What Is the Purpose of Post-Quantum Cryptography?

The purpose of post-quantum cryptography is to keep encryption working after quantum computers can break the encryption we use today. That’s the whole idea in one sentence. The rest of this page explains why today’s encryption is at risk, what post-quantum cryptography replaces, and why the timing matters now even though the threatening machine doesn’t exist yet.

This is a plain-language explainer for a security or IT lead — no advanced math required.

The problem it solves

Most secure communication on the internet relies on public-key cryptography — algorithms like RSA and elliptic-curve cryptography (ECDSA, ECDH). These make two things possible: agreeing on a secret key over an open network (so your traffic can be encrypted) and proving identity with digital signatures (so you know a website or a software update is genuine).

The security of RSA and ECC rests on math problems that are effectively impossible for today’s computers — factoring huge numbers, or solving elliptic-curve discrete logarithms. A large-scale quantum computer running Shor’s algorithm solves exactly those problems efficiently. When such a machine exists, RSA and ECC stop protecting anything.

Post-quantum cryptography (PQC) is a new generation of public-key algorithms built on different math problems — primarily structured lattices — that neither classical nor quantum computers are known to solve efficiently. Same jobs (key exchange, signatures), different foundations, designed to survive quantum attack. PQC runs on the ordinary computers you already have; it is not “quantum cryptography” and needs no quantum hardware.

What is and isn’t at risk

A useful distinction, because “quantum breaks encryption” is only half true:

Public-key crypto — broken. RSA, ECDSA, ECDH, Diffie-Hellman. This is what PQC replaces.
Symmetric crypto — mostly fine. AES and hash functions (SHA-2, SHA-3) are only modestly weakened (by Grover’s algorithm) and stay secure with adequate key sizes — AES-256 is considered safe. You don’t replace these; you just make sure key sizes are large enough.

So the purpose of PQC is narrow and specific: shore up the public-key layer — the part that establishes keys and proves identity.

Why it matters now, before the quantum computer arrives

The natural objection is: “If the machine doesn’t exist yet, why act?” Three reasons:

Harvest now, decrypt later. An adversary can record your encrypted data today and simply store it until a quantum computer can decrypt it. Anything that must stay confidential for years — health records, financial data, legal files, trade secrets — is effectively exposed the moment it crosses the wire, even though the decryption happens later. That’s the sharpest reason to act early, and we cover it in depth in harvest now, decrypt later.
Migration is slow. Replacing cryptography across certificates, protocols, code-signing, and hardware takes years for a real organization. You have to start well before the threat lands.
The standards and deadlines already exist. In August 2024, NIST published the first finished post-quantum standards [SOURCE: NIST FIPS 203/204/205], so the replacement algorithms are ready. And forcing dates are appearing in the real world — Google has stated a goal of deploying post-quantum cryptography by 2029 [SOURCE: Google post-quantum roadmap] [VERIFY: exact 2029 date], and PQC line items are showing up in RFPs and security questionnaires now.

The tools of post-quantum cryptography

NIST standardized the first PQC algorithms as Federal Information Processing Standards:

ML-KEM (FIPS 203) — key establishment. Replaces the key-exchange role of ECDH/RSA. Based on CRYSTALS-Kyber.
ML-DSA (FIPS 204) — digital signatures. Replaces ECDSA/RSA signatures. Based on CRYSTALS-Dilithium.
SLH-DSA (FIPS 205) — a hash-based signature alternative with very conservative security assumptions, based on SPHINCS+.

You’ll find these explained for non-cryptographers in NIST post-quantum standards. In practice the first place most organizations meet them is TLS: modern browsers now negotiate a hybrid ML-KEM key exchange by default (see TLS 1.3 post-quantum).

What this means for you

If you run security or IT, the purpose of post-quantum cryptography translates into a practical mandate: know where you use RSA and ECC, protect anything with a long confidentiality lifetime first, and move to the NIST algorithms — starting with hybrid key exchange, which adds quantum resistance without giving up today’s security.

You don’t have to become a cryptographer to begin. The first useful step is simply seeing your current posture.

Run a free scan of your domain to see whether your public TLS is already quantum-exposed and where your migration starts. It’s an honest check, no signup — and the first concrete step from “I understand the purpose” to “I know my exposure.” From there, the post-quantum readiness self-check walks you through the rest.