From Very Weak to Very Strong: Analyzing Password-Strength Meters

Millions of users are exposed to password-strength meters/checkers at highly popular web services that use userchosen passwords for authentication. Recent studies have found evidence that some meters actually guide users to choose better passwords—which is a rare bit of good news in password research. However, these meters are mostly based on ad-hoc design. At least, as we found, most vendors do not provide any explanation of their design choices, sometimes making them appear to be a black box. We analyze password meters deployed in selected popular websites, by measuring the strength labels assigned to common passwords from several password dictionaries. From this empirical analysis with millions of passwords, we report prominent characteristics of meters as deployed at popular websites. We shed light on how the server-end of some meters functions, provide examples of highly inconsistent strength outcomes for the same password in different meters, along with examples of many weak passwords being labeled as strong or even very strong. These weaknesses and inconsistencies may confuse users in choosing a stronger password, and thus may weaken the purpose of these meters. On the other hand, we believe these findings may help improve existing meters, and possibly make them an effective tool in the long run.