Encrypt and URL disallowed characters - alphanumeric option?

[eluser]antonumia[/eluser]
I am constructing an autologin process whereby restricted users on a site receive an email with a url containing controller/function/encrypted_name/encrypted_password.


I want to avoid allowing more characters in $config['permitted_uri_chars']

Is there a way to restrict the encryption to alphanumeric characters only?

thanks,

anton