The junk marker was a per-key CONSTANT sent in plaintext at a fixed offset in
junk frames. Junk is meant to look like random noise (zapret-style), but a
constant prefix is a recognizable per-user structure: an on-path observer
watching one user sees the same 4 bytes on every junk packet, i.e. an OSTP
fingerprint. (The earlier fix only removed the GLOBAL constant.)
Now the marker rotates every 60s window: junk_marker = HKDF(key, ver, 0x04 ||
window). To an observer the prefix changes each window (no fixed signature),
and a captured marker is only valid for ~1 window — the "bit of protection"
against a leaked marker. Only a key holder can compute it, so an outsider still
can't forge a silently-dropped junk packet (and silent-drop is cheaper than
normal processing anyway, so junk spam was never a DoS lever to begin with).
- core: derive_junk_marker(key, window) + current_junk_window() (60s window),
same version-gated HKDF scheme; junk_marker dropped from DerivedSecrets.
- client: stamps junk with the current window's marker.
- server: checks current AND previous window per key (absorbs ~1 window of
clock skew) before falling through to unauthorized-probe handling.
- Not a wire break: only junk framing changes; real handshake/data untouched.
During mixed rollout, unmatched junk merely logs as a probe (cosmetic).
Applied Kerckhoffs's principle: the protocol's security and obfuscation
now depend SOLELY on the access key. An adversary who reverse-engineers
the binary cannot build a DPI filter without knowing the key.
Changes:
- Replaced hardcoded salt string ('-ostp-psk-salt') with HKDF-SHA256.
The salt is now derived from the key hash itself — no protocol-specific
strings remain in the binary.
- Unified all secret derivation into derive_all_secrets() which produces
PSK, obfuscation key, and handshake padding range from a single HKDF
invocation.
- Handshake padding range is now key-derived: different access keys
produce different size distributions (min: 16-79, max: +48..+175).
A universal size-based filter is impossible without the key.
- HKDF-SHA256 (RFC 5869) implemented inline using existing hmac+sha2
dependencies — no new crate required.
What remains identifiable in the binary:
- 'Noise_NNpsk0_25519_ChaChaPoly_BLAKE2s' — standard Noise pattern
string, shared with many other projects, NOT OSTP-specific.
- Generic HMAC/SHA-256/ChaCha20-Poly1305 code — standard crypto
primitives used by millions of applications.
- Removed stale KeyExchange re-export from crypto/mod.rs (kex.rs
only exports HybridSharedSecret and HybridKex after stub refactor)
- Removed unused imports in ostp-server/lib.rs (AsyncWriteExt,
tcp::OwnedWriteHalf)
- Suppressed dead_code warning on HelperMsg::Log variant (IPC spec)
- Verified: cargo check passes with zero errors and zero warnings
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