Category: Crypto Points: 400 Description:
nc asis-ctf.ir 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.
- Log onto the server, send “Paillier” to the server (this is the cryptosystem’s name);
- Get the secret s;
- Calculate c = s × 2;
- Let the server decrypt c, get the answer m;
- Let the server decrypt 2, get the answer m2;
- Calculate flag = m – m2, you get the flag;
- Convert flag to ascii string. Done!
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.
The flag is encrypted by this code, can you decrypt it after finding the system?
The code is written in python.
I enter the site it gives and find this infomation:
Anyone who has visited our site is the 1234567890th Special prizes are awarded.
Welcome, It seems that you are visiting our website first time :)
Sorry, visit our again later :(
It gives us a string like this:
It ends up with “==”, so I guess it’s base64 encoded. After base64 decode, I don’t get the flag but some strange characters.