More Instant Messaging Interoperability (MIMI) using HTTPS and MLS
draft-ietf-mimi-protocol-00
| Document | Type | Active Internet-Draft (mimi WG) | |
|---|---|---|---|
| Authors | Richard Barnes , Matthew Hodgson , Konrad Kohbrok , Rohan Mahy , Travis Ralston , Raphael Robert | ||
| Last updated | 2024-04-02 | ||
| Replaces | draft-ralston-mimi-protocol | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
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| Additional resources | Mailing list discussion | ||
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draft-ietf-mimi-protocol-00
More Instant Messaging Interoperability R. L. Barnes
Internet-Draft Cisco
Intended status: Standards Track M. Hodgson
Expires: 4 October 2024 The Matrix.org Foundation C.I.C.
K. Kohbrok
Phoenix R&D
R. Mahy
Unaffiliated
T. Ralston
The Matrix.org Foundation C.I.C.
R. Robert
Phoenix R&D
2 April 2024
More Instant Messaging Interoperability (MIMI) using HTTPS and MLS
draft-ietf-mimi-protocol-00
Abstract
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.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://bifurcation.github.io/ietf-mimi-protocol/draft-ralston-mimi-
protocol.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-mimi-protocol/.
Discussion of this document takes place on the More Instant Messaging
Interoperability Working Group mailing list (mailto:mimi@ietf.org),
which is archived at https://mailarchive.ietf.org/arch/browse/mimi/.
Subscribe at https://www.ietf.org/mailman/listinfo/mimi/.
Source for this draft and an issue tracker can be found at
https://github.com/bifurcation/ietf-mimi-protocol.
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Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 4 October 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Known Gaps . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 5
3. Example protocol flow . . . . . . . . . . . . . . . . . . . . 6
3.1. Alice Creates a Room . . . . . . . . . . . . . . . . . . 7
3.2. Alice adds Bob to the Room . . . . . . . . . . . . . . . 8
3.3. Bob adds Cathy to the Room . . . . . . . . . . . . . . . 10
3.4. Cathy Sends a Message . . . . . . . . . . . . . . . . . . 11
3.5. Bob Leaves the Room . . . . . . . . . . . . . . . . . . . 12
3.6. Cathy adds a new device . . . . . . . . . . . . . . . . . 14
4. Services required at each layer . . . . . . . . . . . . . . . 16
4.1. Transport layer . . . . . . . . . . . . . . . . . . . . . 16
4.2. End-to-End Security Layer . . . . . . . . . . . . . . . . 16
4.3. Application Layer . . . . . . . . . . . . . . . . . . . . 17
4.3.1. Server State . . . . . . . . . . . . . . . . . . . . 17
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4.3.2. Participant List Changes . . . . . . . . . . . . . . 18
5. MIMI Endpoints and Framing . . . . . . . . . . . . . . . . . 18
5.1. Directory . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2. Fetch Key Material . . . . . . . . . . . . . . . . . . . 19
5.3. Update Room State . . . . . . . . . . . . . . . . . . . . 23
5.4. Submit a Message . . . . . . . . . . . . . . . . . . . . 25
5.5. Fanout Messages and Room Events . . . . . . . . . . . . . 27
5.6. Claim group key information . . . . . . . . . . . . . . . 29
6. Relation between MIMI state and cryptographic state . . . . . 32
6.1. Room state . . . . . . . . . . . . . . . . . . . . . . . 32
6.2. Cryptographic room representation . . . . . . . . . . . . 32
6.3. Proposal-commit paradigm . . . . . . . . . . . . . . . . 32
6.4. Authenticating proposals . . . . . . . . . . . . . . . . 33
7. MLS Application State Synchronization . . . . . . . . . . . . 33
8. Security Considerations . . . . . . . . . . . . . . . . . . . 37
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2. Informative References . . . . . . . . . . . . . . . . . 39
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
The More Instant Messaging Interoperability (MIMI) transport protocol
enables providers of end-to-end encrypted instant messaging to
interoperate. As described in the MIMI architecture
[I-D.barnes-mimi-arch], group chats and direct messages are described
in terms of "rooms". Each MIMI protocol room is hosted at a single
provider (the "hub" provider"), but allows users from different
providers to become participants in the room. The hub provider is
responsible for ordering and distributing messages, enforcing policy,
and authorizing messages. It also keeps a copy of the room state,
which includes the room policy and participant list, which it can
provide to new joiners. Each provider also stores initial keying
material for its own users (who may be offline).
This document describes the communication among different providers
necessary to support messaging application functionality, for
example:
* Sharing room policy
* Adding and removing participants in a room
* Exchanging secure messages
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In support of these functions, the protocol also has primitives to
fetch initial keying material and fetch the current state of the
underlying end-to-end encryption protocol for the room.
Messages sent inside each room are end-to-end encrypted using the
Messaging Layer Security (MLS) protocol [RFC9420], and each room is
associated with an MLS group. MLS also ensures that clients in a
room agree on the room policy and participation. MLS is integrated
into MIMI in such a way as to ensure that a client is joined to a
room's MLS group only if the client's user is a participant in the
room, and that all clients in the group agree on the state of the
room (including, for example, the room's participant list).
1.1. Known Gaps
In this version of the document, we have tried to capture enough
concrete functionality to enable basic application functionality,
while defining enough of a protocol framework to indicate how to add
other necessary functionality. The following functions are likely to
be needed by the complete protocol, but are not covered here:
Authorization policy: In this document, we introduce a notional
concept of roles for participants, and permissions for roles.
Actual messaging systems have more complex and well-specified
authorization policies about which clients can take which actions
in a room.
Advanced join/leave flows: In this document, all adds / removes /
joins / leaves are initiated from within the group, or by a new
joiner who already has permission to join, as this aligns well
with MLS. Messaging applications support a variety of other
flows, some of which this protocol will need to support.
Consent: In this document, we assume that any required consent has
already been obtained, e.g., a user consenting to be added to a
room by another user. The full protocol will need some mechanisms
for establishing this consent.
Identifiers: Certain entities in the MIMI system need to be
identified in the protocol. In this document, we define a
notional syntax for identifiers, but a more concrete one should be
defined.
Abuse reporting: There is no mechanism in this document for
reporting abusive behavior to a messaging provider.
Identifier resolution: In some cases, the identifier used to
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initiate communications with a user might be different from the
identifier that should be used internally. For example, a user-
visible handle might need to be mapped to a durable internal
identifier. This document provides no mechanism for such
resolution.
Authentication While MLS provides basic message authentication,
users should also be able to (cryptographically) tie the identity
of other users to their respective providers. Further
authentication such as tying clients to their users (or the user's
other clients) may also be desirable.
2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Terms and definitions are inherited from [I-D.barnes-mimi-arch]. We
also make use of terms from the MLS protocol [RFC9420].
