OSU Satellite TV for MITRE eCTF 2025

This repo contains all the code and documentation for team scriptohio from The Ohio State University in MITRE's eCTF 2025.

Highlights of our design:

• 🔐 Strong cryptography
  • 💃 The Salsa20 stream cipher is used for lightning quick symmetric encryption and decryption
  • 🌲 Our binary hash key derivation tree efficiently compresses unique keys per timestamp to create a subscription over a time interval 🕐
  • 📝 All frames are signed and verified with Ed25519 to ensure decoders only read frames from the encoder they came from
• ⚡ Written in Zig
  • 📦 Expansive standard library which includes everything a programmer might need, from string manipulation to secure cryptography
  • 🧪 Unit testing framework built in, allowing for critical sections to have their correctness verified
  • 🚫 Memory safety protections against common issues like indexing out of bounds, double free, integer overflows, and more
  • 🚧 Build system with incredible interoperability with C code. We still use the MSDK, and there are no HAL rewrites in sight 👀

Layout 🌎

Icon	Description
👥	Created by our team
👔	Created by MITRE for the competition, and unmodified by us

• 👥 benches/ - Custom benchmarks to test the speed of the design 🐍
• 👥 decoder/ - Firmware for the decoder ⚡
• 👥 design/ - Host design including encoder 🐍
• 👥 docs/ - Documentation on the system design and security 📖
• 👔 tools/ - Host tools to interact with the decoder 🐍
• 👥 Taskfile.yml - Command runner definitions 📄
• 👥 flake.nix - Convenient Nix development environment ❄️

Usage

A Taskfile is available to easily run common tasks

* 1-gen-secrets: Generate secrets
* 2-build-firmware: Build the firmware image
* 3-flash: Flash the built firmware image onto the Decoder
* 4-gen-subscription: Generate a subscription
* 5-subscribe: Take a subscription file and send it to the Decoder
* 6-list: Asks the Decoder to list the subscriptions that it currently has
* 7-uplink: Encodes sample frames with the custom encoder function and sends them to the satellite
* 8-satellite: Broadcasts all frames received from the uplink to all listening TVs on the host computer
* 9-tv: Receives encoded frames from the satellite and decodes with a Decoder connected to the host computer
* bench-decoder: Benchmark the decoder
* bench-encoder: Benchmark the encoder
* bench-subscription: Benchmark creating a subscription on the decoder
* docs: Build documentation design file into PDF and SVG artifacts
* stress-decoder: Use MITRE's stress test to benchmark the decoder
* stress-encoder: Use MITRE's stress test to benchmark the encoder

On a clean repo, plug in the MAX78000, enter firmware writing mode (the LED should be flashing blue), and run the following command:

task 1-gen-secrets 2-build-firmware 3-flash 4-gen-subscription 5-subscribe 6-list

Then, iterating on the design is typically done with the following command:

task 2-build-firmware 3-flash

Documentation 📖

The design document is available to read as a PDF, or inline below:

View the PDF inline

Code Documentation 📖

Our Zig code makes use of documentation comments which can be read as a webpage. To view it, navigate to the decoder directory and run the following

$ zig build docs
$ python -m http.server -d docs

Then navigate to http://localhost:8000 in your browser.