rfc9750.original   rfc9750.txt 
Network Working Group B. Beurdouche Internet Engineering Task Force (IETF) B. Beurdouche
Internet-Draft Inria & Mozilla Request for Comments: 9750 Inria & Mozilla
Intended status: Informational E. Rescorla Category: Informational E. Rescorla
Expires: 4 February 2025 Windy Hill Systems, LLC ISSN: 2070-1721 Windy Hill Systems, LLC
E. Omara E. Omara
S. Inguva S. Inguva
A. Duric A. Duric
Wire Wire
3 August 2024 February 2025
The Messaging Layer Security (MLS) Architecture The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-15
Abstract Abstract
The Messaging Layer Security (MLS) protocol (I-D.ietf-mls-protocol) The Messaging Layer Security (MLS) protocol (RFC 9420) provides a
provides a Group Key Agreement protocol for messaging applications. Group Key Agreement protocol for messaging applications. MLS is
MLS is meant to protect against eavesdropping, tampering, message meant to protect against eavesdropping, tampering, and message
forgery, and provide Forward Secrecy (FS) and Post-Compromise forgery and to provide Forward Secrecy (FS) and Post-Compromise
Security (PCS). Security (PCS).
This document describes the architecture for using MLS in a general This document describes the architecture for using MLS in a general
secure group messaging infrastructure and defines the security goals secure group messaging infrastructure and defines the security goals
for MLS. It provides guidance on building a group messaging system for MLS. It provides guidance on building a group messaging system
and discusses security and privacy tradeoffs offered by multiple and discusses security and privacy trade-offs offered by multiple
security mechanisms that are part of the MLS protocol (e.g., security mechanisms that are part of the MLS protocol (e.g.,
frequency of public encryption key rotation). The document also frequency of public encryption key rotation). The document also
provides guidance for parts of the infrastructure that are not provides guidance for parts of the infrastructure that are not
standardized by MLS and are instead left to the application. standardized by MLS and are instead left to the application.
While the recommendations of this document are not mandatory to While the recommendations of this document are not mandatory to
follow in order to interoperate at the protocol level, they affect follow in order to interoperate at the protocol level, they affect
the overall security guarantees that are achieved by a messaging the overall security guarantees that are achieved by a messaging
application. This is especially true in the case of active application. This is especially true in the case of active
adversaries that are able to compromise clients, the delivery adversaries that are able to compromise clients, the delivery
service, or the authentication service. service, or the authentication service.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the MLS Working Group
mailing list (mls@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/mls/.
Source for this draft and an issue tracker can be found at
https://github.com/mlswg/mls-architecture.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
This Internet-Draft will expire on 4 February 2025. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9750.
Copyright Notice Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction
2. General Setting . . . . . . . . . . . . . . . . . . . . . . . 4 2. General Setting
2.1. Protocol Overview . . . . . . . . . . . . . . . . . . . . 4 2.1. Protocol Overview
2.2. Abstract Services . . . . . . . . . . . . . . . . . . . . 5 2.2. Abstract Services
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7 3. Overview of Operation
3.1. Step 1: Account Creation . . . . . . . . . . . . . . . . 7 3.1. Step 1: Account Creation
3.2. Step 2: Initial Keying Material . . . . . . . . . . . . . 8 3.2. Step 2: Initial Keying Material
3.3. Step 3: Adding Bob to the Group . . . . . . . . . . . . . 8 3.3. Step 3: Adding Bob to the Group
3.4. Step 4: Adding Charlie to the Group . . . . . . . . . . . 8 3.4. Step 4: Adding Charlie to the Group
3.5. Other Group Operations . . . . . . . . . . . . . . . . . 9 3.5. Other Group Operations
3.6. Proposals and Commits . . . . . . . . . . . . . . . . . . 9 3.6. Proposals and Commits
3.7. Users, Clients, and Groups . . . . . . . . . . . . . . . 10 3.7. Users, Clients, and Groups
4. Authentication Service
4. Authentication Service . . . . . . . . . . . . . . . . . . . 10 5. Delivery Service
5. Delivery Service . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Key Storage and Retrieval
5.1. Key Storage and Retrieval . . . . . . . . . . . . . . . . 12 5.2. Delivery of Messages
5.2. Delivery of Messages . . . . . . . . . . . . . . . . . . 14 5.2.1. Strongly Consistent
5.2.1. Strongly Consistent . . . . . . . . . . . . . . . . . 15 5.2.2. Eventually Consistent
5.2.2. Eventually Consistent . . . . . . . . . . . . . . . . 15 5.2.3. Welcome Messages
5.2.3. Welcome Messages . . . . . . . . . . . . . . . . . . 16 5.3. Invalid Commits
5.3. Invalid Commits . . . . . . . . . . . . . . . . . . . . . 17 6. Functional Requirements
6. Functional Requirements . . . . . . . . . . . . . . . . . . . 18 6.1. Membership Changes
6.1. Membership Changes . . . . . . . . . . . . . . . . . . . 18 6.2. Parallel Groups
6.2. Parallel Groups . . . . . . . . . . . . . . . . . . . . . 19 6.3. Asynchronous Usage
6.3. Asynchronous Usage . . . . . . . . . . . . . . . . . . . 20 6.4. Access Control
6.4. Access Control . . . . . . . . . . . . . . . . . . . . . 20 6.5. Handling Authentication Failures
6.5. Handling Authentication Failures . . . . . . . . . . . . 21 6.6. Recovery After State Loss
6.6. Recovery After State Loss . . . . . . . . . . . . . . . . 22 6.7. Support for Multiple Devices
6.7. Support for Multiple Devices . . . . . . . . . . . . . . 22 6.8. Extensibility
6.8. Extensibility . . . . . . . . . . . . . . . . . . . . . . 22 6.9. Application Data Framing and Type Advertisements
6.9. Application Data Framing and Type Advertisements . . . . 23 6.10. Federation
6.10. Federation . . . . . . . . . . . . . . . . . . . . . . . 23 6.11. Compatibility with Future Versions of MLS
6.11. Compatibility with Future Versions of MLS . . . . . . . . 23 7. Operational Requirements
7. Operational Requirements . . . . . . . . . . . . . . . . . . 24 8. Security and Privacy Considerations
8. Security and Privacy Considerations . . . . . . . . . . . . . 28 8.1. Assumptions on Transport Security Links
8.1. Assumptions on Transport Security Links . . . . . . . . . 28 8.1.1. Integrity and Authentication of Custom Metadata
8.1.1. Integrity and Authentication of Custom Metadata . . . 29 8.1.2. Metadata Protection for Unencrypted Group Operations
8.1.2. Metadata Protection for Unencrypted Group 8.1.3. DoS Protection
Operations . . . . . . . . . . . . . . . . . . . . . 29 8.1.4. Message Suppression and Error Correction
8.1.3. DoS protection . . . . . . . . . . . . . . . . . . . 30 8.2. Intended Security Guarantees
8.1.4. Message Suppression and Error Correction . . . . . . 30 8.2.1. Message Secrecy and Authentication
8.2. Intended Security Guarantees . . . . . . . . . . . . . . 31 8.2.2. Forward and Post-Compromise Security
8.2.1. Message Secrecy and Authentication . . . . . . . . . 31 8.2.3. Non-Repudiation vs. Deniability
8.2.2. Forward and Post-Compromise Security . . . . . . . . 31 8.2.4. Associating a User's Clients
8.2.3. Non-Repudiation vs Deniability . . . . . . . . . . . 33 8.3. Endpoint Compromise
8.2.4. Associating a User's Clients . . . . . . . . . . . . 33 8.3.1. Compromise of Symmetric Keying Material
8.3. Endpoint Compromise . . . . . . . . . . . . . . . . . . . 34 8.3.2. Compromise by an Active Adversary with the Ability to
8.3.1. Compromise of Symmetric Keying Material . . . . . . . 34 Sign Messages
8.3.2. Compromise by an active adversary with the ability to 8.3.3. Compromise of the Authentication with Access to a
sign messages . . . . . . . . . . . . . . . . . . . . 36 Signature Key
8.3.3. Compromise of the authentication with access to a 8.3.4. Security Considerations in the Context of a Full State
signature key . . . . . . . . . . . . . . . . . . . . 37 Compromise
8.3.4. Security consideration in the context of a full state 8.4. Service Node Compromise
compromise . . . . . . . . . . . . . . . . . . . . . 38 8.4.1. General Considerations
8.4. Service Node Compromise . . . . . . . . . . . . . . . . . 39 8.4.2. Delivery Service Compromise
8.4.1. General considerations . . . . . . . . . . . . . . . 39 8.4.3. Authentication Service Compromise
8.4.2. Delivery Service Compromise . . . . . . . . . . . . . 40 8.5. Considerations for Attacks Outside of the Threat Model
8.4.3. Authentication Service Compromise . . . . . . . . . . 42 8.6. Cryptographic Analysis of the MLS Protocol
8.5. Considerations for attacks outside of the threat model . 45 9. IANA Considerations
8.6. Cryptographic Analysis of the MLS Protocol . . . . . . . 45 10. References
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 10.1. Normative References
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 46 10.2. Informative References
10.1. Normative References . . . . . . . . . . . . . . . . . . 46 Contributors
10.2. Informative References . . . . . . . . . . . . . . . . . 46 Authors' Addresses
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction 1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH
The source for this draft is maintained in GitHub. Suggested changes
should be submitted as pull requests at https://github.com/mlswg/mls-
architecture. Instructions are on that page as well. Editorial
changes can be managed in GitHub, but any substantive change should
be discussed on the MLS mailing list.
End-to-end security is used in the vast majority of instant messaging End-to-end security is used in the vast majority of instant messaging
systems, and also deployed in systems for other purposes such as systems and is also deployed in systems for other purposes such as
calling and conferencing. In this context, "end-to-end" captures the calling and conferencing. In this context, "end-to-end" captures the
notion that users of the system enjoy some level of security -- with notion that users of the system enjoy some level of security -- with
the precise level depending on the system design -- even in the face the precise level depending on the system design -- even in the face
of malicious actions by the operator of the messaging system. of malicious actions by the operator of the messaging system.
Messaging Layer Security (MLS) specifies an architecture (this Messaging Layer Security (MLS) specifies an architecture (this
document) and a protocol [I-D.ietf-mls-protocol] for providing end- document) and a protocol [RFC9420] for providing end-to-end security
to-end security in this setting. MLS is not intended as a full in this setting. MLS is not intended as a full instant messaging
instant messaging protocol but rather is intended to be embedded in protocol but rather is intended to be embedded in concrete protocols,
concrete protocols, such as XMPP [RFC6120]. Implementations of the such as the Extensible Messaging and Presence Protocol (XMPP)
MLS protocol will interoperate at the cryptographic level, though [RFC6120]. Implementations of the MLS protocol will interoperate at
they may have incompatibilities in terms of how protected messages the cryptographic level, though they may have incompatibilities in
are delivered, contents of protected messages, and identity/ terms of how protected messages are delivered, contents of protected
authentication infrastructures. The MLS protocol has been designed messages, and identity/authentication infrastructures. The MLS
to provide the same security guarantees to all users, for all group protocol has been designed to provide the same security guarantees to
sizes, including groups of only two clients. all users, for all group sizes, including groups of only two clients.
2. General Setting 2. General Setting
2.1. Protocol Overview 2.1. Protocol Overview
MLS provides a way for _clients_ to form _groups_ within which they MLS provides a way for _clients_ to form _groups_ within which they
can communicate securely. For example, a set of users might use can communicate securely. For example, a set of users might use
clients on their phones or laptops to join a group and communicate clients on their phones or laptops to join a group and communicate
with each other. A group may be as small as two clients (e.g., for with each other. A group may be as small as two clients (e.g., for
simple person to person messaging) or as large as hundreds of simple person-to-person messaging) or as large as hundreds of
thousands. A client that is part of a group is a _member_ of that thousands. A client that is part of a group is a _member_ of that
group. As groups change membership and group or member properties, group. As groups change membership and group or member properties,
they advance from one _epoch_ to another and the cryptographic state they advance from one _epoch_ to another and the cryptographic state
of the group evolves. of the group evolves.
The group is represented as a tree, which represents the members as The group is represented as a tree, which represents the members as
the leaves of a tree. It is used to efficiently encrypt to subsets the leaves of a tree. It is used to efficiently encrypt to subsets
of the members. Each member has a state called a _LeafNode_ object of the members. Each member has a state called a _LeafNode_ object
holding the client's identity, credentials, and capabilities. holding the client's identity, credentials, and capabilities.
Various messages are used in the evolution from epoch to epoch. A Various messages are used in the evolution from epoch to epoch. A
_Proposal_ message proposes a change to be made in the next epoch, _Proposal_ message proposes a change to be made in the next epoch,
such as adding or removing a member. A _Commit_ message initiates a such as adding or removing a member. A _Commit_ message initiates a
new epoch by instructing members of the group to implement a new epoch by instructing members of the group to implement a
collection of proposals. Proposals and Commits are collectively collection of proposals. Proposals and Commits are collectively
called _Handshake messages_. A _KeyPackage_ provides keys that can be called _Handshake messages_. A _KeyPackage_ provides keys that can
used to add the client to a group, including its LeafNode, and be used to add the client to a group, including its LeafNode, and
_Signature Key_. A _Welcome_ message provides a new member to the _Signature Key_. A _Welcome_ message provides a new member to the
group with the information to initialize their state for the epoch in group with the information to initialize their state for the epoch in
which they were added. which they were added.
Of course most (but not all) applications use MLS to send encrypted Of course most (but not all) applications use MLS to send encrypted
group messages. An _application message_ is an MLS message with an group messages. An _application message_ is an MLS message with an
arbitrary application payload. arbitrary application payload.
Finally, a _PublicMessage_ contains an integrity-protected MLS Finally, a _PublicMessage_ contains an integrity-protected MLS
Handshake message, while a _PrivateMessage_ contains a confidential, Handshake message, while a _PrivateMessage_ contains a confidential,
integrity-protected Handshake or application message. integrity-protected Handshake or application message.
skipping to change at page 6, line 6 skipping to change at line 223
* A Delivery Service (DS) which can receive and distribute messages * A Delivery Service (DS) which can receive and distribute messages
between group members. In the case of group messaging, the between group members. In the case of group messaging, the
delivery service may also be responsible for acting as a delivery service may also be responsible for acting as a
"broadcaster" where the sender sends a single message which is "broadcaster" where the sender sends a single message which is
then forwarded to each recipient in the group by the DS. The DS then forwarded to each recipient in the group by the DS. The DS
is also responsible for storing and delivering initial public key is also responsible for storing and delivering initial public key
material required by MLS clients in order to proceed with the material required by MLS clients in order to proceed with the
group secret key establishment that is part of the MLS protocol. group secret key establishment that is part of the MLS protocol.
For presentation purposes, this document treats the AS and DS as For presentation purposes, this document treats the AS and DS as
conventional network services, however MLS does not require a conventional network services. However, MLS does not require a
specific implementation for the AS or DS. These services may reside specific implementation for the AS or DS. These services may reside
on the same server or different servers, they may be distributed on the same server or different servers, they may be distributed
between server and client components, and they may even involve some between server and client components, and they may even involve some
action by users. For example: action by users. For example:
* Several secure messaging services today provide a centralized DS, * Several secure messaging services today provide a centralized DS
and rely on manual comparison of clients' public keys as the AS. and rely on manual comparison of clients' public keys as the AS.
* MLS clients connected to a peer-to-peer network could instantiate * MLS clients connected to a peer-to-peer network could instantiate
a decentralized DS by transmitting MLS messages over that network. a decentralized DS by transmitting MLS messages over that network.
* In an MLS group using a Public Key Infrastructure (PKI) for * In an MLS group using a Public Key Infrastructure (PKI) for
authentication, the AS would comprise the certificate issuance and authentication, the AS would comprise the certificate issuance and
validation processes, both of which involve logic inside MLS validation processes, both of which involve logic inside MLS
clients as well as various existing PKI roles (ex: Certification clients as well as various existing PKI roles (e.g., Certification
Authorities). Authorities).
It is important to note that the Authentication Service can be It is important to note that the Authentication Service can be
completely abstract in the case of a Service Provider which allows completely abstract in the case of a Service Provider that allows MLS
MLS clients to generate, distribute, and validate credentials clients to generate, distribute, and validate credentials themselves.
themselves. As with the AS, the Delivery Service can be completely As with the AS, the Delivery Service can be completely abstract if
abstract if users are able to distribute credentials and messages users are able to distribute credentials and messages without relying
without relying on a central Delivery Service (as in a peer-to-peer on a central Delivery Service (as in a peer-to-peer system). Note,
system). Note, though, that in such scenarios, clients will need to though, that in such scenarios, clients will need to implement logic
implement logic that assures the delivery properties required of the that assures the delivery properties required of the DS (see
DS (see Section 5.2). Section 5.2).
Figure 1 shows the relationship of these concepts, with three clients
and one group, and clients 2 and 3 being part of the group and client
1 not being part of any group.
+----------------+ +--------------+ +----------------+ +--------------+
| Authentication | | Delivery | | Authentication | | Delivery |
| Service (AS) | | Service (DS) | | Service (AS) | | Service (DS) |
+----------------+ +-------+------+ +----------------+ +-------+------+
/ | \ Group / | \ Group
/ ........|........\................ / ........|........\................
/ . | \ . / . | \ .
