Merge track A: VERSION discipline, rekey, migration, rate limiting, O(1) index

src/bin/dosh-client.rs

@@ -17,7 +17,7 @@ use dosh::native::{
 use dosh::protocol::{
     self, AttachOk, AttachReject, BootstrapAttachRequest, CLIENT_TO_SERVER, Frame, Input,
     NativeAuthCheckOkBody, NativeAuthOkBody, NativeClientHelloBody, NativeServerHelloBody,
-    NativeUserAuthBody, PacketKind, Resize, ResumeRequest, SERVER_TO_CLIENT, StreamClose,
+    NativeUserAuthBody, PacketKind, Rekey, Resize, ResumeRequest, SERVER_TO_CLIENT, StreamClose,
     StreamData, StreamOpen, StreamOpenOk, StreamOpenReject, StreamWindowAdjust, TicketAttachBody,
     TicketAttachEnvelope, TicketAttachOkEnvelope,
 };
@@ -2153,6 +2153,9 @@ async fn run_terminal(
         None
     };
 
+    // Retain the previous epoch's key briefly after a rekey so any in-flight
+    // pre-rekey frame still decrypts instead of triggering a needless reconnect.
+    let mut previous_session_key: Option<[u8; 32]> = None;
     let mut recv_buf = vec![0u8; 65535];
     loop {
         tokio::select! {
@@ -2193,7 +2196,16 @@ async fn run_terminal(
                 }
                 match packet.header.kind {
                     PacketKind::Frame | PacketKind::ResumeOk => {
-                        let Ok(plain) = protocol::decrypt_body(&packet, &cred.session_key, SERVER_TO_CLIENT) else {
+                        let decrypted = protocol::decrypt_body(&packet, &cred.session_key, SERVER_TO_CLIENT)
+                            .or_else(|err| {
+                                // Fall back to the previous epoch key for in-flight
+                                // pre-rekey frames before declaring the link stale.
+                                match previous_session_key {
+                                    Some(prev) => protocol::decrypt_body(&packet, &prev, SERVER_TO_CLIENT),
+                                    None => Err(err),
+                                }
+                            });
+                        let Ok(plain) = decrypted else {
                             if let Some(frame) = reconnect(
                                 &socket,
                                 &mut cred,
@@ -2233,6 +2245,48 @@ async fn run_terminal(
                     PacketKind::Pong => {
                         last_packet_at = Instant::now();
                     }
+                    PacketKind::Rekey => {
+                        // Server-initiated transport rekey (spec §11). The Rekey is
+                        // sealed under the current key; once decrypted we derive the
+                        // next key from the shipped fresh material + current key,
+                        // switch atomically (keeping the old key for grace), and
+                        // confirm with a RekeyAck under the NEW key.
+                        let Ok(plain) = protocol::decrypt_body(&packet, &cred.session_key, SERVER_TO_CLIENT) else {
+                            continue;
+                        };
+                        let Ok(rekey) = protocol::from_body::<Rekey>(&plain) else {
+                            continue;
+                        };
+                        let previous_key = cred.session_key;
+                        let previous_key_id = cred.session_key_id;
+                        let Ok(new_key) = dosh::native::derive_rekey_session_key(
+                            &previous_key,
+                            &rekey.rekey_material,
+                            &previous_key_id,
+                            rekey.epoch,
+                        ) else {
+                            continue;
+                        };
+                        // Only accept if our derivation matches the server's id.
+                        if protocol::session_key_id(&new_key) != rekey.new_session_key_id {
+                            continue;
+                        }
+                        previous_session_key = Some(previous_key);
+                        cred.session_key = new_key;
+                        cred.session_key_id = rekey.new_session_key_id;
+                        last_packet_at = Instant::now();
+                        send_seq += 1;
+                        let ack = protocol::encode_encrypted(
+                            PacketKind::RekeyAck,
+                            cred.client_id,
+                            send_seq,
+                            cred.last_rendered_seq,
+                            &cred.session_key,
+                            CLIENT_TO_SERVER,
+                            b"",
+                        )?;
+                        socket.send_to(&ack, addr).await?;
+                    }
                     PacketKind::AttachReject => {
                         let reject: AttachReject = protocol::from_body(&packet.body)?;
                         if reject.reason == "unknown client" {

