csm/crates/client/src/network/mod.rs

use anyhow::Result;
use std::path::PathBuf;
use std::sync::{Arc, Mutex};
use std::time::Duration;
use tokio::net::TcpStream;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tracing::{info, debug, warn};
use csm_protocol::{Frame, MessageType, RegisterRequest, RegisterResponse, HeartbeatPayload, WatermarkConfigPayload, UsbPolicyPayload, DiskEncryptionConfigPayload, PatchScanConfigPayload};
use hmac::{Hmac, Mac};
use sha2::{Sha256, Digest};

use crate::ClientState;

/// Holds senders for all plugin config channels
pub struct PluginChannels {
    pub watermark_tx: tokio::sync::watch::Sender<Option<WatermarkConfigPayload>>,
    pub web_filter_tx: tokio::sync::watch::Sender<crate::web_filter::WebFilterConfig>,
    pub software_blocker_tx: tokio::sync::watch::Sender<crate::software_blocker::SoftwareBlockerConfig>,
    pub popup_blocker_tx: tokio::sync::watch::Sender<crate::popup_blocker::PopupBlockerConfig>,
    pub usb_audit_tx: tokio::sync::watch::Sender<crate::usb_audit::UsbAuditConfig>,
    pub usage_timer_tx: tokio::sync::watch::Sender<crate::usage_timer::UsageConfig>,
    pub usb_policy_tx: tokio::sync::watch::Sender<Option<UsbPolicyPayload>>,
    pub disk_encryption_tx: tokio::sync::watch::Sender<crate::disk_encryption::DiskEncryptionConfig>,
    pub print_audit_tx: tokio::sync::watch::Sender<crate::print_audit::PrintAuditConfig>,
    pub clipboard_control_tx: tokio::sync::watch::Sender<crate::clipboard_control::ClipboardControlConfig>,
    pub patch_tx: tokio::sync::watch::Sender<crate::patch::PluginConfig>,
}

/// Connect to server and run the main communication loop
pub async fn connect_and_run(
    state: &ClientState,
    data_rx: &mut tokio::sync::mpsc::Receiver<Frame>,
    plugins: &PluginChannels,
) -> Result<()> {
    let tcp_stream = TcpStream::connect(&state.server_addr).await?;
    debug!("TCP connected to {}", state.server_addr);

    if state.use_tls {
        let tls_stream = wrap_tls(tcp_stream, &state.server_addr).await?;
        run_comm_loop(tls_stream, state, data_rx, plugins).await
    } else {
        run_comm_loop(tcp_stream, state, data_rx, plugins).await
    }
}

/// Wrap a TCP stream with TLS and certificate pinning.
async fn wrap_tls(stream: TcpStream, server_addr: &str) -> Result<tokio_rustls::client::TlsStream<TcpStream>> {
    let mut root_store = rustls::RootCertStore::empty();

    // Load custom CA certificate if specified
    if let Ok(ca_path) = std::env::var("CSM_TLS_CA_CERT") {
        let ca_pem = std::fs::read(&ca_path)
            .map_err(|e| anyhow::anyhow!("Failed to read CA cert {}: {}", ca_path, e))?;
        let certs = rustls_pemfile::certs(&mut &ca_pem[..])
            .collect::<Result<Vec<_>, _>>()
            .map_err(|e| anyhow::anyhow!("Failed to parse CA cert: {:?}", e))?;
        for cert in certs {
            root_store.add(cert)?;
        }
        info!("Loaded custom CA certificates from {}", ca_path);
    }

    // Always include system roots as fallback
    root_store.extend(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());

    // Check if skip-verify is allowed (only in CSM_DEV mode)
    let skip_verify = std::env::var("CSM_TLS_SKIP_VERIFY").as_deref() == Ok("true")
        && std::env::var("CSM_DEV").is_ok();

    let config = if skip_verify {
        warn!("TLS certificate verification DISABLED — CSM_DEV mode only!");
        rustls::ClientConfig::builder()
            .dangerous()
            .with_custom_certificate_verifier(Arc::new(NoVerifier))
            .with_no_client_auth()
    } else {
        // Build standard verifier with pinning wrapper
        let inner = rustls::client::WebPkiServerVerifier::builder(Arc::new(root_store))
            .build()
            .map_err(|e| anyhow::anyhow!("Failed to build TLS verifier: {:?}", e))?;

