Zarcade

Zarcade is an arcade that teaches Zcash privacy. Every machine is a small game that moves real practice money on the Zcash test network. Built for the Zecathon, Track 05 Wildcard.

The ask

You are asked to send a small payment. You pick: hide it, or send it in the open. Then you see exactly what a stranger can see afterwards. You install nothing and hold no keys.

Proof it is real

Every round broadcasts a real transaction to the Zcash testnet. Two early ones, both mined on 2026-10-11:

• Hidden: 952a1959…f3299c77 in block 4,488,884. The explorer labels it a fully private Ironwood pool transaction: all amounts, senders and recipients encrypted.¹
• Open: 7ffa8ea6…45bb914f2 in block 4,488,905. The explorer gives it a privacy score of 0 out of 100 and names the amount and the recipient.¹

What you see

How a round works

1. You pick hide or open.
2. The house wallet sends 1,000 zatoshis of testnet money. The amount and the destination are fixed on the server, never by the player.
3. Hidden: the payment lands in the shielded pool. Open: it lands at a transparent address, in full public view.
4. You get the receipt: the transaction id and, once it is mined, the block.
5. The next player can go right away. The pot is split into confirmed small coins ahead of time, so nobody waits for one large coin to settle.

Hide versus open, side by side

	Hidden	Open
Explorer shows the amount	No	Yes
Explorer shows the sender	No	No. The coins come out of the private pool, so the payment never names who sent it
Explorer shows the recipient	No	Yes
A private note can ride along	Yes	No. The network refuses memos on transparent payments
The explorer's own verdict	"Fully private… all amounts, senders, and recipients are encrypted"	Privacy score 0/100

How it is built

flowchart LR
  P[Player picks hide or open] --> R["POST /api/game/unseen/send"]
  R --> G{Three guards}
  G -->|refuse| X[No money moves]
  G -->|pass| W[Rust sidecar with the house wallet]
  W -->|signed transaction| C[(Zcash testnet)]

Loading

The front end is Next.js 16 with React 19 and TypeScript. The wallet is a Rust sidecar built on axum and librustzcash. Its seed never leaves the sidecar's data directory. On a dev machine it listens on 127.0.0.1:9070, which only that machine can reach. On Railway it has no public door at all, and its send endpoint carries three locks of its own (D-094): a service token, the game's exact payment policy (the house's own two addresses, the fixed round amount, nothing else), and a daily budget that survives restarts.

Exactly one route can move money: POST /api/game/unseen/send. It accepts {"choice": "hide"} or {"choice": "open"} and nothing else. Unexpected fields are refused, not ignored. Three guards run before the wallet is touched: a global pause, a per-visitor hourly share, and a daily ceiling for the whole arcade.

Real numbers, measured on this machine:²

• Fees follow ZIP-317: 5,000 zatoshis per logical action.
• One zero-knowledge proof takes about 12 seconds to build.
• A full round takes 21 to 29 seconds. Confirmation waits ran 34 to 99 seconds against blocks that average 23.6 seconds.
• 76 tests pin the money rules down.

The house wallet at the moment of writing (GET /api/house/status, address fields trimmed):

{
  "ok": true,
  "refilling": false,
  "network": "testnet",
  "syncedTo": 4492642,
  "zatoshis": 11428000,
  "taz": "0.11428000",
  "spendableZatoshis": 11412000,
  "pendingChangeZatoshis": 0,
  "transparentZatoshis": 16000,
  "readyForRound": true
}

Field note: the faucet that paid on a dead chain

On 2026-10-04 the public testnet activated NU7 at height 4,465,026. The well-known faucets kept paying on the abandoned pre-NU7 fork, so their coins are invisible to a wallet on the canonical chain. We claimed a deposit that, as far as our sidecar was concerned, never existed. The fix was finding a faucet on the living chain and re-funding. It was logged as D-027 in DECISIONS.md before any code changed.

