Future Quantum Computers Could Link Private Zcash Swaps. Zakura Proposes a Fix

If you swap private ZEC through NEAR Intents, one of your shielded Zcash addresses ends up on a public blockchain. A powerful enough quantum computer could one day use that single address to link every payment your account has received.

Zakura, a team building Zcash software, says it has a fix that wallets could add in the coming weeks. The risk is to privacy, not to your coins.

What is the problem?

Zakura set it out in an engineering post by Adam Tucker. Zcash’s shielded transactions are already private against future quantum attackers “in many ordinary cases”, it says. “But until Tachyon ships, there is a glaring hole: a single published shielded address lets a quantum attacker link every payment that address’s key has ever received.”

“The most concerning case is swaps through NEAR Intents, where the user’s shielded address is posted on NEAR’s public chain,” Zakura writes.

NEAR Intents is a cross-chain swap service. When you swap ZEC for another token, your wallet gives NEAR a refund address in case the swap fails or some ZEC goes unused. That address is “written to NEAR’s public chain as soon as the deposit registers, whether or not a refund ever happens”, according to Zakura.

Why could a quantum computer link my payments?

A Zcash wallet can create many addresses that look unrelated, but they usually share one secret, the incoming viewing key. That key lets a wallet spot and read payments sent to it.

On today’s computers, the key cannot be worked out from an address. Zakura says “a sufficiently powerful quantum computer running Shor’s algorithm” could do it. An attacker who cracked the key could see every payment to the account and link them together, CoinDesk reported, including the amounts and the private notes, or memos, attached to them.

Zakura flags the risk of “harvest now, decrypt later” attacks, where data is collected today and cracked years later. An address posted somewhere public and permanent “will still be there whenever a powerful enough quantum computer exists”, it says.

Are my funds at risk?

No, according to both sources. “Spending takes a separate key, so the funds themselves would stay safe,” CoinDesk reported. Zakura’s fix also leaves the spending key “untouched”.

And the threat is not here yet. “No quantum computer has been demonstrated that can recover these keys,” CoinDesk wrote, and such machines “are still likely years away”.

What would Zakura change?

The idea is a one-time viewing key for each address that has to be published. Zakura says this is done “with the account’s spending key untouched, so an attacker who recovers one address’s key learns only the payments made to that address.”

“On the day a sufficiently powerful quantum computer exists, an attacker can break one posted refund address and learn about that one refund,” Zakura writes. “It doesn’t tie that refund to Alice’s other payments, or to her account.”

Why not just use a new account for each swap?

Zakura considered that and rejected it. Funds would be split across many accounts, which hardware wallets handle badly.

According to the post, Keystone’s Zcash support “only allows signing from account index 0”, and Ledger “can only spend from one account per transaction”. Approving several transactions for one payment “wasn’t an acceptable UX to us”, Zakura says.

How would a restored wallet find its refunds?

Extra keys make restoring a wallet from its seed slower, because each key means another scan of the chain. Zakura’s answer uses private information retrieval (PIR), a technique that lets a wallet query a database without revealing what it asked for.

NEAR’s bridge encrypts all its outgoing Zcash payments with the same “all zero” outgoing viewing key, Zakura says, so payouts can be indexed by address. A restoring wallet checks small public filters, then looks up only the addresses that were paid. “The server never learns which address she asked about,” the post says.

For swaps into ZEC, the wallet checks addresses in order and stops after 30 unused addresses in a row.

When can I use it?

Not yet. “Wallets using Zakura’s open source cryptography and wallet libraries, such as Vizor, will be able to integrate this one-time ivk feature for software wallets in the coming weeks,” Zakura says. “Hardware wallets will follow once their firmware supports it.”

CoinDesk said that would put the fix in place ahead of the January target Zcash developers set earlier this month for quantum-resistant payments.

Our read

This is TSN’s analysis. This is a narrow, practical patch for a long-term privacy risk. It targets the one place where Zcash users’ addresses are reliably made public. For compliance teams, it is a reminder that cross-chain swap services can leave lasting public records for privacy-coin users. Whether it matters in practice depends on adoption: until wallets ship it, each new swap through NEAR Intents still posts an address that, on Zakura’s analysis, a future quantum computer could use to link the whole account.

Sources

• Zakura: Quantum Unlinkability for Zcash Swaps (Adam Tucker)
• CoinDesk: Zcash developers plan a fix to keep ZEC swaps private from future quantum computers, 19 October 2026

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