Harden native protocol: version discipline, rekey, migration, rate limit, O(1) client index

Track A protocol hardening for native-v1.

1. Protocol VERSION discipline (protocol.rs, native.rs, dosh-server.rs,
   dosh-client.rs): bump protocol::VERSION to 2 and NATIVE_PROTOCOL_VERSION to 2.
   Foreign wire-version datagrams now get a clear named "protocol version
   mismatch - upgrade dosh" reject from the server instead of a silent timeout
   (peek_foreign_wire_version + VERSION_MISMATCH_REASON). The native handshake's
   plaintext protocol_version is also checked server-side before any crypto via
   check_native_protocol_version, returning a typed ProtocolVersionMismatch.

2. Transport rekey (spec section 11/9): server rotates a client's traffic key
   after rekey_after_packets OR rekey_after_secs (config knobs). Rotated keys are
   derived independently of the handshake keys from the current key plus fresh
   server CSPRNG material shipped confidentially in an AEAD Rekey packet
   (derive_rekey_session_key). The previous epoch's key is retained briefly so
   in-flight pre-rekey packets still decrypt (matched by session_key_id), and
   stale-epoch packets are ignored, not fatal. Client handles Rekey/RekeyAck.

3. Connection migration (spec section 11): every authenticated, replay-accepted
   packet now migrates client.endpoint to the new source address (input, resize,
   ping, ack, resume, stream*, rekey-ack), not just resume. Ping now verifies its
   AEAD tag before acting so migration cannot be spoofed.

4. Native auth rate limiting: per-source-IP token bucket
   (native_auth_rate_limit_per_minute) enforced in handle_native_client_hello
   BEFORE any X25519/Ed25519 work; over-limit hellos get a non-crypto reject.

5. Speed: replaced O(sessions x clients) per-packet linear scans with an O(1)
   HashMap<conn_id, session_name> index in ServerState, kept in sync on every
   client insert/remove (attach handlers, detach, remove_client, cleanup reap,
   session-exit drain).

New config keys (ServerConfig, with defaults): rekey_after_packets = 100000,
rekey_after_secs = 3600.

Tests: version-mismatch reject (no hang), rate-limit flood reject, rekey
round-trip end-to-end + key-derivation unit tests, source-address migration,
client-index sync + cleanup purge. fmt and full test suite green.

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

src/config.rs

@@ -28,6 +28,14 @@ pub struct ServerConfig {
     pub authorized_keys: Vec<String>,
     #[serde(default = "default_native_auth_rate_limit_per_minute")]
     pub native_auth_rate_limit_per_minute: u32,
+    /// Rotate a client's transport traffic key after this many packets in the
+    /// current epoch (spec §11). `0` disables the packet-count trigger.
+    #[serde(default = "default_rekey_after_packets")]
+    pub rekey_after_packets: u64,
+    /// Rotate a client's transport traffic key after this many wall-clock seconds
+    /// in the current epoch (spec §11). `0` disables the time trigger.
+    #[serde(default = "default_rekey_after_secs")]
+    pub rekey_after_secs: u64,
     #[serde(default = "default_true")]
     pub allow_tcp_forwarding: bool,
     #[serde(default)]
@@ -61,6 +69,8 @@ impl Default for ServerConfig {
             host_key: default_host_key(),
             authorized_keys: default_authorized_keys(),
             native_auth_rate_limit_per_minute: default_native_auth_rate_limit_per_minute(),
+            rekey_after_packets: default_rekey_after_packets(),
+            rekey_after_secs: default_rekey_after_secs(),
             allow_tcp_forwarding: true,
             allow_remote_forwarding: false,
             allow_remote_non_loopback_bind: false,
@@ -175,6 +185,18 @@ fn default_native_auth_rate_limit_per_minute() -> u32 {
     30
 }
 
+fn default_rekey_after_packets() -> u64 {
+    // Rotate well before any AEAD nonce-reuse concern; ChaCha20-Poly1305 with a
+    // per-direction monotonic seq is safe far beyond this, but a bounded epoch
+    // keeps forward-secrecy windows small.
+    100_000
+}
+
+fn default_rekey_after_secs() -> u64 {
+    // One hour per epoch by default.
+    3600
+}
+
 fn default_auth_preference() -> String {
     "native,ssh".to_string()
 }