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What is post-quantum cryptography? | ExpressVPN Glossary

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Post-quantum cryptography

What is post-quantum cryptography?

Post-quantum cryptography (PQC) is a type of cryptography designed to remain secure in a future with powerful quantum computers. It refers to new public-key methods and digital-signature algorithms, intended to replace today’s widely used public-key algorithms, which could become unsafe against quantum attacks.

Why post-quantum cryptography matters

The following factors contribute to the importance of PQC:

Quantum threats: Large-scale quantum computers may be able to break widely used public-key encryption and signature schemes. Sensitive data encrypted and stored today could be decrypted later (“harvest now, decrypt later”), creating long-term privacy and security exposure.
Long-term privacy and resilience: PQC helps preserve confidentiality and trustworthy authentication over long time horizons, including for data and systems that must remain secure for many years.
Ongoing transition and infrastructure protection: Governments and organizations are beginning large-scale transitions as standards finalize and adoption grows. Migration takes years, and PQC helps protect communications, identity systems, and critical digital infrastructure as cryptography is upgraded.

How post-quantum cryptography works

PQC uses mathematical problems believed to be resistant to attacks by large-scale quantum computers. Common approaches include lattice-based schemes (used for key establishment and some digital signatures) and hash-based schemes for digital signatures.

These algorithms are designed to replace or complement current public-key systems. During migration, they are often deployed in hybrid key-establishment designs that combine a post-quantum algorithm with a classical algorithm to reduce transition risk. They're also intended to work with existing network protocols, which can negotiate supported algorithm suites, though deployments may require updates and accommodations for larger keys or messages to maintain interoperability.

The National Institute of Standards and Technology (NIST) has published its first PQC standards. They include the Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) for key establishment. ML-KEM was originally submitted as CRYSTALS-Kyber. They also include the Module-Lattice-Based Digital Signature Algorithm (ML-DSA) for digital signatures. ML-DSA was originally submitted as CRYSTALS-Dilithium. NIST also standardized the Stateless Hash-Based Digital Signature Algorithm (SLH-DSA) from SPHINCS+. SLH-DSA is a hash-based digital signature scheme. A flow chart explaining how Post-quantum cryptography works.

Key features of quantum-resistant algorithms

Quantum-resistant algorithms are designed for:

High computational difficulty: Based on problems believed to be hard for both classical and quantum computers, with no efficient attacks currently known at standard security levels.
Quantum resilience: Designed to avoid the weaknesses exploited by major quantum algorithms, including Shor’s algorithm, which threatens Rivest-Shamir-Adleman (RSA) and many elliptic-curve systems, and, where relevant, Grover’s algorithm (which can reduce the effective security margin of some symmetric and hash-based constructions).
Functional coverage: Provide the core public-key functions required in modern systems, primarily key establishment and digital signatures.
Different structures and larger keys: Use different mathematical foundations than RSA or elliptic curve cryptography (ECC), commonly relying on structures such as lattices or hash-based constructions, and often requiring larger key, ciphertext, or signature sizes than traditional public-key schemes.

Post-quantum cryptography vs. classical cryptography

	Classical public-key cryptography	Post-quantum cryptography
Algorithms in scope	RSA, ECC, Diffie–Hellman	Lattice-based and hash-based schemes (among others)
Quantum exposure	Vulnerable to large-scale quantum attacks against public-key methods	Intended to resist known quantum attacks targeting public-key cryptography
Deployment maturity	Widely deployed and highly interoperable across systems	Adoption is ongoing; rollout differs by ecosystem and vendor
Operational impact	Typically, smaller keys/messages and predictable performance	Some algorithms use larger keys/messages and can have different performance and compatibility across devices and software stacks

Challenges and limitations of post-quantum cryptography

Larger keys and messages: Some post-quantum schemes use larger public keys, ciphertexts, or signatures, which can increase bandwidth, storage, and handshake/certificate sizes.
Performance and platform variation: Speed and memory use can differ across servers, desktops, mobile devices, and constrained hardware. Implementation choices can significantly affect latency and resource consumption.
Evolving standards and ecosystems: Core standards exist, but guidance continues to develop, and supporting software libraries, protocol profiles, and compliance requirements may change as adoption expands.
Migration and operational risk: Transitions must be carefully managed to avoid introducing vulnerabilities, such as misconfiguration, inconsistent algorithm support across systems, or weak integration during staged rollouts.

FAQ

When will quantum computers break current encryption?

No reliable date exists. Most public guidance treats the point at which quantum computers can break today’s widely used public-key cryptography, for example, Rivest-Shamir-Adleman (RSA) and many elliptic-curve systems, as uncertain. This is why standards bodies and national cybersecurity agencies recommend planning and migrating before a deadline is forced by events.

Should individuals worry about post-quantum security now?

For most individuals, post-quantum security is not an urgent day-to-day concern yet because quantum computers capable of breaking widely used public-key cryptography are not available at the needed scale. The main near-term risk is long-lived sensitive data that could be collected now and decrypted later, which is why major platforms and organizations are migrating to standardized post-quantum algorithms.

Are VPNs adopting post-quantum algorithms?

Yes, some virtual private network (VPN) providers have begun adopting post-quantum algorithms. For example, ExpressVPN’s Lightway protocol includes post-quantum protection via hybrid key exchange and has been upgraded to the Module Lattice–Based Key-Encapsulation Mechanism (ML-KEM), the National Institute of Standards and Technology (NIST)'s standardized key-encapsulation mechanism derived from Kyber. ExpressVPN also offers a WireGuard option with post-quantum key establishment using ML-KEM in the handshake.

Is PQC the same as quantum cryptography?

No. Post-quantum cryptography (PQC) refers to classical (non-quantum) cryptographic algorithms designed to resist attacks by quantum computers and that run on standard hardware. Quantum cryptography refers to cryptographic techniques that use quantum physics (most commonly quantum key distribution) to achieve security properties.

What is hybrid encryption?

Hybrid encryption (in the post-quantum transition context) generally means combining a classical and a post-quantum key-establishment method, then combining their outputs, so that the negotiated session key remains secure as long as at least one of the component methods remains unbroken.