Biblio

Abstract Despite decades of research into developing abstract security advice and improving interfaces, users still struggle to make passwords. Users frequently create passwords that are predictable for attackers [1, 9] or make other decisions (e.g., reusing the same password across accounts) that harm their security [2, 8]. In this thesis,1 I use data-driven methods to better understand how users choose passwords and how attackers guess passwords. I then combine these insights into a better password-strength meter that provides real-time, data-driven feedback about the user's password. I first quantify the impact on password security and usability of showing users different password-strength meters that score passwords using basic heuristics. I find in a 2,931- participant online study that meters that score passwords stringently and present their strength estimates visually lead users to create stronger passwords without significantly impacting password memorability [6]. Second, to better understand how attackers guess passwords, I perform comprehensive experiments on password-cracking approaches. I find that simply running these approaches in their default configuration is insufficient, but considering multiple well-configured approaches in parallel can serve as a proxy for guessing by an expert in password forensics [9]. The third and fourth sections of this thesis delve further into how users choose passwords. Through a series of analyses, I pinpoint ways in which users structure semantically significant content in their passwords [7]. I also examine the relationship between users' perceptions of password security and passwords' actual security, finding that while users often correctly judge the security impact of individual password characteristics, wide variance in their understanding of attackers may lead users to judge predictable passwords as sufficiently strong [5]. Finally, I integrate these insights into an open-source2 password-strength meter that gives users data-driven feedback about their specific password. This meter uses neural networks [3] and numerous carefully combined heuristics to score passwords and generate data-driven text feedback about a given password. I evaluate this meter through a ten-participant laboratory study and 4,509-participant online study [4]. Under the more common password-composition policy we tested, we find that the data-driven meter with detailed feedback leads users to create more secure, and no less memorable, passwords than a meter with only a bar as a strength indicator. In sum, the objective of this thesis is to demonstrate how integrating data-driven insights about how users create and how attackers guess passwords into a tool that presents real-time feedback can help users make better passwords.

Despite their ubiquity, many password meters provide inaccurate strength estimates. Furthermore, they do not explain to users what is wrong with their password or how to improve it. We describe the development and evaluation of a data-driven password meter that provides accurate strength measurement and actionable, detailed feedback to users. This meter combines neural networks and numerous carefully combined heuristics to score passwords and generate data-driven text feedback about the user's password. We describe the meter's iterative development and final design. We detail the security and usability impact of the meter's design dimensions, examined through a 4,509-participant online study. Under the more common password-composition policy we tested, we found that the data-driven meter with detailed feedback led users to create more secure, and no less memorable, passwords than a meter with only a bar as a strength indicator.