feat(chat): add stateless ciphertext relay

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
+    };
+}