diff --git a/crates/quicprochat-p2p/src/lib.rs b/crates/quicprochat-p2p/src/lib.rs
index fa8c705..7df1577 100644
--- a/crates/quicprochat-p2p/src/lib.rs
+++ b/crates/quicprochat-p2p/src/lib.rs
@@ -18,6 +18,7 @@ pub mod announce_protocol;
 pub mod broadcast;
 pub mod envelope;
 pub mod envelope_v2;
+pub mod mls_lite;
 pub mod identity;
 pub mod link;
 pub mod mesh_router;
diff --git a/crates/quicprochat-p2p/src/mls_lite.rs b/crates/quicprochat-p2p/src/mls_lite.rs
new file mode 100644
index 0000000..e81f880
--- /dev/null
+++ b/crates/quicprochat-p2p/src/mls_lite.rs
@@ -0,0 +1,550 @@
+//! MLS-Lite: Lightweight symmetric encryption for constrained mesh links.
+//!
+//! MLS-Lite provides group encryption without the overhead of full MLS:
+//! - Pre-shared group secret (exchanged out-of-band: QR code, NFC, voice)
+//! - ChaCha20-Poly1305 symmetric encryption (same as MLS application messages)
+//! - Per-message nonce derived from epoch + sequence
+//! - Replay protection via sequence numbers
+//! - Optional Ed25519 signatures for sender authentication
+//!
+//! # Security Properties
+//!
+//! - **Confidentiality**: ChaCha20-Poly1305 (256-bit key)
+//! - **Integrity**: Poly1305 MAC
+//! - **Replay protection**: Sequence numbers
+//! - **Sender authentication (optional)**: Ed25519 signatures
+//!
+//! # NOT Provided (vs full MLS)
+//!
+//! - Automatic post-compromise security (requires manual key rotation)
+//! - Automatic forward secrecy (only per-epoch, not per-message)
+//! - Key agreement (keys are pre-shared)
+//!
+//! # Wire Format
+//!
+//! See [`MlsLiteEnvelope`] for the compact envelope structure.
+
+use chacha20poly1305::{
+    aead::{Aead, KeyInit},
+    ChaCha20Poly1305, Nonce,
+};
+use hkdf::Hkdf;
+use rand::RngCore;
+use serde::{Deserialize, Serialize};
+use sha2::Sha256;
+use std::collections::HashMap;
+
+use crate::address::MeshAddress;
+use crate::identity::MeshIdentity;
+
+/// Maximum replay window size (track last N sequence numbers per sender).
+const REPLAY_WINDOW_SIZE: usize = 64;
+
+/// MLS-Lite group state.
+pub struct MlsLiteGroup {
+    /// 8-byte group identifier.
+    group_id: [u8; 8],
+    /// Current epoch (incremented on key rotation).
+    epoch: u16,
+    /// 32-byte symmetric encryption key (derived from group_secret + epoch).
+    encryption_key: [u8; 32],
+    /// 7-byte nonce prefix (derived from group_secret).
+    nonce_prefix: [u8; 7],
+    /// Next sequence number for sending.
+    next_seq: u32,
+    /// Replay protection: track seen (sender_addr, seq) pairs.
+    replay_window: HashMap<MeshAddress, ReplayWindow>,
+}
+
+/// Sliding window for replay detection.
+struct ReplayWindow {
+    /// Highest sequence number seen.
+    max_seq: u32,
+    /// Bitmap of seen sequence numbers in window.
+    seen: u64,
+}
+
+impl ReplayWindow {
+    fn new() -> Self {
+        Self { max_seq: 0, seen: 0 }
+    }
+
+    /// Check if sequence number is valid (not replayed).
+    /// Returns true if valid, false if replayed or too old.
+    fn check_and_update(&mut self, seq: u32) -> bool {
+        if seq == 0 {
+            // Seq 0 is always allowed once (first message)
+            if self.max_seq == 0 && self.seen == 0 {
+                self.seen = 1;
+                return true;
+            }
+        }
+
+        if seq > self.max_seq {
+            // New highest sequence
+            let shift = (seq - self.max_seq).min(64);
+            self.seen = self.seen.checked_shl(shift as u32).unwrap_or(0);
+            self.seen |= 1; // Mark current as seen
+            self.max_seq = seq;
+            true
+        } else if self.max_seq - seq >= REPLAY_WINDOW_SIZE as u32 {
+            // Too old
+            false
+        } else {
+            // Within window — check bitmap
+            let idx = (self.max_seq - seq) as u32;
+            let bit = 1u64 << idx;
+            if self.seen & bit != 0 {
+                false // Already seen
+            } else {
+                self.seen |= bit;
+                true
+            }
+        }
+    }
+}
+
+/// Result of decryption.
