The beta version of Threema or Threema Work 2.0 for desktop stores the following data on your computer:
All data is stored on your computer in encrypted form and protected with a password of your choice that you set when linking the beta version of Threema 2.0 for desktop to your mobile device (a strong password is recommended). Each time you use the beta version of Threema 2.0 for desktop, you have to enter your password to access your chats, contacts, etc.
