Separate three related subjects
Quantum cryptography studies ways to use quantum information for cryptographic tasks. Its best-known application is quantum key distribution, or QKD: two endpoints use quantum signals and classical communication to establish secret key material. That key can then be used by an encryption system.
| Topic | What changes | Reader’s question |
|---|---|---|
| Quantum computing | The model of computation used for certain algorithms. | Which problem is being solved, at what scale and with what error correction? |
| Quantum key distribution | How physical signals contribute to shared secret key material. | What are the channel, device and authentication assumptions? |
| Post-quantum cryptography | Classical cryptographic algorithms designed to resist known quantum attacks. | Which standardized algorithm and implemented protocol are supported? |
On a narrow screen, scroll the table sideways. Keyboard users can focus the table and use the arrow keys.
A news story may discuss one, two or all three. Identify the demonstrated result before drawing a conclusion about the security of a messaging app, certificate or stored archive.
Follow a simplified BB84 exchange
Charles Bennett and Gilles Brassard introduced the BB84 protocol in 1984. Their original paper, republished by its authors is a useful historical source. The central idea uses quantum states prepared and measured in different bases.
- Alice randomly selects bits and preparation bases, then sends the corresponding quantum signals.
- Bob independently chooses measurement bases. Measurements made in a mismatched basis do not reliably reproduce Alice’s bit.
- Over an authenticated classical channel, they compare which bases they used and retain appropriate matching-basis results. They keep the remaining bit values private.
- They reveal a sample to estimate errors, discard those sample bits, and decide whether the observed conditions permit proceeding under the protocol’s security analysis.
- Error correction and privacy amplification produce a shorter final key while accounting for information that may have leaked.
This outline omits the substantial details needed for implementation, including finite sample sizes, imperfect sources and detectors, and the exact security proof. A diagram of two optical boxes is only the beginning of a deployment.
Interpret an eavesdropping example carefully
Real equipment introduces losses and errors even without an attacker. A security analysis turns measured data and explicit device assumptions into a bound on information leakage and a permitted key length. Read the experimental method and proof assumptions together.
A headline reporting a long-distance link may describe transmission distance, raw detections, sifted bits or final secret-key rate. Ask which quantity was measured and over what conditions and duration. These measurements answer different engineering questions.
Authentication and devices remain essential
The parties need to establish that the classical messages come from the intended peer. QKD alone does not authenticate the source. Endpoints also hold keys and plaintext and need physical and software protection.
The NSA’s discussion of QKD limitations identifies authentication, specialized equipment, implementation assurance and availability concerns. Its recommendation against QKD for National Security Systems unless the limitations are overcome has that specific scope. It also provides concrete questions for understanding the engineering gap between a proof and a system.
A practical evaluation should state which nodes are trusted, how faults are detected, whether service stops when keys are unavailable, and how an attacker’s possible access to equipment is modeled. These topics are developed in the quantum key infrastructure guide.
Understand the post-quantum alternative
Post-quantum cryptography runs on conventional computers. It aims to provide key establishment and signatures using problems believed to resist known quantum attacks. NIST finalized ML-KEM, ML-DSA and SLH-DSA standards in 2024; the NIST PQC project records standards and subsequent work.
An organization preparing for migration can inventory certificates, protocols, libraries and products, identify information that must remain confidential for many years, and track supported upgrades. A standardized algorithm still needs an appropriate protocol, interoperable implementation and operational testing.
QKD is a physical key-distribution approach. PQC changes classical cryptographic mechanisms. Both deserve precise descriptions, with decisions based on the application, trust assumptions and current evidence rather than a shared “quantum-safe” label.