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Applications and Real-Time More Instant Messaging Interoperability room policy This document describes a set of concrete room policies for the More Instant Messaging Interoperability (MIMI) Working Group. It describes several independent properties and policy attributes which can be combined to model a wide range of chat and multimedia conference types. 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 The MIMI architecture describes how each room has an associated policy. Providers offer a "policy envelope" of supported and allowed policy settings, from which the creator of a room selects a specific room policy. The room policy might further allow individual participants to make specific choices (for example, allowing but not requiring read-message notifications), while constraining other choices (for example, prohibiting self-deleting messages). Individual users can examine the room policy to determine if it is consistent with policies they accept either before or immediately on joining a room. Making rooms interoperable across existing clients is challenging, as rooms and clients can support different policies and capabilities across vendors and providers. Our goal is to balance the policy and authorization goals of the room with the policy and authorization goals of the end user, so we can support a broad range of vendors and providers. Each room is owned by one provider at a time. The owning provider controls the range of acceptable policies. The user responsible for the room can further choose among the acceptable policies. Users (regardless if on other providers) can either accept the policies of the room or not. However we want to make it as easy as possible for clients from other providers to comply with the room policy primitives without enumerating specific features or requiring all clients implementations to present an identical user experience. An important aspect of this is the system of configurable Role-based access control with granular capabilities per role (described in ). Each user in the participant list (defined in ) has exactly one role. By virtue of a user's credential, a user might also be preauthorized with a specific role as described in .

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Room ID: An identifier which uniquely identifies a room. User ID: An internal identifier which uniquely identifies a user. Nickname: The identifier by which a user is referred inside a room. Depending on the context it may be a display name, handle, pseudonym, or temporary identifier. The nickname in one room need not correlate with the nickname for the same user in a different room. Client ID: An internal identifier which uniquely identifies one client/device instance of one user account. Persistent vs. Temporary rooms: A temporary room is no longer joinable once the last participant exits whereas a persistent room is not destroyed when the last participant exist. As MLS has no notion of a group with no members, a persistent room could consist of a sequence of distinct MLS groups, zero or one of which would exist at a time.

Moderation Terms Knock: To request entry into a room. Ban: To remove a user from a room such that the user is not allowed to re-enter the room (until and unless the ban has been removed). It is distinct from merely removing a user from a room. Kick: To temporarily remove a participant's clients from a room. The user is allowed to re-enter the room at any time. Voice (noun): The privilege to send messages in a room. Revoke Voice: To remove the permission to send messages in a room. Grant Voice: To grant the permission to send messages in a room.

Room Capabilities Membership-Approach: The overall approach of membership authorization in a room, which could be open, members-only (administrated), fixed-membership, or parent-dependent.

• Open room: Typically an open room can be joined by any non-banned user. It can be represented solely by a permissive set of roles as defined in .
• Members-Only room: A members-only room can only be joined by a user in the particpant list, or who is pre-authorized. Authorized users can add or remove users to the room. In an enterprise context, it is also common (but not required) for users from a particular domain, group, or workgroup to be pre-authorized to add themselves to a Members-Only room. It can be represented solely by a set of less permissive roles as defined in .
• Fixed-Membership room: Fixed-membership rooms have the list of participants specified when they are created. Other users cannot be added. Ordinary users cannot leave or be removed, however a user can remove all its clients from the associated MLS group. The most common case of a fixed-membership room is a 1:1 conversation. This room membership style is used to implement Direct Message (DM) and Group DM features. Only a single fixed-membership room can exist for any unique set of participants.
• Parent-dependent room: In a parent-dependent room, the list participants of the room must be a strict subset of the participants of the parent room. If a user leaves or is removed from the parent room, that user is automatically removed from any parent-dependent rooms of that parent.

Role-Based Access Control The Role-Based Access Control component contains a list of all the roles in the room, and the capabilities associated with them. It contains a role_index, which is used to refer to the role elsewhere. (Note that role indexes might not be contiguous.) The role_index zero is reserved to refer to a participant that does not (yet) or no longer appears (or will no longer appear) in the participant list. The component also contains a role_name (a human-readable text string name for the role), and a role_description (another string, which can have zero length). Each Role also can contain constraints on the minimum and maximum number of participants, and the minimum and maximum number of active participants. If the minimum number is zero, there is no minimum number of participants for that particular role. If there is no maximum number of participants for a particular role, that parameter is absent.

• If the maximum number of active participants is zero, then no participants are allowed to have clients in the room's MLS group.

The authorized_role_changes field is used to provide fine-grained control about which transitions are allowed when adding and removing participants and when moving participants to new roles, including banning/unbanning, and promoting/demoting to or from roles with moderator or administrator privileges. A more detailed discussion is in the description of the specific capabilities in the next section.

• This design results in each participant only having a single role at a time, with a single list of capabilities and an explicit list of allowed role transitions. It makes the authorization process for a verifier consistent regardless of the complexity of the set of authorization rules.

Some examples are provided in . RoleData is the format of the data field inside the ComponentData struct for the Role-Based Access Control component in the app_data_dictionary GroupContext extension defined in . ; } SingleSourceRoleChangeTargets; struct { uint32 role_index; opaque role_name; opaque role_description; CapabilityType role_capabilities; uint32 minimum_participants_constraint; optional uint32 maximum_participants_constraint; uint32 minimum_active_participants_constraint; optional uint32 maximum_active_participants_constraint; SingleSourceRoleChangeTargets authorized_role_changes; } Role; struct { Role roles; } RoleData; RoleData RoleUpdate; ]]> RoleUpdate (which has the same format as RoleData) is the format of the update field inside the AppDataUpdate struct in an AppDataUpdate Proposal for the Role-Based Access Control component. If the contents of the update field are valid and if the proposer is authorized to generate such an update, the value of the update field completely replaces the value of the data field.