Throughout this document, the examples use the TLS Presentation
Language [RFC8446] and the semantics of HTTP [RFC7231] respectively
as placeholder a set of binary encoding mechanism and transport
semantics.
The protocol layering of the MIMI transport protocol is as follows:
1. An application layer that enables messaging functionality
2. A security layer that provides end-to-end security guarantees:
* Confidentiality for messages
* Authentication of actors making changes to rooms
* Agreement on room state across the clients involved in a room
3. A transport layer that provides secure delivery of protocol
objects between servers.
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+------------------------+
| Application |
| +----------------+
| | E2E Security |
+-------+----------------+
| Transport |
+------------------------+
Figure 1: MIMI protocol layering
MIMI uses MLS [RFC9420] for end-to-end security, using the MLS
AppSync proposal type to efficiently synchronize room state across
the clients involved in a room. The MIMI transport is based on HTTPS
over mutually-authenticated TLS.
3. Example protocol flow
This section walks through a basic scenario that illustrates how a
room works in the MIMI protocol. The scenario involves the following
actors:
* Service providers a.example, b.example, and c.example represented
by servers ServerA, ServerB, and ServerC respectively
* Users Alice (alice), Bob (bob) and Cathy (cathy) of the service
providers a.example, b.example, and c.example respectively.
* Clients ClientA1, ClientA2, ClientB1, etc. belonging to these
users
* A room clubhouse hosted by hub provider a.example where the three
users interact.
Inside the protocol, each provider is represented by a domain name in
the host production of the authority of a MIMI URI [RFC3986].
Specific hosts or servers are represented by domain names, but not by
MIMI URIs. Examples of different types of identifiers represented in
a MIMI URI are shown in the table below:
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+=================+==============================+
| Identifier type | Example URI |
+=================+==============================+
| Provider | mimi://a.example |
+-----------------+------------------------------+
| User | mimi://a.example/u/alice |
+-----------------+------------------------------+
| Client | mimi://a.example/d/ClientA1 |
+-----------------+------------------------------+
| Room | mimi://a.example/r/clubhouse |
+-----------------+------------------------------+
| MLS group | mimi://a.example/g/clubhouse |
+-----------------+------------------------------+
Table 1: MIMI URI examples
As noted in [I-D.barnes-mimi-arch], the MIMI protocol only defines
interactions between service providers' servers. Interactions
between clients and servers within a service provider domain are
shown here for completeness, but surrounded by [[ double brackets ]].
3.1. Alice Creates a Room
The first step in the lifetime of a MIMI room is its creation on the
hub server. This operation is local to the service provider, and
does not entail any MIMI protocol operations. However, it must
establish the initial state of the room, which is then the basis for
protocol operations related to the room.
For authorization purposes, MIMI uses permissions based on room-
defined roles. For example, a room might have a role named "admin",
which has canAddUser, canRemoveUser, and canSetUserRole permisions.
Here, we assume that Alice uses ClientA1 to create a room with the
following base policy properties:
* Room Identifier: mimi://a.example/r/clubhouse
* Roles: admin = [canAddUser, canRemoveUser, canSetUserRole]
And the following participant list:
* Participants: [[mimi://a.example/u/alice, "admin"]]
ClientA1 also creates an MLS group with group ID mimi://a.example/g/
clubhouse and ensures via provider-local operations that Alice's
other clients are members of this MLS group.
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3.2. Alice adds Bob to the Room
Adding Bob to the room entails operations at two levels. First,
Bob's user identity must be added to the room's participant list.
Second, Bob's clients must be added to the room's MLS group.
The process of adding Bob to the room thus begins by Alice fetching
key material for Bob's clients. Alice then updates the room by
sending an MLS Commit over the following proposals:
* An AppSync proposal updating the room state by adding Bob to the
participant list
* Add proposals for Bob's clients
The MIMI protocol interactions are between Alice's server ServerA and
Bob's server ServerB. ServerB stores KeyPackages on behalf of Bob's
devices. ServerA performs the key material fetch on Alice's behalf,
and delivers the resulting KeyPackages to Alice's clients. Both
ServerA and ServerB remember the sources of the KeyPackages they
handle, so that they can route a Welcome message for those
KeyPackages to the proper recipients -- ServerA to ServerB, and
ServerB to Bob's clients.
*NOTE:* In the full protocol, it will be necessary to have consent
and access control on these operations. We have elided that step
here in the interest of simplicity.
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ClientA1 ServerA ServerB ClientB*
| | | |
| | | Store KPs |
| | |<~~~~~~~~~~~~~~+
| | |<~~~~~~~~~~~~~~+
| Request KPs | | |
+~~~~~~~~~~~~~~>| /keyMaterial | |
| +-------------->| |
| | 200 OK | |
| KPs |<--------------+ |
|<~~~~~~~~~~~~~~+ | |
| | | |
ClientB*->ServerB: [[ Store KeyPackages ]]
ClientA1->ServerA: [[ request KPs for Bob ]]
ServerA->ServerB: POST /keyMaterial KeyMaterialRequest
ServerB: Verify that Alice is authorized to fetch KeyPackages
ServerB: Mark returned KPs as reserved for Alice's use
ServerB->ServerA: 200 OK KeyMaterialResponse
ServerA: Remember that these KPs go to b.example
ServerA->ClientA1: [[ KPs ]]
Figure 2: Alice Fetches KeyPackages for Bob's Clients
ClientA1 ServerA ServerB ClientB*
| | | |
| Commit, etc. | | |
+~~~~~~~~~~~~~~>| | |
| Accepted | /notify | |
|<~~~~~~~~~~~~~~+-------------->| |
| | 200 OK | Welcome, Tree |
| |<--------------+~~~~~~~~~~~~~~>|
| | +~~~~~~~~~~~~~~>|
| | | |
ClientA1: Prepare Commit over AppSync(+Bob), Add*
ClientA1->ServerA: [[ Commit, Welcome, GroupInfo?, RatchetTree? ]]
ServerA: Verify that AppSync, Adds are allowed by policy
ServerA: Identifies Welcome domains based on KP hash in Welcome
ServerA->ServerB: POST /notify/a.example/r/clubhouse Intro{ Welcome, RatchetTree? }
ServerB: Recognizes that Welcome is adding Bob to room clubhouse
ServerB->ClientB*: [[ Welcome, RatchetTree? ]]
Figure 3: Alice Adds Bob to the Room and Bob's Clients to the MLS
Group
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3.3. Bob adds Cathy to the Room
The process of adding Bob was a bit abbreviated because Alice is a
user of the hub service provider. When Bob adds Cathy, we see the
full process, involving the same two steps (KeyPackage fetch followed
by Add), but this time indirected via the hub server ServerA. Also,
now that there are users on ServerB involved in the room, the hub
ServerA will have to distribute the Commit adding Cathy and Cathy's
clients to ServerB as well as forwarding the Welcome to ServerC.
