ASIS-CTF-Finals-2014/Ultra Secure -Writeup

Category: Crypto Points: 400 Description:

Connect there: nc 12445


At the very beginning, I want to say something about this problem. In my opinion, this problem is not worth 400 pts. It can be manually solved  through several steps without writing any scripts or programs!

Ok, Let’s begin!

Firstly, please let me show you the steps to solve the problem, then I will explain these in details.

  1. Log onto the server, send “Paillier” to the server (this is the cryptosystem’s name);
  2. Get the secret s;
  3. Calculate c = s × 2;
  4. Let the server decrypt c, get the answer m;
  5. Let the server decrypt 2, get the answer m2;
  6. Calculate flag = m – m2, you get the flag;
  7. Convert flag to ascii string. Done!

Continue reading


ASIS-CTF-Finals-2014/Important day! -Writeup

Category: Forensics

Points: 100


When did the computer start? Download this file

flag = ASIS_md5(time), time = ~$ date +%Y:%m:%d:%H:%M


After decompressing the file, I use `file` command to find out what this file is. It turns out to be a tcpdump capture file.

I use Wireshark to analyze this dump file. According to the problem, I need to find something related to time.


Continue reading