The data stored in Polkadot Multisig is protected by your Polkadot wallet. This provides a secure authentication method in which you need to sign a message using your wallet in order to log into Polkadot Multisig.
We developed the Sign-in with Substrate (SIWS) library to handle wallet authentication and secure storage without needing to resort to a centralized identity provider like Google. The library was funded for release by a Web3 Foundation Grant.
If you are a dapp developer and would like to integrate SIWS into your own app, please check out the documentation at https://siws.xyz, or the intro article linked below.
Sign-in with Substrate (SIWS) offers a standardized way for Substrate users to use their keys to log in and access off-chain applications and data stores. This enables users to store and access their own data in Web2-style apps, but do so with a self-custodial identity, meaning users do not need to rely on a centralized identity provider such as those which are generally required for Web2 today.
From the article "Sign-in with Substrate: Enabling a new world of self-custodial identity-based applications and services for Polkadot"