feat(mesh): add MeshEnvelopeV2 with truncated 16-byte addresses

S5: Compact envelope format for constrained links:
- 16-byte truncated addresses (MeshAddress) instead of 32-byte keys
- 16-byte truncated content ID
- u16 TTL and u32 timestamp (smaller than V1)
- Priority field (Low/Normal/High/Emergency)
- ~30-50 bytes savings per envelope vs V1

Full public keys are exchanged during announce phase and cached in
routing table. Envelope only needs addresses for routing.

crates/quicprochat-p2p/src/envelope_v2.rs

@@ -0,0 +1,432 @@
+//! Compact mesh envelope using truncated 16-byte addresses.
+//!
+//! [`MeshEnvelopeV2`] is a bandwidth-optimized envelope format for constrained
+//! links (LoRa, serial). It uses [`MeshAddress`] (16 bytes) instead of full
+//! 32-byte public keys, saving 32 bytes per envelope.
+//!
+//! Full public keys are exchanged during the announce phase and cached in the
+//! routing table. The envelope only needs addresses for routing.
+
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use crate::address::MeshAddress;
+use crate::identity::MeshIdentity;
+
+/// Default maximum hops for mesh forwarding.
+const DEFAULT_MAX_HOPS: u8 = 5;
+
+/// Version byte for envelope format detection.
+const ENVELOPE_V2_VERSION: u8 = 0x02;
+
+/// Priority levels for mesh routing.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[repr(u8)]
+pub enum Priority {
+    /// Lowest priority (announce, telemetry).
+    Low = 0,
+    /// Normal priority (regular messages).
+    Normal = 1,
+    /// High priority (important messages).
+    High = 2,
+    /// Emergency priority (always forwarded first).
+    Emergency = 3,
+}
+
+impl Default for Priority {
+    fn default() -> Self {
+        Self::Normal
+    }
+}
+
+impl From<u8> for Priority {
+    fn from(v: u8) -> Self {
+        match v {
+            0 => Self::Low,
+            1 => Self::Normal,
+            2 => Self::High,
+            3 => Self::Emergency,
+            _ => Self::Normal,
+        }
+    }
+}
+
+/// Compact mesh envelope with 16-byte truncated addresses.
+///
+/// # Wire overhead
+///
+/// - Version: 1 byte
+/// - Flags: 1 byte (priority: 2 bits, reserved: 6 bits)
+/// - ID: 16 bytes (truncated from 32)
+/// - Sender: 16 bytes
+/// - Recipient: 16 bytes (or 0 for broadcast)
+/// - TTL: 2 bytes (u16, max ~18 hours)
+/// - Hop count: 1 byte
+/// - Max hops: 1 byte
+/// - Timestamp: 4 bytes (u32, seconds since epoch mod 2^32)
+/// - Signature: 64 bytes
+/// - Payload: variable
+///
+/// **Total fixed overhead: ~122 bytes** (vs ~174 for V1 with full keys)
+/// Savings: ~52 bytes per envelope
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct MeshEnvelopeV2 {
+    /// Format version (0x02 for V2).
+    pub version: u8,
+    /// Flags byte: bits 0-1 = priority, bits 2-7 reserved.
+    pub flags: u8,
+    /// 16-byte truncated content ID (for deduplication).
+    pub id: [u8; 16],
+    /// 16-byte truncated sender address.
+    pub sender_addr: MeshAddress,
+    /// 16-byte truncated recipient address (BROADCAST for all).
+    pub recipient_addr: MeshAddress,
+    /// Encrypted payload (opaque to mesh layer).
+    pub payload: Vec<u8>,
+    /// Time-to-live in seconds (u16, max 65535 = ~18 hours).
+    pub ttl_secs: u16,
+    /// Current hop count.
+    pub hop_count: u8,
+    /// Maximum hops before drop.
+    pub max_hops: u8,
+    /// Unix timestamp (seconds, truncated to u32).
+    pub timestamp: u32,
+    /// Ed25519 signature (64 bytes).
