Fernet.py

# MIT License
#
# Copyright (c) 2022 Mark Qvist / unsigned.io
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

import os
import time

from RNS.Cryptography import HMAC
from RNS.Cryptography import PKCS7
from RNS.Cryptography.AES import AES_128_CBC

class Fernet():
    """
    This class provides a slightly modified implementation of the Fernet spec
    found at: https://github.com/fernet/spec/blob/master/Spec.md

    According to the spec, a Fernet token includes a one byte VERSION and
    eight byte TIMESTAMP field at the start of each token. These fields are
    not relevant to Reticulum. They are therefore stripped from this
    implementation, since they incur overhead and leak initiator metadata.
    """
    FERNET_OVERHEAD  = 48 # Bytes

    @staticmethod
    def generate_key():
        return os.urandom(32)

    def __init__(self, key = None):
        if key == None:
            raise ValueError("Fernet key cannot be None")

        if len(key) != 32:
            raise ValueError("Fernet key must be 32 bytes, not "+str(len(key)))
            
        self._signing_key = key[:16]
        self._encryption_key = key[16:]


    def verify_hmac(self, token):
        if len(token) <= 32:
            raise ValueError("Cannot verify HMAC on token of only "+str(len(token))+" bytes")
        else:
            received_hmac = token[-32:]
            expected_hmac = HMAC.new(self._signing_key, token[:-32]).digest()

            if received_hmac == expected_hmac:
                return True
            else:
                return False


    def encrypt(self, data = None):
        iv = os.urandom(16)
        current_time = int(time.time())

        if not isinstance(data, bytes):
            raise TypeError("Fernet token plaintext input must be bytes")

        ciphertext = AES_128_CBC.encrypt(
            plaintext = PKCS7.pad(data),
            key = self._encryption_key,
            iv = iv,
        )

        signed_parts = iv+ciphertext

        return signed_parts + HMAC.new(self._signing_key, signed_parts).digest()


    def decrypt(self, token = None):
        if not isinstance(token, bytes):
            raise TypeError("Fernet token must be bytes")

        if not self.verify_hmac(token):
            raise ValueError("Fernet token HMAC was invalid")

        iv = token[:16]
        ciphertext = token[16:-32]

        try:
            plaintext = PKCS7.unpad(
                AES_128_CBC.decrypt(
                    ciphertext,
                    self._encryption_key,
                    iv,
                )
            )

            return plaintext

        except Exception as e:
            raise ValueError("Could not decrypt Fernet token")