import hashlib
import math

# 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.

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

def enc(plaintext, key, iv):
    ciphertext = bytes()
    bs = 256/8
    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()
        ciphertext += sxor(k, plaintext[bs*i:][:bs])

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