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Fernet Spec

This document describes version 0x80 (currently the only version) of the fernet format.

Conceptually, fernet takes a user-provided message (an arbitrary sequence of bytes), a key (256 bits), and the current time, and produces a token, which contains the message in a form that can't be read or altered without the key.

To facilitate convenient interoperability, this spec defines the external format of both tokens and keys.

All encryption in this version is done with AES 128 in CBC mode.

All base 64 encoding is done with the "URL and Filename Safe" variant, defined in RFC 4648 as "base64url".

Key Format

A fernet key is the base64url encoding of the following fields:

Signing-key ‖ Encryption-key
  • Signing-key, 128 bits
  • Encryption-key, 128 bits

Token Format

A fernet token is the base64url encoding of the concatenation of the following fields:

Version ‖ Timestamp ‖ IV ‖ Ciphertext ‖ HMAC
  • Version, 8 bits
  • Timestamp, 64 bits
  • IV, 128 bits
  • Ciphertext, variable length, multiple of 128 bits
  • HMAC, 256 bits

Fernet tokens are not self-delimiting. It is assumed that the transport will provide a means of finding the length of each complete fernet token.

Token Fields

Version

This field denotes which version of the format is being used by the token. Currently there is only one version defined, with the value 128 (0x80).

Timestamp

This field is a 64-bit unsigned big-endian integer. It records the number of seconds elapsed between January 1, 1970 UTC and the time the token was created.

IV

The 128-bit Initialization Vector used in AES encryption and decryption of the Ciphertext.

When generating new fernet tokens, the IV must be chosen uniquely for every token. With a high-quality source of entropy, random selection will do this with high probability.

Ciphertext

This field has variable size, but is always a multiple of 128 bits, the AES block size. It contains the original input message, padded and encrypted.

HMAC

This field is the 256-bit SHA256 HMAC, under signing-key, of the concatenation of the following fields:

Version ‖ Timestamp ‖ IV ‖ Ciphertext

Note that the HMAC input is the entire rest of the token verbatim, and that this input is not base64url encoded.

Generating

Given a key and message, generate a fernet token with the following steps, in order:

  1. Record the current time for the timestamp field.
  2. Choose a unique IV.
  3. Construct the ciphertext:
    1. Pad the message to a multiple of 16 bytes (128 bits) per RFC 5652, section 6.3. This is the same padding technique used in PKCS #7 v1.5 and all versions of SSL/TLS (cf. RFC 5246, section 6.2.3.2 for TLS 1.2).
    2. Encrypt the padded message using AES 128 in CBC mode with the chosen IV and user-supplied encryption-key.
  4. Compute the HMAC field as described above using the user-supplied signing-key.
  5. Concatenate all fields together in the format above.
  6. base64url encode the entire token.

Verifying

Given a key and token, to verify that the token is valid and recover the original message, perform the following steps, in order:

  1. base64url decode the token.
  2. Ensure the first byte of the token is 0x80.
  3. If the user has specified a maximum age (or "time-to-live") for the token, ensure the recorded timestamp is not too far in the past.
  4. Recompute the HMAC from the other fields and the user-supplied signing-key.
  5. Ensure the recomputed HMAC matches the HMAC field stored in the token, using a constant-time comparison function.
  6. Decrypt the ciphertext field using AES 128 in CBC mode with the recorded IV and user-supplied encryption-key.
  7. Unpad the decrypted plaintext, yielding the original message.