        let pin_file = pin_file_path();
        let pinned_hashes = load_pinned_hashes(&pin_file);

        let verifier = PinnedCertVerifier {
            inner,
            pin_file,
            pinned_hashes: Arc::new(Mutex::new(pinned_hashes)),
        };

        rustls::ClientConfig::builder()
            .dangerous()
            .with_custom_certificate_verifier(Arc::new(verifier))
            .with_no_client_auth()
    };

    let connector = tokio_rustls::TlsConnector::from(Arc::new(config));

    // Extract hostname from server_addr (strip port)
    let domain = server_addr.split(':').next().unwrap_or("localhost").to_string();
    let server_name = rustls_pki_types::ServerName::try_from(domain.clone())
        .map_err(|e| anyhow::anyhow!("Invalid TLS server name '{}': {:?}", domain, e))?;

    let tls_stream = connector.connect(server_name, stream).await?;
    info!("TLS handshake completed with {}", domain);
    Ok(tls_stream)
}

/// Default pin file path: %PROGRAMDATA%\CSM\server_cert_pin (Windows)
fn pin_file_path() -> PathBuf {
    if let Ok(custom) = std::env::var("CSM_TLS_PIN_FILE") {
        PathBuf::from(custom)
    } else if cfg!(target_os = "windows") {
        std::env::var("PROGRAMDATA")
            .map(|p| PathBuf::from(p).join("CSM").join("server_cert_pin"))
            .unwrap_or_else(|_| PathBuf::from("server_cert_pin"))
    } else {
        PathBuf::from("/var/lib/csm/server_cert_pin")
    }
}

/// Load pinned certificate hashes from file.
/// Format: one hex-encoded SHA-256 hash per line.
fn load_pinned_hashes(path: &PathBuf) -> Vec<String> {
    match std::fs::read_to_string(path) {
        Ok(content) => content.lines()
            .map(|l| l.trim().to_string())
            .filter(|l| !l.is_empty())
            .collect(),
        Err(_) => Vec::new(), // First connection — no pin file yet
    }
}

/// Save a pinned hash to the pin file.
fn save_pinned_hash(path: &PathBuf, hash: &str) {
    if let Some(parent) = path.parent() {
        let _ = std::fs::create_dir_all(parent);
    }
    let _ = std::fs::write(path, format!("{}\n", hash));
}

/// Compute SHA-256 fingerprint of a DER-encoded certificate.
fn cert_fingerprint(cert: &rustls_pki_types::CertificateDer) -> String {
    let mut hasher = Sha256::new();
    hasher.update(cert.as_ref());
    hex::encode(hasher.finalize())
}

/// Certificate verifier with pinning support.
/// On first connection (no stored pin), records the certificate fingerprint.
/// On subsequent connections, verifies the fingerprint matches.
#[derive(Debug)]
struct PinnedCertVerifier {
    inner: Arc<rustls::client::WebPkiServerVerifier>,
    pin_file: PathBuf,
    pinned_hashes: Arc<Mutex<Vec<String>>>,
}

impl rustls::client::danger::ServerCertVerifier for PinnedCertVerifier {
    fn verify_server_cert(
        &self,
        end_entity: &rustls_pki_types::CertificateDer,
        intermediates: &[rustls_pki_types::CertificateDer],
        server_name: &rustls_pki_types::ServerName,
        ocsp_response: &[u8],
        now: rustls_pki_types::UnixTime,
    ) -> Result<rustls::client::danger::ServerCertVerified, rustls::Error> {
        // 1. Standard PKIX verification
        self.inner.verify_server_cert(end_entity, intermediates, server_name, ocsp_response, now)?;

        // 2. Compute certificate fingerprint
        let fingerprint = cert_fingerprint(end_entity);

        // 3. Check against pinned hashes
        let mut pinned = self.pinned_hashes.lock().unwrap();
        if pinned.is_empty() {
            // First connection — record the certificate fingerprint
            info!("Recording server certificate pin: {}...", &fingerprint[..16]);
            save_pinned_hash(&self.pin_file, &fingerprint);
            pinned.push(fingerprint);
        } else if !pinned.contains(&fingerprint) {
            warn!("Certificate pin mismatch! Expected one of {:?}, got {}", pinned, fingerprint);
            return Err(rustls::Error::General(
                "Server certificate does not match pinned fingerprint. Possible MITM attack.".into(),
            ));
        }