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,
@@ -180,6 +190,18 @@ 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()
 }

src/native.rs

@@ -15,7 +15,7 @@ use std::str::FromStr;
 use x25519_dalek::{PublicKey as X25519PublicKey, StaticSecret};
 
 pub const HOST_KEY_ALGORITHM: &str = "dosh-ed25519";
-pub const NATIVE_PROTOCOL_VERSION: u8 = 1;
+pub const NATIVE_PROTOCOL_VERSION: u8 = 2;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct HostPublicKey {
@@ -248,17 +248,55 @@ pub fn sign_server_hello(
     Ok(())
 }
 
+/// Error raised when a native handshake peer speaks a different protocol version.
+///
+/// Carries the peer's advertised version so callers can render an actionable,
+/// named message ("upgrade dosh") rather than letting a mismatch surface as an
+/// opaque decrypt failure or a silent timeout. The `Display` text deliberately
+/// embeds [`crate::protocol::VERSION_MISMATCH_REASON`] so the server's reject and
+/// the client's local error read the same way.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct ProtocolVersionMismatch {
+    pub local: u8,
+    pub remote: u8,
+    pub peer: &'static str,
+}
+
+impl std::fmt::Display for ProtocolVersionMismatch {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "{} ({} speaks native protocol v{}, this build speaks v{})",
+            crate::protocol::VERSION_MISMATCH_REASON,
+            self.peer,
+            self.remote,
+            self.local,
+        )
+    }
+}
+
+impl std::error::Error for ProtocolVersionMismatch {}
+
+/// Verify a peer-advertised native protocol version against this build.
+///
+/// `peer` names whose version was wrong ("client" or "server") for the error
+/// message. Returns a typed [`ProtocolVersionMismatch`] so the caller can both
+/// match on it and print an actionable, upgrade-oriented message.
+pub fn check_native_protocol_version(version: u8, peer: &'static str) -> Result<()> {
+    if version != NATIVE_PROTOCOL_VERSION {
+        return Err(ProtocolVersionMismatch {
+            local: NATIVE_PROTOCOL_VERSION,
+            remote: version,
+            peer,
+        }
+        .into());
+    }
+    Ok(())
+}
+
 pub fn verify_server_hello(client: &NativeClientHello, server: &NativeServerHello) -> Result<()> {
-    anyhow::ensure!(
+    check_native_protocol_version(client.protocol_version, "client")?;
-        client.protocol_version == NATIVE_PROTOCOL_VERSION,
+    check_native_protocol_version(server.protocol_version, "server")?;
-        "unsupported native client protocol {}",
-        client.protocol_version
-    );
-    anyhow::ensure!(
-        server.protocol_version == NATIVE_PROTOCOL_VERSION,
-        "unsupported native server protocol {}",
-        server.protocol_version
-    );
     let transcript = server_hello_transcript(client, server)?;
     verify_host_signature(&server.host_key, &transcript, &server.host_signature)
 }
@@ -369,6 +407,36 @@ pub fn derive_native_session_key(
     crypto::hkdf32(shared.as_bytes(), &salt, b"dosh/native/chacha20poly1305/v1")
 }
 
+/// Derive a rotated transport key for transport rekey (spec §11 / §9).
+///
+/// The rotated key is derived **independently of the handshake traffic keys**:
+/// the input keying material is the current epoch's key (which both peers already
+/// hold) mixed with fresh, server-generated `rekey_material` (32 bytes of CSPRNG
+/// output, delivered confidentially inside an AEAD `Rekey` packet sealed under
+/// the current key). The new `epoch` and the previous epoch's `session_key_id`
+/// salt the derivation so each epoch's key is unique. It never re-derives from
+/// the handshake DH output, satisfying the spec requirement that "rotated session
+/// keys must be derived independently ... from fresh randomness" and "must not
+/// reuse handshake traffic keys."
+///
+/// Both peers run this identically — the client never needs the server's
+/// long-term secret, only the fresh `rekey_material` it receives in the `Rekey`
+/// packet plus the current key it already shares.
+pub fn derive_rekey_session_key(
+    current_key: &[u8; 32],
+    rekey_material: &[u8; 32],
+    previous_session_key_id: &[u8; 16],
+    epoch: u64,
+) -> Result<[u8; 32]> {
+    let mut ikm = Vec::with_capacity(64);
+    ikm.extend_from_slice(current_key);
+    ikm.extend_from_slice(rekey_material);
+    let mut salt = b"dosh/native/rekey/v1".to_vec();
+    salt.extend_from_slice(previous_session_key_id);
+    salt.extend_from_slice(&epoch.to_be_bytes());
+    crypto::hkdf32(&ikm, &salt, b"dosh/native/rekey/chacha20poly1305/v1")
+}
+
 pub fn load_ed25519_identity(path: &Path) -> Result<SigningKey> {
     load_ed25519_identity_with_passphrase(path, None)
 }

