Etymology

Today it means: A conjectured future day on which quantum computing developments will make it possible to defeat the encryption systems that underpin most of the applied cryptography in online use during the late 20th and early 21st centuries, including that which underpins most of the secure communications and most cryptocurrencies to date. (The defense to prevent Q-Day from having catastrophic effects would be the adoption of post-quantum cryptography as soon as possible, even before Q-Day arrives, owing to the HNDL problem.) Full definition →