When storing and accessing sensitive user-data, using the Multi-Index DB API, what considerations should be made to respect the user's privacy?
This is not doable as by the definition all data in a public blockchain is public. If possible store data on the server side and/or only hashes of data in the blockchain.
To run logic against data in a Smart Contract it's going to have to be in a visible state so achieving Privacy would fail.
I'd further question why sensitive or private data would go to chain in the first place as even if it is encrypted via asymmetric or symmetric means the key can always be leaked or cracked when we achieve processing power fast enough, ...
In the current implementation of EOSIO software, the account balances can not be made confidential. Each user's balance is stored in a table alongside their username. In addition to this, any transaction performed by a user would show the from and to fields, so that you could follow who sent tokens to another account.
A semi-workaround to this ...