ClientB1 ServerB ServerA ServerC ClientC*
| | | | |
| | | | Store KPs |
| | | |<~~~~~~~~~~~~~~+
| | | |<~~~~~~~~~~~~~~+
| Request KPs | | | |
+~~~~~~~~~~~~~~>| /keyMaterial | /keyMaterial | |
| +-------------->+-------------->| |
| | 200 OK | 200 OK | |
| KPs |<--------------+<--------------+ |
|<~~~~~~~~~~~~~~+ | | |
| | | | |
ClientC*->ServerC: [[ Store KeyPackages ]]
ClientB1->ServerB: [[ request KPs for Bob ]]
ServerB->ServerA: POST /keyMaterial KeyMaterialRequest
ServerA->ServerC: POST /keyMaterial KeyMaterialRequest
ServerB: Verify that Bob is authorized to fetch KeyPackages
ServerB: Mark returned KPs as reserved for Bob's use
ServerC->ServerA: 200 OK KeyMaterialResponse
ServerA: Remember that these KPs go to b.example
ServerA->ServerB: 200 OK KeyMaterialResponse
ServerB->ClientB1: [[ KPs ]]
Figure 4: Bob Fetches KeyPackages for Cathy's Clients
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Client Client Client Client
B1 ServerB ServerA ServerC C* B* A*
| | | | | | |
| Commit, etc | | | | | |
+~~~~~~~~~~~~>| /update | | | | |
| +---------->| | | | |
| | 200 OK | | | | |
| |<----------+ | | | |
| Accepted | | /notify | Welcome, | | |
|<~~~~~~~~~~~~+ +---------->| Tree | | |
| | | +~~~~~~~~~~>| | |
| | /notify | Commit +~~~~~~~~~~>| | |
| |<----------+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>|
| | Commit | | | | |
| +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>| |
| | | | | | |
ClientB1: Prepare Commit over AppSync(+Cathy), Add*
ClientB1->ServerB: [[ Commit, Welcome, GroupInfo?, RatchetTree? ]]
ServerB->ServerA: POST /update/a.example/r/clubhouse CommitBundle
ServerA: Verify that Adds are allowed by policy
ServerA->ServerB: 200 OK
ServerA->ServerC: POST /notify/a.example/r/clubhouse
Intro{ Welcome, RatchetTree? }
ServerC: Recognizes that Welcome is adding Cathy to clubhouse
ServerC->ClientC*: [[ Welcome, RatchetTree? ]]
ServerA->ServerB: POST /notify/a.example/r/clubhouse Commit
ServerB->ClientB*: [[ Commit ]]
ServerA->ClientA*: [[ Commit ]]
Figure 5: Bob Adds Cathy to the Room and Cathy's Clients to the
MLS Group
3.4. Cathy Sends a Message
Now that Alice, Bob, and Cathy are all in the room, Cathy wants to
say hello to everyone. Cathy's client encapsulates the message in an
MLS PrivateMessage and sends it to ServerC, who forwards it to the
hub ServerA on Cathy's behalf. Assuming Cathy is allowed to speak in
the room, ServerA will forward Cathy's message to the other servers
involved in the room, who distribute it to their clients.
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ClientC1 ServerC ServerA ServerB ClientB* ClientC* ClientA*
| | | | | | |
| Message | | | | | |
+~~~~~~~~~~~~~~>| /submit | | | | |
| +-------------->| | | | |
| | 200 OK | | | | |
| |<--------------+ | | | |
| Accepted | | /notify | | | |
|<~~~~~~~~~~~~~~+ +-------------->| Message | | |
| | | +~~~~~~~~~~~~~~>| | |
| | /notify | | | | |
| |<--------------+ | | | |
| | Message | | | | |
| +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>| |
| | | Message | | | |
| | +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>|
| | | | | | |
ClientC1->ServerC: [[ MLSMessage(PrivateMessage) ]]
ServerC->ServerA: POST /submit/a.example/r/clubhouse MLSMessage(PrivateMessage)
ServerA: Verifies that message is allowed
ServerA->ServerC: POST /notify/a.example/r/clubhouse Message{ MLSMessage(PrivateMessage) }
ServerA->ServerB: POST /notify/a.example/r/clubhouse Message{ MLSMessage(PrivateMessage) }
ServerA->ClientA*: [[ MLSMessage(PrivateMessage) ]]
ServerB->ClientB*: [[ MLSMessage(PrivateMessage) ]]
ServerC->ClientC*: [[ MLSMessage(PrivateMessage) ]]
Figure 6: Cathy Sends a Message to the Room
3.5. Bob Leaves the Room
A user removing another user follows the same flow as adding the
user. The user performing the removal creates an MLS commit covering
Remove proposals for all of the removed user's devices, and an
AppSync proposal updating the room state to remove the removed user
from the room's participant list.
One's own user leaving is slightly more complicated than removing
another user, because the leaving user cannot remove all of their
devices from the MLS group. Instead, the leave happens in three
steps:
1. The leaving client constructs MLS Remove proposals for all of the
user's devices (including the leaving client), and an AppSync
proposal that removes its user from the participant list.
2. The leaving client sends these proposals to the hub. The hub
caches the proposals.
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3. The next time a client attempts to commit, the hub requires the
client to include the cached proposals.
The hub thus guarantees the leaving client that they will be removed
as soon as possible.
ClientB1 ServerB ServerA ServerC ClientC1 C2
| | | | | |
| Proposals | | | | |
+~~~~~~~~~~~~~~>| /update | | | |
| +-------------->| | | |
| | 200 OK | | | |
| |<--------------+ | | |
| Accepted | | /notify | | |
|<~~~~~~~~~~~~~~+ +-------------->| | |
| | | | Proposals | |
| | | +~~~~~~~~~~~~~~>| |
| | | | | |
| | | | Commit(Props) | |
| | | |<~~~~~~~~~~~~~~+ |
| | | /update | | |
| | |<--------------+ | |
| | | 200 OK | | |
| | +-------------->| | |
| | | | Accepted | |
| | | +~~~~~~~~~~~~~~>| |
| | /notify | /notify | | |
| Commit |<--------------+-------------->| Commit | |
|<~~~~~~~~~~~~~~+ | +~~~~~~~~~~~~~~~~~~~~~~>|
| | | +~~~~~~~~~~~~~~>| |
| | | | | |
ClientB1: Prepare Remove*, AppSync(-Bob)
ClientB1->ServerB: [[ Remove*, AppSync ]]
ServerB->ServerA: POST /update/a.example/r/clubhouse Remove*, AppSync
ServerA: Verify that Removes, AppSync are allowed by policy; cache
ServerA->ServerB: 200 OK
ServerA->ServerC: POST /notify/a.example/r/clubhouse Proposals
ServerC1->ClientC1: [[ Proposals ]]
ClientC1->ServerC: [[ Commit(Props), Welcome, GroupInfo?, RatchetTree? ]]
ServerC->ServerA: POST /update/a.example/r/clubhousee CommitBundle
ServerA: Check whether Commit includes queued proposals; accept
ServerA->ServerC: 200 OK
ServerA->ServerB: POST /notify/a.example/r/clubhouse Commit
ServerA->ServerC: POST /notify/a.example/r/clubhouse Commit
ServerB->ClientB1: [[ Commit ]]
ServerC->ClientC2: [[ Commit ]]
ServerC->ClientC1: [[ Commit ]] (up to provider)
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Figure 7: Bob Leaves the Room
3.6. Cathy adds a new device
Many users have multiple clients often running on different devices
(for example a phone, a tablet, and a computer). When a user creates
a new client, that client needs to be able to join all the MLS groups
associated with the rooms in which the user is a participant.
