crypto: use subtle for browser pbkdf2.

lib/crypto/backend-browser.js

@@ -12,15 +12,20 @@ var assert = require('assert');
 var hashjs = require('hash.js');
 var aes = require('./aes');
 var backend = exports;
-var crypto, global;
+var global, crypto, subtle;
 
-if (typeof window !== 'undefined')
-  global = window;
-else if (typeof self !== 'undefined')
-  global = self;
+global = (function() {
+  if (typeof window !== 'undefined')
+    return window;
 
-if (global)
-  crypto = global.crypto || global.msCrypto;
+  if (typeof self !== 'undefined')
+    return self;
+
+  throw new Error('No global found.');
+})();
+
+crypto = global.crypto || global.msCrypto;
+subtle = crypto ? crypto.subtle : null;
 
 backend.hash = function hash(alg, data) {
   return new Buffer(hashjs[alg]().update(data).digest());
@@ -39,7 +44,30 @@ backend.hmac = function hmac(alg, data, salt) {
 
 backend.pbkdf2 = null;
 
-backend.pbkdf2Async = null;
+backend.pbkdf2Async = function pbkdf2Async(key, salt, iter, len, alg) {
+  var algo = { name: 'PBKDF2' };
+  var use = ['deriveBits'];
+  var length = len * 8;
+  var options, promise;
+
+  options = {
+    name: 'PBKDF2',
+    salt: salt,
+    iterations: iter,
+    hash: getHash(alg)
+  };
+
+  promise = subtle.importKey('raw', key, algo, false, use);
+
+  return promise.then(function(key) {
+    return subtle.deriveBits(options, key, length);
+  }).then(function(result) {
+    return new Buffer(result);
+  });
+};
+
+if (!subtle || !subtle.importKey || !subtle.deriveBits)
+  backend.pbkdf2Async = null;
 
 backend.encipher = function encipher(data, key, iv) {
   return aes.cbc.encrypt(data, key, iv);
@@ -71,3 +99,18 @@ if (!crypto || !crypto.getRandomValues) {
     return data;
   };
 }
+
+function getHash(name) {
+  switch (name) {
+    case 'sha1':
+      return 'SHA-1';
+    case 'sha256':
+      return 'SHA-256';
+    case 'sha384':
+      return 'SHA-384';
+    case 'sha512':
+      return 'SHA-512';
+    default:
+      throw new Error('Unknown hash.');
+  }
+}