What harvest-now-decrypt-later means

Harvest-now-decrypt-later (HNDL) — also called store-now-decrypt-later or catch-now-decrypt-later — describes an adversary strategy, not a technology. An adversary captures encrypted or otherwise cryptographically protected data today, stores it, and waits. If a cryptographically relevant quantum computer (CRQC) exists later, the adversary decrypts what was captured years earlier.

This is what makes HNDL easy to under-prioritize: nothing is breached today in any way a monitoring system would detect. The exposure is created the day data is captured, not the day it is decrypted. A record that is genuinely safe to expose in ten years carries none of this risk; a record that must stay confidential for decades might already be exposed.

HNDL (store-now-decrypt-later, catch-now-decrypt-later)

Three names for the same adversary strategy: capture protected data now, decrypt it once a CRQC exists.

CRQC (cryptographically relevant quantum computer)

A quantum computer capable of running Shor's algorithm at a scale that breaks the key sizes deployed in practice today. None exists yet, and this course does not predict when one will.

Confidentiality vs. forgery

HNDL threatens confidentiality only. A signature already verified cannot be retroactively forged; long-lived signing keys need migration for a separate, future-facing reason.

An adversary does not need privileged access to do this. Publicly routable internet traffic, data held by any third party, and durable copies such as backups and logs are all realistic capture targets long before any CRQC exists. Module 2 treats where capture is actually plausible as its own variable, in detail.

For a deeper walkthrough of this exposure model, see Harvest-Now-Decrypt-Later Exposure. The rest of this course builds a repeatable way to answer, for any specific piece of data: is it exposed to HNDL, and if so, how urgently does that matter.