[2019 X-MAS CTF] Santa Knows Crypto

rElGamal.py

from Crypto.Util.number import *
from PRNG import PRNG

class ElGamal(object):
    def __init__(self, length, prime = None):
        self.prng = PRNG(256)
        self.length = length
        self.g = 2
        self.q = prime
        while self.q == None:
                p = self.next_prime(2**(length) + self.prng.get_bits(length))
                if ((2 * p + 1) % 8 == 3 or (2 * p + 1) % 8 == 5) and isPrime(2 * p + 1):
                    self.q = 2 * p + 1

        self.x = self.prng.get_bits(self.length)
        self.h = pow(self.g, self.x, self.q)

    def next_prime(self, x):
        if x & 1 == 0:
            x += 1
        while not isPrime(x):
            x += 2
        return x

    def encrypt(self, message):
        y = self.prng.get_bits(self.length)
        s = pow(self.h, y, self.q)
        c1 = pow(self.g, y, self.q)
        c2 = message * s % self.q
        return y, c1, c2

 

PRNG.py

import os

class PRNG(object):
    def __init__(self, length):
        self.length = length
        self.state = self.getseed()
        self.key = self.getseed()

    def parity(self,x):
        aux = self.length
        while aux > 1:
            x ^= x >> ((aux + 1) / 2)
            aux = (aux+1) / 2

        return x & 1  

    def getseed(self):
        return int(os.urandom(self.length / 8).encode('hex'), 16)

    def next_state(self):
        self.state = (self.state >> 1) | (self.parity(self.state & self.key) << (self.length - 1))

    def get_bit(self):
        output = self.state & 1
        self.next_state()
        return output

    def get_bits(self, bits):
        output = 0
        for i in range(bits):
            output = (output << 1) + self.get_bit()
        return output

 

server.py

from Crypto.Util.number import *
import SocketServer
from ElGamal import ElGamal
from secret import flag
from text import *
import os
from hashlib import sha256
prime = 685221181007655969055643176795598500987539499099103356485206001142535588544010199273848305625469714980556215814571531400059127410834556408213573648640620782264547499664798168588595354413552517947288874272302443155003080155578117949303088899418619338227631822471359675459950492254857920916052407700056096582307
PORT = 2000

class ThreadedTCPRequestHandler(SocketServer.BaseRequestHandler):
    def PoW(self):
        s = os.urandom(10)
        h = sha256(s).hexdigest()
        self.request.sendall("Provide a hex string X such that sha256(X)[-6:] = {}\n".format(h[-6:]))
        inp = self.request.recv(2048).strip().lower()
        is_hex = 1
        for c in inp:
            if not c in '0123456789abcdef':
                is_hex = 0

        if is_hex and sha256(inp.decode('hex')).hexdigest()[-6:] == h[-6:]:
            self.request.sendall('Good, you can continue!\n')
            return True
        else:
            self.request.sendall('Oops, your string didn\'t respect the criterion.\n')
            return False

    def handle(self):
        self.request.settimeout(120)
        if not self.PoW():
            return
        self.request.sendall(intro)
        enc = ElGamal(1024, prime)
        print enc.x
        self.request.sendall(details.format(enc.q, enc.g))
        correct = 0
        while correct < 10:
            m = int(os.urandom(32).encode('hex'), 16) % enc.q
            y, c1, c2 = enc.encrypt(m)
            self.request.sendall(challenge.format(c1, c2))
            self.request.sendall(get_input)
            try:
                x = self.request.recv(1024)
                x = int(x)
                if x == m:
                    self.request.sendall(correct_answer)
                    correct += 1
                else:
                    self.request.sendall(wrong_answer.format(m, y))`
                    correct = 0                    
            except:
                self.request.sendall(bad_input)
                break
        if correct == 10:
            self.request.sendall(flag+'\n')

class ThreadedTCPServer(SocketServer.ThreadingMixIn, SocketServer.TCPServer):
    pass

if __name__ == '__main__':
    server = ThreadedTCPServer(('0.0.0.0', PORT), ThreadedTCPRequestHandler)
    server.allow_reuse_address = True
    server.serve_forever()

 

text.py

intro = 'Welcome!\nIn this challenge you will have to decrypt 10 messages in a row to get the flag!\n'
details = 'Here are some things to help you in your quest!\nq : {}\ng : {}\n'
challenge = 'Decrypt this:\nc1 : {}\nc2 : {}\n'
wrong_answer = "Nope, that's not it!\nhere's what you sohould've sent:\nm : {}\nalso here is your y : {}\n"
correct_answer = 'Hey, you did it, great job!\n'
get_input = 'Send the decrypted message as an integer: '
bad_input = 'Input does not match desired criteria. Aborting!\n'

STEP 1 - Recover h in ElGamal class
After send wrong $m$ to server, server tells both $m$ and $y$.

 

Then We can recover $h^y = m \cdot m^{-1}$.

 

Once we found a pair $y1, y2$ such that $gcd(y1, y2) = 1$, find $v1, v2$ satisfies $v1y1 + v2y2 = 1$ using extended gcd and recover $h = ((h^{y1})^{v1}) \cdot ((h^{y2})^{v2}))$

 

STEP 2 - Recover state, key in PRNG class
This PRNG extracts each bit by XORing specific bit in state.(If key = (1<<128) | (1<<64) | (1<<32), parity is 128-th bit of state $\oplus$ 192-th bit of state $\oplus$ 224-th bit of state)

 

Since key is fixed, with any y, you can easily build a linear equation system which contains 256 equations and 256 variables in GF(2). After solving this, you can get a key.

