Hack.lu-Qualifier-2023/Crypto/Lucky Numbers Writeup

#!/usr/bin/env python
#hacklu23 Baby Crypyo Challenge
import math
import random
from Crypto.Cipher import AES
from Crypto.Random import get_random_bytes
import base64
import os                                                   
    
def add(e): return e+(length-len(e)%length)*chr(length-len(e)%length)
def remove(e): return e[0:-ord(e[-1:])]
length=16 

def main():  
    flag= os.environ["FLAG"]
    print("Starting Challenge")
 
    key=get_random_bytes(32)
    message=add(flag)
    iv=get_random_bytes(length)
    cipher=AES.new(key,AES.MODE_CBC,iv) 
    cipher_bytes=base64.b64encode(iv+cipher.encrypt(message.encode("utf8")))
    print(cipher_bytes.decode())

    for l in range(0,5):
        A=[]
        print("You know the moment when you have this special number that gives you luck? Great cause I forgot mine")
        data2=input()
        print("I also had a second lucky number, but for some reason I don't remember it either :(")
        data3=input()
        v=data2.strip()
        w=data3.strip()
        if not v.isnumeric() or not w.isnumeric():
            print("You sure both of these are numbers?")
            continue
        s=int(data2)
        t=int(data3)
        if s<random.randrange(10000,20000):
            print("I have the feeling the first number might be too small")
            continue
        if s>random.randrange(150000000000,200000000000):
            print("I have the feeling the first number might be too big")
            continue
        if t>42:
            print("I have the feeling the second number might be too big")
            continue

        n=2**t-1
        sent=False
        for i in range(2,int(n**0.5)+1):
             if (n%i) == 0:
                print("The second number didn't bring me any luck...")
                sent = True
                break
        if sent:
            continue
        u=t-1
        number=(2**u)*(2**(t)-1)
        sqrt_num=math.isqrt(s)
        for i in range(1,sqrt_num+1):
            if s%i==0:
                A.append(i)
                if i!=s//i and s//i!=s:
                    A.append(s//i)      
        total=sum(A)
        if total==s==number:
            decoded=base64.b64decode(cipher_bytes)
            cipher=AES.new(key,AES.MODE_CBC,iv)
            decoded_bytes=remove(cipher.decrypt(decoded[length:]))
            print("You found them, well done! Here have something for your efforts: ")
            print(decoded_bytes.decode())
            exit()
        else:
            print("Hm sadge, those don't seem to be my lucky numbers...😞")
    
    print("Math is such a cool concept, let's see if you can use it a little more...")
    exit()
  
if __name__ == "__main__":
    main()

if not v.isnumeric() or not w.isnumeric():
    print("You sure both of these are numbers?")
    continue
s=int(data2)
t=int(data3)
if s<random.randrange(10000,20000):
    print("I have the feeling the first number might be too small")
    continue
if s>random.randrange(150000000000,200000000000):
    print("I have the feeling the first number might be too big")
    continue
if t>42:
    print("I have the feeling the second number might be too big")
    continue

n=2**t-1
sent=False
for i in range(2,int(n**0.5)+1):
     if (n%i) == 0:
        print("The second number didn't bring me any luck...")
        sent = True
        break
if sent:
    continue

u=t-1
number=(2**u)*(2**(t)-1)
sqrt_num=math.isqrt(s)
for i in range(1,sqrt_num+1):
    if s%i==0:
        A.append(i)
        if i!=s//i and s//i!=s:
            A.append(s//i)      
total=sum(A)
if total==s==number:
    # print flag

from Crypto.Util.number import isPrime

def solve():
    answer = []
    for t in range(42):
        n = 2**t-1
        if isPrime(n):
            u = t-1
            s = (2**u)*(2**(t)-1)
            if 10000 <= s <= 200000000000:
                answer.append((s, t))
    return answer

answer = solve(); answer

from pwn import *

r = remote(b"flu.xxx", 10010)

def io(s, t):
    r.sendlineafter(b"Great cause I forgot mine", str(s).encode())
    r.sendlineafter(b"I don't remember it either :(", str(t).encode())
    r.recvuntil(b"for your efforts: \n")
    flag = r.recv()
    print(flag)

io(*answer[0])
r.close()

if s<random.randrange(10000,20000):
    print("I have the feeling the first number might be too small")
    continue