+--------+-+ . +----+-----+ +----------+ . +--------+-+ . +----+-----+ +----------+ .
| Client 1 | . | Client 2 | | Client 3 | . | Client 1 | . | Client 2 | | Client 3 | .
+----------+ . +----------+ +----------+ . +----------+ . +----------+ +----------+ .
. Member 1 Member 2 . . Member 1 Member 2 .
. . . .
.................................. ..................................
Figure 1: A Simplified Messaging System Figure 1: A Simplified Messaging System
Figure 1 shows the relationship of these concepts, with three clients
and one group, and clients 2 and 3 being part of the group and client
1 not being part of any group.
3. Overview of Operation 3. Overview of Operation
Figure 2 shows the formation of an example group consisting of Alice, Figure 2 shows the formation of an example group consisting of Alice,
Bob, and Charlie, with Alice driving the creation of the group. Bob, and Charlie, with Alice driving the creation of the group.
Alice Bob Charlie AS DS Alice Bob Charlie AS DS
Create account ---------------------------------> | Create account ---------------------------------> |
<------------------------------------- Credential | <------------------------------------- Credential |
Create account -----------------------> | Step 1 Create account -----------------------> | Step 1
<--------------------------- Credential | <--------------------------- Credential |
Create account -------------> | Create account -------------> |
<----------------- Credential | <----------------- Credential |
Initial Keying Material -----------------------------------> | Initial Keying Material -----------------------------------> |
Initial Keying Material -------------------------> | Step 2 Initial Keying Material -------------------------> | Step 2
Initial Keying Material ---------------> | Initial Keying Material ---------------> |
Get Bob Initial Keying Material ---------------------------> | Get Bob Initial Keying Material ---------------------------> |
<------------------------------- Bob Initial Keying Material | <------------------------------- Bob Initial Keying Material |
Add Bob to Group ------------------------------------------> | Step 3 Add Bob to Group ------------------------------------------> | Step 3
Welcome (Bob)----------------------------------------------> | Welcome (Bob) ---------------------------------------------> |
<-------------------------------- Add Bob to Group | <-------------------------------- Add Bob to Group |
<----------------------------------- Welcome (Bob) | <----------------------------------- Welcome (Bob) |
Get Charlie Initial Keying Material -----------------------> | Get Charlie Initial Keying Material -----------------------> |
<--------------------------- Charlie Initial Keying Material | <--------------------------- Charlie Initial Keying Material |
Add Charlie to Group --------------------------------------> | Add Charlie to Group --------------------------------------> |
Welcome (Charlie) -----------------------------------------> | Step 4 Welcome (Charlie) -----------------------------------------> | Step 4
<---------------------------- Add Charlie to Group | <---------------------------- Add Charlie to Group |
<----------------- Add Charlie to Group | <----------------- Add Charlie to Group |
<-------------------- Welcome (Charlie) | <-------------------- Welcome (Charlie) |
Figure 2: Group Formation Example Figure 2: Group Formation Example
This process proceeds as follows. This process proceeds as follows.
3.1. Step 1: Account Creation 3.1. Step 1: Account Creation
Alice, Bob, and Charlie create accounts with a service provider and Alice, Bob, and Charlie create accounts with a service provider and
obtain credentials from the AS. This is a one-time setup phase. obtain credentials from the AS. This is a one-time setup phase.
skipping to change at page 8, line 28 skipping to change at line 336
DS to look up his initial keying material. She then generates two DS to look up his initial keying material. She then generates two
messages: messages:
* A message to the entire group (which at this point is just her and * A message to the entire group (which at this point is just her and
Bob) that adds Bob to the group. Bob) that adds Bob to the group.
* A _Welcome_ message just to Bob encrypted with his initial keying * A _Welcome_ message just to Bob encrypted with his initial keying
material that includes the secret keying information necessary to material that includes the secret keying information necessary to
join the group. join the group.
She sends both of these messages to the Delivery Services, which is She sends both of these messages to the Delivery Service, which is
responsible for sending them to the appropriate people. Note that responsible for sending them to the appropriate people. Note that
the security of MLS does not depend on the DS forwarding the Welcome the security of MLS does not depend on the DS forwarding the Welcome
message only to Bob, as it is encrypted for him; it is simply not message only to Bob, as it is encrypted for him; it is simply not
necessary for other group members to receive it. necessary for other group members to receive it.
3.4. Step 4: Adding Charlie to the Group 3.4. Step 4: Adding Charlie to the Group
If Alice then wants to add Charlie to the group, she follows a If Alice then wants to add Charlie to the group, she follows a
similar procedure as with Bob: she first uses the DS to look up his similar procedure as with Bob: She first uses the DS to look up his
initial keying material and then generates two messages: initial keying material and then generates two messages:
* A message to the entire group (consisting of her, Bob, and * A message to the entire group (consisting of her, Bob, and
Charlie) adding Charlie to the group. Charlie) adding Charlie to the group.
* A _Welcome_ message just to Charlie encrypted with his initial * A _Welcome_ message just to Charlie encrypted with his initial
keying material that includes the secret keying information keying material that includes the secret keying information
necessary to join the group. necessary to join the group.
At the completion of this process, we have a group with Alice, Bob, At the completion of this process, we have a group with Alice, Bob,
and Charlie, which means that they share a single encryption key and Charlie, which means that they share a single encryption key that
which can be used to send messages or to key other protocols. can be used to send messages or to key other protocols.
3.5. Other Group Operations 3.5. Other Group Operations
Once the group has been created, clients can perform other actions, Once the group has been created, clients can perform other actions,
such as: such as:
* sending a message to everyone in the group * sending a message to everyone in the group
* receiving a message from someone in the group * receiving a message from someone in the group
* adding one or more clients to an existing group * adding one or more clients to an existing group
* remove one or more members from an existing group * removing one or more members from an existing group
* updating their own key material * updating their own key material
* leave a group (by asking to be removed) * leaving a group (by asking to be removed)
Importantly, MLS does not itself enforce any access control on group Importantly, MLS does not itself enforce any access control on group
operations. For instance, any member of the group can send a message operations. For instance, any member of the group can send a message
to add a new member or to evict an existing member. This is in to add a new member or to evict an existing member. This is in
contrast to some designs in which there is a single group controller contrast to some designs in which there is a single group controller
who can modify the group. MLS-using applications are responsible for who can modify the group. MLS-using applications are responsible for
setting their own access control policies. For instance, if only the setting their own access control policies. For instance, if only the
group administrator is allowed to change group members, then it is group administrator is allowed to change group members, then it is
the responsibility of the application to inform members of this the responsibility of the application to inform members of this
policy and who the administrator is. policy and who the administrator is.
3.6. Proposals and Commits 3.6. Proposals and Commits
The general pattern for any change in the group state (e.g., to add The general pattern for any change in the group state (e.g., to add
or remove a user) is that it consists of two messages: or remove a user) is that it consists of two messages:
Proposal This message describes the change to be made (e.g., add Bob Proposal: This message describes the change to be made (e.g., add
to the group) but does not effect a change. Bob to the group) but does not effect a change.
Commit This message changes the group state to include the changes Commit: This message changes the group state to include the changes
described in a set of proposals. described in a set of proposals.
The simplest pattern is for a client to just send a Commit which The simplest pattern is for a client to just send a Commit which
contains one or more Proposals, for instance Alice could send a contains one or more Proposals; for instance, Alice could send a
Commit with the Proposal Add(Bob) embedded to add Bob to the group. Commit with the Proposal Add(Bob) embedded to add Bob to the group.
However, there are situations in which one client might send a However, there are situations in which one client might send a
proposal and another might send the commit. For instance, Bob might proposal and another might send the commit. For instance, Bob might
wish to remove himself from the group and send a Remove Proposal to wish to remove himself from the group and send a Remove Proposal to
do so (see Section 12.1.3 of [RFC9420]). Because Bob cannot send the do so (see Section 12.1.3 of [RFC9420]). Because Bob cannot send the
Commit, an existing member must do so. Commits can apply to multiple Commit, an existing member must do so. Commits can apply to multiple
valid Proposals, in which case all the listed changes are applied. valid Proposals, in which case all the listed changes are applied.
It is also possible for a Commit to apply to an empty set of It is also possible for a Commit to apply to an empty set of
Proposals in which case it just updates the cryptographic state of Proposals, in which case it just updates the cryptographic state of
the group without changing its membership. the group without changing its membership.
3.7. Users, Clients, and Groups 3.7. Users, Clients, and Groups
While it's natural to think of a messaging system as consisting of While it's natural to think of a messaging system as consisting of
groups of users, possibly using different devices, in MLS the basic groups of users, possibly using different devices, in MLS the basic
unit of operation is not the user but rather the "client". Formally, unit of operation is not the user but rather the "client". Formally,
a client is a set of cryptographic objects composed of public values a client is a set of cryptographic objects composed of public values
such as a name (an identity), a public encryption key, and a public such as a name (an identity), a public encryption key, and a public
signature key. As usual, a user demonstrates ownership of the client signature key. As usual, a user demonstrates ownership of the client
by demonstrating knowledge of the associated secret values. by demonstrating knowledge of the associated secret values.
In some messaging systems, clients belonging to the same user must In some messaging systems, clients belonging to the same user must
all share the same signature key pair, but MLS does not assume this; all share the same signature key pair, but MLS does not assume this;
instead a user may have multiple clients with the same identity and instead, a user may have multiple clients with the same identity and
different keys. In this case, each client will have its own different keys. In this case, each client will have its own
cryptographic state, and it is up to the application to determine how cryptographic state, and it is up to the application to determine how
to present this situation to users. For instance, it may render to present this situation to users. For instance, it may render
messages to and from a given user identically regardless of which messages to and from a given user identically regardless of which
client they are associated with, or may choose to distinguish them. client they are associated with, or it may choose to distinguish
them.
When a client is part of a Group, it is called a Member. A group in When a client is part of a Group, it is called a Member. A group in
MLS is defined as the set of clients that have knowledge of the MLS is defined as the set of clients that have knowledge of the
shared group secret established in the group key establishment phase. shared group secret established in the group key establishment phase.
Note that until a client has been added to the group and contributed Note that until a client has been added to the group and contributed
to the group secret in a manner verifiable by other members of the to the group secret in a manner verifiable by other members of the
group, other members cannot assume that the client is a member of the group, other members cannot assume that the client is a member of the
group; for instance, the newly added member might not have received group; for instance, the newly added member might not have received
the Welcome message or been unable to decrypt it for some reason. the Welcome message or might not have been able to decrypt it for
some reason.
4. Authentication Service 4. Authentication Service
The Authentication Service (AS) has to provide three services: The Authentication Service (AS) has to provide three services:
1. Issue credentials to clients that attest to bindings between 1. Issue credentials to clients that attest to bindings between
identities and signature key pairs identities and signature key pairs.
2. Enable a client to verify that a credential presented by another 2. Enable a client to verify that a credential presented by another
client is valid with respect to a reference identifier client is valid with respect to a reference identifier.
3. Enable a group member to verify that a credential represents the 3. Enable a group member to verify that a credential represents the
same client as another credential same client as another credential.
A member with a valid credential authenticates its MLS messages by A member with a valid credential authenticates its MLS messages by
signing them with the private key corresponding to the public key signing them with the private key corresponding to the public key
bound by its credential. bound by its credential.
The AS is considered an abstract layer by the MLS specification and The AS is considered an abstract layer by the MLS specification; part
part of this service could be, for instance, running on the members' of this service could be, for instance, running on the members'
devices, while another part is a separate entity entirely. The devices, while another part is a separate entity entirely. The
following examples illustrate the breadth of this concept: following examples illustrate the breadth of this concept:
* A PKI could be used as an AS [RFC5280]. The issuance function * A PKI could be used as an AS [RFC5280]. The issuance function
would be provided by the certificate authorities in the PKI, and would be provided by the certificate authorities in the PKI, and
the verification function would correspond to certificate the verification function would correspond to certificate
verification by clients. verification by clients.
* Several current messaging applications rely on users verifying * Several current messaging applications rely on users verifying
each other's key fingerprints for authentication. In this each other's key fingerprints for authentication. In this
scenario, the issuance function is simply the generation of a key scenario, the issuance function is simply the generation of a key
pair (i.e., a credential is just an identifier and public key, pair (i.e., a credential is just an identifier and public key,
with no information to assist in verification). The verification with no information to assist in verification). The verification
function is the application function that enables users to verify function is the application function that enables users to verify
keys. keys.
* In a system based on [CONIKS] end user Key Transparency (KT) [KT], * In a system based on end-user Key Transparency (KT) [CONIKS] [KT],
the issuance function would correspond to the insertion of a key the issuance function would correspond to the insertion of a key
in a KT log under a user's identity. The verification function in a KT log under a user's identity. The verification function
would correspond to verifying a key's inclusion in the log for a would correspond to verifying a key's inclusion in the log for a
claimed identity, together with the KT log's mechanisms for a user claimed identity, together with the KT log's mechanisms for a user
to monitor and control which keys are associated with their to monitor and control which keys are associated with their
identity. identity.
By the nature of its roles in MLS authentication, the AS is invested By the nature of its roles in MLS authentication, the AS is invested
with a large amount of trust and the compromise of the AS could allow with a large amount of trust and the compromise of the AS could allow
an adversary to, among other things, impersonate group members. We an adversary to, among other things, impersonate group members. We
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that all clients belonging to a given user have the same signature that all clients belonging to a given user have the same signature
key (in fact, having duplicate signature keys in a group is key (in fact, having duplicate signature keys in a group is
forbidden). A member can also rotate the signature key they use forbidden). A member can also rotate the signature key they use
within a group. These mechanisms allow clients to use different within a group. These mechanisms allow clients to use different
signature keys in different contexts and at different points in time, signature keys in different contexts and at different points in time,
providing unlinkability and post-compromise security benefits. Some providing unlinkability and post-compromise security benefits. Some
security trade-offs related to this flexibility are discussed in the security trade-offs related to this flexibility are discussed in the
security considerations. security considerations.
In many applications, there are multiple MLS clients that represent a In many applications, there are multiple MLS clients that represent a
single entity, for example a human user with a mobile and desktop single entity -- for example, a human user with a mobile and desktop
version of an application. Often the same set of clients is version of an application. Often, the same set of clients is
represented in exactly the same list of groups. In applications represented in exactly the same list of groups. In applications
where this is the intended situation, other clients can check that a where this is the intended situation, other clients can check that a
user is consistently represented by the same set of clients. This user is consistently represented by the same set of clients. This
would make it more difficult for a malicious AS to issue fake would make it more difficult for a malicious AS to issue fake
credentials for a particular user because clients would expect the credentials for a particular user because clients would expect the
credential to appear in all groups of which the user is a member. If credential to appear in all groups of which the user is a member. If
a client credential does not appear in all groups after some a client credential does not appear in all groups after some
relatively short period of time, clients have an indication that the relatively short period of time, clients have an indication that the
credential might have been created without the user's knowledge. Due credential might have been created without the user's knowledge. Due
to the asynchronous nature of MLS, however, there may be transient to the asynchronous nature of MLS, however, there may be transient
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Section 3.7, users may own multiple clients, each with their own Section 3.7, users may own multiple clients, each with their own
keying material. Each KeyPackage is specific to an MLS version and keying material. Each KeyPackage is specific to an MLS version and
ciphersuite, but a client may want to offer support for multiple ciphersuite, but a client may want to offer support for multiple
protocol versions and ciphersuites. As such, there may be multiple protocol versions and ciphersuites. As such, there may be multiple
KeyPackages stored by each user for a mix of protocol versions, KeyPackages stored by each user for a mix of protocol versions,
ciphersuites, and end-user devices. ciphersuites, and end-user devices.
When a client wishes to establish a group or add clients to a group, When a client wishes to establish a group or add clients to a group,
it first contacts the Delivery Service to request KeyPackages for it first contacts the Delivery Service to request KeyPackages for
each other client, authenticates the KeyPackages using the signature each other client, authenticates the KeyPackages using the signature
keys, includes the KeyPackages in Add Proposals, encrypts the keys, includes the KeyPackages in Add Proposals, and encrypts the
information needed to join the group (the _GroupInfo_ object) with an information needed to join the group (the _GroupInfo_ object) with an
ephemeral key, then separately encrypts the ephemeral key with the ephemeral key; it then separately encrypts the ephemeral key with the
init_key from each KeyPackage. When a client requests a KeyPackage init_key from each KeyPackage. When a client requests a KeyPackage
in order to add a user to a group, the Delivery Service should in order to add a user to a group, the Delivery Service should
provide the minimum number of KeyPackages necessary to satisfy the provide the minimum number of KeyPackages necessary to satisfy the
request. For example, if the request specifies the MLS version, the request. For example, if the request specifies the MLS version, the
DS might provide one KeyPackage per supported ciphersuite, even if it DS might provide one KeyPackage per supported ciphersuite, even if it
has multiple such KeyPackages to enable the corresponding client to has multiple such KeyPackages to enable the corresponding client to
be added to multiple groups before needing to upload more fresh be added to multiple groups before needing to upload more fresh
KeyPackages. KeyPackages.
In order to avoid replay attacks and provide forward secrecy for In order to avoid replay attacks and provide forward secrecy for
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KeyPackage that is specially designated by the client to be used KeyPackage that is specially designated by the client to be used
multiple times. Clients are responsible for providing new multiple times. Clients are responsible for providing new
KeyPackages as necessary in order to minimize the chance that the KeyPackages as necessary in order to minimize the chance that the
"last resort" KeyPackage will be used. "last resort" KeyPackage will be used.
