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The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-15

Document Type Active Internet-Draft (mls WG)
Authors Benjamin Beurdouche , Eric Rescorla , Emad Omara , Srinivas Inguva , Alan Duric
Last updated 2024-09-03 (Latest revision 2024-08-03)
Replaces draft-omara-mls-architecture
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Initial working group documents for architecture and key management
Sep 2022
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draft-ietf-mls-architecture-15
Network Working Group                                      B. Beurdouche
Internet-Draft                                           Inria & Mozilla
Intended status: Informational                               E. Rescorla
Expires: 4 February 2025                         Windy Hill Systems, LLC
                                                                E. Omara
                                                                        
                                                               S. Inguva
                                                                        
                                                                A. Duric
                                                                    Wire
                                                           3 August 2024

            The Messaging Layer Security (MLS) Architecture
                     draft-ietf-mls-architecture-15

Abstract

   The Messaging Layer Security (MLS) protocol (I-D.ietf-mls-protocol)
   provides a Group Key Agreement protocol for messaging applications.
   MLS is meant to protect against eavesdropping, tampering, message
   forgery, and provide Forward Secrecy (FS) and Post-Compromise
   Security (PCS).

   This document describes the architecture for using MLS in a general
   secure group messaging infrastructure and defines the security goals
   for MLS.  It provides guidance on building a group messaging system
   and discusses security and privacy tradeoffs offered by multiple
   security mechanisms that are part of the MLS protocol (e.g.,
   frequency of public encryption key rotation).  The document also
   provides guidance for parts of the infrastructure that are not
   standardized by MLS and are instead left to the application.

   While the recommendations of this document are not mandatory to
   follow in order to interoperate at the protocol level, they affect
   the overall security guarantees that are achieved by a messaging
   application.  This is especially true in the case of active
   adversaries that are able to compromise clients, the delivery
   service, or the authentication service.

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/.

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   Source for this draft and an issue tracker can be found at
   https://github.com/mlswg/mls-architecture.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 4 February 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  General Setting . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Abstract Services . . . . . . . . . . . . . . . . . . . .   5
   3.  Overview of Operation . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Step 1: Account Creation  . . . . . . . . . . . . . . . .   7
     3.2.  Step 2: Initial Keying Material . . . . . . . . . . . . .   8
     3.3.  Step 3: Adding Bob to the Group . . . . . . . . . . . . .   8
     3.4.  Step 4: Adding Charlie to the Group . . . . . . . . . . .   8
     3.5.  Other Group Operations  . . . . . . . . . . . . . . . . .   9
     3.6.  Proposals and Commits . . . . . . . . . . . . . . . . . .   9
     3.7.  Users, Clients, and Groups  . . . . . . . . . . . . . . .  10

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   4.  Authentication Service  . . . . . . . . . . . . . . . . . . .  10
   5.  Delivery Service  . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Key Storage and Retrieval . . . . . . . . . . . . . . . .  12
     5.2.  Delivery of Messages  . . . . . . . . . . . . . . . . . .  14
       5.2.1.  Strongly Consistent . . . . . . . . . . . . . . . . .  15
       5.2.2.  Eventually Consistent . . . . . . . . . . . . . . . .  15
       5.2.3.  Welcome Messages  . . . . . . . . . . . . . . . . . .  16
     5.3.  Invalid Commits . . . . . . . . . . . . . . . . . . . . .  17
   6.  Functional Requirements . . . . . . . . . . . . . . . . . . .  18
     6.1.  Membership Changes  . . . . . . . . . . . . . . . . . . .  18
     6.2.  Parallel Groups . . . . . . . . . . . . . . . . . . . . .  19
     6.3.  Asynchronous Usage  . . . . . . . . . . . . . . . . . . .  20
     6.4.  Access Control  . . . . . . . . . . . . . . . . . . . . .  20
     6.5.  Handling Authentication Failures  . . . . . . . . . . . .  21
     6.6.  Recovery After State Loss . . . . . . . . . . . . . . . .  22
     6.7.  Support for Multiple Devices  . . . . . . . . . . . . . .  22
     6.8.  Extensibility . . . . . . . . . . . . . . . . . . . . . .  22
     6.9.  Application Data Framing and Type Advertisements  . . . .  23
     6.10. Federation  . . . . . . . . . . . . . . . . . . . . . . .  23
     6.11. Compatibility with Future Versions of MLS . . . . . . . .  23
   7.  Operational Requirements  . . . . . . . . . . . . . . . . . .  24
   8.  Security and Privacy Considerations . . . . . . . . . . . . .  28
     8.1.  Assumptions on Transport Security Links . . . . . . . . .  28
       8.1.1.  Integrity and Authentication of Custom Metadata . . .  29
       8.1.2.  Metadata Protection for Unencrypted Group
               Operations  . . . . . . . . . . . . . . . . . . . . .  29
       8.1.3.  DoS protection  . . . . . . . . . . . . . . . . . . .  30
       8.1.4.  Message Suppression and Error Correction  . . . . . .  30
     8.2.  Intended Security Guarantees  . . . . . . . . . . . . . .  31
       8.2.1.  Message Secrecy and Authentication  . . . . . . . . .  31
       8.2.2.  Forward and Post-Compromise Security  . . . . . . . .  31
       8.2.3.  Non-Repudiation vs Deniability  . . . . . . . . . . .  33
       8.2.4.  Associating a User's Clients  . . . . . . . . . . . .  33
     8.3.  Endpoint Compromise . . . . . . . . . . . . . . . . . . .  34
       8.3.1.  Compromise of Symmetric Keying Material . . . . . . .  34
       8.3.2.  Compromise by an active adversary with the ability to
               sign messages . . . . . . . . . . . . . . . . . . . .  36
       8.3.3.  Compromise of the authentication with access to a
               signature key . . . . . . . . . . . . . . . . . . . .  37
       8.3.4.  Security consideration in the context of a full state
               compromise  . . . . . . . . . . . . . . . . . . . . .  38
     8.4.  Service Node Compromise . . . . . . . . . . . . . . . . .  39
       8.4.1.  General considerations  . . . . . . . . . . . . . . .  39
       8.4.2.  Delivery Service Compromise . . . . . . . . . . . . .  40
       8.4.3.  Authentication Service Compromise . . . . . . . . . .  42
     8.5.  Considerations for attacks outside of the threat model  .  45
     8.6.  Cryptographic Analysis of the MLS Protocol  . . . . . . .  45
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46

