Tenjō

Demo

About

Concert ballots and scarce drops make fans lose again and again, and every ballot forgets them. About 2.2 million people applied for the first Switch 2 lottery in Japan. By the fifth lottery, Nintendo had added a rule by hand: only people who had entered before and never won. That’s a pity system. Gacha games call it 天井, tenjō, the ceiling: keep trying and your luck builds. Tenjō makes that ceiling a primitive for any ballot. Each real person enters once. Your name goes in the draw once, plus once for every ballot you’ve lost in the same series (a tour, a shop, a product line), up to six chances. A win resets you to one. Yui, our example fan, has lost three ballots on a tour, so her next entry goes in four times. On paid drops you lock a refundable deposit: win and it pays for your seat, lose and it comes straight back. A pity counter only works if nobody can cheat it, and the two cheats need two networks. World ID stops one fan becoming fifty accounts that farm losses: each entrant proves they’re one unique human, and no name, email or document is stored. Sui stops anyone, us included, from fiddling the draw, the money or the counts: a Move package draws with sui::random, holds deposits in escrow, refunds every loser in the settlement transaction and keeps the loss ledger on-chain. A real World ID starts fresh in every drop, so a passkey on the fan’s phone remembers their losses. It never buys a second entry. For organisers (ticketing platforms, fan clubs, shops running lottery sales), Tenjō replaces three hacks: play-hour rules to stop bots, chasing winners who don’t pay, and “it’s rigged” complaints. Winners get a Ticket that can’t be transferred, which matters in Japan, where reselling specified event tickets above face value has been banned since 2019. It’s live at tenjo-azure.vercel.app on Sui testnet, with 22 World ID entries verified on production so far.

How it's made

TypeScript end to end: Next.js 16 and React 19 on Vercel, Neon Postgres (PGlite locally) and a Sui Move 2024 package, tenjo::ballot. World ID: IDKit 4.3 in the browser. The server issues an RP-signed request, valid for five minutes and bound to one drop and purpose, then forwards the proof byte for byte to World’s v4 verify API. A 200 is never enough: the configured credential’s own result must succeed with a matching nullifier, action, environment, nonce and signal, and the request is consumed in the same database transaction as the entry. The nullifier is canonicalised and hashed into a 32-hex anonymous code. World ID 4 nullifiers are single-use per action, so real World IDs get one action per drop and a fresh code each time. To carry their losses we added WebAuthn passkeys (SimpleWebAuthn): the passkey signs over that drop’s World ID request and is checked before World is asked, so a refusal never spends the proof. A per-drop identity table stops a second passkey from buying a second entry. Sui: a shared Series keeps the loss ledger (Table<code, u64>) and each Drop<T> is an escrow vault over any coin type. For a paid entry, the server signs an Ed25519 permit over drop, code and sender after the World ID check, and ballot::enter rebuilds it with the real sender, so a copied permit fails. draw is a non-public entry function taking &Random that only stores a 32-byte seed. Its gas never depends on the outcome, so an unlucky draw can’t be aborted and retried. settle is public and deterministic: weighted picks without replacement from blake2b256(seed ‖ i), the winners’ deposits paid to the organiser as one coin, every loser refunded, winners with a wallet given a Ticket with key but no store, and every pity count updated, all in one transaction. The drop page re-runs settle in TypeScript from the seed and checks it matches. The server talks to Sui over gRPC with @mysten/sui and serialises organiser transactions with a Postgres advisory lock; fans pay through dApp Kit. Postgres mirrors the chain for fast pages. 31 TypeScript tests (one drives a software authenticator holding a real key), 22 Move tests and Playwright browser tests.