+#[derive(Debug)]
+pub enum DecryptResult {
+    /// Successfully decrypted plaintext.
+    Success(Vec<u8>),
+    /// Decryption failed (wrong key, corrupted, etc).
+    DecryptionFailed,
+    /// Replay detected (sequence number already seen).
+    ReplayDetected,
+    /// Signature verification failed.
+    SignatureFailed,
+}
+
+impl MlsLiteGroup {
+    /// Create a new MLS-Lite group from a pre-shared secret.
+    ///
+    /// The `group_secret` should be at least 32 bytes of high-entropy data.
+    /// It can be:
+    /// - Randomly generated and shared via QR code
+    /// - Derived from a password via Argon2id
+    /// - Exported from a full MLS group's epoch secret
+    pub fn new(group_id: [u8; 8], group_secret: &[u8], epoch: u16) -> Self {
+        let (encryption_key, nonce_prefix) = Self::derive_keys(group_secret, &group_id, epoch);
+
+        Self {
+            group_id,
+            epoch,
+            encryption_key,
+            nonce_prefix,
+            next_seq: 0,
+            replay_window: HashMap::new(),
+        }
+    }
+
+    /// Derive encryption key and nonce prefix from group secret and epoch.
+    fn derive_keys(group_secret: &[u8], group_id: &[u8; 8], epoch: u16) -> ([u8; 32], [u8; 7]) {
+        let salt = b"quicprochat-mls-lite-v1";
+        let hk = Hkdf::<Sha256>::new(Some(salt), group_secret);
+
+        // Include epoch in the info to get different keys per epoch
+        let mut info = Vec::with_capacity(10);
+        info.extend_from_slice(group_id);
+        info.extend_from_slice(&epoch.to_be_bytes());
+
+        let mut okm = [0u8; 39]; // 32 bytes key + 7 bytes nonce prefix
+        hk.expand(&info, &mut okm)
+            .expect("HKDF expand should not fail with valid length");
+
+        let mut key = [0u8; 32];
+        let mut prefix = [0u8; 7];
+        key.copy_from_slice(&okm[..32]);
+        prefix.copy_from_slice(&okm[32..39]);
+
+        (key, prefix)
+    }
+
+    /// Rotate to a new epoch with a new group secret.
+    pub fn rotate(&mut self, new_secret: &[u8], new_epoch: u16) {
+        let (key, prefix) = Self::derive_keys(new_secret, &self.group_id, new_epoch);
+        self.encryption_key = key;
+        self.nonce_prefix = prefix;
+        self.epoch = new_epoch;
+        self.next_seq = 0;
+        self.replay_window.clear();
+    }
+
+    /// Encrypt a plaintext payload.
+    ///
+    /// Returns `(ciphertext, nonce_suffix, seq)`.
+    /// The ciphertext includes the 16-byte Poly1305 tag.
+    pub fn encrypt(&mut self, plaintext: &[u8]) -> anyhow::Result<(Vec<u8>, [u8; 5], u32)> {
+        let seq = self.next_seq;
+        self.next_seq = self.next_seq.wrapping_add(1);
+
+        // Build nonce: 7-byte prefix + 5-byte suffix (1 byte random + 4 byte seq)
+        let mut nonce_suffix = [0u8; 5];
+        rand::thread_rng().fill_bytes(&mut nonce_suffix[..1]);
+        nonce_suffix[1..].copy_from_slice(&seq.to_be_bytes());
+
+        let mut nonce_bytes = [0u8; 12];
+        nonce_bytes[..7].copy_from_slice(&self.nonce_prefix);
+        nonce_bytes[7..].copy_from_slice(&nonce_suffix);
+        let nonce = Nonce::from_slice(&nonce_bytes);
+
+        let cipher = ChaCha20Poly1305::new_from_slice(&self.encryption_key)
+            .expect("key length is 32 bytes");
+
+        let ciphertext = cipher
+            .encrypt(nonce, plaintext)
+            .map_err(|e| anyhow::anyhow!("encryption failed: {e}"))?;
+
+        Ok((ciphertext, nonce_suffix, seq))
+    }
+
+    /// Decrypt a ciphertext.
