crates/rshc/src/cmd/serve_ssh.rs

@@ -45,8 +45,9 @@ pub async fn run(
         }
     };
 
-    // Load the operator's SSH key once. It serves as both the SSH host key and,
-    // via its public half, an always-allowed client key.
+    // Load the operator's SSH key once. Its public half is folded into the
+    // client allowlist (an always-allowed client key). It is NOT used as the
+    // SSH host key — see load_or_create_host_key.
     let operator_key = load_operator_key(&cfg)?;
 
     // Build the set of client public keys allowed to connect: the backend's
@@ -70,13 +71,8 @@ pub async fn run(
         ..Default::default()
     };
 
-    // Derive the SSH host key from the operator key, round-tripped through
-    // OpenSSH encoding (russh uses a different ssh-key version than we do).
-    let host_pem = operator_key
-        .to_openssh(ssh_key::LineEnding::LF)
-        .context("encode host key")?;
-    let host_key = russh::keys::PrivateKey::from_openssh(host_pem.as_bytes())
-        .map_err(|e| anyhow!("parse host key: {e}"))?;
+    // Dedicated, persistent SSH host key (never the operator's auth key).
+    let host_key = load_or_create_host_key()?;
     config.keys.push(host_key);
     let config = Arc::new(config);
 
@@ -129,6 +125,53 @@ fn load_operator_key(cfg: &Config) -> Result<ssh_key::PrivateKey> {
     Ok(key)
 }
 
+/// Load the persistent serve-ssh host key, generating an Ed25519 one on first
+/// use. This is a dedicated key, distinct from the operator's auth key, so the
+/// listener has a stable identity without cross-context key reuse.
+fn load_or_create_host_key() -> Result<russh::keys::PrivateKey> {
+    let path = Config::serve_ssh_host_key_path();
+    let key: ssh_key::PrivateKey = if path.exists() {
+        let raw = std::fs::read(&path).with_context(|| format!("read {}", path.display()))?;
+        ssh_key::PrivateKey::from_openssh(&raw).context("parse host key")?
+    } else {
+        let k = ssh_key::PrivateKey::random(&mut rand::rngs::OsRng, ssh_key::Algorithm::Ed25519)
+            .map_err(|e| anyhow!("generate host key: {e}"))?;
+        let pem = k.to_openssh(ssh_key::LineEnding::LF).context("encode host key")?;
+        write_private(&path, pem.as_bytes())?;
+        ui::print_info(&format!("generated serve-ssh host key at {}", path.display()));
+        k
+    };
+    ui::print_info(&format!(
+        "host key fingerprint {}",
+        key.fingerprint(ssh_key::HashAlg::Sha256)
+    ));
+    // Round-trip into russh's ssh-key version (russh re-exports an incompatible
+    // ssh-key release, so we cannot hand it our value directly).
+    let pem = key.to_openssh(ssh_key::LineEnding::LF).context("encode host key")?;
+    russh::keys::PrivateKey::from_openssh(pem.as_bytes()).map_err(|e| anyhow!("parse host key: {e}"))
+}
+
+/// Write private-key bytes to `path` with `0600` permissions, creating the
+/// parent directory if needed.
+fn write_private(path: &std::path::Path, bytes: &[u8]) -> Result<()> {
+    use std::io::Write;
+    use std::os::unix::fs::OpenOptionsExt;
+    if let Some(parent) = path.parent() {
+        std::fs::create_dir_all(parent)
+            .with_context(|| format!("create {}", parent.display()))?;
+    }
+    let mut f = std::fs::OpenOptions::new()
+        .write(true)
+        .create(true)
+        .truncate(true)
+        .mode(0o600)
+        .open(path)
+        .with_context(|| format!("create {}", path.display()))?;
+    f.write_all(bytes)
+        .with_context(|| format!("write {}", path.display()))?;
+    Ok(())
+}
+
 /// SHA-256 fingerprints of every public key allowed to connect: the backend's
 /// authorized_keys plus the operator's own key.
 async fn build_allowed_fingerprints(

crates/rshc/src/config.rs

@@ -43,4 +43,14 @@ impl Config {
     pub fn ssh_key_path(&self) -> PathBuf {
         PathBuf::from(shellexpand::tilde(&self.ssh_key_file).to_string())
     }
+
+    /// Path to the dedicated, persistent serve-ssh host key, kept next to the
+    /// config file (respecting `RSHC_CONFIG_PATH`).
+    pub fn serve_ssh_host_key_path() -> PathBuf {
+        Self::path()
+            .parent()
+            .map(|p| p.to_path_buf())
+            .unwrap_or_default()
+            .join("serve_ssh_host_ed25519")
+    }
 }