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

Zakura, a developer of Zcash full node software, has proposed a way to stop a future quantum computer from linking users' private Zcash payments, in an engineering post by Zakura developer Adam Tucker announced on Sunday.

The approach needs no network upgrade, Zakura said in a tweet introducing what it called "post-quantum unlinkable swaps for Zcash."

Zcash's shielded transactions hide the sender, recipient and amount. Each shielded account has a single incoming viewing key behind all of its addresses, which its wallet uses to find and decrypt payments sent to any of them.

A classical computer cannot recover that key from an address. But a sufficiently powerful quantum computer running Shor's algorithm could, according to the post, and would then be able to see the value and memo of every payment the account has received and tie them together.

"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," Tucker wrote, referring to the proposed scalability upgrade for Zcash that its developers say would also deliver full post-quantum privacy.

The biggest risk comes from swaps through NEAR Intents, a cross-chain swap service, according to the post. Each swap out of ZEC writes one of the user's shielded addresses to NEAR's public chain as a refund address, whether or not a refund is ever made. The addresses look different, but they all belong to the same account, so breaking one would expose all of them.

Around $600 million to $700 million had flowed through Zcash's NEAR Intents route by May, Zcash Open Development Lab CEO Josh Swihart said at the time.

Zakura's fix derives a separate incoming viewing key for each address that has to be published, while leaving the account's spending key untouched. A quantum attacker who breaks one posted address would learn only about the payments to that address.

"If a quantum computer ever breaks a posted swap address, it will not be able to link this to any other transaction," Tucker wrote in a Twitter thread.

To stop the extra keys slowing down restores from a seed phrase, the wallet looks up NEAR's payouts using private information retrieval, a technique that lets it query a server without revealing which addresses it is asking about.

Zcash's quantum roadmap

The proposal follows Zakura's plan for quantum-safe transparent addresses earlier this month, which also uses private information retrieval and said post-quantum signature opcodes were expected in Zcash in January. Swihart said in May that Zcash was aiming for full post-quantum status within 12 to 18 months.

Concern over the cryptography that secures crypto wallets is growing, with Ethereum Foundation researcher Justin Drake and Ethereum co-founder Vitalik Buterin warning that AI could break it before quantum computers do.

Wallets using Zakura's open-source libraries, such as Vizor, will be able to integrate the one-time keys for software wallets in the coming weeks, and hardware wallets will follow once their firmware supports it, the post said.

ZEC was trading at $1,165 Monday morning, up 3.6% on the day per Decrypt data.