+    pub signature: [u8; 64],
+}
+
+impl MeshEnvelopeV2 {
+    /// Create and sign a new compact mesh envelope.
+    pub fn new(
+        identity: &MeshIdentity,
+        recipient_addr: MeshAddress,
+        payload: Vec<u8>,
+        ttl_secs: u16,
+        max_hops: u8,
+        priority: Priority,
+    ) -> Self {
+        let sender_addr = MeshAddress::from_public_key(&identity.public_key());
+        let hop_count = 0u8;
+        let max_hops = if max_hops == 0 { DEFAULT_MAX_HOPS } else { max_hops };
+        let timestamp = (SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap_or_default()
+            .as_secs() & 0xFFFF_FFFF) as u32;
+
+        let id = Self::compute_id(
+            &sender_addr,
+            &recipient_addr,
+            &payload,
+            ttl_secs,
+            max_hops,
+            timestamp,
+        );
+
+        let flags = (priority as u8) & 0x03;
+
+        let mut envelope = Self {
+            version: ENVELOPE_V2_VERSION,
+            flags,
+            id,
+            sender_addr,
+            recipient_addr,
+            payload,
+            ttl_secs,
+            hop_count,
+            max_hops,
+            timestamp,
+            signature: [0u8; 64],
+        };
+
+        let signable = envelope.signable_bytes();
+        let sig = identity.sign(&signable);
+        envelope.signature = sig;
+
+        envelope
+    }
+
+    /// Create for broadcast (recipient = all zeros).
+    pub fn broadcast(
+        identity: &MeshIdentity,
+        payload: Vec<u8>,
+        ttl_secs: u16,
+        max_hops: u8,
+        priority: Priority,
+    ) -> Self {
+        Self::new(identity, MeshAddress::BROADCAST, payload, ttl_secs, max_hops, priority)
+    }
+
+    /// Compute the 16-byte truncated content ID.
+    fn compute_id(
+        sender_addr: &MeshAddress,
+        recipient_addr: &MeshAddress,
+        payload: &[u8],
+        ttl_secs: u16,
+        max_hops: u8,
+        timestamp: u32,
+    ) -> [u8; 16] {
+        let mut hasher = Sha256::new();
+        hasher.update(sender_addr.as_bytes());
+        hasher.update(recipient_addr.as_bytes());
+        hasher.update(payload);
+        hasher.update(ttl_secs.to_le_bytes());
+        hasher.update([max_hops]);
+        hasher.update(timestamp.to_le_bytes());
+        let hash = hasher.finalize();
+        let mut id = [0u8; 16];
+        id.copy_from_slice(&hash[..16]);
+        id
+    }
+
+    /// Bytes to sign/verify (excludes signature and hop_count).
+    fn signable_bytes(&self) -> Vec<u8> {
+        let mut buf = Vec::with_capacity(64 + self.payload.len());
+        buf.push(self.version);
+        buf.push(self.flags);
+        buf.extend_from_slice(&self.id);
+        buf.extend_from_slice(self.sender_addr.as_bytes());
+        buf.extend_from_slice(self.recipient_addr.as_bytes());
+        buf.extend_from_slice(&self.payload);
+        buf.extend_from_slice(&self.ttl_secs.to_le_bytes());
+        buf.push(self.max_hops);
+        buf.extend_from_slice(&self.timestamp.to_le_bytes());
+        buf
+    }
+
+    /// Verify the signature using the sender's full public key.
+    ///
+    /// The caller must have the sender's full key (from announce/routing table).
+    pub fn verify_with_key(&self, sender_public_key: &[u8; 32]) -> bool {
+        // First check that the address matches the key
+        if !self.sender_addr.matches_key(sender_public_key) {
+            return false;
+        }
+        let signable = self.signable_bytes();
+        quicprochat_core::IdentityKeypair::verify_raw(sender_public_key, &signable, &self.signature)
+            .is_ok()
+    }
+
+    /// Get the priority level.
