I am building a Blockchain-based project on supply chain management, where each user and company would have to sign up with their email addresses and will have a usual password. Each time a new record is submitted onto the Blockchain, a key-pair is issued, which determines the ownership of the right person. My question is, what is the most efficient way to make sure the user can use this application on different systems without me having to store the private keys on any centralised database(vulnerable to attack) and is able to fetch the data on that applications too?

P.S. I understand how backup works in wallets, but we're assuming a common user without much knowledge of the tech behind. Thanks in advance.