
    documentation of individual modes.

A cipher mode with a nonce.

.. attribute:: nonce

    :type: :term:`bytes-like`

    Exact requirements of the nonce are described by the documentation of
    individual modes.

A cipher mode with an authentication tag.

.. attribute:: tag

    :type: :term:`bytes-like`

    Exact requirements of the tag are described by the documentation of
    individual modes.

.. versionadded:: 2.1

A cipher mode with a tweak.

.. attribute:: tweak

    :type: :term:`bytes-like`

    Exact requirements of the tweak are described by the documentation of
    individual modes.

Exceptions

.. currentmodule:: cryptography.exceptions

This is raised if an authenticated encryption tag fails to verify during decryption.