Harvest now, decrypt later

When two systems open an encrypted connection, they first agree on a secret key, then use it to scramble everything that follows. The key agreement is protected by public-key cryptography such as RSA or elliptic curves. An attacker who captures that traffic today cannot read it, because they cannot recover the key.

But nothing stops them from saving the captured traffic. Once a quantum computer capable of breaking RSA or elliptic curves exists, they run it against the stored key agreement, recover the secret, and decrypt the whole recorded session. The data was encrypted the entire time. It was still lost, just on a delay.

This is the distinction that decides what to fix first. Encryption does two different jobs, and only one of them is retroactively vulnerable.

• Key exchange is exposed. The shared secret set up today can be recovered later, which decrypts data that was recorded today. This is where harvest-now-decrypt-later lives.
• Signatures are not. A digital signature proves who you are right now. Forging one after the fact is useless, because the moment it needed to be trusted has already passed. Signatures still need to migrate, but they are not a recording risk.

The practical takeaway: post-quantum key exchange is the urgent move. It is exactly what an external readiness scan can measure, because a completed post-quantum handshake is visible from the outside.

The exposure scales with how long your data must stay secret. If a captured session only matters for a day, a decryption years from now is harmless. If it must stay confidential for a decade or more, it is exposed the moment it crosses a network today. That includes:

• Health and genetic records, which stay sensitive for a lifetime.
• Financial records, contracts, and anything under a long retention obligation.
• Government and defense communications.
• Trade secrets, source code, and intellectual property.

The defense is to move key exchange to a post-quantum method now, so that anything recorded from this point forward stays protected even against a future quantum computer. In practice that means enabling hybrid ML-KEM key exchange, which combines the classical exchange with the post-quantum one. Modern browsers and servers already support it; the work is turning it on and confirming it is actually negotiated.