perf(P6B.T6): offload Argon2 + userKey unseal to Web Worker

PIN-unlock used to freeze the renderer for ~1-2 s on mid-hardware while
the moderate-preset Argon2id KDF + sealed-key secretbox open ran on the
main thread. Push that work into a Vite-bundled ESM Web Worker so the
unlock screen stays responsive.

The worker (apps/desktop/src/workers/crypto.worker.ts) bundles its own
libsodium-wrappers-sumo instance and registers a fresh CryptoBackend
inside the worker realm. Client wrapper (apps/desktop/src/lib/cryptoWorker.ts)
spawns a one-shot worker per unlock — workers are cheap, PIN-unlock is
once-per-session, and one-shot avoids the request-id bookkeeping that the
existing decrypt.worker needs for high-volume per-message decrypts.

Falls back to inline main-thread openUserKey when the Worker constructor
is unavailable (vitest's jsdom) or when worker spawn / round-trip fails
(strict CSP). All 14 desktop + 71 shared tests still pass — the existing
loadOrUnlockUserKey test exercises the inline-fallback branch.

Private key bytes are transferred (zero-copy) back to the main thread,
detaching the worker-side ArrayBuffer view on transfer.

Build emits crypto.worker-<hash>.js (~2.5 MB, mostly libsodium WASM glue
duplicated from the main bundle). Acceptable trade-off for the unblocked
UI; a future change could lazy-load libsodium on the main thread to drop
the duplication.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src/lib/cryptoWorker.ts

@@ -0,0 +1,73 @@
+// Main-thread wrapper around the crypto Web Worker (Argon2id pwhash +
+// sealed user-key open). Each unlock attempt spawns a fresh one-shot worker
+// — workers are cheap and the pwhash is a one-time cost per login, so we
+// avoid the bookkeeping needed for a persistent request queue.
+//
+// Falls back to inline (main-thread) `openUserKey` when the `Worker`
+// constructor is unavailable (e.g. vitest's jsdom environment, strict CSPs).
+// The fallback path is identical in semantics to the worker path; the only
+// difference is whether it blocks the main thread.
+//
+// Why not a long-lived worker? The KDF cost dwarfs the spawn cost (~1-2 s
+// vs. a few ms), and PIN-unlock happens at most once per session. Keeping a
+// worker resident would also require a request-id correlation map which the
+// decrypt.worker uses (because per-message decrypts are high-volume).
+
+import { openUserKey } from '@chat-app/shared/crypto';
+
+import type {
+  OpenUserKeyInput,
+  OpenUserKeyResult,
+} from '../workers/crypto.worker';
+
+type WorkerResponse =
+  | { ok: true; result: OpenUserKeyResult }
+  | { ok: false; error: string };
+
+export type { OpenUserKeyInput, OpenUserKeyResult };
+
+export async function openUserKeyInWorker(
+  input: OpenUserKeyInput,
+): Promise<OpenUserKeyResult> {
+  const worker = new Worker(
+    new URL('../workers/crypto.worker.ts', import.meta.url),
+    { type: 'module' },
+  );
+  try {
+    return await new Promise<OpenUserKeyResult>((resolve, reject) => {
+      worker.addEventListener('message', (ev: MessageEvent<WorkerResponse>) => {
+        const msg = ev.data;
+        if (msg && msg.ok) resolve(msg.result);
+        else reject(new Error(msg?.error ?? 'crypto worker returned malformed response'));
+      });
+      worker.addEventListener('error', (ev: ErrorEvent) => {
+        reject(new Error(ev.message || 'crypto worker error'));
+      });
+      worker.postMessage({ op: 'openUserKey', input });
+    });
+  } finally {
+    worker.terminate();
+  }
+}
+
+// Public entry point: route through the worker when possible, fall back to
+// the synchronous-on-main-thread path otherwise. Callers should prefer this
+// over importing `openUserKey` directly so we get the worker speedup
+// everywhere it's available.
+export async function openUserKeyMaybeWorker(
+  input: OpenUserKeyInput,
+): Promise<Uint8Array> {
+  if (typeof Worker === 'undefined') {
+    return openUserKey(input);
+  }
+  try {
+    const result = await openUserKeyInWorker(input);
+    return result.privateKey;
+  } catch (err) {
+    // If the worker spawn or message round-trip fails (e.g. CSP blocks
+    // module workers in some packaging modes), fall back to inline so the
+    // unlock still succeeds — just with a brief main-thread hitch.
+    console.warn('[cryptoWorker] worker path failed, falling back to inline', err);
+    return openUserKey(input);
+  }
+}

apps/desktop/src/lib/userIdentity.ts

@@ -4,10 +4,11 @@ import {
 } from '@chat-app/shared/auth';
 import {
   generateRecoveryCode, generateUserKeyPair, getCryptoBackend, normalizeRecoveryCode,
-  openUserKey, sealUserKey,
+  sealUserKey,
 } from '@chat-app/shared/crypto';
 import { migrateOwnLegacyBundles } from '@chat-app/shared/chat';
 
