Add hostile-network tests, parser fuzzing, and CI fuzz job

Track B (milestone 5 / §16 hardening):

- tests/hostile_network.rs: in-process UDP relay/shim between a test
  client and a real dosh-server that can drop, reorder, and duplicate
  datagrams, and rebind its upstream socket mid-session. Asserts:
  session survives loss/reorder, duplicated/replayed Input is applied at
  most once, stale packets after resume are ignored (not fatal), and a
  client source-address change preserves the session.
- tests/parser_robustness.rs: deterministic randomized tests throwing
  garbage at every reachable public parser (packet decode, from_body for
  all protocol/native structs, authorized_keys, known_hosts, host-key
  line, ssh-ed25519 blob, bootstrap, attach ticket); asserts none panic.
- fuzz/: standalone cargo-fuzz crate (own [workspace], non-default
  member) with libfuzzer-sys harnesses for packet decode, from_body,
  authorized_keys, known_hosts, handshake structs+verifiers, and attach
  tickets. README documents the run command.
- .github/workflows/ci.yml: add fuzz-smoke job (nightly + cargo-fuzz,
  short -max_total_time per target, tolerant if tooling unavailable);
  existing fmt/test/build/bench steps unchanged.

cargo fmt --check and cargo test are green.

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

fuzz/fuzz_targets/attach_ticket.rs

@@ -0,0 +1,20 @@
+#![no_main]
+//! Fuzz the attach-ticket and bootstrap decoders. These open server-sealed
+//! AEAD blobs and base64 bootstrap envelopes from cache / wire material that an
+//! attacker may corrupt; none may panic.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::auth::{decode_bootstrap, open_attach_ticket, verify_attach_ticket};
+
+fuzz_target!(|data: &[u8]| {
+    let secret = [0x11u8; 32];
+    let psk = [0x22u8; 32];
+
+    let _ = open_attach_ticket(&secret, data);
+    let _ = verify_attach_ticket(&secret, data, &psk, "default", "read-write");
+
+    if let Ok(text) = std::str::from_utf8(data) {
+        let _ = decode_bootstrap(text);
+    }
+});

fuzz/fuzz_targets/authorized_keys.rs

@@ -0,0 +1,18 @@
+#![no_main]
+//! Fuzz the authorized_keys parser, including its option lexer and the
+//! ssh-ed25519 public-key blob parser it depends on. None may panic.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::native::{parse_authorized_keys, parse_ssh_ed25519_public_blob};
+
+fuzz_target!(|data: &[u8]| {
+    // The blob parser operates directly on raw bytes.
+    let _ = parse_ssh_ed25519_public_blob(data);
+
+    // The line parser operates on text; only feed valid UTF-8 (lossless),
+    // matching how the file is read in production via read_to_string.
+    if let Ok(text) = std::str::from_utf8(data) {
+        let _ = parse_authorized_keys(text);
+    }
+});

fuzz/fuzz_targets/from_body.rs

@@ -0,0 +1,62 @@
+#![no_main]
+//! Fuzz `protocol::from_body` (bincode deserialization) for every protocol and
+//! native struct that is decoded from untrusted wire bytes. None may panic.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::auth::{AttachTicketPlain, BootstrapResponse, SealedAttachTicket};
+use dosh::native::{
+    HostPublicKey, NativeAuthOk, NativeClientHello, NativeServerHello, NativeUserAuth,
+};
+use dosh::protocol::{
+    self, AttachOk, AttachReject, BootstrapAttachRequest, Frame, Input, NativeAuthCheckOkBody,
+    NativeAuthOkBody, NativeClientHelloBody, NativeServerHelloBody, NativeUserAuthBody, Resize,
+    ResumeRequest, StreamClose, StreamData, StreamEof, StreamOpen, StreamOpenOk, StreamOpenReject,
+    StreamWindowAdjust, TicketAttachBody, TicketAttachEnvelope, TicketAttachOkEnvelope,
+};
+
+macro_rules! try_body {
+    ($data:expr, $ty:ty) => {
+        let _ = protocol::from_body::<$ty>($data);
+    };
+}
+
+fuzz_target!(|data: &[u8]| {
+    // protocol.rs structs
+    try_body!(data, BootstrapAttachRequest);
+    try_body!(data, TicketAttachEnvelope);
+    try_body!(data, TicketAttachBody);
+    try_body!(data, TicketAttachOkEnvelope);
+    try_body!(data, AttachOk);
+    try_body!(data, AttachReject);
+    try_body!(data, ResumeRequest);
+    try_body!(data, Input);
+    try_body!(data, Resize);
+    try_body!(data, Frame);
+    try_body!(data, StreamOpen);
+    try_body!(data, StreamOpenOk);
+    try_body!(data, StreamOpenReject);
+    try_body!(data, StreamData);
+    try_body!(data, StreamWindowAdjust);
+    try_body!(data, StreamEof);
+    try_body!(data, StreamClose);
+
+    // native handshake wrapper bodies
+    try_body!(data, NativeClientHelloBody);
+    try_body!(data, NativeServerHelloBody);
+    try_body!(data, NativeUserAuthBody);
+    try_body!(data, NativeAuthOkBody);
+    try_body!(data, NativeAuthCheckOkBody);
+
+    // bare native handshake structs
+    try_body!(data, NativeClientHello);
+    try_body!(data, NativeServerHello);
+    try_body!(data, NativeUserAuth);
+    try_body!(data, NativeAuthOk);
+    try_body!(data, HostPublicKey);
+
+    // auth.rs structs
+    try_body!(data, BootstrapResponse);
+    try_body!(data, SealedAttachTicket);
+    try_body!(data, AttachTicketPlain);
+});

