Post-quantum cryptography (PQC)

Post-quantum cryptography comprises algorithms designed to remain secure against attackers equipped with large quantum computers, which would break today's RSA and elliptic-curve schemes. NIST standardized the first PQC algorithms in 2024, including ML-DSA for signatures (FIPS 204) and ML-KEM for key encapsulation (FIPS 203), starting the industry's largest cryptographic migration.

The migration is unusually awkward because the new algorithms are not drop-in. ML-DSA signatures are kilobytes where ECDSA signatures are tens of bytes, which changes packet sizes, storage budgets and anything that assumed a signature fits in a header. Hybrid deployment, running a classical and a post-quantum signature side by side, is the common answer while confidence in the new schemes accumulates.

Timelines are governed by data lifetime rather than by when a quantum computer appears. Anything that must stay verifiable for a decade is already inside the window.

Signature size is the property that breaks assumptions written years earlier. Protocols that assumed a signature fits in an HTTP header, a QR code or a fixed-width database column need resizing rather than re-encoding, and those assumptions are usually undocumented, discovered by exceeding a limit in production. Auditing where signatures are stored and transported is the cheapest early step in a migration.

Hybrid deployment is the common transition strategy because it fails safe in both directions: the classical layer keeps existing verifiers working, and the post-quantum layer protects the artifact if the classical scheme falls. It costs size and complexity, which is why hybrid signatures are a transition mechanism rather than a permanent state.

Prioritization follows data lifetime rather than sensitivity. Anything that must remain verifiable or confidential for a decade is already inside the exposure window described by harvest now, decrypt later, while a short-lived session token is not, and sorting systems by that question produces a more defensible plan than sorting them by how important they feel.