Federation API

Matrix homeservers use the Federation APIs (also known as server-server APIs) to communicate with each other. Homeservers use these APIs to push messages to each other in real-time, to request historic messages from each other, and to query profile and presence information about users on each other's servers.

The APIs are implemented using HTTPS GETs and PUTs between each of the servers. These HTTPS requests are strongly authenticated using public key signatures at the TLS transport layer and using public key signatures in HTTP Authorization headers at the HTTP layer.

There are three main kinds of communication that occur between homeservers:

Persisted Data Units (PDUs):

These events are broadcast from one homeserver to any others that have joined the same room (identified by Room ID). They are persisted in long-term storage and record the history of messages and state for a room.

Like email, it is the responsibility of the originating server of a PDU to deliver that event to its recipient servers. However PDUs are signed using the originating server's private key so that it is possible to deliver them through third-party servers.

Ephemeral Data Units (EDUs):
These events are pushed between pairs of homeservers. They are not persisted and are not part of the history of a room, nor does the receiving homeserver have to reply to them.
Queries:
These are single request/response interactions between a given pair of servers, initiated by one side sending an HTTPS GET request to obtain some information, and responded by the other. They are not persisted and contain no long-term significant history. They simply request a snapshot state at the instant the query is made.

EDUs and PDUs are further wrapped in an envelope called a Transaction, which is transferred from the origin to the destination homeserver using an HTTPS PUT request.

1   Specification version

This version of the specification is generated from matrix-doc as of Git commit HEAD,bdc9360.

2   Server Discovery

2.1   Resolving Server Names

Each matrix homeserver is identified by a server name consisting of a DNS name and an optional TLS port.

server_name = dns_name [ ":" tls_port]
dns_name = <host, see [RFC 3986], Section 3.2.2>
tls_port = *DIGIT

If the port is present then the server is discovered by looking up an AAAA or A record for the DNS name and connecting to the specified TLS port. If the port is absent then the server is discovered by looking up a _matrix._tcp SRV record for the DNS name. If this record does not exist then the server is discovered by looking up an AAAA or A record on the DNS name and taking the default fallback port number of 8448. Homeservers may use SRV records to load balance requests between multiple TLS endpoints or to failover to another endpoint if an endpoint fails.

2.2   Retrieving Server Keys

2.2.1   Version 2

Each homeserver publishes its public keys under /_matrix/key/v2/server/. Homeservers query for keys by either getting /_matrix/key/v2/server/ directly or by querying an intermediate notary server using a /_matrix/key/v2/query API. Intermediate notary servers query the /_matrix/key/v2/server/ API on behalf of another server and sign the response with their own key. A server may query multiple notary servers to ensure that they all report the same public keys.

This approach is borrowed from the Perspectives Project, but modified to include the NACL keys and to use JSON instead of XML. It has the advantage of avoiding a single trust-root since each server is free to pick which notary servers they trust and can corroborate the keys returned by a given notary server by querying other servers.

2.2.1.1   Publishing Keys

Homeservers publish the allowed TLS fingerprints and signing keys in a JSON object at /_matrix/key/v2/server/{key_id}. The response contains a list of verify_keys that are valid for signing federation requests made by the server and for signing events. It contains a list of old_verify_keys which are only valid for signing events. Finally the response contains a list of TLS certificate fingerprints to validate any connection made to the server.

A server may have multiple keys active at a given time. A server may have any number of old keys. It is recommended that servers return a single JSON response listing all of its keys whenever any key_id is requested to reduce the number of round trips needed to discover the relevant keys for a server. However a server may return a different responses for a different key_id.

The tls_certificates contain a list of hashes of the X.509 TLS certificates currently used by the server. The list must include SHA-256 hashes for every certificate currently in use by the server. These fingerprints are valid until the millisecond POSIX timestamp in valid_until_ts.

The verify_keys can be used to sign requests and events made by the server until the millisecond POSIX timestamp in valid_until_ts. If a homeserver receives an event with a origin_server_ts after the valid_until_ts then it should request that key_id for the originating server to check whether the key has expired.

The old_verify_keys can be used to sign events with an origin_server_ts before the expired_ts. The expired_ts is a millisecond POSIX timestamp of when the originating server stopped using that key.