+    pub fn priority(&self) -> Priority {
+        Priority::from(self.flags & 0x03)
+    }
+
+    /// Check if broadcast (recipient is all zeros).
+    pub fn is_broadcast(&self) -> bool {
+        self.recipient_addr.is_broadcast()
+    }
+
+    /// Check if expired.
+    pub fn is_expired(&self) -> bool {
+        let now = (SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap_or_default()
+            .as_secs() & 0xFFFF_FFFF) as u32;
+        // Handle u32 wraparound (every ~136 years)
+        let elapsed = now.wrapping_sub(self.timestamp);
+        elapsed > self.ttl_secs as u32
+    }
+
+    /// Can this envelope be forwarded?
+    pub fn can_forward(&self) -> bool {
+        self.hop_count < self.max_hops && !self.is_expired()
+    }
+
+    /// Create a forwarded copy with hop_count incremented.
+    pub fn forwarded(&self) -> Self {
+        let mut copy = self.clone();
+        copy.hop_count = copy.hop_count.saturating_add(1);
+        copy
+    }
+
+    /// Serialize to compact CBOR.
+    pub fn to_wire(&self) -> Vec<u8> {
+        let mut buf = Vec::new();
+        ciborium::into_writer(self, &mut buf).expect("CBOR serialization should not fail");
+        buf
+    }
+
+    /// Deserialize from CBOR.
+    pub fn from_wire(bytes: &[u8]) -> anyhow::Result<Self> {
+        let env: Self = ciborium::from_reader(bytes)?;
+        if env.version != ENVELOPE_V2_VERSION {
+            anyhow::bail!("unexpected envelope version: {}", env.version);
+        }
+        Ok(env)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn test_identity() -> MeshIdentity {
+        MeshIdentity::generate()
+    }
+
+    #[test]
+    fn create_and_verify() {
+        let id = test_identity();
+        let recipient_key = [0xBBu8; 32];
+        let recipient_addr = MeshAddress::from_public_key(&recipient_key);
+
+        let env = MeshEnvelopeV2::new(
+            &id,
+            recipient_addr,
+            b"hello compact".to_vec(),
+            3600,
+            5,
+            Priority::Normal,
+        );
+
+        assert_eq!(env.version, ENVELOPE_V2_VERSION);
+        assert_eq!(env.hop_count, 0);
+        assert!(env.verify_with_key(&id.public_key()));
+        assert!(!env.is_expired());
+        assert!(env.can_forward());
+    }
+
+    #[test]
+    fn broadcast_envelope() {
+        let id = test_identity();
+        let env = MeshEnvelopeV2::broadcast(
+            &id,
+            b"announcement".to_vec(),
+            300,
+            8,
+            Priority::Low,
+        );
+
+        assert!(env.is_broadcast());
+        assert_eq!(env.priority(), Priority::Low);
+        assert!(env.verify_with_key(&id.public_key()));
+    }
+
+    #[test]
+    fn forwarded_still_verifies() {
+        let id = test_identity();
+        let env = MeshEnvelopeV2::new(
+            &id,
+            MeshAddress::from_bytes([0xCC; 16]),
+            b"forward me".to_vec(),
+            3600,
+            5,
+            Priority::High,
+        );
+
+        let fwd = env.forwarded();
+        assert_eq!(fwd.hop_count, 1);
+        assert!(fwd.verify_with_key(&id.public_key()));
+
+        let fwd2 = fwd.forwarded();
+        assert_eq!(fwd2.hop_count, 2);
+        assert!(fwd2.verify_with_key(&id.public_key()));
+    }
+
+    #[test]
+    fn cbor_roundtrip() {
+        let id = test_identity();
+        let env = MeshEnvelopeV2::new(
+            &id,
+            MeshAddress::from_bytes([0xDD; 16]),
+            b"roundtrip test".to_vec(),
+            1800,
+            4,
+            Priority::Emergency,
+        );
+
+        let wire = env.to_wire();
