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Encrypted SMS and malware scanning
(f-droid.org)
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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It works with 2G, sure, but I'm not sure how it would work on dumb phones: how messages would be decrypted or encrypted on a dumb phone, without that app?
Ah, yes, good one. (I feel dumb now, time to catch some sleep) ๐