  Knock Codes are a knowledge-based unlock authentication scheme used on LG
smartphones where a user enters a code by tapping or "knocking" a sequence on a
2x2 grid. While a lesser used authentication method, as compared to PINs or
Android patterns, there is likely a large number of Knock Code users; we
estimate, 700,000--2,500,000 in the US alone. In this paper, we studied Knock
Codes security asking participants to select codes on mobile devices in three
settings: a control treatment, a blocklist treatment, and a treatment with a
larger, 2x3 grid. We find that Knock Codes are significantly weaker than other
deployed authentication, e.g., PINs or Android patterns. In a simulated
attacker setting, 2x3 grids offered no additional security, but blocklisting
was more beneficial, making Knock Codes' security similar to Android patterns.
Participants expressed positive perceptions of Knock Codes, but usability was
challenged. SUS values were "marginal" or "ok" across treatments. Based on
these findings, we recommend deploying blacklists for selecting a Knock Code
because it improves security but has limited impact on usability perceptions.
