Harvest now, decrypt later

Quantum computers capable of running Shor's algorithm on real keys do not yet exist. One might think the threat is far off. That is a mistake: it is already active today.

Harvest now, decrypt later

The attack has a name: harvest now, decrypt later.

The idea is devastatingly simple. An adversary does not need a quantum computer today to harm you. They only need to:

1. record, starting now, all the encrypted traffic they care about — diplomatic communications, medical records, industrial secrets;
2. store it patiently, for years;
3. decrypt it later, the day a sufficient quantum computer becomes available.

Today's encryption therefore does not protect data against an attacker who plays the long game.

The timeline of the risk

AUJOURD'HUI                          JOUR J (ordinateur quantique)
    |                                        |
    v                                        v
[ Trafic RSA/ECC chiffré ]  ---enregistré-->  [ Déchiffré d'un coup ]
    |                                        |
    |<------ stockage silencieux ---------->|
    |         (5, 10, 20 ans ?)             |

Any secret captured today and still sensitive on "D-day" is retroactively compromised.

Which data is at risk?

The decisive criterion is the lifetime of the secret. Let us compare:

Data	Sensitivity lifetime	Exposed?
One-time access code	a few minutes	no
Bank card number	a few years	at risk
Medical record	several decades	yes
State secret, patent	30 years and more	yes

Data with a long lifetime must be protected right now by post-quantum cryptography, even if "D-day" has not yet arrived.

Why the urgency is real

The rule is sometimes stated this way (Mosca's rule): if you add the time during which a piece of data must stay secret to the time needed to migrate systems, and this total exceeds the time before the quantum computer arrives, then you are already too late.

Migrating an entire infrastructure takes years. Waiting for the quantum computer before acting means having already lost all the data that has been harvested.

In summary

"Harvest now, decrypt later" means an adversary records encrypted traffic today in order to decrypt it once a quantum computer is available. The threat is therefore immediate for any data with a long lifetime. Hence the urgency to migrate without waiting for "D-day".