Strengths, limits, and reality

QKD is a powerful but often misunderstood technology. This lesson clarifies its strengths and limits, and clearly distinguishes it from post-quantum cryptography.

What QKD really guarantees

QKD offers unconditional security for the key exchange: under the laws of quantum physics, no eavesdropper can obtain the key without being detected, whatever their computing power (even quantum).

Be careful: QKD does not encrypt the data. The key obtained must be used with a classical cipher: a one-time pad (provably secure if the key is as long as the message) or, in practice, AES.

The concrete limits

• A dedicated quantum channel: individual photons must be transmitted, via a dedicated optical fiber or in free space (satellite/laser line of sight). You cannot do QKD over the ordinary Internet.
• Range: photons are absorbed in the fiber. Without quantum repeaters (still experimental), the range is limited to about 100 to 500 km.
• Cost: specialized hardware (photon sources, detectors) and dedicated infrastructure, so for now reserved for sensitive links.
• Implementation attacks: the theory is safe, but real hardware has flaws — multi-photon pulses (the PNS attack), detector blinding by a laser. These attacks target the imperfections, not the principle.

QKD is not post-quantum cryptography

This is the most frequent confusion. Both respond to the threat of quantum computers, but in opposite ways:

Critère	Distribution quantique (QKD)	Cryptographie post-quantique
Nature	Matérielle : photons, physique	Logicielle : algorithmes
Sécurité fondée sur	les lois de la physique	des problèmes mathématiques durs
Détecte l'espion ?	oui (par le QBER)	non
Infrastructure	canal quantique dédié	réseaux existants (Internet)
Portée	limitée (~100-500 km)	illimitée (logiciel)
Rôle	échanger une clé	chiffrer, signer, échanger

Post-quantum cryptography is purely software: it consists of new algorithms (based on Euclidean lattices, codes, etc.) resistant to Shor's algorithm, and deployable on today's Internet without special hardware. QKD, on the other hand, requires a physical infrastructure but offers eavesdropping detection.

In summary

QKD provides unconditional security for the key exchange, to be combined with a one-time pad or AES for encryption. Its limits are practical: dedicated quantum channel, range of about 100 to 500 km, cost, and implementation attacks. It must not be confused with post-quantum cryptography, which is software, works on existing networks, but does not detect the eavesdropper.