Category: Crypto

Points: 300

Author: Mpex

Description:

After a long day, you sit around a campfire in the wild wild web with a few Sioux you met today. To celebrate friendship one of them takes out his wooden peace pipe and minutes later everyone seems to be pretty dizzy. You remember that their war chief “Makawee” started something to say about a secret tipi filled with fire-water (the good stuff). But when he noticed your interest he immediately stopped talking. You recall that “Makawee” spoke with “Wahkoowah” about that issue, but it ended with a fight. Since then Makawee wouldnt talk to Wahkoowah anymore. While they argued “Chapawee” wrote something down. Maybe you can exploit their dizzyness to find out the location of the tipi.

wildwildweb.fluxfingers.net 1432 
wildwildweb.fluxfingers.net 1433 
wildwildweb.fluxfingers.net 1434

Write-up

Connecting to wildwildweb.fluxfingers.net 1432 gives us protocol description:

                         ====================
                        +Description of Truth+
                         ====================

Wahkoowah                                                              Makawee



* choose random r_w out of Q                      choose random r_m out of Q *
* Compute                                                            Compute *
  t_w = pubk_m ^ r_w % p                              t_m = pubk_w ^ r_m % p *
* send t_w to m                                                send t_m to w *


                                       t_w
                            ------------------------>

                                       t_m
                            <------------------------

* Compute shared secret
  key = t_m ^ (privk_w ^ -1) * g ^ r_w % p


                                                       Compute shared secret *
                                    key = t_w ^ (privk_m ^ -1) * g ^ r_m % p


* if talking to m
  send token = enc_key(auth)
                                       token
                             ------------------------>

                                                           if token is valid *
                                                                   send flag *
                                            flag
                             <------------------------



 More Information
 ----------------


privkey = some_value % p
pubkey = g ^ privkey % p

g = 2

p = ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e08
8a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b
0a6df25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b
0bff5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece4
5b3dc2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23
dca3ad961c62f356208552bb9ed529077096966d670c354e4abc9804f174
6c08ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28f
b5c55df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa
051015728e5a8aacaa68ffffffffffffffff

We also get associated public keys for Chapawee, Makawee and Wahkoowah. Additionaly we can register our own key. Looking at the protocol a Man-In-The-Middle attack seems like good approach:

  • Register public key and get Wahkoowah
  • Go to Makawee and pretend to be Wahkoowah, go to Wahkoowah and pretend to be Makawee
  • Relay messages to get the flag

    nc wildwildweb.fluxfingers.net 1434 Hi, I’m, Makawee, and you are? Too bright here… Wahkoowah I dont talk to you anymore. That thing with my daughter…

There goes our attack approach. Let us try to eliminate encryption by pretending our public key is zero - that is we send p as t_w.

Hi, I'm Wahkoowah. Who are you? Too foggy...
Makawee
Oh its you, Im so sorry. Can we talk now?
This is your key of truth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With magic I did this: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We continue our conversation, right?
ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67
cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6d
f25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff
5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece45b3d
c2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23dca3
ad961c62f356208552bb9ed529077096966d670c354e4abc9804f1746c08
ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28fb5c5
5df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa0510
15728e5a8aacaa68ffffffffffffffff
Now I can almost see it, the sheer truth
cnJEBmiBlUkXirI0DXnu5jo6FGGTNOrDrLS3Go5xJvyuuExro8I6169GjWCT492C4LI=

It worked! Wahkoowah accepted 0 as correct token. Now let us try the same with Makawee and use the answer from Wahkoowah…

Hi, I'm, Makawee, and you are? Too bright here...
test
test ... do I know you?
This is your key of truth
fffffffffffffffffffffffffffffffffffff
With magic I did this: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grml...
ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67
cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6d
f25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff
5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece45b3d
c2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23dca3
ad961c62f356208552bb9ed529077096966d670c354e4abc9804f1746c08
ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28fb5c5
5df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa0510
15728e5a8aacaa68ffffffffffffffff
Bit more truth is missing
cnJEBmiBlUkXirI0DXnu5jo6FGGTNOrDrLS3Go5xJvyuuExro8I6169GjWCT492C4LI=
I knew you are able to see IT. Lets get drunk, I tell you where
flag{FreeBoozeForEverone-Party!}

Flag FreeBoozeForEverone-Party!

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