ForgeFed/ActivityPub Federation in Vervis
=========================================

At the time of writing, here's the current status of federation implemented in
Vervis.

Summary:

* To post a comment on a local ticket (same server), log in and browse
  to the ticket's page and use the ticket reply form, the regular way it's been
  on Vervis.
* To post a comment on a remote ticket (other server), log in and browse to the
  /publish page, fill the form and the comment will be delivered to the right
  place

For more details, read below.

## Federation triggered by regular UI

* Ticket comments are federated. If you submit a ticket comment, local and
  remote users who previously commented on the same ticket will get your
  comment delivered to their inboxes. The user who created the ticket's project
  will have it delivered to them too.
* If you comment on a ticket, you automatically become a ticket follower, and
  all future comments on the ticket will be delivered to your inbox.
* You can see users' outboxes.
* You can see your inbox.
* If you create a project, all comments on all tickets of the project will be
  delivered to your inbox.
* There is UI for notifications about comments on tickets you commented on or
  whose projects you created. However there's no JS to display them in
  real-time and no email integration.

The ticket comment UI allows to see tickets and comments, and if you're logged
in, you can post new comments. If you wish to post a comment on a ticket hosted
on another server, not the one on which your account is hosted, see the
dedicated federation pages listed below.

## GET endpoints

`GET /publish`

A page where you can write and publish a ticket comment, either on a local
ticket (i.e. a ticket on a project hosted on the same server as your account)
or on a remote ticket (i.e. a ticket on a project hosted on some other server).

`GET /inbox`

A test page that displays received activities and the result of their
processing.

`GET /s/joe/inbox`

A page that displays your personal inbox. It should list all ticket comments on
projects you've created and and ticket comments on tickets you previously
commented on.

`GET /s/joe/outbox`

A page that displays your personal outbox. It should list all the activities
you're published, all ticket comments you've made.

## POST endpoints

`POST /s/joe/outbox`

Personal endpoint for publishing ticket comments. When you submit the form in
the /publish page, this is where it is sent. In the future you'll be able to
see the content of your outbox, and other people will be able to see the public
items in your outbox.

You can access this endpoint without using the /publish page, but Vervis
doesn't have OAuth2 support yet, so you'll need to log in first and grab the
cookie, and send it along with the request.

`POST /s/joe/inbox`

Personal endpoint to which other servers deliver ticket comments for you to
see. These are comments on tickets on which you previously commented, and thus
automatically became a follower of thosr tickets.

`POST /s/joe/p/proj/inbox`

Per-project inbox, to which projects receive ticket comments from other
servers. If someone on another server publishes a comment on your project, then
your project will receive the comment at this endpoint and the comment will be
displayed when you visit the ticket page.

## Spec

Federation in Vervis is done using ActivityPub. Below comes a description of
the details that aren't already common on the Fediverse. The details are
written informally in the form of short simple proposals.

### (A) Authentication

Vervis uses HTTP Signatures to authenticate messages received in inboxes. The
Host, (request-target), Date and Digest headers are required to be present and
used in the signature, and the Digest header must be verified by computing the
hash of the request body. Other headers may need signing too, as specified in
the proposals below.

The `publicKeyPem` field maps to the PEM encoding of the key. The PEM encoding
contains not just the key itself, but also a code specifying the key type. The
Fediverse de-facto standard is RSA, more precisely PKCS#1 v1.5, and used with
the SHA-256 hash algorithm. This is often referred to as RSA-SHA256.

#### (1) Actor key(s) in a separate document

Allow an actor's signing key to be a separate document, rather than embedded in
the actor document. In Vervis, the use of that is for server-scope keys (see
proposal below), but otherwise, an embedded key is just as good.