Intermediate notary servers should cache a response for half of its remaining life time to avoid serving a stale response. Originating servers should avoid returning responses that expire in less than an hour to avoid repeated requests for an about to expire certificate. Requesting servers should limit how frequently they query for certificates to avoid flooding a server with requests.

If a server goes offline intermediate notary servers should continue to return the last response they received from that server so that the signatures of old events sent by that server can still be checked.

Key Type Description
server_name String DNS name of the homeserver.
verify_keys Object Public keys of the homeserver for verifying digital signatures.
old_verify_keys Object The public keys that the server used to use and when it stopped using them.
signatures Object Digital signatures for this object signed using the verify_keys.
tls_fingerprints Array of Objects Hashes of X.509 TLS certificates used by this this server encoded as Unpadded Base64.
valid_until_ts Integer POSIX timestamp when the list of valid keys should be refreshed.
{
    "old_verify_keys": {
        "ed25519:auto1": {
            "expired_ts": 922834800000,
            "key": "Base+64+Encoded+Old+Verify+Key"
        }
    },
    "server_name": "example.org",
    "signatures": {
        "example.org": {
            "ed25519:auto2": "Base+64+Encoded+Signature"
        }
    },
    "tls_fingerprints": [
        {
            "sha256": "Base+64+Encoded+SHA-256-Fingerprint"
        }
    ],
    "valid_until_ts": 1052262000000,
    "verify_keys": {
        "ed25519:auto2": {
            "key": "Base+64+Encoded+Signature+Verification+Key"
        }
    }
}

2.2.1.2   Querying Keys Through Another Server

Servers may offer a query API _matrix/key/v2/query/ for getting the keys for another server. This API can be used to GET at list of JSON objects for a given server or to POST a bulk query for a number of keys from a number of servers. Either way the response is a list of JSON objects containing the JSON published by the server under _matrix/key/v2/server/ signed by both the originating server and by this server.

The minimum_valid_until_ts is a millisecond POSIX timestamp indicating when the returned certificate will need to be valid until to be useful to the requesting server. This can be set using the maximum origin_server_ts of an batch of events that a requesting server is trying to validate. This allows an intermediate notary server to give a prompt cached response even if the originating server is offline.

This API can return keys for servers that are offline be using cached responses taken from when the server was online. Keys can be queried from multiple servers to mitigate against DNS spoofing.

Requests:

GET /_matrix/key/v2/query/${server_name}/${key_id}/?minimum_valid_until_ts=${minimum_valid_until_ts} HTTP/1.1

POST /_matrix/key/v2/query HTTP/1.1
Content-Type: application/json

{
    "server_keys": {
        "$server_name": {
            "$key_id": {
                "minimum_valid_until_ts": $posix_timestamp
            }
        }
    }
}

Response:

HTTP/1.1 200 OK
Content-Type: application/json
{
    "server_keys": [
       # List of responses with same format as /_matrix/key/v2/server
       # signed by both the originating server and this server.
    ]
}

2.2.2   Version 1

Warning

Version 1 of key distribution is obsolete

Homeservers publish their TLS certificates and signing keys in a JSON object at /_matrix/key/v1.

Key Type Description
server_name String DNS name of the homeserver.
verify_keys Object Public keys of the homeserver for verifying digital signatures.
signatures Object Digital signatures for this object signed using the verify_keys.
tls_certificate String The X.509 TLS certificate used by this this server encoded as Unpadded Base64.
{
    "server_name": "example.org",
    "signatures": {
        "example.org": {
            "ed25519:auto": "Base+64+Encoded+Signature"
        }
    },
    "tls_certificate": "Base+64+Encoded+DER+Encoded+X509+TLS+Certificate"
    "verify_keys": {
        "ed25519:auto": "Base+64+Encoded+Signature+Verification+Key"
    }
}

When fetching the keys for a server the client should check that the TLS certificate in the JSON matches the TLS server certificate for the connection and should check that the JSON signatures are correct for the supplied verify_keys

3   Transactions

Warning

This section may be misleading or inaccurate.

The transfer of EDUs and PDUs between homeservers is performed by an exchange of Transaction messages, which are encoded as JSON objects, passed over an HTTP PUT request. A Transaction is meaningful only to the pair of homeservers that exchanged it; they are not globally-meaningful.

