FEP-ef61: Portable Objects

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Summary

Portable ActivityPub objects with server-independent IDs.

Motivation

Usage of HTTP(S) URLs as identifiers has a major drawback: when the server disappears, everyone who uses it loses their identities and data.

The proposed solution should satisfy the following constraints:

User's identity and data should not be tied to a single server.
Users should have a choice between full control over their identity and data, and delegation of control to a trusted party.
Implementing the solution in existing software should be as simple as possible. Changes to ActivityPub data model should be kept to a minimum.
The solution should be compatible with existing and emerging decentralized identity and storage systems.
The solution should be transport-agnostic.

History

Nomadic identity mechanism makes identity independent from a server and was originally part of the Zot federation protocol.

Streams (2021) made nomadic accounts available via the Nomad protocol, which supported ActivityStreams serialisation.

FEP-c390 (2022) introduced a decentralized identity solution compatible with ActivityPub. It enabled permissionless migration of followers between servers, but didn't provide full data portability.

Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119.

Identifiers

An ActivityPub object can be made portable by using an identifier that is not tied to a single server. This proposal describes a new identifier type that has this property and is compatible with the ActivityPub specification.

ap:// URLs

ap:// URL is constructed according to the URI specification, but with a Decentralized Identifier in place of the authority:

ap://did:example:123456/path/to/object?name=value#fragment-id
\_/  \________________/ \____________/ \________/ \_________/
 |           |                |            |           |
scheme   authority           path        query     fragment

The URI scheme MUST be ap.
The authority component MUST be a valid DID.
The path is REQUIRED. It MUST be treated as an opaque string.
The query is OPTIONAL. To avoid future conflicts, implementers SHOULD NOT use parameter names that are not defined in this proposal.
The fragment is OPTIONAL.

DID methods

Implementers MUST support the did:key method. Other DID methods SHOULD NOT be used, as it might hinder interoperability.

[!NOTE] The following additional DID methods are being considered: did:web, did:dns, did:webvh (formerly did:tdw) and did:fedi.

DID documents SHOULD contain Ed25519 public keys represented as verification methods with Multikey type (as defined in the Controller Documents specification).

Any DID URL capabilities of a DID method MUST be ignored when working with ap:// URLs.

Dereferencing ap:// URLs

To dereference an ap:// URL, the client MUST make HTTP GET request to a gateway endpoint at well-known location /.well-known/apgateway. The ap:// prefix MUST be removed from the URL and the rest of it appened to a gateway URL. The client MUST specify an Accept header with the application/ld+json; profile="https://www.w3.org/ns/activitystreams" media type.

Example of a request to a gateway:

GET https://social.example/.well-known/apgateway/did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/path/to/object

ActivityPub objects identified by ap:// URLs can be stored on multiple servers simultaneously.

If object identified by ap:// URL is stored on the server, it MUST return a response with status 200 OK containing the requested object. The value of a Content-Type header MUST be application/ld+json; profile="https://www.w3.org/ns/activitystreams".

If object identified by ap:// URL is not stored on the server, it MUST return 404 Not Found.

If object is not public, the server MUST return 404 Not Found unless the request has a HTTP signature and the signer is allowed to view the object.

[!NOTE] This document describes web gateways, which use HTTP transport. However, the data model and authentication mechanism are transport-agnostic and other types of gateways could exist.

Authentication and authorization

Authentication and authorization are performed in accordance with FEP-fe34 origin-based security model.

The origin of an ap:// URL is computed by the following algorithm:

Let uri-scheme be the ap string.
Let uri-host be the authority component of the URL.
Let uri-port be the number 0.
Return the triple (uri-scheme, uri-host, uri-port).

And the origin of a DID URL is computed by the following algorithm:

Let uri-scheme be the ap string.
Let uri-host be the DID component of the DID URL.
Let uri-port be the number 0.
Return the triple (uri-scheme, uri-host, uri-port).

Actors, activities and objects identified by ap:// URLs MUST contain FEP-8b32 integrity proofs. Collections identified by ap:// URLs MAY contain integrity proofs. If collection doesn't contain an integrity proof, another authentication method MUST be used.

The value of verificationMethod property of the proof MUST be a DID URL where the DID matches the authority component of the ap:// URL.

[!NOTE] This document uses terms "actor", "activity", "collection" and "object" according to the classification given in FEP-2277.

Portable actors

An actor object identified by ap:// URL MUST have a gateways property containing an ordered list of gateways where the latest version of that actor object can be retrieved. Each item in the list MUST be an HTTP(S) URL with empty path, query and fragment components. The list MUST contain at least one item.

One identity (represented by DID) can control multiple actors (which are differentiated by the path component of an ap:// URL).

