diff --git a/src/message.js b/src/message.js
index a5a31856..a7d6854d 100644
--- a/src/message.js
+++ b/src/message.js
@@ -243,10 +243,10 @@ Message.prototype.encrypt = function(keys, passwords) {
 export function encryptSessionKey(sessionKey, symAlgo, publicKeys, passwords) {
 
   /** Convert to arrays if necessary */
-  if (publicKeys && !Array.prototype.isPrototypeOf(publicKeys)) {
+  if (publicKeys && !util.isArray(publicKeys)) {
     publicKeys = [publicKeys];
   }
-  if (passwords && !Array.prototype.isPrototypeOf(passwords)) {
+  if (passwords && !util.isArray(passwords)) {
     passwords = [passwords];
   }
 
@@ -455,7 +455,7 @@ export function fromText(text, filename) {
  * @static
  */
 export function fromBinary(bytes, filename) {
-  if (!Uint8Array.prototype.isPrototypeOf(bytes)) {
+  if (!util.isUint8Array(bytes)) {
     throw new Error('Data must be in the form of a Uint8Array');
   }
 
diff --git a/src/util.js b/src/util.js
index 9cb1eca1..4e429b38 100644
--- a/src/util.js
+++ b/src/util.js
@@ -26,6 +26,34 @@
 import config from './config';
 
 export default {
+
+  isString: function(data) {
+    return typeof data === 'string' || String.prototype.isPrototypeOf(data);
+  },
+
+  isArray: function(data) {
+    return Array.prototype.isPrototypeOf(data);
+  },
+
+  isUint8Array: function(data) {
+    return Uint8Array.prototype.isPrototypeOf(data);
+  },
+
+  isEmailAddress: function(data) {
+    if (!this.isString(data)) {
+      return false;
+    }
+    const re = /^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
+    return re.test(data);
+  },
+
+  isUserId: function(data) {
+    if (!this.isString(data)) {
+      return false;
+    }
+    return / </.test(data) && />$/.test(data);
+  },
+
   readNumber: function (bytes) {
     var n = 0;
 
