How Proof of Location Works
Explanation of how 360-degree challenger selection improves accuracy
Last updated
Explanation of how 360-degree challenger selection improves accuracy
Last updated
Location verification is a critical component in many modern applications, from supply chain tracking to decentralized finance. Traditional methods often rely on trusted hardware or centralized authorities, creating single points of failure. Witness Chain's innovative Proof of Location protocol takes a fundamentally different approach, leveraging network physics to provide robust location verification without specialized hardware.
At the heart of Witness Chain's system is a remarkably elegant concept: internet delay can serve as a reliable proxy for physical distance. The protocol works through these key steps:
A network of geographically distributed challengers send signed UDP packets to a prover claiming to be at a specific location
Both parties cryptographically sign these packets, creating an immutable record of the interaction
The measured network delay is converted to an estimated physical distance
By combining measurements from multiple challengers, the system triangulates the prover's actual location
This approach provides strong guarantees against manipulation while requiring no specialized hardware on the prover's side.
The critical innovation in Witness Chain's approach is how challengers are calibrated. Rather than using theoretical models, each challenger builds an empirical delay-distance curve based on real-world measurements with other known challengers to ensure guarantees against Byzantine behaviour by the prover.
What makes this approach particularly powerful is the use of monotone curves - as network delay increases, the estimated distance never decreases. This mathematical property enables the system to provide concrete location guarantees even when facing adversarial provers attempting to manipulate the system.
For calibration, each challenger measures Internet delays with respect to other challengers whose locations are known. Thus a challenger has a series of delay-distance points as shown in Fig. 1 from its measurements. From these measurements a challenger computes a monotone curve shown by the solid line in Fig. 1. The monotone curve has the property that with increasing delay the distance is non-decreasing. This monotone curve enables us to offer location guarantees in case of byzantine prover as illustrated in [1].
Once each challenger has estimated the prover's distance, Witness Chain employs geometric principles to verify location claims:
Each challenger creates a probability circle with radius equal to its estimated distance
The prover must be located within the intersection of all these circles
The "Location Uncertainty" is defined as the maximum possible distance between the claimed location and the edge of this intersection area
This approach provides a mathematically rigorous bound on location accuracy rather than just a best guess.
After each challenger estimates a distance for the delay measured to the prover, we can compute the final location of the prover as follows. As we use a monotone delay-distance curve for the challenger, the prover will be within the circle of radius equal to estimated distance of the challenger centred at the location of the challenger as shown in Fig. 2.
Accordingly, if we aggregate across the multiple challengers, the prover will be within the intersection of all the circles from all the challengers. Thus the farthest distance the prover can be from its claimed location is maximum distance between the prover and the periphery of the intersection area, marked as “Location Uncertainty” in Fig. 2. This location uncertainty is the guarantee of our proof of location protocol. For more technical details refer [1].
Witness Chain's global network of challengers enables a powerful optimization: strategic challenger selection. By choosing challengers distributed in a 360-degree pattern around the prover's claimed location, the system dramatically reduces the size of the intersection area and consequently the location uncertainty.
This strategic selection represents a significant improvement over random challenger assignment, substantially enhancing location verification accuracy without requiring additional infrastructure, as explained in Fig 3
This technology has far-reaching implications for applications requiring trustworthy location verification:
Decentralized finance protocols with location-dependent features
Proof of presence for events or activities / geo-fenced marketing campaigns.
Geographic access control for sensitive services
As Witness Chain's challenger network continues to expand globally, the protocol's accuracy and resilience will only improve, opening new possibilities for location-verified applications across industries.
