Because the user's credentials travel via a number of intermediate servers, not under the control of the home institution of the user, it is important that the credentials are protected. This requirement limits the types of authentication methods that can be used. Basically there are two categories of useful authentication methods: those that use credentials in the form of some public-key mechanism with certificates and those that use so-called tunnelled authentication. Most institutions use a tunnelled authentication method that only requires server certificates. These server certificates are used to set up a secure tunnel between the mobile device and the authentication server, through which the user credentials are securely transported.