diff --git a/framework/base/InvalidParamException.php b/framework/base/InvalidParamException.php
new file mode 100644
index 0000000..852546d
--- /dev/null
+++ b/framework/base/InvalidParamException.php
@@ -0,0 +1,28 @@
+<?php
+/**
+ * InvalidParamException class file.
+ *
+ * @link http://www.yiiframework.com/
+ * @copyright Copyright &copy; 2008 Yii Software LLC
+ * @license http://www.yiiframework.com/license/
+ */
+
+namespace yii\base;
+
+/**
+ * InvalidParamException represents an exception caused by invalid parameters passed to a method.
+ *
+ * @author Qiang Xue <qiang.xue@gmail.com>
+ * @since 2.0
+ */
+class InvalidParamException extends Exception
+{
+	/**
+	 * @return string the user-friendly name of this exception
+	 */
+	public function getName()
+	{
+		return \Yii::t('yii|Invalid Parameter');
+	}
+}
+
diff --git a/framework/util/PasswordHelper.php b/framework/util/PasswordHelper.php
new file mode 100644
index 0000000..5b29cc9
--- /dev/null
+++ b/framework/util/PasswordHelper.php
@@ -0,0 +1,161 @@
+<?php
+/**
+ * StringHelper class file.
+ *
+ * @link http://www.yiiframework.com/
+ * @copyright Copyright &copy; 2008 Yii Software LLC
+ * @license http://www.yiiframework.com/license/
+ */
+
+namespace yii\util;
+
+use yii\base\Exception;
+use yii\base\InvalidParamException;
+
+/**
+ * PasswordHelper provides a simple API for secure password hashing and verification.
+ *
+ * PasswordHelper uses the Blowfish hash algorithm available in many PHP runtime
+ * environments through the PHP [crypt()](http://php.net/manual/en/function.crypt.php)
+ * built-in function. As of Dec 2012 it is the strongest algorithm available in PHP
+ * and the only algorithm without some security concerns surrounding it. For this reason,
+ * PasswordHelper fails to initialize when run in and environment that does not have
+ * crypt() and its Blowfish option. Systems with the option include:
+ *
+ * 1. Most *nix systems since PHP 4 (the algorithm is part of the library function crypt(3));
+ * 2. All PHP systems since 5.3.0;
+ * 3. All PHP systems with the [Suhosin patch](http://www.hardened-php.net/suhosin/).
+ *
+ * For more information about password hashing, crypt() and Blowfish, please read
+ * the Yii Wiki article [Use crypt() for password storage](http://www.yiiframework.com/wiki/425/use-crypt-for-password-storage/)
+ * and the PHP RFC [Adding simple password hashing API](http://wiki.php.net/rfc/password_hash).
+ *
+ * PasswordHelper throws an exception if the Blowfish hash algorithm is not
+ * available in the runtime PHP's crypt() function. It can be used as follows
+ *
+ * Generate a hash from a password:
+ *
+ * ~~~
+ * $hash = PasswordHelper::hashPassword($password);
+ * ~~~
+ *
+ * This hash can be stored in a database (e.g. `CHAR(64) CHARACTER SET latin1` on MySQL). The
+ * hash is usually generated and saved to the database when the user enters a new password.
+ * But it can also be useful to generate and save a hash after validating a user's
+ * password in order to change the cost or refresh the salt.
+ *
+ * To verify a password, fetch the user's saved hash from the database (into `$hash`) and:
+ *
+ * ~~~
+ * if (PasswordHelper::verifyPassword($password, $hash) {
+ *     // password is good
+ * } else {
+ *     // password is bad
+ * }
+ * ~~~
+ *
+ * @author Tom Worster <fsb@thefsb.org>
+ * @since 2.0
+ */
+
+class PasswordHelper
+{
+	/**
+	 * Generate a secure hash from a password and a random salt.
+	 *
+	 * Uses the PHP [crypt()](http://php.net/manual/en/function.crypt.php) built-in function
+	 * with the Blowfish hash option.
+	 *
+	 * @param string $password The password to be hashed.
+	 * @param integer $cost Cost parameter used by the Blowfish hash algorithm.
