In case you are having troubles logging in (e.g. in cases where you use Google method to login go Xiaomi servers), you can easily install the modified version of Zepp Life application. In this modified application, you can see the Auth Key for your bracelet in the Settings screen. Feel free to just copy it and paste this Auth Key to Mi Band Tools - Settings - Advanced - Auth Key.
Authorization key (auth key) is pairing code generated by official Xiaomi servers and no application can work correctly without it. If you want to retrieve it automatically, remove all characters from Mi Band Tools - Settings - Advanced - Mi Band Auth Key and OK. You'll be redirected to Xiaomi login form and after logging-in, your auth key will be automatically set.
Remember that you must login into your Xiaomi account if the application asks you to do so. Without this permission, we're not able to communicate with your bracelet and many features of the application won't work (e.g. vibrations, watch faces, button control, weather, ...). This one-time procedure is necessary to obtain authorization key (auth key) that is generated by official Xiaomi servers and no application can work correctly without this permission. In case you already know your auth key, you can enter it manually in Mi Band Tools - Settings - Advanced - Mi Band Auth Key.
Your credentials are sent only to Xiaomi servers and are not stored on this device! We do only store the authorization key that you can see in the settings and is not used for anything but initializing the encrypted communication channel with your bracelet. This key does not contain any of your or any other sensitive information.