.. Copyright 2016 OpenMarket Ltd .. Copyright 2017 New Vector Ltd .. .. Licensed under the Apache License, Version 2.0 (the "License"); .. you may not use this file except in compliance with the License. .. You may obtain a copy of the License at .. .. http://www.apache.org/licenses/LICENSE-2.0 .. .. Unless required by applicable law or agreed to in writing, software .. distributed under the License is distributed on an "AS IS" BASIS, .. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. .. See the License for the specific language governing permissions and .. limitations under the License. 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. .. contents:: Table of Contents .. sectnum:: Specification version --------------------- This version of the specification is generated from `matrix-doc `_ as of Git commit `{{git_version}} `_. Server Discovery ---------------- Resolving Server Names ~~~~~~~~~~~~~~~~~~~~~~ Each matrix homeserver is identified by a server name consisting of a DNS name and an optional TLS port. .. code:: server_name = dns_name [ ":" tls_port] dns_name = 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. Server implementation ~~~~~~~~~~~~~~~~~~~~~~ {{version_ss_http_api}} Retrieving Server Keys ~~~~~~~~~~~~~~~~~~~~~~ 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. .. _Perspectives Project: https://web.archive.org/web/20170702024706/https://perspectives-project.org/ 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. ==================== =================== ====================================== .. code:: json { "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" } } } 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: .. code:: 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: .. code:: 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. ] } 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`_. ==================== =================== ====================================== .. code:: json { "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`` 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. 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. ==================== =================== ====================================== .. code:: json { "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.) 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) 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, The originating homeserver, PDU ids and String, Object) hashes of the most recent PDUs the Triplets 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 ==================== ================== ======================================= .. code:: json { "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. 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 ``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. ======================== ============ ========================================= Authorization of PDUs ~~~~~~~~~~~~~~~~~~~~~ Whenever a server receives an event from a remote server, the receiving server must check that the event is allowed by the authorization rules. These rules depend on the state of the room at that event. Definitions +++++++++++ Required Power Level A given event type has an associated *required power level*. This is given by the current ``m.room.power_levels`` event. The event type is either listed explicitly in the ``events`` section or given by either ``state_default`` or ``events_default`` depending on if the event is a state event or not. Invite Level, Kick Level, Ban Level, Redact Level The levels given by the ``invite``, ``kick``, ``ban``, and ``redact`` properties in the current ``m.room.power_levels`` state. Each defaults to 50 if unspecified. Target User For an ``m.room.member`` state event, the user given by the ``state_key`` of the event. Rules +++++ The rules governing whether an event is authorized depend solely on the state of the room at the point in the room graph at which the new event is to be inserted. The types of state events that affect authorization are: - ``m.room.create`` - ``m.room.member`` - ``m.room.join_rules`` - ``m.room.power_levels`` Servers should not create new events that reference unauthorized events. However, any event that does reference an unauthorized event is not itself automatically considered unauthorized. Unauthorized events that appear in the event graph do *not* have any effect on the state of the room. .. Note:: This is in contrast to redacted events which can still affect the state of the room. For example, a redacted ``join`` event will still result in the user being considered joined. 1. If type is ``m.room.create``, allow if and only if has depth 0 and it has no previous events - *i.e.* it is the first event in the room. #. If type is ``m.room.member``: a. If ``membership`` is ``join``: i. If the previous event in the room graph is an ``m.room.create``, the depth is 1 and the ``state_key`` is the creator, allow. #. If the ``sender`` does not match ``state_key``, reject. #. If the user's current membership state is ``invite`` or ``join``, allow. #. If the ``join_rule`` is ``public``, allow. #. Otherwise, reject. #. If ``membership`` is ``invite``: i. If the ``sender``'s current membership state is not ``joined``, reject. #. If *target user*'s current membership state is ``join`` or ``ban``, reject. #. If the ``sender``'s power level is greater than or equal to the *invite level*, allow. #. Otherwise, reject. #. If ``membership`` is ``leave``: i. If the ``sender`` matches ``state_key``, allow if and only if that user's current membership state is ``invite`` or ``join``. #. If the ``sender``'s current membership state is not ``joined``, reject. #. If the *target user*'s current membership state is ``ban``, and the ``sender``'s power level is less than the *ban level*, reject. #. If the ``sender``'s power level is greater than or equal to the *kick level*, and the *target user*'s power level is less than the ``sender``'s power level, allow. #. Otherwise, reject. #. If ``membership`` is ``ban``: i. If the ``sender``'s current membership state is not ``joined``, reject. #. If the ``sender``'s power level is greater than or equal to the *ban level*, and the *target user*'s power level is less than the ``sender``'s power level, allow. #. Otherwise, reject. #. Otherwise, the membership