Example:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/data-integrity/v1",
    "https://w3id.org/fep/ef61"
  ],
  "type": "Person",
  "id": "ap://did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/actor",
  "inbox": "ap://did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/actor/inbox",
  "outbox": "ap://did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/actor/outbox",
  "gateways": [
    "https://server1.example",
    "https://server2.example"
  ],
  "proof": {
    "type": "DataIntegrityProof",
    "cryptosuite": "eddsa-jcs-2022",
    "created": "2023-02-24T23:36:38Z",
    "verificationMethod": "did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2#z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2",
    "proofPurpose": "assertionMethod",
    "proofValue": "..."
  }
}

Location hints

When ActivityPub object containing a reference to another actor is being constructed, implementations SHOULD provide a list of gateways where specified actor object can be retrieved. This list MAY be provided using the gateways query parameter. Each gateway address MUST be URL-endcoded, and if multiple addresses are present they MUST be separated by commas.

Example:

ap://did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/actor?gateways=https%3A%2F%2Fserver1.example,https%3A%2F%2Fserver2.example

This URL indicates that object can be retrieved from two gateways:

https://server1.example
https://server2.example

Implementations MUST discard query parameters when comparing ap:// identifiers and treat identifiers with different query parameter values as equal.

Inboxes and outboxes

Servers and clients MUST use gateways to deliver activities to inboxes or outboxes. Servers specified in the gateways property of an actor object MUST accept POST requests to respective gateway URLs.

Example:

POST https://social.example/.well-known/apgateway/did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/actor/inbox

Delivered activities might be not portable. If delivered activity is portable (has ap:// identifier), the server MUST verify its FEP-8b32 integrity proof. If the server does not accept deliveries on behalf of an actor, it MUST return 405 Method Not Allowed.

ActivityPub clients MAY follow FEP-ae97 to publish activities. In this case, the client MAY deliver signed activity to multiple outboxes, located on different servers.

Upon receiving an activity in actor's outbox, server SHOULD forward it to outboxes located on other servers where actor's data is stored. An activity MUST NOT be forwarded from outbox more than once.

Upon receiving an activity in actor's inbox, server SHOULD forward it to inboxes located on other servers where actor's data is stored.

Collections

Collections associated with portable actors (such as inbox and outbox collections) MAY not have FEP-8b32 integrity proofs. Consuming implementations MUST NOT process unsecured collections retrieved from servers that are not listed in the gateways array of the actor document.

Portable objects

Example:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/data-integrity/v1",
    "https://w3id.org/fep/ef61"
  ],
  "type": "Note",
  "id": "ap://did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/objects/dc505858-08ec-4a80-81dd-e6670fd8c55f",
  "attributedTo": "ap://did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/actor?gateways=https%3A%2F%2Fserver1.example,https%3A%2F%2Fserver2.example",
  "inReplyTo": "ap://did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK/objects/f66a006b-fe66-4ca6-9a4c-b292e33712ec",
  "content": "Hello!",
  "attachment": [
    {
      "type": "Image",
      "url": "hl:zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n",
      "mediaType": "image/png",
      "digestMultibase": "zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n"
    }
  ],
  "to": [
    "ap://did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK/actor"
  ],
  "proof": {
    "type": "DataIntegrityProof",
    "cryptosuite": "eddsa-jcs-2022",
    "created": "2023-02-24T23:36:38Z",
    "verificationMethod": "did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2#z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2",
    "proofPurpose": "assertionMethod",
    "proofValue": "..."
  }
}

Media

Integrity of an external resource is attested with a digest. When a portable object contains a reference to an external resource (such as image), it MUST also contain a digestMultibase property representing the integrity digest of that resource. The digest MUST be computed using the SHA-256 algorithm.

The URI of an external resource SHOULD be a hashlink.

Example of an Image attachment:

{
  "type": "Image",
  "url": "hl:zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n",
  "mediaType": "image/png",
  "digestMultibase": "zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n"
}

After retrieving a resource, the client MUST verify its integrity by computing its digest and comparing the result with the value encoded in digestMultibase property.

Resources attached to portable objects using hashlinks can be stored by gateways. To retrieve a resource from a gateway, the client MUST make an HTTP GET request to the gateway endpoint at well-known location /.well-known/apgateway. The value of a hashlink URI MUST be appended to the gateway base URL.

