“Harvest Now Decrypt Later”: Examining Post-Quantum Cryptography and the Data Privacy Risks for Distributed Ledger Networks

This paper analyzes the risks posed by future-state quantum computers, specifically the “harvest now decrypt later” (HNDL) risk. We review foundational concepts of quantum computing to address the present and ongoing threat of HNDL to currently protected data. We use the Bitcoin network as an illustrative example to study the implications of HNDL for distributed ledger cryptocurrency networks that rely upon traditional cryptography. We posit that while cryptocurrency distributed ledger network maintainers could successfully deploy post-quantum cryptography (PQC) mitigations to protect the network’s security and data integrity against a future-state quantum computer, data privacy of the network’s previously recorded transactions remains vulnerable against a future-state quantum computer due to HNDL. The difficulty in protecting data privacy lies in the risk that a bad actor can obtain a distributed ledger replica, harvest the data, and in the fullness of time reveal previously obfuscated and confidential data using a sufficiently powerful quantum computer. The authors highlight this gap in data privacy protection and note the shortage of mitigations for the data privacy risks associated with the HNDL threat within distributed ledger networks.