神のミソ汁
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kaminomisosiru.bsky.social
神のミソ汁
@kaminomisosiru.bsky.social
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Cryptographer (Ph.D. from Tokyo Tech), interested in practical cryptographic protocols in the real-world
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kaminomisosiru.bsky.social
Our paper on the security analysis of the post-quantum Signal handshake protocols has been accepted at Usenix Security 2025!
eprint.ing.bot ePrint Updates @eprint.ing.bot · Jan 13
Bundled Authenticated Key Exchange: A Concrete Treatment of (Post-Quantum) Signal’s Handshake Protocol (Keitaro Hashimoto, Shuichi Katsumata, Thom Wiggers) ia.cr/2025/040
Abstract. The Signal protocol relies on a special handshake protocol, formerly X3DH and now PQXDH, to set up secure conversations. Prior analyses of these protocols (or proposals for post-quantum alternatives) have all used highly tailored models to the individual protocols and generally made ad-hoc adaptations to “standard” AKE definitions, making the concrete security attained unclear and hard to compare between similar protocols. Indeed, we observe that some natural Signal handshake protocols cannot be handled by these tailored models. In this work, we introduce Bundled Authenticated Key Exchange (BAKE), a concrete treatment of the Signal handshake protocol. We formally model prekey bundles and states, enabling us to define various levels of security in a unified model. We analyze Signal’s classically secure X3DH and harvest-now-decrypt-later-secure PQXDH, and show that they do not achieve what we call optimal security (as is documented). Next, we introduce RingXKEM, a fully post-quantum Signal handshake protocol achieving optimal security; as RingXKEM shares states among many prekey bundles, it could not have been captured by prior models. Lastly, we provide a security and efficiency comparison of X3DH, PQXDH, and RingXKEM. Image showing part 2 of abstract.
January 13, 2025 at 3:09 PM