diff --git a/docs/PROTOCOL.md b/docs/PROTOCOL.md
index b5d8f43..4a18209 100644
--- a/docs/PROTOCOL.md
+++ b/docs/PROTOCOL.md
@@ -82,13 +82,47 @@ Payload:
   "hostPeerId": "string or null",
   "controlMode": "everyone | host-only",
   "controllers": ["peerId"],
-  "capabilities": ["host-control", "co-host"]
+  "capabilities": ["host-control", "co-host", "chat"]
 }
 ```
 
 `room_data` is sent to the joining socket. It is not the general broadcast used
 for every later room update.
 
+## Ephemeral encrypted chat
+
+Relays advertise chat support with `"chat"` in `room_data.capabilities`. Clients
+must not infer support from another field.
+
+### `chat_message`
+
+Client to relay:
+
+```json
+{ "ciphertext": "<unpadded-base64url>" }
+```
+
+`ciphertext` contains a 12-byte AES-GCM IV followed by ciphertext and the 16-byte
+authentication tag. The relay validates only canonical base64url and byte bounds.
+It cannot inspect plaintext.
+
+Relay to every current room peer, including the sender:
+
+```json
+{
+  "id": "<server-generated UUID>",
+  "senderId": "<server-stamped peer ID>",
+  "timestamp": 1710000000000,
+  "ciphertext": "<unpadded-base64url>"
+}
+```
+
+Client-provided `id`, `senderId`, `timestamp`, or plaintext fields are discarded.
+The relay keeps no message collection and `room_data` contains no chat history.
+Messages are limited to 10 per socket per 10 seconds in addition to the global event
+budget. There are no typing, read-receipt, history, or chat-specific peer-management
+events.
+
 ## Room Leave
 
 ### `leave_room` (client -> server)
@@ -378,5 +412,6 @@ If sender and target are still in the same room, the relay emits:
 
 - `host-control`
 - `co-host`
+- `chat`
 
 Clients should treat a missing or unknown capabilities list as unsupported.
diff --git a/scripts/test-server-ws.mjs b/scripts/test-server-ws.mjs
index 5c1186c..32ae992 100644
--- a/scripts/test-server-ws.mjs
+++ b/scripts/test-server-ws.mjs
@@ -1,4 +1,5 @@
 import assert from 'node:assert/strict';
+import { Buffer } from 'node:buffer';
 import http from 'node:http';
 import { createRequire } from 'node:module';
 import { fileURLToPath } from 'node:url';
@@ -70,6 +71,38 @@ try {
     assert.equal(capEv, 'room_data');
     assert.ok(Array.isArray(capData.capabilities) && capData.capabilities.includes('host-control'),
         'ROOM_DATA advertises the host-control capability');
+    assert.ok(capData.capabilities.includes('chat'), 'ROOM_DATA advertises the chat capability');
+    assert.equal(capData.chatHistory, undefined, 'ROOM_DATA never contains chat history');
+    close();
+    resetConnectionRate();
+
+    // --- Encrypted chat is a live-only canonical relay ---
+    const chatRoom = 'chat-'+Date.now();
+    const chat1 = await c(), chat2 = await c();
+    await j(chat1, chatRoom, 'alice'); await j(chat2, chatRoom, 'bob');
+    chat1._m.length = chat2._m.length = 0;
+    const ciphertext = Buffer.alloc(64, 7).toString('base64url');
+    s(chat1, 'chat_message', {
+        ciphertext,
+        id: 'spoofed-id', senderId: 'mallory', timestamp: 1, text: 'plaintext'
+    });
+    const [chat1Event, chat1Data] = await a(chat1);
+    const [chat2Event, chat2Data] = await a(chat2);
+    assert.equal(chat1Event, 'chat_message', 'sender receives canonical chat envelope');
+    assert.equal(chat2Event, 'chat_message', 'peer receives canonical chat envelope');
+    assert.deepEqual(chat2Data, chat1Data, 'all peers receive the same canonical envelope');
+    assert.equal(chat1Data.senderId, 'alice', 'relay stamps senderId');
+    assert.equal(chat1Data.ciphertext, ciphertext, 'relay preserves ciphertext');
+    assert.equal(chat1Data.text, undefined, 'relay drops plaintext fields');
+    assert.notEqual(chat1Data.id, 'spoofed-id', 'relay replaces client IDs');
+    assert.ok(Number.isFinite(chat1Data.timestamp) && chat1Data.timestamp > 1, 'relay stamps timestamp');
+
+    const late = await c();
+    s(late, 'join_room', { roomId: chatRoom, peerId: 'late', protocolVersion: '1.0.0' });
+    const [lateEvent, lateRoomData] = await a(late);
+    assert.equal(lateEvent, 'room_data');
+    assert.equal(lateRoomData.chatHistory, undefined, 'late joiner gets no chat backlog');
