What Is Post-Quantum Cryptography?

01Why Do Quantum Computers Threaten Encryption?

Most secure communication on the internet relies on public-key cryptography. When you connect to a site over HTTPS, the browser and server agree on a secret key using RSA or elliptic-curve-based algorithms. The security of these algorithms rests on the fact that factoring large numbers or solving similar mathematical problems is practically impossible for classical computers.

Quantum computers work with a different computational logic. Shor's algorithm, described in 1994, shows that a sufficiently large and stable quantum computer could solve these problems efficiently. Such a machine does not exist yet and when it will appear is uncertain; but when it does, a significant portion of today's public-key systems will lose their security.

If you are curious about how quantum computers work and where they stand today, you can start with our post on what a quantum computer is.

02The Harvest Now, Decrypt Later Risk

If quantum computers don't exist yet, why worry now? The answer lies in the scenario known as harvest now, decrypt later. An attacker can record and store encrypted traffic today and decrypt those records in the future once they have access to a quantum computer powerful enough.

This risk depends on how long the data needs to stay secret. A campaign announcement loses its value after a few weeks; but health records, contracts, trade secrets or identity details must stay confidential for years. For organizations that handle this kind of data, transition planning is a topic for today.

A simple calculation helps: add the time the data must stay secret to the time it will take to move your systems to the new algorithms; if the sum exceeds the estimated time a powerful quantum computer could appear, you need to start preparing now. In large organizations, infrastructure transitions like this often take years.

03Which Algorithms Are Affected?

The effect of quantum computers is not the same on all types of encryption. While public-key algorithms are at serious risk, symmetric encryption and hash functions are more resilient.

04The New PQC Standards

The US National Institute of Standards and Technology (NIST) put candidate algorithms from around the world through an open evaluation process lasting years and published the first quantum-resistant encryption standards in 2024. The process and documents are open to everyone on NIST's PQC project page.

• ML-KEM (FIPS 203): for key exchange; based on the CRYSTALS-Kyber algorithm
• ML-DSA (FIPS 204): for digital signatures; based on the CRYSTALS-Dilithium algorithm
• SLH-DSA (FIPS 205): a hash-based backup digital signature; based on the SPHINCS+ algorithm

What PQC Is and Is Not

A concept often confused when asking what PQC is is quantum key distribution (QKD). QKD is a physical method that requires special hardware and fiber lines. Post-quantum cryptography, by contrast, consists entirely of software algorithms that run on existing computers and internet infrastructure, which is why it can be deployed at scale.

05How Are HTTPS and SSL Affected?

An HTTPS connection involves two separate operations: the parties agreeing on a session key (key exchange) and the server proving its identity (certificate signature). The “harvest now, decrypt later” risk concerns key exchange in particular; that is why the transition began on this side first.

Major browsers and some infrastructure providers have started to support hybrid key exchange, which uses classical elliptic curve together with ML-KEM. The hybrid approach keeps the protection of the classical algorithm even if an unexpected weakness is found in the new one. The move of certificate signatures to quantum-resistant algorithms is planned as a longer process.

We explained how certificates and HTTPS work today in our post on what an SSL certificate is. The good news for site owners is this: up-to-date server software, browsers and CDN services take care of most of this transition in the background.

openssl s_client -connect www.ornek.com:443 -groups X25519MLKEM768 < /dev/null

06Preparing for Post-Quantum Encryption

The post-quantum encryption transition is not a job to be done overnight but a transformation spread over years. The most valuable thing organizations can do today is to know where they use which encryption and keep their systems ready for change.

1. Build a cryptography inventory: certificates, VPN, e-mail, database encryption, API connections
2. Prioritize data that must stay secret for a long time and the systems that carry it
3. Keep up to date your server, operating system and libraries; the transition comes largely through updates
4. Prefer long keys (for example AES-256) in symmetric encryption
5. Ask your software and suppliers about their PQC roadmaps
6. In newly developed systems, apply the crypto agility principle that makes it easy to change algorithms

07Why Does Crypto Agility Matter?

Crypto agility is a system's ability to change the encryption algorithm it uses at the configuration level, without burying it deep in the code. A weakness may be found tomorrow in an algorithm considered secure today; agile systems adapt quickly in that case.

In software solutions developed specifically for a company, this principle is put into practice by performing encryption and signing through up-to-date, maintained libraries in a central layer. In an e-commerce platform that handles payment and customer data, the same approach means seamless adaptation to future standard changes.

A similar transformation is under way in authentication. Passkey technology, which is based on public-key cryptography, has a structure that can be updated with the same agile approach as standards evolve toward quantum-resistant signature algorithms.

08A Realistic View for Businesses

A small or mid-sized business doesn't need to do its own cryptography research today. What matters is keeping the servers, certificates and software it uses up to date and making sure providers are following this transition. The server software, TLS settings and certificate renewals of a live site are a natural part of the regular maintenance under speed and performance optimization.

Organizations that handle data requiring long-term confidentiality, such as those in healthcare, law or finance, should start transition planning earlier and follow national and international guidance on the subject. Asking for a current TLS configuration from the start when planning a new corporate website or customer portal is also a sound step.