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


Raina Samuel, New Jersey Institute of Technology; Philipp Markert, Ruhr University Bochum; Adam J. Aviv, The George Washington University; Iulian Neamtiu, New Jersey Institute of Technology


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 in an online study 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. Blocklisting, on the other hand, was more beneficial, making Knock Codes' security similar to Android patterns. Participants expressed positive perceptions of Knock Codes, yet usability was challenged. SUS values were "marginal" or "ok" across treatments. Based on these findings, we recommend deploying blocklists for selecting a Knock Code because they improve security but have a limited impact on usability perceptions.

Open Access Media

USENIX is committed to Open Access to the research presented at our events. Papers and proceedings are freely available to everyone once the event begins. Any video, audio, and/or slides that are posted after the event are also free and open to everyone. Support USENIX and our commitment to Open Access.

@inproceedings {255654,
author = {Raina Samuel and Philipp Markert and Adam J. Aviv and Iulian Neamtiu},
title = {Knock, Knock. {Who{\textquoteright}s} There? On the Security of {LG{\textquoteright}s} Knock Codes},
booktitle = {Sixteenth Symposium on Usable Privacy and Security (SOUPS 2020)},
year = {2020},
isbn = {978-1-939133-16-8},
pages = {37--59},
url = {},
publisher = {USENIX Association},
month = aug

Presentation Video