[j_s]

Watch the 33c3 presentation linked elsewhere in this discussion, or just skip to solutions/Q&A: https://youtu.be/WW1-xnTIDjQ?t=35m05s

You can also read the paper: https://petsymposium.org/2017/papers/issue2/paper18-2017-2-s...

Here is the blurb from their talk:

Cross-device tracking (XDT) technologies are currently the "Holy Grail" for marketers because they allow to track the user's visited content across different devices to then push relevant, more targeted content. For example, if a user clicks on a particular advertisement while browsing the web at home, the advertisers are very interested in collecting this information to display, later on, related advertisements on other devices belonging to the same user (e.g., phone, tablet).

Currently, the most recent innovation in this area is ultrasonic cross-device tracking (uXDT), which is the use of the ultrasonic spectrum as a communication channel to "pair" devices for the aforementioned tracking purposes. Technically, this pairing happens through a receiver application installed on the phone or tablet. The business model is that users will receive rewards or useful services for keeping those apps active, pretty much like it happens for proximity-marketing apps (e.g., Shopkick), where users receive deals for walk-ins recorded by their indoor-localizing apps.

-- https://www.blackhat.com/eu-16/briefings.html#talking-behind...