I'm currently developing an Android (and probably iOS in the future) application for my company.
I was wondering what is the most unique data identifier to authenticate the users. A data that cannot be repeated through users. For example:
Email? That user can log in with another phone using the email and password
Phone number? Could be the most unique one but it would required to verify the phone and I will have to setup a SMS validation service like WhatsApp
IMEI? It pretty much validates the unique phone but it can be spoofed or replaced. Although I don't know if the application required permissions for this.
Maybe a mix of all this methods?
My main goal is to save this data as a database and make it the primary key of it and with this know exactly who's the user that it's really using the company web services.