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Actor dispatcher

In ActivityPub, actors are entities that can perform activities. You can register an actor dispatcher so that Fedify can dispatch an appropriate actor by its bare handle (i.e., handle without @ prefix and domain suffix). Since the actor dispatcher is the most significant part of the Fedify, it is the first thing you need to do to make Fedify work.

An actor dispatcher is a callback function that takes a Context object and a bare handle, and returns an actor object. The actor object can be one of the following:

The below example shows how to register an actor dispatcher:

typescript
import { createFederation, Person } from "@fedify/fedify";

const federation = createFederation({
  // Omitted for brevity; see the related section for details.
});

federation.setActorDispatcher("/users/{handle}", async (ctx, handle) => {
  // Work with the database to find the actor by the handle.
  if (user == null) return null;  // Return null if the actor is not found.
  return new Person({
    id: ctx.getActorUri(handle),
    preferredUsername: handle,
    // Many more properties; see the next section for details.
  });
});

In the above example, the setActorDispatcher() method registers an actor dispatcher for the /users/{handle} path. This pattern syntax follows the URI Template specification.

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By registering the actor dispatcher, Federation.fetch() automatically deals with WebFinger requests for the actor.

Key properties of an Actor

Despite ActivityPub declares every property of an actor as optional, in practice, you need to set some of them to make the actor work properly with the existing ActivityPub implementations. The following shows the key properties of an Actor object:

id

The id property is the URI of the actor. It is a required property in ActivityPub. You can use the Context.getActorUri() method to generate the dereferenceable URI of the actor by its bare handle.

preferredUsername

The preferredUsername property is the bare handle of the actor. For the most cases, it is okay to set the preferredUsername property to the string taken from the handle parameter of the actor dispatcher.

name

The name property is the full name of the actor.

summary

The summary property is usually a short biography of the actor.

url

The url property usually refers to the actor's profile page.

published

The published property is the date and time when the actor was created. Note that Fedify represents the date and time in the Temporal.Instant value.

inbox

The inbox property is the URI of the actor's inbox. You can use the Context.getInboxUri() method to generate the URI of the actor's inbox.

See the Inbox listeners section for details.

outbox

The outbox property is the URI of the actor's outbox. You can use the Context.getOutboxUri() method to generate the URI of the actor's outbox.

followers

The followers property is the URI of the actor's followers collection. You can use the Context.getFollowersUri() method to generate the URI of the actor's followers collection.

following

The following property is the URI of the actor's following collection. You can use the Context.getFollowingUri() method to generate the URI of the actor's following collection.

endpoints

The endpoints property is an Endpoints instance, an object that contains the URIs of the actor's endpoints. The most important endpoint is the sharedInbox. You can use the Context.getInboxUri() method with no arguments to generate the URI of the actor's shared inbox:

typescript
new Endpoints({ sharedInbox: ctx.getInboxUri() })

publicKey

The publicKey property contains the public key of the actor. It is a CryptographicKey instance. This property is usually used for verifying HTTP Signatures.

See the next section for details.

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In theory, an actor has multiple publicKeys, but in practice, the most implementations have trouble with multiple keys. Therefore, it is recommended to set only one key in the publicKey property. Usually, it contains the first RSA-PKCS#1-v1.5 public key of the actor.

If you need to set multiple keys, you can use the assertionMethods property instead.

assertionMethods

This API is available since Fedify 0.10.0.

The assertionMethods property contains the public keys of the actor. It is an array of Multikey instances. This property is usually used for verifying Object Integrity Proofs.

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Usually, the assertionMethods property contains the Ed25519 public keys of the actor. Although it is okay to include RSA-PKCS#1-v1.5 public keys too, those RSA-PKCS#1-v1.5 keys are not used for verifying Object Integrity Proofs.

Public keys of an Actor

In order to sign and verify the activities, you need to set the publicKey property of the actor. The publicKey property contains a CryptographicKey instance, and usually you don't have to create it manually. Instead, you can register a key pairs dispatcher through the setKeyPairsDispatcher() method so that Fedify can dispatch appropriate key pairs by the actor's bare handle:

typescript
federation.setActorDispatcher("/users/{handle}", async (ctx, handle) => {
  // Work with the database to find the actor by the handle.
  if (user == null) return null;  // Return null if the actor is not found.
  return new Person({
    id: ctx.getActorUri(handle),
    preferredUsername: handle,
    // Context.getActorKeyPairs() method dispatches the key pairs of an actor
    // by the handle, and returns an array of key pairs in various formats.
    // In this example, we only use first CryptographicKey.
    publicKey: (await ctx.getActorKeyPairs(handle))[0].cryptographicKey,
    // Many more properties; see the previous section for details.
  });
})
  .setKeyPairsDispatcher(async (ctxData, handle) => {
    // Work with the database to find the key pair by the handle.
    if (user == null) return [];  // Return null if the key pair is not found.
    // Return the loaded key pair.  See the below example for details.
    return [{ publicKey, privateKey }];
  });

In the above example, the setKeyPairsDispatcher() method registers a key pairs dispatcher. The key pairs dispatcher is a callback function that takes context data and a bare handle, and returns an array of CryptoKeyPair object which is defined in the Web Cryptography API.

Usually, you need to generate key pairs for each actor when the actor is created (i.e., when a new user is signed up), and securely store an actor's key pairs in the database. The key pairs dispatcher should load the key pairs from the database and return them.

How to generate key pairs and store them in the database is out of the scope of this document, but here's a simple example of how to generate a key pair and store it in a Deno KV database in form of JWK:

typescript
import { generateCryptoKeyPair, exportJwk } from "@fedify/fedify";

const kv = await Deno.openKv();
const { privateKey, publicKey } =
  await generateCryptoKeyPair("RSASSA-PKCS1-v1_5");
await kv.set(["keypair", handle], {
  privateKey: await exportJwk(privateKey),
  publicKey: await exportJwk(publicKey),
});

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Fedify currently supports two key types:

HTTP Signatures is a de facto standard for signing ActivityPub activities, and Object Integrity Proofs is a new standard for verifying the integrity of the objects in the fediverse. While HTTP Signatures is widely supported in the fediverse, it's limited to the RSA-PKCS#1-v1.5 algorithm, and unusable for forwarding from inbox and several other cases.

If your federated app needs to support both HTTP Signatures and Object Integrity Proofs, you need to generate both RSA-PKCS#1-v1.5 and Ed25519 key pairs for each actor, and store them in the database.

Here's an example of how to load a key pair from the database too:

typescript
import { importJwk } from "@fedify/fedify";

federation
  .setActorDispatcher("/users/{handle}", async (ctx, handle) => {
    // Omitted for brevity; see the previous example for details.
  })
  .setKeyPairsDispatcher(async (ctxData, handle) => {
    const kv = await Deno.openKv();
    const entry = await kv.get<{ privateKey: JsonWebKey; publicKey: JsonWebKey }>(
      ["keypair", handle],
    );
    if (entry == null || entry.value == null) return [];
    return [
      {
        privateKey: await importJwk(entry.value.privateKey, "private"),
        publicKey: await importJwk(entry.value.publicKey, "public"),
      }
    ];
  });