stack-the-flags-2020-writeups

Writeups for the Challenges that iWanFlagz solved

Project maintained by iWanFlagz Hosted on GitHub Pages — Theme by mattgraham

[Cryptography] Can COViD steal Bob’s idea?

The challenge provides a .pcapng file. I use Wireshark to open this file.

I found three interesting data in this file.

  1. Set filter: tcp.stream eq 0
  2. Right-click the packet, Follow -> TCP Stream

From the image below, I know that they used a shared Diffie-Hellman key to encrypt their data. This means that g is the base, p is the modulo, g^a is the public key of a and g^b is the public key of b.

  1. Set filter: tcp.stream eq 1
  2. Right-click the packet, Follow -> TCP Stream

The user retrieves a copy of CryptoDesign.zip file.

  1. Set filter: tcp.stream eq 2
  2. Right-click the packet, Follow -> TCP Stream

This is the raw data of CryptoDesign.zip. I can extract the data by selecting raw and then select Save As. The zip file is password-protected (probably using shared diffie-hellman key).

So, I have to figure out the shared key.

Here is some background knowledge about Diffie-Hellman (From: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange):

So, I need to find the value of a and b in order to find the value of the shared key. To do this, I used Discrete Logarithm calculator.

So, I found the value of a and b:

a = 211631375588570729261040810141700746731
b = 164628728413072046550711086534226032119

With the value of a and b, I can find the value of the shared key.

Flag: govtech-csg{246544130863363089867058587807471986686}

References: https://github.com/tudorog/ctf-writeups/blob/master/RITSEC%20CTF%202018/crypto/DarkPearAi.md