src/protocol.rs

@@ -5,8 +5,14 @@ use anyhow::{Context, Result, bail};
 use serde::{Deserialize, Serialize};
 
 pub const MAGIC: &[u8; 4] = b"DOSH";
-pub const VERSION: u8 = 1;
+pub const VERSION: u8 = 2;
 pub const HEADER_LEN: usize = 58;
+
+/// Stable, user-facing reason string the server puts in an `AttachReject` when a
+/// native handshake arrives carrying a `protocol_version` it cannot speak. The
+/// client recognizes this prefix and surfaces a clear "upgrade dosh" message
+/// instead of the generic transport rejection or, worse, a silent timeout.
+pub const VERSION_MISMATCH_REASON: &str = "protocol version mismatch — upgrade dosh";
 const HEADER_AAD_LEN: usize = HEADER_LEN - 2;
 pub const CLIENT_TO_SERVER: u32 = 1;
 pub const SERVER_TO_CLIENT: u32 = 2;
@@ -39,6 +45,8 @@ pub enum PacketKind {
     StreamEof = 23,
     StreamClose = 24,
     NativeAuthCheckOk = 25,
+    Rekey = 26,
+    RekeyAck = 27,
 }
 
 impl TryFrom<u8> for PacketKind {
@@ -71,6 +79,8 @@ impl TryFrom<u8> for PacketKind {
             23 => Self::StreamEof,
             24 => Self::StreamClose,
             25 => Self::NativeAuthCheckOk,
+            26 => Self::Rekey,
+            27 => Self::RekeyAck,
             _ => bail!("unknown packet kind {value}"),
         })
     }
@@ -110,7 +120,11 @@ impl Header {
             bail!("bad magic");
         }
         if input[4] != VERSION {
-            bail!("bad protocol version {}", input[4]);
+            bail!(
+                "{} (peer wire protocol v{}, this build speaks v{VERSION})",
+                VERSION_MISMATCH_REASON,
+                input[4]
+            );
         }
         let kind = PacketKind::try_from(input[5])?;
         let flags = u16::from_be_bytes(input[6..8].try_into().unwrap());
@@ -133,6 +147,20 @@ impl Header {
     }
 }
 
+/// Cheaply inspect a datagram that carries our [`MAGIC`] but whose wire
+/// [`VERSION`] byte differs from this build's. Returns the peer's advertised
+/// wire version when (and only when) the packet is a Dosh packet we cannot
+/// otherwise decode because of a version skew, so the receiver can answer with a
+/// clear, named version-mismatch reject instead of dropping it (a silent
+/// timeout for the peer). Returns `None` for our own version, foreign magic, or
+/// runt packets.
+pub fn peek_foreign_wire_version(input: &[u8]) -> Option<u8> {
+    if input.len() < 5 || &input[..4] != MAGIC || input[4] == VERSION {
+        return None;
+    }
+    Some(input[4])
+}
+
 #[derive(Debug, Clone)]
 pub struct Packet {
     pub header: Header,
@@ -369,6 +397,20 @@ pub struct StreamClose {
     pub stream_id: u64,
 }
 
+/// Server→client transport rekey, sealed under the *current* session key.
+///
+/// Carries the fresh server-generated `rekey_material` and the new `epoch`; both
+/// peers feed these into [`crate::native::derive_rekey_session_key`] to compute
+/// the next traffic key. `new_session_key_id` is the id the next epoch's packets
+/// will carry, so the client can recognize and switch atomically. The client
+/// replies with a [`PacketKind::RekeyAck`] encrypted under the *new* key.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Rekey {
+    pub epoch: u64,
+    pub rekey_material: [u8; 32],
+    pub new_session_key_id: [u8; 16],
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Frame {
     pub session: String,

