The objective of this thesis is to investigate through objective metrics the usability of a new system-generated authentication mechanism, based on the concept of implicit memory, with the purpose to overcome the unreliable subjective human component in both authentication and usability evaluation. The first part of this work aims to analyse the state of the art of authentication systems, presenting their advantages, disadvantages and usability issues. Particular attention is given to system-generated authentication mechanisms, which permits to address the human related vulnerabilities (e.g. predictability) and to provide a greater degree of security. In this context, novel system-generated authentication mechanisms, based on the concept of implicit memory, are examined.