FINDING · DEFENSE
The BiFEP datastream construction uses a 'wrapper' layer with authenticated DATA, DUMMY, and FIN frames over an independently keyed inner FEP per direction, revealing only a public sending schedule ΓX(t) and quantized closing epoch from the set Ecl — the exact private half-close time is hidden behind continued cover traffic. Empirical passive diagnostics on the Rust implementation show byte entropy of 7.999279–7.999749 bits/byte, maximum absolute byte correlation of 0.004668, and a random-forest ROC-AUC of 0.485 (95% CI [0.428, 0.542]) distinguishing DATA from ChaCha20 — not significantly better than chance.
From 2026-chen-closing-loop-bidirectional — Closing the Loop: Bidirectional Fully Encrypted Protocols · §IV; §VIII-D, Table IV · 2026 · arXiv preprint
Implications
- Use authenticated typed frames (DATA/DUMMY/FIN) inside an inner FEP rather than relying on length-only shaping — this prevents cross-direction semantic leakage while keeping wire bytes indistinguishable from uniform random.
- Separate the private close-readiness event from the public close time by continuing full scheduled output through the next permitted close bucket (quantized close), so connection teardown carries no timing signal.
Tags
Extracted by claude-sonnet-4-6 — review before relying.