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   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  46
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  46
     10.2.  Informative References . . . . . . . . . . . . . . . . .  46
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  49
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50

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
   systems, and also deployed in systems for other purposes such as
   calling and conferencing.  In this context, "end-to-end" captures the
   notion that users of the system enjoy some level of security -- with
   the precise level depending on the system design -- even in the face
   of malicious actions by the operator of the messaging system.

   Messaging Layer Security (MLS) specifies an architecture (this
   document) and a protocol [I-D.ietf-mls-protocol] for providing end-
   to-end security in this setting.  MLS is not intended as a full
   instant messaging protocol but rather is intended to be embedded in
   concrete protocols, such as XMPP [RFC6120].  Implementations of the
   MLS protocol will interoperate at the cryptographic level, though
   they may have incompatibilities in terms of how protected messages
   are delivered, contents of protected messages, and identity/
   authentication infrastructures.  The MLS protocol has been designed
   to provide the same security guarantees to all users, for all group
   sizes, including groups of only two clients.

2.  General Setting

2.1.  Protocol Overview

   MLS provides a way for _clients_ to form _groups_ within which they
   can communicate securely.  For example, a set of users might use
   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
   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
   group.  As groups change membership and group or member properties,
   they advance from one _epoch_ to another and the cryptographic state
   of the group evolves.

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   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
   of the members.  Each member has a state called a _LeafNode_ object
   holding the client's identity, credentials, and capabilities.

   Various messages are used in the evolution from epoch to epoch.  A
   _Proposal_ message proposes a change to be made in the next epoch,
   such as adding or removing a member.  A _Commit_ message initiates a
   new epoch by instructing members of the group to implement a
   collection of proposals.  Proposals and Commits are collectively
   called _Handshake messages_. A _KeyPackage_ provides keys that can be
   used to add the client to a group, including its LeafNode, and
   _Signature Key_. A _Welcome_ message provides a new member to the
   group with the information to initialize their state for the epoch in
   which they were added.

   Of course most (but not all) applications use MLS to send encrypted
   group messages.  An _application message_ is an MLS message with an
   arbitrary application payload.

   Finally, a _PublicMessage_ contains an integrity-protected MLS
   Handshake message, while a _PrivateMessage_ contains a confidential,
   integrity-protected Handshake or application message.

   For a more detailed explanation of these terms, please consult the
   MLS protocol specification [RFC9420].

2.2.  Abstract Services

   MLS is designed to operate within the context of a messaging service,
   which may be a single service provider, a federated system, or some
   kind of peer-to-peer system.  The service needs to provide two
   services that facilitate client communication using MLS:

   *  An Authentication Service (AS) which is responsible for attesting
      to bindings between application-meaningful identifiers and the
      public key material used for authentication in the MLS protocol.
      The AS must also be able to generate credentials that encode these
      bindings and validate credentials provided by MLS clients.

