PlaidCTF 2013 – Crypto 250 Compression Writeup
On the recently concluded PlaidCTF (which was an awesome competition) by PPP there was a problem. Here it goes:
Question: We managed to get the source code for an encryption service running at 220.127.116.11:4433.
I have listed the python source provided below:
#!/usr/bin/python import os import struct import SocketServer import zlib from Crypto.Cipher import AES from Crypto.Util import Counter # Not the real keys! ENCRYPT_KEY = '0000000000000000000000000000000000000000000000000000000000000000'.decode('hex') # Determine this key. # Character set: lowercase letters and underscore PROBLEM_KEY = 'XXXXXXXXXXXXXXXXXXXX' def encrypt(data, ctr): aes = AES.new(ENCRYPT_KEY, AES.MODE_CTR, counter=ctr) return aes.encrypt(zlib.compress(data)) class ProblemHandler(SocketServer.StreamRequestHandler): def handle(self): nonce = os.urandom(8) self.wfile.write(nonce) ctr = Counter.new(64, prefix=nonce) while True: data = self.rfile.read(4) if not data: break try: length = struct.unpack('I', data) if length > (1<<20): break data = self.rfile.read(length) data += PROBLEM_KEY ciphertext = encrypt(data, ctr) self.wfile.write(struct.pack('I', len(ciphertext))) self.wfile.write(ciphertext) except: break class ReusableTCPServer(SocketServer.ForkingMixIn, SocketServer.TCPServer): allow_reuse_address = True if __name__ == '__main__': HOST = '0.0.0.0' PORT = 4433 SocketServer.TCPServer.allow_reuse_address = True server = ReusableTCPServer((HOST, PORT), ProblemHandler) server.serve_forever()
The key on this challenge is to see that the stream encryption is being done on the compressed input. In the source provided, if the user input is similar to the secret value in the PROBLEM_DATA variable then the zlib.compress() function would show a reduced length ciphertext. This is somewhat (and I use the term loosely) similar to the CRIME vulnerability. The AES Counter mode RFC has the implementation details of the cipher. So I wrote the following script.
import socket import sys from itertools import * import struct def display(msg,numbytes): #print >>sys.stderr, 'received "%s"' % msg #print >>sys.stderr, 'bytes "%d"' % numbytes print >>sys.stderr, 'bytes %d ' % numbytes + msg.encode('hex') # Create a TCP/IP socket sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) # Connect the socket to the port where the server is listening server_address = ('18.104.22.168', 4433) print >>sys.stderr, 'connecting to %s port %s' % server_address sock.connect(server_address) #mesage len = 20 lowercase and underscore letters try: amount_received = 0 nonce = sock.recv(8) amount_received += len(nonce) # Send data #strng = 'crime_some' #minciphlen = 1000 #strng = 'crimes_pays' #strng = 'so_' #strng = 'crime_some_times_pays' #strng = 'somet_' strng = 'cr' minchar = '' ciphlen = 1000 sampleset = 'hijklmnopqrstuvwxyz_abdefgc' #while True: strng = strng + minchar minciphlen = ciphlen minchar = '' for s in map("".join,permutations(sampleset,1)): #message = nonce + (strng + s)*10 #'\x00'*11 + s message = strng + s datalen = struct.pack('I',len(message)) # datalen = '\xe4\x00\x00\x00' sock.sendall(datalen) #print >>sys.stderr, 'sending '+ message sock.sendall(message) #print >>sys.stderr, 'message sent' amount_received = 0 # Look for the response data = sock.recv(4) amount_received += len(data) ciphlen = struct.unpack('I', data) #print >>sys.stderr, message + ' ' amount_received = 0 if ciphlen <= minciphlen: minciphlen = ciphlen minchar = s print str(ciphlen) + ' It is ' + strng + minchar data = sock.recv(ciphlen) #display(data,ciphlen) finally: print >>sys.stderr, 'closing socket' sock.close()
When you connect to the service it provides you the nonce, so I prepended the nonce to the plaintext. The above script shows the plaintext and the length of the cipher text. To start off with this, you start with a string of length 1, and see which is the smallest length response, that gives your first character. Then in the
variable above, you add that character and run again, and the lowest length ciphertext tells you the next character and so on. I noticed that sometimes the output had a few characters with the lowest length. So I tried each of them and ended up with the following flag: