Either use a key on a USB device or enter a (long!) password. There are much more clever solutions (two-factor authentication involving sending an SMS to the mo

Either use a key on a USB device or enter a (long!) password. There are much more clever solutions (two-factor authentication involving sending an SMS to the mobile phone of the account holder, "delegate" keys that are valid for a limited amount of time, etc), but even the basic stuff works reasonably well.