It's best to use one of the standard callbacks to intercept a save (or create, or update) and change something to an attribute.

My code for a login system in which an password entered by a new user is encrypted before being saved to the database: in the model:

function beforeCreate()
{
  $this->encryptPassword();
  return true;
}

function encryptPassword()
{
   $this->set('password_encrypted', sha1($this->get('password')));
}

But in this example the encrypted password is kept in another attribute (password_encrypted) than the original (password)

More info on the Active Record callbacks in the API docs