“If I distribute keys through a PQC-secured channel, I’m doing QKD.”
No – you are not.
Using post-quantum cryptography (PQC) to protect key exchanges is essential and strongly recommended. But it remains classical cryptography, very different from Quantum Key Distribution (QKD), which relies on the physics of single photons – and, in some architectures, on entangled photons.
Let’s clarify both, then look at the underlying physics.
What Post Quantum Cryptography (PQC) Actually Is
- Uses classical algorithms (lattice-based, code-based, etc.) designed to resist attacks from large-scale quantum computers.
- Runs over today’s infrastructure (IP networks, TLS, VPNs, etc.) as new key encapsulation mechanisms (KEMs) and digital signatures.
- Derives its security from hard mathematical problems, not from quantum physics.
When you upgrade TLS, IPSec or your key management to PQC, you are still in the world of software and classical channels. The “quantum” is in the attacker model, not in the way signals propagate.
What Quantum Key Distribution (QKD) Actually Is
It is of completely different nature:
- It uses single photons transmitted over a quantum channel (optical fiber or free space).
- Information is encoded in quantum degrees of freedom (polarization, phase, etc.).
- Any attempt to measure or copy those photons by an eavesdropper inevitably disturbs their quantum state, which can be detected.
Some QKD protocols (like Ekert’91) leverage entangled photons: pairs of photons created in a joint quantum state, such that their measurement results are strongly correlated in a way no classical mechanism can reproduce.
This is where the deep physics comes in.
Entangled Photons and True Randomness
In entanglement-based QKD, a source emits pairs of entangled photons towards two distant parties (often called Alice and Bob). Each measures their photon in randomly chosen bases.
Key points:
- Entanglement means the pair is described by a single quantum state, not by two independent states with pre-defined values.
- When Alice and Bob measure, each individual result is fundamentally random. There is no hidden internal “setting” determining the outcome in advance.
- Yet, when they later compare part of their data, they observe correlations stronger than any classical “common origin” explanation would allow.
This is not just interpretation or philosophy; it is a precise, testable statement captured by the Bell inequalities.
From Bohr vs Einstein to Bell Inequalities
Historically:
- Einstein, Podolsky and Rosen (EPR) argued in 1935 that quantum mechanics might be incomplete, suggesting that measurement outcomes could be explained by “hidden variables” stemming from a common cause in the past, avoiding “spooky action at a distance”.
- Niels Bohr defended the standard quantum view: outcomes are intrinsically probabilistic, and the quantum description is complete without such hidden variables.
In 1964, John Bell translated this philosophical debate into physics by deriving the Bell inequalities: any theory based on local hidden variables (pre-determined properties + no faster-than-light influence) must satisfy certain statistical bounds on measurement correlations.
Quantum mechanics predicts that entangled photons can violate these inequalities.
In the early 1980s, the French physicist Alain Aspect and his team performed landmark experiments with entangled photons that violated Bell inequalities, in agreement with quantum predictions and in conflict with local hidden-variable explanations.
The conclusion is central for QKD:
- The measurement outcomes are not just the consequence of a common origin with unknown parameters.
- They are genuinely random, and the correlations have no classical analogue.
- This randomness can be harnessed to generate cryptographic keys whose values are not determined anywhere “in advance” – not even by the device manufacturer.
How This Secures QKD
In entanglement-based QKD:
- Alice and Bob measure entangled photons in randomly chosen bases.
- A subset of the results is used to test error rates and, if desired, violation of Bell inequalities.
- If an eavesdropper (Eve) tries to intercept or manipulate the photons, she unavoidably changes the statistics:
- If the observed data remain within expected quantum limits, Alice and Bob can be confident that no significant information has leaked, and they distill a secret key from the remaining data.
Thus the security of QKD is anchored in experimental facts (Bell tests, disturbance detection) and quantum laws (no-cloning theorem, measurement disturbance), not in the assumed hardness of a mathematical problem.
PQC vs QKD: Different Foundations, Complementary Roles
To summarize:
PQC:
- Classical algorithms, classical channels.
- Security = no known efficient attack (classical or quantum) on the underlying math.
- Mostly a software upgrade; scalable across the Internet.
QKD:
- Quantum hardware + quantum channel (photons, entanglement).
- Security = grounded in quantum physics, true randomness, and, for entanglement-based schemes, experimentally verified violation of Bell inequalities.
- Infrastructure-intensive; targeted at specific high-value links.
Using a PQC-secured channel to distribute keys is excellent practice and quantum-resistant but it is not QKD. In QKD, the photons and their quantum properties are not a metaphor; they are the core of the security model.
For a quantum-resilient architecture, the realistic vision is PQC everywhere, possibly complemented by QKD on selected strategic links where the stronger, physics-based guarantees justify the cost and complexity.
Amine ISMAIL Ph.D
VP Technology access at Ekinops
