actor.md

Actor

The endpoint describing an actor, e.g. https://mymath.rocks/activitypub/bovine returns

{
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://w3id.org/security/v1"
    ],
    "id": "https://mymath.rocks/activitypub/bovine",
    "inbox": "https://mymath.rocks/activitypub/bovine/inbox",
    "name": "bovine",
    "outbox": "https://mymath.rocks/activitypub/bovine/outbox",
    "preferredUsername": "bovine",
    "publicKey": {
        "id": "https://mymath.rocks/activitypub/bovine#main-key",
        "owner": "https://mymath.rocks/activitypub/bovine",
        "publicKeyPem": "-----BEGIN PUBLIC KEY-----....-----END PUBLIC KEY-----\n"
    },
    "type": "Application"
}

Two comments on this:

• publicKey is required see HTTP Signatures
• preferredUsername is required to interact with Mastodon. The reason is explained below

PreferredUsername and Mastodon

Mastodon doesn't seem to identify actors with their url, e.g. https://mymath.rocks/activitypub/bovine, instead it identifies them with their "username", e.g. bovine@mymath.rocks. This means that when looking up bovine@mymath.rocks on Mastodon the following requests happen:

GET /.well-known/webfinger?resource=acct:bovine@mymath.rocks
GET /activitypub/bovine
GET /.well-known/webfinger?resource=acct:$PREFERRED_USERNAME@mymath.rocks
...

This means that in order to interact with Mastodon, you need both the webfinger endpoint and specify the preferredUsername property in the actor object. Neither is required by ActivityPub as far as I can tell.