tests/integration_smoke.rs

@@ -1246,3 +1246,314 @@ fn resume_updates_udp_endpoint_for_roaming() {
         "expected output on resumed socket, got {text:?}"
     );
 }
+
+#[test]
+fn transport_rekey_round_trip_keeps_session_alive() {
+    use dosh::native::derive_rekey_session_key;
+    use dosh::protocol::Rekey;
+
+    let dir = tempfile::tempdir().unwrap();
+    let port = free_udp_port();
+    let config = write_server_config(&dir, port);
+    // Rotate after a single packet so a rekey fires almost immediately.
+    let mut raw = fs::read_to_string(&config).unwrap();
+    raw.push_str("rekey_after_packets = 1\n");
+    fs::write(&config, raw).unwrap();
+    let mut server = start_server(&dir, &config);
+
+    let (socket, bootstrap, ok) = direct_attach(&config, port, "read-write");
+    socket
+        .set_read_timeout(Some(Duration::from_millis(300)))
+        .unwrap();
+
+    // Track the live transport key; it rotates when a Rekey arrives.
+    let mut current_key = bootstrap.session_key;
+    let mut current_key_id = protocol::session_key_id(&current_key);
+    let mut send_seq = 2u64;
+
+    // Produce some output so the server starts sending frames (and rekeys).
+    let input = Input {
+        bytes: b"printf DOSH_REKEY_ONE\\n\n".to_vec(),
+    };
+    send_encrypted(
+        &socket,
+        port,
+        PacketKind::Input,
+        ok.client_id,
+        send_seq,
+        0,
+        &current_key,
+        &protocol::to_body(&input).unwrap(),
+    );
+    send_seq += 1;
+
+    // Drive the loop: decrypt frames under the live key, and when a Rekey lands,
+    // adopt the new epoch key and ack it. Confirm a post-rekey input still works.
+    let mut rekeyed = false;
+    let mut saw_post_rekey_output = false;
+    let mut buf = [0u8; 65535];
+    let deadline = std::time::Instant::now() + Duration::from_secs(8);
+    while std::time::Instant::now() < deadline {
+        let Ok((n, _)) = socket.recv_from(&mut buf) else {
+            continue;
+        };
+        let Ok(packet) = protocol::decode(&buf[..n]) else {
+            continue;
+        };
+        match packet.header.kind {
+            PacketKind::Rekey => {
+                let plain =
+                    protocol::decrypt_body(&packet, &current_key, SERVER_TO_CLIENT).unwrap();
+                let rekey: Rekey = protocol::from_body(&plain).unwrap();
+                let new_key = derive_rekey_session_key(
+                    &current_key,
+                    &rekey.rekey_material,
+                    &current_key_id,
+                    rekey.epoch,
+                )
+                .unwrap();
+                assert_eq!(
+                    protocol::session_key_id(&new_key),
+                    rekey.new_session_key_id,
+                    "client-derived rekey key id must match the server's"
+                );
+                current_key = new_key;
+                current_key_id = rekey.new_session_key_id;
+                // Ack under the NEW key.
+                send_encrypted(
+                    &socket,
+                    port,
+                    PacketKind::RekeyAck,
+                    ok.client_id,
+                    send_seq,
+                    0,
+                    &current_key,
+                    b"",
+                );
+                send_seq += 1;
+                rekeyed = true;
+                // Now send a fresh input under the new key.
+                let input = Input {
+                    bytes: b"printf DOSH_REKEY_TWO\\n\n".to_vec(),
+                };
+                send_encrypted(
+                    &socket,
+                    port,
+                    PacketKind::Input,
+                    ok.client_id,
+                    send_seq,
+                    0,
+                    &current_key,
+                    &protocol::to_body(&input).unwrap(),
+                );
+                send_seq += 1;
+            }
+            PacketKind::Frame | PacketKind::ResumeOk => {
