Mastodon largely follows the ActivityPub server-to-server specification but it makes uses of some non-standard extensions, some of which are required for interacting with Mastodon at all.
In Mastodon, users are identified by a `username` and `domain` pair (e.g., `Gargron@mastodon.social`).
This is used both for discovery and for unambiguously mentioning users across the fediverse. Furthermore, this is part of Mastodon's database design from its very beginnings.
As a result, Mastodon requires that each ActivityPub actor uniquely maps back to an `acct:` URI that can be resolved via WebFinger.
In order to authenticate activities, Mastodon relies on HTTP Signatures, signing every `POST` and `GET` request to other ActivityPub implementations on behalf of the user authoring an activity (for `POST` requests) or an actor representing the Mastodon server itself (for most `GET` requests).
Mastodon requires all `POST` requests to be signed, and MAY require `GET` requests to be signed, depending on the configuration of the Mastodon server.
