#!/usr/bin/python3

import random
from math import gcd

def encrypt(dt):
    mod = 256
    while True:
        a = random.randint(1,mod)
        if gcd(a, mod) == 1: break
    b = random.randint(1,mod)

    res = b''
    for byte in dt:
        enc = (a*byte + b) % mod
        res += bytes([enc])
    return res

dt = open('letter.pdf', 'rb').read()

res = encrypt(dt)

f = open('encrypted.bin', 'wb')
f.write(res)
f.close()