Chalk Chain: Proof-of-Presence Pay for Teachers

Inspiration

In several sub-Saharan African countries, roughly one in four teachers is absent on a given school day. Paying teachers for each day they can prove they were in class works: a randomized trial in India (Duflo, Hanna & Ryan, 2012) cut absence by 21 percentage points. But the proof gets faked, and photo-attendance apps are gamed with old photos. We wanted proof that's cheap on any phone and hard to fake.

What it does

1. The app turns a Solana block hash only seconds old into three words.
2. The teacher chalks them on the board and photographs the class.
3. Our Solana program rejects the photo if it was sealed more than 90 seconds after the words existed.
4. A vision service checks the words, the head count and that the photo isn't of a screen. Hashing catches reused photos.
5. Each verified photo earns a USDC bonus at the end of the day.

Surprise re-checks, timed by future block hashes, make the teacher add new words under the old ones. Each set depends on the previous photo, so the board becomes a hash chain written in chalk. Teachers never see a wallet or pay a fee.

How we built it

• Program: Anchor 1.1.2 (Rust). Checks Solana's recent block hashes on-chain to enforce the time window, and pays USDC from a program-owned vault.
• Backend: Hono relayer that pays transaction fees, but only for our own instructions.
• Vision: FastAPI + Claude, with decoy words to catch misreadings, plus perceptual hashing (PDQ) to catch reused photos.
• App: React + Vite installable web app.
• Shared library: @solana/kit 8. The word derivation matches byte for byte in Rust, TypeScript and Python.

Challenges we ran into

• Anchor 1.x and Kit 8 are newer than most tutorials.
• The list of recent block hashes is too big to decode normally on-chain, so we parse it by hand.
• Reading chalk handwriting is unreliable, so we made it multiple choice with decoys.
• A fee-paying relayer is a wallet anyone can call, so we locked it to our instructions.

Accomplishments that we're proud of

• The full loop runs on a real Solana validator: check-in, verify, re-check and payout.
• Late photos, reused photos and forged transactions all fail.
• 111 unit tests plus a 39-check end-to-end run.

What we learned

Solana's most underrated feature may be its clock: a fresh, unpredictable, verifiable value every few hundred milliseconds. We also learned that asking a model "which of these words do you see?" works much better than asking "what does this say?"

What's next for Chalk Chain: Proof-of-Presence Pay for Teachers

A 10–20 school pilot with an NGO, SMS check-in for weak connections, cash-out to mobile money, face blurring on the phone, and the same proof for community health workers.

Built With

• .env
• .log
• blockchain
• claude
• computers
• crypto
• json
• math
• openai
• png
• programming
• python
• rust
• solana
• technology
• typescript