Switching SIMs means switching accounts, because phone numbers are usernames, right? Why would the application hide this by default? Also, why would the key ch

Switching SIMs means switching accounts, because phone numbers are usernames, right? Why would the application hide this by default? Also, why would the key change be hidden by default when the phone number doesn't change? This is the actual mechanism described by Tobias Boelter in his report: https://tobi.rocks/2016/04/whats-app-retransmission-vulnerability/ https://tobi.rocks/2016/04/whats-app-retransmission-vulnerab... Is this done to facilitate the "same SIM, lost local storage" scenario? Then the phone number is not only username but also a password and anybody able to steal the phone (or just the number) can impersonate a WhatsApp user. Is this the level of security these experts are protecting?