Ask HN: Encrypted Group Mailing List Service?

I am looking for a web app with the following features:

1) It should enable long-form, small group communication, just like a private forum or mailing list. It should support at least 100 users in a group. 2) Client-side encryption -- should be zero-knowledge on the server-side. 3) Usable via a desktop. 4) Does not require a phone number or smart phone to sign up.

I'm not looking for something secure against the NSA or FBI, I figure nothing is secure against the government if they really want you. I am just looking for something that will protect my privacy in the face of attacks by normal hackers, leaks, rogue employees at the hosting company, etc.

Does anybody know of a service that meets the above requirements?