Zakura Proposes a One-Time Viewing Key for Every Zcash Swap

A Zcash client developer has proposed giving every ZEC swap its own viewing key, so that a quantum computer breaking one address could not read the rest of a user's history. The developer is Zakura, and the design went up on X on 18 October. It covers swaps routed through NEAR Intents, a cross-chain service that publishes a refund address to NEAR's public blockchain when a deposit lands. That address stays there whether or not a refund is ever needed. Nothing about the Zcash network itself would change.

What one key currently exposes

A Zcash account derives many addresses, and in current wallets those addresses share one incoming viewing key, the key that decrypts payments received. Shor's algorithm, published in 1994, is the method a large quantum computer would use to work backward from a public key to the secret behind it. Applied to a single published swap address, it would hand over the whole account: every payment in and every memo attached. A one-time key confines that to one swap. Spending is unaffected, because spending takes a different key.

The accounts do not describe the same key

How precise the fix is depends on who reported it. CoinDesk, filing at 1:23 a.m. New York time on 19 October, called it the swap's own private key in one sentence and its own viewing key in another. The Crypto Times is narrower. Only the rivk value changes, it says, derived with a ZIP 32-style keyed hash plus a Zakura domain byte, a purpose and an index, while the components that authorise spending stay put. Bitcoin.com split the difference and called it a one-time incoming viewing key and refund address. Two accounts name Adam Tucker, a Zakura principal engineer, as the author of the design post. CoinDesk names nobody.

Recovering a wallet without scanning the chain

Hundreds of one-time keys create a restore problem, and the proposal answers it with private information retrieval. Public filters tell a wallet whether a given address was ever paid. A filter can flag an address that received nothing, but it never misses one that did, so candidates are ruled out locally. Encrypted lookups then locate the real refund without the server learning which address was queried. Without that, a restore would mean scanning the whole Zcash chain once per key. The Crypto Times adds detail no other account carries: the design stops after 30 consecutive unused addresses, and it declines to give each swap its own account because Keystone signs only from account index 0 and Ledger allows one account per transaction. Those constraints stand unverified elsewhere.

What has not been dated

The work is listed as in review, with no deployment date. Wallets built on Zakura's open-source libraries could pick the feature up within weeks on every account's telling, with Vizor named as a likely first; hardware wallets wait on firmware. Zakura describes the change as a stopgap until Project Tachyon ships, and Tachyon is not live. CoinDesk reported that Zcash developers have set a January goal for quantum-resistant payments, announced earlier this month, without saying which January. No account read here fixes the year. One outlet puts cumulative ZEC volume through NEAR Intents above $1.5 billion, citing a NEAR figure from July, and that number appears in no other account. The attack this guards against has never been demonstrated by any machine built so far.