import random
from Crypto.Util.number import *

class LCG:
    def __init__(self, seed, a, b, m):
        self.state = seed
        self.a = a
        self.b = b
        self.m = m
    def next(self):
        self.state = (self.a * self.state + self.b) % self.m
        return self.state

myseed = random.randint(2,2**31)
lcg = LCG(myseed, 1103515245, 12345, 2**31)

f=file('flag.png','r')
allbytes = f.read()
f.close()

encryptedflag = ''
for i in range(0, len(allbytes), 4):
    pseudorand = lcg.next()
    plaintextchunk = allbytes[i:i+4]
    cipherchunk_as_int = pseudorand ^ bytes_to_long(plaintextchunk)
    encryptedflag += long_to_bytes(cipherchunk_as_int, 4)

f=file('flag.png.enc','w')
f.write(encryptedflag)
f.close()