Shared key on the Android platform

Question

The platform has several keys that sign apps during build time (platform, shared, media. etc.) one of these keys is the shared key.

For what purpose do apps like Dialer ,Contacts and Launcher need to be signed with the same key? (Launcher and Contacts , for example, don't have a sharedUserId so I don't see why not use different keys for each)

Answer 1

Shared key was used at the time of Android cupcake, only because, at that time there were low-end devices, which required running home (i.e Launcher app) across process with Contacts apps in same process.

But using LOCAL_CERTIFICATE := shared in Android.mk only really matters if the shared user id is set in Manifest.xml, otherwise launcher will still get its own uid.

Also There's no other reason for using shared key between Launcher, Contacts app,