diff --git a/crates/wzp-client/src/birthday.rs b/crates/wzp-client/src/birthday.rs
new file mode 100644
index 0000000..e4a5584
--- /dev/null
+++ b/crates/wzp-client/src/birthday.rs
@@ -0,0 +1,350 @@
+//! Birthday attack for hard NAT traversal.
+//!
+//! When both peers are behind symmetric NATs with random port
+//! allocation, standard hole-punching fails because neither side
+//! can predict the other's external port. This module implements
+//! the birthday-paradox approach:
+//!
+//! 1. **Acceptor** opens N sockets, STUN-probes each to learn
+//!    their external ports, reports them to the Dialer.
+//! 2. **Dialer** sprays QUIC connect attempts to the Acceptor's
+//!    reported ports + random ports on the Acceptor's IP.
+//! 3. Birthday paradox: with N=64 ports and M=256 probes across
+//!    65536 ports, collision probability is high.
+//!
+//! In practice, the Acceptor's STUN-probed ports are known
+//! exactly (not random), so the Dialer targets them first —
+//! making this more like "spray-and-pray with a hit list" than
+//! a pure birthday attack.
+
+use std::net::{Ipv4Addr, SocketAddr};
+use std::time::{Duration, Instant};
+
+use crate::stun;
+
+/// Configuration for the birthday attack.
+#[derive(Debug, Clone)]
+pub struct BirthdayConfig {
+    /// Number of sockets the Acceptor opens (default: 32).
+    /// Each socket gets STUN-probed to learn its external port.
+    /// More = higher chance of collision, but more resource usage.
+    pub acceptor_ports: u16,
+    /// Number of QUIC connect attempts the Dialer makes (default: 128).
+    /// Spread across the Acceptor's known ports + random ports.
+    pub dialer_probes: u16,
+    /// Rate limit: ms between consecutive probes (default: 20ms = 50/s).
+    pub probe_interval_ms: u16,
+    /// Overall timeout for the birthday attack phase.
+    pub timeout: Duration,
+    /// STUN config for probing external ports.
+    pub stun_config: stun::StunConfig,
+}
+
+impl Default for BirthdayConfig {
+    fn default() -> Self {
+        Self {
+            acceptor_ports: 32,
+            dialer_probes: 128,
+            probe_interval_ms: 20,
+            timeout: Duration::from_secs(8),
+            stun_config: stun::StunConfig {
+                servers: vec!["stun.l.google.com:19302".into()],
+                timeout: Duration::from_secs(2),
+            },
+        }
+    }
+}
+
+/// Result of the Acceptor's port-opening phase.
+#[derive(Debug, Clone, serde::Serialize)]
+pub struct AcceptorPorts {
+    /// External IP (from STUN).
+    pub external_ip: Option<Ipv4Addr>,
+    /// List of (local_port, external_port) for each opened socket.
+    pub ports: Vec<PortMapping>,
+    /// How many sockets we attempted to open.
+    pub attempted: u16,
+    /// How many STUN probes succeeded.
+    pub succeeded: u16,
+}
+
+/// A single socket's local↔external port mapping.
+#[derive(Debug, Clone, serde::Serialize)]
+pub struct PortMapping {
+    pub local_port: u16,
+    pub external_port: u16,
+}
+
+/// Open N sockets and STUN-probe each to discover external ports.
+///
+/// Returns the set of known external ports that the Dialer should
+/// target. Each socket stays open (bound) so the NAT mapping
+/// remains active until the returned `PortGuard` is dropped.
+///
+/// The sockets are returned so the caller can keep them alive
+/// during the attack. Dropping them closes the NAT pinholes.
+pub async fn open_acceptor_ports(
+    config: &BirthdayConfig,
+) -> (AcceptorPorts, Vec<tokio::net::UdpSocket>) {
+    let mut sockets = Vec::new();
+    let mut mappings = Vec::new();
+    let mut external_ip: Option<Ipv4Addr> = None;
+    let mut succeeded: u16 = 0;
+
+    let stun_server = match config.stun_config.servers.first() {
+        Some(s) => match stun::resolve_stun_server(s).await {
+            Ok(a) => Some(a),
+            Err(_) => None,
+        },
+        None => None,
+    };
+
+    for _ in 0..config.acceptor_ports {
+        // Bind to random port
+        let sock = match tokio::net::UdpSocket::bind("0.0.0.0:0").await {
+            Ok(s) => s,
+            Err(_) => continue,
+        };
+        let local_port = match sock.local_addr() {
+            Ok(a) => a.port(),
+            Err(_) => continue,
+        };
+
+        // STUN probe to learn external port
+        if let Some(stun_addr) = stun_server {
+            match stun::stun_reflect(&sock, stun_addr, config.stun_config.timeout).await {
+                Ok(ext_addr) => {
+                    if external_ip.is_none() {
+                        if let std::net::IpAddr::V4(ip) = ext_addr.ip() {
+                            external_ip = Some(ip);
+                        }
+                    }
+                    mappings.push(PortMapping {
+                        local_port,
+                        external_port: ext_addr.port(),
+                    });
+                    succeeded += 1;
+                }
+                Err(e) => {
+                    tracing::debug!(local_port, error = %e, "birthday: STUN probe failed for socket");
+                }
+            }
+        }
+
+        sockets.push(sock);
+    }
+
+    tracing::info!(
+        attempted = config.acceptor_ports,
+        succeeded,
+        external_ip = ?external_ip,
+        "birthday: acceptor ports opened"
+    );
+
+    let result = AcceptorPorts {
+        external_ip,
+        ports: mappings,
+        attempted: config.acceptor_ports,
+        succeeded,
+    };
+
+    (result, sockets)
+}
+
+/// Generate the list of target addresses for the Dialer to spray.
