LNC 2025 Challenge Submission Template

In order to automate the deployment of challenges, the challenge committee has defined a standard folder structure (with specific metadata files) to facilitate an easier way to deploy challenges. This document defines how we plan on translating challenge folders into actual CTFd challenges.

Flag Format

To ensure consistency, LNC has defined a flag format for all challenge creators to follow. The flag should match the regex expression of ^LNC25\{.*\}$. An example of a flag that matches the said regex expression is LNC25{th1s_1s_A_fl4g}.

Folder Template

The challenge & file structure should look like this:

challenge_name/
    |
    ├─ dist/
    │       ├─ files_to_give_to_participants
    |
    ├─ service/
    │       ├─ Dockerfile
    │       ├─ files_to_host_challenge
    |
    ├─ solution/
    │       ├─ writeup.md
    │       ├─ solution_files
    |
    ├─ chall.json
    ├─ README.md

Additional folders can be added, but they will be ignored by the deployment script. Each of the folders defined above serve a specific purpose as documented below:

dist/

All files within the dist/ folder will be given to the participants, so don't leak any sensitive information here. This folder can be left empty if your challenge has no files to give to the participants.

service/

This folder will contain all the deployment-related files. A Dockerfile must be present to tell us how to build your challenge. Refer to the examples folder in this repository for how service folders should look like.

solution/

This folder should contain a writeup.md file, with additional assets like images or solve scripts to assist the reader in solving your challenge. While the writeup.md filename is not necessary, it's best to have a naming convention to follow.

chall.json

This file contains additional challenge metadata for deployment to CTFd. The full specs on how to craft this file is documented in the section below.

Chall.JSON Structure

This file should contain the following compulsory fields:

Field	Type	Description
name	str	Challenge name. Should match folder name when possible.
author	str	The challenge author's name.
category	str	One of the following categories (crypto, forensics, misc, osint, pwn, re, web). If unsure, put it as misc. This is case sensitive!!
difficulty	str	One of the following difficulties (easy, medium, hard, insane). This is case sensitive!!
description	str	A description of the challenge. Encode any newlines with \n. Note: If you have a service, the connection URL will be automatically filled in below.
discord	str	The challenge author's discord username.
flag	str	The challenge flag. Should match the regex LNC25\{.*\}$

The file can also contain the following optional fields. Some or all of these fields can be left out in your chall.json.

Field	Type	Description
port	int	The port that your service exposes, between 1 and 65535. Both port and the service/ folder should exist or an error would be thrown.
requirements	List[str]	A list of strings containing the name of the challenges a user has to complete before unlocking the current challenge.
hints	List[obj]	A list of objects containing hints. Each hint object should have a cost (int) and description (str). An example hint object can be found in the examples section below.

Chall.json Examples

Bare minimum chall.json

{
    "name": "Lorem Ipsum",
    "author": "John Doe",
    "category": "misc",
    "difficulty": "easy",
    "description": "This is a multi-line\ndescription.",
    "discord": "john.doe",
    "flag": "LNC25{f4k3_fl4g_f0r_t3st1ng}"
}

Challenge with a service

Let's assume a Flask challenge with the following code:

from flask import Flask

app = Flask (__name__)

# ...Flask stuff...

app.run(port=8080)

Your chall.json should look similar to:

{
    "name": "Web Challenge",
    "author": "John Doe",
    "category": "web",
    "difficulty": "easy",
    "description": "I made a web application! Hack me.", // Connection URL will be appended by the script
    "discord": "john.doe",
    "flag": "LNC25{f4k3_fl4g_f0r_t3st1ng}",
    "port": 8080 // Same as the Python script
}

Adding hints

"hints": [
  {
    "cost": 10,
    "description": "Lorem ipsum dolo amet ..."
  }
]

Adding requirements

// challenge-1/chall.json
{
    "name": "Challenge 1",
    "author": "John Doe",
    "category": "misc",
    "difficulty": "easy",
    "description": "This is a multi-line\ndescription.",
    "discord": "john.doe",
    "flag": "LNC25{f4k3_fl4g_f0r_t3st1ng}"
}

// challenge-2/chall.json
{
    "name": "Challenge 2",
    "author": "John Doe",
    "category": "misc",
    "difficulty": "medium",
    "description": "This is a multi-line\ndescription.",
    "discord": "john.doe",
    "flag": "LNC25{f4k3_fl4g_f0r_t3st1ng_aga1n}",
    "requirements": ["Challenge 1"]
}

Frequently Asked Questions (FAQs)

Q: If I am creating a static challenge (forensics, crypto, etc.), do I need to include a service folder?
A: No, you do not need to include a service folder if your challenge is static.

Q: I'm having trouble submitting my challenge, what should I do?
A: Please contact your school's LNC 2025 representative for assistance, alternatively you can contact the challenge team by opening a ticket on our Discord server.