   *  A Delivery Service (DS) which can receive and distribute messages
      between group members.  In the case of group messaging, the
      delivery service may also be responsible for acting as a
      "broadcaster" where the sender sends a single message which is
      then forwarded to each recipient in the group by the DS.  The DS
      is also responsible for storing and delivering initial public key
      material required by MLS clients in order to proceed with the
      group secret key establishment that is part of the MLS protocol.

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   For presentation purposes, this document treats the AS and DS as
   conventional network services, however MLS does not require a
   specific implementation for the AS or DS.  These services may reside
   on the same server or different servers, they may be distributed
   between server and client components, and they may even involve some
   action by users.  For example:

   *  Several secure messaging services today provide a centralized DS,
      and rely on manual comparison of clients' public keys as the AS.

   *  MLS clients connected to a peer-to-peer network could instantiate
      a decentralized DS by transmitting MLS messages over that network.

   *  In an MLS group using a Public Key Infrastructure (PKI) for
      authentication, the AS would comprise the certificate issuance and
      validation processes, both of which involve logic inside MLS
      clients as well as various existing PKI roles (ex: Certification
      Authorities).

   It is important to note that the Authentication Service can be
   completely abstract in the case of a Service Provider which allows
   MLS clients to generate, distribute, and validate credentials
   themselves.  As with the AS, the Delivery Service can be completely
   abstract if users are able to distribute credentials and messages
   without relying on a central Delivery Service (as in a peer-to-peer
   system).  Note, though, that in such scenarios, clients will need to
   implement logic that assures the delivery properties required of the
   DS (see Section 5.2).

        +----------------+    +--------------+
        | Authentication |    |   Delivery   |
        |  Service (AS)  |    | Service (DS) |
        +----------------+    +-------+------+
                             /        |       \            Group
                            / ........|........\................
                           /  .       |         \              .
                 +--------+-+ .  +----+-----+    +----------+  .
                 | Client 1 | .  | Client 2 |    | Client 3 |  .
                 +----------+ .  +----------+    +----------+  .
                              .   Member 1        Member 2     .
                              .                                .
                              ..................................

                  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.

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3.  Overview of Operation

   Figure 2 shows the formation of an example group consisting of Alice,
   Bob, and Charlie, with Alice driving the creation of the group.

Alice     Bob       Charlie                     AS        DS

Create account --------------------------------->                |
<------------------------------------- Credential                |
          Create account ----------------------->                | Step 1
          <--------------------------- Credential                |
                    Create account ------------->                |
                    <----------------- Credential                |

Initial Keying Material ----------------------------------->     |
          Initial Keying Material ------------------------->     | Step 2
                    Initial Keying Material --------------->     |

Get Bob Initial Keying Material --------------------------->     |
<------------------------------- Bob Initial Keying Material     |
Add Bob to Group ------------------------------------------>     | Step 3
Welcome (Bob)---------------------------------------------->     |
          <-------------------------------- Add Bob to Group     |
          <----------------------------------- Welcome (Bob)     |

Get Charlie Initial Keying Material ----------------------->     |
<--------------------------- Charlie Initial Keying Material     |
Add Charlie to Group -------------------------------------->     |
Welcome (Charlie) ----------------------------------------->     | Step 4
          <---------------------------- Add Charlie to Group     |
                     <----------------- Add Charlie to Group     |
                     <-------------------- Welcome (Charlie)     |

                  Figure 2: Group Formation Example

   This process proceeds as follows.

3.1.  Step 1: Account Creation

   Alice, Bob, and Charlie create accounts with a service provider and
   obtain credentials from the AS.  This is a one-time setup phase.

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3.2.  Step 2: Initial Keying Material

   Alice, Bob, and Charlie authenticate to the DS and store some initial
   keying material which can be used to send encrypted messages to them
   for the first time.  This keying material is authenticated with their
   long-term credentials.  Although in principle this keying material
   can be reused for multiple senders, in order to provide forward
   secrecy it is better for this material to be regularly refreshed so
   that each sender can use a new key.

3.3.  Step 3: Adding Bob to the Group

   When Alice wants to create a group including Bob, she first uses the
   DS to look up his initial keying material.  She then generates two
   messages:

   *  A message to the entire group (which at this point is just her and
      Bob) that adds Bob to the group.

   *  A _Welcome_ message just to Bob encrypted with his initial keying
      material that includes the secret keying information necessary to
      join the group.

   She sends both of these messages to the Delivery Services, which is
   responsible for sending them to the appropriate people.  Note that
   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
   necessary for other group members to receive it.

3.4.  Step 4: Adding Charlie to the Group

   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
   initial keying material and then generates two messages:

   *  A message to the entire group (consisting of her, Bob, and
      Charlie) adding Charlie to the group.

   *  A _Welcome_ message just to Charlie encrypted with his initial
      keying material that includes the secret keying information
      necessary to join the group.

