/**

     * PBKDF2 加密函数

     * 参考标准

     * @link https://www.ietf.org/rfc/rfc2898.txt

     *

     * php官方函数将在php5.5发布

     * @see http://php.net/manual/en/function.hash-pbkdf2.php

     * example: pbkdf2("sha256", 'password', 'salt', 1, 20);

     * result:120fb6cffcf8b32c43e7 (与php5.5内置的pbkdf2函数输出一至)

     *

     * @param string $algo             The hash algorithm to use. Recommended: SHA256

     * @param string $password         The password to use for the derivation.

     * @param string $salt             The salt to use for the derivation.

     * @param int     $iterations    The number of internal iterations to perform for the derivation.

     * @param int     $length        The length of the derived key to output. If 0, the length of the supplied algorithm is used.

     * @param boolean $raw_output    When set to TRUE, outputs raw binary data. FALSE outputs lowercase hexits.

     * @return string

     */

    public static function pbkdf2($algo, $password, $salt, $iterations, $length= 0, $raw_output = false)

    {

        $algo = strtolower($algo);

        if(!in_array($algo, hash_algos(), true))

            throw new Exception('PBKDF2 ERROR: Invalid hash algorithm.');

        if($iterations <= 0)

            throw new Exception('PBKDF2 ERROR: Invalid parameters.');

        // hash方式默认长度，对一个空值进行hash计算，得出

        // length in octets of pseudorandom function output, a positive integer

        $hlen = strlen(hash($algo, null, true));

        if ($length <=0) $length = $hlen;

        if ($length > (pow(2,32)-1)*$hlen)

            throw new Exception('PBKDF2 ERROR: derived key too long.');

        // 如果生成的加密值长度低于取值长度，就将加密再执行N遍以补足数据长度

        // N=取值长度除以加密方式对应的值长度加一法取整

        $block_count = ceil($length / $hlen);

        $output = "";

        for($i = 1; $i <= $block_count; $i++) {

            // $i encoded as 4 bytes, big endian.

            $last = $salt . pack("N", $i);

            // first iteration

            $last = $xorsum = hash_hmac($algo, $last, $password, true);

            // perform the other $count - 1 iterations

            for ($j = 1; $j < $iterations; $j++) {

                $xorsum ^= ($last = hash_hmac($algo, $last, $password, true));

            }

            $output .= $xorsum;

        }

        if($raw_output)

            return substr($output, 0, $length);

        else

            return substr(bin2hex($output), 0, $length);

    }

　　/*　　
　　　　hashPassword("PBKDF2WithHmacSHA1","123456","abcdefg",1024,128);
　　*/
　　static public byte[] hashPassword(String algorithm, String password, String salt, int iterations, int len) throws Exception{

        PBEKeySpec pbeKeySpec = new PBEKeySpec(password.toCharArray(), salt.getBytes(), iterations, len);

        SecretKeyFactory kFactory=SecretKeyFactory.getInstance(algorithm);

        SecretKey secretKey = kFactory.generateSecret(pbeKeySpec);

        byte[] buff = secretKey.getEncoded();

        return buff;

    }