Zcash Developer Zakura Proposes Fix to Stop Quantum Computers Linking Shielded Swaps

What it is

Zakura, a Zcash full node developer, announced on 18 October 2026 a proposal for "post-quantum unlinkable swaps" in an engineering post by Zakura developer Adam Tucker. It gives each published shielded address its own incoming viewing key (IVK), so a future quantum attacker who breaks one address learns nothing about the account's other payments. The page describes a proposal and does not report a shipped release.

How it works

A Zcash shielded account has one IVK behind all of its addresses. A classical computer cannot recover it from an address. A sufficiently powerful quantum computer running Shor's algorithm could, according to the post. The attacker would then see the value and memo of every payment the account ever received and could link them.

Swaps out of ZEC through NEAR Intents write one of the user's shielded addresses to NEAR's public chain as a refund address. This happens whether or not a refund is ever made. The addresses look different but share one account, so breaking one exposes all of them.

The fix derives a separate IVK for each address that must be published. The account's spending key is unchanged. Per Tucker, a broken swap address could not be linked to any other transaction.

Extra keys could slow seed-phrase restores. To avoid that, the wallet looks up NEAR payouts using private information retrieval (PIR), which lets it query a server without revealing which addresses it asks about.

Preconditions / Affected

• The exposure is greatest for users who swap out of ZEC through NEAR Intents. Zcash Open Development Lab CEO Josh Swihart said in May 2026 that about $600 million to $700 million had flowed through that route.
• Tucker describes the gap as lasting "until Tachyon ships". Tachyon is a proposed Zcash scalability upgrade whose developers say it would also deliver full post-quantum privacy.

What to do

• The change needs no network upgrade.
• Wallets built on Zakura's open-source libraries, such as Vizor, can add one-time keys for software wallets within weeks, per the post.
• Hardware wallets follow once their firmware supports it.

Caveats

• The post gives no date for a quantum computer able to run this attack.
• It covers swap addresses only and does not protect addresses already published under a shared IVK.
• Zakura's earlier plan for quantum-safe transparent addresses expected post-quantum signature opcodes in Zcash in January. Swihart said in May that full post-quantum status was targeted within 12 to 18 months.