“Harvest Now, Decrypt Later.”

This sentence, known as the most chilling maxim among security experts, penetrates the essence of the encryption threat we face in 2025. Hackers and hostile nations are indiscriminately collecting encrypted confidential data that cannot be decrypted right now. They are waiting. Waiting for the day when a powerful Quantum Computer appears that can tear apart existing RSA or Elliptic Curve Cryptography (ECC) like a piece of paper—that is, ‘Q-Day’.

Many people dismiss the commercialization of quantum computers as a distant future event. However, from a security perspective, Q-Day has already begun. If national secrets or corporate 10-year R&D data encrypted and transmitted today are decrypted in 5 or 10 years, can that be considered safe? For information with a long data Shelf-life, the quantum threat is not a ‘future problem’ but a ‘crisis of today’. In this article, we will examine the status and response strategies of Post-Quantum Cryptography (PQC), the biggest topic in the security industry in 2025.

The End of Existing Encryption Systems

The foundation of all security we use when doing internet banking, shopping, and using messengers lies in ‘Public Key Encryption Algorithms’. Algorithms like RSA or ECC rely on the mathematical difficulty of huge prime factorization problems or discrete logarithm problems. Because it would take longer than the age of the universe for existing supercomputers to solve these problems, we have believed them to be ‘secure’.

However, quantum computers using the principles of quantum mechanics are different. If a quantum computer with enough Qubits to run ‘Shor’s Algorithm’ appears, the public key encryption systems currently in use worldwide will be virtually neutralized. Experts predict that point to be as early as the early 2030s.

“Still more than 5 years left?” We cannot be relieved. The time it takes to replace the world’s IT infrastructure with a new encryption system, the ‘migration period’, takes several years or more. If Q-Day arrives before infrastructure replacement is complete, global financial and communication networks will fall into chaos. This is why intelligence agencies around the world, including the US White House and South Korea’s National Intelligence Service, are rushing to transition to PQC.

2025, The First Year of PQC Standardization

Fortunately, humanity has been preparing for this threat. The National Institute of Standards and Technology (NIST) in the US has been conducting a global competition to select next-generation encryption algorithms since 2016, and after long verification, finally officially announced the first PQC standard algorithms in August 2024. 2025 is the first year these standards begin to be installed in actual software and hardware.

Selected Core Algorithms

The major PQC algorithms confirmed by NIST are largely divided into two uses.

1. General Encryption and Key Exchange (KEM): ML-KEM (formerly CRYSTALS-Kyber)

   • This is the algorithm that will be used most universally. Using Lattice-based cryptography, it has a mathematical structure that is difficult to solve even with a quantum computer. It is fast and has a relatively small key size, making it suitable for web browsers or communication protocols.

2. Digital Signatures: ML-DSA (formerly CRYSTALS-Dilithium), SLH-DSA (formerly SPHINCS+)

   • Used for identity authentication and integrity verification. ML-DSA is used as the main force, and SLH-DSA, which uses a completely different mathematical principle (hash-based), was selected as a backup in case a mathematical breakthrough is discovered.

Now, big tech companies like Apple, Google, and Microsoft are including these algorithms by default in their OS, browsers, and cloud services. iPhone’s iMessage has already applied its own protocol called PQ3 to respond to quantum threats, and the Chrome browser is also piloting PQC for communication with web servers.

What Corporate Security Managers Must Do: Crypto Agility

Then what should general companies do? Should we tear down and fix all the code right now? The key is to secure flexibility, that is, ‘Crypto Agility’.

1. Cryptographic Asset Identification (Discovery)

The first thing to do is to figure out where and what encryption algorithms our company is using. Surprisingly, many companies do not even know that hard-coded RSA keys planted by developers 10 years ago exist deep in their legacy systems. You need to conduct a complete survey of the encryption status across the system through automated tools and create a ‘Cryptographic Inventory’.

2. Applying Hybrid Mode

PQC still has a short verification period. Theoretically safe, but implementation vulnerabilities may be discovered. Therefore, currently in the transition period, a ‘Hybrid Method’ applying both existing verified encryption (RSA/ECC) and new PQC algorithms doubly is recommended. It’s like putting on two locks. So that even if one is breached, the other can hold.

3. Understanding Vendor Status

You need to check if the VPN equipment, firewalls, cloud services, and SSL certificate authorities you use have a PQC roadmap. Asking “When will you support ML-KEM?” is the start. If it is a solution with no support plan, you should seriously consider replacement.

Another Axis of Quantum Security: Quantum Key Distribution (QKD)

If PQC is a software solution, there is also a hardware approach to building a communication network that is physically impossible to eavesdrop on. It is Quantum Key Distribution (QKD).

QKD uses the ‘No-cloning theorem’ principle of quantum mechanics. The moment a hacker intrudes in the middle of communication and tries to peek at the encryption key information, the quantum state collapses, and the communication parties immediately know the fact of eavesdropping. Theoretically, it provides perfect security where hacking is impossible.

Korea’s three major telecommunications companies (SKT, KT, LG U+) possess world-class technology in this field. QKD equipment has already been introduced and operated in sections requiring ultra-high security, such as national administrative networks, military communication networks, and financial dedicated lines. However, due to cost issues requiring separate dedicated equipment and optical cables, it seems it will take time for it to spread to the internet networks of general companies. Therefore, for the time being, PQC will become the trend, and QKD will coexist as a premium security option for special purposes.

Conclusion: Crisis is Opportunity for the Prepared

2025 is the year the paradigm of security changes. A huge construction project to replace the bottom of the digital trust system we have built up so far has begun.

Q-Day is not a subject of vague fear. It is a predicted future and a risk that can be sufficiently prepared for. Just as we wisely overcame the Y2K problem in 2000, security threats in the quantum computing era can be overcome through thorough preparation and technological response.

Security is not speed but direction. Even if you cannot build a perfect PQC system right now, it is important to set the direction of where our organization’s encryption system should go and take the first step. Is your data equipped with quantum resistance now?

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TechDepend Security Manager Kang Ji-won