Kryptering/Dekryptering

Hoy!

Hittade denna lilla kodsnutt på PHP.NET

<?php

  function bytexor($a,$b,$l)
  {
   $c="";
   for($i=0;$i<$l;$i++) {
    $c.=$a{$i}^$b{$i};
    
   }
   return($c);
  }

  function binmd5($val)
  {
   return(pack("H*",md5($val)));
  }

  function decrypt_md5($msg,$heslo)
  {
   $key=$heslo;$sifra="";
   $key1=binmd5($key);
   while($msg) {
     $m=substr($msg,0,16);
     $msg=substr($msg,16);
     $sifra.=$m=bytexor($m,$key1,16);
     $key1=binmd5($key.$key1.$m);
   }
   echo "\n";
   return($sifra);
  }

  function crypt_md5($msg,$heslo)
  {
   $key=$heslo;$sifra="";
   $key1=binmd5($key);
   while($msg) {
     $m=substr($msg,0,16);
     $msg=substr($msg,16);
     $sifra.=bytexor($m,$key1,16);
     $key1=binmd5($key.$key1.$m);
   }
   echo "\n";
   return($sifra);
  }

// Example of usage...

$message = "This is a very long message, but it is very secret and important 
and we need to keep the contents hidden from nasty people who might want to steal it.";

$key = "secret key";

$crypted = crypt_md5($message, $key);
echo "Encoded = $crypted<BR>"; // returns ¦ý¼=¯ ¶òºÏ`¬ù<ÂH _ëÇ{.‡1º{ïåÉ‘Ñ’JÞV£+ß³jŠeÎ

$uncrypted = decrypt_md5($crypted, $key);
echo "Unencoded = $uncrypted"; // returns This is a very long message (etc)

?>

Den kodar strängarna som den ska, men jag får detta delmeddelande:

Notice: Uninitialized string offset: 8 in c:\inetpub\wwwroot\kryptering.php on line 7

Har försökt deklarera variablerna, men får fel ändå....