I saw @yivi_privacybydesign in #Fdroid the other day and it's got me interested!
By using free IBM research "Idemix" (Identity Mixer) it allows selective data transfer, including zero-knowledge proofs (e.g. that you are over 18 without revealing age.)
Data storage is local only. Initial app development from Radboud Uni Nijmegen, a foundation now owns the brand, and a company continues development...
What say ye fedi?