Veracrypt has clients for Windows, Linux, and macos, I'm pretty sure it can run as a "portable" app in most of them.
And it's FOSS, so even if "the software manufacturer could stop supporting it at anytime" someone else will just maintain or fork it, it's already a fork of TrueCrypt itself.
In regards of security, Nordlocker is a good solution for file encryption with high privacy focus as well. It protects files with easy encryption process (drag and drop into the locker). Only you and people with password can access them.
It has a free option, which allows to encrypt up to 5gb of data for free. I started from this version three months ago ...
With https://whereby.com/ you can provide video meetings in the browser. Your attendees just need to visit your custom link to whereby.
You can also share your screen and for small teams (up to 4 attendees) it is gratis.
This is not a code library but a software recommendation. Anyway, i hope this helps.
As I didn't find exactly what I wanted, I finally spent a few hours building this:
sdfgh, an encrypted notepad
It's a single Python file of < 100 lines of code, so you can easily read the code, and decide if you trust it or not! (very important to be able to read the source easily for security-sensitive encryption programs).
Look at the ccrypt package, which does file level encryption and doesn't cache the decryption key. Since it's scriptable, you can write a wrapper script which caches the key if you'd like or just use it directly as you wish.
Disclaimer: I'm the developer of one of the product (File Lock PEA) mentioned in this answer.
You are looking for a symmetric encryption tool that works on file level. The variety of programs with symmetric cryptography for Linux is, as far as I know, not that big and most of them use PBKDF2 which is not much better than s2k. You can check:
There used to be LockNote for it. However, this is no longer maintained. But if you look for alternatives, you will find many applications for exactly this purpose. There is even a plugin for Notepad++ called NPPCrypt if you use Windows.
The most well known for public private key encryption is OpenSSL. You can download it here.
OpenSSL is a software library for applications that secure communications over computer networks against eavesdropping or need to identify the party at the other end. It is widely used by Internet servers, including the majority of HTTPS websites.
Generate a ...
I think you might want to consider GPG & it's keys, and possibly keyservers too if they might be useful.
It's the only encryption software I can think of now that's already been working well for decades, and will probably be around in future decades
Can have multiple recipients (users) able to decrypt something, or possibly only encrypt a "master key" ...