• Note that in the MIMI environment, changing the definitions of roles is anticipated to be very rare over the lifetime of a room (for example changing a room which has grown dramatically from cooperatively managed by all participants to explicitly moderated or administered).

Changing Role definitions is sufficiently disruptive, that an update to this component is not valid if it appear in the same commit as any Participant List change.

Preauthorized Users Preauthorized users are MIMI users and external senders that have authorization to adopt a role in a room by virtue of certain credential claims or properties, as opposed to being individually enumerated in the participant list. For example, a room for employee benefits might be available to join with the regular participant role to all full-time employees with a residence in a specific country; while anyone working in the human resources department might be able to join the same room as a moderator. This data structure is consulted in two situations: for external joins (external commits) and external proposals when the requester does not already appear in the participant list; and separately when an existing participant explicitly tries to change its own role.

• Only consulting Preauthorized users in these cases prevents several attacks. For example, it prevents an explicitly banned user from rejoining a group based on a preauthorization.

PreAuthData is the format of the data field inside the ComponentData struct for the Preauthorized Participants component in the application_data GroupContext extension. The individual PreAuthRoleEntry rules in PreAuthData are consulted one at a time. A PreAuthRoleEntry matches for a requester when every Claim.claim_id has a corresponding claim in the requester's MLS Credential which exactly matches the corresponding claim_value. When the rules in a Preauthorized users struct match multiple roles, the requesting client receives the first role which matches its claims. ; } ClaimId; struct { ClaimId claim_id; opaque claim_value; } Claim; struct { /* when all claims in the claimset are satisfied, the claimset */ */ is satisfied */ Claim claimset; Role target_role; } PreAuthRoleEntry; struct { PreAuthRoleEntry preauthorized_entries; } PreAuthData; PreAuthData PreAuthUpdate; ]]> PreAuthUpdate (which has the same format as PreAuthData) is the format of the update field inside the AppDataUpdate struct in an AppDataUpdate Proposal for the Preauthorized Participants component. If the contents of the update field are valid and if the proposer is authorized to generate such an update, the value of the update field completely replaces the value of the data field.

• As with the definition of roles, in MIMI it is not expected that the definition of Preauthorized users would change frequently. Instead the claims in the underlying credentials would be modified without modifying the preauthorization policy.

Changing Preauthorized user definitions is sufficiently disruptive, that an update to this component is not valid if it appears in the same commit as any Participant List change, except for user removals. Because the Preauthorized users component usually authorizes non-members, it is also a natural choice for providing concrete authorization for policy enforcing systems incorporated into or which run in coordination with the MIMI Hub provider or specific MLS Distribution Services. For example, a preauthorized role could allow the Hub to remove participants and to ban them, but not to add any users or devices. This unifies the authorization model for members and non-members.

Base Room policy format syntax

Membership-related policy

• TODO: refactor membership_style to be constraints to the role-based access control system

The membership_style of a room can express some additional constraints on membership transitions in a room. It can have one of the following values:

• ordinary (default)
• fixed-membership
• parent-dependent

An ordinary room has no constraints beyond those of the role-based access control system. A fixed-membership room (which can be used for DMs or Group DMs) has a participant list set once at creation time that cannot be added to. A parent-dependent room always has a strict subset of the participants of its parent room. If the membership_style is parent-dependent the parent_room_uri MUST be set with the room ID of the parent. Otherwise the field is zero-length. If multi_device is true (the default), the MLS group may contain multiple clients per user. If false only a single client can be an MLS member at one time. ; bool multi_device; bool persistent_room; ... } BaseRoomPolicy; ]]> If persistent_room is false, the room will be automatically inaccsessible when the corresponding MLS group is destroyed (when there are no clients in the group). If persistent_room is true, the room policy will remain and a client whose user has appropriate authorization can create a new MLS group for the same room. (There is not a 1:1 correlation of MLS group to room ID in a persistent room.)

Delivery and Read notifications, Pseudonyms The delivery_notifications value can be set to "forbidden", "optional", or "required". If the value is set to "optional", the client uses its local configuration to determine if it should send delivery notifications in the group. The read_receipts value can be set to "forbidden", "optional", or "required". If the value is set to "optional", the client uses its local configuration to determine if it should send read receipts in the group. The format for delivery notifications and read receipts is described in Section 5.12 of . If pseudonymous_ids is true, clients in the MLS group are free to use pseudonymous identifiers in their MLS credentials. Otherwise the policy of the room is that "real" long-term identifiers are required in MLS credentials in the room's corresponding MLS group.

Link, Logging, History, and Bot policies ; Uri machine_readable_policy; Uri human_readable_policy; } LoggingPolicy; struct { Optionality history_sharing; Role who_can_share; bool automatically_share; uint32 max_time_period; } HistoryPolicy; struct { opaque name; opaque description; Uri homepage; Role bot_role; bool can_read; bool can_write; bool can_target_message_in_group; bool per_user_content; } Bot; struct { ... bool discoverable; LinkPolicy link_policy; LoggingPolicy logging_policy; HistoryPolicy history_sharing; Bot allowed_bots; ... } RoomPolicy; ]]>

Link policies If discoverable is true, the room is searchable. Presumably this means the the only way to join the room in a client user interface is to be added by an administrator or to use a joining link. Inside the LinkPolicy are several fields that describe the behavior of links.If the on_request field is true, no joining link will be provided in the room policy; the client will need to fetch a joining link out-of-band or generate a valid one for itself. If present, the URI in link_requests can be used by the client to request an invite code. The value of join_link is empty and the other fields are ignored.If the on_request field is false, the join_link field will contain a joining link. If the link will work for multiple users, multiuser is true. The expiration field represents the time, in seconds after the start of the UNIX epoch (1-January-1970) when the link will expire. The link_requests field can be empty.