Each transaction has:
  • An opaque transaction ID.
  • A timestamp (UNIX epoch time in milliseconds) generated by its origin server.
  • An origin and destination server name.
  • A list of "previous IDs".
  • A list of PDUs and EDUs - the actual message payload that the Transaction carries.

3.1   Transaction Fields

Key Type Description
origin String DNS name of homeserver making this transaction.
origin_server_ts Integer Timestamp in milliseconds on originating homeserver when this transaction started.
previous_ids List of Strings List of transactions that were sent immediately prior to this transaction.
pdus List of Objects List of persistent updates to rooms.
edus List of Objects List of ephemeral messages.
{
 "transaction_id":"916d630ea616342b42e98a3be0b74113",
 "ts":1404835423000,
 "origin":"red",
 "prev_ids":["e1da392e61898be4d2009b9fecce5325"],
 "pdus":[...],
 "edus":[...]
}

The prev_ids field contains a list of previous transaction IDs that the origin server has sent to this destination. Its purpose is to act as a sequence checking mechanism - the destination server can check whether it has successfully received that Transaction, or ask for a re-transmission if not.

The pdus field of a transaction is a list, containing zero or more PDUs.[*] Each PDU is itself a JSON object containing a number of keys, the exact details of which will vary depending on the type of PDU. Similarly, the edus field is another list containing the EDUs. This key may be entirely absent if there are no EDUs to transfer.

(* Normally the PDU list will be non-empty, but the server should cope with receiving an "empty" transaction, as this is useful for informing peers of other transaction IDs they should be aware of. This effectively acts as a push mechanism to encourage peers to continue to replicate content.)

4   PDUs

All PDUs have:

  • An ID to identify the PDU itself
  • A room ID that it relates to
  • A declaration of their type
  • A list of other PDU IDs that have been seen recently in that room (regardless of which origin sent them)

4.1   Required PDU Fields

Key Type Description
context String Room identifier
user_id String The ID of the user sending the PDU
origin String DNS name of homeserver that created this PDU
pdu_id String Unique identifier for PDU on the originating homeserver
origin_server_ts Integer Timestamp in milliseconds on origin homeserver when this PDU was created.
pdu_type String PDU event type
content Object The content of the PDU.
prev_pdus List of (String, String, Object) Triplets The originating homeserver, PDU ids and hashes of the most recent PDUs the homeserver was aware of for the room when it made this PDU
depth Integer The maximum depth of the previous PDUs plus one
is_state Boolean True if this PDU is updating room state
{
 "context":"#example:green.example.com",
 "origin":"green.example.com",
 "pdu_id":"a4ecee13e2accdadf56c1025af232176",
 "origin_server_ts":1404838188000,
 "pdu_type":"m.room.message",
 "prev_pdus":[
   ["blue.example.com","99d16afbc8",
       {"sha256":"abase64encodedsha256hashshouldbe43byteslong"}]
 ],
 "hashes":{"sha256":"thishashcoversallfieldsincasethisisredacted"},
 "signatures":{
   "green.example.com":{
     "ed25519:key_version:":"these86bytesofbase64signaturecoveressentialfieldsincludinghashessocancheckredactedpdus"
   }
 },
 "is_state":false,
 "content": {...}
}

In contrast to Transactions, it is important to note that the prev_pdus field of a PDU refers to PDUs that any origin server has sent, rather than previous IDs that this origin has sent. This list may refer to other PDUs sent by the same origin as the current one, or other origins.

Because of the distributed nature of participants in a Matrix conversation, it is impossible to establish a globally-consistent total ordering on the events. However, by annotating each outbound PDU at its origin with IDs of other PDUs it has received, a partial ordering can be constructed allowing causality relationships to be preserved. A client can then display these messages to the end-user in some order consistent with their content and ensure that no message that is semantically in reply of an earlier one is ever displayed before it.

