AGvin

Вот юолее простой пример использования mcrypt_encrypt / mcrypt_decrypt:

<?php
function encrypt_data($key, $text){
  $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
  $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
  $encrypted_text = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_ECB, $iv);
  return $encrypted_text;
}

function decrypt_data($key, $text){
  global $encryptionkey;
  $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
  $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
  $decrypted_text = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_ECB, $iv);
  return $decrypted_text;
}

$key = 'This is a very secret key';
$text = "Meet me at 11 o'clock behind the monument.";
echo $text .':'. mb_strlen ($text)."\n";

$crypttext = encrypt_data ($key,$text);

$decrypttext = decrypt_data ($key, $crypttext) ;
echo  $decrypttext.':'. mb_strlen ($decrypttext)."\n";
var_dump($decrypttext);

В результате выполнения:

[AGvin@localhost public]$ php test.php 
Meet me at 11 o'clock behind the monument.:42
Meet me at 11 o'clock behind the monument.:64
string(64) "Meet me at 11 o'clock behind the monument.\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"

В качестве лечения, можно использовать $decrypttext= rtrim($decrypttext, '\0');

или поправить функцию decrypt_data:

<?php
function decrypt_data($key, $text){
  global $encryptionkey;
  $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
  $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
  $decrypted_text = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_ECB, $iv);
  return rtrim($decrypted_text, '\0');;
}