Private-Owner Counter

A privacy-preserving Midnight Compact counter where ownership is proven with a private secret key without revealing the key on-chain.

Contract Address

Network	Address
Preview	`6a9f45c19b671944eff8cbe3a0f2d3c02511d1b01fbabaff6476682d9d2930c8`
Preprod	`6a9f45c19b671944eff8cbe3a0f2d3c02511d1b01fbabaff6476682d9d2930c8`

Live Demo

https://midnightcounter.vercel.app

What This Does

This contract starts in an unclaimed state. A user can claim ownership by proving they know a secret key without exposing that secret on-chain. Once claimed, the owner may increment the public counter while callers without the matching secret key are rejected.

The contract stores a public hash commitment for ownership and a public count value, while the secret itself remains private and is never revealed to the ledger.

Privacy Model

PUBLIC (on-chain, visible to anyone): the contract state, the public owner hash commitment, and the public count value.
PRIVATE (never on-chain): the secret key returned by the localSecretKey() witness.
PROVED without revealing: the caller demonstrates knowledge of the secret behind the public owner commitment.

The hash commitment is disclosed during claim(), but the underlying secret remains hidden.

Privacy Claim

An on-chain observer can see the counter state, the owner hash commitment, and each successful increment. They cannot see the owner's secret key; increment() proves knowledge of the key matching the commitment without revealing the key.

Tech Stack

Midnight Network
Compact language
Midnight.js SDK and DApp Connector API
React and Vite
Lace wallet
Node.js v22
Docker

Prerequisites

Node.js v22 and npm
Lace wallet installed and configured for Preprod
Compact CLI/compiler 0.31.1
Docker running locally
Access to the Midnight Preview or Preprod network
A funded Preprod wallet for deployment

Run Locally

git clone <repository-url>
cd Midnight_counter
nvm use 22
npm ci
cp .env.example .env
npm run dev

The app uses Lace's DApp Connector to request a Preprod connection, show the wallet address, and clear the local connection state on disconnect. It reports missing Lace, a declined connection, and a network mismatch.

VITE_CONTRACT_ADDRESS is reserved for the deployed Preprod contract. The current frontend does not yet read the on-chain count, generate a proof, or submit a transaction. The increment button remains disabled because the generated contract runtime and transaction providers are not wired up; setting the address alone will not enable it.

Deploy Frontend

After deploying the Compact contract to Preprod, configure Vercel and deploy:

npx vercel login
npx vercel link
npx vercel env add VITE_CONTRACT_ADDRESS production
npx vercel env add VITE_MIDNIGHT_NETWORK_ID production
npx vercel --prod

Enter the deployed Preprod contract address when prompted for VITE_CONTRACT_ADDRESS, and preprod for VITE_MIDNIGHT_NETWORK_ID. Add the production URL to the Live Demo section above.

Compile Contract

mkdir -p "$HOME/.local/bin"
curl -fL https://github.com/LFDT-Minokawa/compact/releases/download/compactc-v0.31.1/compactc_v0.31.1_x86_64-unknown-linux-musl.zip -o /tmp/compactc.zip
unzip -o /tmp/compactc.zip -d /tmp/compactc-install
cp /tmp/compactc-install/* "$HOME/.local/bin/"
chmod +x "$HOME/.local/bin"/*
ln -sf "$HOME/.local/bin/compactc" "$HOME/.local/bin/compact"
export PATH="$HOME/.local/bin:$PATH"

Verify the toolchain before compiling:

node --version
compact --version

The challenge prompt's npm install -g @midnight-ntwrk/compact-compiler command currently returns 404 from npm. This project uses the direct compiler installation and script flow required by Compact 0.31.1.

Compile the contract and generate the managed artifacts:

npm run compile

The generated managed/ directory contains the contract interface, circuits, and proving and verification keys.

Demo Video

Placeholder: add the recording link after filming. Record the requested circuit flow only after proof generation and transaction submission are integrated; those actions are not available in the current frontend.

Connect Lace and show the wallet address.
Call the circuit and show proof-generation loading.
Show the on-chain result after submission.
Point out that the private input is never displayed.

Run Tests

npm test

The test suite validates the contract logic, generated compiler metadata, public ledger transitions, and the private witness model. A full transaction-level test requires the Midnight runtime, proof server, and a funded wallet.