4.2   State Update PDU Fields

PDUs fall into two main categories: those that deliver Events, and those that synchronise State. For PDUs that relate to State synchronisation, additional keys exist to support this:

Key Type Description
state_key String Combined with the pdu_type this identifies the which part of the room state is updated
required_power_level Integer The required power level needed to replace this update.
prev_state_id String The PDU id of the update this replaces.
prev_state_origin String The homeserver of the update this replaces.
user_id String The user updating the state.
{...,
 "is_state":true,
 "state_key":TODO-doc
 "required_power_level":TODO-doc
 "prev_state_id":TODO-doc
 "prev_state_origin":TODO-doc
}

5   EDUs

EDUs, by comparison to PDUs, do not have an ID, a room ID, or a list of "previous" IDs. The only mandatory fields for these are the type, origin and destination homeserver names, and the actual nested content.

Key Type Description
edu_type String The type of the ephemeral message.
content Object Content of the ephemeral message.
{
 "edu_type":"m.presence",
 "origin":"blue",
 "destination":"orange",
 "content":{...}
}

6   Protocol URLs

Warning

This section may be misleading or inaccurate.

All these URLs are name-spaced within a prefix of:

/_matrix/federation/v1/...

For active pushing of messages representing live activity "as it happens":

PUT .../send/<transaction_id>/
  Body: JSON encoding of a single Transaction
  Response: TODO-doc

The transaction_id path argument will override any ID given in the JSON body. The destination name will be set to that of the receiving server itself. Each embedded PDU in the transaction body will be processed.

To fetch all the state of a given room:

GET .../state/<room_id>/
  Response: JSON encoding of a single Transaction containing multiple PDUs

Retrieves a snapshot of the entire current state of the given room. The response will contain a single Transaction, inside which will be a list of PDUs that encode the state.

To fetch a particular event:

GET .../event/<event_id>/
  Response: JSON encoding of a partial Transaction containing the event

Retrieves a single event. The response will contain a partial Transaction, having just the origin, origin_server_ts and pdus fields; the event will be encoded as the only PDU in the pdus list.

To backfill events on a given room:

GET .../backfill/<room_id>/
  Query args: v, limit
  Response: JSON encoding of a single Transaction containing multiple PDUs

Retrieves a sliding-window history of previous PDUs that occurred on the given room. Starting from the PDU ID(s) given in the "v" argument, the PDUs that preceded it are retrieved, up to a total number given by the "limit" argument.

To stream events all the events:

GET .../pull/
  Query args: origin, v
  Response: JSON encoding of a single Transaction consisting of multiple PDUs

Retrieves all of the transactions later than any version given by the "v" arguments.

To make a query:

GET .../query/<query_type>
  Query args: as specified by the individual query types
  Response: JSON encoding of a response object

Performs a single query request on the receiving homeserver. The Query Type part of the path specifies the kind of query being made, and its query arguments have a meaning specific to that kind of query. The response is a JSON-encoded object whose meaning also depends on the kind of query.

To join a room:

GET .../make_join/<room_id>/<user_id>
  Response: JSON encoding of a join proto-event

PUT .../send_join/<room_id>/<event_id>
  Response: JSON encoding of the state of the room at the time of the event

Performs the room join handshake. For more information, see "Joining Rooms" below.

7   Joining Rooms

When a new user wishes to join room that the user's homeserver already knows about, the homeserver can immediately determine if this is allowable by inspecting the state of the room, and if it is acceptable, it can generate, sign, and emit a new m.room.member state event adding the user into that room. When the homeserver does not yet know about the room it cannot do this directly. Instead, it must take a longer multi-stage handshaking process by which it first selects a remote homeserver which is already participating in that room, and uses it to assist in the joining process. This is the remote join handshake.

This handshake involves the homeserver of the new member wishing to join (referred to here as the "joining" server), the directory server hosting the room alias the user is requesting to join with, and a homeserver where existing room members are already present (referred to as the "resident" server).

In summary, the remote join handshake consists of the joining server querying the directory server for information about the room alias; receiving a room ID and a list of join candidates. The joining server then requests information about the room from one of the residents. It uses this information to construct a m.room.member event which it finally sends to a resident server.

Conceptually these are three different roles of homeserver. In practice the directory server is likely to be resident in the room, and so may be selected by the joining server to be the assisting resident. Likewise, it is likely that the joining server picks the same candidate resident for both phases of event construction, though in principle any valid candidate may be used at each time. Thus, any join handshake can potentially involve anywhere from two to four homeservers, though most in practice will use just two.

