SimpleDiscordCrypt oleh An0

Encrypt your messages in the chat! This extension helps you in security and privacy by allowing you to encrypt your messages in Discord making sure that it's only you and the recipient(s) who ever read them!
Works from the browser or on your computer, without BetterDiscord.

Once you open discord you will get a window to create your local database, once that's done you can toggle encryption using the lock icon added to the channel's header and you can select the key used besides that. You can access the menu by right clicking the aforementioned lock icon. DM channels have different options than group channels.
For more information visit: https://gitlab.com/An0/SimpleDiscordCrypt

It is 100% open-source!

(all your data is stored locally on your computer so make sure to use the export function for backups)