<?php
/*
    このファイルの関数は、PHPマニュアルページより拝借したものに、akiが少しだけ手を加えたものです。
    http://jp.php.net/manual/ja/function.md5.php

    オリジナル感謝 → Alexander Valyalkin
    01-Jul-2004 05:41 
    Below is MD5-based block cypher (MDC-like), which works in 128bit CFB mode.
    It is very useful to encrypt secret data before transfer it over the network.
    $iv_len - initialization vector's length.
    0 <= $iv_len <= 512
*/

/**
 * @access  public
 * @return  string
 */
function md5_encrypt($plain_text, $password, $iv_len = 16)
{
   $password = adjustPassword($password, $iv_len); // added by aki
   
   $plain_text .= "\x13";
   $n = strlen($plain_text);
   if ($n % 16) $plain_text .= str_repeat("\0", 16 - ($n % 16));
   $i = 0;
   $enc_text = get_rnd_iv($iv_len);
   $iv = substr($password ^ $enc_text, 0, 512);
   while ($i < $n) {
       $block = substr($plain_text, $i, 16) ^ pack('H*', md5($iv));
       $enc_text .= $block;
       $iv = substr($block . $iv, 0, 512) ^ $password;
       $i += 16;
   }
   return base64_encode($enc_text);
}

/**
 * @access  public
 * @return  string
 */
function md5_decrypt($enc_text, $password, $iv_len = 16)
{
   $password = adjustPassword($password, $iv_len); // added by aki
   
   $enc_text = base64_decode($enc_text);
   $n = strlen($enc_text);
   $i = $iv_len;
   $plain_text = '';
   $iv = substr($password ^ substr($enc_text, 0, $iv_len), 0, 512);
   while ($i < $n) {
       $block = substr($enc_text, $i, 16);
       $plain_text .= $block ^ pack('H*', md5($iv));
       $iv = substr($block . $iv, 0, 512) ^ $password;
       $i += 16;
   }
   return preg_replace('/\\x13\\x00*$/', '', $plain_text);
}

/**
 * @access  private
 * @return  string
 */
function get_rnd_iv($iv_len)
{
   $iv = '';
   while ($iv_len-- > 0) {
       $iv .= chr(mt_rand() & 0xff);
   }
   return $iv;
}

/**
 * $password（salt）の長さが $iv_len を超えていたら md5() した後、カットして収める
 *
 * @author  aki
 * @since   2007/07/02
 * @access  private
 * @return  string
 */
function adjustPassword($password, $iv_len)
{
    if (strlen($password) > $iv_len) {
        $password = substr(md5($password), 0, $iv_len);
    }
    return $password;
}

/******************************************/
/*
$plain_text = 'very secret string';
$password = 'very secret password';
echo "plain text is: [${plain_text}]<br />\n";
echo "password is: [${password}]<br />\n";

$enc_text = md5_encrypt($plain_text, $password);
echo "encrypted text is: [${enc_text}]<br />\n";

$plain_text2 = md5_decrypt($enc_text, $password);
echo "decrypted text is: [${plain_text2}]<br />\n";
*/

