Room Version 2

This room version builds on version 1 with an improved state resolution algorithm.

Client considerations

There are no client considerations introduced in this room version. Clients which implement the redaction algorithm locally should refer to the redactions section below for a full overview of the algorithm.

Server implementation components

Room version 2 uses the base components of room version 1, changing only the state resolution algorithm.

State resolution

[New in this version]

The room state S′(E) after an event E is defined in terms of the room state S(E) before E, and depends on whether E is a state event or a message event:

• If E is a message event, then S′(E) = S(E).
• If E is a state event, then S′(E) is S(E), except that its entry corresponding to E’s event_type and state_key is replaced by E’s event_id.

The room state S(E) before E is the resolution of the set of states {S′(E₁), S′(E₂), …} consisting of the states after each of E’s prev_events {E₁, E₂, …}, where the resolution of a set of states is given in the algorithm below.

Definitions

The state resolution algorithm for version 2 rooms uses the following definitions, given the set of room states {S₁, S₂, …}:

Power events A power event is a state event with type m.room.power_levels or m.room.join_rules, or a state event with type m.room.member where the membership is leave or ban and the sender does not match the state_key. The idea behind this is that power events are events that might remove someone’s ability to do something in the room.

Unconflicted state map and conflicted state set The unconflicted state map is the state where the value of each key exists and is the same in each state S_i. The conflicted state set is the set of all other state events. Note that the unconflicted state map only has one event per (event_type, state_key), whereas the conflicted state set may have multiple events.

Auth difference The auth difference is calculated by first calculating the full auth chain for each state S_i, that is the union of the auth chains for each event in S_i, and then taking every event that doesn’t appear in every auth chain. If C_i is the full auth chain of S_i, then the auth difference is  ∪ C_i −  ∩ C_i.

Full conflicted set The full conflicted set is the union of the conflicted state set and the auth difference.

Reverse topological power ordering The reverse topological power ordering of a set of events is the lexicographically smallest topological ordering based on the DAG formed by auth events. The reverse topological power ordering is ordered from earliest event to latest. For comparing two topological orderings to determine which is the lexicographically smallest, the following comparison relation on events is used: for events x and y, x < y if

1. x’s sender has greater power level than y’s sender, when looking at their respective auth_events; or
2. the senders have the same power level, but x’s origin_server_ts is less than y’s origin_server_ts; or
3. the senders have the same power level and the events have the same origin_server_ts, but x’s event_id is less than y’s event_id.

The reverse topological power ordering can be found by sorting the events using Kahn’s algorithm for topological sorting, and at each step selecting, among all the candidate vertices, the smallest vertex using the above comparison relation.

Mainline ordering Given an m.room.power_levels event P, the mainline of P is the list of events generated by starting with P and recursively taking the m.room.power_levels events from the auth_events, ordered such that P is last. Given another event e, the closest mainline event to e is the first event encountered in the mainline when iteratively descending through the m.room.power_levels events in the auth_events starting at e. If no mainline event is encountered when iteratively descending through the m.room.power_levels events, then the closest mainline event to e can be considered to be a dummy event that is before any other event in the mainline of P for the purposes of condition 1 below.

The mainline ordering based on P of a set of events is the ordering, from smallest to largest, using the following comparison relation on events: for events x and y, x < y if

1. the closest mainline event to x appears before the closest mainline event to y; or
2. the closest mainline events are the same, but x’s origin_server_ts is less than y’s origin_server_ts; or
3. the closest mainline events are the same and the events have the same origin_server_ts, but x’s event_id is less than y’s event_id.

Iterative auth checks The iterative auth checks algorithm takes as input an initial room state and a sorted list of state events, and constructs a new room state by iterating through the event list and applying the state event to the room state if the state event is allowed by the authorization rules. If the state event is not allowed by the authorization rules, then the event is ignored. If a (event_type, state_key) key that is required for checking the authorization rules is not present in the state, then the appropriate state event from the event’s auth_events is used if the auth event is not rejected.

Algorithm

The resolution of a set of states is obtained as follows:

1. Take all power events and any events in their auth chains, recursively, that appear in the full conflicted set and order them by the reverse topological power ordering.
2. Apply the iterative auth checks algorithm, starting from the unconflicted state map, to the list of events from the previous step to get a partially resolved state.
3. Take all remaining events that weren’t picked in step 1 and order them by the mainline ordering based on the power level in the partially resolved state obtained in step 2.
4. Apply the iterative auth checks algorithm on the partial resolved state and the list of events from the previous step.
5. Update the result by replacing any event with the event with the same key from the unconflicted state map, if such an event exists, to get the final resolved state.

Rejected events

Events that have been rejected due to failing auth based on the state at the event (rather than based on their auth chain) are handled as usual by the algorithm, unless otherwise specified.

Note that no events rejected due to failure to auth against their auth chain should appear in the process, as they should not appear in state (the algorithm only uses events that appear in either the state sets or in the auth chain of the events in the state sets).

Rejected auth events are deliberately excluded from use in the iterative auth checks, as auth events aren’t re-authed (although non-auth events are) during the iterative auth checks.

Unchanged from v1

The following sections have not been modified since v1, but are included for completeness.

