Abstract or Summary:
Challenge questions are commonly used as a backup should users forget their "main" authentication secret. Such questions are notoriously difficult to design properly, and have sometimes allowed intruders to access the system via a back door simply by engaging in some online research about the victim [33]. Most challenge questions rely on a user's knowledge of their early life, something which tends not to deteriorate over time [15]. Unfortunately, this kind of information can also be discovered by a determined attacker.

We developed a challenge protocol in which a set of pictorial cues are used to prompt answers, rather than using the standard mechanism based on textual questions. The prompts solicit associative memories that need not represent factual information (information that aids an attacker in mounting targeted observation attacks) and serve as a stronger cue to aid the recall.

Our results reveal that the solution has comparable security with that of traditional challenge questions (when considering external attackers), and suggests additional benefits from posing three or more questions serially. Furthermore, we obtained a 13% increase in the memorability of our (name-based) answers, while our results suggest enhancements could help improve the recall of place-based answers. We conclude by discussing how further modifications could achieve gains on the usability front.