*RECOMMENDATION:* Ensure that "last resort" KeyPackages don't get *RECOMMENDATION:* Ensure that "last resort" KeyPackages don't get
used by provisioning enough standard KeyPackages. used by provisioning enough standard KeyPackages.
*RECOMMENDATION:* Rotate "last resort" KeyPackages as soon as *RECOMMENDATION:* Rotate "last resort" KeyPackages as soon as
possible after being used or if they have been stored for a possible after being used or if they have been stored for a
prolonged period of time. Overall, avoid reusing last resort prolonged period of time. Overall, avoid reusing "last resort"
KeyPackages as much as possible. KeyPackages as much as possible.
*RECOMMENDATION:* Ensure that the client for which a last resort *RECOMMENDATION:* Ensure that the client for which a "last resort"
KeyPackage has been used is updating leaf keys as early as KeyPackage has been used is updating leaf keys as early as
possible. possible.
Overall, it needs to be noted that key packages need to be updated Overall, it needs to be noted that key packages need to be updated
when signature keys are changed. when signature keys are changed.
5.2. Delivery of Messages 5.2. Delivery of Messages
The main responsibility of the Delivery Service is to ensure delivery The main responsibility of the Delivery Service is to ensure delivery
of messages. Some MLS messages need only be delivered to specific of messages. Some MLS messages need only be delivered to specific
clients (e.g., a Welcome message initializing a new member's state), clients (e.g., a Welcome message initializing a new member's state),
while others need to be delivered to all the members of a group. The while others need to be delivered to all the members of a group. The
Delivery Service may enable the latter delivery pattern via unicast Delivery Service may enable the latter delivery pattern via unicast
channels (sometimes known as "client fanout"), broadcast channels channels (sometimes known as "client fanout"), broadcast channels
("server fanout"), or a mix of both. ("server fanout"), or a mix of both.
For the most part, MLS does not require the Delivery Service to For the most part, MLS does not require the Delivery Service to
deliver messages in any particular order. Applications can set deliver messages in any particular order. Applications can set
policies that control their tolerance for out-of-order messages (see policies that control their tolerance for out-of-order messages (see
Section 7), and messages that arrive significantly out-of-order can Section 7), and messages that arrive significantly out of order can
be dropped without otherwise affecting the protocol. There are two be dropped without otherwise affecting the protocol. There are two
exceptions to this. First, Proposal messages should all arrive exceptions to this. First, Proposal messages should all arrive
before the Commit that references them. Second, because an MLS group before the Commit that references them. Second, because an MLS group
has a linear history of epochs, the members of the group must agree has a linear history of epochs, the members of the group must agree
on the order in which changes are applied. Concretely, the group on the order in which changes are applied. Concretely, the group
must agree on a single MLS Commit message that ends each epoch and must agree on a single MLS Commit message that ends each epoch and
begins the next one. begins the next one.
In practice, there's a realistic risk of two members generating In practice, there's a realistic risk of two members generating
Commit messages at the same time, based on the same epoch, and both Commit messages at the same time, based on the same epoch, and both
attempting to send them to the group at the same time. The extent to attempting to send them to the group at the same time. The extent to
which this is a problem, and the appropriate solution, depends on the which this is a problem, and the appropriate solution, depend on the
design of the Delivery Service. Per the CAP theorem [CAPBR], there design of the Delivery Service. Per the CAP theorem [CAPBR], there
are two general classes of distributed system that the Delivery are two general classes of distributed systems that the Delivery
Service might fall into: Service might fall into:
* Consistent and Partition-tolerant, or Strongly Consistent, systems * Consistent and Partition-tolerant, or Strongly Consistent,
can provide a globally consistent view of data but has the systems, which can provide a globally consistent view of data but
inconvenience of clients needing to handle rejected messages; have the inconvenience of clients needing to handle rejected
messages.
* Available and Partition-tolerant, or Eventually Consistent, * Available and Partition-tolerant, or Eventually Consistent,
systems continue working despite network issues but may return systems, which continue working despite network issues but may
different views of data to different users. return different views of data to different users.
Strategies for sequencing messages in strongly and eventually Strategies for sequencing messages in strongly and eventually
consistent systems are described in the next two subsections. Most consistent systems are described in the next two subsections. Most
Delivery Services will use the Strongly Consistent paradigm but this Delivery Services will use the Strongly Consistent paradigm, but this
remains a choice that can be handled in coordination with the client remains a choice that can be handled in coordination with the client
and advertized in the KeyPackages. and advertised in the KeyPackages.
However, note that a malicious Delivery Service could also reorder However, note that a malicious Delivery Service could also reorder
messages or provide an inconsistent view to different users. The messages or provide an inconsistent view to different users. The
"generation" counter in MLS messages provides per-sender loss "generation" counter in MLS messages provides per-sender loss
detection and ordering that cannot be manipulated by the DS, but this detection and ordering that cannot be manipulated by the DS, but this
does not provide complete protection against partitioning. A DS can does not provide complete protection against partitioning. A DS can
cause a partition in the group by partitioning key exchange messages; cause a partition in the group by partitioning key exchange messages;
this can be detected only by out-of-band comparison (e.g., confirming this can be detected only by out-of-band comparison (e.g., confirming
that all clients have the same epoch_authenticator value). A that all clients have the same epoch_authenticator value). A
mechanism for more robust protections is discussed in mechanism for more robust protections is discussed in [EXTENSIONS].
[I-D.ietf-mls-extensions].
Other forms of Delivery Service misbehavior are still possible that Other forms of Delivery Service misbehavior are still possible that
are not easy to detect. For instance, a Delivery Service can simply are not easy to detect. For instance, a Delivery Service can simply
refuse to relay messages to and from a given client. Without some refuse to relay messages to and from a given client. Without some
sort of side information, other clients cannot generally detect this sort of side information, other clients cannot generally detect this
form of Denial of Service (DoS) attack. form of Denial-of-Service (DoS) attack.
5.2.1. Strongly Consistent 5.2.1. Strongly Consistent
With this approach, the Delivery Service ensures that some types of With this approach, the Delivery Service ensures that some types of
incoming messages have a linear order and all members agree on that incoming messages have a linear order and all members agree on that
order. The Delivery Service is trusted to break ties when two order. The Delivery Service is trusted to break ties when two
members send a Commit message at the same time. members send a Commit message at the same time.
As an example, there could be an "ordering server" Delivery Service As an example, there could be an "ordering server" Delivery Service
that broadcasts all messages received to all users and ensures that that broadcasts all messages received to all users and ensures that
all clients see messages in the same order. This would allow clients all clients see messages in the same order. This would allow clients
to only apply the first valid Commit for an epoch and ignore to only apply the first valid Commit for an epoch and ignore
subsequent ones. Clients that send a Commit would then wait to apply subsequent Commits. Clients that send a Commit would then wait to
it until it is broadcast back to them by the Delivery Service, apply it until it is broadcast back to them by the Delivery Service,
assuming they do not receive another Commit first. assuming that they do not receive another Commit first.
Alternatively, the Delivery Service can rely on the epoch and Alternatively, the Delivery Service can rely on the epoch and
content_type fields of an MLSMessage to provide an order only to content_type fields of an MLSMessage to provide an order only to
handshake messages, and possibly even filter or reject redundant handshake messages, and possibly even filter or reject redundant
Commit messages proactively to prevent them from being broadcast. Commit messages proactively to prevent them from being broadcast.
There is some risk associated with filtering, which is discussed There is some risk associated with filtering; this is discussed
further in Section 5.3. further in Section 5.3.
5.2.2. Eventually Consistent 5.2.2. Eventually Consistent
With this approach, the Delivery Service is built in a way that may With this approach, the Delivery Service is built in a way that may
be significantly more available or performant than a strongly be significantly more available or performant than a strongly
consistent system, but offers weaker consistency guarantees. consistent system, but offers weaker consistency guarantees.
Messages may arrive to different clients in different orders and with Messages may arrive to different clients in different orders and with
varying amounts of latency, which means clients are responsible for varying amounts of latency, which means clients are responsible for
reconciliation. reconciliation.
This type of Delivery Service might arise, for example, when group This type of Delivery Service might arise, for example, when group
members are sending each message to each other member individually, members are sending each message to each other member individually or
or when a distributed peer-to-peer network is used to broadcast when a distributed peer-to-peer network is used to broadcast
messages. messages.
Upon receiving a Commit from the Delivery Service, clients can Upon receiving a Commit from the Delivery Service, clients can do
either: either of the following:
1. Pause sending new messages for a short amount of time to account 1. Pause sending new messages for a short amount of time to account
for a reasonable degree of network latency and see if any other for a reasonable degree of network latency and see if any other
Commits are received for the same epoch. If multiple Commits are Commits are received for the same epoch. If multiple Commits are
received, the clients can use a deterministic tie-breaking policy received, the clients can use a deterministic tie-breaking policy
to decide which to accept, and then resume sending messages as to decide which to accept, and then resume sending messages as
normal. normal.
2. Accept the Commit immediately but keep a copy of the previous 2. Accept the Commit immediately but keep a copy of the previous
group state for a short period of time. If another Commit for a group state for a short period of time. If another Commit for a
past epoch is received, clients use a deterministic tie-breaking past epoch is received, clients use a deterministic tie-breaking
policy to decide if they should continue using the Commit they policy to decide if they should continue using the Commit they
originally accepted or revert and use the later one. Note that originally accepted or revert and use the later one. Note that
any copies of previous or forked group states must be deleted any copies of previous or forked group states must be deleted
within a reasonable amount of time to ensure the protocol within a reasonable amount of time to ensure that the protocol
provides forward-secrecy. provides forward secrecy.
If the Commit references an unknown proposal, group members may need If the Commit references an unknown proposal, group members may need
to solicit the Delivery Service or other group members individually to solicit the Delivery Service or other group members individually
for the contents of the proposal. for the contents of the proposal.
5.2.3. Welcome Messages 5.2.3. Welcome Messages
Whenever a commit adds new members to a group, MLS requires the Whenever a commit adds new members to a group, MLS requires the
committer to send a Welcome message to the new members. Applications committer to send a Welcome message to the new members. Applications
should ensure that Welcome messages are coupled with the tie-breaking should ensure that Welcome messages are coupled with the tie-breaking
logic for commits, discussed in Section 5.2.1 and Section 5.2.2. logic for commits (see Sections 5.2.1 and 5.2.2). That is, when
That is, when multiple commits are sent for the same epoch, multiple commits are sent for the same epoch, applications need to
applications need to ensure that only Welcome messages corresponding ensure that only Welcome messages corresponding to the commit that
to the commit that "succeeded" are processed by new members. "succeeded" are processed by new members.
This is particularly important when groups are being reinitialized. This is particularly important when groups are being reinitialized.
When a group is reinitialized, it is restarted with a different When a group is reinitialized, it is restarted with a different
protocol version and/or ciphersuite but identical membership. protocol version and/or ciphersuite but identical membership.
Whenever an authorized member sends and commits a ReInit proposal, Whenever an authorized member sends and commits a ReInit proposal,
this immediately freezes the existing group and triggers the creation this immediately freezes the existing group and triggers the creation
of a new group with a new group_id. of a new group with a new group_id.
Ideally, the new group would be created by the same member that Ideally, the new group would be created by the same member that
committed the ReInit proposal (including sending Welcome messages for committed the ReInit proposal (including sending Welcome messages for
the new group to all of the previous group's members). However this the new group to all of the previous group's members). However, this
operation is not always atomic, so it's possible for a member to go operation is not always atomic, so it's possible for a member to go
offline after committing a ReInit proposal but before creating the offline after committing a ReInit proposal but before creating the
new group. If this happens, it's necessary for another member to new group. If this happens, it's necessary for another member to
continue the reinitialization by creating the new group and sending continue the reinitialization by creating the new group and sending
out Welcome messages. out Welcome messages.
This has the potential to create a race condition, where multiple This has the potential to create a race condition, where multiple
members try to continue the reinitialization at the same time, and members try to continue the reinitialization at the same time, and
members receive multiple Welcome messages for each attempt at members receive multiple Welcome messages for each attempt at
reinitializing the same group. Ensuring that all members agree on reinitializing the same group. Ensuring that all members agree on
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form described in Section 16.12 of [RFC9420]. form described in Section 16.12 of [RFC9420].
In situations where the DS is attempting to filter redundant Commits, In situations where the DS is attempting to filter redundant Commits,
the DS might update its internal state under the assumption that a the DS might update its internal state under the assumption that a
Commit has succeeded and thus end up in a state inconsistent with the Commit has succeeded and thus end up in a state inconsistent with the
members of the group. For example, the DS might think that the members of the group. For example, the DS might think that the
current epoch is now n+1 and reject any commits from other epochs, current epoch is now n+1 and reject any commits from other epochs,
while the members think the epoch is n, and as a result, the group is while the members think the epoch is n, and as a result, the group is
stuck -- no member can send a Commit that the DS will accept. stuck -- no member can send a Commit that the DS will accept.
Such “desynchronization” problems can arise even when the Delivery Such "desynchronization" problems can arise even when the Delivery
Service takes no stance on which Commit is "correct" for an epoch. Service takes no stance on which Commit is "correct" for an epoch.
The DS can enable clients to choose between Commits, for example by The DS can enable clients to choose between Commits, for example by
providing Commits in the order received and allow clients to reject providing Commits in the order received and allow clients to reject
any Commits that violate their view of the group's policies. As any Commits that violate their view of the group's policies. As
such, all honest and correctly-implemented clients will arrive at the such, all honest and correctly implemented clients will arrive at the
same "first valid Commit" and choose to process it. Malicious or same "first valid Commit" and choose to process it. Malicious or
buggy clients that process a different Commit will end up in a forked buggy clients that process a different Commit will end up in a forked
view of the group. view of the group.
When these desynchronizations happen, the application may choose to When these desynchronizations happen, the application may choose to
take action to restore the functionality of the group. These actions take action to restore the functionality of the group. These actions
themselves can have security implications. For example, a client themselves can have security implications. For example, a client
developer might have a client automatically rejoin a group, using an developer might have a client automatically rejoin a group, using an
external join, when it processes an invalid Commit. In this external join, when it processes an invalid Commit. In this
operation, however, the client trusts that the GroupInfo provided by operation, however, the client trusts that the GroupInfo provided by
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and DoS implications. For example, if a recovery mechanism relies on and DoS implications. For example, if a recovery mechanism relies on
external joins, a malicious member that deliberately posts an invalid external joins, a malicious member that deliberately posts an invalid
Commit could also post a corrupted GroupInfo object in order to Commit could also post a corrupted GroupInfo object in order to
prevent victims from rejoining the group. Thus, careful analysis of prevent victims from rejoining the group. Thus, careful analysis of
security implications should be made for any system for recovering security implications should be made for any system for recovering
from desynchronization. from desynchronization.
6. Functional Requirements 6. Functional Requirements
MLS is designed as a large-scale group messaging protocol and hence MLS is designed as a large-scale group messaging protocol and hence
aims to provide both performance and security (e.g. integrity and aims to provide both performance and security (e.g., integrity and
confidentiality) to its users. Messaging systems that implement MLS confidentiality) to its users. Messaging systems that implement MLS
provide support for conversations involving two or more members, and provide support for conversations involving two or more members, and
aim to scale to groups with tens of thousands of members, typically they aim to scale to groups with tens of thousands of members,
including many users using multiple devices. typically including many users using multiple devices.
6.1. Membership Changes 6.1. Membership Changes
MLS aims to provide agreement on group membership, meaning that all MLS aims to provide agreement on group membership, meaning that all
group members have agreed on the list of current group members. group members have agreed on the list of current group members.
Some applications may wish to enforce ACLs to limit addition or Some applications may wish to enforce Access Control Lists (ACLs) to
removal of group members, to privileged clients or users. Others may limit addition or removal of group members, to privileged clients or
wish to require authorization from the current group members or a users. Others may wish to require authorization from the current
subset thereof. Such policies can be implemented at the application group members or a subset thereof. Such policies can be implemented
layer, on top of MLS. Regardless, MLS does not allow for or support at the application layer, on top of MLS. Regardless, MLS does not
addition or removal of group members without informing all other allow for or support addition or removal of group members without
members. informing all other members.
Membership of an MLS group is managed at the level of individual Membership of an MLS group is managed at the level of individual
clients. In most cases, a client corresponds to a specific device clients. In most cases, a client corresponds to a specific device
used by a user. If a user has multiple devices, the user will used by a user. If a user has multiple devices, the user will
generally be represented in a group by multiple clients (although generally be represented in a group by multiple clients (although
applications could choose to have devices share keying material). If applications could choose to have devices share keying material). If
an application wishes to implement operations at the level of users, an application wishes to implement operations at the level of users,
it is up to the application to track which clients belong to a given it is up to the application to track which clients belong to a given
user and ensure that they are added / removed consistently. user and ensure that they are added/removed consistently.
MLS provides two mechanisms for changing the membership of a group. MLS provides two mechanisms for changing the membership of a group.
The primary mechanism is for an authorized member of the group to The primary mechanism is for an authorized member of the group to
send a Commit that adds or removes other members. The second send a Commit that adds or removes other members. The second
mechanism is an "external join": A member of the group publishes mechanism is an "external join": A member of the group publishes
certain information about the group, which a new member can use to certain information about the group, which a new member can use to
construct an "external" Commit message that adds the new member to construct an "external" Commit message that adds the new member to
the group. (There is no similarly unilateral way for a member to the group. (There is no similarly unilateral way for a member to
leave the group; they must be removed by a remaining member.) leave the group; they must be removed by a remaining member.)