+    assert.equal(late._m.some(raw => raw.includes('chat_message')), false, 'late joiner receives no old message');
     close();
     resetConnectionRate();
 
diff --git a/server/chat.js b/server/chat.js
new file mode 100644
index 0000000..6792d24
--- /dev/null
+++ b/server/chat.js
@@ -0,0 +1,24 @@
+import { randomUUID } from 'node:crypto';
+
+export const CHAT_CIPHERTEXT_MAX_BYTES = 2028;
+export const CHAT_CIPHERTEXT_MIN_BYTES = 29;
+
+export function normalizeChatCiphertext(value) {
+    if (typeof value !== 'string' || !/^[A-Za-z0-9_-]+$/.test(value) || value.length % 4 === 1) return null;
+    const bytes = Buffer.from(value, 'base64url');
+    if (bytes.length < CHAT_CIPHERTEXT_MIN_BYTES || bytes.length > CHAT_CIPHERTEXT_MAX_BYTES) return null;
+    if (bytes.toString('base64url') !== value) return null;
+    return value;
+}
+
+export function createChatEnvelope(data, senderId, now = Date.now, createId = randomUUID) {
+    if (!data || typeof data !== 'object' || typeof senderId !== 'string' || !senderId) return null;
+    const ciphertext = normalizeChatCiphertext(data.ciphertext);
+    if (!ciphertext) return null;
+    return {
+        id: createId(),
+        senderId,
+        timestamp: now(),
+        ciphertext
+    };
+}
diff --git a/server/chat.test.mjs b/server/chat.test.mjs
new file mode 100644
index 0000000..51907fb
--- /dev/null
+++ b/server/chat.test.mjs
@@ -0,0 +1,43 @@
+import { describe, expect, it } from 'vitest';
+import { Buffer } from 'node:buffer';
+import {
+    CHAT_CIPHERTEXT_MAX_BYTES,
+    createChatEnvelope,
+    normalizeChatCiphertext
+} from './chat.js';
+
+const encoded = (length) => Buffer.alloc(length, 7).toString('base64url');
+
+describe('encrypted chat relay envelopes', () => {
+    it('accepts canonical base64url within the encrypted message bounds', () => {
+        expect(normalizeChatCiphertext(encoded(29))).toBe(encoded(29));
+        expect(normalizeChatCiphertext(encoded(CHAT_CIPHERTEXT_MAX_BYTES))).toBe(encoded(CHAT_CIPHERTEXT_MAX_BYTES));
+    });
+
+    it('rejects malformed, padded, too-small, and oversized payloads', () => {
+        expect(normalizeChatCiphertext('<script>')).toBeNull();
+        expect(normalizeChatCiphertext(`${encoded(29)}=`)).toBeNull();
+        expect(normalizeChatCiphertext(encoded(28))).toBeNull();
+        expect(normalizeChatCiphertext(encoded(CHAT_CIPHERTEXT_MAX_BYTES + 1))).toBeNull();
+        expect(normalizeChatCiphertext('A')).toBeNull();
+    });
+
+    it('stamps authoritative metadata and keeps only ciphertext from the sender', () => {
+        const ciphertext = encoded(64);
+        const envelope = createChatEnvelope({
+            ciphertext,
+            id: 'spoofed',
+            senderId: 'mallory',
+            timestamp: 1,
+            text: 'plaintext must not survive'
+        }, 'alice', () => 1234, () => 'server-id');
+
+        expect(envelope).toEqual({
+            id: 'server-id',
+            senderId: 'alice',
+            timestamp: 1234,
+            ciphertext
+        });
+        expect(JSON.stringify(envelope)).not.toContain('plaintext');
+    });
+});
diff --git a/server/index.js b/server/index.js
index 175369c..a172cc2 100644
--- a/server/index.js
+++ b/server/index.js
@@ -5,6 +5,7 @@ import { Server } from 'socket.io';
 import crypto from 'crypto';
 import dotenv from 'dotenv';
 import { EVENTS, OFFICIAL_SERVER_TOKEN, PROTOCOL_VERSION, CONTROL_MODES, CAPABILITIES } from '../shared/constants.js';
+import { createChatEnvelope } from './chat.js';
 import {
     buildHealthPayload,
     checkCooldown,
@@ -19,6 +20,7 @@ import {
     connectionCounts,
     failedAuthAttempts,
     eventCounts,
+    chatMessageCounts,
     healthCounts,
     adminMetricsAuthCounts,
     roomListCooldowns,
@@ -28,6 +30,7 @@ import {
     recordAuthFailure,
     checkConnectionRate,
     checkEventRate,
+    checkChatMessageRate,
     checkHealthRate,
     checkAdminMetricsAuthRate,
     checkLeaveRoomRate,
@@ -171,7 +174,7 @@ const HOST_ONLY_GATED_EVENTS = new Set([
 // Features this relay supports, advertised to clients in ROOM_DATA so they can
 // enable matching UI/behavior only when the server actually backs it. Append a
 // flag here when a new server-gated feature ships (e.g. co-host promotion).
