feat: add traffic analysis resistance (Phase 7.7 + F8)

- Core: add pad_uniform/unpad_uniform for configurable boundary padding
  and generate_decoy for indistinguishable decoy messages
- Server: add traffic_resistance module with payload padding, timing
  jitter, and background decoy traffic generator (feature-gated)
- P2P: add mesh traffic_resistance module with padded envelopes and
  mesh decoy injection (feature-gated)
- All gated behind --features traffic-resistance
- 22 new tests across core (8), server (4), and P2P (5)

crates/quicproquo-p2p/src/lib.rs

@@ -16,6 +16,8 @@ pub mod broadcast;
 pub mod envelope;
 pub mod identity;
 pub mod store;
+#[cfg(feature = "traffic-resistance")]
+pub mod traffic_resistance;
 
 use std::sync::{Arc, Mutex};
 

crates/quicproquo-p2p/src/traffic_resistance.rs

@@ -0,0 +1,204 @@
+//! Mesh traffic analysis resistance — uniform envelope padding and decoy injection.
+//!
+//! When the `traffic-resistance` feature is enabled:
+//!
+//! 1. [`MeshEnvelope`] payloads are padded to a configurable boundary (default 256 bytes)
+//!    before signing, so all envelopes on the wire have uniform-sized payloads.
+//! 2. A background decoy injector periodically creates fake mesh envelopes and stores
+//!    them in the mesh store, making real vs. decoy traffic indistinguishable.
+
+use std::sync::{Arc, Mutex};
+
+use rand::Rng;
+use tokio::sync::Notify;
+
+use crate::envelope::MeshEnvelope;
+use crate::identity::MeshIdentity;
+use crate::store::MeshStore;
+
+/// Configuration for mesh traffic analysis resistance.
+#[derive(Clone, Debug)]
+pub struct MeshTrafficConfig {
+    /// Padding boundary in bytes for envelope payloads.
+    pub padding_boundary: usize,
+
+    /// Mean interval in milliseconds between decoy envelope injections.
+    /// Set to 0 to disable.
+    pub decoy_interval_ms: u64,
+}
+
+impl Default for MeshTrafficConfig {
+    fn default() -> Self {
+        Self {
+            padding_boundary: quicproquo_core::padding::DEFAULT_PADDING_BOUNDARY,
+            decoy_interval_ms: 5000,
+        }
+    }
+}
+
+/// Pad a mesh payload to the nearest boundary before wrapping in a [`MeshEnvelope`].
+pub fn pad_mesh_payload(payload: &[u8], boundary: usize) -> Vec<u8> {
+    quicproquo_core::padding::pad_uniform(payload, boundary)
+}
+
+/// Create a [`MeshEnvelope`] with a uniformly padded payload.
+pub fn padded_envelope(
+    identity: &MeshIdentity,
+    recipient_key: &[u8],
+    payload: Vec<u8>,
+    ttl_secs: u32,
+    max_hops: u8,
+    boundary: usize,
+) -> MeshEnvelope {
+    let padded = pad_mesh_payload(&payload, boundary);
+    MeshEnvelope::new(identity, recipient_key, padded, ttl_secs, max_hops)
+}
+
+/// Spawn a background task that injects decoy mesh envelopes into the store.
+///
+/// Decoy envelopes have random recipient keys and empty (padded) payloads.
+/// They are indistinguishable from real padded envelopes on the wire.
+pub fn spawn_mesh_decoy_generator(
+    identity: MeshIdentity,
+    store: Arc<Mutex<MeshStore>>,
+    config: MeshTrafficConfig,
+    shutdown: Arc<Notify>,
+) -> tokio::task::JoinHandle<()> {
+    tokio::spawn(async move {
+        if config.decoy_interval_ms == 0 {
+            shutdown.notified().await;
+            return;
+        }
+
+        let base_interval = std::time::Duration::from_millis(config.decoy_interval_ms);
+
+        loop {
+            let jitter_factor: f64 = rand::thread_rng().gen_range(0.5..1.5);
+            let interval = base_interval.mul_f64(jitter_factor);
+
+            tokio::select! {
+                () = tokio::time::sleep(interval) => {}
+                () = shutdown.notified() => {
+                    tracing::debug!("mesh decoy generator shutting down");
+                    return;
+                }