`GET /users/aviva/keys/key1`

```json
{ "@context":     "https://w3id.org/security/v1"
, "@id":          "https://example.dev/users/aviva/keys/key1"
, "@type":        "Key"
, "owner":        "https://example.dev/users/aviva"
, "publicKeyPem": "-----BEGIN PUBLIC KEY----- ..."
}
```

`GET /users/aviva`

```json
{ "@context":
    [ "https://www.w3.org/ns/activitystreams"
    , "https://w3id.org/security/v1"
    ]
, "id":                "https://example.dev/users/aviva"
, "type":              "Person"
, "preferredUsername": "aviva"
, "name":              "Aviva"
, "inbox":             "https://example.dev/users/aviva/inbox"
, "outbox":            "https://example.dev/users/aviva/outbox"
, "publicKey":         "https://example.dev/users/aviva/keys/key1"
}
```

Authentication requirements:

- The `keyId` from the signature header matches the `@id` in the document you
  receive
- The and key and the owner actor IDs are on the same host
- They key specifies the `owner`, and the owner actor's `publicKey` links back
  to the key

#### (2) Multiple actor keys

Allow an actor to specify more than one key, or no key at all. This means that
when you examine the owner actor of the key, you verify the actor links back to
the key by checking that the key is listed among the actor's keys (instead of
requiring/expecting only a single key to be specified by the actor).

The reason this is used in Vervis is for key rotation using a pair of
server-cope keys (see proposal below).

When used along with proposal A.1, each key may be either embedded in the
document, or a URI specifying the ID of a key defined in a separate document.

Actors that never need to post activities can simply not specify any keys at
all.

`GET /users/aviva`

```json
{ "@context":
    [ "https://www.w3.org/ns/activitystreams"
    , "https://w3id.org/security/v1"
    ]
, "id":                "https://example.dev/users/aviva"
, "type":              "Person"
, "preferredUsername": "aviva"
, "name":              "Aviva"
, "inbox":             "https://example.dev/users/aviva/inbox"
, "outbox":            "https://example.dev/users/aviva/outbox"
, "publicKey":
    [ { "id":           "https://example.dev/users/aviva#main-key"
      , "type":         "Key"
      , "owner":        "https://example.dev/users/aviva"
      , "publicKeyPem": "-----BEGIN PUBLIC KEY----- ..."
      }
    , "https://example.dev/users/aviva/extra-keys/extra-key1"
    , "https://example.dev/users/aviva/extra-keys/extra-key2"
    ]
}
```

#### (3) Server-scope actor key

Allows to have actor keys that can be used to sign (and verify) activities of
any actor on the server, not limited to any specific actor. That allows to have
some small constant number of keys on the server, which is very easy to manage
and makes key rotations very cheap. It also saves storage of many local and
remote actor keys.

In the common Fediverse situation, there's a separate key for each actor, but
all of these actor keys are managed by a single entity, the server. The
signatures aren't made on users' devices using private keys they keep to
themselves. They're made by the server, using private keys the server
generates.

Server-scope keys are made by the server too. The server makes the signatures,
using a private key it generates and maintains. The server is the owner of the
key, and a part of the signed message is the ID of the actor on whose behalf
the message is being sent. Since the actor isn't specified by the key, the
actor ID is instead placed in a HTTP header. And the actor still has to list
the key under `publicKey` as usual.

`GET /key1`

```json
{ "@context":
    [ "https://w3id.org/security/v1"
    , { "isShared": "https://peers.community/as2-ext#isShared"
      }
    ]
, "@id":          "https://example.dev/key1"
, "@type":        "Key"
, "owner":        "https://example.dev"
, "isShared":     true
, "publicKeyPem": "-----BEGIN PUBLIC KEY----- ..."
}
```

`GET /users/aviva`

```json
{ "@context":
    [ "https://www.w3.org/ns/activitystreams"
    , "https://w3id.org/security/v1"
    ]
, "id":                "https://example.dev/users/aviva"
, "type":              "Person"
, "preferredUsername": "aviva"
, "name":              "Aviva"
, "inbox":             "https://example.dev/users/aviva/inbox"
, "outbox":            "https://example.dev/users/aviva/outbox"
, "publicKey":
    [ { "id":           "https://example.dev/users/aviva#main-key"
      , "type":         "Key"
      , "owner":        "https://example.dev/users/aviva"
      , "publicKeyPem": "-----BEGIN PUBLIC KEY----- ..."
      }
    , "https://example.dev/users/aviva/extra-keys/extra-key1"
    , "https://example.dev/users/aviva/extra-keys/extra-key2"
    , "https://example.dev/key1"
    ]
}
```

Requirements for a server-scope key:

