A debate on X this week exposed a core question for on-chain privacy: when quantum computers are able to break elliptic-curve cryptography (ECC), will they be able to retroactively deanonymize every transaction ever made of privacy coins like Zcash?

Nic Carter, co-founder of Coin Metrics and partner at Castle Island Ventures, argued that the answer is effectively yes for most privacy coins. “For privacy coins, even if they migrate to post-quantum cryptographic schemes, all historical transactions prior to that migration can be decrypted,” he said on October 30, 2025. “So all historical txns will be stripped of privacy in >~5y. Everything is built on ECC.”

Carter’s point is based on “harvest now, decrypt later.” Attackers don’t need to break you today. They just copy the data now and cr

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