Fernet (symmetric encryption)

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 MultiFernet.

class cryptography.fernet.Fernet(key)[source]

This class provides both encryption and decryption facilities.

>>> from cryptography.fernet import Fernet
>>> key = Fernet.generate_key()
>>> f = Fernet(key)
>>> token = f.encrypt(b"my deep dark secret")
>>> token
>>> f.decrypt(token)
'my deep dark secret'
Parameters:key (bytes) – A URL-safe base64-encoded 32-byte key. This must be kept secret. Anyone with this key is able to create and read messages.
classmethod generate_key()[source]

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.

Parameters:data (bytes) – 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.


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.

decrypt(token, ttl=None)[source]
  • token (bytes) – The Fernet token. This is the result of calling encrypt().
  • ttl (int) – 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.

  • 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.
  • TypeError – This exception is raised if token is not bytes.
class cryptography.fernet.MultiFernet(fernets)[source]

New in version 0.7.

This class implements key rotation for Fernet. It takes a list of Fernet instances, and implements the same API:

>>> 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
>>> f.decrypt(token)
'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 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.

class cryptography.fernet.InvalidToken[source]

See 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 PBKDF2HMAC, bcrypt or Scrypt.

>>> import base64
>>> import os
>>> from cryptography.fernet import Fernet
>>> from cryptography.hazmat.backends import default_backend
>>> 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=100000,
...     backend=default_backend()
... )
>>> key = base64.urlsafe_b64encode(kdf.derive(password))
>>> f = Fernet(key)
>>> token = f.encrypt(b"Secret message!")
>>> token
>>> f.decrypt(token)
'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 100,000 iterations which is what Django recommended in 2014.


Fernet is built on top of a number of standard cryptographic primitives. Specifically it uses:

  • AES in CBC mode with a 128-bit key for encryption; using PKCS7 padding.
  • HMAC using SHA256 for authentication.
  • Initialization vectors are generated using os.urandom().

For complete details consult the specification.


Fernet is ideal for encrypting data that easily fits in memory. As a design feature it does not expose unauthenticated bytes. Unfortunately, this makes it generally unsuitable for very large files at this time.