Warning! I no longer support this plugin after version 3.5.9.9! Please ask questions on wordpress support page.

This article is based on this one and the latest version of plugin. Now you can easily encrypt your links without changing plugin code.

1) Ensure that you have php-mcrypt installed:

For CentOS, type

sudo yum install php-mcrypt

For ubuntu, type

sudo apt-get install php5-mcrypt

For windows, enable this extension in php.ini:

extension=php_mcrypt.dll

2) Create file wp-content/uploads/custom-parser.php and add here the following:

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<?php
if(!defined('DB_NAME'))
  die('Error: Plugin "wp-noexternallinks" does not support standalone calls, damned hacker.');
 
#include base parser
include_once(ABSPATH . 'wp-content/plugins/wp-noexternallinks/wp-noexternallinks-parser.php');
 
class custom_parser extends wp_noexternallinks_parser
{
  var $salt,$iv;
  function init_crypt()
  {
    if(!function_exists('mcrypt_create_iv'))
      die('php-mcrypt library not installed, can not process links!');
    $this->salt ='ms8-sRt-kBs5-s1wQ';
    $this->iv=mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB), MCRYPT_RAND);
  }
  function encode_link($url)
  {
    if(!$this->salt)
      $this->init_crypt();
    return trim(base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $this->salt, $url, MCRYPT_MODE_ECB, $this->iv)));
  }
  function decode_link($url)
  { 
    if(!$this->salt)
      $this->init_crypt();
    return trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $this->salt, base64_decode($url), MCRYPT_MODE_ECB, $this->iv));
  }
}
?>

Do not forget to change $salt to any custom value! Also, add statistic handling from the original function if you use statistics.