Zcash Builder Zakura Proposes Per-Swap Viewing Keys to Close a Quantum Privacy Gap

Introduction

Zcash’s privacy model hinges on a single incoming viewing key that can decrypt every payment to a shielded address. While classical computers can’t derive this key, a future quantum computer running Shor’s algorithm could, potentially exposing the full history of any compromised address. Zakura, a team developing Zcash node and wallet software, has drafted a design that assigns a unique viewing key to each address used in a NEAR Intents swap. The proposal aims to limit the impact of a quantum breach to the individual swap rather than the entire account.

Why Swaps Reveal a Single Key

In normal Zcash usage, a user shares a shielded address privately with a payer. NEAR Intents, a cross‑chain swap service, works differently: the wallet supplies a refund address that the NEAR blockchain publishes on‑chain once the deposit is logged. The same address is retained even if the swap fails, and the same incoming viewing key governs all such addresses. If a quantum adversary ever recovers that key, every payment linked to it—including amounts and memos—becomes visible.

The Per‑Swap Viewing Key Design

Zakura’s approach modifies only the randomness that seeds the incoming viewing key. By hashing a swap‑specific index, the wallet derives a fresh viewing key for each swap while keeping the spending key unchanged, so funds remain in a single account. This design avoids the UX pitfalls of opening a new account per swap—hardware wallets like Keystone and Ledger currently support only the first account or a single account per transaction.

Key technical points:

• Indexed Key Derivation: Each swap’s index is hashed to produce a distinct viewing key.
• Encrypted Memo Storage: The wallet records the swap index in an encrypted memo, enabling it to locate the correct key later.
• Private Information Retrieval (PIR): When checking NEAR bridge payouts, the wallet uses PIR to query the blockchain without revealing which addresses it is interested in.
• Address Scanning Limits: For inbound swaps, the wallet scans addresses sequentially and stops after encountering 30 consecutive unused addresses, reducing scan overhead.

Path to Adoption

The design is currently listed as “in review” on Zakura’s engineering page and has not yet shipped. Software wallets that integrate Zakura’s libraries—such as Vizor—could implement the per‑swap keys within weeks, according to the proposal’s author. Hardware wallet support will follow once firmware updates accommodate the new key‑derivation scheme.

Practical Takeaways

• Quantum‑Resilience Increment: Even before Zcash’s planned Tachyon upgrade, per‑swap viewing keys provide a pragmatic mitigation against a future quantum threat.
• Minimal UX Disruption: Users retain a single spending account, preserving the familiar wallet experience while enhancing privacy.
• Implementation Timeline: Expect software wallet updates in the near term; hardware wallet updates will lag pending firmware releases.

--- Zcash users and developers should monitor Zakura’s repository for the final design release and prepare to test the new per‑swap viewing key functionality as it becomes available.