Zcash developers plan a fix to keep ZEC swaps private from future quantum computers

The privacy-focused cryptocurrency Zcash is taking significant steps to future-proof its transactions against the theoretical threat posed by quantum computers. While Zcash is designed to offer private payments, a specific vulnerability has been identified within cross-chain swapping services like NEAR Intents. This vulnerability could, in a quantum computing future, allow an attacker to link a user's entire transaction history from a single, publicly posted refund address.

NEAR Intents

NEAR Intents functions as a crucial bridge for users looking to swap tokens across various blockchains, including ZEC. When a user initiates a swap involving private ZEC, the service temporarily posts one of their private Zcash addresses on NEAR’s public chain. This is done to facilitate potential refunds of unused ZEC, ensuring that if a swap fails or is incomplete, the funds can be returned to the correct origin.

The core issue arises because this refund address remains on the public chain even after a successful swap, creating a persistent data point. While this poses no threat to privacy with current computational technology, it represents a potential weak link that could be exploited by sufficiently powerful quantum computers, undermining the fundamental privacy promise of Zcash.

Shor's algorithm

The theoretical threat to Zcash's privacy, and indeed to much of modern cryptography, stems from the potential application of Shor's algorithm. Published in 1994, this algorithm demonstrates how a quantum computer could efficiently factor large numbers, a task that is computationally infeasible for classical computers. In the context of cryptocurrencies, Shor's algorithm could be used to reverse-engineer public keys from addresses, thereby exposing the underlying viewing keys.

For Zcash, an attacker who cracks a viewing key from a single published address could then link all payments associated with that key, revealing transaction amounts and private memos. This is a concern not just for Zcash but also for older cryptocurrencies like Bitcoin, where public keys are already on-chain. Recent advancements, such as Google's Willow quantum chip producing near-perfect components for such attacks, underscore the urgency of developing quantum-resistant solutions, even if fully capable machines are still years away.

Zakura

Zakura, a newer iteration of the Zcash network's software, is spearheading the proposed fix to mitigate this quantum threat. The primary innovation is to assign a unique viewing key to every ZEC swap conducted through NEAR Intents. This means that even if a future quantum computer were to crack a refund address and its associated viewing key, it would only expose the details of that single swap, leaving the user's broader payment history secure.

Furthermore, Zakura addresses the practical challenge of managing hundreds of unique keys for a user who has made numerous swaps. It proposes a private information retrieval technique, allowing wallets to query a database for refund information without revealing which specific addresses they are checking. This ensures both privacy and usability, with compatible software wallets like Vizor expected to integrate this open-source feature within weeks, aligning with Zcash developers' January target for quantum-resistant payments.