  In this paper, we provide the first comprehensive study of user-chosen 4- and
6-digit PINs (n=1220) collected on smartphones with participants being
explicitly primed for device unlocking. We find that against a throttled
attacker (with 10, 30, or 100 guesses, matching the smartphone unlock setting),
using 6-digit PINs instead of 4-digit PINs provides little to no increase in
security, and surprisingly may even decrease security. We also study the
effects of blocklists, where a set of "easy to guess" PINs is disallowed during
selection. Two such blocklists are in use today by iOS, for 4-digits (274 PINs)
as well as 6-digits (2910 PINs). We extracted both blocklists compared them
with four other blocklists, including a small 4-digit (27 PINs), a large
4-digit (2740 PINs), and two placebo blocklists for 4- and 6-digit PINs that
always excluded the first-choice PIN. We find that relatively small blocklists
in use today by iOS offer little or no benefit against a throttled guessing
attack. Security gains are only observed when the blocklists are much larger,
which in turn comes at the cost of increased user frustration. Our analysis
suggests that a blocklist at about 10% of the PIN space may provide the best
balance between usability and security.