Logging policies Inside the LoggingPolicy, the logging field can be forbidden, optional, or required. If logging is forbidden then the other fields are empty. If logging is required, the list of logging_clients needs to contain at least one logging URI. Each provider should have no more than one logging client at a time in a room. The machine_readable_policy and human_readable_policy fields optionally contain pointers to the owning provider's machine readable and human readable logging policies, respectively. If logging is optional and there is at least one logging_client then logging is active for the room.

Chat history policies Inside the HistoryPolicy, if history_sharing is forbidden, this means that clients (including bots) are expected to not to share chat history with new joiners, in which case who_can_share is empty, automatically_share is false, and max_time_period is zero. Otherwise who_can_share is a list of roles that are authorized to share history (for example, only admins and owners can share). The values of none and outcast cannot be used in who_can_share. If automatically_share is true, clients can share history with new joiners without user initiation. The history that is shared is limited to max_time_period seconds worth of history.

Chat bot policies Inside the RoomPolicy there is a list of allowed_bots. Each of which has several fields. The name, description, and homepage are merely descriptive. The bot_role indicates if the chat bot would be treated as a system-user, owner, admin, regular_user, or visitor. The can_read and can_write fields indicate if the chat bot is allowed to read messages or send messages in the MLS group, respectively. If can_target_message_in_group is true it indicates that the chat bot can send an MLS targeted message (see Section 2.2 of ) or use a different conversation or out-of-band channel to send a message to specific individual users in the room. If per_user_content is true, the chat bot is allowed to send messages with distinct content to each member. (For example a poker bot could deal a different hand to each user in a chat).Users could set policies to reject or leave groups with bots rights that are inconsistent with the user's privacy goals.

Operational policy Section 7 of the defines a set of operational policy considerations that influence interoperability of MLS clients. MIMI explicitly address a handful of the issues in the document by taking a position on ordering (Proposals referenced in a Commit need to be received before the Commit; the Commit entering a new epoch needs to be received before any other messages in that epoch), privacy of handshake messages (handshakes can be a PublicMessage or SemiPrivateMessage), and GroupInfo storage (committers need to provide a valid GroupInfo to the Hub). The rest of these issues are described here. Just because a topic is listed does not mean that a room needs to take a position; nor different rooms on a Hub need to have different policies for these items.

Some MLS-related policy that could be tied to a room

• any mandatory or forbidden MLS extensions.
• which proposals are valid to have in a commit, including but not limited to:
  • when, and under what circumstances, a reinitialization proposal is allowed.
  • when proposals from external senders are allowed and how to authorize those proposals.
  • when external joiners are allowed and how to authorize those external commits.
  • which other proposal types are allowed.
• when members should commit pending proposals in a group.
• when two credentials represent the same client.
• how long to allow a member to stay in a group without updating its leaf keys before removing them.
• When and how to pad messages.
• When to send a reinitialization proposal.
• How often clients should update their leaf keys.
• Whether to prefer sending full commits or partial/empty commits.
• Whether there should be a required_capabilities extension in groups.
• minimum and maximum lifetime of KeyPackages
• if last resort KeyPackages are allowed
• how long to store resumption PSK (how much time and how many epochs)
• minimum and maximum number past epochs to keep
• how long to keep unused nonce and key pairs for a sender
• maximum number of unused key pairs to keep
• maximum number of steps that clients will move a secret tree ratchet forward in response to a single message before rejecting it
• tolerance to out of order app messages
• tolerance to out of order handshake messages
• handshakes may be which of PublicMessage, PrivateMessage, or SemiPrivateMessage.
• if external joiners are allowed
• if external proposals are allowed
  • if so, who can submit
  • which member(s) are responsible for submitting pending proposals
• how a joiner gets access to the ratchet_tree

Not relevant to MIMI (between client and its provider)

• how many KPs to keep active
• how group IDs are constructed
• which ciphersuites are acceptable.

Areas for future works Which credential types are allowed/required How to protect and share the GroupInfo objects needed for external joins. If an application wishes to detect and possibly discipline members that send malformed commits with the intention of corrupting a group's state, there must be a method for reporting and validating malformed commits. MLS requires the following parameters to be defined, which must be the same for two implementations to interoperate: Which media types are required to send and required to understand in MIMI. What Additional authenticated data, can/should be sent unencrypted in an otherwise encrypted message. Application-level identifiers of public key material (specifically the application_id extension as defined in Section 5.3.3 of [RFC9420]).

Role Capabilities As described in , each role has a list of capabilities, which in rare cases could be empty. When we say that the holder of a capability can take some action, we mean that whatever entity is taking the action (a participant, a potential future participant, or an external party) has a specific entry in the Participant List struct and a corresponding role--or is preauthorized to take action with a specific role via the Preauthorized Users struct--and that the role_capabilities list contains the relevant capability. Unless otherwise specified, capabilities apply both to sending a set of consistent MLS proposals that could be committed by any member of the corresponding MLS group, and to sending an MLS commit containing a set of consistent MLS proposals.

