I’ve written quite a few posts about passwords, mainly focusing on poor implementations, bugs and leaks from large websites. I’ve also written on the difficulty of guessing PINs, multi-word phrases and personal knowledge questions. How hard are passwords to guess? How does guessing difficulty compare between different groups of users? How does it compare to potential replacement technologies? I’ve been working on the answers to these questions for much of the past two years, culminating in my PhD dissertation on the subject and a new paper at this year’s IEEE Symposium on Security and Privacy (Oakland) which I presented yesterday. My approach is simple: don’t assume any semantic model for the distribution of passwords (Markov models and probabilistic context-free-grammars have been proposed, amongst others), but instead learn the distribution of passwords with lots of data and use this to estimate the efficiency of an hypothetical guesser with perfect knowledge. It’s been a long effort requiring new mathematical techniques and the largest corpus of passwords ever collected for research. My results provide some new insight on the nature of password selection and a good framework for future research on authentication using human-chosen distributions of secrets. Continue reading The science of password guessing →

As any computer user already knows, passwords are a usability disaster: you are basically told to “pick something you can’t remember, then don’t write it down“, which is worse than impossible if you must also use a different password for every account. Moreover, security-wise, passwords can be shoulder-surfed, keylogged, eavesdropped, brute-forced and phished. Notable industry insiders have long predicted their demise. Over the past couple of decades, dozens of alternative schemes have been proposed. Yet here we are in 2012, still using more and more password-protected accounts every year. Why? Can’t we do any better? Don’t the suggested replacements offer any improvements?

The paper I am about to present at the IEEE Symposium on Security and Privacy in San Francisco (Oakland 2012), grown out of the “related work” section of my earlier Pico paper and written with coauthors Joe Bonneau, Cormac Herley and Paul van Oorschot, offers a structured and well-researched answer that, according to peer review, “should have considerable influence on the research community”. It offers, as its subtitle says, a framework for comparative evaluation of password replacement schemes.

We build a large 2D matrix. Across the columns we define a broad spectrum of 25 benefits that a password replacement scheme might potentially offer, starting with USABILITY benefits, such as being easy to learn, or not requiring a memory effort from the user, and SECURITY benefits, such as resilience to shoulder-surfing or to phishing. These two broad categories, and the tension between them, are relatively well-understood: it’s easy to provide more usability by offering less security and vice versa. But we also introduce a third category, DEPLOYABILITY, that measures how easy it would be to deploy the scheme on a global scale, taking into account such benefits as cost per user, compatibility with deployed web infrastructure and accessibility to people with disabilities.

Next, in the rows, we identify 35 representative schemes covering 11 broad categories, from password managers through federated authentication to hardware tokens and biometric schemes. We then carefully rate each scheme individually, with various cross-checks to preserve accuracy and consistency, assessing for each benefit whether the given scheme offers, almost offers or does not offer the benefit. The resulting colourful matrix allows readers to compare features at a glance and to recognize general patterns that would otherwise be easily missed.

Contrary to the optimistic claims of scheme authors, who often completely ignore some evaluation criteria when they assert that their scheme is a definite improvement, none of the examined schemes does better than passwords on every benefit when rated on all 25 benefits of this objective benchmark.

From the concise overview offered by the summary matrix we distil key high level insights, such as why we are still using passwords in 2012 and are probably likely to continue to do so for quite a while.

How can we make progress? It has been observed that many people repeat the mistakes of history because they didn’t understand the history book. In the field of password replacements, it looks like a good history book still needed to be written! As pointed out during peer review, our work will be a foundational starting point for further research in the area and a useful sanity check for future password replacement proposals.

An extended version of the paper is available as a tech report.