   At the completion of this process, we have a group with Alice, Bob,
   and Charlie, which means that they share a single encryption key
   which can be used to send messages or to key other protocols.

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3.5.  Other Group Operations

   Once the group has been created, clients can perform other actions,
   such as:

   *  sending a message to everyone in the group

   *  receiving a message from someone in the group

   *  adding one or more clients to an existing group

   *  remove one or more members from an existing group

   *  updating their own key material

   *  leave a group (by asking to be removed)

   Importantly, MLS does not itself enforce any access control on group
   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
   contrast to some designs in which there is a single group controller
   who can modify the group.  MLS-using applications are responsible for
   setting their own access control policies.  For instance, if only the
   group administrator is allowed to change group members, then it is
   the responsibility of the application to inform members of this
   policy and who the administrator is.

3.6.  Proposals and Commits

   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:

   Proposal  This message describes the change to be made (e.g., add Bob
      to the group) but does not effect a change.

   Commit  This message changes the group state to include the changes
      described in a set of proposals.

   The simplest pattern is for a client to just send a Commit which
   contains one or more Proposals, for instance Alice could send a
   Commit with the Proposal Add(Bob) embedded to add Bob to the group.
   However, there are situations in which one client might send a
   proposal and another might send the commit.  For instance, Bob might
   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
   Commit, an existing member must do so.  Commits can apply to multiple
   valid Proposals, in which case all the listed changes are applied.

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   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
   the group without changing its membership.

3.7.  Users, Clients, and Groups

   While it's natural to think of a messaging system as consisting of
   groups of users, possibly using different devices, in MLS the basic
   unit of operation is not the user but rather the "client".  Formally,
   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
   signature key.  As usual, a user demonstrates ownership of the client
   by demonstrating knowledge of the associated secret values.

   In some messaging systems, clients belonging to the same user must
   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
   different keys.  In this case, each client will have its own
   cryptographic state, and it is up to the application to determine how
   to present this situation to users.  For instance, it may render
   messages to and from a given user identically regardless of which
   client they are associated with, or may choose to distinguish them.

   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
   shared group secret established in the group key establishment phase.
   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
   group, other members cannot assume that the client is a member of the
   group; for instance, the newly added member might not have received
   the Welcome message or been unable to decrypt it for some reason.

4.  Authentication Service

   The Authentication Service (AS) has to provide three services:

   1.  Issue credentials to clients that attest to bindings between
       identities and signature key pairs

   2.  Enable a client to verify that a credential presented by another
       client is valid with respect to a reference identifier

   3.  Enable a group member to verify that a credential represents the
       same client as another credential

   A member with a valid credential authenticates its MLS messages by
   signing them with the private key corresponding to the public key
   bound by its credential.

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   The AS is considered an abstract layer by the MLS specification and
   part of this service could be, for instance, running on the members'
   devices, while another part is a separate entity entirely.  The
   following examples illustrate the breadth of this concept:

   *  A PKI could be used as an AS [RFC5280].  The issuance function
      would be provided by the certificate authorities in the PKI, and
      the verification function would correspond to certificate
      verification by clients.

   *  Several current messaging applications rely on users verifying
      each other's key fingerprints for authentication.  In this
      scenario, the issuance function is simply the generation of a key
      pair (i.e., a credential is just an identifier and public key,
      with no information to assist in verification).  The verification
      function is the application function that enables users to verify
      keys.

   *  In a system based on [CONIKS] end user Key Transparency (KT) [KT],
      the issuance function would correspond to the insertion of a key
      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
      claimed identity, together with the KT log's mechanisms for a user
      to monitor and control which keys are associated with their
      identity.

   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
   an adversary to, among other things, impersonate group members.  We
   discuss security considerations regarding the compromise of the
   different AS functions in detail in Section 8.4.3.

   The association between members' identities and their signature keys
   is fairly flexible in MLS.  As noted above, there is no requirement
   that all clients belonging to a given user have the same signature
   key (in fact, having duplicate signature keys in a group is
   forbidden).  A member can also rotate the signature key they use
   within a group.  These mechanisms allow clients to use different
   signature keys in different contexts and at different points in time,
   providing unlinkability and post-compromise security benefits.  Some
   security trade-offs related to this flexibility are discussed in the
   security considerations.

   In many applications, there are multiple MLS clients that represent a
   single entity, for example a human user with a mobile and desktop
   version of an application.  Often the same set of clients is
   represented in exactly the same list of groups.  In applications
   where this is the intended situation, other clients can check that a

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   user is consistently represented by the same set of clients.  This
   would make it more difficult for a malicious AS to issue fake
   credentials for a particular user because clients would expect the
   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
   relatively short period of time, clients have an indication that the
   credential might have been created without the user's knowledge.  Due
   to the asynchronous nature of MLS, however, there may be transient
   inconsistencies in a user's client set, so correlating users' clients
   across groups is more of a detection mechanism than a prevention
   mechanism.