+	 * The higher the value of cost,
+	 * the longer it takes to generate the hash and to verify a password against it. Higher cost
+	 * therefore slows down a brute-force attack. For best protection against brute for attacks,
+	 * set it to the highest value that is tolerable on production servers. The time taken to
+	 * compute the hash doubles for every increment by one of $cost. So, for example, if the
+	 * hash takes 1 second to compute when $cost is 14 then then the compute time varies as
+	 * 2^($cost - 14) seconds.
+	 * @throws Exception on bad password parameter or cost parameter
+	 * @return string The password hash string, ASCII and not longer than 64 characters.
+	 */
+	public static function hashPassword($password, $cost = 13)
+	{
+		$salt = static::generateSalt($cost);
+		$hash = crypt($password, $salt);
+
+		if (!is_string($hash) || strlen($hash) < 32) {
+			throw new Exception('Unknown error occurred while generating hash.');
+		}
+
+		return $hash;
+	}
+
+	/**
+	 * Verifies a password against a hash.
+	 * @param string $password The password to verify.
+	 * @param string $hash The hash to verify the password against.
+	 * @return boolean whether the password is correct.
+	 * @throws InvalidParamException on bad password or hash parameters or if crypt() with Blowfish hash is not available.
+	 */
+	public static function verifyPassword($password, $hash)
+	{
+		if (!is_string($password) || $password === '') {
+			throw new InvalidParamException('Password must be a string and cannot be empty.');
+		}
+
+		if (!preg_match('/^\$2[axy]\$(\d\d)\$[\./0-9A-Za-z]{22}/', $hash, $matches) || $matches[1] < 4 || $matches[1] > 30) {
+			throw new InvalidParamException('Hash is invalid.');
+		}
+
+		$test = crypt($password, $hash);
+		$n = strlen($test);
+		if (strlen($test) < 32 || $n !== strlen($hash)) {
+			return false;
+		}
+
+		// Use a for-loop to compare two strings to prevent timing attacks. See:
+		// http://codereview.stackexchange.com/questions/13512
+		$check = 0;
+		for ($i = 0; $i < $n; ++$i) {
+			$check |= (ord($test[$i]) ^ ord($hash[$i]));
+		}
+
+		return $check === 0;
+	}
+
+	/**
+	 * Generates a salt that can be used to generate a password hash.
+	 *
+	 * The PHP [crypt()](http://php.net/manual/en/function.crypt.php) built-in function
+	 * requires, for the Blowfish hash algorithm, a salt string in a specific format:
+	 * "$2a$", "$2x$" or "$2y$", a two digit cost parameter, "$", and 22 characters
+	 * from the alphabet "./0-9A-Za-z".
+	 *
+	 * @param integer $cost the cost parameter
+	 * @return string the random salt value.
+	 * @throws InvalidParamException if the cost parameter is not between 4 and 30
+	 */
+	protected static function generateSalt($cost = 13)
+	{
+		$cost = (int)$cost;
+		if ($cost < 4 || $cost > 30) {
+			throw new InvalidParamException('Cost must be between 4 and 31.');
+		}
+
+		// Get 20 * 8bits of pseudo-random entropy from mt_rand().
+		$rand = '';
+		for ($i = 0; $i < 20; ++$i) {
+			$rand .= chr(mt_rand(0, 255));
+		}
+
+		// Add the microtime for a little more entropy.
+		$rand .= microtime();
+		// Mix the bits cryptographically into a 20-byte binary string.
+		$rand = sha1($rand, true);
+		// Form the prefix that specifies Blowfish algorithm and cost parameter.
+		$salt = sprintf("$2y$%02d$", $cost);
+		// Append the random salt data in the required base64 format.
+		$salt .= str_replace('+', '.', substr(base64_encode($rand), 0, 22));
+		return $salt;
+	}
+}
\ No newline at end of file
diff --git a/todo.md b/todo.md
index d4bc275..c80da5a 100644
--- a/todo.md
+++ b/todo.md
@@ -23,6 +23,8 @@
 	* DateValidator: should we use CDateTimeParser, or simply use strtotime()?
 	* CompareValidator::clientValidateAttribute(): depends on CHtml::activeId()
 
+memo
+	* Minimal PHP version required: 5.3.7 (http://www.php.net/manual/en/function.crypt.php)
 ---
 
 - base