I imagine a simple device with these properties:

  • compact (credit-card sized; fits in wallet)
  • independent (does not, cannot and need not connect to another computer to operate)
  • does not depend on user memory
  • biometrically authenticated (probably thumbprint)
  • driven by one or more of: chemical energy (battery), kinetic energy, solar energy

Why build such a complicate system? Complicate systems break down easier. Always settle for simple. All you need is to do is prove who you are.