+    ///
+    /// `sender_addr` is used for replay detection.
+    pub fn decrypt(
+        &mut self,
+        ciphertext: &[u8],
+        nonce_suffix: &[u8; 5],
+        sender_addr: MeshAddress,
+    ) -> DecryptResult {
+        // Extract sequence number from nonce suffix
+        let seq = u32::from_be_bytes([
+            nonce_suffix[1],
+            nonce_suffix[2],
+            nonce_suffix[3],
+            nonce_suffix[4],
+        ]);
+
+        // Replay check
+        let window = self.replay_window.entry(sender_addr).or_insert_with(ReplayWindow::new);
+        if !window.check_and_update(seq) {
+            return DecryptResult::ReplayDetected;
+        }
+
+        // Build nonce
+        let mut nonce_bytes = [0u8; 12];
+        nonce_bytes[..7].copy_from_slice(&self.nonce_prefix);
+        nonce_bytes[7..].copy_from_slice(nonce_suffix);
+        let nonce = Nonce::from_slice(&nonce_bytes);
+
+        let cipher = ChaCha20Poly1305::new_from_slice(&self.encryption_key)
+            .expect("key length is 32 bytes");
+
+        match cipher.decrypt(nonce, ciphertext) {
+            Ok(plaintext) => DecryptResult::Success(plaintext),
+            Err(_) => DecryptResult::DecryptionFailed,
+        }
+    }
+
+    /// Current epoch.
+    pub fn epoch(&self) -> u16 {
+        self.epoch
+    }
+
+    /// Group ID.
+    pub fn group_id(&self) -> &[u8; 8] {
+        &self.group_id
+    }
+}
+
+/// Compact MLS-Lite envelope for constrained links.
+///
+/// # Wire overhead (approximate)
+///
+/// - Version: 1 byte
+/// - Flags: 1 byte
+/// - Group ID: 8 bytes
+/// - Sender addr: 4 bytes (truncated further for constrained)
+/// - Seq: 4 bytes
+/// - Epoch: 2 bytes
+/// - Nonce suffix: 5 bytes
+/// - Ciphertext: variable (payload + 16 byte tag)
+/// - Signature (optional): 64 bytes
+///
+/// **Minimum overhead without signature: ~41 bytes**
+/// **Minimum overhead with signature: ~105 bytes**
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct MlsLiteEnvelope {
+    /// Format version (0x03 for MLS-Lite).
+    pub version: u8,
+    /// Flags: bit 0 = has_signature, bits 1-2 = priority.
+    pub flags: u8,
+    /// 8-byte group identifier.
+    pub group_id: [u8; 8],
+    /// 4-byte truncated sender address (first 4 bytes of MeshAddress).
+    pub sender_addr: [u8; 4],
+    /// Sequence number.
+    pub seq: u32,
+    /// Key epoch.
+    pub epoch: u16,
+    /// 5-byte nonce suffix.
+    pub nonce: [u8; 5],
+    /// Encrypted payload (includes 16-byte Poly1305 tag).
+    pub ciphertext: Vec<u8>,
+    /// Optional Ed25519 signature (64 bytes).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub signature: Option<[u8; 64]>,
+}
+
+/// MLS-Lite envelope version byte.
+const MLS_LITE_VERSION: u8 = 0x03;
+
+impl MlsLiteEnvelope {
+    /// Create a new MLS-Lite envelope (without signature).
+    pub fn new(
+        identity: &MeshIdentity,
+        group: &mut MlsLiteGroup,
+        plaintext: &[u8],
+        sign: bool,
+    ) -> anyhow::Result<Self> {
+        let (ciphertext, nonce, seq) = group.encrypt(plaintext)?;
+
+        let sender_full = MeshAddress::from_public_key(&identity.public_key());
+        let mut sender_addr = [0u8; 4];
+        sender_addr.copy_from_slice(&sender_full.as_bytes()[..4]);
+
+        let flags = if sign { 0x01 } else { 0x00 };
+
+        let mut envelope = Self {
+            version: MLS_LITE_VERSION,
+            flags,
+            group_id: *group.group_id(),
+            sender_addr,
+            seq,
+            epoch: group.epoch(),
+            nonce,
+            ciphertext,
+            signature: None,
+        };
+
+        if sign {
+            let signable = envelope.signable_bytes();
+            let sig = identity.sign(&signable);
+            envelope.signature = Some(sig);
+        }
+
+        Ok(envelope)
+    }
+
+    /// Bytes to sign (everything except signature).
