According to an engineering blog post by Zakura developer Adam Tucker on Sunday, Zakura, a developer of Zcash full nodes, has proposed a method aimed at preventing future quantum computers from linking users' private Zcash payments.
Zakura stated that this solution does not require a network upgrade. The company referred to it as "post-quantum unlinkable swaps for Zcash" in a tweet.
Zcash's shielded transactions conceal the sender, recipient, and amount. Each shielded account has a single inbound viewing key behind all its addresses, which the wallet uses to find and decrypt payments sent to those addresses.
A regular computer cannot recover the key from an address. However, according to this article, if a sufficiently powerful quantum computer runs Shor's algorithm, it could achieve this, thereby revealing the amount and notes of every payment received by the account and linking them together.
Tucker wrote: "Before Tachyon launches, there is a clear vulnerability: a publicly exposed protected address is sufficient for quantum attackers to link all payments received by that address’s key." He was referring to the scaling upgrade proposed in Zcash, which developers say will also bring full post-quantum privacy.
The article states that the greatest risk comes from exchanges conducted via NEAR Intents, a cross-chain exchange service. Each time ZEC is exchanged out, the system writes one of the user’s protected addresses to NEAR’s public chain as a refund address—regardless of whether a refund actually occurs. Although these addresses appear different, they all belong to the same account; thus, if one is compromised, all addresses are exposed.
At the time, Josh Swihart, CEO of the Zcash Open Development Lab, stated that as of May, approximately $600 million to $700 million in funds had flowed through Zcash’s NEAR Intents routing.
Zakura's fix will derive a separate inbound viewing key for each address that must be made public, while keeping the account's spending key unchanged. This way, even if a quantum attacker compromises a publicly exposed address, they can only learn about payments associated with that specific address.
Tucker wrote in a tweet: "If a quantum computer one day breaks a publicly exposed exchange address, it will not be able to link that to any other transaction."
To avoid slowing down wallet recovery from the mnemonic phrase due to additional keys, the wallet uses private information retrieval to locate NEAR payment information. This technology allows the wallet to query a server without revealing which addresses it is inquiring about.
Zcash's Quantum Roadmap
This proposal builds on the quantum-safe transparent address scheme proposed by Zakura earlier this month. The scheme also employs private information retrieval and states that Zcash expects to introduce post-quantum signature opcodes in January. Swihart stated in May that Zcash aims to achieve full post-quantum readiness within 12 to 18 months.
Concerns about the cryptographic security of cryptocurrency wallets are rising. Ethereum Foundation researcher Justin Drake and Ethereum co-founder Vitalik Buterin have warned that artificial intelligence could break it before quantum computers do.
The article states that wallets using the Zakura open-source library, such as Vizor, will be able to integrate this one-time-use key into software wallets in the coming weeks; hardware wallets will follow once their firmware supports it.
According to Decrypt data, ZEC opened at $1,165 on Monday, up 3.6% for the day.