Client         Joining                Directory       Resident
               Server                 Server          Server

join request -->
               |
               directory request ------->
               <---------- directory response
               |
               make_join request ----------------------->
               <------------------------------- make_join response
               |
               send_join request ----------------------->
               <------------------------------- send_join response
               |
<---------- join response

The first part of the handshake usually involves using the directory server to request the room ID and join candidates. This is covered in more detail on the directory server documentation, below. In the case of a new user joining a room as a result of a received invite, the joining user's homeserver could optimise this step away by picking the origin server of that invite message as the join candidate. However, the joining server should be aware that the origin server of the invite might since have left the room, so should be prepared to fall back on the regular join flow if this optimisation fails.

Once the joining server has the room ID and the join candidates, it then needs to obtain enough information about the room to fill in the required fields of the m.room.member event. It obtains this by selecting a resident from the candidate list, and requesting the make_join endpoint using a GET request, specifying the room ID and the user ID of the new member who is attempting to join.

The resident server replies to this request with a JSON-encoded object having a single key called event; within this is an object whose fields contain some of the information that the joining server will need. Despite its name, this object is not a full event; notably it does not need to be hashed or signed by the resident homeserver. The required fields are:

Key Type Description
type String The value m.room.member
auth_events List An event-reference list containing the authorization events that would allow this member to join
content Object The event content
depth Integer (this field must be present but is ignored; it may be 0)
origin String The name of the resident homeserver
origin_server_ts Integer A timestamp added by the resident homeserver
prev_events List An event-reference list containing the immediate predecessor events
room_id String The room ID of the room
sender String The user ID of the joining member
state_key String The user ID of the joining member

The content field itself must be an object, containing:

Key Type Description
membership String The value join

The joining server now has sufficient information to construct the real join event from these protoevent fields. It copies the values of most of them, adding (or replacing) the following fields:

Key Type Description
event_id String A new event ID specified by the joining homeserver
origin String The name of the joining homeserver
origin_server_ts Integer A timestamp added by the joining homeserver

This will be a true event, so the joining server should apply the event-signing algorithm to it, resulting in the addition of the hashes and signatures fields.

To complete the join handshake, the joining server must now submit this new event to an resident homeserver, by using the send_join endpoint. This is invoked using the room ID and the event ID of the new member event.

The resident homeserver then accepts this event into the room's event graph, and responds to the joining server with the full set of state for the newly- joined room. This is returned as a two-element list, whose first element is the integer 200, and whose second element is an object which contains the following keys:

Key Type Description
auth_chain List A list of events giving the authorization chain for this join event
state List A complete list of the prevailing state events at the instant just before accepting the new m.room.member event

8   Backfilling

Once a homeserver has joined a room, it receives all the events emitted by other homeservers in that room, and is thus aware of the entire history of the room from that moment onwards. Since users in that room are able to request the history by the /messages client API endpoint, it's possible that they might step backwards far enough into history before the homeserver itself was a member of that room.

To cover this case, the federation API provides a server-to-server analog of the /messages client API, allowing one homeserver to fetch history from another. This is the /backfill API.

To request more history, the requesting homeserver picks another homeserver that it thinks may have more (most likely this should be a homeserver for some of the existing users in the room at the earliest point in history it has currently), and makes a /backfill request. The parameters of this request give an event ID that the requesting homeserver wishes to obtain, and a number specifying how many more events of history before that one to return at most.

The response to this request is an object with the following keys:

Key Type Description
pdus List A list of events
origin String The name of the resident homeserver
origin_server_ts Integer A timestamp added by the resident homeserver

The list of events given in pdus is returned in reverse chronological order; having the most recent event first (i.e. the event whose event ID is that requested by the requestor in the v parameter).

9   Authentication

9.1   Request Authentication

Every HTTP request made by a homeserver is authenticated using public key digital signatures. The request method, target and body are signed by wrapping them in a JSON object and signing it using the JSON signing algorithm. The resulting signatures are added as an Authorization header with an auth scheme of X-Matrix. Note that the target field should include the full path starting with /_matrix/..., including the ? and any query parameters if present, but should not include the leading https:, nor the destination server's hostname.