Authorization rules

The types of state events that affect authorization are:

• m.room.create
• m.room.member
• m.room.join_rules
• m.room.power_levels
• m.room.third_party_invite

The rules are as follows:

1. If type is m.room.create:
   1. If it has any previous events, reject.
   2. If the domain of the room_id does not match the domain of the sender, reject.
   3. If content.room_version is present and is not a recognised version, reject.
   4. If content has no creator field, reject.
   5. Otherwise, allow.
2. Reject if event has auth_events that:
   1. have duplicate entries for a given type and state_key pair
   2. have entries whose type and state_key don’t match those specified by the auth events selection algorithm described in the server specification.
3. If event does not have a m.room.create in its auth_events, reject.
4. If type is m.room.aliases:
   1. If event has no state_key, reject.
   2. If sender’s domain doesn’t matches state_key, reject.
   3. Otherwise, allow.
5. If type is m.room.member:
   1. If no state_key key or membership key in content, reject.
   2. If membership is join:
      1. If the only previous event is an m.room.create and the state_key is the creator, allow.
      2. If the sender does not match state_key, reject.
      3. If the sender is banned, reject.
      4. If the join_rule is invite then allow if membership state is invite or join.
      5. If the join_rule is public, allow.
      6. Otherwise, reject.
   3. If membership is invite:
      1. If content has third_party_invite key:
         1. If target user is banned, reject.
         2. If content.third_party_invite does not have a signed key, reject.
         3. If signed does not have mxid and token keys, reject.
         4. If mxid does not match state_key, reject.
         5. If there is no m.room.third_party_invite event in the current room state with state_key matching token, reject.
         6. If sender does not match sender of the m.room.third_party_invite, reject.
         7. If any signature in signed matches any public key in the m.room.third_party_invite event, allow. The public keys are in content of m.room.third_party_invite as:
            1. A single public key in the public_key field.
            2. A list of public keys in the public_keys field.
         8. Otherwise, reject.
      2. If the sender’s current membership state is not join, reject.
      3. If target user’s current membership state is join or ban, reject.
      4. If the sender’s power level is greater than or equal to the invite level, allow.
      5. Otherwise, reject.
   4. If membership is leave:
      1. If the sender matches state_key, allow if and only if that user’s current membership state is invite or join.
      2. If the sender’s current membership state is not join, reject.
      3. If the target user’s current membership state is ban, and the sender’s power level is less than the ban level, reject.
      4. 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.
      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 ban level, and the target user’s power level is less than the sender’s power level, allow.
      3. Otherwise, reject.
   6. Otherwise, the membership is unknown. Reject.
6. If the sender’s current membership state is not join, reject.
7. If type is m.room.third_party_invite:
   1. Allow if and only if sender’s current power level is greater than or equal to the invite level.
8. If the event type’s required power level is greater than the sender’s power level, reject.
9. If the event has a state_key that starts with an @ and does not match the sender, reject.
10. If type is m.room.power_levels:
    1. If users key in content is not a dictionary with keys that are valid user IDs with values that are integers (or a string that is an integer), reject.
    2. If there is no previous m.room.power_levels event in the room, allow.
    3. For the keys users_default, events_default, state_default, ban, redact, kick, 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.
    4. For each entry being added, changed or removed in both the events and users keys:
       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.
    5. For each entry being changed under the users key, other than the sender’s own entry:
       1. If the current value is equal to the sender’s current power level, reject.
    6. Otherwise, allow.
11. If type is m.room.redaction:
    1. If the sender’s power level is greater than or equal to the redact level, allow.
    2. If the domain of the event_id of the event being redacted is the same as the domain of the event_id of the m.room.redaction, allow.
    3. Otherwise, reject.
12. Otherwise, allow.

Event format

Events in rooms of this version have the following structure:

{
  "auth_events": [
    "$af232176:example.org",
    {
      "sha256": "abase64encodedsha256hashshouldbe43byteslong"
    }
  ],
  "content": {
    "key": "value"
  },
  "depth": 12,
  "event_id": "$a4ecee13e2accdadf56c1025:example.com",
  "hashes": {
    "sha256": "thishashcoversallfieldsincasethisisredacted"
  },
  "origin": "example.com",
  "origin_server_ts": 1404838188000,
  "prev_events": [
    "$af232176:example.org",
    {
      "sha256": "abase64encodedsha256hashshouldbe43byteslong"
    }
  ],
  "redacts": "$def456:matrix.org",
  "room_id": "!UcYsUzyxTGDxLBEvLy:example.org",
  "sender": "@alice:example.com",
  "signatures": {
    "example.com": {
      "ed25519:key_version:": "these86bytesofbase64signaturecoveressentialfieldsincludinghashessocancheckredactedpdus"
    }
  },
  "state_key": "my_key",
  "type": "m.room.message",
  "unsigned": {
    "age": 4612,
    "key": "value",
    "prev_content": {
      "displayname": "Bob",
      "membership": "join"
    },
    "prev_sender": "@someone:example.org",
    "redacted_because": "Inappropriate content",
    "replaces_state": "$state_event:example.org"
  }
}

Canonical JSON

Servers MUST NOT strictly enforce the JSON format specified in the appendices for the reasons described there.

Redactions

Upon receipt of a redaction event, the server must strip off any keys not in the following list:

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

The content object must also be stripped of all keys, unless it is one of one of the following event types:

• m.room.member allows key membership.
• m.room.create allows key creator.
• m.room.join_rules allows key join_rule.
• m.room.power_levels allows keys ban, events, events_default, kick, redact, state_default, users, users_default.
• m.room.aliases allows key aliases.
• m.room.history_visibility allows key history_visibility.