-const SERVER_CAPABILITIES = [CAPABILITIES.HOST_CONTROL, CAPABILITIES.CO_HOST];
+const SERVER_CAPABILITIES = [CAPABILITIES.HOST_CONTROL, CAPABILITIES.CO_HOST, CAPABILITIES.CHAT];
 
 // M-4: minimum interval between CONTROL_MODE changes per room. Stops a rapidly
 // toggling host from thrashing every guest's UI (locked/unlocked/locked...) and
@@ -844,8 +847,31 @@ io.on('connection', (socket) => {
         }
     });
 
+    socket.on(EVENTS.CHAT_MESSAGE, (data) => {
+        try {
+            if (!checkEventRate(socket.id) || !checkChatMessageRate(socket.id)) {
+                log('SECURITY', `Encrypted chat rate limit exceeded for socket: ${socket.id}`);
+                socket.disconnect(true);
+                return;
+            }
+
+            const mapping = socketToRoom.get(socket.id);
+            if (!mapping) return;
+            const room = rooms.get(mapping.roomId);
+            if (!room) return;
+
+            const envelope = createChatEnvelope(data, mapping.peerId);
+            if (!envelope) return;
+            room.lastActivity = Date.now();
+            io.to(mapping.roomId).emit(EVENTS.CHAT_MESSAGE, envelope);
+        } catch (err) {
+            log('ERROR', `CHAT_MESSAGE handler error: ${err.message}`);
+        }
+    });
+
     socket.on('disconnect', () => {
         eventCounts.delete(socket.id);
+        chatMessageCounts.delete(socket.id);
         roomListCooldowns.delete(socket.id);
         leaveRoomCounts.delete(socket.id);
         const mapping = socketToRoom.get(socket.id);
diff --git a/server/rate-limiter.js b/server/rate-limiter.js
index aec4171..0f8cc33 100644
--- a/server/rate-limiter.js
+++ b/server/rate-limiter.js
@@ -12,6 +12,8 @@ export const CONNECTION_RATE_LIMIT = 10;           // max new connections per IP
 export const CONNECTION_RATE_WINDOW_MS = 60000;    // 1 minute
 export const EVENT_RATE_LIMIT = 50;                // max relayed events per socket per window
 export const EVENT_RATE_WINDOW_MS = 10000;         // 10 seconds
+export const CHAT_MESSAGE_RATE_LIMIT = 10;         // max encrypted chat messages per socket per window
+export const CHAT_MESSAGE_RATE_WINDOW_MS = 10000;  // 10 seconds
 export const HEALTH_RATE_WINDOW_MS = 60000;        // 1 minute
 export const ADMIN_METRICS_AUTH_WINDOW_MS = 60000; // 1 minute
 export const LEAVE_ROOM_RATE_LIMIT = 10;           // max LEAVE_ROOM events per socket per window
@@ -20,6 +22,7 @@ export const LEAVE_ROOM_RATE_WINDOW_MS = 60000;    // 1 minute
 export const connectionCounts = new Map(); // ip -> { count, resetTime }
 export const failedAuthAttempts = new Map(); // Map<IP+RoomID, {count, lastAttempt}>
 export const eventCounts = new Map(); // socketId -> { count, resetTime }
+export const chatMessageCounts = new Map(); // socketId -> { count, resetTime }
 export const healthCounts = new Map(); // ip -> { count, resetTime }
 export const adminMetricsAuthCounts = new Map(); // ip -> { count, resetTime }
 export const roomListCooldowns = new Map(); // socketId -> last allowed timestamp
@@ -28,6 +31,7 @@ export const leaveRoomCounts = new Map();    // socketId -> { count, resetTime }
 export const rateLimitDenied = {
     connections: 0,
     events: 0,
+    chatMessages: 0,
     health: 0,
     adminMetricsAuth: 0,
     roomList: 0,
@@ -104,6 +108,17 @@ export function checkEventRate(socketId) {
     return false;
 }
 
+export function checkChatMessageRate(socketId) {
+    const now = Date.now();
+    const entry = chatMessageCounts.get(socketId) || { count: 0, resetTime: now + CHAT_MESSAGE_RATE_WINDOW_MS };
+    if (now > entry.resetTime) { entry.count = 0; entry.resetTime = now + CHAT_MESSAGE_RATE_WINDOW_MS; }
+    entry.count++;
+    chatMessageCounts.set(socketId, entry);
+    if (entry.count <= CHAT_MESSAGE_RATE_LIMIT) return true;