Membership Capabilities The membership capabilities below allow authorized holders to update the Participant list, or change the active participants (by removing and adding MLS clients corresponding to those participants), or both.

• canAddParticipant - the holder of this capability can add another user, that is not already in the participant list, to the participant list. (This capability does not apply to the holder adding itself.) The authorized_role_changes list in the holder's role is consulted to authorize the added user's target role. The authorized_role_changes list MUST have an entry where the authorized_role_changes.from_role_index equals zero, and that entry's target_role_indexes list includes the target role. The proposed action is only authorized if the action respects both the maximum_participants_constraint (if present) and maximum_active_participants_constraint (if present) for the added user's target role. When the participant list addition for the target role is authorized, the holder is also authorized to add any MLS clients matching the added user to the room's MLS group .
• canAddOwnClient - a holder of this capability that is in the participant list, can add its own client (via an external commit or external proposal); and can add other clients that share the same user identity (via Add proposals) if the holder's client is already a member of the corresponding MLS group.
• canAddSelf - the holder of this capability can use an external commit or external proposal to add itself to the participant list. (The holder MUST NOT already appear in the participant list). Its usage differs slightly based on in which role it appears.
  • When canAddSelf appears on role zero, any user who is not already in the participant list can add itself, with certain provisions. The holder consults the authorized_role_changes list for an entry with from_role_index equal to zero. The holder can add itself with any non-zero target_role_indexes from that entry, if the action respects both the maximum_participants_constraint (if present) and maximum_active_participants_constraint (if present) for the added user's target role.
  • When canAddSelf appears on a non-zero role, a client can only become the holder of this capability via the Preauthorized users mechanism. The authorized_role_changes list in the target role MUST have an entry where the from_role_index is zero and the target_role_indexes contains the target role. In addition, the action MUST respect both the maximum_participants_constraint (if present) and maximum_active_participants_constraint (if present) for the added user's target role.
• canUseJoinCode - the holder of this capability can externally join a room using a join code for that room, provided the join code is valid, the join code refers to a valid target role, and both the maximum_participants_constraint (if present) and maximum_active_participants_constraint (if present) constraints are respected.
• canRemoveParticipant - the holder of this capability can propose a) the removal of another user (excluding itself) from the participant list, and b) removal of all of that user's clients, as a single action. There MUST NOT be any clients of the removed user in the MLS group after the corresponding commit. A proposer holding this capability consults its role's authorized_role_changes entries for an entry where from_role_index matches the target user's current role; if the target_role_indexes for that entry contains zero, and the minimum_participants_constraint and minimum_active_participants_constraint are satisfied, the proposal is authorized.
• canRemoveOwnClient - the holder of this capability can propose to remove its own client using an MLS Remove or SelfRemove proposal without changing the Participant list. Due to restrictions in MLS which insure the consistency of the group, this action cannot be committed by the leaving user. If the minimum_active_participants_constraint is satisfied, the proposal is authorized.
• canRemoveSelf - the holder of this capability can propose to remove itself from (i.e. leave) the participant list; it MUST simultaneously propose to remove all of its remaining clients from the corresponding MLS group. Due to restrictions in MLS which insure the consistency of the group, this action cannot be committed by the leaving user. A proposer holding this capability consults its role's authorized_role_changes entries for an entry where from_role_index matches its current role; if the target_role_indexes for that entry contains zero, and the minimum_participants_constraint and minimum_active_participants_constraint are satisfied, the proposal is authorized.
• canKick - the holder of this capability can propose removal of another participant's clients, without changing the Participant List. If the minimum_active_participants_constraint is satisfied, the proposal is authorized.
• canChangeUserRole - the holder of this capability is authorized to change the role of another participant (but not itself), according to the holder's authorized_role_changes list, from a role represented by an entry where the target's current role matches from_role_index to any of the non-zero target_role_indexes in the same element of authorized_role_changes. The minimum_participants_constraint and minimum_active_participants_constraint for the target user's current role, and the maximum_participants_constraint (if present) and maximum_active_participants_constraint (if present) for the target user's target role must also be satisfied.
• canChangeOwnRole - the holder of this capability is authorized to change its own role to the first non-zero role it matches in the Preauthorized users component (see ). The authorized_role_changes list is not consulted. The minimum_participants_constraint and minimum_active_participants_constraint for the holder's original role, and the maximum_participants_constraint (if present) and maximum_active_participants_constraint (if present) for the holder's target role must also be satisfied.
• canBan - the holder of this capability can propose to "ban" another user. Specifically, a successful ban changes the target user's role to a special "banned" role (if it exists), and removes all the banned user's clients. The "banned" role always has role_index = 1 and role_name = "banned" (without quotes).
  • A "banned" role does not have to exist in a room, but to use the canBan and canUnban capabilities, the role needs to exist exactly as described above. While holding canChangeUserRole and canKick capabilities would allow the same action, it could potentially allow the holder other actions which might be undesirable in some contexts, such as kicking clients without banning.
  A proposer holding this capability consults its role's authorized_role_changes entries for an entry where from_role_index matches the target user's current role; if the target_role_indexes for that entry contains the role_index 1; that role_name = "banned" for the role with role_index = 1, and the minimum_participants_constraint and minimum_active_participants_constraint are satisfied, the proposal is authorized.
• canUnban - the holder of this capability can propose to "unban" another user. Specifically, a successful unban changes the target user's role from role_index = 1 to another non-zero role_index allowed by the holder's authorized_role_changes list. Adding clients for that unbanned user is not authorized by this capability. The authorization of this capability is identical to the canChangeUserRole capability, except that the from_role_index for the unbanned user MUST be 1, and the role_name of role 1 MUST be "banned".

