diff --git a/packages/app/src/utils/crypto.ts b/packages/app/src/utils/crypto.ts
new file mode 100644
index 00000000..c6f39ae1
--- /dev/null
+++ b/packages/app/src/utils/crypto.ts
@@ -0,0 +1,144 @@
+/**
+ * AES-256-GCM encryption utilities using Web Crypto API.
+ * PBKDF2 key derivation with 100K+ iterations.
+ */
+
+const PBKDF2_ITERATIONS = 100_000
+const SALT_LENGTH = 16
+const IV_LENGTH = 12
+
+export async function generateSalt(): Promise<Uint8Array> {
+  return crypto.getRandomValues(new Uint8Array(SALT_LENGTH))
+}
+
+export async function deriveKey(password: string, salt: Uint8Array): Promise<CryptoKey> {
+  const enc = new TextEncoder()
+  const keyMaterial = await crypto.subtle.importKey(
+    'raw',
+    enc.encode(password),
+    'PBKDF2',
+    false,
+    ['deriveKey'],
+  )
+
+  return crypto.subtle.deriveKey(
+    {
+      name: 'PBKDF2',
+      salt,
+      iterations: PBKDF2_ITERATIONS,
+      hash: 'SHA-256',
+    },
+    keyMaterial,
+    { name: 'AES-GCM', length: 256 },
+    false,
+    ['encrypt', 'decrypt'],
+  )
+}
+
+export interface EncryptedPayload {
+  ciphertext: ArrayBuffer
+  iv: Uint8Array
+}
+
+export async function encrypt(data: string, key: CryptoKey): Promise<EncryptedPayload> {
+  const enc = new TextEncoder()
+  const iv = crypto.getRandomValues(new Uint8Array(IV_LENGTH))
+  const ciphertext = await crypto.subtle.encrypt(
+    { name: 'AES-GCM', iv },
+    key,
+    enc.encode(data),
+  )
+  return { ciphertext, iv }
+}
+
+export async function decrypt(
+  ciphertext: ArrayBuffer,
+  iv: Uint8Array,
+  key: CryptoKey,
+): Promise<string> {
+  const plaintext = await crypto.subtle.decrypt(
+    { name: 'AES-GCM', iv },
+    key,
+    ciphertext,
+  )
+  return new TextDecoder().decode(plaintext)
+}
+
+/**
+ * Convenience: encrypt a string and return a storable format.
+ * Returns base64-encoded JSON with iv + ciphertext.
+ */
+export async function encryptToString(data: string, key: CryptoKey): Promise<string> {
+  const { ciphertext, iv } = await encrypt(data, key)
+  const combined = new Uint8Array(iv.length + ciphertext.byteLength)
+  combined.set(iv)
+  combined.set(new Uint8Array(ciphertext), iv.length)
+  return bufferToBase64(combined)
+}
+
+/**
+ * Convenience: decrypt a base64-encoded encrypted string.
+ */
+export async function decryptFromString(encoded: string, key: CryptoKey): Promise<string> {
+  const combined = base64ToBuffer(encoded)
+  const iv = combined.slice(0, IV_LENGTH)
+  const ciphertext = combined.slice(IV_LENGTH)
+  return decrypt(ciphertext.buffer, iv, key)
+}
+
+function bufferToBase64(buffer: Uint8Array): string {
+  let binary = ''
+  for (let i = 0; i < buffer.length; i++) {
+    binary += String.fromCharCode(buffer[i])
+  }
+  return btoa(binary)
+}
+
+function base64ToBuffer(base64: string): Uint8Array {
+  const binary = atob(base64)
+  const bytes = new Uint8Array(binary.length)
+  for (let i = 0; i < binary.length; i++) {
+    bytes[i] = binary.charCodeAt(i)
+  }
+  return bytes
+}
+
+/**
+ * Check if encryption should be enabled.
+ * Disabled in dev mode with VITE_DISABLE_CRYPTO=true.
+ */
+export function isCryptoEnabled(): boolean {
+  try {
+    return import.meta.env.VITE_DISABLE_CRYPTO !== 'true'
+  } catch {
+    return true
+  }
+}
+
+/**
+ * Session key management — held in memory only.
