Quantum and security terms, answered plainly
One term per page: the short answer first, then why it matters, with sources. For the bigger picture, start with the library or the banking guide.
Concepts
What is Q-day?
The day a quantum computer could break today's public-key cryptography, and why nobody needs its date to start preparing.
What is a cryptographically relevant quantum computer?
The machine that would break RSA and elliptic-curve cryptography, and how it differs from the quantum computers in the news.
What is a logical qubit?
Why one reliable qubit is built from many noisy ones, and why headlines usually count the wrong kind.
What is Shor's algorithm?
The quantum algorithm that would break RSA and elliptic-curve cryptography, explained without equations.
What is Grover's algorithm?
A square-root speed-up for search, and why it calls for longer keys rather than new algorithms.
Defences
What is ML-KEM?
The main post-quantum standard for agreeing on secret keys, and where it's already in use.
What is hybrid key exchange?
Traditional and post-quantum key exchange combined, so a connection holds if either one does.
What is ML-DSA?
The main post-quantum standard for digital signatures, and why its size is the real cost.
What is SLH-DSA?
The conservative signature standard built only on hash functions, and where its large signatures make sense.
What is FN-DSA?
The compact signature algorithm NIST is standardising from Falcon, and why it's harder to get right.
What is HQC?
NIST's backup for ML-KEM, built on different mathematics, and why nothing needs to wait for it.
What is quantum key distribution, and how is it different from post-quantum cryptography?
Physics versus maths: two very different answers to the quantum threat, and which one banks are adopting.
What is crypto-agility?
The ability to change cryptography without rebuilding systems, and why it outlasts the quantum migration.
Rules and timelines
What is NIST IR 8547?
NIST's draft plan to retire quantum-vulnerable algorithms, and why its 2030 and 2035 dates matter beyond the US.
What is CNSA 2.0?
The US national security algorithm suite and schedule, and why it reaches banks through their vendors.