I put this PGP app on my phone yesterday and it integrates real well on K9 Mail for encrypted messages on an insecure medium:
https://f-droid.org/en/packages/com.pgpony.android/
This is their main site:
Is this any different than using Proton Mail as that is what I been using for years?
Proton offers something called external pgp or external encryption or some might call it password protected email for sending to non Proton people.
Yeah, this is just a new spin on an old technology. PGP encryption started in 2004 but has made advances. This app gives u 4 ways of encrypting messages and it can work off a Yubico NFC card they say (I didn’t get this part working). I did get it working off my phone and u can cut and paste messages into their app.
I also have protonmail and I think for the possible non-tech recipient that it is easier to have them reply to a password encrypted email than to try to get PGPony setup.
I don’t think it is difficult to setup but I bought 2 BraX3 phones that people I work with scoff at! 