In MLS in order to initiate joining a group the joining client needs
to get the current GroupInfo and ratchet_tree, and then send an
External Commit to the hub. In MIMI, the hub keeps or reconstructs a
copy of the GroupInfo, assuming that other clients may not be
available to assist the client with joining.
For Cathy's new client (ClientC3) to join the MLS group and therefore
fully participate in the room with Alice, ClientC3 needs to fetch the
MLS GroupInfo, and then generate an External Commit adding ClientC3.
Cathy's new client sends the External Commit to the room's MLS group
by sending an /update to the room.
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ClientC3 ServerC ServerA ServerB ClientB* ClientC* ClientA*
| | | | | | |
| Fetch | | | | | |
| GroupInfo | | | | | |
|~~~~~~~~~~~~~~>| /groupInfo | | | | |
| |-------------->| | | | |
| GroupInfo + | 200 OK | | | | |
| tree |<--------------| | | | |
|<~~~~~~~~~~~~~~| | | | | |
| | | | | | |
| External | | | | | |
| Commit, etc. | | | | | |
+~~~~~~~~~~~~~~>| /update | | | | |
| +-------------->| | | | |
| | 200 OK | | | | |
| |<--------------+ | | | |
| Accepted | | Commit | | | |
|<~~~~~~~~~~~~~~| +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>|
| | | | | | |
| | | /notify | | | |
| + +-------------->| Commit | | |
| | | +~~~~~~~~~~~~~~>| | |
| | /notify | | | | |
| |<--------------+ | | | |
| | Commit | | | | |
| +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>| |
| | | | | | |
ClientC3->ServerC: [[ request current GroupInfo ]]
ServerC->ServerA: POST /groupInfo/a.example/clubhouse
ServerA: Verify that ClientC3 has authorization to join the room
ServerA->ServerC: 200 OK w/ current GroupInfo and ratchet tree
ServerC->ClientC3: [[ GroupInfo, tree ]]
ClientC3: Prepare External Commit Add*
ClientC3->ServerC: [[ Commit, GroupInfo?, RatchetTree? ]]
ServerC->ServerA: POST /update/a.example/clubhouse CommitBundle
ServerA: Verify that Commit is valid and ClientC3 is authorized
ServerA->ServerC: 200 OK
ServerC->ClientC3: [[ External Commit was accepted ]]
ServerA->ClientA*: [[ Commit ]]
ServerA->ServerB: POST /notify/a.example/clubhouse Commit
ServerB->ClientB*: [[ Commit ]]
ServerA->ServerC: POST /notify/a.example/clubhouse Commit
ServerC->ClientC*: [[ Commit ]]
Figure 8: Cathy Adds a new Client
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4. Services required at each layer
4.1. Transport layer
MIMI servers communicate using HTTPS. The HTTP request MUST identify
the source and target providers for the request, in the following
way:
* The target provider is indicated using a Host header [RFC9110].
If the provider is using a non-standard port, then the port
component of the Host header is ignored.
* The source provider is indicated using a From header [RFC9110].
The mailbox production in the From header MUST use the addr-spec
variant, and the local-part of the address MUST contain the fixed
string mimi. Thus, the content of the From header will be
mimi@a.example, where a.example is the domain name of the source
provider.
*NOTE*: The use of the From header field here is not really well-
aligned with its intended use. The WG should consider whether
this is correct, or whether a new header field would be better.
Perhaps something like "From-Host" to match Host?
The TLS connection underlying the HTTPS connection MUST be mutually
authenticated. The certificates presented in the TLS handshake MUST
authenticate the source and target provider domains, according to
[RFC6125].
The bodies of HTTP requests and responses are defined by the
individual endpoints defined in Section 4.3.
4.2. End-to-End Security Layer
Every MIMI room has an MLS group associated to it, which provides
end-to-end security guarantees. The clients participating in the
room manage the MLS-level membership by sending Commit messages
covering Add and Remove proposals.
Every application message sent within a room is authenticated and
confidentiality-protected by virtue of being encapsulated in an MLS
PrivateMessage object.
MIMI uses the MLS application state synchronization mechanism
(Section 7) to ensure that the clients involved in a MIMI room agree
on the state of the room. Each MIMI message that changes the state
of the room is encapsulated in an AppSync proposal and transmitted
inside an MLS PublicMessage object.
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The PublicMessage encapsulation provides sender authentication,
including the ability for actors outside the group (e.g., servers
involved in the room) to originate AppSync proposals. Encoding room
state changes in MLS proposals ensures that a client will not process
a commit that confirms a state change before processing the state
change itself.
*TODO*: A little more needs to be said here about how MLS is used.
For example: What types of credential are required / allowed? If
servers are going to be allowed to introduce room changes, how are
their keys provisioned as external signers? Need to maintain the
membership and the list of queued proposals.
4.3. Application Layer
Servers in MIMI provide a few functions that enable messaging
applications. All servers act as publication points for key material
used to add their users to rooms. The hub server for a room tracks
the state of the room, and controls how the room's state evolves,
e.g., by ensuring that changes are compliant with the room's policy.
Non-hub servers facilitate interactions between their clients and the
hub server.
In this section, we describe the state that servers keep. The
following top level section describes the HTTP endpoints exposed to
enable these functions.
4.3.1. Server State
Every MIMI server is a publication point for users' key material, via
the keyMaterial endpoint discussed in Section 5.2. To support this
endpoint, the server stores a set of KeyPackages, where each
KeyPackage belongs to a specific user and device.
Each KeyPackage includes a list of its MLS client's capabilities (MLS
protocol versions, cipher suites, extensions, proposal types, and
credential types). When claiming KeyPackages, the requester includes
the list of RequiredCapabilites to ensure the new joiner is
compatible with and capable of participating in the corresponding
room.
The hub server for the room stores the state of the room, comprising:
* The _base policy_ of the room, which does not depend on the
specific participants in the room. For example, this includes the
room roles and their permissions.
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* The _participant list_: a list of the users who are participants
of the room, and each user's role in the room.
*TODO*: We need a more full description of the room, room state
syntax.
