Every user, post, community, etc. on each of these services is represented as an ActivityPub object. Let’s take the example of following your Lemmy user from Mastodon. I can search for @Gohos@lemmy.ml in Mastodon and in the background my Mastodon instance will issue the following WebFinger request to lemmy.ml: https://lemmy.ml/.well-known/webfinger?resource=acct:Gohos@lemmy.ml. This will return the following JSON: -
{
"subject": "acct:Gohos@lemmy.ml",
"links": [
{
"rel": "http://webfinger.net/rel/profile-page",
"type": "text/html",
"href": "https://lemmy.ml/u/Gohos"
},
{
"rel": "self",
"type": "application/activity+json",
"href": "https://lemmy.ml/u/Gohos",
"properties": {
"https://www.w3.org/ns/activitystreams#type": "Person"
}
}
]
}
From here I can request the application/activity+json URL at https://lemmy.ml/u/Gohos and if I request JSON I get: -
{
"@context": [
"https://www.w3.org/ns/activitystreams",
"https://w3id.org/security/v1",
{
"lemmy": "https://join-lemmy.org/ns#",
"litepub": "http://litepub.social/ns#",
"pt": "https://joinpeertube.org/ns#",
"sc": "http://schema.org/",
"ChatMessage": "litepub:ChatMessage",
"commentsEnabled": "pt:commentsEnabled",
"sensitive": "as:sensitive",
"matrixUserId": "lemmy:matrixUserId",
"postingRestrictedToMods": "lemmy:postingRestrictedToMods",
"removeData": "lemmy:removeData",
"stickied": "lemmy:stickied",
"moderators": {
"@type": "@id",
"@id": "lemmy:moderators"
},
"expires": "as:endTime",
"distinguished": "lemmy:distinguished",
"language": "sc:inLanguage",
"identifier": "sc:identifier"
}
],
"type": "Person",
"id": "https://lemmy.ml/u/Gohos",
"preferredUsername": "Gohos",
"inbox": "https://lemmy.ml/u/Gohos/inbox",
"outbox": "https://lemmy.ml/u/Gohos/outbox",
"publicKey": {
"id": "https://lemmy.ml/u/Gohos#main-key",
"owner": "https://lemmy.ml/u/Gohos",
"publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAwDc9TbmcvR/eRx0zuXl5\nqUu8vwcU3K1/70BqLdae/HtbjqXoHbVUI3exSVOTLVmPQ4oHjf6Lq5axyRRwihTh\nbDgWYSBCqZ07L6VrgXVg2APCkFpX32XlbFbbMQDXf+kodj6YrzwelXEJ03eTDKaa\nuFkFt0Uelu1k0AZVydGYT3U2iZ0jNGvpUMWQycTJ/k0r8n61JLuEMkvkVlH4ZDGb\nGTjwjLRds/zawe8FAK/Grn+AW/UTmW/1kBLcqhkeRYzWhyrVy+/f9jf03s0rW92w\n8PWM02AAE9edIpnK2XgElfkVWJenQzw7WDp045XPHMlUh5iizAJGcPAzdeEVNLq8\nGQIDAQAB\n-----END PUBLIC KEY-----\n"
},
"endpoints": {
"sharedInbox": "https://lemmy.ml/inbox"
},
"published": "2023-06-03T17:30:16.990908+00:00"
}
This includes two important properties: inbox and outbox. Using ActivityPub I now know I can send you a message by POSTing a request to your inbox and I can fetch posts, comments, etc. you’ve made via your outbox.
Your public key is also included. This is how, if you sent me a message, I can verify that the message actually came from you.
That’s a very brief overview but I hope that helps!
If you implement IP over smoke signals then perhaps :)