cscg20-cry1

CSCG 2020 Challenge 'Intro to Crypto 1'
git clone https://git.sinitax.com/sinitax/cscg20-cry1
Log | Files | Refs | sfeed.txt

solve.py (573B)

      1#!/usr/bin/env python3
      2
      3from Crypto.PublicKey import RSA
      4from Crypto.Util.number import long_to_bytes
      5from math import isqrt, inf
      6from tqdm import tqdm
      7
      8pubkey = RSA.importKey(open("pubkey.pem").read())
      9
     10# Small primes can be found with naive approach.
     11
     12for i in tqdm(range(3, isqrt(pubkey.n), 2), total=inf):
     13    if pubkey.n % i == 0:
     14        p = i
     15        break
     16else:
     17    raise
     18
     19q = pubkey.n // p
     20phi = (p-1)*(q-1)
     21d = pow(pubkey.e, -1, phi)
     22
     23ciphertext = int(open("message.txt").read())
     24plaintext = pow(ciphertext, d, pubkey.n)
     25
     26print(long_to_bytes(plaintext).decode())