Internetwache CTF 2016: Code 60 - It's Prime Time!

Description

We all know that prime numbers are quite important in cryptography. Can you help me to find some?

Write-up

어떤 자연수 n보다 큰 첫 번째 소수를 보내주면 된다. 다음 code를 실행했다.

from pwn import *

p = remote('188.166.133.53', 11059)
primes = [] #list of prime numbers below 1000 (omitted due to readability)
cnt = 0

while 1:
  cnt += 1
  print cnt

  if cnt == 101:
    data = p.recvall()
    print data
    exit(0)

  data = p.recvuntil(':')
  print data
  data = p.recvuntil(':')
  print data

  idx = data.find('after')

  n = int(data[idx + 6 : -1])

  print n

  for i in primes:
    if n < i:
      p.send(str(i))
      break

100번 맞았더니 flag를 줬다.

Flag

IW{Pr1m3s_4r3_!mp0rt4nt}