AI disclosure: drafted by Basis Desk's AI newsroom and machine-checked against the primary sources listed below — how we use AI. Originally published on Basis Desk.
The proposed protocol modification isolates swap addresses using private information retrieval to prevent future quantum computers from de-anonymizing historical wallet payments.
Key points
- Zakura introduced a one-time incoming viewing key (ivk) architecture to shield Zcash swap users from future quantum decryption attacks.
- Cross-chain swaps via NEAR Intents previously exposed user addresses on public ledgers, creating vulnerabilities to Shor's algorithm.
- The solution integrates private information retrieval (PIR) to maintain fast wallet seed recovery without splitting hardware wallet balances.
Zakura published an engineering proposal on Oct. 19, 2026, detailing a defense against future quantum decryption attacks targeting $ZEC cross-chain swaps [1]. The vulnerability affects users who publish shielded addresses to public ledgers during automated swap processes, particularly through protocols like NEAR Intents [1].
Under Zcash's current cryptographic design, shielded transactions hide recipient and public-key data from classical and quantum observers on the native ledger [1]. However, when an address is permanently published to an external public blockchain—such as a refund destination recorded on NEAR's public network—an adversary with a sufficiently advanced quantum computer running Shor's algorithm could derive the user's incoming viewing key ($ivk$) [1]. Because standard Zcash accounts rely on a single $ivk$ across all diversified addresses, compromising that key would expose the value, memo, and linkability of every transaction the account has ever received [1].
Isolating Addresses Without Account Fragmentation
To eliminate the risk of chain-wide de-anonymization prior to the launch of Zcash's Tachyon upgrade, Zakura designed a "one-time $ivk$" architecture that wallets can adopt directly [1]. Standard naive workarounds involve generating a new account for each swap, but hardware wallet limitations make that approach unwieldy [1]. For instance, Keystone hardware signers only access account index 0, while Ledger devices require separate transactional approvals for every account index [1].
Zakura's method instead modifies key derivation by producing a unique randomized secret ($rivk$) per swap index via keyed hashing, while leaving spending keys ($ask$) and nullifier derivation keys ($nk$) untouched [1]. This ensures that all incoming funds remain spendable from a single unified balance without manual multi-account approvals [1]. If a quantum computer subsequently cracks an exposed swap address, the attacker recovers only the isolated viewing key for that specific transaction [1].
To resolve wallet-recovery scanning overhead created by multiple viewing keys, Zakura implemented private information retrieval (PIR), adapting lookup mechanisms previously deployed for Zcash coinholder voting [1]. The update arrives as infrastructure operators expand zero-knowledge integrations, following steps such as when Gemini migrated Zcash infrastructure to Zakura node capabilities [1]. Zakura indicated that client software can integrate the derivation pattern immediately ahead of long-term post-quantum upgrades [1].
Sources
- Quantum Unlinkability for Zcash Swaps — Zakura — Primary document (discovered)
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