Zcash full node developer Zakura has published an experimental implementation designed to stop a future quantum computer from linking shielded swap payment histories, without requiring a network upgrade. The proposal, introduced by Zakura engineer Adam Tucker on October 18, derives separate incoming viewing keys for swap refund and receiving addresses while preserving the account's spending authority.
Zcash shielded transactions hide the sender, recipient and amount. However, each shielded account has a single incoming viewing key behind all its addresses. According to Zakura, a sufficiently powerful quantum computer running Shor's algorithm could eventually recover that key from one published address and then reveal the value and memo of every payment the account has received, tying them together. As Tucker wrote, until the proposed Tachyon scalability upgrade ships, "a single published shielded address lets a quantum attacker link every payment that address's key has ever received."
The main exposure is through NEAR Intents, a cross-chain swap service. Each swap out of ZEC writes one of the user's shielded addresses to NEAR's public blockchain as a refund address, whether or not a refund is ever made; swaps into ZEC also publish the receiving address. These addresses can appear unrelated while sharing the same incoming viewing key. Around $600 million to $700 million had flowed through Zcash's NEAR Intents route by May, according to Zcash Open Development Lab CEO Josh Swihart.
Zakura's fix isolates published swap addresses by assigning each a separate viewing key. If a future quantum computer breaks one posted address, it would learn only the payments associated with that key, not the account's broader incoming history. The spending key remains unchanged, so users can still spend funds received through those separate keys from the same account.
To solve wallet restoration, outbound swaps leave an encrypted memo in a note belonging to the user's funding transaction that records the index used to derive the refund key. Incoming swaps instead check deterministically derived addresses in sequence. The receiver-discovery system uses public filters checked locally and private information retrieval, or PIR, to identify relevant transactions without telling a server which addresses are being queried. This prevents a readable series of lookup requests from revealing that several addresses belong to the same wallet.
The project remains experimental. Zakura says the key derivation requires cryptographic review before live addresses are issued, hardware signers need firmware qualification, and the code does not yet establish that wallets have enabled the feature for ordinary users. Integration into compatible software wallets, including Vizor, is anticipated in the coming weeks, though that is a development estimate rather than a confirmed release date. The proposal is separate from January’s planned protections for transparent Zcash payments and cannot remove addresses already disclosed publicly.
ZEC was trading at $1,165 Monday morning, up 3.6% on the day, according to Decrypt data.
