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Techras II

Twin Oracle

Category Difficulty Score Solves First 🩸
Crypto Baby 👶 71 69 TheBacterias

Code / Description

1
Thanks to the guidance on the Techras challenge, I've learned a lot. I think I finally understand when to discard a number versus when to reuse it. To be honest, I feel like I've got it all figured out now with Techras II. Don't you agree?

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#!/usr/bin/env python3

from Crypto.Util.number import *
from string import *
from flag import flag

def pad(flag):
    r = len(flag) % 8
    if r != 0:
        flag = flag[:-1] + (8 - r) * printable[:72][getRandomRange(0, 71)].encode() + flag[-1:]
    return flag

def genkey(nbit):
    e = getPrime(64)
    p, q = [getPrime(nbit) for _ in ':)']
    n = p * q
    return (e, n), (p, q)

def encrypt(msg, pubkey):
    e, n = pubkey
    msg = pad(msg)
    m = bytes_to_long(msg)
    c = pow(m, e, n)
    return c

nbit = 1024
pubkey, _ = genkey(nbit)

e, n = pubkey
print(f'n = {n}')
for _ in range(25):
    print(f'c = {encrypt(flag, (e, n))}')
    e += 2
print(f'e = {e}')

Overview

Challenge Analysis

Solution

Final Code

Flag

1
ASIS{Us3___L4r9eR___PexP!}

Authors

Kourosh Rajabzadeh