When a client requests key material via the hub, the hub records the
KeyPackageRef values for the returned KeyPackages, and the identity
of the provider from which they were received. This information is
then used to route Welcome message to the proper provider.
4.3.2. Participant List Changes
The participant list can be changed by adding or removing users, or
changing a user's role. These changes are described without a
specific syntax as a list of adds, removes, and role changes:
Add: ["mimi://d.example/u/diana", "admin"],
["mimi://e.example/u/eric", "admin"],
Remove: ["mimi://b.example/u/bob"],
SetRole: [["mimi://c.example/u/cathy", "admin"]]
Figure 9: Changing the state of the room
To put these changes into effect, a client or server encodes them in
an AppSync proposal, signs the proposal as a PublicMessage, and
submits them to the update endpoint on the hub.
5. MIMI Endpoints and Framing
This section describes the specific endpoints necessary to provide
the functionality in the example flow. The framing for each endpoint
includes a protocol so that different variations of the end-to-end
encryption can be used.
*TODO*: Determine the what needs to be included in the protocol.
MIMI version, e2e protocol version, etc.
The syntax of the MIMI protocol messages are described using the TLS
presentation language format (Section 3 of [RFC8446]).
enum {
reserved(0),
mls10(1),
(255)
} Protocol;
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5.1. Directory
Like the ACME protocol (See Section 7.1.1 of [RFC8555]), the MIMI
protocol uses a directory document to convey the HTTPS URLs used to
reach certain endpoints (as opposed to hard coding the endpoints).
The directory URL is discovered using the mimi-protocol-directory
well-known URI. The response is a JSON document with URIs for each
type of endpoint.
GET /.well-known/mimi-protocol-directory
{
"keyMaterial":
"https://mimi.example.com/v1/keyMaterial/{targetUser}",
"update": "https://mimi.example.com/v1/update{roomId}",
"notify": "https://mimi.example.com/v1/notify/{roomId}",
"submitMessage":
"https://mimi.example.com/v1/submitMessage/{roomId}",
"groupInfo":
"https://mimi.example.com/v1/groupInfo/{roomId}"
}
5.2. Fetch Key Material
This action attempts to claim initial keying material for all the
clients of a single user at a specific provider. The keying material
is designed for use in a single room and may not be reused. It uses
the HTTP POST method.
POST /keyMaterial/{targetUser}
The target user's URI is listed in the request path. KeyPackages
requested using this primitive MUST be sent via the hub provider of
whatever room they will be used in. (If this is not the case, the
hub provider will be unable to forward a Welcome message to the
target provider).
The path includes the target user. The request body includes the
protocol (currently just MLS 1.0), and the requesting user. When the
request is being made in the context of adding the target user to a
room, the request MUST include the room ID for which the KeyPackage
is intended, as the target may have only granted consent for a
specific room.
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For MLS, the request includes a non-empty list of acceptable MLS
ciphersuites, and an MLS RequiredCapabilities object (which contains
credential types, non-default proposal types, and extensions)
required by the requesting provider (these lists can be an empty).
The request body has the following form.
struct {
opaque uri<V>;
} IdentifierUri;
struct {
Protocol protocol;
IdentifierUri requestingUser;
IdentifierUri targetUser;
IdentifierUri roomId;
select (protocol) {
case mls10:
CipherSuite acceptableCiphersuites<V>;
RequiredCapabilities requiredCapabilities;
};
} KeyMaterialRequest;
The response contains a user status code that indicates keying
material was returned for all the user's clients (success), that
keying material was returned for some of their clients
(partialSuccess), or a specific user code indicating failure. If the
user code is success or partialSuccess, each client is enumerated in
the response. Then for each client with a _client_ success code, the
structure includes initial keying material (a KeyPackage for MLS
1.0). If the client's code is nothingCompatible, the client's
capabilities are optionally included (The client's capabilities could
be omitted for privacy reasons.)
If the _user_ code is noCompatibleMaterial, the provider MAY populate
the clients list. For any other user code, the provider MUST NOT
populate the clients list.
Keying material provided from one response MUST NOT be provided in
any other response. The target provider MUST NOT provide expired
keying material (ex: an MLS KeyPackage containing a LeafNode with a
notAfter time past the current date and time).
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enum {
success(0);
partialSuccess(1);
incompatibleProtocol(2);
noCompatibleMaterial(3);
userUnknown(4);
noConsent(5);
noConsentForThisRoom(6);
userDeleted(7);
(255)
} KeyMaterialUserCode;
enum {
success(0);
keyMaterialExhausted(1),
nothingCompatible(2),
(255)
} KeyMaterialClientCode;
struct {
KeyMaterialClientCode clientStatus;
IdentifierUri clientUri;
select (protocol) {
case mls10:
select (clientStatus) {
case success:
KeyPackage keyPackage;
case nothingCompatible:
optional<Capabilities> clientCapabilities;
};
};
} ClientKeyMaterial;
struct {
Protocol protocol;
KeyMaterialUserCode userStatus;
IdentifierUri userUri;
ClientKeyMaterial clients<V>;
} KeyMaterialResponse;
The semantics of the KeyMaterialUserCode are as follows:
* success indicates that key material was provided for every client
of the target user.
* partialSuccess indicates that key material was provided for at
least one client of the target user.
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* incompatibleProtocol indicates that either one of providers
supports the protocol requested, or none of the clients of the
target user support the protocol requested.
* noCompatibleMaterial indicates that none of the clients was able
to supply key material compatible with the requiredCapabilities
field in the request.
* userUnknown indicates that the target user is not known to the
target provider.
* noConsent indicates that the requester does not have consent to
fetch key material for the target user. The target provider can
use this response as a catch all and in place of other status
codes such as userUnknown if desired to preserve the privacy of
its users.
* noConsentForThisRoom indicates that the target user might have
allowed a request for another room, but does not for this room.
If the provider does not wish to make this distinction, it can
return noConsent instead.
* userDeleted indicates that the target provider wishes the
requester to know that the target user was previously a valid user
of the system and has been deleted. A target provider can of
course use userUnknown if the provider does wish to keep or
specify this distinction.
The semantics of the KeyMaterialClientCode are as follows:
* success indicates that key material was provided for the specified
client.
* keyMaterialExhausted indicates that there was no keying material
available for the specified client.
* nothingCompatible indicates that the specified clients had no key
material compatible with the requiredCapabilities field in the
request.
At minimum, as each MLS KeyPackage is returned to a requesting
provider (on behalf of a requesting IM client), the target provider
needs to associate its KeyPackageRef with the target client and the
hub provider needs to associate its KeyPackageRef with the target
provider. This ensures that Welcome messages can be correctly routed
to the target provider and client. These associations can be deleted
after a Welcome message is forwarded or after the KeyPackage
leaf_node.lifetime.not_after time has passed.
