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Since GnuPG is available for Android, why does no one makes an app that encrypts every message using an RSA key and just swaps the keys with each "friend", so for example:

  • John has a limit of 100 friends.
  • John has 1 RSA public key.
  • Only John has the secret key.

People who would message John would only get his public key, therefore when they write to John, their message will be encrypted using John's public RSA key. When a message arrives, John's cellphone decrypts every message using his secret key. Keys will be stored on each cellphone and generated at their cellphone.

Please enlighten me.

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    Short story: people have (e.g., TextSecure). Long story: these haven't really taken off, because the average person either doesn't know how to choose between systems that claim to protect their communications, doesn't care all that much about the effectiveness of those protections, or both.
    – Stephen Touset
    Aug 5, 2014 at 20:49
  • Relevant update: there are now plenty of these apps (though they mostly don't use RSA; elliptic curve crypto for signing and key exchange is much better). Signal (evolution of TextSecure) is the gold standard, and its protocol has been adopted by WhatsApp and Facebook Messenger, but there are other apps offering zero-trust end-to-end encryption as well (though few if any require pre-sharing the public key, preferring instead the ratcheting algorithm that Signal uses).
    – CBHacking
    Jul 18, 2022 at 9:50

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Telegram is a texting app whose "secret chats" supposedly run on public key encryption or at least Diffie-Hellman key exchange to use encryption. Other than that, I don't really know about something similar that has actually taken off.

The problem with encryption is that your ordinary person still doesn't consider themselves affected by the massive number of parties who might want their data, whether they are governments, corporations or independent individuals. They think having an honest, law-abiding lifestyle is a total safeguard from mass surveillance. I'm just a 9-to-5 office worker, why should I be affected? This is why your ordinary layman is not concerned about crypto. Admittedly, however, the NSA mass surveillance scandal, and its secondary exposure of other mass surveillance programs like Google or Facebook's, have resulted in laypeople taking privacy and security much more seriously. For example, when Facebook purchased WhatsApp, there was a mass migration of people from WhatsApp to Telegram.

There is, however, a second problem associated with crypto: people view it as hard, arcane and highly academic stuff, like rocket science or quantum mechanics. Just look at how wrong can Hollywood get crypto. Therefore, people just don't use it. Even with tools that simplify using cryptosystems as much as TrueCrypt, GPA and Enigmail on computers, or APG on smartphones, people still think crypto is hard to use by an end user and therefore they don't use it. It really doesn't help that the user himself must take measures to make sure crypto will actually work. When people share passwords as a ritual of love and write them on a Post-it on the monitor, you have large numbers of people who might as well be sending absolutely everything in plaintext.

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TextSecure (FOSS), Wickr, Telegram, ChatSecure (Gibberbot, previously) (FOSS), Threema,

Just to name a few of the big names.

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