encrypt.py

#!/usr/bin/python3
from __future__ import print_function

def encrypt(in_file,out_file,key=[255],blocksize=4096):
  """
  This function takes a file a xor each byte with 255
  It will then write this "anti-file" to the disk
  NOTE: the file is loaded and written in one pass, so its size should not
  exceed the available memory!
  args:
    in_file (str): Name of the input file
    out_file (str): Name of the output file
  NOTE: This encryption is NOT safe and should NOT be used
  to protect your data!
  """
  offset = 0
  with open(in_file,"rb") as fi:
    with open(out_file,"wb") as fo:
      while True:
        # bytearrays are mutable unlike bytes
        content = bytearray(fi.read(blocksize)) # Read chunks of the file
        if len(content) == 0: # Means we are done!
          break
        for i,b in enumerate(content):
          content[i] = b ^ key[(offset+i)%len(key)] # ^ means bitwise XOR
        fo.write(content)
        offset += len(content)
  print("Encrypted",offset,"bytes")

if __name__ == "__main__":
  import sys
  if sys.version_info.major < 3:
    # For input to return a string and not execute the input in Python 2
    input = raw_input
  # If calling the file directly: prompt the files and run it once
  f_in = input("File to encrypt?> ")
  f_out = input("File to write (will be overwritten!)> ")
  key = bytearray(input("Key? (leave empty for 255)> "),"utf-8") or [255]
  encrypt(f_in,f_out,key)