- Its `owner` is the top-level URI of the server, of the form `https://HOST`
- The `isShared` property is `true`
- The key is in its own document, not embedded in an actor

Requirements for authentication using a server-scope key:

- The actor ID is specified in the `ActivityPub-Actor` HTTP header
- The actor and key are on the same server
- That header is included in the HTTP Signature in the `Signature` header
- That actor lists the key (as one of the keys) under `publicKey`
- In the payload, i.e. the activity in the request body, the activity's actor
  is the same one specified in the `ActivityPub-Actor` (unless the activity is
  forwarded, see proposal B.2 about inbox forwarding)

#### (4) Actor key expiration and revocation

Allow to improve the secure handling of signing keys by supporting expiration
and revocation. Expiration means the key specifies a time at which it stops
being valid, and once that time comes, signatures made by that key are
considered invalid. Revocation similary means the key specifies a time at which
it stops being valid.

`GET /users/aviva/keys/key1`

```json
{ "@context":     "https://w3id.org/security/v1"
, "@id":          "https://example.dev/users/aviva/keys/key1"
, "@type":        "Key"
, "owner":        "https://example.dev/users/aviva"
, "created":      "2019-01-13T11:00:00+0000"
, "expires":      "2021-01-13T11:00:00+0000"
, "publicKeyPem": "-----BEGIN PUBLIC KEY----- ..."
}
```

Requirement: When verifying a signature, compare `expires` and `revoked`, if
one of them or both of them are present, to the current time. If at least one
of the 2 times is the current time or earlier, then consider the signature
invalid. If using a cached version of the key, try to HTTP GET the key and try
to authenticate once more, because it's possible the key has been replaced with
a new valid one.

#### (5) Ed25519 actor keys

Allows actor keys to be [Ed25519](https://ed25519.cr.yp.to) keys, by allowing
the `publicKeyPem` field to simply contain a PEM encoded Ed25519 public key.
The [HTTP Signatures draft](https://tools.ietf.org/html/draft-cavage-http-signatures-11#appendix-E.2)
lists more algorithms; we could support them too. This proposal just suggests
that we all start supporting Ed25519 in addition to RSA.

#### (6) HTTP Signature draft 11

The draft linked above, from April 2019, makes some changes and
recommendations. This proposal suggests we adopt them:

- For the `algorithm` parameter, use the value `hs2019`, or none, and start
  deprecating the old values (such as `rsa-sha256`).
- The new `created` and `expires` parameters seem to be mostly useful to web
  browser based clients, while our usage of HTTP Signatures is between servers.
  So perhaps they aren't very useful here. But if someone finds them useful,
  let's support them.
- Support at least Ed25519 in addition to RSA, see proposal A.5 above.

#### (7) Key rotation using a pair of server-scope keys

Allows to easily and computationally-cheaply perform periodic key rotation.

Rationale:

If you deliver an activity and then rotate the key, the target servers will
want to fetch the old key to verify your signatures, but, the old key has been
replaced, so they will fail to authenticate your requests. When using per-actor
keys, it's possible to try waiting for a time the user is inactive (which is
hopefully common because most people probably sleep for a few hours every day),
and use that as a safer chance to rotate the key. During the quiet time, other
servers will have had enough time to process their activity inbox queues, and
by the time we rotate, nobody will want the old key anymore.

The weakness of that solution is that:

- It's limited to periods of inactivity, which may limit rotation to once per
  day or less (what if you want to rotate more often? Hmm is there a good
  reason to? I'm not sure, just saying hypothetically)
- It doesn't work for users that don't have inactivity periods, e.g. a user
  that uses scheduled activities, automatic responses etc.
- It involves the computation of generating a new key for every user every day
  (assuming we don't want to rotate more often), which I suppose can be
  somewhat heavy, especially for RSA (but I haven't done any measurements)
- It involves lots of network activity because other servers will be fetching
  the new rotated keys all the time, keys can't be cached for days or weeks or
  more if they keep being replaced every day or every hour (but I haven't done
  measurements of the effect on the amount of network requests)

The proposal:

