Challenge was available at (password: wctf2018, SHA1 HASH: 7efe988b1f9fe283e4f3d9bd073d1a97a93f51ee).


Encrypted message for user "admin":


admin public key:

n = 483901264006946269405283937218262944021205510033824140430120406965422208942781742610300462772237450489835092525764447026827915305166372385721345243437217652055280011968958645513779764522873874876168998429546523181404652757474147967518856439439314619402447703345139460317764743055227009595477949315591334102623664616616842043021518775210997349987012692811620258928276654394316710846752732008480088149395145019159397592415637014390713798032125010969597335893399022114906679996982147566245244212524824346645297637425927685406944205604775116409108280942928854694743108774892001745535921521172975113294131711065606768927
e = 65537


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Organize those rectangular things that take physical space!

A website is given along with its source where you can create an account and add books with some metadata.

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Challenge (46 solves, 210 points)

You discover this cat enthusiast chat app, but the annoying thing about it is that you’re always banned when you start talking about dogs. Maybe if you would somehow get to know the admin’s password, you could fix that.

We are given a chat website. When entering it we are redirected to a random room and are greeted by a message.

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Challenge (74 solves, 158 points)

This crypto experiment will help you decrypt an RSA encrypted message.
(Attachment containing, flag.txt, key_pub.pem)
nc 1337

Looking at the description and the given files we can guess, that flag.txt is the flag encrypted with RSA under key_pub.pem, which turns out to be correct. Furthermore we can guess that is running on the server, which is also correct.

It seems our goal is to use the server to decrypt the flag for us.

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Challenge: “Win the game 1,000,000 times to get the flag.”

To get things started, I ran the apk in Anbox. We are greeted by a Tic-Tac-Toe implementation. Now from the challenge we know that we have to win the game 1 million times to get the flag. Being an avid gamer, I took this challenge and just went on a 1 million games winning streak, thanks for reading this Write-Up.

Jokes aside, this would not be a challenge, as the used AI seems to play really random, so we basically win after 3 turns anyway. So lets get us a real challenge.

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We receive the following info:

Building the future web, together.

After loading the page we are presented with:

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We were given a binary smcauth together with a verilog file smcauth_syn.v and a server address. It can act as server:

./smcauth verify --secret aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa --netlist smcauth_syn.v [--listen ip:port]

and client:

./smcauth auth --secret aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa --netlist smcauth_syn.v [--verifier ip:port]

The client tells us whether the secret was "correct", or rather whether the circuit specified by --netlist evaluates to true, taking the client's and server's secret as input.

May 14 13:31:42.775 INFO authentication successful

May 14 13:31:57.274 WARN authentication failed

At this point, it is pretty obvious that we need to obtain the server's secret.

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First a disclaimer, we did not actually solve this challenge during the competition, but the servers were left running…

A server is provided: nc 22555

It greets us with the following text:

| Welcome to the Yunnyit crypto task! |
| Options:                            |
| [M]ixed encryption function of FLAG |
| [D]ecrypting cipher                 |
| [E]ncryption & decryption function  |
| [F]LAG encrypting...                |
| [Q]uit                              |
Submit a printable string X, such that sha256(X)[-6:] = 92730d

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