“Harvest now, decrypt later” is the risk that an attacker could collect encrypted information today and keep it until a future cryptographically relevant quantum computer can decrypt it. That computer does not exist yet, and NIST says no one knows when one will. The reason to act now is the combination of data that must remain confidential for years and the time it takes to update cryptography across products, services, and systems.
What “harvest now, decrypt later” means
In an HNDL attack, the attacker records encrypted communications or data now, without necessarily being able to read them. If a future quantum computer can break the public-key cryptography protecting that information, the attacker could decrypt the stored material later.
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The danger is therefore about confidentiality over time, not a claim that current encryption has already been broken. It matters most when information will still be sensitive years from now: the value of a secret may outlast the protection currently used to keep it private.
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Consider information that would cause harm if exposed long after it was sent or stored. If it must stay confidential for many years, an organization has to account for the possibility that it could be collected now and exposed later. Data with a shorter confidentiality lifetime may present a different priority. The key question is how long the data needs protection, not simply whether it is encrypted today.
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Why the timing is uncertain
NIST’s explainer says, “No one knows how long it will take to build a cryptographically relevant quantum computer.” That uncertainty is not evidence that such a computer is imminent, nor is it a reason to assume the threat can be ignored. NIST also notes that, historically, integrating new algorithms into information systems after standardization can take 10 to 20 years. That is an observation about past integration timelines, not a forecast that every post-quantum migration will take that long.
What post-quantum cryptography changes
Post-quantum cryptography (PQC) means cryptographic algorithms designed to resist attacks from both conventional computers and quantum computers. It is intended to let systems protect information and verify digital signatures even if a cryptographically relevant quantum computer becomes available.
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In August 2024, the U.S. Secretary of Commerce approved three NIST standards. They do different jobs: one establishes shared secret keys, while the other two provide digital signatures. A key-establishment method and a signature method are not interchangeable; systems need the appropriate cryptographic function for each use.
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|---|---|---|
| FIPS 203: ML-KEM | Key encapsulation mechanism (KEM) | Establishing a shared secret key across a public channel. Derived from CRYSTALS-KYBER. |
| FIPS 204: ML-DSA | Digital signature | Helping authenticate a signer and detect unauthorized changes. Derived from CRYSTALS-Dilithium. |
| FIPS 205: SLH-DSA | Digital signature | Helping authenticate a signer and detect unauthorized changes. Derived from SPHINCS+. |
NIST’s 2026 project information says the finalized standards can and should be put into use now. NIST is also continuing standardization work, so these three approved standards should be distinguished from algorithms that remain candidates or may appear in future standards.
How to begin a PQC migration
NIST’s National Cybersecurity Center of Excellence (NCCoE) identifies cryptographic asset discovery and inventory as a good starting point. The practical sequence below synthesizes its guidance on visibility, risk management, interoperability, and migration; it is not a mandatory checklist prescribed for every organization.
- Find where cryptography is used. Inventory applications, network protocols, certificates, products, and services that rely on cryptography. Include dependencies supplied or operated by vendors, not only systems your team built.
- Record what each use protects. Map each cryptographic use to the data, process, or system it supports. Note how long confidentiality or authenticity must be maintained and which public-key algorithms are involved.
- Prioritize by exposure and lifetime. Give attention first to high-value, long-lived sensitive data and systems that depend on quantum-vulnerable public-key algorithms. An inventory helps distinguish urgent cases from uses that can be addressed later.
- Ask providers about supported plans. Work with product and service providers to understand their PQC roadmaps, affected components, and how updates will be delivered. A change to one component can depend on changes elsewhere in a service or protocol.
- Test before broad rollout. Check interoperability with connected systems and assess implementation performance in the environments where the cryptography will run. A standard’s existence does not by itself establish that every implementation will work with every existing dependency.
- Build for future change. Maintain crypto agility: the ability to update cryptographic choices as standards, implementation needs, and system dependencies change. Keep the inventory current so future migrations are easier to plan.
Why inventory is more than a list of algorithms
A useful inventory connects cryptography to real systems and data. Knowing that an application uses a particular algorithm is not enough if the organization does not know which service depends on it, what information it protects, or who can update it. NIST NCCoE’s work is framed around two related needs: cryptographic visibility and risk management, and interoperability and benchmarking.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the 2035 transition means—and does not mean
NIST’s project page describes a transition under which quantum-vulnerable algorithms will be deprecated and ultimately removed from NIST standards by 2035, with high-risk systems moving earlier. This is a timeline for the transition of algorithms in NIST standards. It is not a universal legal deadline requiring every private organization to complete its migration by 2035.
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What organizations should avoid assuming
- Do not treat the threat as a present-day decryption capability. HNDL describes a possible future payoff from data collected now; it does not mean quantum computers can already break today’s public-key cryptography.
- Do not wait for a precise arrival date. NIST says the timing of a cryptographically relevant quantum computer is unknown. Planning can instead start with data sensitivity, secrecy lifetime, and migration dependencies.
- Do not treat the three standards as substitutes. ML-KEM is for establishing a shared secret; ML-DSA and SLH-DSA are for digital signatures. Choose and integrate algorithms according to the cryptographic function a system needs.
- Do not assume standardization completes migration. Products, services, protocols, and connected systems still need updates and interoperability checks.
- Do not interpret 2035 as a deadline for all businesses. NIST’s stated date concerns its standards transition, with high-risk systems moving earlier.
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