# OmniVox Protocol v2 — Normative Specification

Version 2, 2026-09. Supersedes the v1 wire format (byte-compatible with the
original C++ implementation); v1 traffic is rejected at the handshake and
`PARITY-CONTRACT.md` is retired alongside it.

## Threat model

The PSK (a scrypt hash of a shared password) authenticates peers. The network
is hostile: it can observe, inject, replay, reorder, and truncate. Goals:
session confidentiality and integrity, replay protection, forward secrecy
for recorded traffic, and no attacker-controlled work before authentication.

## Connection lifecycle

```
TCP connect
   │
   ├── HELLO exchange (plaintext, fixed size, timeout: handshake_timeout_ms)
   ├── key schedule    (HKDF over ephemeral DH + PSK)
   ├── framed messages (AEAD, per-direction sequence numbers)
   └── close (any timeout / violation closes immediately)
```

## HELLO (both directions, 69 bytes, no attacker-controlled lengths)

```
bytes  0..4    magic   "OVX2"
byte      4    version 2
bytes  5..37   x25519 ephemeral public key
bytes 37..69   session nonce (32 random bytes)
```

The client sends first. The server validates magic, version, and key
encoding **before replying**; any violation closes the connection. The
server rejects an all-zero DH output (small-order contribution).

## Key schedule

```
dh   = X25519(eph_local, eph_peer)
prk  = HKDF-SHA256-Extract(salt = nonce_client ‖ nonce_server, ikm = PSK)
okm  = HKDF-SHA256-Expand(prk, info = "omnivox-v2" ‖ hello_client ‖ hello_server, 64)
key_c2s = okm[0..32]      key_s2c = okm[32..64]
```

The client's send key is `key_c2s`, receive key `key_s2c`; reversed for the
server. Direction separation means neither peer can present itself forged
frames (v1's single symmetric key could). The ephemeral DH gives forward
secrecy against later PSK compromise; the PSK authenticates against
off-path attackers. A weak password remains guessable by an active attacker
with the PSK budget to grind scrypt — use random 32-byte PSKs; a PAKE is the
future upgrade if passwords must stay human.

## Framing (both directions, after key confirmation)

```
u32 LE length   (0 < len ≤ max_message_size, default cap 64 KiB)
AeadPacket:
  nonce  [u8; 12]   four zero bytes ‖ u64 BE sequence number
  tag    [u8; 16]   Poly1305 tag
  len    u64 LE     (cereal vector length, retained for payload lineage)
  ct     [len]      ChaCha20-Poly1305 ciphertext of the serialized payload
```

AEAD is ChaCha20-Poly1305 (RFC 8439) with the packet's own nonce as the IETF
nonce and empty AAD — identical to v1's construction, now keyed per session
per direction.

### Sequence discipline (the replay defense)

Each sender's counter starts at 1 and increments by exactly 1 per frame; a
failed write must not consume a number. The receiver's first expected value
is 1. A frame whose decoded nonce is not the expected value is a protocol
violation: **log and close the connection.** There is no reorder window.

### Payload encoding

The plaintext is the `Cereal` binary encoding of the message type — raw
little-endian fields, `u64` length prefixes for byte vectors, no archive
header — unchanged from the C++ original.

## Limits and timeouts (all server- and client-side)

| Knob | Default | Violation |
|---|---|---|
| handshake_timeout_ms | 10000 | close |
| idle_timeout_ms (silence between messages) | 15000 | close |
| body_stall_ms (mid-body silence) | 5000 | close |
| send_timeout_ms (peer not reading) | 5000 | close that peer |
| max_message_size | 65536 | close on header |
| max_connections (server) | 1024 | new connections wait |

Corrupt frames with the *correct* sequence number (failed AEAD or payload
decode) are logged and skipped; the connection continues. Anything else —
sequence violations, unreasonable headers, stalls — is a disconnect.

## Security properties

| Property | Mechanism | Tested by |
|---|---|---|
| Confidentiality | ChaCha20-Poly1305 per-session keys | `protocol_v2::frames_from_another_session_are_undecryptable` |
| Replay defense | strict sequence numbers | `protocol_v2::replayed_frame_is_dropped_and_the_connection_dies` |
| Session binding | nonces enter the salt/info | same cross-session test |
| Forward secrecy | ephemeral X25519 per connection | design (single-connection compromise leaks only it) |
| 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` |
| Liveness both roles | idle + stall timeouts client **and** server | `malicious_server_silence_times_out_the_client`, `header_stall_times_out` |
| Slow-client containment | per-connection locks + send timeout | design (no shared writer mutex) |
| DoS containment | connection semaphore, message caps, callback panic isolation | `panicking_callback_does_not_kill_the_connection` |

## Refusals

Release builds refuse to start with the original demo key (scrypt of
`SecurePassword123` / salt `01…08`) unless `allow_insecure_defaults` is set.