HNDL Attacks: Your Data Is Already Being Harvested

· Threat Intel · 6 min read

The silent threat

Imagine a burglar who photographs every safe in your building, knowing that in a few years, they'll have a universal key. They don't need to break in today. They just need to record what's there.

That's exactly what Harvest Now, Decrypt Later (HNDL) attacks do.

State-sponsored actors - primarily from China, Russia, and other nation-states - are intercepting and storing encrypted network traffic at massive scale. The encryption protecting this traffic (RSA, ECDSA, ECDHE) is computationally secure today. But once cryptographically relevant quantum computers arrive, every bit of that stored traffic becomes readable.

The scale of the problem

According to intelligence community estimates:

  • Nation-state actors have been reported to intercept and store encrypted traffic at major internet exchange points, with some programs believed to have been active since at least 2015
  • Large volumes of encrypted traffic are estimated to have been stockpiled by various intelligence agencies
  • Financial transactions, healthcare records, government communications, and corporate IP are all targets

The critical question for any CISO: How many years of your organization's encrypted traffic are sitting in an adversary's data warehouse?

Calculating your HNDL window

Your HNDL exposure window is calculated as:

```

HNDL Window = (Year quantum arrives) - (Year traffic was intercepted)

```

If quantum computers can break RSA-2048 by 2030, and your traffic has been intercepted since 2018, your HNDL window is 12 years of exposed data.

This means:

  • Every financial transaction from 2018–2030
  • Every email encrypted with RSA/ECDSA
  • Every VPN session using non-PQC key exchange
  • Every API call between your services

All of it becomes readable.

What makes you a target?

HNDL attacks are strategic, not opportunistic. High-value targets include:

  • Financial services - Transaction data, trading algorithms, customer records
  • Healthcare - Patient records, clinical trial data, genomic data
  • Government - Classified communications, diplomatic cables
  • Defense - Military communications, weapons system data
  • Technology - Intellectual property, source code, research data

If your organization holds data that will be valuable in 5–10 years, you're a target.

Defending against HNDL

The only defense against HNDL is migrating to post-quantum cryptography before quantum computers arrive. Specifically:

  1. Deploy PQC key exchange (ML-KEM) on all external-facing endpoints. This ensures that new traffic is protected against future quantum decryption, even if intercepted today.
  1. Upgrade TLS to hybrid mode - combine classical key exchange (ECDHE) with post-quantum key exchange (ML-KEM) for defense-in-depth.
  1. Prioritize by data sensitivity - not all data has long-term value. Focus PQC migration on systems handling data with 10+ year sensitivity.
  1. Rotate existing keys - even if you can't deploy PQC immediately, rotating keys and certificates reduces the window of exposure.

The urgency is real

HNDL is not a future threat. It's a present-tense attack that compounds every day you delay PQC migration. Every day of RSA-encrypted traffic is another day of captured data that adversaries will eventually decrypt.

The cost of migration is measured in months and dollars. The cost of inaction is measured in decades of compromised data.

Qypher quantifies your HNDL exposure, prioritises the migration, and generates the post-quantum key-exchange artifacts (hybrid ML-KEM configurations) your team deploys. Get your quantum risk score →