[2607.20800] Classical Acceptance Is Not Hybrid Authentication: Measuring X.509 Verifier Semantics in Post-Quantum Migration

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arXiv:2607.20800 (cs)

[Submitted on 23 Jul 2026]

Title:Classical Acceptance Is Not Hybrid Authentication: Measuring X.509 Verifier Semantics in Post-Quantum Migration

Authors:Taesung Kim, Boheung Chung, Keonwoo Kim, Yousung Kang<br>View a PDF of the paper titled Classical Acceptance Is Not Hybrid Authentication: Measuring X.509 Verifier Semantics in Post-Quantum Migration, by Taesung Kim and 3 other authors

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Abstract:A relying party validating a hybrid X.509 certificate --- carrying both a classical and a post-quantum credential --- must distinguish whether its accepting judgment rests on the post-quantum evidence or only on the classical path. To preserve compatibility, the separable designs place that evidence where classical path validation may ignore it. A verifier can then validate the classical path and accept while the post-quantum evidence never bears on the decision --- a valid classical result silently promoted to a hybrid conclusion it did not establish. We measure this across eight path-validation stacks (seven independent codebases), in nine validation modes, over six certificate schemes. Under a hybrid-required policy, nearly every stack parsing a separable hybrid certificate accepts on the classical path without making the post-quantum evidence outcome-bearing; one enforcing mode instead fractures interoperability over a signature-input encoding not yet interoperably profiled; and stacks that verify post-quantum signatures still do not enforce the binding by default: the gap is structural, not explained by missing primitive capability alone. Under lifecycle desynchronization the downgrade is realized: when a bound post-quantum credential is revoked while the classical certificate stays valid, the default path still accepts, because the bound credential lies outside the decision's scope. Binding success is not authentication success. We contribute a specification-derived verifier model and an executable, policy-parametric reference contract --- what a verifier must recognize, verify, make outcome-bearing, and check before reporting a validation as hybrid --- with a diagnosis of why standards do not require it.

Comments:<br>37 pages, 1 figure, 7 tables. Submitted to Computers & Security

Subjects:

Cryptography and Security (cs.CR)

Cite as:<br>arXiv:2607.20800 [cs.CR]

(or<br>arXiv:2607.20800v1 [cs.CR] for this version)

https://doi.org/10.48550/arXiv.2607.20800

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arXiv-issued DOI via DataCite (pending registration)

Submission history<br>From: Taesung Kim [view email]<br>[v1]<br>Thu, 23 Jul 2026 00:04:32 UTC (120 KB)

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View a PDF of the paper titled Classical Acceptance Is Not Hybrid Authentication: Measuring X.509 Verifier Semantics in Post-Quantum Migration, by Taesung Kim and 3 other authors<br>View PDF<br>HTML (experimental)<br>TeX Source

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