]> The Matrix.org Foundation C.I.C.

travisr@matrix.org

The Matrix.org Foundation C.I.C.

matthew@matrix.org

Applications and Real-Time More Instant Messaging Interoperability matrix linearized interoperability messaging mimi Matrix is an existing openly specified decentralized secure communications protocol able to provide a framework for instant messaging interoperability. However, the existing model can be complex to reason about for simple interoperability usecases. With modifications to the room model, Matrix can support those simpler usecases more easily. This document explores "Linearized Matrix": the modified room model still backed by Matrix. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the More Instant Messaging Interoperability Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Alongside messaging, Matrix operates as an openly federated communications protocol for VoIP, IoT, and more. The existing Matrix network uses fully decentralized access control within rooms (conversations) and is highly extensible in its structure. These features are not critically important to a strict focus on messaging interoperability, however. This document describes "Linearized Matrix": a modified room model based upon Matrix's existing room model. This document does not explore how to interconnect Linearized Matrix with the existing Matrix room model - interested readers may wish to review MSC3995 within the Matrix Specification process.

Conventions and Definitions This document uses where possible. This document additionally uses the following definitions:

• Room: Synonymous with "conversation" from I-D.ralston-mimi-terminology.
• Room Member: Synonymous with "conversation member" from I-D.ralston-mimi-terminology.
• State Event: Synonymous with "conversation property" from I-D.ralston-mimi-terminology. A state event is a subclass of an event.

Further terms are introduced in-context within this document. TODO: We should move/copy those definitions up here anyways.

Architecture For a given conversation/room: | | | Provider/Server | | Provider/Server | | A |<---------( events )--------+ B | +-----+------------+ Server-Server API +------------------+ | ^ | | +------------------+ | +-----------------( events )--------+ | | | Provider/Server | +-----------------------( events )------->| C | +--------------+---+ ^ | | | | V .+-----------. | Client C | '------------' ]]> In this diagram, Server A is acting as a hub for the other two servers. Servers B and C do not converse directly when sending events to the room: those events are instead sent to the hub which then distributes them back out to all participating servers. Clients are shown in the diagram here for demonstrative purposes only. No client-server API is specified as part of Linearized Matrix, and the clients can be pre-existing or newly created for messaging. The objects given to clients are implementation-dependent, though for simplicity may be events. This leads to two distinct roles:

• Hub server: the server responsible for holding conversation history on behalf of other servers in the room.
• Participant server: any non-hub server. This server is not required to persist conversation history as it can fetch it from the hub if needed.

OPEN QUESTION: Should we support having multiple hubs for increased trust between participant and hub? (participant can pick the hub it wants to use rather than being forced to use a single hub)

Server names / domain names Throughout this document servers are referred to as having a "domain name" or "server name". A server name MUST be compliant with RFC 1123 (Section 2.1) . TODO: Should we incorporate Matrix's IPv6 extension, or are we able to assume that everyone will be using non-literal hostnames? TODO: Do we really need to make this case sensitive? Matrix does, but is that correct?

Rooms A room is a conceptual place where users send and receive events. Events are sent to a room, and all users which have sufficient access will receive that event. Rooms have a single internal "Room ID" to identify them from another room: : ]]> For example, !abc:example.org. The opaque portion of the room ID, called the localpart, must not be empty and must consist entirely of the characters [0-9a-zA-Z._~-]. The domain portion of a room ID does NOT indicate the room is "hosted" or served by that domain. The domain is used as a namespace to prevent another server from maliciously taking over a room. The server represented by that domain may no longer be participating in the room. The total length (including the sigil and domain) of a room ID MUST NOT exceed 255 characters. Room IDs are case sensitive.

Users As described by , a user is typically a human which operates a client. In Linearized Matrix, all users have a User ID to distinguish them: : ]]> The localpart portion of the user ID is expected to be human-readable, MUST NOT be empty, and MUST consist solely of [0-9a-z._=-/] characters. Note that user IDs cannot contain uppercase letters in the localpart. The domain portion indicates which server allocated the ID, or would allocate the resource if the user doesn't exist yet. @alice:first.example.org is a different user on a different server from @alice:second.example.org, for example. The total length (including the sigil and domain) of a user ID MUST NOT exceed 255 characters. User IDs are case sensitive. Note: User IDs are sometimes informally referenced as "MXIDs", short for "Matrix User IDs". Author's note: This draft assumes that an external system will resolve phone number to user ID, somehow. Or that @18005552222:example.org will resolve to +1 800 555 2222 on a given server, or similar.

