ActivityPub federation in Mitra

Mitra largely follows the ActivityPub server-to-server specification but it makes uses of some non-standard extensions, some of which are required for interacting with it.

The following activities are supported:

Accept(Follow)
Reject(Follow)
Undo(Follow)
Create(Note)
Delete(Note)
Like(Note)
Undo(Like)
Announce(Note)
Undo(Announce)
Update(Note)
Follow(Person)
Update(Person)
Move(Person)
Delete(Person)
Add(Person)
Remove(Person)

And these additional standards:

Activities are implemented in way that is compatible with Pleroma, Mastodon and other popular ActivityPub servers.

Supported FEPs:

Object integrity proofs

All outgoing activities are signed with actor's key in accordance with FEP-8b32 document.

Example:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://w3id.org/security/data-integrity/v1"
  ],
  "actor": "https://example.com/users/alice",
  "cc": [],
  "id": "https://example.com/objects/0185f5f8-10b5-1b69-f45e-25f06792f411",
  "object": "https://example.net/users/bob/posts/141892712081205472",
  "proof": {
    "created": "2023-01-28T01:22:40.183273595Z",
    "proofPurpose": "assertionMethod",
    "proofValue": "z5djAdMSrV...",
    "type": "MitraJcsRsaSignature2022",
    "verificationMethod": "https://example.com/users/alice#main-key"
  },
  "to": [
    "https://example.net/users/bob",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type":"Like"
}

Supported proof suites

MitraJcsRsaSignature2022

Canonicalization algorithm: JCS
Hashing algorithm: SHA-256
Signature algorithm: RSASSA-PKCS1-v1_5

MitraJcsEip191Signature2022

Canonicalization algorithm: JCS
Hashing algorithm: KECCAK-256 (EIP-191)
Signature algorithm: ECDSA (EIP-191)

MitraJcsEd25519Signature2022

Canonicalization algorithm: JCS
Hashing algorithm: BLAKE2b-512
Signature algorithm: EdDSA

Custom emojis

Custom emojis are implemented as described in Mastodon documentation: https://docs.joinmastodon.org/spec/activitypub/#emoji.

Profile extensions

Cryptocurrency addresses

Cryptocurrency addresses are represented as PropertyValue attachments where name attribute is a currency symbol prefixed with $:

{
  "name": "$XMR",
  "type": "PropertyValue",
  "value": "8Ahza5RM4JQgtdqvpcF1U628NN5Q87eryXQad3Fy581YWTZU8o3EMbtScuioQZSkyNNEEE1Lkj2cSbG4VnVYCW5L1N4os5p"
}

Identity proofs

Identity proofs are represented as attachments of IdentityProof type:

{
  "name": "<did>",
  "type": "IdentityProof",
  "signatureAlgorithm": "<proof-type>",
  "signatureValue": "<proof>"
}

Supported proof types:

EIP-191 (Ethereum personal signatures)
Minisign

FEP-c390 identity proofs are not supported yet.

Account migrations

After registering an account its owner can upload the list of followers and start the migration process. The server then sends Move activity to each follower:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams"
  ],
  "actor": "https://server2.com/users/alice",
  "id": "https://server2.com/activities/00000000-0000-0000-0000-000000000001",
  "object": "https://server1.com/users/alice",
  "target": "https://server2.com/users/alice",
  "to": [
    "https://example.com/users/bob"
  ],
  "type": "Move"
}

Where object is an ID of old account and target is an ID of new account. Actors identified by object and target properties must have at least one identity key in common to be considered aliases. Upon receipt of such activity, actors that follow object should un-follow it and follow target instead.

Subscription events

Local actor profiles have subscribers property which points to the collection of actor's paid subscribers.

The Add activity is used to notify the subscriber about successful subscription payment. Upon receipt of this activity, the receiving server should add specified object to actors's subscribers collection (specified in target property):

{
  "@context": [
    "https://www.w3.org/ns/activitystreams"
  ],
  "actor": "https://example.com/users/alice",
  "id": "https://example.com/activities/00000000-0000-0000-0000-000000000001",
  "object": "https://example.com/users/bob",
  "target": "https://example.com/users/alice/collections/subscribers",
  "to": [
    "https://example.com/users/bob"
  ],
  "type": "Add"
}

The Remove activity is used to notify the subscriber about expired subscription. Upon receipt of this activity, the receiving server should remove specified object from actors's subscribers collection (specified in target property):

{
  "@context": [
    "https://www.w3.org/ns/activitystreams"
  ],
  "actor": "https://example.com/users/alice",
  "id": "https://example.com/activities/00000000-0000-0000-0000-000000000002",
  "object": "https://example.com/users/bob",
  "target": "https://example.com/users/alice/collections/subscribers",
  "to": [
    "https://example.com/users/bob"
  ],
  "type": "Remove"
}

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