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5.3. Update Room State
Adds, removes, and policy changes to the room are all forms of
updating the room state. They are accomplished using the update
transaction which is used to update the room base policy,
participation list, or its underlying MLS group. It uses the HTTP
POST method.
POST /update/{roomId}
Any change to the participant list or room policy (including
authorization policy) is communicated via an AppSync proposal type
with the applicationId of mimiParticipantList or mimiRoomPolicy
respectively. When adding a user, the proposal containing the
participant list change MUST be committed either before or
simultaneously with the corresponding MLS operation.
Removing an active user from a participant list or banning an active
participant likewise also happen simultaneously with any MLS changes
made to the commit removing the participant.
A hub provider which observes that an active participant has been
removed or banned from the room, MUST prevent any of its clients from
sending or receiving any additional application messages in the
corresponding MLS group; MUST prevent any of those clients from
sending Commit messages in that group; and MUST prevent it from
sending any proposals except for Remove and SelfRemove
[I-D.ietf-mls-extensions] proposals for its users in that group.
The update request body is described below:
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enum {
reserved(0),
full(1),
(255)
} RatchetTreeRepresentation;
struct {
RatchetTreeRepresentation representation;
select (representation) {
case full:
Node ratchet_tree<V>;
};
} RatchetTreeOption;
struct {
select (room.protocol) {
case mls10:
PublicMessage proposalOrCommit;
select (proposalOrCommit.content.content_type) {
case commit:
/* Both the Welcome and GroupInfo omit the ratchet_tree */
optional<Welcome> welcome;
GroupInfo groupInfo;
RatchetTreeOption ratchetTreeOption;
case proposal:
PublicMessage moreProposals<V>; /* a list of additional proposals */
};
};
} UpdateRequest;
For example, in the first use case described in the Protocol
Overview, Alice creates a Commit containing an AppSync proposal
adding Bob (mimi://b.example/b/bob), and Add proposals for all Bob's
MLS clients. Alice includes the Welcome message which will be sent
for Bob, a GroupInfo object for the hub provider, and complete
ratchet_tree extension.
The response body is described below:
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enum {
success(0),
wrongEpoch(1),
notAllowed(2),
invalidProposal(3),
(255)
} UpdateResponseCode;
struct {
UpdateResponseCode responseCode;
string errorDescription;
select (responseCode) {
case success:
/* the hub acceptance time (in milliseconds from the UNIX epoch) */
uint64 acceptedTimestamp;
case wrongEpoch:
/* current MLS epoch for the MLS group */
uint64 currentEpoch;
case invalidProposal:
ProposalRef invalidProposals<V>;
};
} UpdateRoomResponse
The semantics of the UpdatedResponseCode values are as follows:
* success indicates the UpdateRequest was accepted and will be
distributed.
* wrongEpoch indicates that the hub provider is using a different
epoch. The currentEpoch is provided in the response.
* notAllowed indicates that some type of policy or authorization
prevented the hub provider from accepting the UpdateRequest.
* invalidProposal indicates that at least one proposal is invalid.
A list of invalidProposals is provided in the response.
5.4. Submit a Message
End-to-end encrypted (application) messages are submitted to the hub
for authorization and eventual fanout using an HTTP POST request.
POST /submitMessage/{roomId}
The request body is as follows:
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struct {
Protocol protocol;
select(protocol) {
case mls10:
/* PrivateMessage containing an application message */
MLSMessage appMessage;
};
} SubmitMessageRequest;
If the protocol is MLS 1.0, the request body is an MLSMessage with a
WireFormat of PrivateMessage (an application message).
The response merely indicates if the message was accepted by the hub
provider.
enum {
accepted(0),
notAllowed(1),
epochTooOld(2),
(255)
} SubmitResponseCode;
struct {
Protocol protocol;
select(protocol) {
case mls10:
SubmitResponseCode statusCode;
select (statusCode) {
case success:
/* the hub acceptance time
(in milliseconds from the UNIX epoch) */
uint64 acceptedTimestamp;
case epochTooOld:
/* current MLS epoch for the MLS group */
uint64 currentEpoch;
};
};
} SubmitMessageResponse;
The semantics of the SubmitResponseCode values are as follows:
* success indicates the SubmitMessageRequest was accepted and will
be distributed.
* notAllowed indicates that some type of policy or authorization
prevented the hub provider from accepting the UpdateRequest. This
could include nonsensical inputs such as an MLS epoch more recent
than the hub's.
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* epochTooOld indicates that the hub provider is using a new MLS
epoch for the group. The currentEpoch is provided in the
response.
*ISSUE:* Do we want to offer a distinction between regular
application messages and ephemeral applications messages (for
example "is typing" notifications), which do not need to be queued
at the target provider.
5.5. Fanout Messages and Room Events
If the hub provider accepts an application or handshake message
(proposal or commit) message, it forwards that message to all other
providers with active participants in the room and all local clients
which are active members. This is described as fanning the message
out. One can think of fanning a message out as presenting an ordered
list of MLS-protected events to the next "hop" toward the receiving
client.
An MLS Welcome message is sent to the providers and local users
associated with the KeyPackageRef values in the secrets array of the
Welcome. In the case of a Welcome message, a RatchetTreeOption is
also included in the FanoutMessage.
The hub provider also fans out any messages which originate from
itself (ex: MLS External Proposals).
The hub can include multiple concatenated FanoutMessage objects
relevant to the same room. This endpoint uses the HTTP POST method.
POST /notify/{roomId}
struct {
/* the hub acceptance time (in milliseconds from the UNIX epoch) */
uint64 timestamp;
select (protocol) {
case mls10:
/* A PrivateMessage containing an application message,
a PublicMessage containing a proposal or commit,
or a Welcome message. */
MLSMessage message;
select (message.wire_format) {
case welcome:
RatchetTreeOption ratchetTreeOption;
};
};
} FanoutMessage;
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*NOTE:* Correctly fanning out Welcome messages relies on the hub
and target providers storing the KeyPackageRef of claimed
KeyPackages.
A client which receives a success to either an UpdateRoomResponse or
a SubmitMessageResponse can view this a commitment from the hub
provider that the message will eventually be distributed to the
group. The hub is not expected to forward the client's own message
to the client or its provider. However, the client and its provider
need to be prepared to receive the client's (effectively duplicate)
message. This situation can occur during failover in high
availability recovery scenarios.
Clients that are being removed SHOULD receive the corresponding
Commit message, so they can recognize that they have been removed and
clean up their internal state. A removed client might not receive a
commit if it was removed as a malicious or abusive client, or if it
obviously deleted.
The response to a FanoutMessage contains no body. The HTTP response
code indicates if the messages in the request were accepted (201
response code), or if there was an error. The hub need not wait for
a response before sending the next fanout message.