Devices Each user can have zero or more devices/active clients. These devices are intended to be members of the MLS group and thus have their own key package material associated with them. TODO: Do we need to define grammar and such for device IDs, or is that covered by MLS already?

Events All data exchanged over Linearized Matrix is expressed as an "event". Each client action (such as sending a message) correlates with exactly one event. All events have a type to distinguish them, and use reverse domain name notation to namespace custom events (for example, org.example.appname.eventname). Event types specified by Linearized Matrix itself use m. as their namespace. When events are traversing a transport to another server they are often referred to as a Persistent Data Unit or PDU. An event has many other fields:

• room_id (string; required) - The room ID for where the event is being sent.
• type (string; required) - A UTF-8 string to distinguish different data types being carried by events. All event types use a reverse domain name notation to namespace themselves (for example, org.example.appname.eventname). Event types specified by Linearized Matrix itself use m as their namespace (for example, m.room.member).
• state_key (string; optional) - A UTF-8 string to further distinguish an event type from other related events. Only specified on State Events (discussed later). Can be empty.
• sender (string; required) - The user ID which is sending this event.
• origin_server_ts (integer; required) - The milliseconds since the unix epoch for when this event was created.
• hub_server (string; technically optional) - The domain name of the hub server which is sending this event to the remainder of the room. Note that all events created within Linearized Matrix will have this field set.
• content (object; required) - The event content. The schema of this is specific to the event type, and should be considered untrusted data until verified otherwise. Malicious servers and clients can, for example, exclude important fields, use invalid value types, or otherwise attempt to disrupt a client - receivers should treat the event with care while processing.
• hashes (object; required) - Keyed by hash algorithm, the content hash for the event.
• signatures (object; required) - Keyed first by domain name then by key ID, the signatures for the event.
• auth_events (array of strings; required) - The event IDs which prove the sender is able to send this event in the room. Which specific events are put here are defined by the auth events selection algorithm.
• prev_events (array of strings; required) - The event IDs which precede the event. Note that all events generated within Linearized Matrix will only ever have a single event ID here.
• unsigned (object; optional) - Additional metadata not covered by the signing algorithm.

Note that an event ID is not specified on the schema. Event IDs are calculated to ensure accuracy and consistency between servers. To calculate an event ID, calculate the reference hash of the event, encode it using URL-safe Unpadded Base64, and prefix it with the event ID sigil, $. If both the sender and receiver are implementing the algorithms correctly, the event ID will be the same. When different, the receiver will have issues accepting the event (none of the auth_events will make sense, for example). Both sender and receiver should review their algorithm implementation to verify everything is according to the specification in this case. Events are treated as JSON within the protocol, but can be encoded and represented by any binary-compatible format. Additional overhead may be introduced when converting between formats, however. An example may be: " }, "signatures": { "first.example.org": { "ed25519:1": "" }, "second.example.org": { "ed25519:1": "" } }, "auth_events": ["$first", "$second"], "prev_events": ["$parent"], "unsigned": { "arbitrary": "fields" } } ]]>

Linearized PDU The hub server is responsible for ensuring events are linearly added to the room from all participants, which means participants cannot set fields such as prev_events on their events. Additionally, participant servers are not expected to store past conversation history or even "current state" for the room, further making participants unable to reliably populate auth_events and prev_events. To avoid these problems, the participant server does not populate the following fields on events they are sending to the hub:

• auth_events - the participant cannot reliably determine what allows it to send the event.
• prev_events - the participant cannot reliably know what event precedes theirs.
• hashes - the hashes cover the above two fields.

The participant server will receive an echo of the fully-formed event from the hub once appended. To ensure authenticity, the participant server signs this "Linearized PDU" or "LPDU" using the normal event signing algorithm. TODO: While a signature is great, it doesn't cover the content. We need to fix hashes to actually support an LPDU hash alongside a full-blown content hash.

State events State events track metadata for the room, such as name, topic, and members. State is keyed by a tuple of type and state_key, noting that an empty string is a valid state key. State in the room with the same key-tuple will be overwritten. State events are otherwise processed like regular events in the room: they're appended to the room history and can be referenced by that room history. "Current state" is the state at the time being considered (which is often the implied HEAD of the room). In Linearized Matrix, a simple approach to calculating current state is to iterate over all events in order, overwriting the key-tuple for state events in an adjacent map. That map becomes "current state" when the loop is finished.

