What is a quantum computer, and why does it matter for encryption?

A quantum computer uses the physics of very small particles to work on certain mathematical problems in a different way from the computer on your desk. For most everyday work it offers no advantage at all. For a small number of specific problems, it could be dramatically faster, and some of those problems are the ones today’s encryption relies on.

How it differs from today’s computers

An ordinary computer stores information as bits, each either 0 or 1. A quantum computer uses quantum bits, or qubits, which can hold a combination of 0 and 1 at once and can be linked so that they behave as a single system. A well-designed quantum algorithm uses those properties to make wrong answers cancel out and right answers stand out.

That only helps for problems with the right mathematical structure. It won’t make spreadsheets faster or replace the servers in your data centre. It matters for security because a few of the problems it suits happen to be at the heart of how systems agree keys and sign data.

Why encryption is affected

Much of today’s security relies on public-key cryptography: the mathematics that lets two systems agree a secret key, or lets someone sign a document, without ever meeting. Methods such as RSA and elliptic-curve cryptography are safe today because the underlying problems would take ordinary computers an impractically long time to solve.

A capable quantum computer running Shor’s algorithm could solve those problems efficiently, which would break the public-key methods most systems use. Symmetric encryption, which protects the data itself once a key has been agreed, is far less affected; does quantum computing break all encryption? explains the difference.

NIST, the US standards body, describes quantum computers as machines that “may be years or decades away but could eventually break many of today’s widely used cryptographic systems.”

Why nobody can give you a date

The day a quantum computer can break today’s public-key encryption is often called Q-Day. The G7’s financial cyber experts describe the trajectory of quantum computing development as uncertain. We don’t predict an arrival date, and you should be wary of anyone who does.

What we can say with confidence is that preparing takes years, and some data is exposed already. Encrypted data captured today can be stored and decrypted later, which is why data stolen today, decrypted later matters now rather than on Q-Day.

Common misunderstandings

• “It will break all encryption.” It threatens public-key methods most; symmetric encryption at adequate key sizes is far less affected.
• “It’s decades away, so it can be ignored.” Nobody knows the date, data captured today can be read later, and the move takes years.
• “Bigger keys will fix it.” Longer keys don’t protect RSA or elliptic-curve methods against a capable quantum computer; replacing the method does.
• “We can buy a product when it arrives.” The hard part is finding where cryptography is used and changing it safely, which no purchase does on its own.

What boards should ask

• Do we know where our business relies on public-key cryptography?
• Which of our data must stay private for many years?
• Which deadlines apply to us or to our customers?
• Who owns our plan, and when will we see the first version?

What to do with that

The replacements exist. NIST published its first three quantum-safe standards in August 2024 and says organizations should begin applying them now. They’re explained in plain words in the glossary.

The practical first step isn’t choosing algorithms. It’s knowing where your business relies on cryptography, and who owns each place, so that when you do change, you change the right things in the right order. Why plan for quantum-safe encryption now sets out where to begin.