"The Same PIN, Just Longer": On the (In)Security of Upgrading PINs from 4 to 6 Digits

Authors: 

Collins W. Munyendo, The George Washington University; Philipp Markert, Ruhr University Bochum; Alexandra Nisenoff, University of Chicago; Miles Grant and Elena Korkes, The George Washington University; Blase Ur, University of Chicago; Adam J. Aviv, The George Washington University

Abstract: 

With the goal of improving security, companies like Apple have moved from requiring 4-digit PINs to 6-digit PINs in contexts like smartphone unlocking. Users with a 4-digit PIN thus must "upgrade" to a 6-digit PIN for the same device or account. In an online user study (n=1010), we explore the security of such upgrades. Participants used their own smartphone to first select a 4-digit PIN. They were then directed to select a 6-digit PIN with one of five randomly assigned justifications. In an online attack that guesses a small number of common PINs (10–30), we observe that 6-digit PINs are, at best, marginally more secure than 4-digit PINs. To understand the relationship between 4- and 6-digit PINs, we then model targeted attacks for PIN upgrades. We find that attackers who know a user's previous 4-digit PIN perform significantly better than those who do not at guessing their 6-digit PIN in only a few guesses using basic heuristics (e.g., appending digits to the 4-digit PIN). Participants who selected a 6-digit PIN when given a "device upgrade" justification selected 6-digit PINs that were the easiest to guess in a targeted attack, with the attacker successfully guessing over 25% of the PINs in just 10 attempts, and more than 30% in 30 attempts. Our results indicate that forcing users to upgrade to 6-digit PINs offers limited security improvements despite adding usability burdens. System designers should thus carefully consider this tradeoff before requiring upgrades.

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BibTeX
@inproceedings {279940,
author = {Collins W. Munyendo and Philipp Markert and Alexandra Nisenoff and Miles Grant and Elena Korkes and Blase Ur and Adam J. Aviv},
title = {"The Same {PIN}, Just Longer": On the ({In)Security} of Upgrading {PINs} from 4 to 6 Digits},
booktitle = {31st USENIX Security Symposium (USENIX Security 22)},
year = {2022},
isbn = {978-1-939133-31-1},
address = {Boston, MA},
pages = {4023--4040},
url = {https://www.usenix.org/conference/usenixsecurity22/presentation/munyendo},
publisher = {USENIX Association},
month = aug,
}