Zcash developers are advancing a targeted privacy enhancement to shield ZEC swap activity from potential future quantum computing risks. The team behind Zakura, a full-node implementation of the Zcash network, has detailed an approach that assigns a unique incoming viewing key to every publicly exposed address used in cross-chain swaps, particularly those routed through NEAR Intents.
This measure addresses a specific vulnerability that could otherwise allow a powerful quantum adversary to connect multiple payments belonging to the same user.
Under normal conditions, Zcash’s shielded transactions already maintain strong privacy properties against classical computers and even quantum ones in many everyday scenarios.
Payments are encrypted in a way that does not place sensitive key material on the blockchain itself.
However, when a shielded address must be published on another chain—as occurs with refund addresses for failed or partial NEAR Intents swaps, or recipient addresses for inbound swaps—that exposure creates a lasting record.
A sufficiently advanced quantum computer employing Shor’s algorithm could, in theory, recover the shared incoming viewing key associated with that address.
Once obtained, the key would reveal the values and memos of every payment ever received by the same Zcash account, effectively linking them together.
The concern falls under the broader “harvest now, decrypt later” threat model, in which public data collected today might be decrypted years from now if quantum capabilities mature.
Rather than requiring users to spin up entirely new accounts for each swap—an approach that creates practical friction for hardware wallets such as Keystone (limited to a single account index) and Ledger (requiring separate transaction approvals)—the Zakura design keeps the core spending key unchanged.
It only varies the randomness used when deriving the incoming viewing key. Each swap therefore receives its own distinct viewing key and corresponding address.
An attacker who later breaks one published address learns only the payments tied to that specific swap; linking it back to the user’s main account would require inverting a hash-based derivation, which remains infeasible even for a quantum computer.
Spending authority stays centralized with the original account key, so funds remain easily accessible.
To keep wallet recovery efficient as the number of one-time keys grows, the system relies on private information retrieval.
Because NEAR bridge transactions on Zcash use a uniform outgoing viewing key, an index can be built that maps addresses to the relevant transactions.
Restored wallets check compact filters first, then privately query the index without revealing which addresses they control.
Encrypted notes left to the user during outgoing swaps record the derivation index, enabling precise reconstruction. Inbound swaps follow a sequential filter-checking process.
Compatible software wallets built on Zakura’s open-source libraries, including Vizor, are expected to support the feature within weeks, with hardware-wallet firmware updates to follow.
No quantum computer capable of recovering these keys has been demonstrated.
The change is designed as a deployable interim step ahead of longer-term post-quantum work such as the Tachyon project and related network upgrades. By isolating exposure to individual swaps without sacrificing usability, the proposal aims to extend practical privacy for ZEC users who interact with cross-chain services.