+///
+/// Priority order:
+/// 1. Acceptor's known external ports (from STUN probes) — highest hit rate
+/// 2. Random ports on the Acceptor's IP — birthday paradox fill
+pub fn generate_dialer_targets(
+    acceptor_ip: Ipv4Addr,
+    known_ports: &[u16],
+    total_probes: u16,
+) -> Vec<SocketAddr> {
+    let mut targets = Vec::with_capacity(total_probes as usize);
+
+    // First: all known ports (guaranteed targets)
+    for &port in known_ports {
+        targets.push(SocketAddr::new(
+            std::net::IpAddr::V4(acceptor_ip),
+            port,
+        ));
+    }
+
+    // Fill remaining with random ports (birthday attack)
+    let remaining = total_probes.saturating_sub(known_ports.len() as u16);
+    if remaining > 0 {
+        use rand::Rng;
+        let mut rng = rand::thread_rng();
+        for _ in 0..remaining {
+            let port = rng.gen_range(1024..=65535u16);
+            let addr = SocketAddr::new(
+                std::net::IpAddr::V4(acceptor_ip),
+                port,
+            );
+            if !targets.contains(&addr) {
+                targets.push(addr);
+            }
+        }
+    }
+
+    targets
+}
+
+/// Run the Dialer side of the birthday attack.
+///
+/// Sprays QUIC connection attempts at the target addresses.
+/// Returns the first successful connection, or None on timeout.
+pub async fn spray_dialer(
+    endpoint: &wzp_transport::Endpoint,
+    targets: &[SocketAddr],
+    call_sni: &str,
+    probe_interval: Duration,
+    timeout: Duration,
+) -> Option<wzp_transport::QuinnTransport> {
+    let start = Instant::now();
+    let mut set = tokio::task::JoinSet::new();
+
+    tracing::info!(
+        target_count = targets.len(),
+        interval_ms = probe_interval.as_millis(),
+        timeout_s = timeout.as_secs(),
+        "birthday: dialer starting spray"
+    );
+
+    // Spray connects with rate limiting
+    for (idx, &target) in targets.iter().enumerate() {
+        if start.elapsed() >= timeout {
+            break;
+        }
+
+        let ep = endpoint.clone();
+        let sni = call_sni.to_string();
+        let client_cfg = wzp_transport::client_config();
+        set.spawn(async move {
+            let result = wzp_transport::connect(&ep, target, &sni, client_cfg).await;
+            (idx, target, result)
+        });
+
+        // Rate limit — don't blast the NAT
+        if idx < targets.len() - 1 {
+            tokio::time::sleep(probe_interval).await;
+        }
+    }
+
+    tracing::info!(
+        spawned = set.len(),
+        elapsed_ms = start.elapsed().as_millis(),
+        "birthday: all probes spawned, waiting for first success"
+    );
+
+    // Wait for first success or all failures
+    let deadline = start + timeout;
+    while let Some(join_res) = tokio::select! {
+        r = set.join_next() => r,
+        _ = tokio::time::sleep_until(tokio::time::Instant::from_std(deadline)) => None,
+    } {
+        match join_res {
+            Ok((idx, target, Ok(conn))) => {
+                tracing::info!(
+                    idx,
+                    %target,
+                    remote = %conn.remote_address(),
+                    elapsed_ms = start.elapsed().as_millis(),
+                    "birthday: HIT! QUIC handshake succeeded"
+                );
+                set.abort_all();
+                return Some(wzp_transport::QuinnTransport::new(conn));
+            }
+            Ok((idx, target, Err(e))) => {
+                tracing::debug!(
+                    idx,
+                    %target,
+                    error = %e,
+                    "birthday: probe failed"
+                );
+            }
+            Err(_) => {}
+        }
+    }
+
+    tracing::info!(
+        elapsed_ms = start.elapsed().as_millis(),
+        "birthday: all probes failed or timed out"
+    );
+    None
+}
+
+// ── Tests ──────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generate_targets_known_ports_first() {