With both mechanisms, changes to the membership are initiated from With both mechanisms, changes to the membership are initiated from
inside the group. When members perform changes directly, this is inside the group. When members perform changes directly, this is
clearly the case. External joins are authorized indirectly, in the clearly the case. External joins are authorized indirectly, in the
sense that a member publishing a GroupInfo object authorizes anyone sense that a member publishing a GroupInfo object authorizes anyone
to join who has access to the GroupInfo object, subject to whatever to join who has access to the GroupInfo object, subject to whatever
access control policies the application applies for external joins. access control policies the application applies for external joins.
Both types of joins are done via a Commit message, which could be Both types of joins are done via a Commit message, which could be
blocked by the DS or rejected by clients if the join is not blocked by the DS or rejected by clients if the join is not
authorized. The former approach requires that Commits be visible to authorized. The former approach requires that Commits be visible to
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6.2. Parallel Groups 6.2. Parallel Groups
Any user or client may have membership in several groups Any user or client may have membership in several groups
simultaneously. The set of members of any group may or may not form simultaneously. The set of members of any group may or may not form
a subset of the members of another group. MLS guarantees that the FS a subset of the members of another group. MLS guarantees that the FS
and PCS goals within a given group are maintained and not weakened by and PCS goals within a given group are maintained and not weakened by
user membership in multiple groups. However, actions in other groups user membership in multiple groups. However, actions in other groups
likewise do not strengthen the FS and PCS guarantees within a given likewise do not strengthen the FS and PCS guarantees within a given
group, e.g., key updates within a given group following a device group, e.g., key updates within a given group following a device
compromise does not provide PCS healing in other groups; each group compromise do not provide PCS healing in other groups; each group
must be updated separately to achieve these security objectives. must be updated separately to achieve these security objectives.
This also applies to future groups that a member has yet to join, This also applies to future groups that a member has yet to join,
which are likewise unaffected by updates performed in current groups. which are likewise unaffected by updates performed in current groups.
Applications can strengthen connectivity among parallel groups by Applications can strengthen connectivity among parallel groups by
requiring periodic key updates from a user across all groups in which requiring periodic key updates from a user across all groups in which
they have membership. they have membership.
MLS provides a pre-shared key (PSK) that can be used to link healing MLS provides a pre-shared key (PSK) that can be used to link healing
properties among parallel groups. For example, suppose a common properties among parallel groups. For example, suppose a common
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conversations protected using MLS can update the group's keys, add or conversations protected using MLS can update the group's keys, add or
remove new members, and send messages without waiting for another remove new members, and send messages without waiting for another
user's reply. user's reply.
Messaging systems that implement MLS have to provide a transport Messaging systems that implement MLS have to provide a transport
layer for delivering messages asynchronously and reliably. layer for delivering messages asynchronously and reliably.
6.4. Access Control 6.4. Access Control
Because all clients within a group (members) have access to the Because all clients within a group (members) have access to the
shared cryptographic material, MLS protocol allows each member of the shared cryptographic material, the MLS protocol allows each member of
messaging group to perform operations. However, every service/ the messaging group to perform operations. However, every service/
infrastructure has control over policies applied to its own clients. infrastructure has control over policies applied to its own clients.
Applications managing MLS clients can be configured to allow for Applications managing MLS clients can be configured to allow for
specific group operations. On the one hand, an application could specific group operations. On the one hand, an application could
decide that a group administrator will be the only member to perform decide that a group administrator will be the only member to perform
add and remove operations. On the other hand, in many settings such add and remove operations. On the other hand, in many settings such
as open discussion forums, joining can be allowed for anyone. as open discussion forums, joining can be allowed for anyone.
While MLS Application messages are always encrypted, MLS handshake While MLS Application messages are always encrypted, MLS handshake
messages can be sent either encrypted (in an MLS PrivateMessage) or messages can be sent either encrypted (in an MLS PrivateMessage) or
unencrypted (in an MLS PublicMessage). Applications may be designed unencrypted (in an MLS PublicMessage). Applications may be designed
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the intermediary. Everything else being equal, using encrypted the intermediary. Everything else being equal, using encrypted
handshake messages provides stronger privacy properties than using handshake messages provides stronger privacy properties than using
unencrypted handshake messages, as it prevents intermediaries from unencrypted handshake messages, as it prevents intermediaries from
learning about the structure of the group. learning about the structure of the group.
If handshake messages are encrypted, any access control policies must If handshake messages are encrypted, any access control policies must
be applied at the client, so the application must ensure that the be applied at the client, so the application must ensure that the
access control policies are consistent across all clients to make access control policies are consistent across all clients to make
sure that they remain in sync. If two different policies were sure that they remain in sync. If two different policies were
applied, the clients might not accept or reject a group operation and applied, the clients might not accept or reject a group operation and
end-up in different cryptographic states, breaking their ability to end up in different cryptographic states, breaking their ability to
communicate. communicate.
*RECOMMENDATION:* Avoid using inconsistent access control policies *RECOMMENDATION:* Avoid using inconsistent access control policies
in the case of encrypted group operations. in the case of encrypted group operations.
MLS allows actors outside the group to influence the group in two MLS allows actors outside the group to influence the group in two
ways: External signers can submit proposals for changes to the group, ways: External signers can submit proposals for changes to the group,
and new joiners can use an external join to add themselves to the and new joiners can use an external join to add themselves to the
group. The external_senders extension ensures that all members agree group. The external_senders extension ensures that all members agree
on which signers are allowed to send proposals, but any other on which signers are allowed to send proposals, but any other
policies must be assured to be consistent as above. policies must be assured to be consistent, as noted above.
*RECOMMENDATION:* Have an explicit group policy setting the *RECOMMENDATION:* Have an explicit group policy setting the
conditions under which external joins are allowed. conditions under which external joins are allowed.
6.5. Handling Authentication Failures 6.5. Handling Authentication Failures
Within an MLS group, every member is authenticated to every other Within an MLS group, every member is authenticated to every other
member by means of credentials issued and verified by the member by means of credentials issued and verified by the
Authentication Service. MLS does not prescribe what actions, if any, Authentication Service. MLS does not prescribe what actions, if any,
an application should take in the event that a group member presents an application should take in the event that a group member presents
an invalid credential. For example, an application may require such an invalid credential. For example, an application may require such
a member to be immediately evicted, or may allow some grace period a member to be immediately evicted or may allow some grace period for
for the problem to be remediated. To avoid operational problems, it the problem to be remediated. To avoid operational problems, it is
is important for all clients in a group to have a consistent view of important for all clients in a group to have a consistent view of
which credentials in a group are valid, and how to respond to invalid which credentials in a group are valid, and how to respond to invalid
credentials. credentials.
*RECOMMENDATION:* Have a uniform credential validation process to *RECOMMENDATION:* Have a uniform credential validation process to
ensure that all group members evaluate other members' credentials ensure that all group members evaluate other members' credentials
in the same way. in the same way.
*RECOMMENDATION:* Have a uniform policy for how invalid *RECOMMENDATION:* Have a uniform policy for how invalid
credentials are handled. credentials are handled.
In some authentication systems, it is possible for a previously-valid In some authentication systems, it is possible for a previously valid
credential to become invalid over time. For example, in a system credential to become invalid over time. For example, in a system
based on X.509 certificates, credentials can expire or be revoked. based on X.509 certificates, credentials can expire or be revoked.
The MLS update mechanisms allow a client to replace an old credential The MLS update mechanisms allow a client to replace an old credential
with a new one. This is best done before the old credential becomes with a new one. This is best done before the old credential becomes
invalid. invalid.
*RECOMMENDATION:* Proactively rotate credentials, especially if a *RECOMMENDATION:* Proactively rotate credentials, especially if a
credential is about to become invalid. credential is about to become invalid.
6.6. Recovery After State Loss 6.6. Recovery After State Loss
Group members whose local MLS state is lost or corrupted can Group members whose local MLS state is lost or corrupted can
reinitialize their state by re-joining the group as a new member and reinitialize their state by rejoining the group as a new member and
removing the member representing their earlier state. An application removing the member representing their earlier state. An application
can require that a client performing such a reinitialization prove can require that a client performing such a reinitialization prove
its prior membership with a PSK that was exported from the prevoius its prior membership with a PSK that was exported from the previous
state. state.
There are a few practical challenges to this approach. For example, There are a few practical challenges to this approach. For example,
the application will need to ensure that all members have the the application will need to ensure that all members have the
required PSK, including any new members that have joined the group required PSK, including any new members that have joined the group
since the epoch in which the PSK was issued. And of course, if the since the epoch in which the PSK was issued. And of course, if the
PSK is lost or corrupted along with the member's other state, then it PSK is lost or corrupted along with the member's other state, then it
cannot be used to recover. cannot be used to recover.
Reinitializing in this way does not provide the member with access to Reinitializing in this way does not provide the member with access to
group messages from during the state loss window, but enables proof group messages exchanged during the state loss window, but it enables
of prior membership in the group. Applications may choose various proof of prior membership in the group. Applications may choose
configurations for providing lost messages to valid group members various configurations for providing lost messages to valid group
that are able to prove prior membership. members that are able to prove prior membership.
6.7. Support for Multiple Devices 6.7. Support for Multiple Devices
It is typically expected for users within a group to own various It is typically expected that users within a group will own various
devices. A new device can be added to a group and be considered as a devices. A new device can be added to a group and be considered as a
new client by the protocol. This client will not gain access to the new client by the protocol. This client will not gain access to the
history even if it is owned by someone who owns another member of the history even if it is owned by someone who owns another member of the
group. MLS does not provide direct support for restoring history in group. MLS does not provide direct support for restoring history in
this case, but applications can elect to provide such a mechanism this case, but applications can elect to provide such a mechanism
outside of MLS. Such mechanisms, if used, may reduce the FS and PCS outside of MLS. Such mechanisms, if used, may reduce the FS and PCS
guarantees provided by MLS. guarantees provided by MLS.
6.8. Extensibility 6.8. Extensibility
The MLS protocol provides several extension points where additional The MLS protocol provides several extension points where additional
information can be provided. Extensions to KeyPackages allow clients information can be provided. Extensions to KeyPackages allow clients
to disclose additional information about their capabilities. Groups to disclose additional information about their capabilities. Groups
can also have extension data associated with them, and the group can also have extension data associated with them, and the group
agreement properties of MLS will confirm that all members of the agreement properties of MLS will confirm that all members of the
group agree on the content of these extensions. group agree on the content of these extensions.
6.9. Application Data Framing and Type Advertisements 6.9. Application Data Framing and Type Advertisements
Application messages carried by MLS are opaque to the protocol; they Application messages carried by MLS are opaque to the protocol; they
can contain arbitrary data. Each application which uses MLS needs to can contain arbitrary data. Each application that uses MLS needs to
define the format of its application_data and any mechanism necessary define the format of its application_data and any mechanism necessary
to determine the format of that content over the lifetime of an MLS to determine the format of that content over the lifetime of an MLS
group. In many applications this means managing format migrations group. In many applications, this means managing format migrations
for groups with multiple members who may each be offline at for groups with multiple members who may each be offline at
unpredictable times. unpredictable times.
*RECOMMENDATION:* Use the default content mechanism defined in *RECOMMENDATION:* Use the default content mechanism defined in
Section 3.3 of [I-D.ietf-mls-extensions], unless the specific Section 3.3 of [EXTENSIONS], unless the specific application
application defines another mechanism which more appropriately defines another mechanism that more appropriately addresses the
addresses the same requirements for that application of MLS. same requirements for that application of MLS.
The MLS framing for application messages also provides a field where The MLS framing for application messages also provides a field where
clients can send information that is authenticated but not encrypted. clients can send information that is authenticated but not encrypted.
Such information can be used by servers that handle the message, but Such information can be used by servers that handle the message, but
group members are assured that it has not been tampered with. group members are assured that it has not been tampered with.
6.10. Federation 6.10. Federation
The protocol aims to be compatible with federated environments. The protocol aims to be compatible with federated environments.
While this document does not specify all necessary mechanisms While this document does not specify all necessary mechanisms
required for federation, multiple MLS implementations can required for federation, multiple MLS implementations can
interoperate to form federated systems if they use compatible interoperate to form federated systems if they use compatible
authentication mechanisms, ciphersuites, application content, and authentication mechanisms, ciphersuites, application content, and
infrastructure functionalities. Federation is described in more infrastructure functionalities. Federation is described in more
detail in [I-D.ietf-mls-federation]. detail in [FEDERATION].
6.11. Compatibility with Future Versions of MLS 6.11. Compatibility with Future Versions of MLS
It is important that multiple versions of MLS be able to coexist in It is important that multiple versions of MLS be able to coexist in
the future. Thus, MLS offers a version negotiation mechanism; this the future. Thus, MLS offers a version negotiation mechanism; this
mechanism prevents version downgrade attacks where an attacker would mechanism prevents version downgrade attacks where an attacker would
actively rewrite messages with a lower protocol version than the ones actively rewrite messages with a lower protocol version than the
originally offered by the endpoints. When multiple versions of MLS messages originally offered by the endpoints. When multiple versions
are available, the negotiation protocol guarantees that the version of MLS are available, the negotiation protocol guarantees that the
agreed upon will be the highest version supported in common by the version agreed upon will be the highest version supported in common
group. by the group.
In MLS 1.0, the creator of the group is responsible for selecting the In MLS 1.0, the creator of the group is responsible for selecting the
best ciphersuite supported across clients. Each client is able to best ciphersuite supported across clients. Each client is able to
verify availability of protocol version, ciphersuites and extensions verify availability of protocol version, ciphersuites, and extensions
at all times once he has at least received the first group operation at all times once it has at least received the first group operation
message. message.
Each member of an MLS group advertises the protocol functionality Each member of an MLS group advertises the protocol functionality
they support. These capability advertisements can be updated over they support. These capability advertisements can be updated over
time, e.g., if client software is updated while the client is a time, e.g., if client software is updated while the client is a
member of a group. Thus, in addition to preventing downgrade member of a group. Thus, in addition to preventing downgrade
attacks, the members of a group can also observe when it is safe to attacks, the members of a group can also observe when it is safe to
upgrade to a new ciphersuite or protocol version. upgrade to a new ciphersuite or protocol version.
7. Operational Requirements 7. Operational Requirements
MLS is a security layer that needs to be integrated with an MLS is a security layer that needs to be integrated with an
application. A fully-functional deployment of MLS will have to make application. A fully functional deployment of MLS will have to make
a number of decisions about how MLS is configured and operated. a number of decisions about how MLS is configured and operated.
Deployments that wish to interoperate will need to make compatible Deployments that wish to interoperate will need to make compatible
decisions. This section lists all of the dependencies of an MLS decisions. This section lists all of the dependencies of an MLS
deployment that are external to the protocol specification, but would deployment that are external to the protocol specification, but would
still need to be aligned within a given MLS deployment, or for two still need to be aligned within a given MLS deployment, or for two
deployments to potentially interoperate. deployments to potentially interoperate.
The protocol has a built-in ability to negotiate protocol versions, The protocol has a built-in ability to negotiate protocol versions,
ciphersuites, extensions, credential types, and additional proposal ciphersuites, extensions, credential types, and additional proposal
types. For two deployments to interoperate, they must have types. For two deployments to interoperate, they must have
overlapping support in each of these categories. The overlapping support in each of these categories. The
required_capabilities extension (Section 7.2 of [RFC9420]) can required_capabilities extension (Section 7.2 of [RFC9420]) can
promote interoperability with a wider set of clients by ensuring that promote interoperability with a wider set of clients by ensuring that
certain functionality continues to be supported by a group, even if certain functionality continues to be supported by a group, even if
the clients in the group aren't currently relying on it. the clients in the group aren't currently relying on it.
MLS relies on the following network services, that need to be MLS relies on the following network services, which need to be
compatible in order for two different deployments based on them to compatible in order for two different deployments based on them to
interoperate. interoperate.
* An *Authentication Service*, described fully in Section 4, defines * An *Authentication Service*, described fully in Section 4, defines
the types of credentials which may be used in a deployment and the types of credentials that may be used in a deployment and
provides methods for: provides methods for:
1. Issuing new credentials with a relevant credential lifetime, 1. Issuing new credentials with a relevant credential lifetime,
2. Validating a credential against a reference identifier, 2. Validating a credential against a reference identifier,
3. Validating whether or not two credentials represent the same 3. Validating whether or not two credentials represent the same
client, and client, and
4. Optionally revoking credentials which are no longer 4. Optionally revoking credentials that are no longer authorized.
authorized.
* A *Delivery Service*, described fully in Section 5, provides * A *Delivery Service*, described fully in Section 5, provides
methods for: methods for:
1. Delivering messages for a group to all members in the group. 1. Delivering messages for a group to all members in the group.
2. Delivering Welcome messages to new members of a group. 2. Delivering Welcome messages to new members of a group.
3. Uploading new KeyPackages for a user's own clients. 3. Uploading new KeyPackages for a user's own clients.
4. Downloading KeyPackages for specific clients. Typically, 4. Downloading KeyPackages for specific clients. Typically,
KeyPackages are used once and consumed. KeyPackages are used once and consumed.