Adjust metadata The holder of each of the following capabilities is authorized to update the Room metadata defined in , changing the relevant field:

• canChangeRoomName
• canChangeRoomDescription
• canChangeRoomAvatar
• canChangeRoomSubject
• canChangeRoomMood

Message Capabilities The capabilities below refer to functionality related to the instant messages, for example sent using the MIMI content format .

• canSendMessage - the holder can send instant messages to the room. Setting specific message fields may require additional capabilities.
• canReceiveMessage - the holder can receive instant messages from the room.
• canCopyMessage - the holder can copy content from a received instant message.
• canReportAbuse - the holder can report a franked instant message as abusive.
• canReplyToMessage - the holder can send a message replying to another message.
• canReactToMessage - the holder can send a reaction, replying to another message, and using the "reaction" disposition.
• canDeleteOwnReaction - the holder can retract (unlike) it own previous reaction.
• canDeleteOtherReaction - the holder can delete the reaction of another user's previous reaction
• canEditOwnMessage - the holder can edit the content of one of its own previously sent messages
• canDeleteOwnMessage - the holder can retract one of its own previously sent messages
• canDeleteOtherMessage - the holder can retract messages for other users.
• canStartTopic - the holder can set the topic for a message
• canReplyInTopic - the holder can send a message replying to a previous message, using the same topic as the original sender.
• canEditOwnTopic - the holder can change the topic of a previously sent message
• canEditOtherTopic - the holder can change the topic of a message previously sent by another user.
• canSendLink - the holder can send an inline link
• canSendLinkPreview - the holder can send an inline link with an associated preview.
• canFollowLink - the holder can open a sent inline link.
• canCopyLink - the holder can copy the URL of a sent inline link.

The Hub can enforce whether a member can send a message. It can also withhold fanout of application messages to clients of a user. The other capabilities in this section can only be enforced by other clients.

Asset Capabilities

• canUploadAttachment - the holder can upload a file with the "attachent" disposition.
• canDownloadAttachment - the holder can download a file with the "attachent" disposition.
• canUploadImage - the holder can upload a file with the media type of "image" and the disposition of "render"
• canDownloadImage - the holder can download a file with the media type of "image" and the disposition of "render"
• canUploadVideo - the holder can upload a file with the media type of "video" and the disposition of "render"
• canDownloadVideo - the holder can download a file with the media type of "video" and the disposition of "render"
• canUploadSound - the holder can upload a file with the media type of "audio" and the disposition of "render"
• canDownloadSound - the holder can download a file with the media type of "audio" and the disposition of "render"

Real-time media The MIMI Working has not yet defined requirements for real-time media, however the capabilities below are widely representative of the permissions that would be required.

• canStartCall - the holder can initiate a new real-time call/conference
• canJoinCall - the holder can join an existing real-time call/conference
• canSendAudio - the holder is authorized to contribute audio in a call/conference.
• canReceiveAudio - the holder is authorized to receive audio in a call/conference.
• canSendVideo - the holder is authorized to contribute video in a call/conference.
• canReceiveVideo - the holder is authorized to receive video in a call/conference.
• canShareScreen - the holder is authorized to contribute screen sharing in a call/conference
• canViewSharedScreen - the holder is authorized to receive screen sharing in a call/conference

Disruptive Policy Changes

• canChangeRoomMembershipStyle - the holder is authorized to modify the base room membership style.
• canChangeRoleDefinitions - the holder is authorized to make changes to the definitions of the Roles component.
• canChangePreauthorizedUserList - the holder is authorized to make changes to the Preauthorized Users component.
• canDestroyRoom - the holder is authorized to completely destroy the room.
• canReinitGroup - the holder is authorized to send an MLS ReInit proposal.

Reserved Capabilities The following capability names are reserved for possible future use

• canCreateJoinCode
• canKnock
• canAcceptKnock
• canCreateSubgroup
• canSendDirectMessage
• canTargetMessage
• canChangeOwnName
• canChangeOwnPresence
• canChangeOwnMood
• canChangeOwnAvatar
• canCreateRoom
• canChangeMlsOperationalPolicies
• canChangeOtherPolicyAttribute
• MLS specific
  • update - update policy
  • PSK - psk policy
  • external proposal - general operational policy rules
  • external commit - general operational policy rules

Extensibility of the policy format Finally, The extensibility mechanism allows for future addition of new room policies. ; ExtType type; opaque value; } PolicyExtension; struct { ... PolicyExtension policy_extensions; } RoomPolicy; ]]>

Security Considerations This entire document focuses on authorization policy. TODO More Security

IANA Considerations

Preauthorized users MLS application component TBC

Role definitions MLS application component TBC

New MIMI Role Capabilities registry Create a new registry with the following values assigned sequentially using the reference RFCXXXX.

