up:: 00 Start Here
Start Here
If you’ve never thought about encryption in your life, this is the right place to begin. Nothing here assumes a background in security, computers, or math.
Six questions cover the whole foundation. Answer them in order and you’ll understand the transition better than most people writing about it.
5 things you must understand about Q-Day
- Q-Day is the day a quantum computer can break today’s encryption, and it’s a capability threshold rather than a date on a calendar. Any specific countdown is a sales device.
- Nobody has built the machine. Estimates of what it would take fell from 20 million noisy qubits in a 2019 analysis to under 1 million in a 2025 revision, and the largest machines built so far hold low thousands of noisy qubits.
- The experts give a range rather than a year. A survey of 26 specialists put the chance of a capable machine at 28% to 49% within 10 years, and 51% to 70% within 15.
- Only 1 kind of encryption is affected. The kind that lets 2 strangers agree on a secret and proves a website is genuine breaks completely. The kind that scrambles your content survives with a larger key.
- The date of the machine matters less than the lifetime of your data. Copies made today are read the day it arrives, so anything that has to stay private for 15 years is already exposed.
Source: C. Gidney and M. Ekerå, “How to factor 2048 bit RSA integers in 8 hours using 20 million noisy qubits,” Quantum 5, 433, 2021, arxiv.org; C. Gidney, “How to factor 2048 bit RSA integers with less than a million noisy qubits,” 2025, arxiv.org; Michele Mosca and Marco Piani, “Quantum Threat Timeline Report,” Global Risk Institute, globalriskinstitute.org; NIST, “Report on Post-Quantum Cryptography,” NISTIR 8105, csrc.nist.gov.
The short version:
- Encryption is what makes it possible to say something to one person instead of everyone, and to know a website really is your bank. It’s running constantly and you never notice it.
- It rests on math that’s quick to do forwards and effectively impossible to undo. That difficulty is the only thing keeping your information private.
- A quantum computer can undo one specific kind of that math. The method for doing it was published in 1994 and nobody’s disputed it since.
- The machine that runs the method doesn’t exist yet, and expert surveys put the odds at roughly 28% to 49% within 10 years.
- Nobody has to wait for it to hurt you, because copies of your scrambled data can be stored now and read the day it arrives.
What lives here
- If you read one thing The whole story in about 3 minutes. Start here if you want one page rather than six.
- What’s actually happening? The whole story in about 5 minutes, start to finish.
- What is encryption, and why does everything depend on it? What it’s doing all day, and the list of things that stop working without it.
- What’s a quantum computer? Without physics, and without the description that’s wrong.
- When is this going to happen? The real numbers, including how much the estimates have moved.
- Is this overhyped? The strongest argument against all of this, stated fairly.
- What is technically happening to your data? The mechanism itself, without mathematics: the 2 kinds of encryption in every connection, which one fails, and what an eavesdropper actually records.
Where to go next
- The 13 animations cover this same ground with no reading at all, in teaching order.
- What’s at risk covers which of your information actually matters.
- What to do is the short honest list.
Go deeper into the technical detail
The technical version of this material lives in Foundations MOC and The Threat MOC, written for security professionals. The arguments are where I stop explaining and start making a case, and Is Q-Day Another Y2K is the friendliest one to start with.
These open the Post-Quantum Field Guide, a separate site written for security professionals.
Last verified 2026-08-02 · Maintained by Addie LaMarr, LaMarr Labs.