Tags: crypto prng
Rating: 5.0
import random
import ssl
import urllib2
import urllib
import re
import click
class SecurePrng(object):
p = 4646704883L
def __init__(self, x, y):
# generate seed with 64 bits of entropy
self.x = x
self.y = y
def next(self):
self.x = (2 * self.x + 3) % self.p
self.y = (3 * self.y + 9) % self.p
return (self.x ^ self.y)
def HTTP(url, cookie=None, payload=None):
if cookie:
headers = { 'Cookie': 'session="%s"' % cookie }
else:
headers = {}
if payload:
payload = urllib.urlencode(payload)
gcontext = ssl.SSLContext(ssl.PROTOCOL_TLSv1) # Only for gangstars
req = urllib2.Request(url, headers=headers, data=payload)
# print (headers)
# print (req.__dict__)
return urllib2.urlopen(req, context=gcontext)
def get_cookie():
req = HTTP('https://giant-goannas.ctfcompetition.com/start')
s = req.headers.getheaders('Set-Cookie')[0]
return re.findall('session="([^"]+).*', s)[0]
def submit_number(cookie, number):
print "submit ", number
req = HTTP('https://giant-goannas.ctfcompetition.com/lake', cookie=cookie, payload=dict(number=number))
body = req.read()
next_numbers = re.findall('([0-9]{4,})', body)
print body
if len(next_numbers) == 2:
return False
else:
import ipdb
ipdb.set_trace()
return body
def try_to_guess(cookie):
print "try to guess"
req = HTTP('https://giant-goannas.ctfcompetition.com/lake', cookie=cookie)
body = req.read()
left, right = re.findall('([0-9]{4,})', body)
req2 = HTTP('https://giant-goannas.ctfcompetition.com/lake', cookie=cookie, payload=dict(number=left))
body2 = req2.read()
next_numbers = re.findall('([0-9]{4,})', body2)
if len(next_numbers) == 2:
return int(left), [int(n) for n in next_numbers]
else:
return False, []
print "start the game"
try_to_play_id = 0
while 1:
try_to_play_id += 1
print "Our %d attemption" % try_to_play_id
print "Let's get the cookie"
cookie = get_cookie()
print "Now, guess first number"
number, possible_numbers = try_to_guess(cookie)
if not number:
continue
print "Found", number, "and possible next numbers", possible_numbers
if possible_numbers[0] > 1 * 1500 * 1000 * 1000 or possible_numbers[1] > 1 * 1500 * 1000 * 1000:
print "but skip, we need only small variants"
continue
with click.progressbar(length=SecurePrng.p, label='Recovery prng state') as bar:
for x in xrange(SecurePrng.p):
y = number ^ x
sprng = SecurePrng(x, y)
value = sprng.next()
if value in possible_numbers:
break
if x % 1000 == 0:
bar.update(1000)
print "Found", value, cookie, sprng.__dict__
while 1:
res = submit_number(cookie, value)
if res:
break
value = sprng.next()
print "Finish with", res
break