        Ok(rustls::client::danger::ServerCertVerified::assertion())
    }

    fn verify_tls12_signature(
        &self,
        message: &[u8],
        cert: &rustls_pki_types::CertificateDer,
        dss: &rustls::DigitallySignedStruct,
    ) -> Result<rustls::client::danger::HandshakeSignatureValid, rustls::Error> {
        self.inner.verify_tls12_signature(message, cert, dss)
    }

    fn verify_tls13_signature(
        &self,
        message: &[u8],
        cert: &rustls_pki_types::CertificateDer,
        dss: &rustls::DigitallySignedStruct,
    ) -> Result<rustls::client::danger::HandshakeSignatureValid, rustls::Error> {
        self.inner.verify_tls13_signature(message, cert, dss)
    }

    fn supported_verify_schemes(&self) -> Vec<rustls::SignatureScheme> {
        self.inner.supported_verify_schemes()
    }
}

/// Update pinned certificate hash (called when receiving TlsCertRotate).
pub fn update_cert_pin(new_hash: &str) {
    let pin_file = pin_file_path();
    let mut pinned = load_pinned_hashes(&pin_file);
    if !pinned.contains(&new_hash.to_string()) {
        pinned.push(new_hash.to_string());
        // Keep only the last 2 hashes (current + rotating)
        while pinned.len() > 2 {
            pinned.remove(0);
        }
        // Write all hashes to file
        if let Some(parent) = pin_file.parent() {
            let _ = std::fs::create_dir_all(parent);
        }
        let content = pinned.iter().map(|h| h.as_str()).collect::<Vec<_>>().join("\n");
        let _ = std::fs::write(&pin_file, format!("{}\n", content));
        info!("Updated certificate pin file with new hash: {}...", &new_hash[..16]);
    }
}

/// A no-op certificate verifier for development use (CSM_TLS_SKIP_VERIFY=true).
#[derive(Debug)]
struct NoVerifier;

impl rustls::client::danger::ServerCertVerifier for NoVerifier {
    fn verify_server_cert(
        &self,
        _end_entity: &rustls_pki_types::CertificateDer,
        _intermediates: &[rustls_pki_types::CertificateDer],
        _server_name: &rustls_pki_types::ServerName,
        _ocsp_response: &[u8],
        _now: rustls_pki_types::UnixTime,
    ) -> Result<rustls::client::danger::ServerCertVerified, rustls::Error> {
        Ok(rustls::client::danger::ServerCertVerified::assertion())
    }

    fn verify_tls12_signature(
        &self,
        _message: &[u8],
        _cert: &rustls_pki_types::CertificateDer,
        _dss: &rustls::DigitallySignedStruct,
    ) -> Result<rustls::client::danger::HandshakeSignatureValid, rustls::Error> {
        Ok(rustls::client::danger::HandshakeSignatureValid::assertion())
    }

    fn verify_tls13_signature(
        &self,
        _message: &[u8],
        _cert: &rustls_pki_types::CertificateDer,
        _dss: &rustls::DigitallySignedStruct,
    ) -> Result<rustls::client::danger::HandshakeSignatureValid, rustls::Error> {
        Ok(rustls::client::danger::HandshakeSignatureValid::assertion())
    }

    fn supported_verify_schemes(&self) -> Vec<rustls::SignatureScheme> {
        vec![
            rustls::SignatureScheme::RSA_PKCS1_SHA256,
            rustls::SignatureScheme::RSA_PKCS1_SHA384,
            rustls::SignatureScheme::RSA_PKCS1_SHA512,
            rustls::SignatureScheme::ECDSA_NISTP256_SHA256,
            rustls::SignatureScheme::ECDSA_NISTP384_SHA384,
            rustls::SignatureScheme::RSA_PSS_SHA256,
            rustls::SignatureScheme::RSA_PSS_SHA384,
            rustls::SignatureScheme::RSA_PSS_SHA512,
            rustls::SignatureScheme::ED25519,
        ]
    }
}

