diff --git a/lib/crypto.browser.js b/lib/crypto.browser.js
index d2fd33f..dc78860 100644
--- a/lib/crypto.browser.js
+++ b/lib/crypto.browser.js
@@ -7,16 +7,20 @@
  *
  * @module bitmessage/crypto.browser
  */
-// FIXME(Kagami): Support webkit subtle prefix!
-// TODO(Kagami): Try to support IE11.
 
 "use strict";
 
 require("es6-promise").polyfill();
 var ripemd160 = require("ripemd160");
+var assert = require("./utils").assert;
+
+// Support `webkit` prefix for Safari (not tested yet).
+// TODO(Kagami): Try to support IE11.
+var subtle = window.crypto.subtle || window.crypto.webkitSubtle;
+assert(subtle, "WebCryptoAPI is not supported");
 
 exports.sha512 = function(buf) {
-  return window.crypto.subtle.digest({name: "SHA-512"}, buf).then(function(arr) {
+  return subtle.digest({name: "SHA-512"}, buf).then(function(arr) {
     return new Buffer(new Uint8Array(arr));
   });
 };