Why We Adopted Post-Quantum Cryptography a Decade Before Quantum Computers Arrive (opens in new tab)
Toss Payments’ biggest legacy-overhaul challenge was not the technology itself, but improving security without disrupting tens of thousands of merchants using decades-old integrations. Because payment systems depend on outdated client environments and small businesses with limited technical resources, security upgrades had to be gradual and carefully communicated. The effort ultimately led from modernizing transport security to adopting post-quantum cryptography in 2026. ## The Challenge of Changing a Legacy Payment Network - Toss Payments supports merchants integrated with its PG system for many years, sometimes decades. - Server-side clients are harder to update than browsers, which update automatically to support new standards. - Security changes such as upgrading TLS, removing weak ciphers, or changing encryption can affect every API call, payment window, and server connection. - Many merchants are small businesses without dedicated developers, making complex security requirements difficult to understand and implement. - As a result, security is a shared responsibility: Toss Payments can strengthen its systems, but legacy merchant environments may still leave connections partially exposed. ## Why Existing Encryption Is Becoming Unsafe - Modern HTTPS and payment systems commonly rely on public-key algorithms such as RSA and ECDSA. - These algorithms are considered secure because conventional computers cannot practically factor enormous numbers or solve elliptic-curve problems. - Quantum algorithms have been mathematically shown to solve these problems efficiently once sufficiently powerful quantum computers exist. - This would make current encryption systems vulnerable, undermining decades of digital-security assumptions. ## Q-Day and “Harvest Now, Decrypt Later” - “Q-Day” refers to the point when quantum computers can break today’s widely used encryption. - Attackers can already intercept and store encrypted payment communications that they cannot currently decrypt. - Once quantum computers become practical, the stored data could be decrypted in bulk. - Payment information is especially valuable because it can remain sensitive for years; data transmitted today could be exposed in the 2030s. - The threat therefore requires action before quantum computers are fully operational. ## A Four-Year Security Upgrade Toss Payments chose a phased approach rather than replacing its security stack all at once: - **2022:** Became the first payment gateway in Korea’s PG industry to implement HTTP/3. - **2022–2025:** Removed weak TLS cipher suites. - **2022–2025:** Completed the rollout of TLS 1.3. - **April 2026:** Implemented post-quantum cryptography (PQC). Each stage balanced stronger protection against the risk of disrupting merchant payments. The gradual rollout gave merchants time to update their systems while ensuring that security improvements continued instead of being postponed indefinitely. ## Starting with HTTP/3 - HTTP/3 is a newer web-transport protocol designed to improve speed and stability, especially on unreliable networks. - It requires TLS 1.3, meaning that adopting HTTP/3 also enforces the use of a modern security protocol. - Toss Payments began with HTTP/3 because it offered both performance improvements and a relatively direct path toward stronger encryption. The broader lesson is that legacy security cannot be improved through a single disruptive upgrade. A phased migration, combined with clear communication and preparation for post-quantum cryptography, allows payment providers to raise security standards while keeping existing merchants operational.
