I suppose you are asking about how app is going to handle password? Currently, Login functionality is in development. But you have private key which you can generate different public keys. Each key has its own purpose on platform some as you have said used for (posting - for posting, voting, active - internal market, transfer of Steem ,etc. owner - used to reset/update all other keys, memo - is for creating and reading memos, not fully understood this key yet, but I suppose this can be used for messaging in future). So one private key (password) used for creation of all different keys that are for specific purpose. As for app, private key will be used and it will be encrypted in app.