+                if let Ok(plain) = protocol::decrypt_body(&packet, &current_key, SERVER_TO_CLIENT) {
+                    if let Ok(frame) = protocol::from_body::<Frame>(&plain) {
+                        let text = String::from_utf8_lossy(&frame.bytes);
+                        if rekeyed && text.contains("DOSH_REKEY_TWO") {
+                            saw_post_rekey_output = true;
+                            break;
+                        }
+                    }
+                }
+            }
+            _ => {}
+        }
+    }
+
+    let _ = server.kill();
+    let _ = server.wait();
+
+    assert!(rekeyed, "server never initiated a rekey");
+    assert!(
+        saw_post_rekey_output,
+        "post-rekey input/output round trip failed under the new epoch key"
+    );
+}
+
+#[test]
+fn input_from_new_source_address_migrates_connection() {
+    // Spec §11: the server must accept client source-address migration after ANY
+    // valid authenticated/encrypted packet from a new address, not just resume.
+    let dir = tempfile::tempdir().unwrap();
+    let port = free_udp_port();
+    let config = write_server_config(&dir, port);
+    let mut server = start_server(&dir, &config);
+    let (_old_socket, bootstrap, ok) = direct_attach(&config, port, "read-write");
+
+    // A fresh socket = a new source address (new ephemeral port). The very first
+    // packet from it is an ordinary encrypted Input, not a ResumeRequest.
+    let new_socket = UdpSocket::bind("127.0.0.1:0").unwrap();
+    new_socket
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+    let input = Input {
+        bytes: b"printf DOSH_MIGRATE\\n\n".to_vec(),
+    };
+    let packet = protocol::encode_encrypted(
+        PacketKind::Input,
+        ok.client_id,
+        2,
+        0,
+        &bootstrap.session_key,
+        CLIENT_TO_SERVER,
+        &protocol::to_body(&input).unwrap(),
+    )
+    .unwrap();
+    new_socket
+        .send_to(&packet, format!("127.0.0.1:{port}"))
+        .unwrap();
+
+    // Output frames for this input must now be delivered to the NEW socket,
+    // proving the server migrated `endpoint` off the original address.
+    let text = collect_frame_text(&new_socket, &bootstrap.session_key, 2000);
+
+    let _ = server.kill();
+    let _ = server.wait();
+
+    assert!(
+        text.contains("DOSH_MIGRATE"),
+        "expected server output on the migrated socket, got {text:?}"
+    );
+}
+
+#[test]
+fn native_auth_rate_limit_rejects_flood_before_crypto() {
+    use dosh::native::{NATIVE_PROTOCOL_VERSION, NativeClientHello};
+    use dosh::protocol::NativeClientHelloBody;
+
+    let dir = tempfile::tempdir().unwrap();
+    let port = free_udp_port();
+    let config = write_server_config(&dir, port);
+    // Squeeze the rate limit down to 2/min so a short burst trips it.
+    let mut raw = fs::read_to_string(&config).unwrap();
+    raw.push_str("native_auth_rate_limit_per_minute = 2\n");
+    fs::write(&config, raw).unwrap();
+    write_native_client_auth(&dir, &config);
+    let mut server = start_server(&dir, &config);
+
+    let socket = std::net::UdpSocket::bind("127.0.0.1:0").unwrap();
+    socket
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+    let user = std::env::var("USER").unwrap_or_else(|_| "unknown".to_string());
+    let make_hello = || {
+        let (_, client_public) = dosh::native::generate_native_ephemeral();
+        let hello = NativeClientHello {
+            protocol_version: NATIVE_PROTOCOL_VERSION,
+            client_random: crypto::random_32(),
+            client_ephemeral_public: client_public,
+            requested_host: "local".to_string(),
+            requested_user: user.clone(),
+            requested_session: "default".to_string(),
+            requested_mode: "read-write".to_string(),
+            terminal_size: (80, 24),
+            supported_aead: vec!["chacha20poly1305".to_string()],
