A CTF Infrastructure with a deterministic anti cheat!
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CTF Infrastructure

This project is a full blown CTF infrastructure, aiming to integrate with CTFd as a frontend. It includes:

  • A server side infrastructure built in NixOS
  • A suite of challenges that are unique, tested and whose solution is auto-generated
  • An anti cheat system to prevent flag sharing between teams

Anti Cheat

This is a deterministic anti-cheat/unique binaries generator specially tailored to CTFs. It creates a regex per challenge to load into CTFd. The flag itself is a sha2 of the challenge name plus a secret on top of 4 bytes at a different position for each challenge identifying uniquely the team in a sneaky fashion.

Challenge generation

The setup also includes ways to add junk code to binaries and have more convoluted setup done to them through a python script.
All the flags and binaries are deterministic, so as long as you don't re-roll your secrets you will get the same result each time.
The cheat detection part detects those bytes to see who cheated with who, probabilistically or definitely, or simply who cheated if the identifying bytes cannot be found. The whole idea is to have hashes that looks the most alike possible while still being fairly certain about the origin of said flag.

Setup:

Put your challengess in the folder called "chals". One folder per challenge, each folder must contain a setup.py setting up the challenge final folder and removing unecessary files.
You will end up with a folder tree such as:

chals_out/
├── access_security
│   ├── regex.txt
│   ├── test_team
│   │   ├── access_security
│   │   └── flag.txt
│   └── test_team2
│       ├── access_security
│       └── flag.txt

You can then call the gen_chals.py script which will take a partial export of a CTFd database to generate each of your challenge per team. You'll find the result in the folder chals_out. You can then use those challenges with the infrastructure directly, and the regex.txt generated in CTFd to validate the flags.

You can thus automate deployment of the binaries onto your VMs however you like. This setup works well with binaries that won't be executed remotely too and whatnot.

RE CTF Edition:

This CTF includes the challenges made for the class "Reverse Engineering and Binary exploitation" I taught for the École Supérieure de Génie Informatique, or ESGI. The challenges included are:

  • reverse_rop
  • simple_rop
  • simple_rop_2
  • web_server
  • snake_oil
  • snake_oil_2
  • access_security
  • web_server_2
  • modern_rop

Credits

This CTF infrastructure was made as a part of a class that I taught, I being Gauvain Roussel-Tarbouriech, also commonly known as "GovanifY". I made all the challenges, build system and anti cheat.

Ryan Lahfa made the original infrastructure work in a night(!) along with me, which I later improved before deploying it. Huge thanks goes to him!