- Each server has 2 or more server-scope keys. For simplicity of discussion,
  let's assume a server has exactly 2 keys, key A and key B.
- The server does periodic rotation, but each time, it rotates one of the keys
  and leaves the other intact. It rotates key A, then next time it rotates key
  B, next time it rotates key A again, next time it rotates key B again... and
  so on.
- When signing HTTP requests, the server always uses the newer key. For
  example, if it just rotated key A, it will sign the next requests with key A.
  When time comes for the next rotation, it will rotate key B and stop using
  key A, switching to using key B for signing requests.
- The time frame suggested here for letting other servers finish processing our
  activities in their inbox queues is **one hour**, although this is just a
  suggestion and open to discussion. So it's suggested you do periodic rotation
  at most once an hour (or at least leave a key available for at least an hour
  without change after you stop using it)

That way, when one of the keys is rotated, the other key is still available for
another hour and other servers are able to use it to verify the signatures we
sent. There's no need to wait for users to be inactive, and it's very cheap:
Rotate 1 key per hour. Especially if that key is Ed25519.

### (B) ActivityPub

The following proposals are federation features in Vervis, or plans and ideas
not implemented yet, but they aren't specific to forges, and other kinds of
servers can benefit from them just as much.

#### (1) Non-actor audience

In C2S, the client sends the server an activity (or an object to be wrapped in
a `Create`) that includes addressing properties, and the server delivers the
activity to the listed recipients. The recipients may be listed as URIs or as
embedded objects, and the audience can be anything, any Object, according to
the AS2 vocabulary spec. Not limited to actors or collections. However, there
are 2 kinds of recipients:

- Actors: Recipients that have inboxes and you HTTP deliver the activity to
  them.
- Non-actors: Recipients to whom you don't directly HTTP deliver, but possibly
  you dereference them in some way and a list of more actors to deliver to.
  Usually these non-actor recipients are Collections, and they are resolved
  into lists of actors, and delivery to them sometimes involves inbox
  forwarding.

If the server gets just a list of URIs, some of which are on other servers, it
has to HTTP GET them, and find out that some are actors and some are
collections (or other non-actor objects). It may also do caching, remembering
remote actors and collections in its database to avoid HTTP GETing them every
time, but even then, it involves the initial GET where it fetches them and
remembers in the DB.

Observation: Very possibly, the client is *aware* which recipients are actors
and which aren't. Especially when the non-actors are collections. So, the
client can *hint* the server about that, so that the server doesn't even need
to do the initial GET for recipients already known to be non-actors.

There are 2 ways specified below, for making that hint. One is what Vervis
currently does, and the other is perhaps a better way I'd like to propose as an
alternative, and possibly switch Vervis to that better way.

What Vervis currently does is to use a custom `nonActors` property, which lists
recipients. The client uses that property to provide a list of recipients that
are known to be non-actors. For example if the `to` property is `[x,y,z]` and
the `nonActors` field is `[y]`, then the server can skip trying to GET the `y`
recipient, and attempt delivery only to `x` and `z`.

Another way, which is perhaps better and I'd like to propose, is to allow the
client to specify the type of the recipient in the addressing properties
themselves. For example, instead of this:

```json
[ "https://example.dev/users/martin"
, "https://example.dev/users/martin/followers"
]
```

We could do this:

```json
[ { "id":   "https://example.dev/users/martin"
  , "type": "Person"
  }
, { "id":   "https://example.dev/users/martin/followers"
  , "type": "Collection"
  }
]
```

The problem is:

- There's no `Actor` type in ActivityPub, and if you use some custom actor type
  that the server doesn't recognize, it will have to GET the recipient to be
  sure.
- There could be a custom type that is a subclass of `Collection`, and if the
  server doesn't recognize it, it will have to GET the recipient to be sure.

Since this is just a hint, and it's C2S where the client and server *possibly*
speak the same custom properties, these problems don't make the hint useless,
but they still make this approach inferior in achieving its goal, than the
custom `nonActors` property. The reason I propose it is that it uses object's
types and doesn't require any custom property. The reason Vervis doesn't use it
is that I don't see a clear way to *really* in practice tell actors from
non-actors. It also requires more computation and coding, to figure out which
things are subclasses of some actor type, or collection type, and if that's
required then RDF inference is required, therefore JSON-LD processing is
required. While in the `nonActors` approach, which maybe feels more like a
trick, it's as simple as computing set/list difference, which is generally
trivial to do.