Event types Linearized Matrix defines the following event types:

m.room.create The very first event in the room. It MUST NOT have any auth_events or prev_events, and the domain of the sender MUST be the same as the domain in the room_id. The state_key MUST be an empty string. The content for a create event MUST have at least a room_version field to denote what set of algorithms the room is using. This document as a whole describes a single room version identified as I.1. Implementation note: Currently I.1 is not a real thing. Use org.matrix.i-d.ralston-mimi-linearized-matrix.00 when testing against other Linearized Matrix implementations. This room version may be updated later. TODO: Describe room versions more?

m.room.join_rules Defines whether users can join without an invite and other similar conditions. The state_key MUST be an empty string. The content for a join rules event MUST have at least a join_rule field to denote the join policy for the room. Allowable values are:

• public - anyone can join without an invite.
• knock - users must receive an invite to join, and can request an invite (knock) too.
• invite - users must receive an invite to join.

TODO: Describe restricted (and knock_restricted) rooms?

m.room.member Defines the membership for a user in the room. If the user does not have a membership event then they are presumed to be in the leave state. The state_key MUST be a non-empty string denotating the user ID the membership is affecting. The content for a membership event MUST have at least a membership field to denote the membership state for the user. Allowable values are:

• leave - not participating in the room. If the state_key and sender do not match, this was a kick rather than voluntary leave.
• join - participating in the room.
• knock - requesting an invite to the room.
• invite - invited to participate in the room.
• ban - implies kicked/not participating. Cannot be invited or join the room without being unbanned first (moderator sends a kick, essentially).

The auth rules define how these membership states interact and what legal transitions are possible. For example, preventing users from unbanning themselves falls under the auth rules.

m.room.power_levels Defines what given users can and can't do, as well as which event types they are able to send. The enforcement of these power levels is determined by the auth rules. The state_key MUST be an empty string. The content for a power levels event SHOULD have at least the following:

• ban (integer) - the level required to ban a user. Defaults to 50 if unspecified.
• kick (integer) - the level required to kick a user. Defaults to 50 if unspecified.
• invite (integer) - the level required to invite a user. Defaults to 0 if unspecified.
• redact (integer) - the level required to redact an event sent by another user. Defaults to 50 if unspecified.
• events (map) - keyed by event type string, the level required to send that event type to the room. Defaults to an empty map if unspecified.
• events_default (integer) - the level required to send events in the room. Overridden by the events map. Defaults to 0 if unspecified.
• state_default (integer) - the level required to send state events in the room. Overridden by the events map. Defaults to 50 if unspecified.
• users (map) - keyed by user ID, the level of that user. Defaults to an empty map if unspecified.
• users_default (integer) - the level for users. Overridden by the users map. Defaults to 0 if unspecified.

TODO: Include notifications for at-room here too? Note that if no power levels event is specified in the room then the room creator (sender of the m.room.create state event) has a default power level of 100.

TODO: Other events TODO: m.room.name, m.room.topic, m.room.avatar, m.room.encryption, m.room.history_visibility TODO: Drop m.room.encryption and pack it into the create event instead?

MLS Considerations MIMI has a chartered requirement to use MLS for encryption, and MLS requires that all group members (devices) know of all other devices. If we consider each Matrix room to have an MLS group, we encounter scenarios where the room and group membership might diverge or otherwise not be equivalent. In a traditional Matrix room, membership is not managed at a per-device level but rather a per-user level. Devices are authenticated to use the room by being attached to a user. This model doesn't work in MLS, though. A couple of options present themselves:

1. Keep managing the room state at the server level, as is traditional for Matrix, and define a set of rules/methods for engaging devices/users in the room with the MLS group. Servers have an ability to instruct devices on how/when to add/remove MLS group members, but not an ability to handle the MLS Proposals and Commits directly.
2. Coordinate a room's state at the device level, leaving servers to figure out how to push events between servers (and by extension, other devices). Servers would not have knowledge or ability to reject proposals based on authorization beyond transport-level authenticity concerns.

At this stage of drafting in the document, it is not clear which would be preferred. Both are explored.