+    fn signable_bytes(&self) -> Vec<u8> {
+        let mut buf = Vec::with_capacity(32 + self.ciphertext.len());
+        buf.push(self.version);
+        buf.push(self.flags);
+        buf.extend_from_slice(&self.group_id);
+        buf.extend_from_slice(&self.sender_addr);
+        buf.extend_from_slice(&self.seq.to_le_bytes());
+        buf.extend_from_slice(&self.epoch.to_le_bytes());
+        buf.extend_from_slice(&self.nonce);
+        buf.extend_from_slice(&self.ciphertext);
+        buf
+    }
+
+    /// Verify signature (if present) using sender's full public key.
+    pub fn verify_signature(&self, sender_public_key: &[u8; 32]) -> bool {
+        match &self.signature {
+            None => true, // No signature to verify
+            Some(sig) => {
+                let signable = self.signable_bytes();
+                quicprochat_core::IdentityKeypair::verify_raw(sender_public_key, &signable, sig)
+                    .is_ok()
+            }
+        }
+    }
+
+    /// Whether this envelope has a signature.
+    pub fn has_signature(&self) -> bool {
+        self.flags & 0x01 != 0
+    }
+
+    /// Serialize to 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 != MLS_LITE_VERSION {
+            anyhow::bail!("unexpected MLS-Lite version: {}", env.version);
+        }
+        Ok(env)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn test_identity() -> MeshIdentity {
+        MeshIdentity::generate()
+    }
+
+    #[test]
+    fn encrypt_decrypt_roundtrip() {
+        let secret = b"super secret group key material!";
+        let group_id = [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08];
+
+        let mut alice_group = MlsLiteGroup::new(group_id, secret, 0);
+        let mut bob_group = MlsLiteGroup::new(group_id, secret, 0);
+
+        let plaintext = b"hello from alice";
+        let (ciphertext, nonce, _seq) = alice_group.encrypt(plaintext).expect("encrypt");
+
+        let alice_addr = MeshAddress::from_bytes([0xAA; 16]);
+        match bob_group.decrypt(&ciphertext, &nonce, alice_addr) {
+            DecryptResult::Success(pt) => assert_eq!(pt, plaintext),
+            other => panic!("expected Success, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn replay_detection() {
+        let secret = b"replay test key material here!!!";
+        let group_id = [0x11; 8];
+
+        let mut alice_group = MlsLiteGroup::new(group_id, secret, 0);
+        let mut bob_group = MlsLiteGroup::new(group_id, secret, 0);
+
+        let (ciphertext, nonce, _seq) = alice_group.encrypt(b"msg1").expect("encrypt");
+        let alice_addr = MeshAddress::from_bytes([0xAA; 16]);
+
+        // First decrypt succeeds
+        match bob_group.decrypt(&ciphertext, &nonce, alice_addr) {
+            DecryptResult::Success(_) => {}
+            other => panic!("first decrypt should succeed, got {other:?}"),
+        }
+
+        // Replay attempt fails
+        match bob_group.decrypt(&ciphertext, &nonce, alice_addr) {
+            DecryptResult::ReplayDetected => {}
+            other => panic!("replay should be detected, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn different_epochs_different_keys() {
+        let secret = b"epoch rotation test material!!!";
+        let group_id = [0x22; 8];
+
+        let mut group_e0 = MlsLiteGroup::new(group_id, secret, 0);
+        let mut group_e1 = MlsLiteGroup::new(group_id, secret, 1);
+
+        let (ciphertext_e0, nonce_e0, _) = group_e0.encrypt(b"epoch 0").expect("encrypt");
+
+        // Decrypt with wrong epoch should fail
+        let sender = MeshAddress::from_bytes([0xBB; 16]);
+        match group_e1.decrypt(&ciphertext_e0, &nonce_e0, sender) {
+            DecryptResult::DecryptionFailed => {}
+            other => panic!("wrong epoch should fail decryption, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn envelope_with_signature() {
+        let id = test_identity();
+        let secret = b"envelope signature test material";