+        let ip = Ipv4Addr::new(203, 0, 113, 5);
+        let known = vec![10000, 10001, 10002];
+        let targets = generate_dialer_targets(ip, &known, 10);
+
+        // Known ports should be first
+        assert_eq!(targets[0].port(), 10000);
+        assert_eq!(targets[1].port(), 10001);
+        assert_eq!(targets[2].port(), 10002);
+        // Rest are random
+        assert!(targets.len() <= 10);
+        // All target the right IP
+        assert!(targets.iter().all(|a| a.ip() == std::net::IpAddr::V4(ip)));
+    }
+
+    #[test]
+    fn generate_targets_no_known_all_random() {
+        let ip = Ipv4Addr::new(10, 0, 0, 1);
+        let targets = generate_dialer_targets(ip, &[], 50);
+        assert!(!targets.is_empty());
+        assert!(targets.len() <= 50);
+        // All ports in valid range
+        assert!(targets.iter().all(|a| a.port() >= 1024));
+    }
+
+    #[test]
+    fn generate_targets_more_known_than_total() {
+        let ip = Ipv4Addr::new(10, 0, 0, 1);
+        let known: Vec<u16> = (10000..10100).collect();
+        let targets = generate_dialer_targets(ip, &known, 50);
+        // All 100 known ports included even though total=50
+        assert_eq!(targets.len(), 100);
+    }
+
+    #[test]
+    fn generate_targets_dedup() {
+        let ip = Ipv4Addr::new(10, 0, 0, 1);
+        let targets = generate_dialer_targets(ip, &[], 100);
+        // No duplicates
+        let mut sorted = targets.clone();
+        sorted.sort();
+        sorted.dedup();
+        assert_eq!(sorted.len(), targets.len());
+    }
+
+    #[test]
+    fn default_config() {
+        let cfg = BirthdayConfig::default();
+        assert_eq!(cfg.acceptor_ports, 32);
+        assert_eq!(cfg.dialer_probes, 128);
+        assert!(cfg.timeout.as_secs() > 0);
+    }
+
+    #[test]
+    fn acceptor_ports_serializes() {
+        let result = AcceptorPorts {
+            external_ip: Some(Ipv4Addr::new(203, 0, 113, 5)),
+            ports: vec![PortMapping { local_port: 12345, external_port: 54321 }],
+            attempted: 32,
+            succeeded: 1,
+        };
+        let json = serde_json::to_string(&result).unwrap();
+        assert!(json.contains("54321"));
+        assert!(json.contains("203.0.113.5"));
+    }
+}
diff --git a/crates/wzp-client/src/featherchat.rs b/crates/wzp-client/src/featherchat.rs
index 646744a..db3bede 100644
--- a/crates/wzp-client/src/featherchat.rs
+++ b/crates/wzp-client/src/featherchat.rs
@@ -133,6 +133,7 @@ pub fn signal_to_call_type(signal: &SignalMessage) -> CallSignalType {
         SignalMessage::MediaPathReport { .. } => CallSignalType::Offer, // control-plane
         SignalMessage::CandidateUpdate { .. } => CallSignalType::IceCandidate, // mid-call re-gather
         SignalMessage::HardNatProbe { .. } => CallSignalType::IceCandidate, // hard NAT coordination
+        SignalMessage::HardNatBirthdayStart { .. } => CallSignalType::IceCandidate, // birthday attack
         SignalMessage::QualityDirective { .. } => CallSignalType::Offer, // relay-initiated
     }
 }
diff --git a/crates/wzp-client/src/lib.rs b/crates/wzp-client/src/lib.rs
index 3c3d94e..98191ca 100644
--- a/crates/wzp-client/src/lib.rs
+++ b/crates/wzp-client/src/lib.rs
@@ -34,6 +34,7 @@ pub mod featherchat;
 pub mod handshake;
 pub mod dual_path;
 pub mod metrics;
+pub mod birthday;
 pub mod ice_agent;
 pub mod netcheck;
 pub mod portmap;
diff --git a/crates/wzp-proto/src/packet.rs b/crates/wzp-proto/src/packet.rs
index bcc08e8..dc8e301 100644
--- a/crates/wzp-proto/src/packet.rs
+++ b/crates/wzp-proto/src/packet.rs
@@ -967,6 +967,19 @@ pub enum SignalMessage {
         external_ip: String,
     },
 
+    /// Birthday attack coordination — Acceptor tells Dialer which
+    /// ports it has open. The Dialer then sprays QUIC connects to
+    /// these ports (and optionally random ports) on the Acceptor's IP.