+            supported_user_key_algorithms: vec!["ssh-ed25519".to_string()],
+            cached_host_key_fingerprint: None,
+            attach_ticket_envelope: None,
+            requested_env: Vec::new(),
+        };
+        protocol::encode_plain(
+            PacketKind::NativeClientHello,
+            [0u8; 16],
+            1,
+            0,
+            &protocol::to_body(&NativeClientHelloBody { hello }).unwrap(),
+        )
+        .unwrap()
+    };
+
+    let mut reasons = Vec::new();
+    // Burst of hellos; with a 2/min budget the later ones must be rejected.
+    for _ in 0..6 {
+        socket
+            .send_to(&make_hello(), format!("127.0.0.1:{port}"))
+            .unwrap();
+        let mut buf = [0u8; 65535];
+        if let Ok((n, _)) = socket.recv_from(&mut buf) {
+            let packet = protocol::decode(&buf[..n]).unwrap();
+            if packet.header.kind == PacketKind::AttachReject {
+                let reject: AttachReject = protocol::from_body(&packet.body).unwrap();
+                reasons.push(reject.reason);
+            }
+        }
+    }
+
+    let _ = server.kill();
+    let _ = server.wait();
+
+    assert!(
+        reasons.iter().any(|r| r.contains("rate limit")),
+        "expected a rate-limit reject within the burst, got {reasons:?}"
+    );
+}
+
+#[test]
+fn native_hello_with_mismatched_protocol_version_gets_named_reject_not_hang() {
+    use dosh::native::{NATIVE_PROTOCOL_VERSION, NativeClientHello};
+    use dosh::protocol::{NativeClientHelloBody, VERSION_MISMATCH_REASON};
+
+    let dir = tempfile::tempdir().unwrap();
+    let port = free_udp_port();
+    let config = write_server_config(&dir, port);
+    write_native_client_auth(&dir, &config);
+    let mut server = start_server(&dir, &config);
+
+    let socket = std::net::UdpSocket::bind("127.0.0.1:0").unwrap();
+    socket
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+
+    // Same wire VERSION as the server (so the datagram decodes), but an
+    // incompatible native handshake protocol_version. This is the spec's
+    // plaintext negotiation point: the server must answer with a clear, named
+    // reject rather than letting the client time out.
+    let hello = NativeClientHello {
+        protocol_version: NATIVE_PROTOCOL_VERSION.wrapping_add(1),
+        client_random: crypto::random_32(),
+        client_ephemeral_public: [3u8; 32],
+        requested_host: "local".to_string(),
+        requested_user: std::env::var("USER").unwrap_or_else(|_| "unknown".to_string()),
+        requested_session: "default".to_string(),
+        requested_mode: "read-write".to_string(),
+        terminal_size: (80, 24),
+        supported_aead: vec!["chacha20poly1305".to_string()],
+        supported_user_key_algorithms: vec!["ssh-ed25519".to_string()],
+        cached_host_key_fingerprint: None,
+        attach_ticket_envelope: None,
+        requested_env: Vec::new(),
+    };
+    let packet = protocol::encode_plain(
+        PacketKind::NativeClientHello,
+        [0u8; 16],
+        1,
+        0,
+        &protocol::to_body(&NativeClientHelloBody { hello }).unwrap(),
+    )
+    .unwrap();
+    socket
+        .send_to(&packet, format!("127.0.0.1:{port}"))
+        .unwrap();
+
+    let mut buf = [0u8; 65535];
+    // recv with the 2s read timeout above: a hang would surface as a recv error
+    // here instead of a reject, which is exactly what this test guards against.
+    let (n, _) = socket
+        .recv_from(&mut buf)
+        .expect("server must answer a version-mismatched hello, not hang");
+    let packet = protocol::decode(&buf[..n]).unwrap();
+    let reject: AttachReject = protocol::from_body(&packet.body).unwrap();
+
+    let _ = server.kill();
+    let _ = server.wait();
+
+    assert_eq!(packet.header.kind, PacketKind::AttachReject);
+    assert!(
+        reject.reason.contains(VERSION_MISMATCH_REASON),
+        "expected a named protocol-version-mismatch reject, got {:?}",
+        reject.reason
+    );
+}

