Privacy

What Hushh Protocol hides on Arc, and what it does not. Stated plainly, because privacy claims you can not check are worthless.

What stays private

No on-chain link between deposit and withdraw. The recipient withdraws with a Groth16 zero-knowledge proof that a note is in the Merkle tree, without saying which leaf. The withdraw transaction carries a nullifier, never the commitment from your deposit.
Payments to the same person do not share an on-chain tag. Each payment is its own Poseidon commitment. Ten payments are ten unrelated notes in the pool.
Amounts carry no distinctive signal. Notes use fixed USDC sizes (for example 1, 10, 50, 100), so an amount alone cannot match a deposit to a withdraw.

Amounts are standardized, not encrypted. A 50 USDC note is a visible 50 USDC ERC-20 transfer into the pool. Privacy comes from everyone using the same sizes, not from hiding balances.
Username resolution is off-chain. Looking up @username hits the Hushh Protocol backend. The server can see who looked up whom. It does not learn which on-chain note resulted.
Merkle roots are posted by a relayer. An indexer watches deposits, builds the Poseidon tree, and posts roots on-chain. That operator is trusted for liveness and for posting correct roots. Decentralizing it is future work.
The crowd is still small. Unlinkability hides you among deposits in the pool. While volume is low, that crowd is thin, and it strengthens as usage grows.
Trusted setup is demo-grade. Groth16 needs a ceremony. The current keys are fine for Arc Testnet demos, not for production mainnet custody of large funds.

Network: Arc Testnet (chain id 5042002). Explorer: testnet.arcscan.app.