Step 1 sign JSON:

 {
     "method": "GET",
     "uri": "/target",
     "origin": "origin.hs.example.com",
     "destintation": "destination.hs.example.com",
     "content": { JSON content ... },
     "signatures": {
         "origin.hs.example.com": {
             "ed25519:key1": "ABCDEF..."
         }
     }
}

Step 2 add Authorization header:

GET /target HTTP/1.1
Authorization: X-Matrix origin=origin.example.com,key="ed25519:key1",sig="ABCDEF..."
Content-Type: application/json

{ JSON content ... }

Example python code:

def authorization_headers(origin_name, origin_signing_key,
                          destination_name, request_method, request_target,
                          content_json=None):
    request_json = {
         "method": request_method,
         "uri": request_target,
         "origin": origin_name,
         "destination": destination_name,
    }

    if content_json is not None:
        request["content"] = content_json

    signed_json = sign_json(request_json, origin_name, origin_signing_key)

    authorization_headers = []

    for key, sig in signed_json["signatures"][origin_name].items():
        authorization_headers.append(bytes(
            "X-Matrix origin=%s,key=\"%s\",sig=\"%s\"" % (
                origin_name, key, sig,
            )
        ))

    return ("Authorization", authorization_headers)

9.2   Response Authentication

Responses are authenticated by the TLS server certificate. A homeserver should not send a request until it has authenticated the connected server to avoid leaking messages to eavesdroppers.

9.3   Client TLS Certificates

Requests are authenticated at the HTTP layer rather than at the TLS layer because HTTP services like Matrix are often deployed behind load balancers that handle the TLS and these load balancers make it difficult to check TLS client certificates.

A homeserver may provide a TLS client certificate and the receiving homeserver may check that the client certificate matches the certificate of the origin homeserver.

11   State Conflict Resolution

Note

This section is a work in progress.

12   Presence

The server API for presence is based entirely on exchange of the following EDUs. There are no PDUs or Federation Queries involved.

Performing a presence update and poll subscription request:

EDU type: m.presence

Content keys:
  push: (optional): list of push operations.
    Each should be an object with the following keys:
      user_id: string containing a User ID
      presence: "offline"|"unavailable"|"online"|"free_for_chat"
      status_msg: (optional) string of free-form text
      last_active_ago: milliseconds since the last activity by the user

  poll: (optional): list of strings giving User IDs

  unpoll: (optional): list of strings giving User IDs

The presence of this combined message is two-fold: it informs the recipient server of the current status of one or more users on the sending server (by the push key), and it maintains the list of users on the recipient server that the sending server is interested in receiving updates for, by adding (by the poll key) or removing them (by the unpoll key). The poll and unpoll lists apply changes to the implied list of users; any existing IDs that the server sent as poll operations in a previous message are not removed until explicitly requested by a later unpoll.

On receipt of a message containing a non-empty poll list, the receiving server should immediately send the sending server a presence update EDU of its own, containing in a push list the current state of every user that was in the original EDU's poll list.

Sending a presence invite:

EDU type: m.presence_invite

Content keys:
  observed_user: string giving the User ID of the user whose presence is
    requested (i.e. the recipient of the invite)
  observer_user: string giving the User ID of the user who is requesting to
    observe the presence (i.e. the sender of the invite)

Accepting a presence invite:

EDU type: m.presence_accept

Content keys - as for m.presence_invite

Rejecting a presence invite:

EDU type: m.presence_deny

Content keys - as for m.presence_invite

13   Profiles

The server API for profiles is based entirely on the following Federation Queries. There are no additional EDU or PDU types involved, other than the implicit m.presence and m.room.member events (see section below).

Querying profile information:

Query type: profile

Arguments:
  user_id: the ID of the user whose profile to return
  field: (optional) string giving a field name

Returns: JSON object containing the following keys:
  displayname: string of free-form text
  avatar_url: string containing an HTTP-scheme URL

If the query contains the optional field key, it should give the name of a result field. If such is present, then the result should contain only a field of that name, with no others present. If not, the result should contain as much of the user's profile as the homeserver has available and can make public.

14   Directory

The server API for directory queries is also based on Federation Queries.

Querying directory information:

Query type: directory

Arguments:
  room_alias: the room alias to query

Returns: JSON object containing the following keys:
  room_id: string giving the underlying room ID the alias maps to
  servers: list of strings giving the join candidates

The list of join candidates is a list of server names that are likely to hold the given room; these are servers that the requesting server may wish to use as resident servers as part of the remote join handshake. This list may or may not include the server answering the query.