tests/protocol_auth.rs

@@ -126,6 +126,139 @@ fn attach_ticket_is_sealed_and_verifies_scope() {
     );
 }
 
+#[test]
+fn peek_foreign_wire_version_flags_only_version_skew() {
+    let key = crypto::random_32();
+    let mut packet = protocol::encode_encrypted(
+        PacketKind::Input,
+        crypto::random_16(),
+        1,
+        0,
+        &key,
+        CLIENT_TO_SERVER,
+        b"hi",
+    )
+    .unwrap();
+    // A correctly framed packet for this build is not "foreign".
+    assert_eq!(protocol::peek_foreign_wire_version(&packet), None);
+    // Bumping the wire version byte makes it undecodable but recognizable.
+    packet[4] = protocol::VERSION.wrapping_add(7);
+    assert_eq!(
+        protocol::peek_foreign_wire_version(&packet),
+        Some(protocol::VERSION.wrapping_add(7))
+    );
+    assert!(protocol::decode(&packet).is_err());
+    // Non-Dosh datagrams and runts are ignored.
+    assert_eq!(protocol::peek_foreign_wire_version(b"XXXX\x01"), None);
+    assert_eq!(protocol::peek_foreign_wire_version(b"DOS"), None);
+}
+
+#[test]
+fn rekey_key_derivation_agrees_and_is_independent_per_epoch() {
+    use dosh::native::derive_rekey_session_key;
+
+    // The handshake/current key both peers already share.
+    let current_key = crypto::random_32();
+    let current_id = protocol::session_key_id(&current_key);
+    // Fresh server-generated material, delivered confidentially in the Rekey.
+    let material = crypto::random_32();
+
+    // Both peers derive identically from shared current key + shipped material.
+    let server_view = derive_rekey_session_key(&current_key, &material, &current_id, 1).unwrap();
+    let client_view = derive_rekey_session_key(&current_key, &material, &current_id, 1).unwrap();
+    assert_eq!(server_view, client_view);
+
+    // The rotated key must not equal the handshake/current key.
+    assert_ne!(server_view, current_key);
+
+    // A different epoch (or different material) yields a different key.
+    let next_epoch = derive_rekey_session_key(&current_key, &material, &current_id, 2).unwrap();
+    assert_ne!(server_view, next_epoch);
+    let other_material = crypto::random_32();
+    let other = derive_rekey_session_key(&current_key, &other_material, &current_id, 1).unwrap();
+    assert_ne!(server_view, other);
+}
+
+#[test]
+fn rekey_round_trip_decrypts_old_and_new_epoch_packets() {
+    use dosh::native::derive_rekey_session_key;
+
+    let key_epoch0 = crypto::random_32();
+    let id0 = protocol::session_key_id(&key_epoch0);
+    let conn_id = crypto::random_16();
+
+    // Pre-rekey packet sealed under epoch-0 key.
+    let pre = protocol::encode_encrypted(
+        PacketKind::Frame,
+        conn_id,
+        5,
+        0,
+        &key_epoch0,
+        protocol::SERVER_TO_CLIENT,
+        b"before",
+    )
+    .unwrap();
+
+    // Rotate to epoch 1.
+    let material = crypto::random_32();
+    let key_epoch1 = derive_rekey_session_key(&key_epoch0, &material, &id0, 1).unwrap();
+    let post = protocol::encode_encrypted(
+        PacketKind::Frame,
+        conn_id,
+        6,
+        0,
+        &key_epoch1,
+        protocol::SERVER_TO_CLIENT,
+        b"after",
+    )
+    .unwrap();
+
+    let pre = protocol::decode(&pre).unwrap();
+    let post = protocol::decode(&post).unwrap();
+
+    // Each epoch's key carries its own session_key_id; the receiver picks the
+    // right one and both decrypt correctly.
+    assert_eq!(pre.header.session_key_id, id0);
+    assert_eq!(
+        pre.header.session_key_id,
+        protocol::session_key_id(&key_epoch0)
+    );
+    assert_eq!(
+        post.header.session_key_id,
+        protocol::session_key_id(&key_epoch1)
+    );
+    assert_eq!(
+        protocol::decrypt_body(&pre, &key_epoch0, protocol::SERVER_TO_CLIENT).unwrap(),
+        b"before"
+    );
+    assert_eq!(
+        protocol::decrypt_body(&post, &key_epoch1, protocol::SERVER_TO_CLIENT).unwrap(),
+        b"after"
+    );
+
+    // A stale-epoch packet under the wrong key is rejected via session_key_id
+    // BEFORE any AEAD work — ignorable, not a fatal decrypt error.
+    let err = protocol::decrypt_body(&post, &key_epoch0, protocol::SERVER_TO_CLIENT).unwrap_err();
+    assert!(err.to_string().contains("session key id"));
+}
+
+#[test]
+fn check_native_protocol_version_names_the_mismatch() {
+    use dosh::native::{NATIVE_PROTOCOL_VERSION, check_native_protocol_version};
+    check_native_protocol_version(NATIVE_PROTOCOL_VERSION, "server").unwrap();
+    let err = check_native_protocol_version(NATIVE_PROTOCOL_VERSION.wrapping_add(1), "server")
+        .unwrap_err();
+    let message = err.to_string();
+    assert!(
+        message.contains(protocol::VERSION_MISMATCH_REASON),
+        "expected actionable upgrade message, got {message:?}"
+    );
+    assert!(
+        message.contains("server"),
+        "should name the wrong peer: {message:?}"
+    );
+}
+
 #[test]
 fn replay_window_rejects_duplicates_but_allows_bounded_out_of_order() {
     let mut replay = ReplayWindow::new(8);