Avoid accessing undefined variable, avoid reassigning window, cleanup…

… redundant window code

src/crypt/decrypter.js

@@ -11,9 +11,7 @@ import { getSelfScope } from '../utils/get-self-scope';
 
 // see https://stackoverflow.com/a/11237259/589493
 /* eslint-disable-next-line no-undef */
-const window = getSelfScope(); // safeguard for code that might run both on worker and main thread
-
-const { performance, crypto } = window;
+const global = getSelfScope(); // safeguard for code that might run both on worker and main thread
 
 class Decrypter {
   constructor (observer, config, { removePKCS7Padding = true } = {}) {
@@ -24,7 +22,7 @@ class Decrypter {
     // built in decryptor expects PKCS7 padding
     if (removePKCS7Padding) {
       try {
-        const browserCrypto = crypto || window.crypto;
+        const browserCrypto = global.crypto;
         this.subtle = browserCrypto.subtle || browserCrypto.webkitSubtle;
       } catch (e) {}
     }