Encrypted String Changes Every Time (very bad!)

[eluser]gtech[/eluser]
First thing I can think of, if you use 1 way encryption (eg md5 or sha1) the encoded string is the same each time

$this->load->library('encrypt');
$msg = 'My secret message';
$encrypted_string = $this->encrypt->sha1($msg);
echo 'encrypted: '.$encrypted_string;
echo '<br />';
echo '<br />';

BUT you wont be able to decrypt the string