How to implement a basic ActivityPub server

Today we’ll be looking at how to connect the protocols powering Mastodon in the simplest way possible to enter the federated network. We will use static files, standard command-line tools, and some simple Ruby scripting, although the functionality should be easily adaptable to other programming languages. First, what’s the end goal of this exercise? We want to send a Mastodon user a message from our own, non-Mastodon server. So what are the ingredients required? The message itself will be formatted with ActivityPub, and it must be attributed to an ActivityPub actor. The actor must be discoverable via Webfinger, and the delivery itself must be cryptographically signed by the actor.