Zcash developers at Zakura have outlined a new privacy mechanism designed to limit how much transaction information a future quantum computer could expose when users swap ZEC through NEAR Intents. The proposal introduces a separate incoming viewing key for each publicly exposed swap address, allowing users to keep their existing spending keys and wallet account intact. The goal is to prevent a single address from becoming the entry point for linking every payment received by the same Zcash account.
The concern centers on quantum computing and the possibility of “harvest now, decrypt later” attacks. An attacker could collect public blockchain data today and attempt to exploit advances in quantum computing in the future. Zcash’s shielded transactions already provide substantial privacy protections in many circumstances, but Zakura says publicly disclosed shielded addresses create a specific risk before broader quantum-resistant protections arrive through Tachyon.
That risk becomes relevant when users rely on cross-chain services. NEAR Intents swaps can require a Zcash refund address to be published on NEAR’s public blockchain. Even if a swap completes successfully and no refund is needed, the address may remain permanently visible. Zakura’s proposal seeks to reduce the potential damage if an attacker eventually derives the viewing key associated with that address.
How One-Time Viewing Keys Limit Exposure
Under the existing arrangement described by Zakura, multiple diversified addresses from one Zcash account share an incoming viewing key. Someone who obtains that key can identify payments received by the account, including their values and memos. A sufficiently powerful quantum computer running Shor’s algorithm could potentially recover the relevant key material from a publicly exposed address, according to the technical explanation.
Zakura’s proposed solution derives a different incoming viewing key for every swap address. The wallet changes the randomness used in the viewing-key derivation while keeping the account’s spending authority and other key components unchanged. As a result, compromising one published address should expose payments associated with that specific address rather than revealing the full history of incoming payments across the account.
This approach also avoids forcing users to create a separate account for every swap. Zakura says that a multi-account workaround would introduce practical problems for hardware wallets. Keystone’s Zcash support is described as limited to signing from account index zero, while Ledger requires separate transactions to spend from different accounts. Managing funds across many accounts could therefore complicate routine payments and transaction approvals.
The proposed design instead preserves a single account for spending. Each swap receives its own viewing key, but funds returned to the associated address remain spendable through the account’s existing spending key. This separates the ability to view payments from the authority to move funds, which is central to the design.
Private Recovery Without Scanning Every Transaction
Creating a new viewing key for every swap introduces another challenge: wallet recovery. A wallet restored from its seed would otherwise need to test every swap-specific key against the blockchain’s transaction history. That scanning burden would increase as the user completed more swaps.
Zakura proposes using private information retrieval (PIR) to make recovery more efficient. NEAR bridge transactions on Zcash reportedly use a shared all-zero outgoing viewing key, which allows the team to build an index linking relevant recipient addresses to transactions. A restored wallet can check published filters to identify potentially paid addresses and then privately query the index for the corresponding transaction.
PIR is intended to conceal the wallet’s query from the server. The wallet sends an encrypted request, the server computes a response without learning which address is being queried, and the wallet decrypts the result. This allows the wallet to locate relevant payments without downloading and scanning the entire transaction history using every swap-specific viewing key.
Recovery also relies on encrypted notes that the wallet leaves for itself during outgoing swaps. These notes record the index used to derive each refund address, allowing a restored wallet to reconstruct the appropriate key. Swaps into ZEC require a separate method because the wallet may not create an outgoing Zcash transaction with a change note. In that case, the wallet checks addresses in sequence against published filters and stops after finding 30 consecutive unused addresses.
Zakura says software wallets using its open-source cryptography and wallet libraries, including Vizor, are expected to be able to integrate the feature in the coming weeks, with hardware-wallet support following when firmware permits. The implementation is not yet described as universally deployed. If adopted and independently assessed, the design could offer Zcash users a practical way to reduce the consequences of future quantum attacks on publicly exposed swap addresses without abandoning the convenience of a single wallet account.
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