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Zcash developer Zakura, which maintains full-node software for the network, has published a proposal to give every shielded address exposed during a NEAR Intents swap its own incoming viewing key, limiting how much a future quantum computer could learn from any single published address. Zakura developer Adam Tucker described the design in an engineering post announced on Sunday, Decrypt reported.
The problem, as Tucker framed it, is that one leaked address can compromise a whole account’s history. “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,” he wrote, referring to the proposed Zcash scalability upgrade its developers say would also deliver full post-quantum privacy. Zakura said in a tweet that the result is “post-quantum unlinkable swaps for Zcash” with no network upgrades required.
Also readZcash's New Zakura Common Cuts Private Transaction Time to Under 200 Milliseconds
Key facts
- Each shielded Zcash account currently uses one incoming viewing key behind all of its addresses; a quantum computer running Shor’s algorithm could, in theory, recover it and decrypt the value and memo of every payment the account received.
- Every swap out of ZEC through NEAR Intents writes one of the user’s shielded addresses to NEAR’s public chain as a refund address, even when no refund is ever made.
- Zakura’s fix derives a separate viewing key per published address while leaving the account’s spending key untouched, so breaking one address reveals only payments to that address.
- Wallets built on Zakura’s open-source libraries, such as Vizor, will be able to add the feature in the coming weeks, with hardware wallets to follow once firmware supports it.
- ZEC was trading at $1,165 on Monday morning, up 3.6% on the day, according to Decrypt data.
Why the swap refund address is the weak point
Shielded Zcash transactions conceal the sender, recipient and amount, and a wallet can generate many addresses that appear unrelated. They still belong to one account, though, and share the same incoming viewing key, which a conventional computer cannot reverse from a published address but a sufficiently powerful quantum machine theoretically could.
NEAR Intents, a cross-chain swap service, writes the refund address into its public chain when a deposit is registered. The addresses differ from one swap to another, but all trace back to the same key. Tucker wrote that if a quantum computer ever breaks a posted swap address, “it will not be able to link this to any other transaction.”
Also readNear Intents Recovers Full $3.8M After 48-Hour Ultimatum
Crypto.news adds detail on the exposure: it concerns transaction privacy, not direct control of funds, because Zcash uses separate keys to authorize spending, so a recovered viewing key would not by itself let an attacker move coins. Crypto.news also reported that the issue cuts both ways, since swaps into ZEC publish the receiving shielded address. It cited Aurora Intents data showing more than $19 million processed across 1,718 swaps tied to the zkSNARKS auction in September, more than half of the assets converted and deposited into the ZEC-denominated sale. Decrypt, citing Zcash Open Development Lab CEO Josh Swihart, reported that roughly $600 million to $700 million had flowed through Zcash’s NEAR Intents route by May.
How the one-time keys work
Under the proposal, a wallet derives a fresh key for each swap address from its existing account credentials, the purpose of the address and an index identifying the swap, using a keyed hash construction already employed in Zcash’s ZIP 32 key system with a separate domain identifier. Authorization and nullifier keys stay as they are, so funds arriving at any of those addresses remain spendable through the original account.
Zakura argued against the simpler alternative of a new account per swap. Crypto.news reported that Keystone’s Zcash implementation can sign only from account index 0, while Ledger can reach multiple accounts but needs separate transactions and approvals for each, creating friction for users whose funds are spread across many swap accounts.
To keep wallet restores from slowing as swap keys pile up, the design relies on private information retrieval, letting a wallet query a server for the location of matching transactions without revealing which addresses it is asking about. Zakura said the technique builds on the lattice-based method developed for its transparent wallet lookups and Zcash coinholder voting.
Why it matters
Zcash’s privacy claim depends on shielded addresses staying unlinkable, and cross-chain services that publish those addresses create a permanent record an attacker could collect today and exploit later. The proposal also sidesteps the usual trade-off of separate accounts, which hardware wallets handle poorly, by keeping everything under one spending key. It sits alongside Zakura’s quantum-safe transparent address plan from earlier this month and Zcash’s separate work on post-quantum signature opcodes, targeted for January, which address spending authorization rather than historical payment linking. Swihart said in May that Zcash was aiming for full post-quantum status within 12 to 18 months.
What to watch
Zakura has not set a confirmed release date. The next tangible step is integration by software wallets using its libraries, including Vizor, followed by hardware wallet support once firmware can handle the new key derivation. Crypto.news also noted that Google reported on Oct. 15 a 40-fold reduction in quantum computing errors using a technique it calls magic state cultivation, while stressing that no quantum computer able to recover Zcash’s elliptic-curve keys has been publicly demonstrated.
Sources: Decrypt, Crypto.news
