Among conditions we tested, a 16-character minimum with
no additional requirements provides the most entropy while
proving more usable on many measures than the strongest
alternative. Dictionary checks rule out the majority of pass-
words that can be easily cracked using heuristics, but should
use large cracking dictionaries if they are to eliminate almost
all vulnerable passwords. Unfortunately, dictionary checks
also significantly increase user frustration.
Most participants write down or otherwise store their passwords.
Interestingly, however, we find that storage is correlated
with use of higher-entropy passwords.
We identified several common misperceptions in how password
entropy is thought to be affected by users’ passwordcomposition
strategies. 1) Adding numbers to passwords is
thought to add little entropy; we found, by contrast, a lot of
entropy in numbers. 2) Dictionary checks, although otherwise
useful, add much less entropy than expected. 3) Unexpectedly,
users typically create passwords that exceed minimum
requirements, thus increasing password entropy.
We believe our results are immediately actionable for system
administrators considering how to balance security and usability
in their password-composition policies. Future work
collecting and analyzing additional empirical data could provide
even more detailed information about these tradeoffs.