+    HardNatBirthdayStart {
+        call_id: String,
+        /// Number of sockets the Acceptor opened.
+        acceptor_port_count: u16,
+        /// External ports discovered via STUN (the "hit list").
+        acceptor_ports: Vec<u16>,
+        /// Acceptor's external IP.
+        external_ip: String,
+    },
+
     // ── Phase 4: cross-relay direct-call signaling ────────────────────
 
     /// Phase 4: relay-to-relay envelope for forwarding direct-call
diff --git a/crates/wzp-relay/src/main.rs b/crates/wzp-relay/src/main.rs
index 1de7db9..539ebec 100644
--- a/crates/wzp-relay/src/main.rs
+++ b/crates/wzp-relay/src/main.rs
@@ -1443,8 +1443,9 @@ async fn main() -> anyhow::Result<()> {
                                     }
                                 }
 
-                                // Hard NAT: forward HardNatProbe to call peer
-                                // (same forwarding pattern as CandidateUpdate).
+                                // Hard NAT: forward HardNatProbe + HardNatBirthdayStart
+                                // to call peer (same pattern as CandidateUpdate).
+                                SignalMessage::HardNatBirthdayStart { ref call_id, .. } |
                                 SignalMessage::HardNatProbe { ref call_id, .. } => {
                                     let (peer_fp, peer_relay_fp) = {
                                         let reg = call_registry.lock().await;
diff --git a/desktop/src-tauri/src/lib.rs b/desktop/src-tauri/src/lib.rs
index 20f081c..be7e76e 100644
--- a/desktop/src-tauri/src/lib.rs
+++ b/desktop/src-tauri/src/lib.rs
@@ -1350,10 +1350,62 @@ fn do_register_signal(
                         let mut sig = signal_state.lock().await;
                         sig.peer_hard_nat_probe = Some(PeerHardNatInfo {
                             external_ip: ip,
-                            port_sequence,
-                            allocation,
+                            port_sequence: port_sequence.clone(),
+                            allocation: allocation.clone(),
                         });
                     }
+
+                    // If peer has a random/symmetric NAT and WE are the
+                    // Acceptor, open birthday attack ports and send
+                    // BirthdayStart so the peer can spray us.
+                    if allocation == "random" || allocation.starts_with("sequential") {
+                        let state_bg = signal_state.clone();
+                        let app_bg = app_clone.clone();
+                        let call_id_bg = call_id.clone();
+                        tokio::spawn(async move {
+                            let config = wzp_client::birthday::BirthdayConfig::default();
+                            let (result, _sockets) = wzp_client::birthday::open_acceptor_ports(&config).await;
+                            if result.succeeded > 0 {
+                                let ext_ports: Vec<u16> = result.ports.iter().map(|p| p.external_port).collect();
+                                let ext_ip = result.external_ip
+                                    .map(|ip| ip.to_string())
+                                    .unwrap_or_default();
+                                emit_call_debug(&app_bg, "birthday:acceptor_ports_opened", serde_json::json!({
+                                    "succeeded": result.succeeded,
+                                    "external_ip": ext_ip,
+                                    "ports": ext_ports,
+                                }));
+                                let sig = state_bg.lock().await;
+                                if let Some(ref t) = sig.transport {
+                                    let _ = t.send_signal(&wzp_proto::SignalMessage::HardNatBirthdayStart {
+                                        call_id: call_id_bg,
+                                        acceptor_port_count: result.succeeded,
+                                        acceptor_ports: ext_ports,
+                                        external_ip: ext_ip,
+                                    }).await;
+                                }
+                                // Keep _sockets alive for 10s so NAT mappings persist
+                                tokio::time::sleep(std::time::Duration::from_secs(10)).await;
+                            }
+                        });
+                    }
+                }
+                Ok(Some(SignalMessage::HardNatBirthdayStart { call_id, acceptor_port_count, acceptor_ports, external_ip })) => {
+                    tracing::info!(
+                        %call_id,
+                        acceptor_port_count,
+                        port_count = acceptor_ports.len(),
+                        %external_ip,
+                        "signal: HardNatBirthdayStart from peer"
+                    );
+                    emit_call_debug(&app_clone, "recv:HardNatBirthdayStart", serde_json::json!({
+                        "call_id": call_id,
+                        "acceptor_port_count": acceptor_port_count,
+                        "acceptor_ports": acceptor_ports,
+                        "external_ip": external_ip,
+                    }));
+                    // TODO: trigger dialer spray when birthday attack
+                    // is integrated into the race waterfall
                 }
                 Ok(Some(SignalMessage::ReflectResponse { observed_addr })) => {
                     // "STUN for QUIC" response — the relay told us our