/// Main communication loop over any read+write stream
async fn run_comm_loop<S>(
    mut stream: S,
    state: &ClientState,
    data_rx: &mut tokio::sync::mpsc::Receiver<Frame>,
    plugins: &PluginChannels,
) -> Result<()>
where
    S: AsyncReadExt + AsyncWriteExt + Unpin,
{
    // Send registration
    let register = RegisterRequest {
        device_uid: state.device_uid.clone(),
        hostname: hostname::get()
            .map(|h| h.to_string_lossy().to_string())
            .unwrap_or_else(|_| "unknown".to_string()),
        registration_token: state.registration_token.clone(),
        os_version: get_os_info(),
        mac_address: None,
    };

    let frame = Frame::new_json(MessageType::Register, &register)?;
    stream.write_all(&frame.encode()).await?;
    info!("Registration request sent");

    let mut buffer = vec![0u8; 65536];
    let mut read_buf = Vec::with_capacity(65536);
    // Clamp heartbeat interval to sane range [5, 3600] to prevent CPU spin or effective disable
    let heartbeat_secs = state.config.heartbeat_interval_secs.clamp(5, 3600);
    let mut heartbeat_interval = tokio::time::interval(Duration::from_secs(heartbeat_secs));
    heartbeat_interval.tick().await; // Skip first tick

    // HMAC key — set after receiving RegisterResponse
    let mut device_secret: Option<String> = state.device_secret.clone();

    loop {
        tokio::select! {
            // Read from server
            result = stream.read(&mut buffer) => {
                let n = result?;
                if n == 0 {
                    return Err(anyhow::anyhow!("Server closed connection"));
                }
                read_buf.extend_from_slice(&buffer[..n]);

                // Guard against unbounded buffer growth from a malicious server
                if read_buf.len() > 1_048_576 {
                    return Err(anyhow::anyhow!("Read buffer exceeded 1MB, server may be malicious"));
                }

                // Process complete frames
                loop {
                    match Frame::decode(&read_buf)? {
                        Some(frame) => {
                            let consumed = frame.encoded_size();
                            read_buf.drain(..consumed);
                            // Capture device_secret from registration response
                            if frame.msg_type == MessageType::RegisterResponse {
                                if let Ok(resp) = frame.decode_payload::<RegisterResponse>() {
                                    device_secret = Some(resp.device_secret.clone());
                                    crate::save_device_secret(&resp.device_secret);
                                    info!("Device secret received and persisted, HMAC enabled for heartbeats");
                                }
                            }
                            handle_server_message(frame, plugins)?;
                        }
                        None => break, // Incomplete frame, wait for more data
                    }
                }
            }

            // Send queued data
            frame = data_rx.recv() => {
                let frame = frame.ok_or_else(|| anyhow::anyhow!("Channel closed"))?;
                stream.write_all(&frame.encode()).await?;
            }

            // Heartbeat
            _ = heartbeat_interval.tick() => {
                let timestamp = chrono::Utc::now().to_rfc3339();
                let hmac_value = compute_hmac(device_secret.as_deref(), &state.device_uid, &timestamp);
                let heartbeat = HeartbeatPayload {
                    device_uid: state.device_uid.clone(),
                    timestamp,
                    hmac: hmac_value,
                };
                let frame = Frame::new_json(MessageType::Heartbeat, &heartbeat)?;
                stream.write_all(&frame.encode()).await?;
                debug!("Heartbeat sent (hmac={})", !heartbeat.hmac.is_empty());
            }
        }
    }
}

