Some of the chat programs do ok at this.
The most general magically functional program is
which leverages social media for encrypted chat.
GPG, the hippy version of PGP, that venerable encrypt-my-email-or-whatever thingy.
The documentation is purest nerdview, but it’s worth it to push through
and learn the tricks,
such as encryption and identity verification.
And it’s worth learning to do properly, because doing it badly just exposes
you to other risks.
Pro-tip: There are many different versions of gnupg,
and it’s an ancient obsession of the internet to document it
obsessively and very badly.
Make sure that the version you see documented matches the version you have.
gpgtools: OSXGUI for GPG. Seems to integrate keychain,
which means trusting apple if you use it.
offline master key,
which also includes a HOWTO guide for running your key off a USB stick,
which is great if you don’t want to get screwed every time your laptop dies.