is unknown. Reject. #. If the ``sender``'s current membership state is not ``joined``, reject. #. If the event type's *required power level* is greater than the ``sender``'s power level, reject. #. If type is ``m.room.power_levels``: a. For each of the keys ``users_default``, ``events_default``, ``state_default``, ``ban``, ``redact``, ``kick``, ``invite``, as well as each entry being changed under the ``events`` or ``users`` keys: i. If the current value is higher than the ``sender``'s current power level, reject. #. If the new value is higher than the ``sender``'s current power level, reject. #. For each entry being changed under the ``users`` key, other than the ``sender``'s own entry: i. If the current value is equal to the ``sender``'s current power level, reject. #. Otherwise, allow. #. If type is ``m.room.redact``: #. If the ``sender``'s power level is greater than or equal to the *redact level*, allow. #. If the ``sender`` of the event being redacted is the same as the ``sender`` of the ``m.room.redact``, allow. #. Otherwise, reject. #. Otherwise, allow. .. NOTE:: Some consequences of these rules: * Unless you are a member of the room, the only permitted operations (apart from the intial create/join) are: joining a public room; accepting or rejecting an invitation to a room. * To unban somebody, you must have power level greater than or equal to both the kick *and* ban levels, *and* greater than the target user's power level. .. TODO-spec I think there is some magic about 3pid invites too. 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. ======================== ============ ========================================= .. code:: json { "edu_type":"m.presence", "origin":"blue", "destination":"orange", "content":{...} } 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// 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// 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// 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// 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 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// Response: JSON encoding of a join proto-event PUT .../send_join// 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. 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 ============== ===== ============ .. TODO-spec - (paul) I don't really understand why the full auth_chain events are given here. What purpose does it serve expanding them out in full, when surely they'll appear in the state anyway? 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). .. TODO-spec Specify (or remark that it is unspecified) how the server handles divergent history. DFS? BFS? Anything weirder? Inviting to a room ------------------ When a user wishes to invite an other user to a local room and the other user is on a different server, the inviting server will send a request to the invited server:: PUT .../invite/{roomId}/{eventId} The required fields in the JSON body are: ==================== ======== ============ Key Type Description ==================== ======== ============ ``room_id`` String The room ID of the room. Must be the same as the room ID specified in the path. ``event_id`` String The ID of the event. Must be the same as the event ID specified in the path. ``type`` String The value ``m.room.member``. ``auth_events`` List An event-reference list containing the IDs of the authorization events that would allow this member to be invited in the room. ``content`` Object The content of the event. ``depth`` Integer The depth of the event. ``origin`` String The name of the inviting homeserver. ``origin_server_ts`` Integer A timestamp added by the inviting homeserver. ``prev_events`` List An event-reference list containing the IDs of the immediate predecessor events. ``sender`` String The Matrix ID of the user who sent the original `m.room.third_party_invite`. ``state_key`` String The Matrix ID of the invited user. ``signatures`` Object The signature of the event from the origin server. ``unsigned`` Object An object containing the properties that aren't part of the signature's computation. ==================== ======== ============ Where the ``content`` key contains the content for the ``m.room.member`` event specified in the `Client-Server API`_. Note that the ``membership`` property of the content must be ``invite``. Upon receiving this request, the invited homeserver will append its signature to the event and respond to the request with the following JSON body:: [ 200, "event": {...} ] Where ``event`` contains the event signed by both homeservers, using the same JSON keys as the initial request on ``/invite/{roomId}/{eventId}``. Note that, except for the ``signatures`` object (which now contains an additional signature), all of the event's keys remain the same as in the event initially provided. This response format is due to a typo in Synapse, the first implementation of Matrix's APIs, and is preserved to maintain compatibility. Now that the event has been signed by both the inviting homeserver and the invited homeserver, it can be sent to all of the users in the room. Third-party invites ------------------- When an user wants to invite another user in a room but doesn't know the Matrix ID to invite, they can do so using a third-party identifier (e.g. an e-mail or a phone number). This identifier and its bindings to Matrix IDs are verified by an identity server implementing the `Identity Service API`_. Cases where an association exists for a third-party identifier ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If the third-party identifier is already bound to a Matrix ID, a lookup request on the identity server will return it. The invite is then processed by the inviting homeserver as a standard ``m.room.member`` invite event. This is the simplest case. Cases where an association doesn't exist for a third-party identifier ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If the third-party identifier isn't bound to any Matrix ID, the inviting homeserver will request the identity server to store an invite for this identifier and to deliver it to whoever binds it to its Matrix ID. It will also send a ``m.room.third_party_invite`` event in the room to specify a display name, a token and public keys the identity server provided as a response to the invite storage