https://hawkpost.co/en-us/help
Opensource with Github:
https://github.com/whitesmith/hawkpost
The way it works is like this:
It fetches your public key.
When the box is open and the secrets submitted, all the content is encrypted on the client side. (with openpgp.js )
The server then signs (experimental) the encrypted content.
Finally the server forwards it to your e-mail address.
Openpgp.js – see https://openpgpjs.org/
Also – similar solution: https://encrypt.to/
with Github: https://github.com/encrypt-to/
—
also interesting – https://browserpgp.github.io/
with https://github.com/browserPGP/browserPGP.github.io