5.  Delivery Service

   The Delivery Service (DS) plays two major roles in MLS:

   *  As a directory service providing the initial keying material for
      clients to use.  This allows a client to establish a shared key
      and send encrypted messages to other clients even if they're
      offline.

   *  Routing MLS messages among clients.

   While MLS depends on correct behavior by the Authentication Service
   in order to provide endpoint authentication and hence confidentiality
   of the group key, these properties do not depend on correct behavior
   by the DS; even a malicious DS cannot add itself to groups or recover
   the group key.  However, depending precisely on how MLS is used, the
   DS may be able to determine group membership or prevent changes to
   the group from taking place (e.g., by blocking group change
   messages).

5.1.  Key Storage and Retrieval

   Upon joining the system, each client stores its initial cryptographic
   key material with the Delivery Service.  This key material, called a
   KeyPackage, advertises the functional abilities of the client such as
   supported protocol versions, supported extensions, and the following
   cryptographic information:

   *  A credential from the Authentication Service attesting to the
      binding between the identity and the client's signature key.

   *  The client's asymmetric encryption public key.

   All the parameters in the KeyPackage are signed with the signature
   private key corresponding to the credential.  As noted in
   Section 3.7, users may own multiple clients, each with their own

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   keying material.  Each KeyPackage is specific to an MLS version and
   ciphersuite, but a client may want to offer support for multiple
   protocol versions and ciphersuites.  As such, there may be multiple
   KeyPackages stored by each user for a mix of protocol versions,
   ciphersuites, and end-user devices.

   When a client wishes to establish a group or add clients to a group,
   it first contacts the Delivery Service to request KeyPackages for
   each other client, authenticates the KeyPackages using the signature
   keys, includes the KeyPackages in Add Proposals, encrypts the
   information needed to join the group (the _GroupInfo_ object) with an
   ephemeral key, then separately encrypts the ephemeral key with the
   init_key from each KeyPackage.  When a client requests a KeyPackage
   in order to add a user to a group, the Delivery Service should
   provide the minimum number of KeyPackages necessary to satisfy the
   request.  For example, if the request specifies the MLS version, the
   DS might provide one KeyPackage per supported ciphersuite, even if it
   has multiple such KeyPackages to enable the corresponding client to
   be added to multiple groups before needing to upload more fresh
   KeyPackages.

   In order to avoid replay attacks and provide forward secrecy for
   messages sent using the initial keying material, KeyPackages are
   intended to be used only once.  The Delivery Service is responsible
   for ensuring that each KeyPackage is only used to add its client to a
   single group, with the possible exception of a "last resort"
   KeyPackage that is specially designated by the client to be used
   multiple times.  Clients are responsible for providing new
   KeyPackages as necessary in order to minimize the chance that the
   "last resort" KeyPackage will be used.

      *RECOMMENDATION:* Ensure that "last resort" KeyPackages don't get
      used by provisioning enough standard KeyPackages.

      *RECOMMENDATION:* Rotate "last resort" KeyPackages as soon as
      possible after being used or if they have been stored for a
      prolonged period of time.  Overall, avoid reusing last resort
      KeyPackages as much as possible.

      *RECOMMENDATION:* Ensure that the client for which a last resort
      KeyPackage has been used is updating leaf keys as early as
      possible.

   Overall, it needs to be noted that key packages need to be updated
   when signature keys are changed.

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5.2.  Delivery of Messages

   The main responsibility of the Delivery Service is to ensure delivery
   of messages.  Some MLS messages need only be delivered to specific
   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
   Delivery Service may enable the latter delivery pattern via unicast
   channels (sometimes known as "client fanout"), broadcast channels
   ("server fanout"), or a mix of both.

   For the most part, MLS does not require the Delivery Service to
   deliver messages in any particular order.  Applications can set
   policies that control their tolerance for out-of-order messages (see
   Section 7), and messages that arrive significantly out-of-order can
   be dropped without otherwise affecting the protocol.  There are two
   exceptions to this.  First, Proposal messages should all arrive
   before the Commit that references them.  Second, because an MLS group
   has a linear history of epochs, the members of the group must agree
   on the order in which changes are applied.  Concretely, the group
   must agree on a single MLS Commit message that ends each epoch and
   begins the next one.

   In practice, there's a realistic risk of two members generating
   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
   which this is a problem, and the appropriate solution, depends on the
   design of the Delivery Service.  Per the CAP theorem [CAPBR], there
   are two general classes of distributed system that the Delivery
   Service might fall into:

   *  Consistent and Partition-tolerant, or Strongly Consistent, systems
      can provide a globally consistent view of data but has the
      inconvenience of clients needing to handle rejected messages;

   *  Available and Partition-tolerant, or Eventually Consistent,
      systems continue working despite network issues but may return
      different views of data to different users.