 

State is more easy. Extract LSB 256-bit of last Y and key(Beware of endian. Must be reversed on LSB 256-bit of last Y). Make another PRNG class adn extract dummy 1024-bit. Then state is synchronized with in server.

 

sage code for extracting key

Y = 89983791425551027210400346476092936344826533636016877225918481462124667356462216675054392621920865356045823186208305570456929667777504930092311160509140608128950177361952489418169806815449239730008355253892404586754110522969077661227581488968435597924500263911858546622164629597080557119791160012652564123666
bb = rev_bit(Y)
sz = 256

print str(Y)[:6], "done"

mat = [[0]*sz for i in range(sz)]
for i in range(sz):
    for j in range(sz):
        mat[i][j] = int(bb[1+i+j])

A = Matrix(IntegerModRing(2),mat)
Ainv = A.inverse()
vec = [0]*sz
for i in range(sz):
    vec[i] = int(bb[i])

b = vector(IntegerModRing(2), vec)
ans = A.solve_right(b)
print(ans)

key = 0
for i in ans:
    z = Integer(i)
    key = (key<<1) | z
print "key ", key

 

solver.py

from hashlib import sha256
from Crypto.Util.number import *
import random, socket



############ my socket ###############
def interactive(socket):
  print("[+] interactive mode")
  while True:
    rr = socket.recv(2**16).decode()
    if not rr:
      print("[!] socket closed")
      return None
    print(rr)
    socket.send((input('> ')+'\n').encode())

def remote(ip, port):
  sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  print("[+] Connecting to {}:{}".format(ip,port))
  sock.connect((ip,port))
  print("[+] Done!")
  return sock

def sendline(socket, msg):
  if type(msg) == str: msg = msg.encode()
  if msg[-1] != b'\n': msg += b'\n'
  socket.send(msg)

def recv(socket):
  return socket.recv(2**16).decode()

class PRNG(object):
  def __init__(self, length, key, state):
    self.length = length
    self.state = state
    self.key = key

  def parity(self,x):
    aux = self.length
    while aux > 1:
      x ^= x >> ((aux + 1) // 2)
      aux = (aux+1) // 2

    return x & 1  

  def next_state(self):
    self.state = (self.state >> 1) | (self.parity(self.state & self.key) << (self.length - 1))

  def get_bit(self):
    output = self.state & 1
    self.next_state()
    return output

  def get_bits(self, bits):
    output = 0
    for i in range(bits):
      output = (output << 1) + self.get_bit()

    return output


def rev_bit(x, sz):
  s = bin(x)[2:].zfill(sz)
  s = s[::-1]
  return int(s, 2)

def egcd(a, b):
  if a == 0:
    return (b, 0, 1)
  g, y, x = egcd(b%a,a)
  return (g, x - (b//a) * y, y)



q = 685221181007655969055643176795598500987539499099103356485206001142535588544010199273848305625469714980556215814571531400059127410834556408213573648640620782264547499664798168588595354413552517947288874272302443155003080155578117949303088899418619338227631822471359675459950492254857920916052407700056096582307


r = remote('challs.xmas.htsp.ro', 10000)
pow_target = recv(r).strip()[-6:]
print(pow_target)
sendline(r, input())

x = recv(r) # Good, you can continue

c1arr = []
c2arr = []
marr = []
yarr = []
sarr = []
i1, i2 = -1, -1

# Step 1. to find h
for step in range(100000):
  print("Collection phase ", step)
  x = recv(r) # c1,c2
  if step == 0: print(x)
  c1 = x[x.find("c1")+5:]
  c1 = int(c1.split('\n')[0].strip())
  c2 = x[x.find("c2")+5:]
  c2 = int(c2.split('\n')[0].strip())
  c1arr.append(c1)
  c2arr.append(c2)
  sendline(r, '1')
  x = recv(r)
  m = x[x.find("m : ")+4:]
  m = int(m.split('\n')[0].strip())
  y = x[x.find("y : ")+4:]
  y = int(y.split('\n')[0].strip())
  s = c2 * inverse(m, q) % q
  sarr.append(s)
  marr.append(m)
  yarr.append(y)
  for i in range(step-1):
    if GCD(yarr[i], yarr[step]) == 1:
      i1, i2 = i, step
      break
  if i1 != -1:
    break

g, v1, v2 = egcd(yarr[i1], yarr[i2])
assert(g==1)
assert(v1*yarr[i1]+v2*yarr[i2]==1)

h = 1
if v1 > 0:
  h = h * pow(sarr[i1], v1, q) % q
else:
  h = h * pow(inverse(sarr[i1],q), -v1, q) % q

if v2 > 0:
  h = h * pow(sarr[i2], v2, q) % q
else:
  h = h * pow(inverse(sarr[i2],q), -v2, q) % q

for i in range(step):
  assert(pow(h,yarr[i],q) == sarr[i])
  assert(c2arr[i] == marr[i]*sarr[i]%q)

key = 0
Y = yarr[-1]
print("Y : ", Y)
key = int(input())
prng = None
# Step 2
for step in range(10):
  print("Challenge phase ", step)
  x = recv(r) # c1,c2
  c1 = x[x.find("c1")+5:]
  c1 = int(c1.split('\n')[0].strip())
  c2 = x[x.find("c2")+5:]
  c2 = int(c2.split('\n')[0].strip())
  if step == 0:
    prng = PRNG(256, key, rev_bit(Y&(2**256-1), 256))
    prng.get_bits(256)
  nexty = prng.get_bits(1024)
  s = pow(h, nexty, q)
  m = c2 * inverse(s, q) % q
  print("guessed m : ", m)
  sendline(r, str(m))
  x = recv(r)
  print(x)

print(recv(r))
print(recv(r))
print(recv(r))

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