+            }
+
+            // Generate a decoy: padded empty payload with a random recipient.
+            let decoy_payload = quicproquo_core::padding::generate_decoy(config.padding_boundary);
+            let mut fake_recipient = [0u8; 32];
+            rand::thread_rng().fill(&mut fake_recipient);
+
+            let envelope = MeshEnvelope::new(
+                &identity,
+                &fake_recipient,
+                decoy_payload,
+                60, // Short TTL.
+                0,
+            );
+
+            match store.lock() {
+                Ok(mut s) => {
+                    let _ = s.store(envelope);
+                    tracing::trace!("mesh decoy envelope injected");
+                }
+                Err(e) => {
+                    tracing::warn!(error = %e, "mesh store lock poisoned in decoy generator");
+                }
+            }
+        }
+    })
+}
+
+#[cfg(test)]
+#[allow(clippy::unwrap_used)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn pad_mesh_payload_boundary_aligned() {
+        let payload = b"hello mesh";
+        let padded = pad_mesh_payload(payload, 256);
+        assert_eq!(padded.len() % 256, 0);
+
+        let unpadded = quicproquo_core::padding::unpad_uniform(&padded).unwrap();
+        assert_eq!(unpadded, payload);
+    }
+
+    #[test]
+    fn padded_envelope_has_uniform_payload() {
+        let id = MeshIdentity::generate();
+        let recipient = [0xAA; 32];
+        let payload = b"short".to_vec();
+
+        let env = padded_envelope(&id, &recipient, payload, 3600, 5, 256);
+        assert_eq!(env.payload.len() % 256, 0);
+        assert!(env.verify());
+
+        // The inner payload should unpad correctly.
+        let unpadded = quicproquo_core::padding::unpad_uniform(&env.payload).unwrap();
+        assert_eq!(unpadded, b"short");
+    }
+
+    #[test]
+    fn padded_envelope_large_payload() {
+        let id = MeshIdentity::generate();
+        let payload = vec![0xBB; 500];
+
+        let env = padded_envelope(&id, &[0xCC; 32], payload.clone(), 60, 3, 256);
+        assert_eq!(env.payload.len() % 256, 0);
+        assert_eq!(env.payload.len(), 512); // 500 + 4 = 504, rounds to 512
+
+        let unpadded = quicproquo_core::padding::unpad_uniform(&env.payload).unwrap();
+        assert_eq!(unpadded, payload);
+    }
+
+    #[tokio::test]
+    async fn mesh_decoy_generator_injects_envelopes() {
+        let id = MeshIdentity::generate();
+        let store = Arc::new(Mutex::new(MeshStore::new(100)));
+        let shutdown = Arc::new(Notify::new());
+
+        let config = MeshTrafficConfig {
+            padding_boundary: 256,
+            decoy_interval_ms: 50,
+        };
+
+        let handle = spawn_mesh_decoy_generator(
+            id,
+            Arc::clone(&store),
+            config,
+            Arc::clone(&shutdown),
+        );
+
+        tokio::time::sleep(std::time::Duration::from_millis(200)).await;
+        shutdown.notify_one();
+        handle.await.unwrap();
+
+        let s = store.lock().unwrap();
+        let (total, _) = s.stats();
+        assert!(total > 0, "decoy generator should have stored at least one envelope");
+    }
+
+    #[tokio::test]
+    async fn mesh_decoy_generator_disabled_when_zero() {
+        let id = MeshIdentity::generate();
+        let store = Arc::new(Mutex::new(MeshStore::new(100)));
+        let shutdown = Arc::new(Notify::new());
+
+        let config = MeshTrafficConfig {
+            padding_boundary: 256,
+            decoy_interval_ms: 0,
+        };
+
+        let handle = spawn_mesh_decoy_generator(
+            id,
+            store,
+            config,
+            Arc::clone(&shutdown),
+        );
+
+        shutdown.notify_one();
+        handle.await.unwrap();
+    }
+}