#### (2) Authenticated inbox forwarding

#### (3) Non-announced following

#### (4) Object nesting depth

#### (5) Object capability authorization tokens

### (C) ForgeFed

#### (1) Actors

#### (2) Authorization and roles

#### (3) Comments

Comments are `Note` objects, published using the `Create` activity.
Requirements, suggestions and details:

- The AS2 `context` property must be specified, and must be a single value, and
  refers to the discussion topic, which is a ticket or a merge request or a
  patch or something else.
- The AS2 `inReplyTo` property must be provided, and must be a single value,
  and either specifies the same value as `context` (which means it's a
  top-level comment under the topic), or specifies another comment, to which it
  replies.
- When receiving a comment with context C and inReplyTo some existing comment
  message M, verify that the context of M is C too
- Some objects that are discussion topics, such as tickets, have a `followers`
  collection, which should include all the previous commenters on the topic (if
  you comment on a ticket, you probably want to start following it to be
  notified on new comments, although this isn't required, so that people can
  opt in and opt out of notifications) as well as anyone who sent a Follow
  activity even without commenting, as well as the topic author (e.g. ticket
  author), and this `followers` collection can be used for comment audience.
- Some objects similarly have a `team` collection (a new proposed ForgeFed
  property). The `team` collection would include people who manage the object.
  While `followers` is usually opt-in, `team` is usually opt-out: For example,
  in a project managed by 3 people, the default could be that all 3 of them get
  notified on every new comment on every new ticket, but once a ticket is
  assigned to one of them, the others can opt out of notifications to reduce
  the noise in their personal inboxes. But how `team` works is
  behind-the-scenes for comments: Just consider it to be a collection of team
  members managing the objects and who want to be notified on new comments.
- Some objects that are discussion topics are actors, or are managed by an
  actor (for example tickets may exist under projects, and projects are
  actors), and when you deliver the comment to them, they store and display it
  in the appropriate discussion page
- Normally, the audience of a new comment would include:
  * The discussion topic (if it's an actor) or the actor that manages it (e.g.
    the project to which the ticket belongs, on which you're commenting)
  * The topic's followers collection
  * The topic's team collection
  * Specific individuals/groups you want to mention or bring the discussion to
    their attention
- This setup with a followers collection means that clients don't need to do
  any digging and querying to figure out who the commenters and team members
  are, and it allows people to opt out of notifications if they previously
  commented on some topic but don't want to be in the discussion anymore. This
  makes it very easy for clients to correctly address comments.

`GET /luke/outbox/A0O8l`

```json
{
    "@context": "https://www.w3.org/ns/activitystreams",
    "id": "https://dev.federated.coop/luke/outbox/A0O8l",
    "type": "Create",
    "to": [
        "https://dev.federated.coop/luke/text-adventure",
        "https://dev.federated.coop/luke/text-adventure/issues/113/followers",
        "https://dev.federated.coop/luke/text-adventure/issues/113/team"
    ],
    "actor": "https://dev.federated.coop/luke",
    "object": {
        "id": "https://dev.federated.coop/luke/comments/L0dRp",
        "type": "Note",
        "attributedTo": "https://dev.federated.coop/luke",
        "context": "https://dev.federated.coop/luke/text-adventure/issues/113",
        "published": "2019-05-26T11:56:50.024267645Z",
        "to": [
            "https://dev.federated.coop/luke/text-adventure",
            "https://dev.federated.coop/luke/text-adventure/issues/113/followers",
            "https://dev.federated.coop/luke/text-adventure/issues/113/team"
        ],
        "content": "That's such a wonderful idea!",
        "inReplyTo": "https://poetry.space/aviva/comments/xN82v"
    }
}
```

TODO:

- `followers` is for actors, it seems, by the AP spec? So, what do we call the
  collection of followers of a ticket?
- `replies` and how the C2S object nesting depth proposal
- Use `nonActors` in the example?
- Content format? HTML? Markdown source? Tags? Referenced ticktes?
- Visibility and privacy?

#### (4) Tickets

#### (5) Patches

#### (6) Merge requests

#### (7) Commits

#### (8) Forks

#### (9) SSH keys

#### (10) Pushes

#### (11) Avatars