Threema offers that. You do a QR code scan, after which a contact is marked as verified (3 green dots). Since Threema has a fixed key per user, these verificati

Threema offers that. You do a QR code scan, after which a contact is marked as verified (3 green dots). Since Threema has a fixed key per user, these verifications are persistent and most people transfer their keys when switching to a new phone.