References Normative References Cisco The More Instant Messaging Interoperability (MIMI) working group is defining a suite of protocols that allow messaging providers to interoperate with one another. This document lays out an overall architecture enumerating the MIMI protocols and how they work together to enable an overall messaging experience. Cisco The Matrix.org Foundation C.I.C. Phoenix R&D Unaffiliated The Matrix.org Foundation C.I.C. Phoenix R&D This document specifies the More Instant Messaging Interoperability (MIMI) transport protocol, which allows users of different messaging providers to interoperate in group chats (rooms), including to send and receive messages, share room policy, and add participants to and remove participants from rooms. MIMI describes messages between providers, leaving most aspects of the provider-internal client- server communication up to the provider. MIMI integrates the Messaging Layer Security (MLS) protocol to provide end-to-end security assurances, including authentication of protocol participants, confidentiality of messages exchanged within a room, and agreement on the state of the room. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Phoenix R&D The Messaging Layer Security (MLS) protocol is an asynchronous group authenticated key exchange protocol. MLS provides a number of capabilities to applications, as well as several extension points internal to the protocol. This document provides a consolidated application API, guidance for how the protocol's extension points should be used, and a few concrete examples of both core protocol extensions and uses of the application API. Rohan Mahy Consulting Services This document describes content semantics common in Instant Messaging (IM) systems and describes a profile suitable for instant messaging interoperability of messages end-to-end encrypted inside the MLS (Message Layer Security) Protocol. Informative References Inria & Mozilla Windy Hill Systems, LLC Wire The Messaging Layer Security (MLS) protocol (I-D.ietf-mls-protocol) provides a Group Key Agreement protocol for messaging applications. MLS is meant to protect against eavesdropping, tampering, message forgery, and provide Forward Secrecy (FS) and Post-Compromise Security (PCS). This document describes the architecture for using MLS in a general secure group messaging infrastructure and defines the security goals for MLS. It provides guidance on building a group messaging system and discusses security and privacy tradeoffs offered by multiple security mechanisms that are part of the MLS protocol (e.g., frequency of public encryption key rotation). The document also provides guidance for parts of the infrastructure that are not standardized by MLS and are instead left to the application. While the recommendations of this document are not mandatory to follow in order to interoperate at the protocol level, they affect the overall security guarantees that are achieved by a messaging application. This is especially true in the case of active adversaries that are able to compromise clients, the delivery service, or the authentication service.

Role examples

Cooperatively administered room This is an example set of role policies, which is suitable for friends and family rooms and small groups of peers in a workgroup or club.

• no_role
  • role_index = 0
  • no capabilities
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = []
• banned
  • role_index = 1
  • no capabilities
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = []
• ordinary_user
  • role_index = 2
  • authorized capabilities
    • canAddParticipant
    • canRemoveParticipant
    • canAddOwnClient
    • canRemoveOwnClient
    • canRemoveSelf
    • canSendMessage
    • canReceiveMessage
    • canCopyMessage
    • canReportAbuse
    • canReplyToMessage
    • canReactToMessage
    • canDeleteOwnReaction
    • canEditOwnMessage
    • canDeleteOwnMessage
    • canStartTopic
    • canReplyInTopic
    • canEditOwnTopic
    • canUploadImage
    • canUploadVideo
    • canUploadSound
    • canUploadAttachment
    • canDownloadImage
    • canDownloadVideo
    • canDownloadSound
    • canDownloadAttachment
    • canSendLink
    • canSendLinkPreview
    • canFollowLink
    • canCopyLink
    • canChangeRoomName
    • canChangeRoomAvatar
    • canChangeRoomSubject
    • canChangeRoomMood
    • canChangeOwnName
    • canChangeOwnPresence
    • canChangeOwnMood
    • canChangeOwnAvatar
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[2]), (2,[0])]
• group_admin
  • role_index = 3
  • authorized capabilities
    • (include all the capabilities authorized for an ordinary_user)
    • canBan
    • canUnBan
    • canKick
    • canRevokeVoice
    • canGrantVoice
    • canChangeUserRole
    • canDeleteOtherMessage
    • canEditOtherTopic
    • canChangeRoomDescription
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[1,2,3]), (1,[0,2,3]), (2,[0,1,3]), (3,[0,1,2])]
• super_admin
  • role_index = 4
  • authorized capabilities
    • (include all the capabilities authorized for a group_admin)
    • canChangeRoomMembershipStyle
    • canChangePreauthorizedUserList
    • canDestroyRoom
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[1,2,3,4]), (1,[0,2,3,4]), (2,[0,1,3,4]), (3,[0,1,2,4]), (4,[0,1,2,3])]
• policy_enforcer
  • role_index = 5
  • capabilities
    • (does not include any other capabilities)
    • canRemoveParticipant
    • canChangeUserRole
    • canBan
    • canUnban
    • canChangeRoleDefinitions
    • canChangePreauthorizedUserList
    • canChangeMlsOperationalPolicies
    • canDestroyRoom
    • canSendMLSReinitProposal
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = 2
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = [(0,[1]), (1,[0]), (2,[0,1]), (3,[0,1]), (4,[0,1])]
  • Notes: can remove a banned user from the list (cleanup) but not restore them

Strictly administered room This is an example set of role policies, which is suitable for friends and family rooms and small groups of peers in a workgroup or club.

