example.md

{
    "id": "https://example.com/activity",
    "type": "Create",
    "to": "https://example.com/followers",
    "published": "2018-01-17T16:23:08Z",
    "object": {
        "id": "https://example.com/status",
        "type": "Note",
        "to": "https://example.com/followers",
        "tag": [{
                "type": "Mention",
                "name": "@example",
                "href": "https://example.com/"
            },
            {
                "type": "Mention",
                "name": "@other",
                "href": "https://other.example.com/"
            }
        ],
        "sensitive": false,
        "published": "2018-01-17T16:23:08Z",
        "inReplyTo": "https://other.example.com/status",
        "content": "beep boop",
        "cc": [
            "https://other.example.com/",
            "https://www.w3.org/ns/activitystreams#Public"
        ],
        "attributedTo": "https://example.com/",
    },
    "cc": [
            "https://other.example.com/",
            "https://www.w3.org/ns/activitystreams#Public"
    ],
    "actor": "https://example.com/",
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://puckipedia.com/render/context"
    ]
}

---

https://example.com/activity

{
    "id": "https://example.com/activity",
    "type": "Create",
    "to": "https://example.com/followers",
    "published": "2018-01-17T16:23:08Z",
    "object": "https://example.com/status",
    "cc": [
            "https://other.example.com/",
            "https://www.w3.org/ns/activitystreams#Public"
    ],
    "actor": "https://example.com/",
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://puckipedia.com/render/context"
    ]
}

---

https://example.com/status

{
    "id": "https://example.com/status",
    "type": "Note",
    "to": "https://example.com/followers",
    "tag": [{
            "type": "Mention",
            "name": "@example",
            "href": "https://example.com/"
        },
        {
            "type": "Mention",
            "name": "@other",
            "href": "https://other.example.com/"
        }
    ],
    "sensitive": false,
    "published": "2018-01-17T16:23:08Z",
    "inReplyTo": "https://other.example.com/status",
    "content": "beep boop",
    "cc": [
        "https://other.example.com/",
        "https://www.w3.org/ns/activitystreams#Public"
    ],
    "attributedTo": "https://example.com/",
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://puckipedia.com/render/context"
    ]
}

jsonld-sigs.md

JSON-LD signatures applied to ActivityPub

(note: this is currently inconsistent with what Mastodon applies. This is a suggestion, for now at least)

Currently, JSON-LD Signatures are defined to apply to an entire document. However, this will not work when stored in a database, as you'd have to exactly unflatten everything back from how it was. This might be difficult, or even impossible. So, my suggestion:

1. Define items as an empty set, and toProcess as a queue containing the root object.

2. While toProcess has an item:

   1. Remove an item from the queue, and iterate over every predicate containing it
   2. If the predicate contains a sub object, add it to toProcess.
   3. If the sub object also has an @id predicate, add it to items

3. For each item in items:

   1. Clone the object, and replace all predicates that contain a sub object with an @id predicate with just its @id
   2. Sign this cloned object, and add this signature to the original object.

To validate the signatures, you can basically do the same, but instead of signing you validate the signature.