Ask HN: User-friendly online “vault” web app to securely share secrets?

I need to share secrets (like API keys), privacy sensitive data, sometimes small files with customers, and they with me. Some of them aren't tech savvy so I don't want to ask them to use encryption tools. I'd really like it if there was some kind of web application I can just give them an account to and that we then have an encrypted conversation on that I can simply delete later on. For legal reasons I'd also want this app hosted in my own country (the Netherlands).

I've come across some enterprise-scale solutions that are almost Dropbox-y and have calendar-synch features and are basically bloated. Because the use case U have is actually quite simple the webapp itself can also be quite simple (I'd think), but security needs to be strong.

Are there webapps like this? What are webapps like this called?

Thanks!