Example of a request:

GET https://social.example/.well-known/apgateway/hl:zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n

Compatibility

Identifiers

ap:// URLs might not be compatible with existing ActivityPub implementations. To provide backward compatibility, gateway-based HTTP(S) URLs of objects can be used instead of their actual ap:// identifiers:

https://social.example/.well-known/apgateway/did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/path/to/object

Publishers MUST use the first gateway from actor's gateways list when constructing compatible identifiers. Consuming implementations that support ap:// URLs MUST remove the part of the URL preceding did: and re-construct the canonical ap:// identifier. Objects with the same canonical identifier, but located on different gateways MUST be treated as different instances of the same object.

Publishers MUST NOT add the gateways query parameter to object IDs if compatible identifiers are used.

When HTTP signatures are necessary for communicating with other servers, each gateway that makes requests on behalf of an actor SHOULD use a separate secret key. The corresponding public keys MUST be added to actor document using the assertionMethod property as described in FEP-521a.

WebFinger addresses

WebFinger address of a portable actor can be obtained by the reverse discovery algorithm described in section 2.2 of ActivityPub and WebFinger report, but instead of taking the hostname from the identifier, it MUST be taken from the first gateway in actor's gateways array.

Discussion

(このセクションは非規範的です。)

'ap' URL syntax

'ap' URLs are not valid URIs per RFC-3986. To make them valid, the authority component can be percent-encoded:

ap://did%3Akey%3Az6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/actor

Discovering locations

Arbitrary paths

The gateways array can contain HTTP(S) URLs with a path component, thus enabling discovery based on the "follow your nose" principle, as opposed to discovery based on a well-known location.

Example of a compatible object ID if the gateway endpoint is https://social.example/ap:

https://social.example/ap/did:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/path/to/object

Alternatives to `gateways` property

This proposal makes use of the gateways property, but the following alternatives are being considered:

aliases and sameAs (containing HTTP(S) URLs of objects)
alsoKnownAs (used for account migrations, so the usage of this property may cause issues)
url (with alternate relation type)

DID services

Instead of specifying gateways in actor document, they can be specified in DID document using DID services. This approach is not compatible with generative DID methods such as did:key, which might be necessary for some types of applications.

Media access control

The proposed approach to referencing media with hashlinks does not support access control: anybody who knows the hash can retrieve the file.

To work around this limitation, a different kind of identifier can be used where digest is combined with the ap:// identifier of its parent document. The gateway will not serve media unless parent document ID is provided, and will check whether request signer has permission to view the document and therefore the attached media.

Compatibility

The following alternatives to gateway-based compatible IDs are being considered:

Use regular HTTP(S) URLs but specify the canonical ap:// URL using the url property (with canonical relation type, as proposed in FEP-fffd). For pointers to other objects such as inReplyTo property, an embedded object with url property can be used instead of a plain URL.
Alter object ID depending on the capabilities of the peer (which can be reported by NodeInfo or some other mechanism).

実装

Streams
Mitra (gateway only)

参考文献

Christine Lemmer Webber, Jessica Tallon, ActivityPub, 2018
S. Bradner, Key words for use in RFCs to Indicate Requirement Levels, 1997
T. Berners-Lee, R. Fielding, L. Masinter, Uniform Resource Identifier (URI): Generic Syntax, 2005
Manu Sporny, Dave Longley, Markus Sabadello, Drummond Reed, Orie Steele, Christopher Allen, Decentralized Identifiers (DIDs) v1.0, 2022
Dave Longley, Dmitri Zagidulin, Manu Sporny, The did:key Method v0.7, 2022
M. Nottingham, Well-Known Uniform Resource Identifiers (URIs), 2019
silverpill, FEP-8b32: Object Integrity Proofs, 2022
silverpill, FEP-ae97: Client-side activity signing, 2023
silverpill, FEP-fe34: Origin-based security model, 2024
A. Barth, The Web Origin Concept, 2011
silverpill, FEP-2277: ActivityPub core types, 2025
M. Sporny, L. Rosenthol, Cryptographic Hyperlinks, 2021
silverpill, FEP-521a: Representing actor's public keys, 2023
a, Evan Prodromou, ActivityPub and WebFinger, 2024
Dave Longley, Manu Sporny, Verifiable Credential Data Integrity 1.0, 2024
Adam R. Nelson, FEP-fffd: Proxy Objects, 2023
Jonne Haß, NodeInfo, 2014

著作権

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FEP-ef61: Portable Objects

Summary

Motivation

History

Requirements

Identifiers

ap:// URLs

DID methods

Dereferencing ap:// URLs

Authentication and authorization

Portable actors

Location hints

Inboxes and outboxes

Collections

Portable objects

Media

Compatibility

Identifiers

WebFinger addresses

Discussion

'ap' URL syntax

Discovering locations

Arbitrary paths

Alternatives to gateways property

DID services

Media access control

Compatibility

実装

参考文献

著作権

Alternatives to `gateways` property