Knock, Knock. Who's There? On the Security of LG's Knock Codes

TL;DR

This study evaluates the security and usability of LG's Knock Codes, revealing they are weaker than PINs and patterns, but security can be improved with blocklisting, while usability remains moderate.

Contribution

The paper provides an empirical analysis of Knock Codes security, comparing different grid sizes and blocklisting, and offers practical recommendations for enhancing security without sacrificing usability.

Findings

Knock Codes are significantly weaker than PINs and Android patterns.

Blocklisting improves Knock Code security, making it comparable to Android patterns.

Usability perceptions of Knock Codes are moderate, with marginal SUS scores.

Abstract

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.