fn handle_server_message(frame: Frame, plugins: &PluginChannels) -> Result<()> {
    match frame.msg_type {
        MessageType::RegisterResponse => {
            let resp: RegisterResponse = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid registration response: {}", e))?;
            info!("Registration accepted by server (server version: {})", resp.config.server_version);
        }
        MessageType::PolicyUpdate => {
            let policy: serde_json::Value = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid policy update: {}", e))?;
            info!("Received policy update: {}", policy);
        }
        MessageType::ConfigUpdate => {
            let update: csm_protocol::ConfigUpdateType = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid config update: {}", e))?;
            match update {
                csm_protocol::ConfigUpdateType::UpdateIntervals { heartbeat, status, asset } => {
                    info!("Config update: intervals heartbeat={}s status={}s asset={}s", heartbeat, status, asset);
                }
                csm_protocol::ConfigUpdateType::TlsCertRotate { new_cert_hash, valid_until } => {
                    info!("Certificate rotation: new hash={}... valid_until={}", &new_cert_hash[..16.min(new_cert_hash.len())], valid_until);
                    update_cert_pin(&new_cert_hash);
                }
                csm_protocol::ConfigUpdateType::SelfDestruct => {
                    warn!("Self-destruct command received (not implemented)");
                }
            }
        }
        MessageType::TaskExecute => {
            warn!("Task execution requested (not yet implemented)");
        }
        MessageType::WatermarkConfig => {
            let config: WatermarkConfigPayload = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid watermark config: {}", e))?;
            info!("Received watermark config: enabled={}", config.enabled);
            plugins.watermark_tx.send(Some(config))?;
        }
        MessageType::UsbPolicyUpdate => {
            let policy: UsbPolicyPayload = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid USB policy: {}", e))?;
            info!("Received USB policy: type={}, enabled={}", policy.policy_type, policy.enabled);
            plugins.usb_policy_tx.send(Some(policy))?;
        }
        MessageType::WebFilterRuleUpdate => {
            let payload: serde_json::Value = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid web filter update: {}", e))?;
            info!("Received web filter rules update");
            let rules: Vec<crate::web_filter::WebFilterRule> = payload.get("rules")
                .and_then(|r| serde_json::from_value(r.clone()).ok())
                .unwrap_or_default();
            let config = crate::web_filter::WebFilterConfig { enabled: true, rules };
            plugins.web_filter_tx.send(config)?;
        }
        MessageType::SoftwareBlacklist => {
            let payload: serde_json::Value = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid software blacklist: {}", e))?;
            info!("Received software blacklist update");
            let blacklist: Vec<crate::software_blocker::BlacklistEntry> = payload.get("blacklist")
                .and_then(|r| serde_json::from_value(r.clone()).ok())
                .unwrap_or_default();
            let whitelist: Vec<String> = payload.get("whitelist")
                .and_then(|r| serde_json::from_value(r.clone()).ok())
                .unwrap_or_default();
            let config = crate::software_blocker::SoftwareBlockerConfig {
                enabled: true,
                blacklist,
                whitelist,
            };
            plugins.software_blocker_tx.send(config)?;
        }
        MessageType::PopupRules => {
            let payload: serde_json::Value = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid popup rules: {}", e))?;
            info!("Received popup blocker rules update");
            let rules: Vec<crate::popup_blocker::PopupRule> = payload.get("rules")
                .and_then(|r| serde_json::from_value(r.clone()).ok())
                .unwrap_or_default();
            let config = crate::popup_blocker::PopupBlockerConfig { enabled: true, rules };
            plugins.popup_blocker_tx.send(config)?;
        }
        MessageType::DiskEncryptionConfig => {
            let config: DiskEncryptionConfigPayload = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid disk encryption config: {}", e))?;
            info!("Received disk encryption config: enabled={}, interval={}s", config.enabled, config.report_interval_secs);
            let plugin_config = crate::disk_encryption::DiskEncryptionConfig {
                enabled: config.enabled,
                report_interval_secs: config.report_interval_secs,
            };
            plugins.disk_encryption_tx.send(plugin_config)?;
        }
        MessageType::PluginEnable => {
            let payload: csm_protocol::PluginControlPayload = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid plugin enable: {}", e))?;
            info!("Plugin enabled: {}", payload.plugin_name);
            // Route to appropriate plugin channel based on plugin_name
            handle_plugin_control(&payload, plugins, true)?;
        }
        MessageType::PluginDisable => {
            let payload: csm_protocol::PluginControlPayload = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid plugin disable: {}", e))?;
            info!("Plugin disabled: {}", payload.plugin_name);
            handle_plugin_control(&payload, plugins, false)?;
        }
        MessageType::ClipboardRules => {
            let payload: csm_protocol::ClipboardRulesPayload = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid clipboard rules: {}", e))?;
            info!("Received clipboard rules update: {} rules", payload.rules.len());
            let config = crate::clipboard_control::ClipboardControlConfig {
                enabled: true,
                rules: payload.rules,
            };
            plugins.clipboard_control_tx.send(config)?;
        }
        MessageType::PatchScanConfig => {
            let config: PatchScanConfigPayload = frame.decode_payload()
                .map_err(|e| anyhow::anyhow!("Invalid patch scan config: {}", e))?;
            info!("Received patch scan config: enabled={}, interval={}s", config.enabled, config.scan_interval_secs);
            let plugin_config = crate::patch::PluginConfig {
                enabled: config.enabled,
                scan_interval_secs: config.scan_interval_secs,
            };
            plugins.patch_tx.send(plugin_config)?;
        }
        _ => {
            debug!("Unhandled message type: {:?}", frame.msg_type);
        }
    }
    Ok(())
}

