Python C Extension for the ECB Encryption Practical

This library provides some utility functions that can be used to operate on images as a single bytes literal -- a sequence of pixels where each pixel is represented as three bytes (R, G, B).

It allows to define a simple function which operates on a bytes literal and apply the function to the pixels body of the image, generating a valid image as output.

The purpose of this library is to allow to visually demonstrate the effects of different encryption methods on images. For this reason, this library also provides functions to encrypt the body using a few different encryption techniques (stream cipher, ECB encryption).

This is part of the Cyber Security Practicals.

Copyright (c)2016 Alfio Emanuele Fresta

Requirements

• Python 3.4+,
• Linux, OS X or (probably) Windows.

Install

sudo pip3 install git+https://github.com/AlfioEmanueleFresta/practical-ecb-lib.git

or,

git clone https://github.com/AlfioEmanueleFresta/practical-ecb-lib.git
cd practical-ecb-lib/
sudo pip3 install ./

Test

cd practical-ecb-lib/
python3 tests.py

Limitations

• This library is for demonstration purposes only. The encryption function is free to return a different number of bytes (e.g. ECB encryption, which requires padding to a multiple of 16 bytes). When saved, the extra padding bytes will be truncated to make the resulting output a valid image. The decrypter therefore needs to apply padding again, and the union of these two factors will cause the last pixels of the image to be corrupted.

• This library is not efficient for large images. Currently this library uses PIL's Image.putdata which writes the image pixel by pixel, and is as slow as it can possibly get. May be fixed at a later time.

• The stream cipher implementation is weak. Again, demonstration purposes only. The implementation in the library simply uses a one-way hashing function to imitate the behaviour of a stream cipher. Its only purpose is to produce an image which looks like one encrypted using a proper stream cipher implementation.

Usage

This is the original image used for the examples:

examples/tux.png

Stream cipher encryption

from cp_ecb import encrypt_image_file, decrypt_image_file,\
                   get_stream_cipher


cipher = get_stream_cipher(seed="This is my seed")

encrypt_image_file("examples/tux.png", cipher, "otp-encrypted.png")
decrypt_image_file("otp-encrypted.png", cipher, "otp-decrypted.png")

examples/otp.png

ECB encryption

from cp_ecb import encrypt_image_file, decrypt_image_file,\
                   get_ecb_encrypter, get_ecb_decrypter

key = "This is my secret key."

encrypter = get_ecb_encrypter(key)
decrypter = get_ecb_decrypter(key)

encrypt_image_file("examples/tux.png", encrypter, "ecb-encrypted.png")
decrypt_image_file("ecb-encrypted.png", decrypter, "ecb-decrypted.png")

examples/ecb.png

Custom function: Caesar Cipher

from cp_ecb import encrypt_image_file, decrypt_image_file


offset = 128  # The 'key' in Caesar Cipher
caesar = lambda image: [(pixel + offset) % 0xff for pixel in image]

encrypt_image_file("examples/tux.png", caesar, "caesar.png")

examples/caesar.png

Custom function: colours inverter

from cp_ecb import encrypt_image_file, decrypt_image_file


# Define an function which operates on byte literals
inverter = lambda image: [0xff - pixel for pixel in image]  # Invert colours

encrypt_image_file("examples/tux.png", inverter, "inverted.png")

Custom function: Steganography example

from cp_ecb import decrypt_image_file

OFF, ON = 0x00, 0xFF
decrypter = lambda image: [OFF if pixel % 2 else ON for pixel in image]

decrypt_image_file("examples/tux-secret.png", decrypter, "tux-message.png")

Extracting a secret message from the lowest order bits of each colour value in the image.

->

examples/tux-secret.png -> examples/tux-message.png