+        let restored = MeshEnvelopeV2::from_wire(&wire).expect("deserialize");
+
+        assert_eq!(env.id, restored.id);
+        assert_eq!(env.sender_addr, restored.sender_addr);
+        assert_eq!(env.recipient_addr, restored.recipient_addr);
+        assert_eq!(env.payload, restored.payload);
+        assert_eq!(env.ttl_secs, restored.ttl_secs);
+        assert_eq!(env.hop_count, restored.hop_count);
+        assert_eq!(env.max_hops, restored.max_hops);
+        assert_eq!(env.timestamp, restored.timestamp);
+        assert_eq!(env.signature, restored.signature);
+        assert_eq!(env.priority(), Priority::Emergency);
+    }
+
+    #[test]
+    fn measure_v2_overhead() {
+        let id = test_identity();
+        let recipient_addr = MeshAddress::from_bytes([0xEE; 16]);
+
+        println!("=== MeshEnvelopeV2 Wire Overhead (CBOR) ===");
+
+        // Empty payload
+        let env_empty = MeshEnvelopeV2::new(&id, recipient_addr, vec![], 3600, 5, Priority::Normal);
+        let wire_empty = env_empty.to_wire();
+        println!("Payload   0B: wire {} bytes (overhead: {} bytes)", wire_empty.len(), wire_empty.len());
+        let v2_overhead = wire_empty.len();
+
+        // Compare to V1
+        let v1_env = crate::envelope::MeshEnvelope::new(
+            &id,
+            &[0xEE; 32],
+            vec![],
+            3600,
+            5,
+        );
+        let v1_wire = v1_env.to_wire();
+        println!("V1 empty:     {} bytes", v1_wire.len());
+        println!("V2 savings:   {} bytes ({:.1}%)", 
+            v1_wire.len() - v2_overhead,
+            ((v1_wire.len() - v2_overhead) as f64 / v1_wire.len() as f64) * 100.0);
+
+        // 10-byte payload
+        let env_10 = MeshEnvelopeV2::new(&id, recipient_addr, b"hello mesh".to_vec(), 3600, 5, Priority::Normal);
+        let wire_10 = env_10.to_wire();
+        println!("Payload  10B: wire {} bytes", wire_10.len());
+
+        // 100-byte payload
+        let env_100 = MeshEnvelopeV2::new(&id, recipient_addr, vec![0x42; 100], 3600, 5, Priority::Normal);
+        let wire_100 = env_100.to_wire();
+        println!("Payload 100B: wire {} bytes", wire_100.len());
+
+        // V2 should save ~30-50 bytes due to truncated addresses and IDs
+        assert!(v2_overhead < v1_wire.len(), "V2 should be smaller than V1");
+        assert!(v2_overhead < 150, "V2 overhead should be under 150 bytes");
+    }
+
+    #[test]
+    fn wrong_key_fails_verification() {
+        let id = test_identity();
+        let env = MeshEnvelopeV2::new(
+            &id,
+            MeshAddress::from_bytes([0xFF; 16]),
+            b"verify me".to_vec(),
+            3600,
+            5,
+            Priority::Normal,
+        );
+
+        // Wrong key should fail
+        let wrong_key = [0x42u8; 32];
+        assert!(!env.verify_with_key(&wrong_key));
+
+        // Correct key should pass
+        assert!(env.verify_with_key(&id.public_key()));
+    }
+
+    #[test]
+    fn priority_levels() {
+        let id = test_identity();
+
+        for prio in [Priority::Low, Priority::Normal, Priority::High, Priority::Emergency] {
+            let env = MeshEnvelopeV2::new(
+                &id,
+                MeshAddress::BROADCAST,
+                b"prio test".to_vec(),
+                60,
+                3,
+                prio,
+            );
+            assert_eq!(env.priority(), prio);
+        }
+    }
+}

crates/quicprochat-p2p/src/lib.rs

@@ -17,6 +17,7 @@ pub mod announce;
 pub mod announce_protocol;
 pub mod broadcast;
 pub mod envelope;
+pub mod envelope_v2;
 pub mod identity;
 pub mod link;
 pub mod mesh_router;