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Fernet (symmetric encryption) ============================= .. currentmodule:: cryptography.fernet Fernet guarantees that a message encrypted using it cannot be manipulated or read without the key. `Fernet`_ is an implementation of symmetric (also known as "secret key") authenticated cryptography. Fernet also has support for implementing key rotation via :class:`MultiFernet`. .. class:: Fernet(key) This class provides both encryption and decryption facilities. .. doctest:: >>> from cryptography.fernet import Fernet >>> key = Fernet.generate_key() >>> f = Fernet(key) >>> token = f.encrypt(b"my deep dark secret") >>> token b'...' >>> f.decrypt(token) b'my deep dark secret' :param key: A URL-safe base64-encoded 32-byte key. This **must** be kept secret. Anyone with this key is able to create and read messages. :type key: bytes or str .. classmethod:: generate_key() Generates a fresh fernet key. Keep this some place safe! If you lose it you'll no longer be able to decrypt messages; if anyone else gains access to it, they'll be able to decrypt all of your messages, and they'll also be able forge arbitrary messages that will be authenticated and decrypted. .. method:: encrypt(data) Encrypts data passed. The result of this encryption is known as a "Fernet token" and has strong privacy and authenticity guarantees. :param bytes data: The message you would like to encrypt. :returns bytes: A secure message that cannot be read or altered without the key. It is URL-safe base64-encoded. This is referred to as a "Fernet token". :raises TypeError: This exception is raised if ``data`` is not ``bytes``. .. note:: The encrypted message contains the current time when it was generated in *plaintext*, the time a message was created will therefore be visible to a possible attacker. .. method:: encrypt_at_time(data, current_time) .. versionadded:: 3.0 Encrypts data passed using explicitly passed current time. See :meth:`encrypt` for the documentation of the ``data`` parameter, the return type and the exceptions raised. The motivation behind this method is for the client code to be able to test token expiration. Since this method can be used in an insecure manner one should make sure the correct time (``int(time.time())``) is passed as ``current_time`` outside testing. :param int current_time: The current time. .. note:: Similarly to :meth:`encrypt` the encrypted message contains the timestamp in *plaintext*, in this case the timestamp is the value of the ``current_time`` parameter. .. method:: decrypt(token, ttl=None) Decrypts a Fernet token. If successfully decrypted you will receive the original plaintext as the result, otherwise an exception will be raised. It is safe to use this data immediately as Fernet verifies that the data has not been tampered with prior to returning it. :param bytes or str token: The Fernet token. This is the result of calling :meth:`encrypt`. :param int ttl: Optionally, the number of seconds old a message may be for it to be valid. If the message is older than ``ttl`` seconds (from the time it was originally created) an exception will be raised. If ``ttl`` is not provided (or is ``None``), the age of the message is not considered. :returns bytes: The original plaintext. :raises cryptography.fernet.InvalidToken: If the ``token`` is in any way invalid, this exception is raised. A token may be invalid for a number of reasons: it is older than the ``ttl``, it is malformed, or it does not have a valid signature. :raises TypeError: This exception is raised if ``token`` is not ``bytes`` or ``str``. .. method:: decrypt_at_time(token, ttl, current_time) .. versionadded:: 3.0 Decrypts a token using explicitly passed current time. See :meth:`decrypt` for the documentation of the ``token`` and ``ttl`` parameters (``ttl`` is required here), the return type and the exceptions raised. The motivation behind this method is for the client code to be able to test token expiration. Since this method can be used in an insecure manner one should make sure the correct time (``int(time.time())``) is passed as ``current_time`` outside testing. :param int current_time: The current time. .. method:: extract_timestamp(token) .. versionadded:: 2.3 Returns the timestamp for the token. The caller can then decide if the token is about to expire and, for example, issue a new token. :param bytes or str token: The Fernet token. This is the result of calling :meth:`encrypt`. :returns int: The Unix timestamp of the token. :raises cryptography.fernet.InvalidToken: If the ``token``'s signature is invalid this exception is raised. :raises TypeError: This exception is raised if ``token`` is not ``bytes`` or ``str``. .. class:: MultiFernet(fernets) .. versionadded:: 0.7 This class implements key rotation for Fernet. It takes a ``list`` of :class:`Fernet` instances and implements the same API with