shark/shark.py

import hashlib
import math
import os
import random

# Shark is a sha256+xor based encryption.
# I made it because I want to try to break it.
# (Precisely: Show it does not provide semantic security, because it is not IND-CPA-secure)
# This will work iff I succeed in building a PPT-discriminator for sha256 from randomness
# As my first approach this discriminator will be based on an LSTM-network.

bs = int(256/8)

def xor(ta,tb):
    return bytes(a ^ b for a, b in zip(ta, tb))

def genIV():
    return random.randint(0, 2**(bs-1)).to_bytes(bs, byteorder='big')

def enc(plaintext, key, iv):
    ciphertext = bytes()
    for i in range(math.ceil(len(plaintext)/bs)):
        m = hashlib.sha256()
        m.update(xor(key, iv + i.to_bytes(bs, byteorder='big')))
        k = m.digest()
        iv = (int.from_bytes(iv, byteorder='big')+1).to_bytes(bs, byteorder='big')
        ciphertext += xor(k, plaintext[bs*i:][:bs].ljust(bs, b'0'))
    return ciphertext

def dec(ciphertext, key, iv):
    return enc(ciphertext, key, iv)

def getSample(length, src=None, key=b'VerySecureKeyMustKeepSecretDontTellAnyone'):
    if src==None:
        src = random.random() > 0.5
    if not src:
        r = os.urandom(length*bs)
        return (r, 0)
    else:
        iv = genIV()
        b = bytes(length*bs)
        return (enc(b, key, iv), 1)