diff --git a/test/general/index.js b/test/general/index.js
index a9684c6b..b0b1312c 100644
--- a/test/general/index.js
+++ b/test/general/index.js
@@ -1,4 +1,5 @@
 describe('General', function () {
+  require('./util.js');
   require('./basic.js');
   require('./armor.js');
   require('./key.js');
diff --git a/test/general/util.js b/test/general/util.js
new file mode 100644
index 00000000..75d3354b
--- /dev/null
+++ b/test/general/util.js
@@ -0,0 +1,151 @@
+'use strict';
+
+var openpgp = typeof window !== 'undefined' && window.openpgp ? window.openpgp : require('../../dist/openpgp');
+
+var chai = require('chai'),
+  expect = chai.expect;
+
+describe('Util unit tests', function() {
+
+  beforeEach(function() {});
+
+  afterEach(function() {});
+
+  describe('isString', function() {
+    it('should return true for type "string"', function() {
+      var data = 'foo';
+      expect(openpgp.util.isString(data)).to.be.true;
+    });
+    it('should return true for type String', function() {
+      var data = String('foo');
+      expect(openpgp.util.isString(data)).to.be.true;
+    });
+    it('should return true for inherited type of String', function() {
+      function MyString() {}
+      MyString.prototype = Object.create(String.prototype);
+      var data = new MyString();
+      expect(openpgp.util.isString(data)).to.be.true;
+    });
+    it('should return true for empty string', function() {
+      var data = '';
+      expect(openpgp.util.isString(data)).to.be.true;
+    });
+    it('should return false for undefined', function() {
+      var data;
+      expect(openpgp.util.isString(data)).to.be.false;
+    });
+    it('should return false for Object', function() {
+      var data = {};
+      expect(openpgp.util.isString(data)).to.be.false;
+    });
+  });
+
+  describe('isArray', function() {
+    it('should return true for []', function() {
+      var data = [];
+      expect(openpgp.util.isArray(data)).to.be.true;
+    });
+    it('should return true for type Array', function() {
+      var data = Array();
+      expect(openpgp.util.isArray(data)).to.be.true;
+    });
+    it('should return true for inherited type of Array', function() {
+      function MyArray() {}
+      MyArray.prototype = Object.create(Array.prototype);
+      var data = new MyArray();
+      expect(openpgp.util.isArray(data)).to.be.true;
+    });
+    it('should return false for undefined', function() {
+      var data;
+      expect(openpgp.util.isArray(data)).to.be.false;
+    });
+    it('should return false for Object', function() {
+      var data = {};
+      expect(openpgp.util.isArray(data)).to.be.false;
+    });
+  });
+
+  describe('isUint8Array', function() {
+    it('should return true for type Uint8Array', function() {
+      var data = new Uint8Array();
+      expect(openpgp.util.isUint8Array(data)).to.be.true;
+    });
+    it('should return true for inherited type of Uint8Array', function() {
+      function MyUint8Array() {}
+      MyUint8Array.prototype = new Uint8Array();
+      var data = new MyUint8Array();
+      expect(openpgp.util.isUint8Array(data)).to.be.true;
+    });
+    it('should return false for undefined', function() {
+      var data;
+      expect(openpgp.util.isUint8Array(data)).to.be.false;
+    });
+    it('should return false for Object', function() {
+      var data = {};
+      expect(openpgp.util.isUint8Array(data)).to.be.false;
+    });
+  });
+
+  describe('isEmailAddress', function() {
+    it('should return true for valid email address', function() {
+      var data = 'test@example.com';
+      expect(openpgp.util.isEmailAddress(data)).to.be.true;
+    });
+    it('should return false for invalid email address', function() {
+      var data = 'Test User <test@example.com>';
+      expect(openpgp.util.isEmailAddress(data)).to.be.false;
+    });
+    it('should return false for invalid email address', function() {
+      var data = 'test@examplecom';
+      expect(openpgp.util.isEmailAddress(data)).to.be.false;
+    });
+    it('should return false for invalid email address', function() {
+      var data = 'testexamplecom';
+      expect(openpgp.util.isEmailAddress(data)).to.be.false;
+    });
+    it('should return false for empty string', function() {
+      var data = '';
+      expect(openpgp.util.isEmailAddress(data)).to.be.false;
+    });
+    it('should return false for undefined', function() {
+      var data;
+      expect(openpgp.util.isEmailAddress(data)).to.be.false;
+    });
+    it('should return false for Object', function() {
+      var data = {};
+      expect(openpgp.util.isEmailAddress(data)).to.be.false;
+    });
+  });
+
+  describe('isUserId', function() {
+    it('should return true for valid user id', function() {
+      var data = 'Test User <test@example.com>';
+      expect(openpgp.util.isUserId(data)).to.be.true;
+    });
+    it('should return false for invalid user id', function() {
+      var data = 'Test User test@example.com>';
+      expect(openpgp.util.isUserId(data)).to.be.false;
+    });
+    it('should return false for invalid user id', function() {
+      var data = 'Test User <test@example.com';
+      expect(openpgp.util.isUserId(data)).to.be.false;
+    });
+    it('should return false for invalid user id', function() {
+      var data = 'Test User test@example.com';
+      expect(openpgp.util.isUserId(data)).to.be.false;
+    });
+    it('should return false for empty string', function() {
+      var data = '';
+      expect(openpgp.util.isUserId(data)).to.be.false;
+    });
+    it('should return false for undefined', function() {
+      var data;
+      expect(openpgp.util.isUserId(data)).to.be.false;
+    });
+    it('should return false for Object', function() {
+      var data = {};
+      expect(openpgp.util.isUserId(data)).to.be.false;
+    });
+  });
+
+});