   Strategies for sequencing messages in strongly and eventually
   consistent systems are described in the next two subsections.  Most
   Delivery Services will use the Strongly Consistent paradigm but this
   remains a choice that can be handled in coordination with the client
   and advertized in the KeyPackages.

   However, note that a malicious Delivery Service could also reorder
   messages or provide an inconsistent view to different users.  The
   "generation" counter in MLS messages provides per-sender loss
   detection and ordering that cannot be manipulated by the DS, but this

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   does not provide complete protection against partitioning.  A DS can
   cause a partition in the group by partitioning key exchange messages;
   this can be detected only by out-of-band comparison (e.g., confirming
   that all clients have the same epoch_authenticator value).  A
   mechanism for more robust protections is discussed in
   [I-D.ietf-mls-extensions].

   Other forms of Delivery Service misbehavior are still possible that
   are not easy to detect.  For instance, a Delivery Service can simply
   refuse to relay messages to and from a given client.  Without some
   sort of side information, other clients cannot generally detect this
   form of Denial of Service (DoS) attack.

5.2.1.  Strongly Consistent

   With this approach, the Delivery Service ensures that some types of
   incoming messages have a linear order and all members agree on that
   order.  The Delivery Service is trusted to break ties when two
   members send a Commit message at the same time.

   As an example, there could be an "ordering server" Delivery Service
   that broadcasts all messages received to all users and ensures that
   all clients see messages in the same order.  This would allow clients
   to only apply the first valid Commit for an epoch and ignore
   subsequent ones.  Clients that send a Commit would then wait to apply
   it until it is broadcast back to them by the Delivery Service,
   assuming they do not receive another Commit first.

   Alternatively, the Delivery Service can rely on the epoch and
   content_type fields of an MLSMessage to provide an order only to
   handshake messages, and possibly even filter or reject redundant
   Commit messages proactively to prevent them from being broadcast.
   There is some risk associated with filtering, which is discussed
   further in Section 5.3.

5.2.2.  Eventually Consistent

   With this approach, the Delivery Service is built in a way that may
   be significantly more available or performant than a strongly
   consistent system, but offers weaker consistency guarantees.
   Messages may arrive to different clients in different orders and with
   varying amounts of latency, which means clients are responsible for
   reconciliation.

   This type of Delivery Service might arise, for example, when group
   members are sending each message to each other member individually,
   or when a distributed peer-to-peer network is used to broadcast
   messages.

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   Upon receiving a Commit from the Delivery Service, clients can
   either:

   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
       Commits are received for the same epoch.  If multiple Commits are
       received, the clients can use a deterministic tie-breaking policy
       to decide which to accept, and then resume sending messages as
       normal.

   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
       past epoch is received, clients use a deterministic tie-breaking
       policy to decide if they should continue using the Commit they
       originally accepted or revert and use the later one.  Note that
       any copies of previous or forked group states must be deleted
       within a reasonable amount of time to ensure the protocol
       provides forward-secrecy.

   If the Commit references an unknown proposal, group members may need
   to solicit the Delivery Service or other group members individually
   for the contents of the proposal.

5.2.3.  Welcome Messages

   Whenever a commit adds new members to a group, MLS requires the
   committer to send a Welcome message to the new members.  Applications
   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.
   That is, when multiple commits are sent for the same epoch,
   applications need to ensure that only Welcome messages corresponding
   to the commit that "succeeded" are processed by new members.

   This is particularly important when groups are being reinitialized.
   When a group is reinitialized, it is restarted with a different
   protocol version and/or ciphersuite but identical membership.
   Whenever an authorized member sends and commits a ReInit proposal,
   this immediately freezes the existing group and triggers the creation
   of a new group with a new group_id.

   Ideally, the new group would be created by the same member that
   committed the ReInit proposal (including sending Welcome messages for
   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
   offline after committing a ReInit proposal but before creating the
   new group.  If this happens, it's necessary for another member to
   continue the reinitialization by creating the new group and sending
   out Welcome messages.

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   This has the potential to create a race condition, where multiple
   members try to continue the reinitialization at the same time, and
   members receive multiple Welcome messages for each attempt at
   reinitializing the same group.  Ensuring that all members agree on
   which reinitialization attempt is "correct" is key to prevent this
   from causing forks.

5.3.  Invalid Commits

   Situations can arise where a malicious or buggy client sends a Commit
   that is not accepted by all members of the group, and the DS is not
   able to detect this and reject the Commit.  For example, a buggy
   client might send an encrypted Commit with an invalid set of
   proposals, or a malicious client might send a malformed Commit of the
   form described in Section 16.12 of [RFC9420].

