How to fail to read a Fediverse post

Posted 27 October 2024 by Natalie

So I'm trying to add support for automatically embedding Fediverse posts on this blog, right? And yeah I could use the Mastodon API for it, but I'd really rather be able to make it work with anything that speaks ActivityPub. So okay, I look up the ActivityPub spec. It's a bit confusing, but I figure out how to at make a GET request for an ActivityPub resource: just add Content-Type: application/ld+json; profile="https://www.w3.org/ns/activitystreams". I can even make a request for one of my statuses and it works! Fantastic!

Just to verify, I make a request to another server, and that's where things start getting hairy. Instead of a nice JSON representation of the post, I get back a 401 Unauthorized with a body that says "Request not signed". This is a public post, but after some digging it turns out Mastodon (and by extension ActivityPub as a whole) has a "secure mode" where all requests have to be signed.

Signed by what though? This is a distributed system—there's no central authority to distribute authorization in the first place.

To answer that I looked in the spec. I searched for "signature" and found nothing particularly relevant. Eventually I found the Authentication and Authorization section, which says "Unfortunately at the time of standardization, there are no strongly agreed upon mechanisms for authentication." Well, shit. Clearly some people agree enough for it to be implemented, but I guess not enough for it to be actually specified!

This does, mercifully, link to a wiki page that purports to lay out "some possible directions" and, under the Server to Server section, seems to describe the scheme that this StackOverflow post describes as "an odd, somewhat well-known ActivityPub quirk"^[1]. This wiki page may not be an official specification, but at the very least it describes the publicKey field that I can see in the actor JSON on Mastodon.social.

This is all fundamentally busywork. Because the whole thing is decentralized, the receiving instance has no choice but to trust whatever public key a new requesting instance provides. All this scheme really does is prove that someone making requests runs a server somewhere that speaks basic ActivityPub, and even then this constraint only exists for ActivityPub requests—HTTP requires no authentication at all.

{
  "id": "https://mastodon.social/users/nex3",
  "type": "Person",
  /* ... */
  "publicKey": {
    "id": "https://mastodon.social/users/nex3#main-key",
    "owner": "https://mastodon.social/users/nex3",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----\n"
  }
}

Or does it? The publicKey link there points to https://web-payments.org/vocabs/security#publicKey, a URL that at time of writing is refusing HTTPS connections entirely. Naturally I looked it up on archive.org, only to find that the specification for the publicKey field is not only totally different from what I'm observing in the wild, its one-sentence specification is essentially useless: "A public key property is used to specify a URL that contains information about a public key." The example included doesn't even have a publicKey field.

{
  "@context": "https://w3id.org/security/v1",…

#code

How to fail to read a Fediverse post