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Pwnable.kr - coin1

Miracle Yu

— 2 min read

Write-up for coin1 of pwnable.kr

0x00 Puzzle

Mommy, I wanna play a game!
(if your network response time is too slow, try nc 0 9007 inside pwnable.kr server)

Running at : nc pwnable.kr 9007

0x01 Explore

nc

shellshock@ubuntu:~$ nc 0 9007

	---------------------------------------------------
	-              Shall we play a game?              -
	---------------------------------------------------
	
	You have given some gold coins in your hand
	however, there is one counterfeit coin among them
	counterfeit coin looks exactly same as real coin
	however, its weight is different from real one
	real coin weighs 10, counterfeit coin weighes 9
	help me to find the counterfeit coin with a scale
	if you find 100 counterfeit coins, you will get reward :)
	FYI, you have 30 seconds.
	
	- How to play - 
	1. you get a number of coins (N) and number of chances (C)
	2. then you specify a set of index numbers of coins to be weighed
	3. you get the weight information
	4. 2~3 repeats C time, then you give the answer
	
	- Example -
	[Server] N=4 C=2 	# find counterfeit among 4 coins with 2 trial
	[Client] 0 1 		# weigh first and second coin
	[Server] 20			# scale result : 20
	[Client] 3			# weigh fourth coin
	[Server] 10			# scale result : 10
	[Client] 2 			# counterfeit coin is third!
	[Server] Correct!

	- Ready? starting in 3 sec... -
	
N=148 C=8

0x02 Solution

N < 2 ^ C, so bi-search would work:

import subprocess

def read1stLine():
  a = ''
  while len(a)==0 or a[0] != 'N':
    a=coin.stdout.readline()
    print "server said:"+a+'end'
  print 'len->'+str(len(a))+'<-'
  print 'hex->'+(hex(ord(a[0])))+'<-'
  return a

def parseNC(string):
  string = string.split(' ')
  n = int(string[0].split('=')[1])
  c = int(string[1].split('=')[1])
  return n,c

def put(st):
  s = ''
  for i in st:
    s += str(i)
    s += ' '
  s += '\n'
  coin.stdin.write(s)
  print 'put: '+s

def get(noreturn=False):
  a=''
  while a == '':
    a=coin.stdout.readline()
  print 'get: '+a
  if not noreturn:
    return int(a)

def getstr():
  a=''
  while a == '':
    a=coin.stdout.readline()
  print 'get: '+a

def q(st):
  put(st)
  return get()

def game(l,r,c):
  if r==l+1:
    return l,c;
  if q(range(l,l+(r-l)/2))<((r-l)/2*10):
    return game(l,l+(r-l)/2,c+1)
  else:
    return game(l+(r-l)/2,r,c+1)
coin = subprocess.Popen("nc 0 9007".split(' '), stdin=subprocess.PIPE, stdout=subprocess.PIPE)

for rd in range(100):
  a = read1stLine()
  n,c = parseNC(a)
  print 'coin said:' +a+'end'
  print 'n = '+str(n)+' c = '+str(c)

  res,tc=game(0,n,0)
  for i in range(c-tc+1):
    put([res])
  get(True)
get(True)
get(True)
get(True)
get(True)
get(True)

...
get: Correct! (99)

get: Congrats! get your flag

get: b1NaRy_S34rch1nG_1s_3asy_p3asy

Done!

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