+import { openUserKeyMaybeWorker } from './cryptoWorker';
 import { devLocalSecretStore } from './secretStore';
 import { supabase } from './supabase';
 
@@ -71,7 +72,7 @@ export async function loadOrUnlockUserKey(p: UnlockParams): Promise<UnlockOutcom
   if (!sealed || !salt) throw new Error('no recovery blob configured');
   let priv: Uint8Array;
   try {
-    priv = await openUserKey({ sealed, pin: secret, salt, kdfParams: remote.kdfParams });
+    priv = await openUserKeyMaybeWorker({ sealed, pin: secret, salt, kdfParams: remote.kdfParams });
   } catch (err) {
     await recordPinAttempt(supabase, p.userId, false, p.isRecoveryCode === true).catch(() => {});
     throw err;

apps/desktop/src/workers/crypto.worker.ts

@@ -0,0 +1,78 @@
+// Web Worker — runs Argon2id pwhash + sealed user-key open off the main
+// thread. PIN-unlock used to freeze the UI for ~1-2 s on mid-hardware while
+// the moderate-preset KDF ran; pushing it here keeps the unlock screen
+// responsive.
+//
+// The worker bundles its own libsodium-wrappers-sumo instance and registers
+// it as the shared CryptoBackend inside this worker realm — there is no
+// shared state with the main thread, so we initialise once per worker and
+// re-use it across messages (the client wrapper currently spawns one-shot,
+// but the worker is safe to keep alive too).
+//
+// Message protocol (one-shot RPC):
+//   request:  { op: 'openUserKey', input: OpenUserKeyInput }
+//   response: { ok: true,  result: { privateKey: Uint8Array } }
+//           | { ok: false, error: string }
+//
+// The private key bytes are transferred (zero-copy) back to the caller via
+// the structured-clone Transferable list; the worker's view of the buffer is
+// detached on transfer which also clears the only worker-side reference.
+
+/// <reference lib="webworker" />
+
+import { setCryptoBackend, openUserKey } from '@chat-app/shared/crypto';
+import type { KdfParams } from '@chat-app/shared/crypto';
+import sodium from 'libsodium-wrappers-sumo';
+
+import { createLibsodiumBackend } from '../lib/cryptoBackend';
+
+export interface OpenUserKeyInput {
+  sealed: Uint8Array;
+  pin: string;
+  salt: Uint8Array;
+  kdfParams: KdfParams;
+}
+
+export interface OpenUserKeyResult {
+  privateKey: Uint8Array;
+}
+
+type WorkerRequest = { op: 'openUserKey'; input: OpenUserKeyInput };
+type WorkerResponse =
+  | { ok: true; result: OpenUserKeyResult }
+  | { ok: false; error: string };
+
+let backendReady: Promise<void> | null = null;
+
+async function ensureBackend(): Promise<void> {
+  if (!backendReady) {
+    backendReady = (async () => {
+      await sodium.ready;
+      setCryptoBackend(await createLibsodiumBackend());
+    })();
+  }
+  return backendReady;
+}
+
+self.addEventListener('message', (ev: MessageEvent<WorkerRequest>) => {
+  const msg = ev.data;
+  void (async () => {
+    try {
+      if (!msg || msg.op !== 'openUserKey') {
+        throw new Error('unknown op: ' + String((msg as { op?: unknown })?.op));
+      }
+      await ensureBackend();
+      const privateKey = await openUserKey(msg.input);
+      const response: WorkerResponse = { ok: true, result: { privateKey } };
+      const transfers: Transferable[] = [];
+      if (privateKey?.buffer instanceof ArrayBuffer) {
+        transfers.push(privateKey.buffer);
+      }
+      (self as unknown as Worker).postMessage(response, transfers);
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err);
+      const response: WorkerResponse = { ok: false, error: message };
+      (self as unknown as Worker).postMessage(response);
+    }
+  })();
+});