What is RSA encryption?

A public-key encryption method from 1977 whose security relies on how hard it is to factor the product of two large primes.

RSA lets anyone encrypt a message with a public key that only the holder of the matching private key can unlock. It is used for website certificates, software signing and email. The usual key size today is 2048 bits, a number about 617 digits long.

The largest RSA number factored publicly by ordinary computers has 250 digits. Shor’s algorithm on a large enough quantum computer would factor RSA-2048 in hours to days. How close that is: the Q-Day tracker.

Shor's algorithmA quantum algorithm, published by Peter Shor in 1994, that factors large numbers and breaks the encryption most of the internet uses for key exchange and signatures.
Elliptic-curve cryptographyPublic-key cryptography based on a hard problem about points on a curve. It protects most web traffic and Bitcoin.
Post-quantum cryptographyEncryption that runs on ordinary computers and is designed to resist attacks by quantum computers.
Q-DayThe day a quantum computer can break the public-key encryption in wide use today, such as RSA-2048 or 256-bit elliptic curves.