Server-side Room Model In this model, servers handle the room state on behalf of devices. This gives the server an ability to apply access control at a user level, and instruct other devices on when/how to add or remove devices from the underlying MLS group. The server does not have an ability to participate in the MLS group directly. This is how traditional Matrix rooms work by handling state changes (user membership, etc) in cleartext for everyone to see. A user's devices would be tracked and added/removed from the MLS group as needed. The exact rules for how a user's devices become engaged with the MLS group is not yet defined. An advantage over this model compared to client-side is the server is able to reduce the client's traffic by rejecting events earlier and deal with conflicts that may arise, keeping the conversation as linear as possible for the client. A clear disadvantage is that without cross-signing or other cryptographic mechanism, the server would be able to add malicious devices to its users and therefore the MLS group. A precise mitigation strategy is not yet defined by this document, but would involve building verifiable trust in a device before it is allowed to participate in the MLS group. The existing model used by Linearized Matrix is covered by "Event Signing & Authorization" later in this document. TODO: We might also need DMLS to handle some of the server-side conflicts?

Client-side Room Model Here, the room's state is completely managed within the MLS group. This provides a key advantage where servers become message-passing nodes (in essence), but increases implementation complexity on the clients/devices. Much of this model is based around the server-side model discussed above: event authorization rules, redactions, etc still behave the same, but on the client-side instead. The server would likely be responsible for ensuring incoming events are properly signed, but otherwise leave it up to clients to accept or reject them into their internal linked list. A potential consequence of this model is clients needing to implement a conflict resolution algorithm despite having linear room history. This is due to clients receiving MLS messages out of guaranteed order. TODO: This could be DMLS, state res, or both.

Event Signing & Authorization There are a few aspects of an event which verify its authenticity and therefore whether it can be accepted into the room. All of these checks work with the fully-formed PDU for an event. First, the event has a content hash which covers the unredacted content of the event. The purpose of this hash is to ensure that the original contents are not modified, therefore if the hash check fails then the event is redacted before continuing processing. This removes any potentially malicious or malformed details from the event. Second, the event has a reference hash which covers the redacted event. This hash serves as the event's ID and thus any difference in calculation will result in an entirely different event ID. Third, the event must be signed by the domain implied by the sender. In Linearized Matrix, this will usually be the LPDU signature discussed earlier in this document. This signature covers the content hash of the event. TODO: Except the LPDU signature doesn't cover the participant's content hash, because the participant doesn't have a content hash at the moment. Finally, the event must be signed by the hub_server domain if present. This is to ensure that the event has actually been processed by the hub and isn't falsely being advertised as sent by a hub. TODO: Does the hub's signature actually guard anything?

Checks performed upon receipt of a PDU/event When a hub receives an LPDU from a participant it adds the missing fields to create a fully formed PDU then sends that PDU back out to all participants, including the original sender. When a server (hub or participant) receives a PDU, it:

1. Verifies the event is in a valid shape. This will mean ensuring the required schema is met and of the correct type (there is a string type, etc). Note that the event may have additional fields in various places, such as at the top level or within content: the receiver should ensure these additional fields do not cause the event to be invalid. If the event fails this validation, it is dropped.
2. Ensures the required signatures are present and valid. If the event fails this, it is dropped.
3. Ensures the event has a valid content hash. If the event's hash doesn't match, it is redacted before processing further. The server will ultimately persist the redacted copy.
4. Ensures the event passes the authorization rules for the state identified by the event's auth_events. If it fails, it is rejected.
5. Ensures the event passes the authorization rules for the state of the room immediately before where the event would be inserted. If it fails, it is rejected.
6. Ensures the event passes the authorization rules for the current state of the room (which may very well be the same as the step above). If it fails, it is soft-failed.

Rejection Events which are rejected are not relayed to any local clients and are not appended to the room in any way. Within Linearized Matrix, events which reference rejected events are rejected themselves.

Soft failure When an event is "soft-failed" it should not be relayed to any local clients nor be used in auth_events. The event is otherwise handled as per usual.

Authorization rules These are the rules which govern whether an event is accepted into the room, depending on the state events surrounding that event. A given event is checked against multiple different sets of state.

Auth events selection The auth_events on an event MUST consist of the following state events, with the exception of an m.room.create event which has no auth_events.

1. The m.room.create state event.
2. The current m.room.power_levels state event, if any.
3. The sender's current m.room.member state event, if any.
4. If the type is m.room.member:
   1. The target's (state_key) current m.room.member state event, if any.
   2. If content.membership is join or invite, the current m.room.join_rules state event, if any.

TODO: Talk about restricted room joins here?

Auth rules algorithm With consideration for default/calculated power levels, the ordered rules which affect authorization of a given event are: TODO: should we reference m.federate?

