Text-based passwords are the most common mechanism for
authenticating humans to computer systems. To prevent users
from picking passwords that are too easy for an adversary
to guess, system administrators adopt password-composition
policies (e.g., requiring passwords to contain symbols and
numbers). Unfortunately, little is known about the relationship
between password-composition policies and the strength
of the resulting passwords, or about the behavior of users
(e.g., writing down passwords) in response to different policies.
We present a large-scale study that investigates password
strength, user behavior, and user sentiment across four
password-composition policies. We characterize the predictability
of passwords by calculating their entropy, and
find that a number of commonly held beliefs about password
composition and strength are inaccurate. We correlate our
results with user behavior and sentiment to produce several
recommendations for password-composition policies that result
in strong passwords without unduly burdening users.