Zcash Developer Proposes One-Time Viewing Keys to Guard Swaps From Quantum Privacy Risks

Zcash developer Zakura has proposed a new privacy design that would assign a one-time incoming viewing key to every ZEC swap routed through Near Intents, targeting a subtle exposure created when shielded Zcash addresses are posted permanently on Near's public blockchain.

Quantum computers are not cracking Zcash today. Even so, Zakura and the Zcash infrastructure team are designing around the possibility that a sufficiently capable machine eventually could.

Under current practice, multiple diversified Zcash addresses can share the same underlying incoming viewing key. Viewing keys give a wallet the ability to detect and decrypt incoming payments without any spending authority — read-only access, in effect — which is what lets users monitor one account spread across many addresses without exposing control of funds.

A powerful enough quantum computer running Shor's algorithm — a quantum method for solving the discrete-logarithm problems that underpin today's public-key cryptography — could, in theory, recover that key from a publicly exposed address and use it to link payments received across the account. That linking ability cuts against the core property shielded Zcash is built to provide: payments to the same user should not be connectable by outside observers.

One Swap, One Viewing Key

Zakura's proposal limits the blast radius of such a compromise. Under the design, every cross-chain swap receives a separate incoming viewing key and refund address. If quantum technology eventually breaks one publicly posted address, an attacker would learn only about payments tied to that individual swap, rather than gaining visibility into the account's broader payment history.

Crucially, the user's spending key — the credential that actually authorizes moving funds — does not change. Funds received through the one-time address remain spendable from the same account, avoiding the cumbersome alternative of creating separate Zcash accounts for every transaction. That distinction matters for both software and hardware wallets, where splitting funds across numerous accounts could create serious usability problems.

Private Lookups Solve the Recovery Problem

Generating a new viewing key for every swap introduces another challenge: wallet recovery. A restored wallet would otherwise have to scan Zcash's transaction history repeatedly using every swap key it had ever generated.

Zakura proposes solving this with private information retrieval (PIR). Public filters help determine whether a refund address has received funds, while encrypted queries allow the wallet to locate the relevant transaction without the server learning which address is being checked. Compatible software wallets using Zakura's open-source libraries could integrate the feature within weeks, with hardware-wallet support expected later.

Real Usage Makes the Risk Worth Solving

The design is not purely theoretical infrastructure without users. In September, Aurora Intents processed more than $19 million across 1,718 swaps for a Zcash-denominated zkSNARKs auction (zkSNARKs are the zero-knowledge proof system at the heart of Zcash's shielded transactions), demonstrating meaningful demand for cross-chain routes into $ZEC.

That level of activity makes permanently published addresses a privacy issue worth resolving before quantum machines become capable of exploiting them.

Zcash developers are separately preparing the broader NU7 network upgrade, though the changes currently proposed for NU7 do not themselves amount to a comprehensive post-quantum migration.

The bigger signal is cultural: crypto security teams increasingly treat quantum risk as an engineering problem to address now, rather than after the first practical attack appears.

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Source: CryptoNewsNet

Original report: Zcash Proposes New Privacy Layer to Guard Swaps From Quantum Risks