   In situations where the DS is attempting to filter redundant Commits,
   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
   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,
   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.

   Such “desynchronization” problems can arise even when the Delivery
   Service takes no stance on which Commit is "correct" for an epoch.
   The DS can enable clients to choose between Commits, for example by
   providing Commits in the order received and allow clients to reject
   any Commits that violate their view of the group's policies.  As
   such, all honest and correctly-implemented clients will arrive at the
   same "first valid Commit" and choose to process it.  Malicious or
   buggy clients that process a different Commit will end up in a forked
   view of the group.

   When these desynchronizations happen, the application may choose to
   take action to restore the functionality of the group.  These actions
   themselves can have security implications.  For example, a client
   developer might have a client automatically rejoin a group, using an
   external join, when it processes an invalid Commit.  In this
   operation, however, the client trusts that the GroupInfo provided by
   the DS faithfully represents the state of the group, and not, say, an
   earlier state containing a compromised leaf node.  In addition, the
   DS may be able to trigger this condition by deliberately sending the
   victim an invalid Commit.  In certain scenarios, this trust can
   enable the DS or a malicious insider to undermine the post-compromise
   security guarantees provided by MLS.

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   Actions to recover from desynchronization can also have availability
   and DoS implications.  For example, if a recovery mechanism relies on
   external joins, a malicious member that deliberately posts an invalid
   Commit could also post a corrupted GroupInfo object in order to
   prevent victims from rejoining the group.  Thus, careful analysis of
   security implications should be made for any system for recovering
   from desynchronization.

6.  Functional Requirements

   MLS is designed as a large-scale group messaging protocol and hence
   aims to provide both performance and security (e.g. integrity and
   confidentiality) to its users.  Messaging systems that implement MLS
   provide support for conversations involving two or more members, and
   aim to scale to groups with tens of thousands of members, typically
   including many users using multiple devices.

6.1.  Membership Changes

   MLS aims to provide agreement on group membership, meaning that all
   group members have agreed on the list of current group members.

   Some applications may wish to enforce ACLs to limit addition or
   removal of group members, to privileged clients or users.  Others may
   wish to require authorization from the current group members or a
   subset thereof.  Such policies can be implemented at the application
   layer, on top of MLS.  Regardless, MLS does not allow for or support
   addition or removal of group members without informing all other
   members.

   Membership of an MLS group is managed at the level of individual
   clients.  In most cases, a client corresponds to a specific device
   used by a user.  If a user has multiple devices, the user will
   generally be represented in a group by multiple clients (although
   applications could choose to have devices share keying material).  If
   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
   user and ensure that they are added / removed consistently.

   MLS provides two mechanisms for changing the membership of a group.
   The primary mechanism is for an authorized member of the group to
   send a Commit that adds or removes other members.  The second
   mechanism is an "external join": A member of the group publishes
   certain information about the group, which a new member can use to
   construct an "external" Commit message that adds the new 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.)

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   With both mechanisms, changes to the membership are initiated from
   inside the group.  When members perform changes directly, this is
   clearly the case.  External joins are authorized indirectly, in the
   sense that a member publishing a GroupInfo object authorizes anyone
   to join who has access to the GroupInfo object, subject to whatever
   access control policies the application applies for external joins.

   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
   authorized.  The former approach requires that Commits be visible to
   the DS; the latter approach requires that clients all share a
   consistent policy.  In the unfortunate event that an unauthorized
   member is able to join, MLS enables any member to remove them.

   Application setup may also determine other criteria for membership
   validity.  For example, per-device signature keys can be signed by an
   identity key recognized by other participants.  If a certificate
   chain is used to authenticate device signature keys, then revocation
   by the owner adds an alternative mechanism to prompt membership
   removal.

   An MLS group's secrets change on every change of membership, so each
   client only has access to the secrets used by the group while they
   are a member.  Messages sent before a client joins or after they are
   removed are protected with keys that are not accessible to the
   client.  Compromise of a member removed from a group does not affect
   the security of messages sent after their removal.  Messages sent
   during the client's membership are also secure as long as the client
   has properly implemented the MLS deletion schedule, which calls for
   the secrets used to encrypt or decrypt a message to be deleted after
   use, along with any secrets that could be used to derive them.

6.2.  Parallel Groups

   Any user or client may have membership in several groups
   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
   and PCS goals within a given group are maintained and not weakened by
   user membership in multiple groups.  However, actions in other groups
   likewise do not strengthen the FS and PCS guarantees within a given
   group, e.g., key updates within a given group following a device
   compromise does not provide PCS healing in other groups; each group
   must be updated separately to achieve these security objectives.
   This also applies to future groups that a member has yet to join,
   which are likewise unaffected by updates performed in current groups.

