Post-Quantum Cryptography

Post-quantum cryptography (PQC) is cryptography designed to protect digital information against attacks by both conventional computers and future powerful quantum computers. It uses mathematical algorithms that can run on conventional computers but are believed to resist known quantum attacks. Its purpose is to maintain quantum-resistant security for functions such as establishing encryption keys and verifying digital signatures.

Why quantum-resistant security is needed

Many digital systems depend on public-key cryptography to establish secure connections and authenticate communications. A sufficiently capable quantum computer could undermine important algorithms used for these purposes.

• RSA and elliptic-curve cryptography rely on mathematical problems that a sufficiently powerful quantum computer could solve efficiently using Shor's algorithm.
• The threat also affects digital signatures, which help establish authenticity and detect alterations to documents, software and transactions.
• In a harvest now, decrypt later attack, an adversary stores encrypted information today to decrypt it when the necessary quantum capability becomes available.
• Symmetric encryption, such as AES, faces a different quantum threat; suitable key lengths can help preserve security, so quantum computing does not make all existing cryptography equally vulnerable.

How post-quantum cryptography works

PQC replaces vulnerable public-key mechanisms with algorithms based on mathematical problems for which no efficient quantum solution is known. Two important approaches are lattice-based cryptography and hash-based signatures.

• A key-encapsulation mechanism establishes a shared secret that can then be used with symmetric encryption to protect data.
• A post-quantum digital-signature algorithm allows recipients to verify the origin and integrity of signed information.
• The US National Institute of Standards and Technology (NIST) develops standards for post-quantum algorithms after evaluating their security and practical performance.
• PQC differs from quantum key distribution (QKD): PQC uses mathematical algorithms on conventional computing systems, whereas QKD uses quantum physical processes to distribute keys.
• Quantum resistance is a security assessment, not a guarantee of permanent invulnerability; implementation flaws and future cryptanalytic advances remain relevant.

Migration and relevance to digital security

Migration requires identifying where vulnerable cryptography is used and replacing it without disrupting essential services. Crypto-agility means being able to change cryptographic algorithms and implementations as security requirements evolve.

• Organisations should inventory cryptographic dependencies, prioritise information requiring long-term confidentiality and plan upgrades with technology suppliers.
• Testing must address interoperability, performance, key and signature sizes, and secure implementation.
• For banks, government networks and critical information infrastructure, migration helps preserve confidentiality and authentication, but does not replace access controls, patching or incident response.