+    rateLimitDenied.chatMessages++;
+    return false;
+}
+
 export function checkHealthRate(ip) {
     const now = Date.now();
     const entry = healthCounts.get(ip) || { count: 0, resetTime: now + HEALTH_RATE_WINDOW_MS };
@@ -163,6 +178,11 @@ export function startRateLimitCleanup(io) {
                 eventCounts.delete(socketId);
             }
         }
+        for (const [socketId, entry] of chatMessageCounts.entries()) {
+            if (now > entry.resetTime || !io.sockets.sockets.has(socketId)) {
+                chatMessageCounts.delete(socketId);
+            }
+        }
         for (const [socketId, entry] of leaveRoomCounts.entries()) {
             if (now > entry.resetTime || !io.sockets.sockets.has(socketId)) {
                 leaveRoomCounts.delete(socketId);
@@ -189,6 +209,7 @@ export function clearRateLimitMaps() {
     connectionCounts.clear();
     failedAuthAttempts.clear();
     eventCounts.clear();
+    chatMessageCounts.clear();
 
     healthCounts.clear();
     adminMetricsAuthCounts.clear();
diff --git a/server/rate-limiter.test.mjs b/server/rate-limiter.test.mjs
index 9a3bb2c..d5ac4bd 100644
--- a/server/rate-limiter.test.mjs
+++ b/server/rate-limiter.test.mjs
@@ -1,6 +1,10 @@
 import { describe, it, expect, beforeEach, afterEach } from 'vitest';
 import {
     checkLeaveRoomRate,
+    checkChatMessageRate,
+    CHAT_MESSAGE_RATE_LIMIT,
+    CHAT_MESSAGE_RATE_WINDOW_MS,
+    chatMessageCounts,
     LEAVE_ROOM_RATE_LIMIT,
     LEAVE_ROOM_RATE_WINDOW_MS,
     rateLimitDenied,
@@ -99,6 +103,32 @@ describe('LEAVE_ROOM Rate Limiter', () => {
     });
 });
 
+describe('CHAT_MESSAGE Rate Limiter', () => {
+    const socketId = 'chat-socket';
+
+    beforeEach(() => {
+        clearRateLimitMaps();
+        rateLimitDenied.chatMessages = 0;
+    });
+
+    afterEach(() => clearRateLimitMaps());
+
+    it('allows ten messages per ten-second window and blocks the next', () => {
+        for (let i = 0; i < CHAT_MESSAGE_RATE_LIMIT; i++) {
+            expect(checkChatMessageRate(socketId)).toBe(true);
+        }
+        expect(checkChatMessageRate(socketId)).toBe(false);
+        expect(rateLimitDenied.chatMessages).toBe(1);
+    });
+
+    it('resets after the window expires', () => {
+        for (let i = 0; i <= CHAT_MESSAGE_RATE_LIMIT; i++) checkChatMessageRate(socketId);
+        const entry = chatMessageCounts.get(socketId);
+        entry.resetTime = Date.now() - CHAT_MESSAGE_RATE_WINDOW_MS - 1;
+        expect(checkChatMessageRate(socketId)).toBe(true);
+    });
+});
+
 describe('Rate Limit Constants', () => {
     it('should have correct rate limit values', () => {
         expect(LEAVE_ROOM_RATE_LIMIT).toBe(10);
diff --git a/shared/constants.js b/shared/constants.js
index a23f5e0..d19093f 100644
--- a/shared/constants.js
+++ b/shared/constants.js
@@ -57,6 +57,9 @@ export const EVENTS = {
     EPISODE_READY: "episode_ready",      // Response: loaded the episode and paused at 0:00
     EPISODE_LOBBY_CANCEL: "episode_lobby_cancel", // Broadcast: cancel active lobby and resume
 
+    // Ephemeral end-to-end encrypted chat
+    CHAT_MESSAGE: "chat_message", // Ciphertext relay; no server history
+
     // Ping / Latency
     PING: "ping",  // { t: timestamp, target?: peerId } — empty target = server echo
     PONG: "pong"   // server responds with same { t } for client RTT calculation
@@ -77,7 +80,8 @@ export const CONTROL_MODES = {
 // simply ignore the field. Add a flag here as each server-gated feature lands.
 export const CAPABILITIES = {
     HOST_CONTROL: 'host-control',
-    CO_HOST: 'co-host'  // owner promotes guests to additional controllers
+    CO_HOST: 'co-host',  // owner promotes guests to additional controllers
+    CHAT: 'chat'
 };
 
 export const HEARTBEAT_INTERVAL = 15000; // 15s