• no_role
  • role_index = 0
  • authorized capabilities
    • canUseJoinCode
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = [(0,[2])]
• banned
  • role_index = 1
  • no capabilities
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = []
• ordinary_user
  • role_index = 2
  • authorized capabilities
    • canAddOwnClient
    • canAddSelf
    • canRemoveOwnClient
    • canRemoveSelf
    • canChangeOwnRole
    • canSendMessage
    • canReceiveMessage
    • canCopyMessage
    • canReportAbuse
    • canReactToMessage
    • canDeleteOwnReaction
    • canEditOwnMessage
    • canDeleteOwnMessage
    • canStartTopic
    • canReplyInTopic
    • canUploadImage
    • canUploadVideo
    • canUploadSound
    • canUploadAttachment
    • canDownloadImage
    • canDownloadVideo
    • canDownloadSound
    • canDownloadAttachment
    • canSendLink
    • canSendLinkPreview
    • canFollowLink
    • canCopyLink
    • canChangeOwnName
    • canChangeOwnPresence
    • canChangeOwnMood
    • canChangeOwnAvatar
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[2]), (2,[0])]
• group_admin
  • role_index = 3
  • authorized capabilities
    • (include all the capabilities authorized for an ordinary_user)
    • canAddParticipant
    • canRemoveParticipant
    • canBan
    • canUnBan
    • canKick
    • canChangeUserRole
    • canCreateJoinCode - reserved for future use
    • canDeleteOtherReaction
    • canDeleteOtherMessage
    • canEditOwnTopic
    • canEditOtherTopic
    • canChangeRoomDescription
    • canChangeRoomName
    • canChangeRoomAvatar
    • canChangeRoomSubject
    • canChangeRoomMood
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[1,2,3]), (1,[0,2,3]), (2,[0,1,3]), (3,[0,1,2])]
• super_admin
  • role_index = 4
  • authorized capabilities
    • (include all the capabilities authorized for a group_admin)
    • canChangeRoomMembershipStyle
    • canChangeRoleDefinitions
    • canChangePreauthorizedUserList
    • canDestroyRoom
    • canSendMLSReinitProposal
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[1,2,3,4]), (1,[0,2,3,4]), (2,[0,1,3,4]), (3,[0,1,2,4]), (4,[0,1,2,3])]
• policy_enforcer
  • role_index = 5
  • capabilities
    • (does not include any other capabilities)
    • canRemoveParticipant
    • canChangeUserRole
    • canBan
    • canUnban
    • canChangeRoleDefinitions
    • canChangePreauthorizedUserList
    • canChangeMlsOperationalPolicies
    • canDestroyRoom
    • canSendMLSReinitProposal
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = 2
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = [(0,[1]), (1,[0]), (2,[0,1]), (3,[0,1]), (4,[0,1])]
  • Notes: can remove a banned user from the list (cleanup) but not restore them

Moderated room

• no_role
  • role_index = 0
  • authorized capabilities
    • canUseJoinCode
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[2,3])]
• banned
  • role_index = 1
  • no capabilities
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = []
• guest
  • role_index = 2
  • authorized capabilities
    • canRemoveSelf
    • canReceiveMessage
    • canCopyMessage
    • canReactToMessage
    • canDeleteOwnReaction
    • canFollowLink
    • canCopyLink
    • canDownloadImage
    • canDownloadVideo
    • canDownloadSound
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[2]), (2,[0])]
• attendee
  • role_index = 3
  • authorized capabilities
    • (include all the capabilities authorized for a guest)
    • canAddOwnClient
    • canAddSelf
    • canRemoveOwnClient
    • canChangeOwnRole
    • canReportAbuse
    • canReplyInTopic
    • canDownloadAttachment
    • canChangeOwnName
    • canChangeOwnPresence
    • canChangeOwnAvatar
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[3]), (3,[0])]
• speaker
  • role_index = 4
  • authorized capabilities
    • (include all the capabilities authorized for a speaker)
    • canSendMessage
    • canEditOwnMessage
    • canDeleteOwnMessage
    • canStartTopic
    • canUploadImage
    • canUploadVideo
    • canUploadSound
    • canUploadAttachment
    • canSendLink
    • canSendLinkPreview
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[4]), (4,[0])]
• moderator
  • role_index = 5
  • authorized capabilities
    • (include all the capabilities authorized for an ordinary_user)
    • canAddParticipant
    • canRemoveParticipant
    • canBan
    • canUnBan
    • canKick
    • canChangeUserRole
    • canCreateJoinCode - reserved for future use
    • canDeleteOtherReaction
    • canDeleteOtherMessage
    • canEditOwnTopic
    • canEditOtherTopic
    • canChangeRoomName
    • canChangeRoomAvatar
    • canChangeRoomSubject
    • canChangeRoomMood
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[1,2,3,4,5]), (1,[0,2,3,4,5]), (2,[0,1,3,4,5]), (3,[0,1,2,4,5]), (4,[0,1,2,3,5]), (5,[0,1,2,3,4])]
• super_admin
  • role_index = 6
  • authorized capabilities
    • (include all the capabilities authorized for a moderator)
    • canChangeRoomDescription
    • canChangeRoomMembershipStyle
    • canChangeRoleDefinitions
    • canChangePreauthorizedUserList
    • canDestroyRoom
    • canSendMLSReinitProposal
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[1,2,3,4,5,6]), (1,[0,2,3,4,5,6]), (2,[0,1,3,4,5,6]), (3,[0,1,2,4,5,6]), (4,[0,1,2,3,5,6]), (5,[0,1,2,3,4,6]), (6,[0,1,2,3,4,5])]
• policy_enforcer
  • role_index = 7
  • capabilities
    • (does not include any other capabilities)
    • canRemoveParticipant
    • canChangeUserRole
    • canBan
    • canUnban
    • canChangeRoleDefinitions
    • canChangePreauthorizedUserList
    • canChangeMlsOperationalPolicies
    • canDestroyRoom
    • canSendMLSReinitProposal
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = 2
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = [(0,[1]), (1,[0]), (2,[0,1]), (3,[0,1]), (4,[0,1]), (5, [0,1]), (6, [0,1])]
  • Notes: can remove a banned user from the list (cleanup) but not restore them

Multi-organization administered room In this example room policy, Alice from organization A is a super admin. There are per organization user and admin roles for orgs A, B, and C. Organizational admins can only move users to and from their org user role, their org admin role, the no_role; and can ban (but not unban) their own org users. The non-host orgs do not have the canChangeOwnRole and canAddSelf, and are limited to 3 admins per org.