+ */
+let sessionKey: CryptoKey | null = null
+let sessionSalt: Uint8Array | null = null
+
+export function setSessionKey(key: CryptoKey, salt: Uint8Array): void {
+  sessionKey = key
+  sessionSalt = salt
+}
+
+export function getSessionKey(): CryptoKey | null {
+  return sessionKey
+}
+
+export function getSessionSalt(): Uint8Array | null {
+  return sessionSalt
+}
+
+export function clearSessionKey(): void {
+  sessionKey = null
+  sessionSalt = null
+}
+
+export function hasSessionKey(): boolean {
+  return sessionKey !== null
+}
diff --git a/packages/app/src/utils/idb-storage.ts b/packages/app/src/utils/idb-storage.ts
new file mode 100644
index 00000000..852ff162
--- /dev/null
+++ b/packages/app/src/utils/idb-storage.ts
@@ -0,0 +1,100 @@
+import type { Conversation } from '@aiui/core/types/message'
+import {
+  isCryptoEnabled,
+  getSessionKey,
+  encryptToString,
+  decryptFromString,
+} from './crypto'
+
+const DB_NAME = 'aiui-store'
+const DB_VERSION = 1
+const STORE_NAME = 'conversations'
+
+let dbPromise: Promise<IDBDatabase> | null = null
+
+export function openDB(): Promise<IDBDatabase> {
+  if (dbPromise) return dbPromise
+  dbPromise = new Promise<IDBDatabase>((resolve, reject) => {
+    const request = indexedDB.open(DB_NAME, DB_VERSION)
+    request.onupgradeneeded = () => {
+      const db = request.result
+      if (!db.objectStoreNames.contains(STORE_NAME)) {
+        const store = db.createObjectStore(STORE_NAME, { keyPath: 'id' })
+        store.createIndex('updatedAt', 'updatedAt', { unique: false })
+      }
+    }
+    request.onsuccess = () => resolve(request.result)
+    request.onerror = () => {
+      dbPromise = null
+      reject(request.error)
+    }
+  })
+  return dbPromise
+}
+
+interface EncryptedRecord {
+  id: string
+  updatedAt: number
+  encrypted: string
+}
+
+async function encryptConversation(conv: Conversation): Promise<EncryptedRecord | Conversation> {
+  const key = getSessionKey()
+  if (!isCryptoEnabled() || !key) return conv
+  const encrypted = await encryptToString(JSON.stringify(conv), key)
+  return { id: conv.id, updatedAt: conv.updatedAt, encrypted }
+}
+
+async function decryptConversation(record: EncryptedRecord | Conversation): Promise<Conversation | null> {
+  if (!('encrypted' in record)) return record as Conversation
+  const key = getSessionKey()
+  if (!key) return null
+  try {
+    const json = await decryptFromString(record.encrypted, key)
+    return JSON.parse(json) as Conversation
+  } catch {
+    return null // Wrong passphrase or corrupted data
+  }
+}
+
+export async function saveConversation(conv: Conversation): Promise<void> {
+  const db = await openDB()
+  const record = await encryptConversation(conv)
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction(STORE_NAME, 'readwrite')
+    tx.objectStore(STORE_NAME).put(record)
+    tx.oncomplete = () => resolve()
+    tx.onerror = () => reject(tx.error)
+  })
+}
+
+export async function loadAllConversations(): Promise<Map<string, Conversation>> {
+  const db = await openDB()
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction(STORE_NAME, 'readonly')
+    const request = tx.objectStore(STORE_NAME).index('updatedAt').getAll()
+    request.onsuccess = async () => {
+      const map = new Map<string, Conversation>()
+      for (const record of request.result) {
+        const conv = await decryptConversation(record)
+        if (conv) map.set(conv.id, conv)
+      }
+      resolve(map)
+    }
+    request.onerror = () => reject(request.error)
+  })
+}
+
+export async function deleteConversation(id: string): Promise<void> {
+  const db = await openDB()
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction(STORE_NAME, 'readwrite')
+    tx.objectStore(STORE_NAME).delete(id)
+    tx.oncomplete = () => resolve()
+    tx.onerror = () => reject(tx.error)
+  })
+}
+
+export function isIDBAvailable(): boolean {
+  return typeof indexedDB !== 'undefined'
+}