* Additional services may or may not be required depending on the * Additional services may or may not be required, depending on the
application design: application design:
- In cases where group operations are not encrypted, the DS has - In cases where group operations are not encrypted, the DS has
the ability to observe and maintain a copy of the public group the ability to observe and maintain a copy of the public group
state. In particular, this is useful for clients that do not state. In particular, this is useful for (1) clients that do
have the ability to send the full public state in a Welcome not have the ability to send the full public state in a Welcome
message when inviting a user, or for a client that needs to message when inviting a user or (2) a client that needs to
recover from losing their state. Such public state can contain recover from losing their state. Such public state can contain
privacy sensitive information such as group members' privacy-sensitive information such as group members'
credentials and related public keys, hence services need to credentials and related public keys; hence, services need to
carefully evaluate the privacy impact of storing this data on carefully evaluate the privacy impact of storing this data on
the DS. the DS.
- If external joiners are allowed, there must be a method to - If external joiners are allowed, there must be a method for
publish a serialized GroupInfo object (with an external_pub publishing a serialized GroupInfo object (with an external_pub
extension) that corresponds to a specific group and epoch, and extension) that corresponds to a specific group and epoch, and
keep that object in sync with the state of the group. for keeping that object in sync with the state of the group.
- If an application chooses not to allow external joining, it may - If an application chooses not to allow external joining, it may
instead provide a method for external users to solicit group instead provide a method for external users to solicit group
members (or a designated service) to add them to a group. members (or a designated service) to add them to a group.
- If the application uses PSKs that members of a group may not - If the application uses PSKs that members of a group may not
have access to (e.g., to control entry into the group or to have access to (e.g., to control entry into the group or to
prove membership in the group in the past, as in Section 6.6) prove membership in the group in the past, as discussed in
there must be a method for distributing these PSKs to group Section 6.6), there must be a method for distributing these
members who might not have them, for instance if they joined PSKs to group members who might not have them -- for instance,
the group after the PSK was generated. if they joined the group after the PSK was generated.
- If an application wishes to detect and possibly discipline - If an application wishes to detect and possibly discipline
members that send malformed commits with the intention of members that send malformed commits with the intention of
corrupting a group's state, there must be a method for corrupting a group's state, there must be a method for
reporting and validating malformed commits. reporting and validating malformed commits.
MLS requires the following parameters to be defined, which must be MLS requires the following parameters to be defined, which must be
the same for two implementations to interoperate: the same for two implementations to interoperate:
* The maximum total lifetime that is acceptable for a KeyPackage. * The maximum total lifetime that is acceptable for a KeyPackage.
* How long to store the resumption PSK for past epochs of a group. * How long to store the resumption PSK for past epochs of a group.
* The degree of tolerance that's allowed for out-of-order message * The degree of tolerance that's allowed for out-of-order message
delivery: delivery:
- How long to keep unused nonce and key pairs for a sender - How long to keep unused nonce and key pairs for a sender.
- A maximum number of unused key pairs to keep. - A maximum number of unused key pairs to keep.
- A maximum number of steps that clients will move a secret tree - A maximum number of steps that clients will move a secret tree
ratchet forward in response to a single message before ratchet forward in response to a single message before
rejecting it. rejecting it.
- Whether to buffer messages that aren't able to be understood - Whether to buffer messages that aren't yet able to be
yet due to other messages not arriving first, and if so, how understood due to other messages not arriving first and, if so,
many and for how long. For example, Commit messages that how many and for how long -- for example, Commit messages that
arrive before a proposal they reference, or application arrive before a proposal they reference or application messages
messages that arrive before the Commit starting an epoch. that arrive before the Commit starting an epoch.
If implementations differ in these parameters, they will interoperate If implementations differ in these parameters, they will interoperate
to some extent but may experience unexpected failures in certain to some extent but may experience unexpected failures in certain
situations, such as extensive message reordering. situations, such as extensive message reordering.
MLS provides the following locations where an application may store MLS provides the following locations where an application may store
arbitrary data. The format and intention of any data in these arbitrary data. The format and intention of any data in these
locations must align for two deployments to interoperate: locations must align for two deployments to interoperate:
* Application data, sent as the payload of an encrypted message. * Application data, sent as the payload of an encrypted message.
* Additional authenticated data, sent unencrypted in an otherwise * Additional authenticated data, sent unencrypted in an otherwise
encrypted message. encrypted message.
* Group IDs, as decided by group creators and used to uniquely * Group IDs, as decided by group creators and used to uniquely
identify a group. identify a group.
* Application-level identifiers of public key material (specifically * Application-level identifiers of public key material
the application_id extension as defined in Section 5.3.3 of (specifically, the application_id extension as defined in
[RFC9420]). Section 5.3.3 of [RFC9420]).
MLS requires the following policies to be defined, which restrict the MLS requires the following policies to be defined, which restrict the
set of acceptable behavior in a group. These policies must be set of acceptable behaviors in a group. These policies must be
consistent between deployments for them to interoperate: consistent between deployments for them to interoperate:
* A policy on which ciphersuites are acceptable. * A policy on which ciphersuites are acceptable.
* A policy on any mandatory or forbidden MLS extensions. * A policy on any mandatory or forbidden MLS extensions.
* A policy on when to send proposals and commits in plaintext * A policy on when to send proposals and commits in plaintext
instead of encrypted. instead of encrypted.
* A policy for which proposals are valid to have in a commit, * A policy for which proposals are valid to have in a commit,
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group. group.
* A policy of how to protect and share the GroupInfo objects needed * A policy of how to protect and share the GroupInfo objects needed
for external joins. for external joins.
* A policy for when two credentials represent the same client. Note * A policy for when two credentials represent the same client. Note
that many credentials may be issued attesting the same identity that many credentials may be issued attesting the same identity
but for different signature keys, because each credential but for different signature keys, because each credential
corresponds to a different client owned by the same application corresponds to a different client owned by the same application
user. However, one device may control multiple signature keys -- user. However, one device may control multiple signature keys --
for instance if they have keys corresponding to multiple for instance, if the device has keys corresponding to multiple
overlapping time periods -- but should still only be considered a overlapping time periods -- but should still only be considered a
single client. single client.
* A policy on how long to allow a member to stay in a group without * A policy on how long to allow a member to stay in a group without
updating its leaf keys before removing them. updating its leaf keys before removing them.
Finally, there are some additional application-defined behaviors that Finally, there are some additional application-defined behaviors that
are partially an individual application's decision but may overlap are partially an individual application's decision but may overlap
with interoperability: with interoperability:
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MLS adopts the Internet threat model [RFC3552] and therefore assumes MLS adopts the Internet threat model [RFC3552] and therefore assumes
that the attacker has complete control of the network. It is that the attacker has complete control of the network. It is
intended to provide the security services described in Section 8.2 in intended to provide the security services described in Section 8.2 in
the face of attackers who can: the face of attackers who can:
* Monitor the entire network. * Monitor the entire network.
* Read unprotected messages. * Read unprotected messages.
* Can generate, inject and delete any message in the unprotected * Generate, inject, and delete any message in the unprotected
transport layer. transport layer.
While MLS should be run over a secure transport such as QUIC While MLS should be run over a secure transport such as QUIC
[RFC9000] or TLS [RFC8446], the security guarantees of MLS do not [RFC9000] or TLS [RFC8446], the security guarantees of MLS do not
depend on the transport. This departs from the usual design practice depend on the transport. This departs from the usual design practice
of trusting the transport because MLS is designed to provide security of trusting the transport because MLS is designed to provide security
even in the face of compromised network elements, especially the DS. even in the face of compromised network elements, especially the DS.
Generally, MLS is designed under the assumption that the transport Generally, MLS is designed under the assumption that the transport
layer is present to keep metadata private from network observers, layer is present to keep metadata private from network observers,
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In addition, these guarantees are intended to degrade gracefully in In addition, these guarantees are intended to degrade gracefully in
the presence of compromise of the transport security links as well as the presence of compromise of the transport security links as well as
of both clients and elements of the messaging system, as described in of both clients and elements of the messaging system, as described in
the remainder of this section. the remainder of this section.
8.1. Assumptions on Transport Security Links 8.1. Assumptions on Transport Security Links
As discussed above, MLS provides the highest level of security when As discussed above, MLS provides the highest level of security when
its messages are delivered over an encrypted transport. The main use its messages are delivered over an encrypted transport. The main use
of the secure transport layer for MLS is to protect the already of the secure transport layer for MLS is to protect the already-
limited amount of metadata. Very little information is contained in limited amount of metadata. Very little information is contained in
the unencrypted header of the MLS protocol message format for group the unencrypted header of the MLS protocol message format for group
operation messages, and application messages are always encrypted in operation messages, and application messages are always encrypted in
MLS. MLS.
*RECOMMENDATION:* Use transports that provide reliability and *RECOMMENDATION:* Use transports that provide reliability and
metadata confidentiality whenever possible, e.g., by transmitting metadata confidentiality whenever possible, e.g., by transmitting
MLS messages over a protocol such as TLS [RFC8446] or QUIC MLS messages over a protocol such as TLS [RFC8446] or QUIC
[RFC9000]. [RFC9000].
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Having no secure channel to exchange MLS messages can have a serious Having no secure channel to exchange MLS messages can have a serious
impact on privacy when transmitting unencrypted group operation impact on privacy when transmitting unencrypted group operation
messages. Observing the contents and signatures of the group messages. Observing the contents and signatures of the group
operation messages may lead an adversary to extract information about operation messages may lead an adversary to extract information about
the group membership. the group membership.
*RECOMMENDATION:* Never use the unencrypted mode for group *RECOMMENDATION:* Never use the unencrypted mode for group
operations without using a secure channel for the transport layer. operations without using a secure channel for the transport layer.
8.1.3. DoS protection 8.1.3. DoS Protection
In general we do not consider Denial of Service (DoS) resistance to In general, we do not consider DoS resistance to be the
be the responsibility of the protocol. However, it should not be responsibility of the protocol. However, it should not be possible
possible for anyone aside from the Delivery Service to perform a for anyone aside from the Delivery Service to perform a trivial DoS
trivial DoS attack from which it is hard to recover. This can be attack from which it is hard to recover. This can be achieved
achieved through the secure transport layer. through the secure transport layer.
In the centralized setting, DoS protection can typically be performed In the centralized setting, DoS protection can typically be performed
by using tickets or cookies which identify users to a service for a by using tickets or cookies which identify users to a service for a
certain number of connections. Such a system helps in preventing certain number of connections. Such a system helps in preventing
anonymous clients from sending arbitrary numbers of group operation anonymous clients from sending arbitrary numbers of group operation
messages to the Delivery Service or the MLS clients. messages to the Delivery Service or the MLS clients.
*RECOMMENDATION:* Use credentials uncorrellated with specific *RECOMMENDATION:* Use credentials uncorrelated with specific users
users to help prevent DoS attacks, in a privacy preserving manner. to help prevent DoS attacks, in a manner that preserves privacy.
Note that the privacy of these mechanisms has to be adjusted in Note that the privacy of these mechanisms has to be adjusted in
accordance with the privacy expected from secure transport links. accordance with the privacy expected from secure transport links.
(See more discussion in the next section.) (See more discussion in the next section.)
8.1.4. Message Suppression and Error Correction 8.1.4. Message Suppression and Error Correction
As noted above, MLS is designed to provide some robustness in the As noted above, MLS is designed to provide some robustness in the
face of tampering within the secure transport, i.e., tampering by the face of tampering within the secure transport, i.e., tampering by the
Delivery Service. The confidentiality and authenticity properties of Delivery Service. The confidentiality and authenticity properties of
MLS prevent the DS from reading or writing messages. MLS also MLS prevent the DS from reading or writing messages. MLS also
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Each encrypted MLS message carries a "generation" number which is a Each encrypted MLS message carries a "generation" number which is a
per-sender incrementing counter. If a group member observes a gap in per-sender incrementing counter. If a group member observes a gap in
the generation sequence for a sender, then they know that they have the generation sequence for a sender, then they know that they have
missed a message from that sender. MLS also provides a facility for missed a message from that sender. MLS also provides a facility for
group members to send authenticated acknowledgments of application group members to send authenticated acknowledgments of application
messages received within a group. messages received within a group.
As discussed in Section 5, the Delivery Service is trusted to select As discussed in Section 5, the Delivery Service is trusted to select
the single Commit message that is applied in each epoch from among the single Commit message that is applied in each epoch from among
the ones sent by group members. Since only one Commit per epoch is the Commits sent by group members. Since only one Commit per epoch
meaningful, it's not useful for the DS to transmit multiple Commits is meaningful, it's not useful for the DS to transmit multiple
to clients. The risk remains that the DS will use the ability Commits to clients. The risk remains that the DS will use the
maliciously. ability maliciously.
While it is difficult or impossible to prevent a network adversary While it is difficult or impossible to prevent a network adversary
from suppressing payloads in transit, in certain infrastructures such from suppressing payloads in transit, in certain infrastructures such
as banks or governments settings, unidirectional transports can be as banks or government settings, unidirectional transports can be
used and be enforced via electronic or physical devices such as used and be enforced via electronic or physical devices such as
diodes. This can lead to payload corruption which does not affect diodes. This can lead to payload corruption, which does not affect
the security or privacy properties of the MLS protocol but does the security or privacy properties of the MLS protocol but does
affect the reliability of the service. In that case specific affect the reliability of the service. In that case, specific
measures can be taken to ensure the appropriate level of redundancy measures can be taken to ensure the appropriate level of redundancy
and quality of service for MLS. and quality of service for MLS.
8.2. Intended Security Guarantees 8.2. Intended Security Guarantees
MLS aims to provide a number of security guarantees, covering MLS aims to provide a number of security guarantees, covering
authentication, as well as confidentiality guarantees to different authentication, as well as confidentiality guarantees to different
degrees in different scenarios. degrees in different scenarios.
8.2.1. Message Secrecy and Authentication 8.2.1. Message Secrecy and Authentication
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MLS provides additional protection regarding secrecy of past messages MLS provides additional protection regarding secrecy of past messages
and future messages. These cryptographic security properties are and future messages. These cryptographic security properties are
Forward Secrecy (FS) and Post-Compromise Security (PCS). Forward Secrecy (FS) and Post-Compromise Security (PCS).
FS means that access to all encrypted traffic history combined with FS means that access to all encrypted traffic history combined with
access to all current keying material on clients will not defeat the access to all current keying material on clients will not defeat the
secrecy properties of messages older than the oldest key of the secrecy properties of messages older than the oldest key of the
compromised client. Note that this means that clients have to delete compromised client. Note that this means that clients have to delete
the appropriate keys as soon as they have been used with the expected the appropriate keys as soon as they have been used with the expected
message, otherwise the secrecy of the messages and the security for message; otherwise, the secrecy of the messages and the security of
MLS is considerably weakened. MLS are considerably weakened.
PCS means that if a group member's state is compromised at some time PCS means that if a group member's state is compromised at some time
t1 but the group member subsequently performs an update at some time t1 but the group member subsequently performs an update at some time
t2, then all MLS guarantees apply to messages sent by the member t2, then all MLS guarantees apply to messages sent by the member
after time t2, and by other members after they have processed the after time t2 and sent by other members after they have processed the
update. For example, if an attacker learns all secrets known to update. For example, if an attacker learns all secrets known to
Alice at time t1, including both Alice's long-term secret keys and Alice at time t1, including both Alice's long-term secret keys and
all shared group keys, but Alice performs a key update at time t2, all shared group keys, but Alice performs a key update at time t2,
then the attacker is unable to violate any of the MLS security then the attacker is unable to violate any of the MLS security
properties after the updates have been processed. properties after the updates have been processed.
Both of these properties are satisfied even against compromised DSs Both of these properties are satisfied even against compromised DSs
and ASs in the case where some other mechanism for verifying keys is and ASes in the case where some other mechanism for verifying keys is
in use, such as Key Transparency [KT]. in use, such as Key Transparency [KT].
Confidentiality is mainly ensured on the client side. Because Confidentiality is mainly ensured on the client side. Because FS and
Forward Secrecy (FS) and Post-Compromise Security (PCS) rely on the PCS rely on the active deletion and replacement of keying material,
active deletion and replacement of keying material, any client which any client that is persistently offline may still be holding old
is persistently offline may still be holding old keying material and keying material and thus be a threat to both FS and PCS if it is
thus be a threat to both FS and PCS if it is later compromised. later compromised.
MLS partially defends against this problem by active members MLS partially defends against this problem by active members
including freshness, however not much can be done on the inactive including freshness. However, not much can be done on the inactive
side especially in the case where the client has not processed side especially in the case where the client has not processed
messages. messages.
*RECOMMENDATION:* Mandate key updates from clients that are not *RECOMMENDATION:* Mandate key updates from clients that are not
otherwise sending messages and evict clients which are idle for otherwise sending messages and evict clients that are idle for too
too long. long.
These recommendations will reduce the ability of idle compromised These recommendations will reduce the ability of idle compromised
clients to decrypt a potentially long set of messages that might have clients to decrypt a potentially long set of messages that might have
followed the point of the compromise. been sent after the point of compromise.
The precise details of such mechanisms are a matter of local policy The precise details of such mechanisms are a matter of local policy
and beyond the scope of this document. and beyond the scope of this document.
8.2.3. Non-Repudiation vs Deniability 8.2.3. Non-Repudiation vs. Deniability
MLS provides strong authentication within a group, such that a group MLS provides strong authentication within a group, such that a group
member cannot send a message that appears to be from another group member cannot send a message that appears to be from another group
member. Additionally, some services require that a recipient be able member. Additionally, some services require that a recipient be able
to prove to the service provider that a message was sent by a given to prove to the service provider that a message was sent by a given
client, in order to report abuse. MLS supports both of these use client, in order to report abuse. MLS supports both of these use
cases. In some deployments, these services are provided by cases. In some deployments, these services are provided by
mechanisms which allow the receiver to prove a message's origin to a mechanisms that allow the receiver to prove a message's origin to a
third party. This is often called "non-repudiation". third party. This is often called "non-repudiation".
