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.
GCHQ web-app for encryption, anyone? CyberChef!