+        let group_id = [0x33; 8];
+
+        let mut group = MlsLiteGroup::new(group_id, secret, 0);
+
+        let envelope = MlsLiteEnvelope::new(&id, &mut group, b"signed message", true)
+            .expect("create envelope");
+
+        assert!(envelope.has_signature());
+        assert!(envelope.verify_signature(&id.public_key()));
+
+        // Wrong key should fail
+        let wrong_key = [0x42u8; 32];
+        assert!(!envelope.verify_signature(&wrong_key));
+    }
+
+    #[test]
+    fn envelope_without_signature() {
+        let id = test_identity();
+        let secret = b"unsigned envelope test material!";
+        let group_id = [0x44; 8];
+
+        let mut group = MlsLiteGroup::new(group_id, secret, 0);
+
+        let envelope = MlsLiteEnvelope::new(&id, &mut group, b"no sig", false)
+            .expect("create envelope");
+
+        assert!(!envelope.has_signature());
+        assert!(envelope.signature.is_none());
+    }
+
+    #[test]
+    fn envelope_cbor_roundtrip() {
+        let id = test_identity();
+        let secret = b"cbor roundtrip test material!!!!";
+        let group_id = [0x55; 8];
+
+        let mut group = MlsLiteGroup::new(group_id, secret, 0);
+
+        let envelope = MlsLiteEnvelope::new(&id, &mut group, b"roundtrip", true)
+            .expect("create envelope");
+
+        let wire = envelope.to_wire();
+        let restored = MlsLiteEnvelope::from_wire(&wire).expect("deserialize");
+
+        assert_eq!(envelope.version, restored.version);
+        assert_eq!(envelope.flags, restored.flags);
+        assert_eq!(envelope.group_id, restored.group_id);
+        assert_eq!(envelope.sender_addr, restored.sender_addr);
+        assert_eq!(envelope.seq, restored.seq);
+        assert_eq!(envelope.epoch, restored.epoch);
+        assert_eq!(envelope.nonce, restored.nonce);
+        assert_eq!(envelope.ciphertext, restored.ciphertext);
+        assert_eq!(envelope.signature, restored.signature);
+    }
+
+    #[test]
+    fn measure_mls_lite_overhead() {
+        let id = test_identity();
+        let secret = b"overhead measurement test secret";
+        let group_id = [0x66; 8];
+
+        let mut group = MlsLiteGroup::new(group_id, secret, 0);
+
+        println!("=== MLS-Lite Wire Overhead (CBOR) ===");
+
+        // Without signature
+        let env_no_sig = MlsLiteEnvelope::new(&id, &mut group, b"", false)
+            .expect("create");
+        let wire_no_sig = env_no_sig.to_wire();
+        // Overhead = wire - payload - 16 byte tag
+        let overhead_no_sig = wire_no_sig.len() - 16; // tag is in ciphertext
+        println!("No signature, 0B payload: {} bytes (overhead: {})", wire_no_sig.len(), overhead_no_sig);
+
+        // With signature
+        let env_sig = MlsLiteEnvelope::new(&id, &mut group, b"", true)
+            .expect("create");
+        let wire_sig = env_sig.to_wire();
+        let overhead_sig = wire_sig.len() - 16;
+        println!("With signature, 0B payload: {} bytes (overhead: {})", wire_sig.len(), overhead_sig);
+
+        // 10-byte payload without sig
+        let env_10 = MlsLiteEnvelope::new(&id, &mut group, b"hello mesh", false)
+            .expect("create");
+        let wire_10 = env_10.to_wire();
+        println!("No signature, 10B payload: {} bytes", wire_10.len());
+
+        // Compare to MeshEnvelope V1
+        let v1_env = crate::envelope::MeshEnvelope::new(
+            &id,
+            &[0x77; 32],
+            b"hello mesh".to_vec(),
+            3600,
+            5,
+        );
+        let v1_wire = v1_env.to_wire();
+        println!("MeshEnvelope V1, 10B payload: {} bytes", v1_wire.len());
+        println!("MLS-Lite savings (no sig): {} bytes", v1_wire.len() as i32 - wire_10.len() as i32);
+
+        assert!(overhead_no_sig < 50, "MLS-Lite overhead without sig should be under 50 bytes");
+        assert!(overhead_sig < 120, "MLS-Lite overhead with sig should be under 120 bytes");
+    }
+}