Roughly speaking, "deniability" is the opposite of "non-repudiation", Roughly speaking, "deniability" is the opposite of "non-repudiation",
i.e., the property that it is impossible to prove to a third party i.e., the property that it is impossible to prove to a third party
that a message was sent by a given sender. MLS does not make any that a message was sent by a given sender. MLS does not make any
claims with regard to deniability. It may be possible to operate MLS claims with regard to deniability. It may be possible to operate MLS
in ways that provide certain deniability properties, but defining the in ways that provide certain deniability properties, but defining the
specific requirements and resulting notions of deniability requires specific requirements and resulting notions of deniability requires
further analysis. further analysis.
8.2.4. Associating a User's Clients 8.2.4. Associating a User's Clients
When a user has multiple devices, the base MLS protocol only When a user has multiple devices, the base MLS protocol only
describes how to operate each device as a distinct client in the MLS describes how to operate each device as a distinct client in the MLS
groups that the user is a member of. As a result, the other members groups that the user is a member of. As a result, the other members
of the group will be able to identify which of a user's devices sent of the group will be able to identify which of a user's devices sent
each message, and therefore which device the user was using at the each message and, therefore, which device the user was using at the
time. Group members would also be able to detect when the user adds time. Group members would also be able to detect when the user adds
or removes authorized devices from their account. For some or removes authorized devices from their account. For some
applications, this may be an unacceptable breach of the user's applications, this may be an unacceptable breach of the user's
privacy. privacy.
This risk only arises when the leaf nodes for the clients in question This risk only arises when the leaf nodes for the clients in question
provide data that can be used to correlate the clients. So one way provide data that can be used to correlate the clients. So, one way
to mitigate this risk is by only doing client-level authentication to mitigate this risk is by only doing client-level authentication
within MLS. If user-level authentication is still desirable, the within MLS. If user-level authentication is still desirable, the
application would have to provide it through some other mechanism. application would have to provide it through some other mechanism.
It is also possible to maintain user-level authentication while It is also possible to maintain user-level authentication while
hiding information about the clients that a user owns. This can be hiding information about the clients that a user owns. This can be
done by having the clients share cryptographic state, so that they done by having the clients share cryptographic state, so that they
appear as a single client within the MLS group. Appearing as a appear as a single client within the MLS group. Appearing as a
single client has the privacy benefits of no longer leaking which single client has the privacy benefits of no longer leaking which
device was used to send a particular message, and no longer leaking device was used to send a particular message and no longer leaking
the user's authorized devices. However, the application would need the user's authorized devices. However, the application would need
to provide a synchronization mechanism so that the clients' state to provide a synchronization mechanism so that the clients' state
remain consistent across changes to the MLS group. Flaws in this remain consistent across changes to the MLS group. Flaws in this
synchronization mechanism may impair the ability of the user to synchronization mechanism may impair the ability of the user to
recover from a compromise of one of their devices. In particular, recover from a compromise of one of their devices. In particular,
state synchronization may make it easier for an attacker to use one state synchronization may make it easier for an attacker to use one
compromised device to establish exclusive control of a user's compromised device to establish exclusive control of a user's
account, locking them out entirely and preventing them from account, locking them out entirely and preventing them from
recovering. recovering.
8.3. Endpoint Compromise 8.3. Endpoint Compromise
The MLS protocol adopts a threat model which includes multiple forms The MLS protocol adopts a threat model that includes multiple forms
of endpoint/client compromise. While adversaries are in a strong of endpoint/client compromise. While adversaries are in a strong
position if they have compromised an MLS client, there are still position if they have compromised an MLS client, there are still
situations where security guarantees can be recovered thanks to the situations where security guarantees can be recovered thanks to the
PCS properties achieved by the MLS protocol. PCS properties achieved by the MLS protocol.
In this section we will explore the consequences and recommendations In this section, we will explore the consequences and recommendations
regarding the following compromise scenarios: regarding the following compromise scenarios:
* The attacker has access to a symmetric encryption key * The attacker has access to a symmetric encryption key.
* The attacker has access to a application ratchet secret * The attacker has access to an application ratchet secret.
* The attacker has access to the group secrets for one group * The attacker has access to the group secrets for one group.
* The attacker has access to a signature oracle for any group * The attacker has access to a signature oracle for any group.
* The attacker has access to the signature key for one group * The attacker has access to the signature key for one group.
* The attacker has access to all secrets of a user for all groups * The attacker has access to all secrets of a user for all groups
(full state compromise) (full state compromise).
8.3.1. Compromise of Symmetric Keying Material 8.3.1. Compromise of Symmetric Keying Material
As described above, each MLS epoch creates a new Group Secret. As described above, each MLS epoch creates a new Group Secret.
These group secrets are then used to create a per-sender Ratchet These group secrets are then used to create a per-sender Ratchet
Secret, which in turn is used to create a per-sender with additional Secret, which in turn is used to create a per-sender with an
data (AEAD) [RFC5116] key that is then used to encrypt MLS Plaintext additional data (Authenticated Encryption with Associated Data
(AEAD)) key [RFC5116] that is then used to encrypt MLS Plaintext
messages. Each time a message is sent, the Ratchet Secret is used to messages. Each time a message is sent, the Ratchet Secret is used to
create a new Ratchet Secret and a new corresponding AEAD key. create a new Ratchet Secret and a new corresponding AEAD key.
Because of the properties of the key derivation function, it is not Because of the properties of the key derivation function, it is not
possible to compute a Ratchet Secret from its corresponding AEAD key possible to compute a Ratchet Secret from its corresponding AEAD key
or compute Ratchet Secret n-1 from Ratchet Secret n. or compute Ratchet Secret n-1 from Ratchet Secret n.
Below, we consider the compromise of each of these pieces of keying Below, we consider the compromise of each of these pieces of keying
material in turn, in ascending order of severity. While this is a material in turn, in ascending order of severity. While this is a
limited kind of compromise, it can be realistic in cases of limited kind of compromise, it can be realistic in cases of
implementation vulnerabilities where only part of the memory leaks to implementation vulnerabilities where only part of the memory leaks to
the adversary. the adversary.
8.3.1.1. Compromise of AEAD Keys 8.3.1.1. Compromise of AEAD Keys
In some circumstances, adversaries may have access to specific AEAD In some circumstances, adversaries may have access to specific AEAD
keys and nonces which protect an Application or a Group Operation keys and nonces that protect an Application message or a Group
message. Compromise of these keys allows the attacker to decrypt the Operation message. Compromise of these keys allows the attacker to
specific message encrypted with that key but no other; because the decrypt the specific message encrypted with that key but no other;
AEAD keys are derived from the Ratchet Secret, it cannot generate the because the AEAD keys are derived from the Ratchet Secret, it cannot
next Ratchet Secret and hence not the next AEAD key. generate the next Ratchet Secret and hence not the next AEAD key.
In the case of an Application message, an AEAD key compromise means In the case of an Application message, an AEAD key compromise means
that the encrypted application message will be leaked as well as the that the encrypted application message will be leaked as well as the
signature over that message. This means that the compromise has both signature over that message. This means that the compromise has both
confidentiality and privacy implications on the future AEAD confidentiality and privacy implications on the future AEAD
encryptions of that chain. In the case of a Group Operation message, encryptions of that chain. In the case of a Group Operation message,
only the privacy is affected, as the signature is revealed, because only the privacy is affected, as the signature is revealed, because
the secrets themselves are protected by HPKE encryption. Note that the secrets themselves are protected by Hybrid Public Key Encryption
under that compromise scenario, authentication is not affected in (HPKE). Note that under that compromise scenario, authentication is
either of these cases. As every member of the group can compute the not affected in either of these cases. As every member of the group
AEAD keys for all the chains (they have access to the Group Secrets) can compute the AEAD keys for all the chains (they have access to the
in order to send and receive messages, the authentication provided by Group Secrets) in order to send and receive messages, the
the AEAD encryption layer of the common framing mechanism is weak. authentication provided by the AEAD encryption layer of the common
Successful decryption of an AEAD encrypted message only guarantees framing mechanism is weak. Successful decryption of an AEAD
that some member of the group sent the message. encrypted message only guarantees that some member of the group sent
the message.
Compromise of the AEAD keys allows the attacker to send an encrypted Compromise of the AEAD keys allows the attacker to send an encrypted
message using that key, but cannot send a message to a group which message using that key, but the attacker cannot send a message to a
appears to be from any valid client since they cannot forge the group that appears to be from any valid client because the attacker
signature. This applies to all the forms of symmetric key compromise cannot forge the signature. This applies to all the forms of
described in Section 8.3.1. symmetric key compromise described in Section 8.3.1.
8.3.1.2. Compromise of Ratchet Secret material 8.3.1.2. Compromise of Ratchet Secret Material
When a Ratchet Secret is compromised, the adversary can compute both When a Ratchet Secret is compromised, the adversary can compute both
the current AEAD keys for a given sender as well as any future keys the current AEAD keys for a given sender as well as any future keys
for that sender in this epoch. Thus, it can decrypt current and for that sender in this epoch. Thus, it can decrypt current and
future messages by the corresponding sender. However, because it future messages by the corresponding sender. However, because it
does not have previous Ratchet Secrets, it cannot decrypt past does not have previous Ratchet Secrets, it cannot decrypt past
messages as long as those secrets and keys have been deleted. messages as long as those secrets and keys have been deleted.
Because of its Forward Secrecy guarantees, MLS will also retain Because of its Forward Secrecy guarantees, MLS will also retain
secrecy of all other AEAD keys generated for _other_ MLS clients, secrecy of all other AEAD keys generated for _other_ MLS clients,
outside this dedicated chain of AEAD keys and nonces, even within the outside this dedicated chain of AEAD keys and nonces, even within the
epoch of the compromise. MLS provides Post-Compromise Security epoch of the compromise. MLS provides Post-Compromise Security
against an active adaptive attacker across epochs for AEAD against an active adaptive attacker across epochs for AEAD
encryption, which means that as soon as the epoch is changed, if the encryption, which means that as soon as the epoch is changed, if the
attacker does not have access to more secret material they won't be attacker does not have access to more secret material they won't be
able to access any protected messages from future epochs. able to access any protected messages from future epochs.
8.3.1.3. Compromise of the Group Secrets of a single group for one or 8.3.1.3. Compromise of the Group Secrets of a Single Group for One or
more group epochs More Group Epochs
An adversary who gains access to a set of Group secrets--as when a An adversary who gains access to a set of Group secrets -- for
member of the group is compromised--is significantly more powerful. example, when a member of the group is compromised -- is
In this section, we consider the case where the signature keys are significantly more powerful. In this section, we consider the case
not compromised, which can occur if the attacker has access to part where the signature keys are not compromised, which can occur if the
of the memory containing the group secrets but not to the signature attacker has access to part of the memory containing the group
keys which might be stored in a secure enclave. secrets but not to the signature keys which might be stored in a
secure enclave.
In this scenario, the adversary gains the ability to compute any In this scenario, the adversary gains the ability to compute any
number of Ratchet Secrets for the epoch and their corresponding AEAD number of Ratchet Secrets for the epoch and their corresponding AEAD
encryption keys and thus can encrypt and decrypt all messages for the encryption keys and thus can encrypt and decrypt all messages for the
compromised epochs. compromised epochs.
If the adversary is passive, it is expected from the PCS properties If the adversary is passive, it is expected from the PCS properties
of the MLS protocol that, as soon as the compromised party remediates of the MLS protocol that as soon as the compromised party remediates
the compromise and sends an honest Commit message, the next epochs the compromise and sends an honest Commit message, the next epochs
will provide message secrecy. will provide message secrecy.
If the adversary is active, the adversary can engage in the protocol If the adversary is active, the adversary can engage in the protocol
itself and perform updates on behalf of the compromised party with no itself and perform updates on behalf of the compromised party with no
ability for an honest group to recover message secrecy. However, MLS ability for an honest group to recover message secrecy. However, MLS
provides PCS against active adaptive attackers through its Remove provides PCS against active adaptive attackers through its Remove
group operation. This means that, as long as other members of the group operation. This means that as long as other members of the
group are honest, the protocol will guarantee message secrecy for all group are honest, the protocol will guarantee message secrecy for all
messages exchanged in the epochs after the compromised party has been messages exchanged in the epochs after the compromised party has been
removed. removed.
8.3.2. Compromise by an active adversary with the ability to sign 8.3.2. Compromise by an Active Adversary with the Ability to Sign
messages Messages
If an active adversary has compromised an MLS client and can sign If an active adversary has compromised an MLS client and can sign
messages, two different settings emerge. In the strongest compromise messages, two different scenarios emerge. In the strongest
scenario, the attacker has access to the signing key and can forge compromise scenario, the attacker has access to the signing key and
authenticated messages. In a weaker, yet realistic scenario, the can forge authenticated messages. In a weaker but realistic
attacker has compromised a client but the client signature keys are scenario, the attacker has compromised a client but the client
protected with dedicated hardware features which do not allow direct signature keys are protected with dedicated hardware features that do
access to the value of the private key and instead provide a not allow direct access to the value of the private key and instead
signature API. provide a signature API.
When considering an active adaptive attacker with access to a When considering an active adaptive attacker with access to a
signature oracle, the compromise scenario implies a significant signature oracle, the compromise scenario implies a significant
impact on both the secrecy and authentication guarantees of the impact on both the secrecy and authentication guarantees of the
protocol, especially if the attacker also has access to the group protocol, especially if the attacker also has access to the group
secrets. In that case both secrecy and authentication are broken. secrets. In that case, both secrecy and authentication are broken.
The attacker can generate any message, for the current and future The attacker can generate any message, for the current and future
epochs, until the compromise is remediated and the formerly epochs, until the compromise is remediated and the formerly
compromised client sends an honest update. compromised client sends an honest update.
Note that under this compromise scenario, the attacker can perform Note that under this compromise scenario, the attacker can perform
all operations which are available to a legitimate client even all operations that are available to a legitimate client even without
without access to the actual value of the signature key. access to the actual value of the signature key.
8.3.3. Compromise of the authentication with access to a signature key 8.3.3. Compromise of the Authentication with Access to a Signature Key
The difference between having access to the value of the signature The difference between having access to the value of the signature
key and only having access to a signing oracle is not about the key and only having access to a signing oracle is not about the
ability of an active adaptive network attacker to perform different ability of an active adaptive network attacker to perform different
operations during the time of the compromise, the attacker can operations during the time of the compromise; the attacker can
perform every operation available to a legitimate client in both perform every operation available to a legitimate client in both
cases. cases.
There is a significant difference, however in terms of recovery after There is a significant difference, however, in terms of recovery
a compromise. after a compromise.
Because of the PCS guarantees provided by the MLS protocol, when a Because of the PCS guarantees provided by the MLS protocol, when a
previously compromised client recovers from compromise and performs previously compromised client recovers from compromise and performs
an honest Commit, both secrecy and authentication of future messages an honest Commit, both secrecy and authentication of future messages
can be recovered as long as the attacker doesn't otherwise get access can be recovered as long as the attacker doesn't otherwise get access
to the key. Because the adversary doesn't have the signing key, they to the key. Because the adversary doesn't have the signing key, they
cannot authenticate messages on behalf of the compromised party, even cannot authenticate messages on behalf of the compromised party, even
if they still have control over some group keys by colluding with if they still have control over some group keys by colluding with
other members of the group. other members of the group.
This is in contrast with the case where the signature key is leaked. This is in contrast with the case where the signature key is leaked.
In that case the compromised endpoint needs to refresh its In that case, the compromised endpoint needs to refresh its
credentials and invalidate the old credentials before the attacker credentials and invalidate the old credentials before the attacker
will be unable to authenticate messages. will be unable to authenticate messages.
Beware that in both oracle and private key access, an active adaptive Beware that in both oracle and private key access, an active adaptive
attacker can follow the protocol and request to update its own attacker can follow the protocol and request to update its own
credential. This in turn induces a signature key rotation which credential. This in turn induces a signature key rotation which
could provide the attacker with part or the full value of the private could provide the attacker with part or the full value of the private
key depending on the architecture of the service provider. key, depending on the architecture of the service provider.
*RECOMMENDATION:* Signature private keys should be *RECOMMENDATION:* Signature private keys should be
compartmentalized from other secrets and preferably protected by compartmentalized from other secrets and preferably protected by a
an HSM or dedicated hardware features to allow recovery of the Hardware Security Module (HSM) or dedicated hardware features to
authentication for future messages after a compromise. allow recovery of the authentication for future messages after a
compromise.
*RECOMMENDATION:* When the credential type supports revocation, *RECOMMENDATION:* When the credential type supports revocation,
the users of a group should check for revoked keys. the users of a group should check for revoked keys.
8.3.4. Security consideration in the context of a full state compromise 8.3.4. Security Considerations in the Context of a Full State
Compromise
In real-world compromise scenarios, it is often the case that In real-world compromise scenarios, it is often the case that
adversaries target specific devices to obtain parts of the memory or adversaries target specific devices to obtain parts of the memory or
even the ability to execute arbitrary code in the targeted device. even the ability to execute arbitrary code in the targeted device.
