We could also use the web of trust. An app already downloaded by people you trust is maybe-probably more secure. Google Play Store could allow third party audi

We could also use the web of trust. An app already downloaded by people you trust is maybe-probably more secure. Google Play Store could allow third party audit badges on the apps store page, so different agencies could give an official rating or an approbation level for the app. Basically a "verified" badge ala FB and Twitter, but more organizations than just the app-store owner could give their checks. You would have "Google-approved" badge, "Open-source-approved" badge, "Mozilla-approved" badge, and even "Government-approved" badge. Then you let the user makes his/her own decision based on what entities or friends he/she trusts.