fuzz/fuzz_targets/handshake_structs.rs

@@ -0,0 +1,44 @@
+#![no_main]
+//! Fuzz the native handshake structs and their structural verifiers.
+//!
+//! Beyond plain deserialization (covered by the `from_body` target), this drives
+//! the verifier state machine: if the input happens to decode into the handshake
+//! structs, run `verify_server_hello`, `user_auth_transcript`, and
+//! `verify_native_user_auth` on them. These run on attacker-controlled material
+//! during the handshake and must reject (Err) without panicking.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::native::{
+    NativeClientHello, NativeServerHello, NativeUserAuth, user_auth_transcript,
+    verify_native_user_auth, verify_server_hello,
+};
+use dosh::protocol;
+
+fuzz_target!(|data: &[u8]| {
+    // Split the input into three slices and try to decode each into a handshake
+    // struct. Use a length prefix scheme that is robust to short inputs.
+    if data.len() < 3 {
+        return;
+    }
+    let n = data.len();
+    let a = n / 3;
+    let b = 2 * n / 3;
+    let (chunk_client, chunk_server, chunk_auth) = (&data[..a], &data[a..b], &data[b..]);
+
+    let client: Option<NativeClientHello> = protocol::from_body(chunk_client).ok();
+    let server: Option<NativeServerHello> = protocol::from_body(chunk_server).ok();
+    let auth: Option<NativeUserAuth> = protocol::from_body(chunk_auth).ok();
+
+    if let (Some(client), Some(server)) = (&client, &server) {
+        // Host signature verification over the transcript must not panic.
+        let _ = verify_server_hello(client, server);
+
+        if let Some(auth) = &auth {
+            // Transcript construction and full user-auth verification (signature
+            // check + authorized-key matching) must not panic on garbage.
+            let _ = user_auth_transcript(client, server, auth);
+            let _ = verify_native_user_auth(client, server, auth, &[], None);
+        }
+    }
+});

fuzz/fuzz_targets/known_hosts.rs

@@ -0,0 +1,14 @@
+#![no_main]
+//! Fuzz the known_hosts parser and the host-public-key line parser. None may
+//! panic on arbitrary input.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::native::{parse_host_public_key_line, parse_known_hosts};
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok(text) = std::str::from_utf8(data) {
+        let _ = parse_known_hosts(text);
+        let _ = parse_host_public_key_line(text);
+    }
+});

fuzz/fuzz_targets/packet_decode.rs

@@ -0,0 +1,23 @@
+#![no_main]
+//! Fuzz the wire-packet decoder + decrypt path.
+//!
+//! Mirrors tests/parser_robustness.rs but driven by libFuzzer so the coverage
+//! engine can search for panics in `protocol::decode`, `Header::parse`, and the
+//! decode -> decrypt_body pipeline. The objective is: NO PANICS on any input.
+
+use libfuzzer_sys::fuzz_target;
+
+use dosh::protocol::{self, CLIENT_TO_SERVER, SERVER_TO_CLIENT};
+
+fuzz_target!(|data: &[u8]| {
+    // Header parsing must never panic.
+    let _ = protocol::Header::parse(data);
+
+    // Full decode, then attempt decryption with a fixed key in both directions.
+    // A real attacker controls these bytes; neither path may panic.
+    if let Ok(packet) = protocol::decode(data) {
+        let key = [0x42u8; 32];
+        let _ = protocol::decrypt_body(&packet, &key, CLIENT_TO_SERVER);
+        let _ = protocol::decrypt_body(&packet, &key, SERVER_TO_CLIENT);
+    }
+});