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arXiv · 2509.11695

Time-Based State-Management of Hash-Based Signature CAs for VPN-Authentication

Daniel Herzinger·Linus Heise·Daniel Loebenberger·Matthias Söllner
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Abstract

Advances in quantum computing necessitate migrating the entire technology stack to post-quantum cryptography. This includes IPsec-based VPN connection authentication. Although there is an RFC draft for post-quantum authentication in this setting, the draft does not consider (stateful) hash-based signatures despite their small signature size and trusted long-term security. We propose a design with time-based state-management that assigns VPN devices a certificate authority (CA) based on the hash-based signature scheme XMSS. The CA then issues leaf certificates which are based on classical cryptography but have a short validity time, e. g., four hours. It is to be expected that even large quantum computers will take significantly longer to break the cryptography, making the design quantum-secure. We propose strategies to make the timekeeping more resilient to faults and tampering, as well as strategies to recognize a wrong system time, minimize its potential damage, and quickly recover. The result is an OpenBSD implementation of a quantum-safe and, regarding the leaf certificates, highly flexible VPN authentication design that requires significantly less bandwidth and computational resources compared to existing alternatives.

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Daniel Herzinger, Linus Heise, Daniel Loebenberger, Matthias Söllner. 2025-09-15. Time-Based State-Management of Hash-Based Signature CAs for VPN-Authentication. https://arxiv.org/abs/2509.11695

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