Also, recall that in this setting, the application will often retain Also, recall that in this setting, the application will often retain
the unencrypted messages. If so, the adversary does not have to the unencrypted messages. If so, the adversary does not have to
break encryption at all to access sent and received messages. break encryption at all to access sent and received messages.
Messages may also be sent by using the application to instruct the Messages may also be sent by using the application to instruct the
protocol implementation. protocol implementation.
*RECOMMENDATION:* If messages are stored on the device, they *RECOMMENDATION:* If messages are stored on the device, they
should be protected using encryption at rest, and the keys used should be protected using encryption at rest, and the keys used
should be stored securely using dedicated mechanisms on the should be stored securely using dedicated mechanisms on the
device. device.
*RECOMMENDATION:* If the threat model of the system is against an *RECOMMENDATION:* If the threat model of the system is against an
adversary which can access the messages on the device without even adversary that can access the messages on the device without even
needing to attack MLS, the application should delete plaintext and needing to attack MLS, the application should delete plaintext and
ciphertext messages as soon as practical after encryption or ciphertext messages as soon as practical after encryption or
decryption. decryption.
Note that this document makes a clear distinction between the way Note that this document makes a clear distinction between the way
signature keys and other group shared secrets must be handled. In signature keys and other group shared secrets must be handled. In
particular, a large set of group secrets cannot necessarily be particular, a large set of group secrets cannot necessarily be
assumed to be protected by an HSM or secure enclave features. This assumed to be protected by an HSM or secure enclave features. This
is especially true because these keys are frequently used and changed is especially true because these keys are frequently used and changed
with each message received by a client. with each message received by a client.
However, the signature private keys are mostly used by clients to However, the signature private keys are mostly used by clients to
send a message. They also provide strong authentication guarantees send a message. They also provide strong authentication guarantees
to other clients, hence we consider that their protection by to other clients; hence, we consider that their protection by
additional security mechanisms should be a priority. additional security mechanisms should be a priority.
Overall there is no way to detect or prevent these compromises, as Overall, there is no way to detect or prevent these compromises, as
discussed in the previous sections, performing separation of the discussed in the previous sections: Performing separation of the
application secret states can help recovery after compromise, this is application secret states can help recovery after compromise; this is
the case for signature keys but similar concern exists for client's the case for signature keys, but similar concerns exist for a
encryption private keys. client's encryption private keys.
*RECOMMENDATION:* The secret keys used for public key encryption *RECOMMENDATION:* The secret keys used for public key encryption
should be stored similarly to the way the signature keys are should be stored similarly to the way the signature keys are
stored, as keys can be used to decrypt the group operation stored, as keys can be used to decrypt the group operation
messages and contain the secret material used to compute all the messages and contain the secret material used to compute all the
group secrets. group secrets.
Even if secure enclaves are not perfectly secure, or even completely Even if secure enclaves are not perfectly secure or are even
broken, adopting additional protections for these keys can ease completely broken, adopting additional protections for these keys can
recovery of the secrecy and authentication guarantees after a ease recovery of the secrecy and authentication guarantees after a
compromise where, for instance, an attacker can sign messages without compromise where, for instance, an attacker can sign messages without
having access to the key. In certain contexts, the rotation of having access to the key. In certain contexts, the rotation of
credentials might only be triggered by the AS through ACLs, hence be credentials might only be triggered by the AS through ACLs and hence
outside of the capabilities of the attacker. be beyond the capabilities of the attacker.
8.4. Service Node Compromise 8.4. Service Node Compromise
8.4.1. General considerations 8.4.1. General Considerations
8.4.1.1. Privacy of the network connections 8.4.1.1. Privacy of the Network Connections
There are many scenarios leading to communication between the There are many scenarios leading to communication between the
application on a device and the Delivery Service or the application on a device and the Delivery Service or the
Authentication Service. In particular when: Authentication Service -- in particular, when:
* The application connects to the Authentication Service to generate * The application connects to the Authentication Service to generate
or validate a new credential before distributing it. or validate a new credential before distributing it.
* The application fetches credentials at the Delivery Service prior * The application fetches credentials at the Delivery Service prior
to creating a messaging group (one-to-one or more than two to creating a messaging group (one-to-one or more than two
clients). clients).
* The application fetches service provider information or messages * The application fetches service provider information or messages
on the Delivery Service. on the Delivery Service.
* The application sends service provider information or messages to * The application sends service provider information or messages to
the Delivery Service. the Delivery Service.
In all these cases, the application will often connect to the device In all these cases, the application will often connect to the device
via a secure transport which leaks information about the origin of via a secure transport that leaks information about the origin of the
the request such as the IP address and depending on the protocol the request, such as the IP address and -- depending on the protocol --
MAC address of the device. the Media Access Control (MAC) address of the device.
Similar concerns exist in the peer-to-peer use cases of MLS. Similar concerns exist in the peer-to-peer use cases for MLS.
*RECOMMENDATION:* In the case where privacy or anonymity is *RECOMMENDATION:* In the case where privacy or anonymity is
important, using adequate protection such as MASQUE important, using adequate protection such as Multiplexed
[I-D.schinazi-masque-proxy], ToR, or a VPN can improve metadata Application Substrate over QUIC Encryption (MASQUE)
protection. [MASQUE-PROXY], Top-of-Rack (ToR) switches, or a VPN can improve
metadata protection.
More generally, using anonymous credentials in an MLS based More generally, using anonymous credentials in an MLS-based
architecture might not be enough to provide strong privacy or architecture might not be enough to provide strong privacy or
anonymity properties. anonymity properties.
8.4.1.2. Storage of Metadata and Ecryption at rest on the Servers 8.4.1.2. Storage of Metadata and Encryption at Rest on the Servers
In the case where private data or metadata has to be persisted on the In the case where private data or metadata has to be persisted on the
servers for functionality (mappings between identities and push servers for functionality (mappings between identities and push
tokens, group metadata...), it should be stored encrypted at rest and tokens, group metadata, etc.), it should be stored encrypted at rest
only decrypted upon need during the execution. Honest Service and only decrypted upon need during the execution. Honest Service
Providers can rely on such encryption at rest mechanism to be able to Providers can rely on such an "encryption at rest" mechanism to be
prevent access to the data when not using it. able to prevent access to the data when not using it.
*RECOMMENDATION:* Store cryptographic material used for server- *RECOMMENDATION:* Store cryptographic material used for server-
side decryption of sensitive meta-data on the clients and only side decryption of sensitive metadata on the clients and only send
send it when needed. The server can use the secret to open and it when needed. The server can use the secret to open and update
update encrypted data containers after which they can delete these encrypted data containers after which they can delete these keys
keys until the next time they need it, in which case those can be until the next time they need it, in which case those can be
provided by the client. provided by the client.
*RECOMMENDATION:* Rely on group secrets exported from the MLS *RECOMMENDATION:* Rely on group secrets exported from the MLS
session for server-side encryption at rest and update the key session for server-side encryption at rest and update the key
after each removal from the group. Rotate those keys on a regular after each removal from the group. Otherwise, rotate those keys
basis otherwise. on a regular basis.
8.4.2. Delivery Service Compromise 8.4.2. Delivery Service Compromise
MLS is intended to provide strong guarantees in the face of MLS is intended to provide strong guarantees in the face of
compromise of the DS. Even a totally compromised DS should not be compromise of the DS. Even a totally compromised DS should not be
able to read messages or inject messages that will be acceptable to able to read messages or inject messages that will be acceptable to
legitimate clients. It should also not be able to undetectably legitimate clients. It should also not be able to undetectably
remove, reorder or replay messages. remove, reorder, or replay messages.
However, a malicious DS can mount a variety of DoS attacks on the However, a malicious DS can mount a variety of DoS attacks on the
system, including total DoS attacks (where it simply refuses to system, including total DoS attacks (where it simply refuses to
forward any messages) and partial DoS attacks (where it refuses to forward any messages) and partial DoS attacks (where it refuses to
forward messages to and from specific clients). As noted in forward messages to and from specific clients). As noted in
Section 5.2, these attacks are only partially detectable by clients Section 5.2, these attacks are only partially detectable by clients
without an out-of-band channel. Ultimately, failure of the DS to without an out-of-band channel. Ultimately, failure of the DS to
provide reasonable service must be dealt with as a customer service provide reasonable service must be dealt with as a customer service
matter, not via technology. matter, not via technology.
Because the DS is responsible for providing the initial keying Because the DS is responsible for providing the initial keying
material to clients, it can provide stale keys. This does not material to clients, it can provide stale keys. This does not
inherently lead to compromise of the message stream, but does allow inherently lead to compromise of the message stream, but it does
it to attack forward security to a limited extent. This threat can allow it to attack forward security to a limited extent. This threat
be mitigated by having initial keys expire. can be mitigated by having initial keys expire.
Initial keying material (KeyPackages) using the basic Credential type Initial keying material (KeyPackages) using the basic Credential type
is more vulnerable to replacement by a malicious or compromised DS, is more vulnerable to replacement by a malicious or compromised DS,
as there is no built-in cryptographic binding between the identity as there is no built-in cryptographic binding between the identity
and the public key of the client. and the public key of the client.
*RECOMMENDATION:* Prefer a Credential type in KeyPackages which *RECOMMENDATION:* Prefer a Credential type in KeyPackages that
includes a strong cryptographic binding between the identity and includes a strong cryptographic binding between the identity and
its key (for example the x509 Credential type). When using the its key (for example, the x509 Credential type). When using the
basic Credential type take extra care to verify the identity basic Credential type, take extra care to verify the identity
(typically out-of-band). (typically out of band).
8.4.2.1. Privacy of delivery and push notifications 8.4.2.1. Privacy of Delivery and Push Notifications
An important mechanism that is often ignored from the privacy Push-tokens provide an important mechanism that is often ignored from
considerations are the push-tokens. In many modern messaging the standpoint of privacy considerations. In many modern messaging
architectures, applications are using push notification mechanisms architectures, applications are using push notification mechanisms
typically provided by OS vendors. This is to make sure that when typically provided by OS vendors. This is to make sure that when
messages are available at the Delivery Service (or by other messages are available at the Delivery Service (or via other
mechanisms if the DS is not a central server), the recipient mechanisms if the DS is not a central server), the recipient
application on a device knows about it. Sometimes the push application on a device knows about it. Sometimes the push
notification can contain the application message itself which saves a notification can contain the application message itself; this saves a
round trip with the DS. round trip with the DS.
To "push" this information to the device, the service provider and To "push" this information to the device, the service provider and
the OS infrastructures use unique per-device, per-application the OS infrastructures use unique per-device, per-application
identifiers called push-tokens. This means that the push identifiers called push-tokens. This means that the push
notification provider and the service provider have information on notification provider and the service provider have information on
which devices receive information and at which point in time. which devices receive information and at which point in time.
Alternatively, non-mobile applications could use a websocket or Alternatively, non-mobile applications could use a WebSocket or
persistent connection for notifications directly from the DS. persistent connection for notifications directly from the DS.
Even though they can't necessarily access the content, which is Even though they can't necessarily access the content, which is
typically encrypted MLS messages, the service provider and the push typically encrypted MLS messages, the service provider and the push
notification provider have to be trusted to avoid making correlation notification provider have to be trusted to avoid making correlation
on which devices are recipients of the same message. on which devices are recipients of the same message.
For secure messaging systems, push notifications are often sent real- For secure messaging systems, push notifications are often sent in
time as it is not acceptable to create artificial delays for message real time, as it is not acceptable to create artificial delays for
retrieval. message retrieval.
*RECOMMENDATION:* If real time notifications are not necessary, *RECOMMENDATION:* If real-time notifications are not necessary,
one can delay notifications randomly across recipient devices one can delay notifications randomly across recipient devices
using a mixnet or other techniques. using a mixnet or other techniques.
Note that with a legal request to ask the service provider for the Note that with a legal request to ask the service provider for the
push-token associated with an identifier, it is easy to correlate the push-token associated with an identifier, it is easy to correlate the
token with a second request to the company operating the push- token with a second request to the company operating the push-
notification system to get information about the device, which is notification system to get information about the device, which is
often linked with a real identity via a cloud account, a credit card often linked with a real identity via a cloud account, a credit card,
or other information. or other information.
*RECOMMENDATION:* If stronger privacy guarantees are needed with *RECOMMENDATION:* If stronger privacy guarantees are needed with
regard to the push notification provider, the client can choose to regard to the push notification provider, the client can choose to
periodically connect to the Delivery Service without the need of a periodically connect to the Delivery Service without the need of a
dedicated push notification infrastructure. dedicated push notification infrastructure.
Applications can also consider anonymous systems for server fanout Applications can also consider anonymous systems for server fanout
(for example [Loopix]). (for example, [Loopix]).
8.4.3. Authentication Service Compromise 8.4.3. Authentication Service Compromise
The Authentication Service design is left to the infrastructure The Authentication Service design is left to the infrastructure
designers. In most designs, a compromised AS is a serious matter, as designers. In most designs, a compromised AS is a serious matter, as
the AS can serve incorrect or attacker-provided identities to the AS can serve incorrect or attacker-provided identities to
clients. clients.
* The attacker can link an identity to a credential * The attacker can link an identity to a credential.
* The attacker can generate new credentials * The attacker can generate new credentials.
* The attacker can sign new credentials * The attacker can sign new credentials.
* The attacker can publish or distribute credentials * The attacker can publish or distribute credentials.
An attacker that can generate or sign new credentials may or may not An attacker that can generate or sign new credentials may or may not
have access to the underlying cryptographic material necessary to have access to the underlying cryptographic material necessary to
perform such operations. In that last case, it results in windows of perform such operations. In that last case, it results in windows of
time for which all emitted credentials might be compromised. time for which all emitted credentials might be compromised.
*RECOMMENDATION:* Use HSMs to store the root signature keys to *RECOMMENDATION:* Use HSMs to store the root signature keys to
limit the ability of an adversary with no physical access to limit the ability of an adversary with no physical access to
extract the top-level signature private key. extract the top-level signature private key.
Note that historically some systems generate signature keys on the Note that historically some systems generate signature keys on the
Authentication Service and distribute the private keys to clients Authentication Service and distribute the private keys to clients
along with their credential. This is a dangerous practice because it along with their credential. This is a dangerous practice because it
allows the AS or an attacker who has compromised the AS to silently allows the AS or an attacker who has compromised the AS to silently
impersonate the client. impersonate the client.
8.4.3.1. Authentication compromise: Ghost users and impersonations 8.4.3.1. Authentication Compromise: Ghost Users and Impersonations
One important property of MLS is that all Members know which other One important property of MLS is that all Members know which other
members are in the group at all times. If all Members of the group members are in the group at all times. If all Members of the group
and the Authentication Service are honest, no parties other than the and the Authentication Service are honest, no parties other than the
members of the current group can read and write messages protected by members of the current group can read and write messages protected by
the protocol for that Group. the protocol for that Group.
This guarantee applies to the cryptographic identities of the This guarantee applies to the cryptographic identities of the
members. Details about how to verify the identity of a client depend members. Details about how to verify the identity of a client depend
on the MLS Credential type used. For example, cryptographic on the MLS Credential type used. For example, cryptographic
verification of credentials can be largely performed autonomously verification of credentials can be largely performed autonomously
(e.g., without user interaction) by the clients themselves for the (e.g., without user interaction) by the clients themselves for the
x509 Credential type. x509 Credential type.
In contrast, when MLS clients use the basic Credential type, then In contrast, when MLS clients use the basic Credential type, some
some other mechanism must be used to verify identities. For instance other mechanism must be used to verify identities. For instance, the
the Authentication Service could operate some sort of directory Authentication Service could operate some sort of directory server to
server to provide keys, or users could verify keys via an out-of-band provide keys, or users could verify keys via an out-of-band
mechanism. mechanism.
*RECOMMENDATION:* Select the MLS Credential type with the *RECOMMENDATION:* Select the MLS Credential type with the
strongest security which is supported by all target members of an strongest security that is supported by all target members of an
MLS group. MLS group.
*RECOMMENDATION:* Do not use the same signature keypair across *RECOMMENDATION:* Do not use the same signature keypair across
groups. Update all keys for all groups on a regular basis. Do groups. Update all keys for all groups on a regular basis. Do
not preserve keys in different groups when suspecting a not preserve keys in different groups when suspecting a
compromise. compromise.
If the AS is compromised, it could validate a (or generate a new) If the AS is compromised, it could validate a signature keypair (or
signature keypair for an attacker. The attacker could then use this generate a new one) for an attacker. The attacker could then use
keypair to join a group as if it were another of the user's clients. this keypair to join a group as if it were another of the user's
Because a user can have many MLS clients running the MLS protocol, it clients. Because a user can have many MLS clients running the MLS
possibly has many signature keypairs for multiple devices. These protocol, it possibly has many signature keypairs for multiple
attacks could be very difficult to detect, especially in large groups devices. These attacks could be very difficult to detect, especially
where the UI might not reflect all the changes back to the users. If in large groups where the UI might not reflect all the changes back
the application participates in a key transparency mechanism in which to the users. If the application participates in a key transparency
it is possible to determine every key for a given user, then this mechanism in which it is possible to determine every key for a given
would allow for detection of surreptitiously created false user, then this would allow for detection of surreptitiously created
credentials. false credentials.
*RECOMMENDATION:* Make sure that MLS clients reflect all the *RECOMMENDATION:* Make sure that MLS clients reflect all the
membership changes to the users as they happen. If a choice has membership changes to the users as they happen. If a choice has
to be made because the number of notifications is too high, the to be made because the number of notifications is too high, the
client should provide a log of state of the device so that the client should provide a log of state of the device so that the
user can examine it. user can examine it.
