diff --git a/lib/__tests__/plugin-host-api-limits.test.ts b/lib/__tests__/plugin-host-api-limits.test.ts
index 5952fbb49..f4a44dafe 100644
--- a/lib/__tests__/plugin-host-api-limits.test.ts
+++ b/lib/__tests__/plugin-host-api-limits.test.ts
@@ -9,7 +9,7 @@ vi.mock('@/lib/browser-navigation', async (orig) => ({
   apiFetch: (...args: unknown[]) => fetchMock(...args),
 }));
 
-import { dispatchApiCall, PLUGIN_STORAGE_QUOTA_BYTES, safeDownloadName, doGetPublicKeyFromWKD } from '../plugin-sandbox/host-api';
+import { dispatchApiCall, PLUGIN_STORAGE_QUOTA_BYTES, safeDownloadName, doGetPublicKeyFromWKD, getWkdHash } from '../plugin-sandbox/host-api';
 
 function plugin(overrides: Partial<InstalledPlugin> = {}): InstalledPlugin {
   return {
@@ -92,3 +92,10 @@ describe('WKD lookup', () => {
     fetchSpy.mockRestore();
   });
 });
+
+describe('WKD hash', () => {
+  it('matches the RFC example (Joe.Doe)', async () => {
+    // draft-koch-openpgp-webkey-service: "Joe.Doe" → iy9q119eutrkn8s1mk4r39qejnbu3n5q
+    expect(await getWkdHash('Joe.Doe')).toBe('iy9q119eutrkn8s1mk4r39qejnbu3n5q');
+  });
+});
diff --git a/lib/plugin-sandbox/host-api.ts b/lib/plugin-sandbox/host-api.ts
index d437bb2c7..19650ca89 100644
--- a/lib/plugin-sandbox/host-api.ts
+++ b/lib/plugin-sandbox/host-api.ts
@@ -31,7 +31,6 @@ import { fileStorage } from '../plugin-storage';
 import { generateUUID } from '../utils';
 import { AddressBook, ContactCard, Identity } from '../jmap/types';
 import { EncryptionAtRestConfig, PublicKeyInfo, PublicKeyInput, useAccountSecurityStore } from '@/stores/account-security-store';
-import { createHash } from 'crypto';
 
 /**
  * Methods only callable from the privileged (same-origin) tier. These expose
@@ -976,15 +975,16 @@ function zBase32Encode(buffer: Uint8Array): string {
  * Generates the WKD hash (SHA-1 in z-base-32) from the local-part.
  * RFC 9292: Lowercase in UTF-8 before hashing.
  */
-function getWkdHash(localPart: string): string {
+export async function getWkdHash(localPart: string): Promise<string> {
   // NFC normalization + lowercasing
   const normalizedLocal = localPart.normalize('NFC').toLowerCase();
-  
-  // Explicit TextEncoder usage to guarantee UTF-8 bytes
+
+  // Explicit TextEncoder usage to guarantee UTF-8 bytes. WebCrypto, not
+  // Node's crypto: importing that pulled a ~330 KB polyfill into the client.
   const utf8Bytes = new TextEncoder().encode(normalizedLocal);
-  const sha1Buffer = createHash('sha1').update(utf8Bytes).digest();
-  
-  return zBase32Encode(sha1Buffer);
+  const sha1 = await crypto.subtle.digest('SHA-1', utf8Bytes);
+
+  return zBase32Encode(new Uint8Array(sha1));
 }
 
 /**
@@ -1031,7 +1031,7 @@ export async function doGetPublicKeyFromWKD(email: string): Promise<WkdResult> {
   if (!/^[a-z0-9-]+(\.[a-z0-9-]+)+$/.test(domain)) {
     return { status: 'ERROR', error: 'Invalid email address' };
   }
-  const hash = getWkdHash(localPart);
+  const hash = await getWkdHash(localPart);
 
   const urls = [
     `https://openpgpkey.${domain}/.well-known/openpgpkey/hu/${hash}?l=${encodeURIComponent(localPart)}`,