Protocol v2: X25519 handshake, sequence-number nonces, hardened transport
Red-team review of the (byte-exact) v1 port found replay, cross-session frame portability, pre-auth allocation amplification, and a symmetric key that let either peer forge as itself — flaws inherent to the v1 wire format. v2 closes them: - fixed 69-byte OVX2 hello; server validates before replying; v1 and wrong-version peers see an immediate close - HKDF(ephemeral X25519 DH, PSK, transcript) -> direction-separated session keys; forward secrecy per connection - strictly increasing u64 sequence nonces: any replay or reorder drops the connection (regression tests in tests/protocol_v2.rs) - Config: message-size cap (64 KiB default), connection semaphore, idle/body-stall/handshake/send timeouts on both roles (client-side timers retire the old no-client-timeouts quirk) - message processing inline per connection; user callbacks panic- isolated; poison-tolerant mutexes; demo key refused in release builds - PROTOCOL-V2.md supersedes PARITY-CONTRACT.md (retired, kept as port history); Cereal payload encoding is unchanged Payload serialization remains byte-exact with the original C++ (the golden scrypt/cereal vectors still govern). Zero unsafe. Verified live: server<->client handshake, encrypted echo, clean teardown. Cuts over from the C++ implementation, which is deleted (phase 5 of the port contract); it remains in the original gitea repository.
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# OmniVox Protocol v2 — Normative Specification
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Version 2, 2026-09. Supersedes the v1 wire format (byte-compatible with the
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original C++ implementation); v1 traffic is rejected at the handshake and
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`PARITY-CONTRACT.md` is retired alongside it.
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## Threat model
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The PSK (a scrypt hash of a shared password) authenticates peers. The network
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is hostile: it can observe, inject, replay, reorder, and truncate. Goals:
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session confidentiality and integrity, replay protection, forward secrecy
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for recorded traffic, and no attacker-controlled work before authentication.
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## Connection lifecycle
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```
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TCP connect
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│
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├── HELLO exchange (plaintext, fixed size, timeout: handshake_timeout_ms)
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├── key schedule (HKDF over ephemeral DH + PSK)
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├── framed messages (AEAD, per-direction sequence numbers)
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└── close (any timeout / violation closes immediately)
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```
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## HELLO (both directions, 69 bytes, no attacker-controlled lengths)
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```
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bytes 0..4 magic "OVX2"
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byte 4 version 2
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bytes 5..37 x25519 ephemeral public key
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bytes 37..69 session nonce (32 random bytes)
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```
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The client sends first. The server validates magic, version, and key
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encoding **before replying**; any violation closes the connection. The
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server rejects an all-zero DH output (small-order contribution).
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## Key schedule
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```
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dh = X25519(eph_local, eph_peer)
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prk = HKDF-SHA256-Extract(salt = nonce_client ‖ nonce_server, ikm = PSK)
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okm = HKDF-SHA256-Expand(prk, info = "omnivox-v2" ‖ hello_client ‖ hello_server, 64)
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key_c2s = okm[0..32] key_s2c = okm[32..64]
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```
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The client's send key is `key_c2s`, receive key `key_s2c`; reversed for the
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server. Direction separation means neither peer can present itself forged
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frames (v1's single symmetric key could). The ephemeral DH gives forward
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secrecy against later PSK compromise; the PSK authenticates against
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off-path attackers. A weak password remains guessable by an active attacker
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with the PSK budget to grind scrypt — use random 32-byte PSKs; a PAKE is the
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future upgrade if passwords must stay human.
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## Framing (both directions, after key confirmation)
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```
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u32 LE length (0 < len ≤ max_message_size, default cap 64 KiB)
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AeadPacket:
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nonce [u8; 12] four zero bytes ‖ u64 BE sequence number
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tag [u8; 16] Poly1305 tag
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len u64 LE (cereal vector length, retained for payload lineage)
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ct [len] ChaCha20-Poly1305 ciphertext of the serialized payload
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```
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AEAD is ChaCha20-Poly1305 (RFC 8439) with the packet's own nonce as the IETF
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nonce and empty AAD — identical to v1's construction, now keyed per session
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per direction.
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### Sequence discipline (the replay defense)
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Each sender's counter starts at 1 and increments by exactly 1 per frame; a
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failed write must not consume a number. The receiver's first expected value
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is 1. A frame whose decoded nonce is not the expected value is a protocol
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violation: **log and close the connection.** There is no reorder window.
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### Payload encoding
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The plaintext is the `Cereal` binary encoding of the message type — raw
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little-endian fields, `u64` length prefixes for byte vectors, no archive
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header — unchanged from the C++ original.
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## Limits and timeouts (all server- and client-side)
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| Knob | Default | Violation |
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|---|---|---|
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| handshake_timeout_ms | 10000 | close |
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| idle_timeout_ms (silence between messages) | 15000 | close |
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| body_stall_ms (mid-body silence) | 5000 | close |
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| send_timeout_ms (peer not reading) | 5000 | close that peer |
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| max_message_size | 65536 | close on header |
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| max_connections (server) | 1024 | new connections wait |
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Corrupt frames with the *correct* sequence number (failed AEAD or payload
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decode) are logged and skipped; the connection continues. Anything else —
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sequence violations, unreasonable headers, stalls — is a disconnect.
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## Security properties
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| Property | Mechanism | Tested by |
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|---|---|---|
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| Confidentiality | ChaCha20-Poly1305 per-session keys | `protocol_v2::frames_from_another_session_are_undecryptable` |
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| Replay defense | strict sequence numbers | `protocol_v2::replayed_frame_is_dropped_and_the_connection_dies` |
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| Session binding | nonces enter the salt/info | same cross-session test |
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| Forward secrecy | ephemeral X25519 per connection | design (single-connection compromise leaks only it) |
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| No pre-auth work | fixed 69-byte hello, caps only after key confirmation | `wrong_version_hello_is_closed`, `v1_traffic_is_rejected_at_the_handshake` |
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| Liveness both roles | idle + stall timeouts client **and** server | `malicious_server_silence_times_out_the_client`, `header_stall_times_out` |
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| Slow-client containment | per-connection locks + send timeout | design (no shared writer mutex) |
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| DoS containment | connection semaphore, message caps, callback panic isolation | `panicking_callback_does_not_kill_the_connection` |
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## Refusals
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Release builds refuse to start with the original demo key (scrypt of
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`SecurePassword123` / salt `01…08`) unless `allow_insecure_defaults` is set.
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