If the hub server does not contain an HTTP 201 response code, then it
SHOULD retry the request, respecting any guidance provided by the
server in HTTP header fields such as Retry-After. If a follower
server receives a duplicate request to the /notify endpoint, in the
sense of a request from the same hub server with the same request
body as a previous /notify request, then the follower server MUST
return a 201 Accepted response. In such cases, the follower server
SHOULD process only the first request; subsequent duplicate requests
SHOULD be ignored (despite the success response).
*NOTE:* These deduplication provisions require follower servers to
track which request bodies they have received from which hub
servers. Since the matching here is byte-exact, it can be done by
keeping a rolling list of hashes of recent messages.
This byte-exact replay criterion might not be the right
deduplication strategy. There might be situations where it is
valid for the same hub server to send the same payload multiple
times, e.g., due to accidental collisions.
If this is a concern, then an explicit transaction ID could be
introduced. The follower server would still have to keep a list
of recently seen transaction IDs, but deduplication could be done
irrespective of the content of request bodies.
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5.6. Claim group key information
When a client joins an MLS group without an existing member adding
the client to the MLS group, that is called an external join. This
is useful a) when a new client of an existing user needs to join the
groups of all the user's rooms. It can also be used b) when a client
did not have key packages available but their user is already in the
participation list for the corresponding room, c) when joining an
open room, or d) when joining using an external authentication
joining code. In MIMI, external joins are accomplished by fetching
the MLS GroupInfo for a room's MLS group, and then sending an
external commit incorporating the GroupInfo.
The GroupInfoRequest uses the HTTP POST method.
POST /groupInfo/{roomId}
The request provides an MLS credential proving the requesting
client's real or pseudonymous identity. This user identity is used
by the hub to correlate this request with the subsequent external
commit. The credential may constitute sufficient permission to
authorize providing the GroupInfo and later joining the group.
Alternatively, the request can include an optional opaque joining
code, which the requester could use to prove permission to fetch the
GroupInfo, even if it is not yet a participant.
The request also provides a signature public key corresponding to the
requester's credential. It also specifies a CipherSuite which merely
needs to be one ciphersuite in common with the hub. It is needed
only to specify the algorithms used to sign the GroupInfoRequest and
GroupInfoResponse.
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struct {
Protocol protocol;
select (protocol) {
case mls10:
CipherSuite cipher_suite;
SignaturePublicKey requestingSignatureKey;
Credential requestingCredential;
optional opaque joiningCode<V>;
};
} GroupInfoRequestTBS;
struct {
Protocol protocol;
select (protocol) {
case mls10:
CipherSuite cipher_suite;
SignaturePublicKey requestingSignatureKey;
Credential requestingCredential;
opaque joiningCode<V>;
/* SignWithLabel(., "GroupInfoRequestTBS", GroupInfoRequestTBS) */
opaque signature<V>;
};
} GroupInfoRequest;
If successful, the response body contains the GroupInfo and a way to
get the ratchet_tree.
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enum {
reserved(0),
success(1),
notAuthorized(2),
noSuchRoom(3),
(255)
} GroupInfoCode;
struct {
Protocol version;
GroupInfoCode status;
select (protocol) {
case mls10:
CipherSuite cipher_suite;
opaque room_id<V>;
ExternalSender hub_sender;
GroupInfo groupInfo; /* without embedded ratchet_tree */
RatchetTreeOption ratchetTreeOption;
};
} GroupInfoResponseTBS;
struct {
Protocol version;
GroupInfoCode status;
select (protocol) {
case mls10:
CipherSuite cipher_suite;
opaque room_id<V>;
ExternalSender hub_sender;
GroupInfo groupInfo; /* without embedded ratchet_tree */
RatchetTreeOption ratchetTreeOption;
/* SignWithLabel(., "GroupInfoResponseTBS", GroupInfoResponseTBS) */
opaque signature<V>;
};
} GroupInfoResponse;
The semantics of the GroupInfoCode are as follows:
* success indicates that GroupInfo and ratchet tree was provided as
requested.
* notAuthorized indicates that the requester was not authorized to
access the GroupInfo.
* noSuchRoom indicates that the requested room does not exist. If
the hub does not want to reveal if a room ID does not exist it can
use notAuthorized instead.
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*TODO*: Consider adding additional failure codes/semantics for
joining codes (ex: code expired, already used, invalid)
*ISSUE*: What security properties are needed to protect a GroupInfo
object in the MIMI context are still under discussion. It is
possible that the requester only needs to prove possession of their
private key. The GroupInfo in another context might be sufficiently
sensitive that it should be encrypted from the end client to the hub
provider (unreadable by the local provider).
6. Relation between MIMI state and cryptographic state
6.1. Room state
The state of a room consists of its room ID, its base policy, its
participant list (including the role and participation state of each
participant), and the associated end-to-end protocol state (its MLS
group state) that anchors the room state cryptographically.
While all parties involved in a room agree on the room's state during
a specific epoch, the Hub is the arbiter that decides if a state
change is valid, consistent with the room's then-current policy. All
state-changing events are sent to the Hub and checked for their
validity and policy conformance, before they are forwarded to any
follower servers or local clients.
As soon as the Hub accepts an event that changes the room state, its
effect is applied to the room state and future events are validated
in the context of that new state.
The room state is thus changed based on events, even if the MLS
proposal implementing the event was not yet committed by a client.
Note that this only applies to events changing the room state.
6.2. Cryptographic room representation
Each room is represented cryptographically by an MLS group. The Hub
that manages the room also manages the list of group members, i.e.
the list of clients belonging to users currently in the room.
6.3. Proposal-commit paradigm
The MLS protocol follows a proposal-commit paradigm. Any party
involved in a room (follower server, Hub or clients) can send
proposals (e.g. to add/remove/update clients of a user or to re-
initialize the group with different parameters). However, only
clients can send commits, which contain all valid previously sent
proposals and apply them to the MLS group state.
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The MIMI usage of MLS ensures that the Hub, all follower servers and
the clients of all active participants agree on the group state,
which includes the client list and the key material used for message
encryption (although the latter is only available to clients). Since
the group state also includes a copy of the room state at the time of
the most recent commit, it is also covered by the agreement.
6.4. Authenticating proposals
MLS requires that MLS proposals from the Hub and from follower
servers (external senders in MLS terminology) be authenticated using
key material contained in the external_senders extension of the MLS
group. Each MLS group associated with a MIMI room MUST therefore
contain an external_senders extension. That extension MUST contain
at least the Certificate of the Hub.
When a user from a follower server becomes a participant in the room,
the Certificate of the follower server MAY be added to the extension.
When the last participant belonging to a follower server leaves the
room, the certificate of that user MUST be removed from the list.
Changes to the external_senders extension only take effect when the
MLS proposal containing the event is committed by a MIMI commit.
7. MLS Application State Synchronization
*TODO:* This section should be moved to its own document in the MLS
working group.
One of the primary security benefits of MLS is that the MLS key
schedule confirms that the group agrees on certain metadata, such as
the membership of the group. Members that disagree on the relevant
metadata will arrive at different keys and be unable to communicate.