15   Send-to-device messaging

The server API for send-to-device messaging is based on the following EDU. There are no PDUs or Federation Queries involved.

Each send-to-device message should be sent to the destination server using the following EDU:

EDU type: m.direct_to_device

Content keys:
  sender: user ID of the sender

  type: event type for the message

  message_id: unique id for the message: used for idempotence

  messages: The messages to send. A map from user ID, to a map from device ID
      to message body. The device ID may also be *, meaning all known devices
      for the user.

16   Signing Events

Signing events is complicated by the fact that servers can choose to redact non-essential parts of an event.

Before signing the event, the unsigned and signature members are removed, it is encoded as Canonical JSON, and then hashed using SHA-256. The resulting hash is then stored in the event JSON in a hash object under a sha256 key.

def hash_event(event_json_object):

    # Keys under "unsigned" can be modified by other servers.
    # They are useful for conveying information like the age of an
    # event that will change in transit.
    # Since they can be modifed we need to exclude them from the hash.
    unsigned = event_json_object.pop("unsigned", None)

    # Signatures will depend on the current value of the "hashes" key.
    # We cannot add new hashes without invalidating existing signatures.
    signatures = event_json_object.pop("signatures", None)

    # The "hashes" key might contain multiple algorithms if we decide to
    # migrate away from SHA-2. We don't want to include an existing hash
    # output in our hash so we exclude the "hashes" dict from the hash.
    hashes = event_json_object.pop("hashes", {})

    # Encode the JSON using a canonical encoding so that we get the same
    # bytes on every server for the same JSON object.
    event_json_bytes = encode_canonical_json(event_json_bytes)

    # Add the base64 encoded bytes of the hash to the "hashes" dict.
    hashes["sha256"] = encode_base64(sha256(event_json_bytes).digest())

    # Add the "hashes" dict back the event JSON under a "hashes" key.
    event_json_object["hashes"] = hashes
    if unsigned is not None:
        event_json_object["unsigned"] = unsigned
    return event_json_object

The event is then stripped of all non-essential keys both at the top level and within the content object. Any top-level keys not in the following list MUST be removed:

auth_events
depth
event_id
hashes
membership
origin
origin_server_ts
prev_events
prev_state
room_id
sender
signatures
state_key
type

A new content object is constructed for the resulting event that contains only the essential keys of the original content object. If the original event lacked a content object at all, a new empty JSON object is created for it.

The keys that are considered essential for the content object depend on the the type of the event. These are:

type is "m.room.aliases":
  aliases

type is "m.room.create":
  creator

type is "m.room.history_visibility":
  history_visibility

type is "m.room.join_rules":
  join_rule

type is "m.room.member":
  membership

type is "m.room.power_levels":
  ban
  events
  events_default
  kick
  redact
  state_default
  users
  users_default

The resulting stripped object with the new content object and the original hashes key is then signed using the JSON signing algorithm outlined below:

def sign_event(event_json_object, name, key):

    # Make sure the event has a "hashes" key.
    if "hashes" not in event_json_object:
        event_json_object = hash_event(event_json_object)

    # Strip all the keys that would be removed if the event was redacted.
    # The hashes are not stripped and cover all the keys in the event.
    # This means that we can tell if any of the non-essential keys are
    # modified or removed.
    stripped_json_object = strip_non_essential_keys(event_json_object)

    # Sign the stripped JSON object. The signature only covers the
    # essential keys and the hashes. This means that we can check the
    # signature even if the event is redacted.
    signed_json_object = sign_json(stripped_json_object)

    # Copy the signatures from the stripped event to the original event.
    event_json_object["signatures"] = signed_json_oject["signatures"]
    return event_json_object

Servers can then transmit the entire event or the event with the non-essential keys removed. If the entire event is present, receiving servers can then check the event by computing the SHA-256 of the event, excluding the hash object. If the keys have been redacted, then the hash object is included when calculating the SHA-256 instead.

New hash functions can be introduced by adding additional keys to the hash object. Since the hash object cannot be redacted a server shouldn't allow too many hashes to be listed, otherwise a server might embed illict data within the hash object. For similar reasons a server shouldn't allow hash values that are too long.