The Retroactive Decryption Trap: Why Post-Quantum Upgrades Can’t Save Your Past Privacy

“Previous papers in this space tended to be either too theoretical or too optimistic about qubit requirements. This one feels like it’s closing the gap in a way that should make people uncomfortable.”

For his part, Zyskind maintains that moving the entire stack requires post-quantum cryptography (PQC), with lattice-based constructions being the most mature option. While he believes such a move would make mempools safe again, the Fhenix founder still advocates for their encryption.

“While we do that, we might as well start encrypting mempools with PQC encryption and, ideally, with fully homomorphic encryption,” Zyskind explained. “Encrypted mempools solve a bunch of other problems—front-running, MEV extraction, and transaction privacy.”

Zyskind warns that the arrival of a sufficiently powerful quantum computer would not merely “weaken” current zero-knowledge (ZK) systems built on elliptic curve cryptography; it would render them completely obsolete.

“Given a sufficiently powerful quantum computer, any ZK-based system built on elliptic curve cryptography should be considered completely broken,” Zyskind noted. “An attacker can prove false claims, which means they can lie about on-chain state and steal funds. That’s catastrophic.”

However, he pointed out that for standard state transitions and asset transfers, the fix is definitive. Once the Ethereum network and its various layers upgrade to post-quantum secure (PQ-secure) cryptography, the immediate threat of theft is neutralized.

Unlike a hijacked transaction, which is a one-time event, encrypted data stored on a public ledger is permanent. A quantum adversary can wait years to gain the necessary computing power to decrypt historical transactions that were intended to remain private forever.

“All the encrypted data that’s already on-chain, all the transactions that were supposed to be private—a quantum adversary might be able to decrypt those,” Zyskind explained. “So even after you upgrade, users may have their privacy permanently compromised.”

This permanence creates a ticking clock for any protocol handling sensitive data today. For Zyskind and the Fhenix team, this justifies the immediate push for PQ-secure encryption standards before the 2029 deadline arrives.

He concludes with a stark warning for the industry: Users of privacy protocols should operate under the assumption that unless those systems are built from the ground up on PQ-secure encryption, their historical data will eventually be exposed. In the quantum era, privacy isn’t just about protecting the next transaction—it’s about ensuring the past remains buried.