Add security review report for security-scan branch

Documents the focused security review of the protocol-versioning migration
and the cargo-deny policy addition. Result: no high-confidence vulnerabilities
— the versioned_topic XOR transform is entropy-preserving, signature binding
uses the raw topic_id consistently, and the ALPN/domain changes are
handshake-level compatibility only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SECURITY-REVIEW-security-scan.md

@@ -0,0 +1,51 @@
+# Security Review: `security-scan` branch (PeerSpeak)
+
+_Date: 2026-06-18_
+
+**Scope:** Protocol-versioning migration (`src/protocol.rs`, `versioned_topic`,
+ALPN/domain centralization, gossip topic namespacing) and the `deny.toml`
+supply-chain policy addition.
+
+## Result: No high-confidence security vulnerabilities found.
+
+Each plausible attack surface introduced by this branch was investigated and
+confirmed safe:
+
+### 1. `versioned_topic` XOR transform — topic secrecy preserved
+`src/protocol.rs:46`, used at `src/network/gossip.rs:255`
+
+The room `topic_id` is a uniformly random 32-byte secret (`rand::random()`,
+`src/core/mod.rs:1012`) acting as the room capability. XOR-ing it with the public
+constant `GOSSIP_PROTO.to_le_bytes()` cyclically is **bijective and
+entropy-preserving** — the result is still uniformly random; no byte becomes
+predictable and no entropy is lost. The room secret is no more recoverable by an
+observer than before the change (previously the raw `topic_id` was the on-wire
+topic; now it's a trivial public XOR of it). Bijectivity also preserves room
+distinctness, so isolation is not weakened. **Not a vulnerability.**
+
+### 2. Signature topic-binding — no raw/versioned confusion
+`src/network/gossip.rs`
+
+`active_topic_bytes` stores the **raw** `ticket.topic_id` (line 293), and both
+`sign_gossip` and `verify_gossip` bind against that raw value. Only the
+*subscribed* swarm topic (line 255) uses the versioned value. There is one swarm
+per join and every peer signs/verifies against the same raw topic, so no second
+topic exists to enable a raw↔versioned replay/confusion attack. Code matches
+VERSIONING.md's claim. **Not a vulnerability.**
+
+### 3. `GOSSIP_SIG_DOMAIN` — moved verbatim
+Value identical (`"peerspeak-gossip-v1"`, `src/protocol.rs:34`); cross-version
+cryptographic domain separation preserved. **Not a vulnerability.**
+
+### 4. ALPN changes — handshake compatibility only
+Audio `peerspeak-audio` → `peerspeak/audio/1`, friends `/0` → `/1`. No security
+check keys off the old ALPN strings (audio admission is gated by live room
+membership per S8, not the ALPN literal); no residual references to old strings
+in non-test code. **Not a vulnerability.**
+
+### 5. `deny.toml`
+Ignores only two *unmaintained* advisories (`RUSTSEC-2024-0436`,
+`RUSTSEC-2026-0150`) on compile-time/FFI-only crates — documented, and dependency
+advisories are out of scope. **Not a vulnerability.**
+
+The versioning migration is a clean, security-preserving change.