The idea is that nobody ever shares who they were close to. Think of it as walking around in a mask, then if you were sick you opt in to being on a list so ever

The idea is that nobody ever shares who they were close to. Think of it as walking around in a mask, then if you were sick you opt in to being on a list so everyone knows they may have been exposed if they saw someone in "a pink fox mask." Your identity is pseudorandomly generated and cycles every 15 minutes, so you won't be identifiable/trackable - until you choose to share you were sick, and release the inputs to the KDF publicly. A third party app determines who can share they were sick, but the OS appears to require user consent before your information can be shared. Even then, the people who did see you haven't shared any information. The biggest vector of privacy abuse IMHO is that once you have opted in to letting an app check whether you had contact with an affected person, the app is responsible for behavior - informing you, helping you schedule testing, or potentially more abusive behavior like informing you and the state of a mandatory quarantine.