1. Events must be signed by the server denoted by the sender field. Note that this may be an LPDU if the hub_server is specified and not the same server.
2. If hub_server is present, events must be signed by that server.
3. If type is m.room.create:
   1. If it has any prev_events, reject.
   2. If the domain of the room_id is not the same domain as the sender, reject.
   3. If content.room_version is not I.1, reject. TODO: Incorporate room versions properly.
   4. Otherwise, allow.
4. Considering the event's auth_events:
   1. If there are duplicate entries for a given type and state_key pair, reject.
   2. If there are entries whose type and state_key do not match those specified by the auth events selection algorithm, reject.
   3. If there are entries where the referenced event was rejected during receipt, reject.
   4. If there is no m.room.create event among the entries, reject.
5. If type is m.room.member:
   1. If there is no state_key property, or no membership in content, reject.
   2. If membership is join:
      1. If the previous event is an m.room.create event and the state_key is the creator, allow.
      2. If sender does not match state_key, reject.
      3. If the sender is banned, reject.
      4. If the join_rule for m.room.join_rules is invite or knock, then allow if the current membership state is invite or join.
      5. If the join_rule for m.room.join_rules is public, allow.
      6. Otherwise, reject.
   3. If membership is invite:
      1. If the sender's current membership state is not join, reject.
      2. If the target user's (state_key) membership is join or ban, reject.
      3. If the sender's power level is greater than or equal to the power level needed to send invites, allow.
      4. Otherwise, reject.
   4. If membership is leave:
      1. If the sender matches the state_key, allow if and only if that user's current membership state is knock, join, or invite.
      2. If the sender's current membership state is not join, reject.
      3. If the target user's (state_key) current membership state is ban, and the sender's power level is less than the power level needed to ban other users, reject.
      4. If the sender's power level is greater than or equal to the power level needed to kick users, and the target user's (state_key) power level is less than the sender's, allow.
      5. Otherwise, reject.
   5. If membership is ban:
      1. If the sender's current membership state is not join, reject.
      2. If the sender's power level is greater than or equal to the power level needed to ban users, and the target user's (state_key) power level is less than the sender's power level, allow.
      3. Otherwise, reject.
   6. If membership is knock:
      1. If the join_rule for m.room.join_rules is anything other than knock, reject.
      2. If the sender does not match the state_key, reject.
      3. If the sender's current membership state is not ban or join, allow.
      4. Otherwise, reject.
   7. Otherwise, the membership is unknown. Reject.
6. If the sender's current membership state is not join, reject.
7. If the event type's required power level is greater than the sender's power level, reject.
8. If the event has a state_key which starts with an @ and does not match the sender, reject.
9. If type is m.room.power_levels:
   1. If any of the fields users_default, events_default, state_default, ban, redact, kick, or invite in content are present and not an integer, reject.
   2. If events in content is present and not an object with values that are integers, reject.
   3. If the users in content is present and not an object with valid user IDs as keys and integers as values, reject.
   4. If there is no previous m.room.power_levels event in the room, allow.
   5. For the fields users_default, events_default, state_default, ban, redact, kick, and invite, check if they were added, changed, or removed. For each found alteration:
      1. If the current value is higher than the sender's current power level, reject.
      2. If the new value is higher than the sender's current power level, reject.
   6. For each entry being changed in or removed from events:
      1. If the current value is higher than the sender's current power level, reject.
   7. For each entry being added to or changed in events:
      1. If the new value is greater than the sender's current power level, reject.
   8. For each entry being changed in or removed from users, other than the sender's own entry:
      1. If the current value is higher than the sender's current power level, reject.
   9. For each entry being added to or changed in users:
      1. If the new value is greater than the sender's current power level, reject.
   10. Otherwise, allow.
10. Otherwise, allow.

There are some consequences to these rules:

• Unless you are already a member of the room, the only permitted operations (aside from the initial create/join) are being able to join public rooms, accept invites to rooms, and reject invites to rooms.
• To unban another user, the sender must have a power level greater than or equal to both the kick and ban power levels, and greater than the target user's power level.

TODO: If we want to enforce a single hub in a room, we'd do so here with auth rules.

Signing All servers, including hubs and participants, publish an ed25519 signing key to be used by other servers when verifying signatures. TODO: Verify RFC reference. We might be using a slightly different ed25519 key today? See https://hdevalence.ca/blog/2020-10-04-its-25519am

Canonical JSON When signing a JSON object, such as an event, it is important that the bytes be ordered in the same way for everyone. Otherwise, the signatures will never match. To canonicalize a JSON object, use . TODO: Matrix currently doesn't use RFC8785, but it should (or similar).