request. When a third-party identifier with pending invites gets bound to a Matrix ID, the identity server will send a ``POST`` request to the ID's homeserver as described in the `Invitation Storage`_ section of the Identity Service API. The following process applies for each invite sent by the identity server: The invited homeserver will create a ``m.room.member`` invite event containing a special ``third_party_invite`` section containing the token and a signed object, both provided by the identity server. If the invited homeserver is in the room the invite came from, it can auth the event and send it. However, if the invited homeserver isn't in the room the invite came from, it will need to request the room's homeserver to auth the event:: PUT .../exchange_third_party_invite/{roomId} Where ``roomId`` is the ID of the room the invite is for. The required fields in the JSON body are: ==================== ======= ================================================== Key Type Description ==================== ======= ================================================== ``type`` String The event type. Must be `m.room.member`. ``room_id`` String The ID of the room the event is for. Must be the same as the ID specified in the path. ``sender`` String The Matrix ID of the user who sent the original `m.room.third_party_invite`. ``state_key`` String The Matrix ID of the invited user. ``content`` Object The content of the event. ==================== ======= ================================================== Where the ``content`` key contains the content for the ``m.room.member`` event as described in the `Client-Server API`_. Its ``membership`` key must be ``invite`` and its content must include the ``third_party_invite`` object. The inviting homeserver will then be able to authenticate the event. It will send a fully authenticated event to the invited homeserver as described in the `Inviting to a room`_ section above. Once the invited homeserver responded with the event to which it appended its signature, the inviting homeserver will respond with ``200 OK`` and an empty body (``{}``) to the initial request on ``/exchange_third_party_invite/{roomId}`` and send the now verified ``m.room.member`` invite event to the room's members. Verifying the invite ++++++++++++++++++++ When a homeserver receives a ``m.room.member`` invite event for a room it's in with a ``third_party_invite`` object, it must verify that the association between the third-party identifier initially invited to the room and the Matrix ID that claims to be bound to it has been verified without having to rely on a third-party server. To do so, it will fetch from the room's state events the ``m.room.third_party_invite`` event for which the state key matches with the value for the ``token`` key in the ``third_party_invite`` object from the ``m.room.member`` event's content to fetch the public keys initially delivered by the identity server that stored the invite. It will then use these keys to verify that the ``signed`` object (in the ``third_party_invite`` object from the ``m.room.member`` event's content) was signed by the same identity server. Since this ``signed`` object can only be delivered once in the ``POST`` request emitted by the identity server upon binding between the third-party identifier and the Matrix ID, and contains the invited user's Matrix ID and the token delivered when the invite was stored, this verification will prove that the ``m.room.member`` invite event comes from the user owning the invited third-party identifier. Authentication -------------- 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: .. code:: { "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: .. code:: 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: .. code:: python 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) 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. 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. Server-Server Authorization --------------------------- .. TODO-doc - PDU signing (see the Event signing section earlier) - State conflict resolution (see below) State Conflict Resolution ------------------------- .. NOTE:: This section is a work in progress. .. TODO-doc - How do conflicts arise (diagrams?) - How are they resolved (incl tie breaks) - How does this work with deleting current state - How do we reject invalid federation traffic? [[TODO(paul): At this point we should probably have a long description of how State management works, with descriptions of clobbering rules, power levels, etc etc... But some of that detail is rather up-in-the-air, on the whiteboard, and so on. This part needs refining. And writing in its own document as the details relate to the server/system as a whole, not specifically to server-server federation.]] 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 .. TODO-doc - Explain the timing-based round-trip reduction mechanism for presence messages - Explain the zero-byte presence inference logic See also: docs/client-server/model/presence 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. 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. Send-to-device messaging ------------------------ .. TODO: add modules to the federation spec and make this a module 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. 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. .. code:: python 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: .. code:: 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: .. code:: 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: .. code:: python 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. .. TODO [[TODO(markjh): We might want to specify a maximum number of keys for the ``hash`` and we might want to specify the maximum output size of a hash]] [[TODO(markjh) We might want to allow the server to omit the output of well known hash functions like SHA-256 when none of the keys have been redacted]] .. _`Invitation storage`: ../identity_service/unstable.html#invitation-storage .. _`Identity Service API`: ../identity_service/unstable.html .. _`Client-Server API`: ../client_server/unstable.html#m-room-member .. _`Inviting to a room`: #inviting-to-a-room .. _`Canonical JSON`: ../appendices.html#canonical-json .. _`Unpadded Base64`: ../appendices.html#unpadded-base64