# QuietPlace: An Ultrasound-Based Proof of Location Protocol with Strong Identities
## 1 Introduction  
Nowadays, more and more people carry their smartphones and other devices with them throughout the day. As they move from place to place, location-based services (LBS) take their location into consideration in order to influence their behavior. Although smart devices have the ability for accurate geolocation through Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) and Galileo, Wi-Fi networks, and cell towers, the location reported to the services is not validated. When there is a benefit from being at a specific location, the incentive to cheat the location-based service is created. In [1, 2] authors mention Trusted Platform Module, a tamper-resistant hardware component that makes spoofing the reported location impossible. However, this is not currently available in commercial devices and has additional cost. It is also useless if the device cannot obtain its location (i.e., due to jammed GNSS signals). A proof of location (PoL) protocol is a system that assists devices in getting a digitally signed proof of their location, with the help of nearby entities. It can either provide both the localization and the validation [3] or just validate the device's reported location obtained by other sensors [4, 5]. PoL protocols could be used in cases where location validation is required, such as location-based access control [6], location-based content access, customer rewards [1], employee or merchandise tracking, etc. Electronic voting systems could also benefit from the assurance that the voters vote in the areasthey live. On the one hand, recent works on PoL use fixed positioned sensors [7]. [PERSON] et al. [8] use omni-directional or directional propagation of radio signals, where some sensors are placed such that they receive the signal from the prover if it is inside the protected area. On the other hand, other proposed approaches avoid the utilization of fixed sensors [9] as in the work of [PERSON] et al. [10], where the use of unmanned aerial vehicles (UAVs) is examined. [PERSON] et al. propose an electronic alibi system for use in court [11]. In cases where spoofing is not of big concern, such as social network applications, validating one's location contributes to establishing trust in these networks and between users by securing the sincerity of the latter. PoL can be regarded as a technology specifically useful at network edge, as it makes it possible for the various location-aware services to associate users and devices to verifiable locations, while at the same time preserving the necessary level of privacy. At the same time, Internet of Things technologies, strongly coupled within a larger area of LBS [12], are aiming to offer services at the edge and can benefit from such PoL approaches. Limitations of GNNS--such as indoor localization, time overhead to acquire accurate coordinates, limitation for low powered IoT devices, etc.--reveal the need for additional localization services which can be provided through PoL techniques such as the one presented in the current paper. The main contribution and innovation of the paper comprises the following. At first, we extensively present the characteristics of PoL protocols. We identify the difficulty of proving a person's location (and not just of their device) and provide a solution. In the literature, this is referred to as strong identities [13]. A strong identity is a piece of information that identifies the user, is hard to forge and is bound to a PoL protocol session. We propose QuietPlace, a novel PoL protocol that is based on ultrasound and provides strong identities, proving the location of the owner of the device, without exposing though their identity. The strong identity requirement makes collusions between users impossible, even if the two devices share the same user's credentials. This is the common vulnerability of protocols as detailed in the relevant literature.