https://techcrunch.com/2020/04/24/apple-and-google-update-joint-coronavirus-tracing-tech-to-improve-user-privacy-and-developer-flexibility/

Apple and Google have provided a number of updates about the technical details of their joint contact tracing system, which they’re now exclusively referring to as an “exposure notification” technology, since the companies say this is a better way to describe what they’re offering. The system is just one part of a contact tracing system, they note, not the entire thing. Changes include modifications made to the API that the companies say provide stronger privacy protections for individual users, and changes to how the API works that they claim will enable health authorities building apps that make use of it to develop more effective software.

The additional measures being implemented to protect privacy include changing the cryptography mechanism for generating the keys used to trace potential contacts. They’re no longer specifically bound to a 24-hour period, and they’re now randomly generated instead of derived from a so-called “tracing key” that was permanently attached to a device. In theory, with the old system, an advanced enough attack with direct access to the device could potentially be used to figure out how individual rotating keys were generated from the tracing key, though that would be very, very difficult. Apple and Google clarified that it was included for the sake of efficiency originally, but they later realized they didn’t actually need this to ensure the system worked as intended, so they eliminated it altogether.