Signing arbitrary objects Though events receive a lot of signing, it is often necessary for a server to sign arbitary, non-event, payloads as well. For example, in Matrix's existing HTTPS+JSON transport, requests are signed to ensure they came from the source they claim to be. To sign an object, the JSON is canonically encoded without the signatures or unsigned fields. The bytes of the canonically encoded JSON are then signed using the ed25519 signing key for the server. The resulting signature is then encoded using unpadded base64.

Signing events Signing events is very similar to signing an arbitary object, however with a note that an event is first redacted before signing. This ensures that later if the event were to be redacted in the room that the signature check still passes. Note that the content hash covers the event's contents in case of redaction.

Redacting an event All fields at the top level except the following are stripped from the event:

• type
• room_id
• sender
• state_key
• content
• origin_server_ts
• hashes
• signatures
• prev_events
• auth_events
• hub_server

Additionally, some event types retain specific fields under the event's content. All other fields are stripped.

• m.room.create retains all fields in content.
• m.room.member retains membership.
• m.room.join_rules retains join_rule.
• m.room.power_levels retains ban, events, events_default, kick, redact, state_default, users, users_default, and invite.
• m.room.history_visibility retains history_visibility.

Checking a signature If the signatures field is missing, doesn't contain the entity that is expected to have done the signing (a server name), doesn't have a known key ID, or is otherwise structurally invalid then the signature check fails. If decoding the base64 fails, the check fails. If removing the signatures and unsigned properties, canonicalizing the JSON, and verifying the signature fails, the check fails. Otherwise, the check passes.

Hashes An event is covered by two hashes: a content hash and a reference hash. The content hash covers the unredacted event to ensure it was not modified in transit. The reference hash covers the essential fields of the event, including content hashes, and serves as the event's ID.

Content hash calculation

1. Remove any existing unsigned, signatures, and hashes fields.
2. Encode the object using canonical JSON.
3. Hash the resulting bytes with SHA-256 .
4. Encode the hash using unpadded base64.

Reference hash

1. Redact the event.
2. Remove signatures and unsigned fields.
3. Encode the object using canonical JSON.
4. Hash the resulting bytes with SHA-256 .
5. Encode the hash using URL-safe unpadded base64.

Unpadded Base64 Throughout this document, "unpadded base64" is used to represent binary values as strings. Base64 is as specified by , and unpadded base64 simply removes any = padding from the resulting string. Implementations SHOULD accept input with or without padding on base64 values. Section 5 of describes URL-safe base64. The same changes are adopted here. Namely, the 62nd and 63rd characters are replaced with - and _ respectively. The unpadded behaviour is as described above.

Hub transfers TODO: This section, if we want a single canonical hub in the room. Some expected problems in this area are: who signs the transfer event? who sends the transfer event? how does a transfer start? TODO: Is this section better placed in the MSC for now?

Transport TODO: This section, though this is likely (should be?) to be a dedicated I-D. Topics: * Server discovery * Publishing of signing keys * Sending events between servers * Media handling * etc Matrix currently uses an HTTPS+JSON transport for this.

Security Considerations TODO: Expand upon this section.

IANA Considerations The m.* namespace likely needs formal registration in some capacity.

References Normative References The Matrix.org Foundation C.I.C. This document introduces a set of terminology to use when discussing or describing concepts within MIMI. This RFC is an official specification for the Internet community. It incorporates by reference, amends, corrects, and supplements the primary protocol standards documents relating to hosts. [STANDARDS-TRACK] ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. Cryptographic operations like hashing and signing need the data to be expressed in an invariant format so that the operations are reliably repeatable. One way to address this is to create a canonical representation of the data. Canonicalization also permits data to be exchanged in its original form on the "wire" while cryptographic operations performed on the canonicalized counterpart of the data in the producer and consumer endpoints generate consistent results. This document describes the JSON Canonicalization Scheme (JCS). This specification defines how to create a canonical representation of JSON data by building on the strict serialization methods for JSON primitives defined by ECMAScript, constraining JSON data to the Internet JSON (I-JSON) subset, and by using deterministic property sorting. Federal Information Processing Standard, FIPS This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] Informative References The Matrix.org Foundation C.I.C. The Matrix.org Foundation C.I.C.

Acknowledgments Thank you to the Matrix Spec Core Team (SCT), and in particular Richard van der Hoff, for exploring how Matrix rooms could be represented as a linear structure, leading to this document.