Applications based on MLS can integrate their state into this
metadata in order to confirm that the members of an MLS group agree
on application state as well as MLS metadata.
Here, we define two extensions to MLS to facilitate this application
design:
1. A GroupContext extension application_states that confirms
agreement on application state from potentially multiple sources.
2. A new proposal type AppSync that allows MLS group members to
propose changes to the agreed application state.
The application_states extension allows the application to inject
state objects into the MLS key schedule. Changes to this state can
be made out of band, or using the AppSync proposal. Using the
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AppSync proposal ensures that members of the MLS group have received
the relevant state changes before they are reflected in the group's
application_states.
*NOTE:* This design exposes the high-level structure of the
application state to MLS. An alternative design would be to have
the application state be opaque to MLS. There is a trade-off
between generality and the complexity of the API between the MLS
implementation and the application. An opaque design would give
the application more freedom, but require the MLS stack to call
out to the application to get the updated state as part of Commit
processing. This design allows the updates to happen within the
MLS stack, so that no callback is needed, at the cost of forcing
the application state to fit a certain structure. It also
potentially can result in smaller state updates in large groups.
The state for Each applicationId in the application_states needs to
conform to one of four basic types: an ordered array, an unordered
array, a map, or an irreducible blob. This allows the AppSync
proposal to efficiently modify a large application state object.
The content of the application_states extension and the AppSync
proposal are structured as follows:
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enum {
irreducible(0),
map(1),
unorderedList(2),
orderedArray(3),
(255)
} StateType;
struct {
opaque element<V>;
} OpaqueElement;
struct {
opaque elementName<V>;
opaque elementValue<V>;
} OpaqueMapElement;
struct {
uint32 applicationId;
StateType stateType;
select (stateType) {
case irreducible:
OpaqueElement state;
case map:
OpaqueMapElement mapEntries<V>;
case unorderedList:
OpaqueElement unorderedEntries<V>;
case orderedArray:
OpaqueElement orderedEntries<V>;
};
} ApplicationState;
struct {
ApplicationState applicationStates<V>;
} ApplicationStatesExtension;
Figure 10: The `application_state` extension
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struct {
uint32 index;
opaque element<V>;
} ElementWithIndex;
struct {
uint32 applicationId;
StateType stateType;
select (stateType) {
case irreducible:
OpaqueElement newState;
case map:
OpaqueElement removedKeys<V>;
OpaqueMapElement newOrUpdatedElements<V>;
case unorderedList:
uint32 removedIndices<V>;
OpaqueElement addedEntries<V>;
case orderedArray:
ElementWithIndex replacedElements<V>;
uint32 removedIndices<V>;
ElementWithIndex insertedElements<V>;
OpaqueElement appenededEntries<V>;
};
} AppSync;
Figure 11: The AppSync proposal type
The applicationId determines the structure and interpretation of the
contents. of an ApplicationState object. AppSync proposals contain
changes to this state, which the client uses to update the
representation of the state in application_states.
A client receiving an AppSync proposal applies it in the following
way:
* Identify an application_states GroupContext extension which
contains the same application_id state as the AppSync proposal
* Apply the relevant operations (replace, remove, update, append,
insert) according to the stateType to the relevant parts of the
ApplicationState object in application_states extension.
An AppSync for an irreducible state replaces its state element with a
new (possibly empty) newState. An AppSync for a map-based
ApplicationState first removes all the keys in removedKeys and than
replaces or adds the elements in newOrUpdatedElements. An AppSync
for an unorderedList ApplicationState first removes all the indexes
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in removedIndices, then adds the elements in addedEntries. Finally
an AppSync for an orderedArray, replaces all the elements (index-by-
index) in replacedElements, the removes the elements in
removedIndices according to the then order of the array, then inserts
all the elements in insertedElements according to the then order of
the array, then finally appends the appendedEntries (in order). All
indices are zero-based.
Note that the application_states extension is updated directly by
AppSync proposals; a GroupContextExtensions proposal is not
necessary. A proposal list that contains both an AppSync proposal
and a GroupContextExtensions proposal is invalid.
Likewise a proposal list in a Commit MAY contain more than one
AppSync proposal, but no more than one AppSync proposal per
applicationId. The proposals are applied in the order that they are
sent in the Commit.
AppSync proposals do not need to contain an UpdatePath. An AppSync
proposal can be sent by an authorized external sender.
*TODO:* IANA registry for application_id; register extension and
proposal types as safe extensions
8. Security Considerations
The MIMI protocol incorporates several layers of security.
Individual protocol actions are protected against network attackers
with mutually-authenticated TLS, where the TLS certificates
authenticate the identities that the protocol actors assert at the
application layer.
Messages and room state changes are protected end-to-end using MLS.
The protection is "end-to-end" in the sense that messages sent within
the group are confidentiality-protected against all servers involved
in the delivery of those messages, and in the sense that the
authenticity of room state changes is verified by the end clients
involved in the room. The usage of MLS ensures that the servers
facilitating the exchange cannot read messages in the room or falsify
room state changes, even though they can read the room state change
messages.
Each room has an authorization policy that dictates which protocol
actors can perform which actions in the room. This policy is
enforced by the hub server for the room. The actors for whom the
policy is being evaluated authenticate their identities to the hub
server using the MLS PublicMessage signed object format, together
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with the identity credentials presented in MLS. This design means
that the hub is trusted to correctly enforce the room's policy, but
this cost is offset by the simplicity of not having multiple policy
enforcement points.
9. IANA Considerations
10. References
10.1. Normative References
[I-D.barnes-mimi-arch]
Barnes, R., "An Architecture for More Instant Messaging
Interoperability (MIMI)", Work in Progress, Internet-
Draft, draft-barnes-mimi-arch-03, 4 March 2024,
<https://datatracker.ietf.org/doc/html/draft-barnes-mimi-
arch-03>.
[I-D.ietf-mls-extensions]
Robert, R., "The Messaging Layer Security (MLS)
Extensions", Work in Progress, Internet-Draft, draft-ietf-
mls-extensions-03, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
extensions-03>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/rfc/rfc6125>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/rfc/rfc7231>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[RFC9420] Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.
10.2. Informative References
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/rfc/rfc8555>.
Acknowledgments
*TODO*:
Authors' Addresses
Richard L. Barnes
Cisco
Email: rlb@ipv.sx
Matthew Hodgson
The Matrix.org Foundation C.I.C.
Email: matthew@matrix.org
Konrad Kohbrok
Phoenix R&D
Email: konrad.kohbrok@datashrine.de
Rohan Mahy
Unaffiliated
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Email: rohan.ietf@gmail.com
Travis Ralston
The Matrix.org Foundation C.I.C.
Email: travisr@matrix.org
Raphael Robert
Phoenix R&D
Email: ietf@raphaelrobert.com
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