The problem is enabling the user to keep track of the key. These two solutions spring to mind: * Password managers * Physical objects that hold the key (credi

The problem is enabling the user to keep track of the key. These two solutions spring to mind: * Password managers * Physical objects that hold the key (credit cards, access cards) Or, for a non-solution: * Just get angry at anyone who forgets their password, while also insisting they never write it down (a common approach in the bad old days)