fn handle_plugin_control(
    payload: &csm_protocol::PluginControlPayload,
    plugins: &PluginChannels,
    enabled: bool,
) -> Result<()> {
    match payload.plugin_name.as_str() {
        "watermark" => {
            if !enabled {
                // Send disabled config to remove overlay
                plugins.watermark_tx.send(None)?;
            }
            // When enabling, server will push the actual config next
        }
        "web_filter" => {
            if !enabled {
                // Clear hosts rules on disable
                plugins.web_filter_tx.send(crate::web_filter::WebFilterConfig { enabled: false, rules: vec![] })?;
            }
            // When enabling, server will push rules
        }
        "software_blocker" => {
            if !enabled {
                plugins.software_blocker_tx.send(crate::software_blocker::SoftwareBlockerConfig { enabled: false, blacklist: vec![], whitelist: vec![] })?;
            }
        }
        "popup_blocker" => {
            if !enabled {
                plugins.popup_blocker_tx.send(crate::popup_blocker::PopupBlockerConfig { enabled: false, rules: vec![] })?;
            }
        }
        "usb_audit" => {
            if !enabled {
                plugins.usb_audit_tx.send(crate::usb_audit::UsbAuditConfig { enabled: false, monitored_extensions: vec![] })?;
            }
        }
        "usage_timer" => {
            if !enabled {
                plugins.usage_timer_tx.send(crate::usage_timer::UsageConfig { enabled: false, ..Default::default() })?;
            }
        }
        "disk_encryption" => {
            if !enabled {
                plugins.disk_encryption_tx.send(crate::disk_encryption::DiskEncryptionConfig { enabled: false, ..Default::default() })?;
            }
        }
        "print_audit" => {
            if !enabled {
                plugins.print_audit_tx.send(crate::print_audit::PrintAuditConfig { enabled: false, ..Default::default() })?;
            }
        }
        "clipboard_control" => {
            if !enabled {
                plugins.clipboard_control_tx.send(crate::clipboard_control::ClipboardControlConfig { enabled: false, ..Default::default() })?;
            }
        }
        "patch" => {
            if !enabled {
                plugins.patch_tx.send(crate::patch::PluginConfig { enabled: false, scan_interval_secs: 43200 })?;
            }
        }
        _ => {
            warn!("Unknown plugin: {}", payload.plugin_name);
        }
    }
    Ok(())
}

/// Compute HMAC-SHA256 for heartbeat verification.
/// Format: HMAC-SHA256(device_secret, "{device_uid}\n{timestamp}")
fn compute_hmac(secret: Option<&str>, device_uid: &str, timestamp: &str) -> String {
    let secret = match secret {
        Some(s) if !s.is_empty() => s,
        _ => return String::new(),
    };

    type HmacSha256 = Hmac<Sha256>;

    let message = format!("{}\n{}", device_uid, timestamp);
    let mut mac = match HmacSha256::new_from_slice(secret.as_bytes()) {
        Ok(m) => m,
        Err(_) => return String::new(),
    };
    mac.update(message.as_bytes());
    hex::encode(mac.finalize().into_bytes())
}

fn get_os_info() -> String {
    use sysinfo::System;
    let name = System::name().unwrap_or_else(|| "Unknown".to_string());
    let version = System::os_version().unwrap_or_else(|| "Unknown".to_string());
    format!("{} {}", name, version)
}