the exception of one additional method: :meth:`MultiFernet.rotate`: .. doctest:: >>> from cryptography.fernet import Fernet, MultiFernet >>> key1 = Fernet(Fernet.generate_key()) >>> key2 = Fernet(Fernet.generate_key()) >>> f = MultiFernet([key1, key2]) >>> token = f.encrypt(b"Secret message!") >>> token b'...' >>> f.decrypt(token) b'Secret message!' MultiFernet performs all encryption options using the *first* key in the ``list`` provided. MultiFernet attempts to decrypt tokens with each key in turn. A :class:`cryptography.fernet.InvalidToken` exception is raised if the correct key is not found in the ``list`` provided. Key rotation makes it easy to replace old keys. You can add your new key at the front of the list to start encrypting new messages, and remove old keys as they are no longer needed. Token rotation as offered by :meth:`MultiFernet.rotate` is a best practice and manner of cryptographic hygiene designed to limit damage in the event of an undetected event and to increase the difficulty of attacks. For example, if an employee who had access to your company's fernet keys leaves, you'll want to generate new fernet key, rotate all of the tokens currently deployed using that new key, and then retire the old fernet key(s) to which the employee had access. .. method:: rotate(msg) .. versionadded:: 2.2 Rotates a token by re-encrypting it under the :class:`MultiFernet` instance's primary key. This preserves the timestamp that was originally saved with the token. If a token has successfully been rotated then the rotated token will be returned. If rotation fails this will raise an exception. .. doctest:: >>> from cryptography.fernet import Fernet, MultiFernet >>> key1 = Fernet(Fernet.generate_key()) >>> key2 = Fernet(Fernet.generate_key()) >>> f = MultiFernet([key1, key2]) >>> token = f.encrypt(b"Secret message!") >>> token b'...' >>> f.decrypt(token) b'Secret message!' >>> key3 = Fernet(Fernet.generate_key()) >>> f2 = MultiFernet([key3, key1, key2]) >>> rotated = f2.rotate(token) >>> f2.decrypt(rotated) b'Secret message!' :param bytes or str msg: The token to re-encrypt. :returns bytes: A secure message that cannot be read or altered without the key. This is URL-safe base64-encoded. This is referred to as a "Fernet token". :raises cryptography.fernet.InvalidToken: If a ``token`` is in any way invalid this exception is raised. :raises TypeError: This exception is raised if the ``msg`` is not ``bytes`` or ``str``. .. class:: InvalidToken See :meth:`Fernet.decrypt` for more information. Using passwords with Fernet --------------------------- It is possible to use passwords with Fernet. To do this, you need to run the password through a key derivation function such as :class:`~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC`, bcrypt or :class:`~cryptography.hazmat.primitives.kdf.scrypt.Scrypt`. .. doctest:: >>> import base64 >>> import os >>> from cryptography.fernet import Fernet >>> from cryptography.hazmat.primitives import hashes >>> from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC >>> password = b"password" >>> salt = os.urandom(16) >>> kdf = PBKDF2HMAC( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... iterations=480000, ... ) >>> key = base64.urlsafe_b64encode(kdf.derive(password)) >>> f = Fernet(key) >>> token = f.encrypt(b"Secret message!") >>> token b'...' >>> f.decrypt(token) b'Secret message!' In this scheme, the salt has to be stored in a retrievable location in order to derive the same key from the password in the future. The iteration count used should be adjusted to be as high as your server can tolerate. A good default is at least 480,000 iterations, which is what `Django recommends as of December 2022`_. Implementation -------------- Fernet is built on top of a number of standard cryptographic primitives. Specifically it uses: * :class:`~cryptography.hazmat.primitives.ciphers.algorithms.AES` in :class:`~cryptography.hazmat.primitives.ciphers.modes.CBC` mode with a 128-bit key for encryption; using :class:`~cryptography.hazmat.primitives.padding.PKCS7` padding. * :class:`~cryptography.hazmat.primitives.hmac.HMAC` using :class:`~cryptography.hazmat.primitives.hashes.SHA256` for authentication. * Initialization vectors are generated using ``os.urandom()``. For complete details consult the `specification`_. Limitations ----------- Fernet is ideal for encrypting data that easily fits in memory. As a design feature it does not expose unauthenticated bytes. This means that the complete message contents must be available in memory, making Fernet generally unsuitable for very large files at this time. .. _`Fernet`: https://github.com/fernet/spec/ .. _`Django recommends as of December 2022`: https://github.com/django/django/blob/main/django/contrib/auth/hashers.py .. _`specification`: https://github.com/fernet/spec/blob/master/Spec.md
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