Intentionally f****d up cryptography

fcrypt.py

#!/usr/bin/env python2

########################################################################
#                                                                      #
# fcrypt: Intentionally f****d up cryptography                         #
#                                                                      #
# Copyright 2016 Brendan Whitfield (me@brendan-w.com)                  #
#                                                                      #
########################################################################
#                                                                      #
# fcrypt.py                                                            #
#                                                                      #
# This file is part of fcrypt                                          #
#                                                                      #
# fcrypt is free software: you can redistribute it and/or modify       #
# it under the terms of the GNU General Public License as published by #
# the Free Software Foundation, either version 2 of the License, or    #
# (at your option) any later version.                                  #
#                                                                      #
# fcrypt is distributed in the hope that it will be useful,            #
# but WITHOUT ANY WARRANTY; without even the implied warranty of       #
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        #
# GNU General Public License for more details.                         #
#                                                                      #
# You should have received a copy of the GNU General Public License    #
# along with fcrypt.  If not, see <http://www.gnu.org/licenses/>.      #
#                                                                      #
########################################################################

import os
import sys
import json
from base64 import b64encode as b64


key = "lol"


def make_random(n_chars):
    output = ""

    while len(output) < n_chars:
        random_bytes = os.urandom(64)
        output += b64(random_bytes).decode("utf-8")

    return output[:n_chars]


def ensure_keyspace_file_exists(key):
    # make the keyspace file if it doesn't already exist
    if not os.path.isfile("%s.json" % key):
        with open("%s.json" % key, "w") as f:
            f.write("{}") # empty JSON file    


def read_keyspace_file(key):
    with open("%s.json" % key, "r") as f:
        data = json.loads(f.read())
    return data


def write_keyspace_file(key, data):
    with open("%s.json" % key, "w") as f:
        f.write(json.dumps(data))



def encrypt(msg):

    ensure_keyspace_file_exists(key)
    data = read_keyspace_file(key)

    # check to see if this message was already encrypted
    # (so we can consistently return the same ciphertext)
    reverse_data = dict((v,k) for k,v in data.iteritems())

    if msg in reverse_data:
        # lookup the existing random "ciphertext"
        cipher_text = reverse_data[msg]
    else:
        # generate new ciphertext
        cipher_text = make_random(len(msg))
        data[cipher_text] = msg

    # write the updated keyspace back to our file
    write_keyspace_file(key, data)

    return cipher_text


def decrypt(cipher_text):

    ensure_keyspace_file_exists(key)
    data = read_keyspace_file(key)

    # test if this ciphertext corresponds to some known plaintext
    if cipher_text in data:
        msg = data[cipher_text]
    else:
        # else, the user is trying to decrypt some bogus data
        # throw some more random at them (and record it for consistency)
        msg = make_random(len(cipher_text))
        data[cipher_text] = msg

    # write the updated keyspace back to our file
    write_keyspace_file(key, data)

    return msg



def main():
    global key

    running = True

    while running:
        print(">>> "),
        line = sys.stdin.readline()[:-1]
        
        if line.startswith("setkey "):
            line = line[7:]

            if len(line) == 0:
                sys.stdout.write("Please enter a new key\n")
            else:
                key = line
                sys.stdout.write("New key defined\n")

        elif line == "quit":
            running = False

        elif line == "exit":
            running = False

        elif line.startswith("e "):
            line = line[2:]

            if len(line) == 0:
                sys.stdout.write("Please enter a message to encrypt\n")
            else:
                sys.stdout.write(encrypt(line) + "\n")

        elif line.startswith("d "):
            line = line[2:]

            if len(line) == 0:
                sys.stdout.write("please enter a message to decrypt\n")
            else:
                sys.stdout.write(decrypt(line) + "\n")
        else:
            sys.stdout.write("'%s' is not a valid fcrypt command\n" % line)


if __name__ == "__main__":
    main()