Secure

• why not pgp - <https://habr.com/ru/post/460827/>
• keybase - social network and chat with key keybase.io - service for working with keys

example https://gist.github.com/splittingred/54762915cee1844353d2#file-keybase-md

• exchange WormHole - <https://github.com/warner/magic-wormhol> <https://github.com/saljam/webwormhole> <https://habr.com/ru/company/globalsign/blog/500356/>
• online <https://send.firefox.com>, <https://file.pizza/>
• cli https://github.com/FiloSottile/age

• free signal realisation <https://en.wikipedia.org/wiki/OMEMO>