A refund address you never used leaks the rest

Zcash exists to make payments private, and a proposal published this week addresses a gap that undoes part of that when ZEC is swapped across chains. NEAR Intents, a cross-chain swapping service, posts one of the user's private Zcash addresses on NEAR's public chain so it knows where to send back any unused ZEC. The address goes up as soon as the deposit lands and stays there even if the swap completes and no refund is ever needed.

Today that address gives nothing away. Each Zcash address carries a public key built from the wallet's viewing key — the secret a wallet uses to spot payments sent to it — through one-way elliptic-curve mathematics of the same kind that protects bitcoin and ether wallets. Ordinary computers cannot run it in reverse.

A large enough quantum computer could. Shor's algorithm, published in 1994 for exactly that job, would work backward from the address to the key, and from the key to the payment history the wallet was meant to keep private. The same reasoning is what hangs over Satoshi-era bitcoin, whose public keys already sit on chain for anyone to copy; CoinDesk notes that Google said last week its Willow quantum chip had produced near-perfect versions of one component such an attack would burn through.

The proposal from the team behind Zakura, a newer version of the software that runs the Zcash network, is to give every swap made through NEAR Intents its own private key. Spending keys stay with users, while private information retrieval helps wallets locate refunds without revealing which addresses they are checking. Zakura says compatible software wallets could adopt the open-source feature within weeks, ahead of the Zcash developers' January target for quantum-resistant payments.