Lead
The U.S. Federal Reserve released an analytical paper in September 2025 examining the Harvest Now, Decrypt Later (HNDL) strategy and its implications for blockchain networks like Bitcoin. Researchers concluded that even a timely shift to post‑quantum cryptography (PQC) will not protect the privacy of historical on‑chain data due to the blockchain’s immutability.
Key Developments
- The paper analyzes how adversaries can store encrypted or obfuscated data today and decrypt it later once quantum computers are sufficiently powerful.
- Using Bitcoin as a case study, it highlights that the immutability of public ledgers means any information recorded now could be retrospectively exposed.
- The research underscores a critical limit of mitigation: adopting PQC helps secure future activity, but it cannot retroactively shield data already written to the chain.
What is HNDL?
Harvest Now, Decrypt Later refers to the practice of collecting encrypted communications and transaction metadata today with the expectation that future quantum decryption capabilities will render them readable. In the context of public blockchains, this raises acute concerns because:
- Data are permanently recorded and globally accessible.
- Adversaries need only wait for sufficient quantum capability to attempt large‑scale decryption or analysis.
Why historical data remain exposed
The report emphasizes that blockchain immutability is both a strength and a vulnerability. While it preserves integrity, it also ensures that any cryptographic assumptions embedded in past transactions cannot be upgraded in hindsight. As a result, even if networks and wallets adopt post‑quantum algorithms, the visibility and structure of prior on‑chain data could still enable retrospective deanonymization once quantum computers mature.
Context and implications
- The findings elevate concerns around a potential "Q Day"—the moment when quantum capabilities can break widely used cryptography.
- For Bitcoin and other networks relying on traditional cryptographic schemes, the long‑term privacy of users’ historical transactions is at risk under the HNDL model.
- The study reinforces the urgency of accelerating PQC research, deployment roadmaps, and user education—while acknowledging that no future upgrade can fully rewrite the past.
Looking ahead
The paper’s conclusions suggest two parallel priorities for the ecosystem: advance post‑quantum defenses for future transactions and improve privacy‑preserving practices today to minimize exposure over time. As quantum computing progresses, the industry will need coordinated standards, rigorous audits, and clear guidance to balance transparency with user privacy across public ledgers.