1.1   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.

2.1   State resolution

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’(E’), S’(E’’), …} consisting of the states after each of E's prev_events {E’, E’’, …}.

The resolution of a set of states is defined as follows. The resolved state is built up in a number of passes; here we use R to refer to the results of the resolution so far.

• Start by setting R to the union of the states to be resolved, excluding any conflicting events.
• First we resolve conflicts between m.room.power_levels events. If there is no conflict, this step is skipped, otherwise:
  • Assemble all the m.room.power_levels events from the states to be resolved into a list.
  • Sort the list by ascending depth then descending sha1(event_id).
  • Add the first event in the list to R.
  • For each subsequent event in the list, check that the event would be allowed by the authorization rules for a room in state R. If the event would be allowed, then update R with the event and continue with the next event in the list. If it would not be allowed, stop and continue below with m.room.join_rules events.
• Repeat the above process for conflicts between m.room.join_rules events.
• Repeat the above process for conflicts between m.room.member events.
• No other events affect the authorization rules, so for all other conflicts, just pick the event with the highest depth and lowest sha1(event_id) that passes authentication in R and add it to R.

A conflict occurs between states where those states have different event_ids for the same (event_type, state_key). The events thus affected are said to be conflicting events.

2.2   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.

2.3   Event format

Events in version 1 rooms have the following structure:

{
    "auth_events": [
        [
            "$abc123:matrix.org",
            {
                "sha256": "Base64EncodedSha256HashesShouldBe43BytesLong"
            }
        ]
    ],
    "content": {
        "key": "value"
    },
    "depth": 12,
    "event_id": "$a4ecee13e2accdadf56c1025:example.com",
    "hashes": {
        "sha256": "ThisHashCoversAllFieldsInCaseThisIsRedacted"
    },
    "origin": "matrix.org",
    "origin_server_ts": 1234567890,
    "prev_events": [
        [
            "$abc123:matrix.org",
            {
                "sha256": "Base64EncodedSha256HashesShouldBe43BytesLong"
            }
        ]
    ],
    "redacts": "$def456:matrix.org",
    "room_id": "!abc123:matrix.org",
    "sender": "@someone:matrix.org",
    "signatures": {
        "example.com": {
            "ed25519:key_version:": "86BytesOfSignatureOfTheRedactedEvent"
        }
    },
    "state_key": "my_key",
    "type": "m.room.message",
    "unsigned": {
        "key": "value"
    }
}

2.4   Canonical JSON

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