Extrapolating Formal Analysis to Uncover Attacks in Bluetooth Passkey Entry Pairing

—Bluetooth is a leading wireless communication technology used by billions of Internet of Things (IoT) devices today. Its ubiquity demands systematic security scrutiny. A key ingredient in Bluetooth security is secure pairing, which includes Numeric comparison (NC) and Passkey Entry (PE). However, most prior formal efforts have considered only NC, and PE has not yet been formally studied in depth. In this paper, we propose a detailed formal analysis of the PE protocol. In particular, we present a generic formal model, built using Tamarin, to verify the security of PE by precisely capturing the protocol behaviors and attacker capabilities. Encouragingly, it rediscovers three known attacks (confusion attacks, static passcode attacks, and reflection attacks), and more importantly also uncovers two new attacks (group guessing attacks and ghost attacks) spanning across diverse attack vectors (e.g., static variable reuse, multi-threading, reflection, human error, and compromise device). Finally, after applying fixes to each vulnerability, our model further proves the confidentiality and authentication properties of the PE protocol using an inductive base model.