InCTF 2021 | Gold digger - crypto-writeup-public

#inctf2021

import random
from Crypto.Util.number import *

flag=open("flag","rb").read()

def encrypt(msg, N,x):
    msg, ciphertexts = bin(bytes_to_long(msg))[2:], []
    for i in msg:
        while True:
            r = random.randint(1, N)
            if gcd(r, N) == 1:
                bin_r = bin(r)[2:]
                c = (pow(x, int(bin_r + i, 2), N) * r ** 2) % N
                ciphertexts.append(c)
                break
    return ciphertexts

N = 76412591878589062218268295214588155113848214591159651706606899098148826991765244918845852654692521227796262805383954625826786269714537214851151966113019

x = 72734035256658283650328188108558881627733900313945552572062845397682235996608686482192322284661734065398540319882182671287066089407681557887237904496283

flag = (encrypt(flag,N,x))

open("handout.txt","w").write("ct:"+str(flag)+"\n\nN:"+str(N)+"\n\nx:"+str(x))