From 8e578a58c8fee5a1850e7cfb23de796179657ecb Mon Sep 17 00:00:00 2001 From: Christopher Lemmer Webber Date: Thu, 18 Jul 2019 13:12:57 -0400 Subject: [PATCH] Conceptual overview of ActivityPub done --- README.org | 130 ++++++++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 125 insertions(+), 5 deletions(-) diff --git a/README.org b/README.org index 962ec54..49e2a3b 100644 --- a/README.org +++ b/README.org @@ -12,16 +12,106 @@ # - ActivityPub is an actor model protocol. # - The general design can be understood from the overview section of the spec +[[https://www.w3.org/TR/activitypub/][ActivityPub]] is a federated social network protocol. +It is generally fairly easily understood by reading the +[[https://en.wikipedia.org/wiki/Actor_model][Overview section of the standard]]. +In short, just as anyone can host their own email server, anyone can +host their own ActivityPub server, and yet different users on different +servers can interact. +At the time of writing, ActivityPub is seeing major uptake, with +several thousand nodes and several million registered users (with the +caveat that registered users is not the same as active users). +ActivityPub defines both a client-to-server and server-to-server +protocol, but at this time the server-to-server protocol is what is +most popular and is the primary concern of this article. + # - In general, most of the design of ActivityPub is fairly clean, with # a few exceptions -# - sharedInbox is a break from the actor model protocol and was a late -# addition +ActivityPub's core design is fairly clean, following the +[[https://en.wikipedia.org/wiki/Actor_model#Fundamental_concepts][actor model]]. +Different entities on the network can create other actors/objects +(such as someone writing a note) and communicate via message passing. +A core set of behaviors are defined in the spec for common message +types, but the system is extensible so that implementations may define +new terms with minimal ambiguity. +If two instances both understand the same terms, they may be able to +operate using behaviors not defined in the original protocol. +This is called an "open world assumption" and is necessary for a +protocol as general as ActivityPub; it would be extremely egotistical +of the ActivityPub authors to assume that we could predict all future +needs of users.[fn:json-ld] + +# - (json-ld conversations outside of the scope of this particular post) + +Unfortunately (mostly due to time constraints and lack of consensus), +even though most of what is defined in ActivityPub is fairly +clean/simple, ActivityPub needed to be released with "holes in the +spec". +Certain key aspects critical to a functioning ActivityPub server are +not specified: + # - authentication is not specified. The community has settled # on using http signatures for signing requests, though there is no # "clean agreement" on how to attach signatures *to* posts yet. # - authorization is not specified -# - (json-ld conversations outside of the scope of this particular post) + + - Authentication is not specified. Authentication is important to + verify "did this entity really say this thing".[fn:did-you-say-it] + However, the community has mostly converged on using [[https://tools.ietf.org/html/draft-cavage-http-signatures-11][HTTP Signatures]] + to sign requests when delivering posts to other users. + The advantage of HTTP Signatures is that they are extremely simple + to implement and require no normalization of message structure; + simply sign the body (and some headers) as-you-are-sending-it. + The disadvantage of HTTP Signatures is that this signature does + not "stick" to the original post and so cannot be "carried around" + the network. + A minority of implementations have implemented some early versions + of [[https://w3c-dvcg.github.io/ld-proofs/][Linked Data Proofs]] (formerly known as "Lined Data Signatures"), + however this requires access to a normalization algorithm that not + all users have a library for in their language, so Linked Data Proofs + have not as of yet caught on as popularly as HTTP Signatures. + + - Authorization is also not specified. (Authentication and + authorization are frequently confused (especially because in + English, the words are so similar) but mean two very different + things: the former is checking who said/did a thing, the latter is + checking whether they are allowed to do a thing.) As of right now, + authorization tends to be extremely ad-hoc in ActivityPub systems, + sometimes as ad-hoc as unspecified heuristics from tracking who + received messages previously, who sent a message the first time, + and so on. The primary way this is worked around is sadly that + interactions which require richer authorization simply have not + been rolled out onto the ActivityPub network. + +Compounding this situation is the general confusion/belief that +autorization must stem from authentication. +This document aims to show that not only is this not true, it is also +a dangerous assumption with unintended consequences. +An alternative approach based on "object capabilities" is +demonstrated, showing that the actor model itself, if we take it at +its purest form, is itself already a sufficient authorization system. + +# - sharedInbox is a break from the actor model protocol and was a late +# addition + +Unfortunately there is a complication. +At the last minute of ActivityPub's standardization, =sharedInbox= was +added as a form of mutated behavior from the previously described +=publicInbox= (which was a place for servers to share public content). +The motivation of =sharedInbox= is admirable: while ActivityPub is based +on explicit message sending to actors' =inbox= endpoints, if an actor +on server A needs to send a message to 1000 followers on server B, +why should server A make 1000 separate requests when it could do it +in one? +A good point, but the primary mistake in how this one request is made; +rather than sending one message with a listing of all 1000 recipients +on that server (which would preserve the actor model integrity), +it was advocated that servers are already tracking follower information, +so the receiving server can decide whom to send the message to. +Unfortunately this decision breaks the actor model and also our suggested +solution to authorization; see [[https://github.com/WebOfTrustInfo/rwot9-prague/blob/master/topics-and-advance-readings/ap-unwanted-messages.md#org7937fed][MultiBox]] for a suggestion on how we +can solve this. # - What to do about the holes in the spec? Many community members have # asked that we codify current behavior. However, as this document lays @@ -34,14 +124,44 @@ # people who are actively concerned with human rights and the # well-being of marginalized groups. -** The mess we're in +Despite these issues, ActivityPub has achieved major adoption. +ActivityPub has the good fortune that its earliest adopters tended to +be people who cared about human rights and the needs of marginalized +groups, and spam has been relatively minimal. +[fn:json-ld] The technology that ActivityPub uses to accomplish this is + called [[https://json-ld.org/][json-ld]] and admittedly has been one of the most controvercial + decisions in the ActivityPub specification. + Most of the objections have surrounded the unavailability of json-ld + libraries in some languages or the difficulty of mapping an open-world + assumption onto strongly typed systems without an "other data" bucket. + Since a project like ActivityPub must allow for the possibility of + extensions, we cannot escape open-world assumptions. + However, there may be things that can be done to improve happiness + about what extension mechanism is used; these discussions are out of + scope for this particular document, however. +[fn:did-you-say-it] Or more accurately, since users may appoint + someone else to manage posting for them, "was this post really made + by someone who is authorized to speak on behalf of this entity". +** Unwanted messages, from spam to harassment + +# - "there are no nazis on the fediverse" + +# - social networks: breadth vs depth? +# - wholesale borrowing of surveillance capitalist assumptions + +** Freedom of speech also means freedom to filter ** Don't pretend we can prevent what we can't -** Freedom of speech also means freedom to filter +# - introduce ocap community phrase +# - introduce revised version + +# - "the fediverse is not indexed" + +# ** Anti-solutions