*RECOMMENDATION:* Provide a key transparency mechanism for the *RECOMMENDATION:* Provide a key transparency mechanism for the
Authentication Services to allow public verification of the Authentication Service to allow public verification of the
credentials authenticated by this service. credentials authenticated by this service.
While the ways to handle MLS credentials are not defined by the While the ways to handle MLS credentials are not defined by the
protocol or the architecture documents, the MLS protocol has been protocol or the architecture documents, the MLS protocol has been
designed with a mechanism that can be used to provide out-of-band designed with a mechanism that can be used to provide out-of-band
authentication to users. The "authentication_secret" generated for authentication to users. The "authentication_secret" generated for
each user at each epoch of the group is a one-time, per client, each user at each epoch of the group is a one-time, per-client
authentication secret which can be exchanged between users to prove authentication secret that can be exchanged between users to prove
their identity to each other. This can be done for instance using a their identities to each other. This can be done, for instance,
QR code that can be scanned by the other parties. using a QR code that can be scanned by the other parties.
*RECOMMENDATION:* Provide one or more out-of-band authentication *RECOMMENDATION:* Provide one or more out-of-band authentication
mechanisms to limit the impact of an Authentication Service mechanisms to limit the impact of an Authentication Service
compromise. compromise.
We note, again, that the Authentication Service may not be a We note, again, that the Authentication Service may not be a
centralized system, and could be realized by many mechanisms such as centralized system and could be realized by many mechanisms such as
establishing prior one-to-one deniable channels, gossiping, or using establishing prior one-to-one deniable channels, gossiping, or using
trust on first use (TOFU) for credentials used by the MLS Protocol. trust on first use (TOFU) for credentials used by the MLS protocol.
Another important consideration is the ease of redistributing new Another important consideration is the ease of redistributing new
keys on client compromise, which helps recovering security faster in keys on client compromise, which helps recovering security faster in
various cases. various cases.
8.4.3.2. Privacy of the Group Membership 8.4.3.2. Privacy of the Group Membership
Group membership is itself sensitive information and MLS is designed Group membership is itself sensitive information, and MLS is designed
to limit the amount of persistent metadata. However, large groups to limit the amount of persistent metadata. However, large groups
often require an infrastructure which provides server fanout. In the often require an infrastructure that provides server fanout. In the
case of client fanout, the destination of a message is known by all case of client fanout, the destination of a message is known by all
clients, hence the server usually does not need this information. clients; hence, the server usually does not need this information.
However, they may learn this information through traffic analysis. However, they may learn this information through traffic analysis.
Unfortunately, in a server-side fanout model, the Delivery Service Unfortunately, in a server-side fanout model, the Delivery Service
can learn that a given client is sending the same message to a set of can learn that a given client is sending the same message to a set of
other clients. In addition, there may be applications of MLS in other clients. In addition, there may be applications of MLS in
which the group membership list is stored on some server associated which the group membership list is stored on some server associated
with the Delivery Service. with the Delivery Service.
While this knowledge is not a breach of the protocol's authentication While this knowledge is not a breach of the protocol's authentication
or confidentiality guarantees, it is a serious issue for privacy. or confidentiality guarantees, it is a serious issue for privacy.
Some infrastructure keep a mapping between keys used in the MLS Some infrastructures keep a mapping between keys used in the MLS
protocol and user identities. An attacker with access to this protocol and user identities. An attacker with access to this
information due to compromise or regulation can associate unencrypted information due to compromise or regulation can associate unencrypted
group messages (e.g., Commits and Proposals) with the corresponding group messages (e.g., Commits and Proposals) with the corresponding
user identity. user identity.
*RECOMMENDATION:* Use encrypted group operation messages to limit *RECOMMENDATION:* Use encrypted group operation messages to limit
privacy risks whenever possible. privacy risks whenever possible.
In certain cases, the adversary can access specific bindings between In certain cases, the adversary can access specific bindings between
public keys and identities. If the signature keys are reused across public keys and identities. If the signature keys are reused across
groups, the adversary can get more information about the targeted groups, the adversary can get more information about the targeted
user. user.
*RECOMMENDATION:* Ensure that linking between public keys and *RECOMMENDATION:* Ensure that linking between public keys and
identities only happens in expected scenarios. identities only happens in expected scenarios.
8.5. Considerations for attacks outside of the threat model 8.5. Considerations for Attacks Outside of the Threat Model
Physical attacks on devices storing and executing MLS principals are Physical attacks on devices storing and executing MLS principals are
not considered in depth in the threat model of the MLS protocol. not considered in depth in the threat model of the MLS protocol.
While non-permanent, non-invasive attacks can sometimes be equivalent While non-permanent, non-invasive attacks can sometimes be equivalent
to software attacks, physical attacks are considered outside of the to software attacks, physical attacks are considered outside of the
MLS threat model. MLS threat model.
Compromise scenarios typically consist of a software adversary, which Compromise scenarios typically consist of a software adversary, which
can maintain active adaptive compromise and arbitrarily change the can maintain active adaptive compromise and arbitrarily change the
behavior of the client or service. behavior of the client or service.
skipping to change at page 45, line 42 skipping to change at line 2100
*RECOMMENDATION:* Additional steps should be taken to protect the *RECOMMENDATION:* Additional steps should be taken to protect the
device and the MLS clients from physical compromise. In such device and the MLS clients from physical compromise. In such
settings, HSMs and secure enclaves can be used to protect settings, HSMs and secure enclaves can be used to protect
signature keys. signature keys.
8.6. Cryptographic Analysis of the MLS Protocol 8.6. Cryptographic Analysis of the MLS Protocol
Various academic works have analyzed MLS and the different security Various academic works have analyzed MLS and the different security
guarantees it aims to provide. The security of large parts of the guarantees it aims to provide. The security of large parts of the
protocol has been analyzed by [BBN19] (draft 7), [ACDT21] (draft 11) protocol has been analyzed by [BBN19] (regarding MLS Draft 7),
and [AJM20] (draft 12). [ACDT21] (regarding MLS Draft 11), and [AJM20] (regarding MLS Draft
12).
Individual components of various drafts of the MLS protocol have been Individual components of various drafts of the MLS protocol have been
analyzed in isolation and with differing adversarial models, for analyzed in isolation and with differing adversarial models. For
example, [BBR18], [ACDT19], [ACCKKMPPWY19], [AJM20], [ACJM20], and example, [BBR18], [ACDT19], [ACCKKMPPWY19], [AJM20], [ACJM20], and
[AHKM21] analyze the ratcheting tree sub-protocol of MLS that [AHKM21] analyze the ratcheting tree sub-protocol of MLS that
facilitates key agreement, [WPBB22] analyzes the sub-protocol of MLS facilitates key agreement; [WPBB22] analyzes the sub-protocol of MLS
for group state agreement and authentication, while [BCK21] analyzes for group state agreement and authentication; and [BCK21] analyzes
the key derivation paths in the ratchet tree and key schedule. the key derivation paths in the ratchet tree and key schedule.
Finally, [CHK21] analyzes the authentication and cross-group healing Finally, [CHK21] analyzes the authentication and cross-group healing
guarantees provided by MLS. guarantees provided by MLS.
9. IANA Considerations 9. IANA Considerations
This document makes no requests of IANA. This document has no IANA actions.
10. References 10. References
10.1. Normative References 10.1. Normative References
[I-D.ietf-mls-protocol]
Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", Work in Progress, Internet-
Draft, draft-ietf-mls-protocol-20, 27 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
protocol-20>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/rfc/rfc5116>. <https://www.rfc-editor.org/info/rfc5116>.
[RFC9420] Barnes, R., Beurdouche, B., Robert, R., Millican, J., [RFC9420] Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420, Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
July 2023, <https://www.rfc-editor.org/rfc/rfc9420>. July 2023, <https://www.rfc-editor.org/info/rfc9420>.
10.2. Informative References 10.2. Informative References
[ACCKKMPPWY19] [ACCKKMPPWY19]
Alwen, J., Capretto, M., Cueto, M., Kamath, C., Klein, K., Alwen, J., Capretto, M., Cueto, M., Kamath, C., Klein, K.,
Markov, I., Pascual-Perez, G., Pietrzak, K., Walter, M., Markov, I., Pascual-Perez, G., Pietrzak, K., Walter, M.,
and M. Yeo, "Keep the Dirt: Tainted TreeKEM, Adaptively and M. Yeo, "Keep the Dirt: Tainted TreeKEM, Adaptively
and Actively Secure Continuous Group Key Agreement", 2019, and Actively Secure Continuous Group Key Agreement",
Cryptology ePrint Archive, 2019,
<https://eprint.iacr.org/2019/1489>. <https://eprint.iacr.org/2019/1489>.
[ACDT19] Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis, [ACDT19] Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis,
"Security Analysis and Improvements for the IETF MLS "Security Analysis and Improvements for the IETF MLS
Standard for Group Messaging", 2019, Standard for Group Messaging", Cryptology ePrint Archive,
<https://eprint.iacr.org/2019/1189.pdf>. 2019, <https://eprint.iacr.org/2019/1189.pdf>.
[ACDT21] Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis, [ACDT21] Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis,
"Modular Design of Secure Group Messaging Protocols and "Modular Design of Secure Group Messaging Protocols and
the Security of MLS", 2021, the Security of MLS", Cryptology ePrint Archive, 2021,
<https://eprint.iacr.org/2021/1083.pdf>. <https://eprint.iacr.org/2021/1083.pdf>.
[ACJM20] Alwen, J., Coretti, S., Jost, D., and M. Mularczyk, [ACJM20] Alwen, J., Coretti, S., Jost, D., and M. Mularczyk,
"Continuous Group Key Agreement with Active Security", "Continuous Group Key Agreement with Active Security",
2020, <https://eprint.iacr.org/2020/752.pdf>. Cryptology ePrint Archive, 2020,
<https://eprint.iacr.org/2020/752.pdf>.
[AHKM21] Alwen, J., Hartmann, D., Kiltz, E., and M. Mularczyk, [AHKM21] Alwen, J., Hartmann, D., Kiltz, E., and M. Mularczyk,
"Server-Aided Continuous Group Key Agreement", 2021, "Server-Aided Continuous Group Key Agreement", Cryptology
ePrint Archive, 2021,
<https://eprint.iacr.org/2021/1456.pdf>. <https://eprint.iacr.org/2021/1456.pdf>.
[AJM20] Alwen, J., Jost, D., and M. Mularczyk, "On The Insider [AJM20] Alwen, J., Jost, D., and M. Mularczyk, "On The Insider
Security of MLS", 2020, Security of MLS", Cryptology ePrint Archive, 2020,
<https://eprint.iacr.org/2020/1327.pdf>. <https://eprint.iacr.org/2020/1327.pdf>.
[BBN19] Bhargavan, K., Beurdouche, B., and P. Naldurg, "Formal [BBN19] Bhargavan, K., Beurdouche, B., and P. Naldurg, "Formal
Models and Verified Protocols for Group Messaging: Attacks Models and Verified Protocols for Group Messaging: Attacks
and Proofs for IETF MLS", 2019, and Proofs for IETF MLS", HAL ID hal-02425229, 2019,
<https://inria.hal.science/hal-02425229/document>. <https://inria.hal.science/hal-02425229/document>.
[BBR18] Bhargavan, K., Barnes, R., and E. Rescorla, "TreeKEM: [BBR18] Bhargavan, K., Barnes, R., and E. Rescorla, "TreeKEM:
Asynchronous Decentralized Key Management for Large Asynchronous Decentralized Key Management for Large
Dynamic Groups A protocol proposal for Messaging Layer Dynamic Groups - A protocol proposal for Messaging Layer
Security (MLS)", 2018, <https://hal.inria.fr/hal- Security (MLS)", HAL ID hal-02425247, 2018,
02425247/file/treekem+%281%29.pdf>. <https://hal.inria.fr/hal-02425247/file/
treekem+%281%29.pdf>.
[BCK21] Brzuska, C., Cornelissen, E., and K. Kohbrok, [BCK21] Brzuska, C., Cornelissen, E., and K. Kohbrok, "Security
"Cryptographic Security of the MLS RFC, Draft 11", 2021, Analysis of the MLS Key Distribution", Cryptology ePrint
<https://eprint.iacr.org/2021/137.pdf>. Archive, 2021, <https://eprint.iacr.org/2021/137.pdf>.
[CAPBR] Brewer, E., "Towards robust distributed systems [CAPBR] Brewer, E. A., "Towards robust distributed systems
(abstract)", ACM, Proceedings of the nineteenth annual ACM (abstract)", Proceedings of the nineteenth annual ACM
symposium on Principles of distributed computing, symposium on Principles of distributed computing, p. 7,
DOI 10.1145/343477.343502, July 2000, DOI 10.1145/343477.343502, July 2000,
<https://doi.org/10.1145/343477.343502>. <https://dl.acm.org/doi/10.1145/343477.343502>.
[CHK21] Cremers, C., Hale, B., and K. Kohbrok, "The Complexities [CHK21] Cremers, C., Hale, B., and K. Kohbrok, "The Complexities
of Healing in Secure Group Messaging: Why Cross-Group of Healing in Secure Group Messaging: Why Cross-Group
Effects Matter", 2021, Effects Matter", Proceedings of the 30th USENIX Security
Symposium, August 2021,
<https://www.usenix.org/system/files/sec21-cremers.pdf>. <https://www.usenix.org/system/files/sec21-cremers.pdf>.
[CONIKS] Melara, M., Blankstein, A., Bonneau, J., Felten, E., and [CONIKS] Melara, M. S., Blankstein, A., Bonneau, J., Felten, E. W.,
M. Freedman, "CONIKS: Bringing Key Transparency to End and M. J. Freedman, "CONIKS: Bringing Key Transparency to
Users", 2015, End Users", Proceedings of the 24th USENIX Security
Symposium, August 2015,
<https://www.usenix.org/system/files/conference/ <https://www.usenix.org/system/files/conference/
usenixsecurity15/sec15-paper-melara.pdf>. usenixsecurity15/sec15-paper-melara.pdf>.
[I-D.ietf-mls-extensions] [EXTENSIONS]
Robert, R., "The Messaging Layer Security (MLS) Robert, R., "The Messaging Layer Security (MLS)
Extensions", Work in Progress, Internet-Draft, draft-ietf- Extensions", Work in Progress, Internet-Draft, draft-ietf-
mls-extensions-04, 24 April 2024, mls-extensions-06, 19 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls- <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
extensions-04>. extensions-06>.
[I-D.ietf-mls-federation] [FEDERATION]
Omara, E. and R. Robert, "The Messaging Layer Security Omara, E. and R. Robert, "The Messaging Layer Security
(MLS) Federation", Work in Progress, Internet-Draft, (MLS) Federation", Work in Progress, Internet-Draft,
draft-ietf-mls-federation-03, 9 September 2023, draft-ietf-mls-federation-03, 9 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls- <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
federation-03>. federation-03>.
[I-D.schinazi-masque-proxy]
Schinazi, D., "The MASQUE Proxy", Work in Progress,
Internet-Draft, draft-schinazi-masque-proxy-02, 28
February 2024, <https://datatracker.ietf.org/doc/html/
draft-schinazi-masque-proxy-02>.
[KT] McMillion, B., "Key Transparency Architecture", Work in [KT] McMillion, B., "Key Transparency Architecture", Work in
Progress, Internet-Draft, draft-ietf-keytrans- Progress, Internet-Draft, draft-ietf-keytrans-
architecture-01, 4 March 2024, architecture-02, 26 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf- <https://datatracker.ietf.org/doc/html/draft-ietf-
keytrans-architecture-01>. keytrans-architecture-02>.
[Loopix] Piotrowska, A. M., Hayes, J., Elahi, T., Meiser, S., and [Loopix] Piotrowska, A. M., Hayes, J., Elahi, T., Meiser, S., and
G. Danezis, "The Loopix Anonymity System", 2017. G. Danezis, "The Loopix Anonymity System", Proceedings of
the 26th USENIX Security Symposium, August 2017.
[MASQUE-PROXY]
Schinazi, D., "The MASQUE Proxy", Work in Progress,
Internet-Draft, draft-schinazi-masque-proxy-05, 18
February 2025, <https://datatracker.ietf.org/doc/html/
draft-schinazi-masque-proxy-05>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003, DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/rfc/rfc3552>. <https://www.rfc-editor.org/info/rfc3552>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>. <https://www.rfc-editor.org/info/rfc5280>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/rfc/rfc6120>. March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based [RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000, Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021, DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>. <https://www.rfc-editor.org/info/rfc9000>.
[WPBB22] Wallez, T., Protzenko, J., Beurdouche, B., and K. [WPBB22] Wallez, T., Protzenko, J., Beurdouche, B., and K.
Bhargavan, "TreeSync: Authenticated Group Management for Bhargavan, "TreeSync: Authenticated Group Management for
Messaging Layer Security", 2022, Messaging Layer Security", Cryptology ePrint Archive,
<https://eprint.iacr.org/2022/1732.pdf>. 2022, <https://eprint.iacr.org/2022/1732.pdf>.
Contributors Contributors
Richard Barnes Richard Barnes
Cisco Cisco
Email: rlb@ipv.sx Email: rlb@ipv.sx
Katriel Cohn-Gordon Katriel Cohn-Gordon
Meta Platforms Meta Platforms
Email: me@katriel.co.uk Email: me@katriel.co.uk
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