• no_role
  • role_index = 0
  • no capabilities
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = []
• banned
  • role_index = 1
  • no capabilities
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = []
• org_a_user
  • role_index = 2
  • authorized capabilities
    • (all capabilities of org_b_user)
    • canChangeOwnRole
    • canAddSelf
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[2]), (2,[0])]
• org_b_user
  • role_index = 3
  • authorized capabilities
    • canRemoveSelf
    • canAddOwnClient
    • canRemoveOwnClient
    • canSendMessage
    • canReceiveMessage
    • canCopyMessage
    • canReportAbuse
    • canReplyInTopic
    • canReactToMessage
    • canDeleteOwnReaction
    • canEditOwnMessage
    • canSendLink
    • canSendLinkPreview
    • canFollowLink
    • canCopyLink
    • canDownloadImage
    • canDownloadVideo
    • canDownloadSound
    • canDownloadAttachment
    • canChangeOwnName
    • canChangeOwnPresence
    • canChangeOwnAvatar
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[3]), (3,[0])]
• org_c_user
  • role_index = 4
  • authorized capabilities
    • (same capabilities as org_b_user)
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[4]), (4,[0])]
• org_a_admin
  • role_index = 5
  • authorized capabilities
    • (all capabilities of org_b_admin)
    • canChangeOwnRole
    • canAddSelf
  • constraints
    • minimum_participants_constraint = 0
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[2,5]), (2,[0,1,5]), (5,[0,1,2])]
• org_b_admin
  • role_index = 6
  • authorized capabilities
    • (all capabilities of org_b_user)
    • canDeleteOwnMessage
    • canStartTopic
    • canUploadImage
    • canUploadVideo
    • canUploadSound
    • canUploadAttachment
    • canAddParticipant
    • canRemoveParticipant
    • canBan
    • canKick
    • canChangeUserRole
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = 3
    • minimum_active_participants_constraint = 1
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[3,6]), (3,[0,1,6]), (6,[0,1,3])]
• org_c_admin
  • role_index = 7
  • authorized capabilities
    • (all capabilities of org_b_admin)
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = 3
    • minimum_active_participants_constraint = 1
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[4,7]), (4,[0,1,7]), (7,[0,1,4])]
• super_admin
  • role_index = 8
  • authorized capabilities
    • (include all the capabilities authorized for org_a_admin)
    • canUnBan
    • canDeleteOtherReaction
    • canDeleteOtherMessage
    • canEditOwnTopic
    • canEditOtherTopic
    • canChangeRoomDescription
    • canChangeRoomName
    • canChangeRoomAvatar
    • canChangeRoomSubject
    • canChangeRoomMood
    • canChangeRoomMembershipStyle
    • canChangeRoleDefinitions
    • canChangePreauthorizedUserList
    • canDestroyRoom
    • canSendMLSReinitProposal
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = null
    • minimum_active_participants_constraint = 1
    • maximum_active_participants_constraint = null
    • authorized_role_changes = [(0,[1,2,3,4,5,6,7,8]), (1,[0,2,3,4,5,6,7,8]), (2,[0,1,5,8]), (3,[0,1,6]), (4,[0,1,7]), (5,[0,1,2,8]), (6,[0,1,3]), (7,[0,1,4]), (8,[0,1,2,5])]
• policy_enforcer
  • role_index = 9
  • capabilities
    • (does not include any other capabilities)
    • canRemoveParticipant
    • canChangeUserRole
    • canBan
    • canUnban
    • canChangeRoleDefinitions
    • canChangePreauthorizedUserList
    • canChangeMlsOperationalPolicies
    • canDestroyRoom
    • canSendMLSReinitProposal
  • constraints
    • minimum_participants_constraint = 1
    • maximum_participants_constraint = 2
    • minimum_active_participants_constraint = 0
    • maximum_active_participants_constraint = 0
    • authorized_role_changes = [(0,[1]), (1,[0]), (3,[0,1]), (4,[0,1]), (5,[0,1]), (6,[0,1]), (7,[0,1]), (8,[0,1])]
  • Notes: can remove a banned user from the list (cleanup) but not restore them

Complete TLS Presentation Language Syntax ; } Uri; enum { optional(0), required(1), forbidden(2) } Optionality; enum { reserved(0) ordinary(1), fixed-membership(2), parent-dependent(3), (255) } MembershipStyle; struct { Optionality logging; bool enabled; Uri logging_clients; Uri machine_readable_policy; Uri human_readable_policy; } LoggingPolicy; struct { bool on_request; Uri join_link; bool multiuser; uint32 expiration; Uri link_requests; } LinkPolicy; struct { opaque name; opaque description; Uri homepage; Role bot_role; bool can_read; bool can_write; bool can_target_message_in_group; bool per_user_content; } Bot; struct { Optionality history_sharing; Role who_can_share; bool automatically_share; uint32 max_time_period; } HistoryPolicy; enum { null(0), boolean(1), number(2), string(3), jsonObject(4) } ExtType; struct { opaque name; ExtType type; opaque value; } PolicyExtension; struct { MembershipStyle membership_style; bool multi_device; Uri parent_room_uri; bool persistent_room; Optionality delivery_notifications; Optionality read_receipts; bool semi_anonymous_ids; bool discoverable; LinkPolicy link_policy; LoggingPolicy logging_policy; HistoryPolicy history_sharing; Bot allowed_bots; PolicyExtension policy_extensions; } RoomPolicy; RoomPolicy room_policy; ]]>

Acknowledgments TODO acknowledge.