Field note: the bug our own audit caught

The motion question remembers your choice, but a fresh page load ignored it. React 19 refuses to patch attribute mismatches found at hydration, so the server-rendered value stayed stuck. It reproduced in the production build, not just in dev. The fix follows the pattern Next.js documents: an inline script applies the saved choice to the page before the first paint. An earlier note had called the warning harmless. That note was wrong, and the correction is public in ROADMAP.md (D-051).

Field note: the dust floor

A wallet can hold coins it will never spend. Anything at or below the ZIP-317 marginal fee of 5,000 zatoshis is treated as dust and deliberately left out of the balance. We found this by reading the wallet database directly when the numbers did not add up (RESEARCH.md, item I5). It is why the pot split refuses to create any coin at or below the dust floor: a pot of dust is an arcade nobody can play.

Run it locally

You need Node.js 20.9 or newer and a stable Rust toolchain. On Windows, also the Visual Studio Build Tools with the C++ workload.

1. Start the house wallet. It has to run before anything can move money.

   cd sidecar
   cargo run

   The first run creates the wallet and scans the chain from a pinned birthday height. Wait for initial sync complete up to height … before continuing.

2. Fund it with free testnet coins from the Zakura NU7 faucet (https://zakura.com/nu7), sent to the deposit address the sidecar prints. One warning, learned the hard way: faucets paying on the abandoned pre-NU7 fork show money that does not exist on the canonical chain.

3. Before a play session, split the pot so players can follow each other:

   powershell -File sidecar\split-pot.ps1

   Allow about eight minutes for the default 40 coins.

4. Start the site.

   npm install
   npm run dev

   Open the printed URL, then load /api/house/status in the browser. "ok": true means the site can see the wallet.

Full operating details, session checklist and troubleshooting: OPERATING.md.

The look

Gold means lit and public. Cool shadow means sealed and private. There is no red and no green anywhere, because nothing here is graded by colour. The Watcher, the hooded figure on every screen, stands for everyone who can read the public record. Every entrance asks "Motion: more or less?" and remembers the answer. Type: Syne for display, Space Grotesk for prose, Geist Mono for chain data. The full system is written down in STYLEGUIDE.md.

Limits

• Testnet only. No real money, ever.
• Two machines are live: Unseen and The Crowd. Four cabinets are dark and the floor says so: The Proof Puzzle, The Double-Spend Heist, The Auditor's Key, The Vault Run.
• One confirmation cannot be skipped, so round pacing depends on the chain.
• The first sync is slow and the pot split is slower. Both are stated costs, not surprises.
• The house wallet serves one operator. It is not multi-tenant production software.

How this was built

One owner, one AI assistant, and a written agreement: the owner decides, the assistant advises and drafts, and nothing is built until it is written down and approved. Every decision, D-001 through D-054, is logged in DECISIONS.md with its why, its alternatives and who decided. Open questions stay visible until answered. The agreement itself is AGREEMENT.md. The repository lives at github.com/Valorian0108/zarcade.

Repo map

app/           the site: door (/), floor (/arcade), machine (/games/unseen), API routes
components/    the game frame, the Watcher, the door and floor, the motion question
lib/games.ts   the cabinet list, single source of truth
lib/zcash/     the bridge from the site to the sidecar
sidecar/       Rust: the house wallet (its data/ folder is secret and gitignored)
assets/        screenshots for this README
OPERATING.md   running the arcade: pot split, session checklist, troubleshooting
AGREEMENT.md   how owner and assistant work together
DECISIONS.md   every decision with its why
ROADMAP.md     phases and the exact state of each step
STYLEGUIDE.md  the design system
README-PLAN.md the plan this README was written from
VISION.md, SCOPE.md, ARCHITECTURE.md, OUTLINE.md, RESEARCH.md, BRIEF.md

License

MIT. See LICENSE.

Footnotes

1. Explorer labels and privacy scores from testnet.zecblock.com, fetched 2026-10-11. ↩ ↩²

2. Measured during the B4 and B5 build phases on 2026-10-11. Details in RESEARCH.md, items I1, I2, I5 and I7. ↩