NEBULA: A Language - Independent Specification for Opaque Rotating Refresh Tokens

Computer Science > Cryptography and Security [Submitted on 4 Aug 2026] NEBULA: A Language - Independent Specification for Opaque Rotating Refresh Tokens Matteo Teodori Refresh tokens are among the most sensitive credentials in modern authentication systems: long-lived, bearer-style, and sufficient to mint access tokens for days or weeks. RFC 9700, the current Best Current Practice for OAuth 2.0 security, mandates that refresh tokens issued to public clients be rotated on every use with replay (reuse) detection, or be sender-constrained. But the BCP specifies policy, not mechanism: it prescribes no wire format, no storage schema, no ordering of verification steps, no concurrency contract, and no semantics for edge cases such as lost-response retries or key rotation. Implementations may therefore diverge in precisely the corner cases that determine security outcomes. We present NEBULA, a precise, language-independent specification of the RFC 9700 refresh-token model, together with ten conformant reference implementations (TypeScript, Python, Go, Rust, Java, PHP, C#, Ruby, Elixir, Dart). NEBULA tokens are opaque -- a 128-bit public selector and a 256-bit secret verifier, both CSPRNG output, carrying no claims and no signature -- so token validity is a property of server-side state rather than of cryptographic verification. Its conformance methodology publishes the behavioural suite as data rather than as prose: 38 scenarios in one machine-readable file that every implementation executes through a thin per-language runner, so that drift by transcription is structurally excluded. We describe the specification -- including a compare-and-set rotation contract that closes a reproducible bypass of reuse detection under concurrent refresh -- analyse its security properties including its post-quantum posture, and report on cross-language conformance as a method for multi-implementation security specifications. The specification, implementations, and conformance artefacts are open source under the Apache License 2.0. Comments: 14 pages, 1 figure, 2 tables. Specification, conformance vectors and ten reference implementations: this https URL Subjects: Cryptography and Security (cs.CR) ACM classes: K.6.5; E.3 Cite as: arXiv:2608.04115 [cs.CR]   (or arXiv:2608.04115v1 [cs.CR] for this version)   https://doi.org/10.48550/arXiv.2608.04115 Focus to learn more Submission history From: Matteo Teodori [view email] [v1] Tue, 4 Aug 2026 18:10:55 UTC (19 KB) Access Paper: HTML (experimental) view license Current browse context: cs.CR < prev   |   next > new | recent | 2026-08 Change to browse by: cs References & Citations NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)