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   Applications can strengthen connectivity among parallel groups by
   requiring periodic key updates from a user across all groups in which
   they have membership.

   MLS provides a pre-shared key (PSK) that can be used to link healing
   properties among parallel groups.  For example, suppose a common
   member M of two groups A and B has performed a key update in group A
   but not in group B.  The key update provides PCS with regard to M in
   group A.  If a PSK is exported from group A and injected into group
   B, then some of these PCS properties carry over to group B, since the
   PSK and secrets derived from it are only known to the new, updated
   version of M, not to the old, possibly compromised version of M.

6.3.  Asynchronous Usage

   No operation in MLS requires two distinct clients or members to be
   online simultaneously.  In particular, members participating in
   conversations protected using MLS can update the group's keys, add or
   remove new members, and send messages without waiting for another
   user's reply.

   Messaging systems that implement MLS have to provide a transport
   layer for delivering messages asynchronously and reliably.

6.4.  Access Control

   Because all clients within a group (members) have access to the
   shared cryptographic material, MLS protocol allows each member of the
   messaging group to perform operations.  However, every service/
   infrastructure has control over policies applied to its own clients.
   Applications managing MLS clients can be configured to allow for
   specific group operations.  On the one hand, an application could
   decide that a group administrator will be the only member to perform
   add and remove operations.  On the other hand, in many settings such
   as open discussion forums, joining can be allowed for anyone.

   While MLS Application messages are always encrypted, MLS handshake
   messages can be sent either encrypted (in an MLS PrivateMessage) or
   unencrypted (in an MLS PublicMessage).  Applications may be designed
   such that intermediaries need to see handshake messages, for example
   to enforce policy on which commits are allowed, or to provide MLS
   ratchet tree data in a central location.  If handshake messages are
   unencrypted, it is especially important that they be sent over a
   channel with strong transport encryption (see Section 8) in order to
   prevent external attackers from monitoring the status of the group.
   Applications that use unencrypted handshake messages may take
   additional steps to reduce the amount of metadata that is exposed to
   the intermediary.  Everything else being equal, using encrypted

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   handshake messages provides stronger privacy properties than using
   unencrypted handshake messages, as it prevents intermediaries from
   learning about the structure of the group.

   If handshake messages are encrypted, any access control policies must
   be applied at the client, so the application must ensure that the
   access control policies are consistent across all clients to make
   sure that they remain in sync.  If two different policies were
   applied, the clients might not accept or reject a group operation and
   end-up in different cryptographic states, breaking their ability to
   communicate.

      *RECOMMENDATION:* Avoid using inconsistent access control policies
      in the case of encrypted group operations.

   MLS allows actors outside the group to influence the group in two
   ways: External signers can submit proposals for changes to the group,
   and new joiners can use an external join to add themselves to the
   group.  The external_senders extension ensures that all members agree
   on which signers are allowed to send proposals, but any other
   policies must be assured to be consistent as above.

      *RECOMMENDATION:* Have an explicit group policy setting the
      conditions under which external joins are allowed.

6.5.  Handling Authentication Failures

   Within an MLS group, every member is authenticated to every other
   member by means of credentials issued and verified by the
   Authentication Service.  MLS does not prescribe what actions, if any,
   an application should take in the event that a group member presents
   an invalid credential.  For example, an application may require such
   a member to be immediately evicted, or may allow some grace period
   for the problem to be remediated.  To avoid operational problems, it
   is 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
   credentials.

      *RECOMMENDATION:* Have a uniform credential validation process to
      ensure that all group members evaluate other members' credentials
      in the same way.

      *RECOMMENDATION:* Have a uniform policy for how invalid
      credentials are handled.

   In some authentication systems, it is possible for a previously-valid
   credential to become invalid over time.  For example, in a system
   based on X.509 certificates, credentials can expire or be revoked.

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   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
   invalid.

      *RECOMMENDATION:* Proactively rotate credentials, especially if a
      credential is about to become invalid.

6.6.  Recovery After State Loss

   Group members whose local MLS state is lost or corrupted can
   reinitialize their state by re-joining the group as a new member and
   removing the member representing their earlier state.  An application
   can require that a client performing such a reinitialization prove
   its prior membership with a PSK that was exported from the prevoius
   state.

   There are a few practical challenges to this approach.  For example,
   the application will need to ensure that all members have the
   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
   PSK is lost or corrupted along with the member's other state, then it
   cannot be used to recover.

   Reinitializing in this way does not provide the member with access to
   group messages from during the state loss window, but enables proof
   of prior membership in the group.  Applications may choose various
   configurations for providing lost messages to valid group members
   that are able to prove prior membership.

6.7.  Support for Multiple Devices

   It is typically expected for users within a group to own various
   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
   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
   this case, but applications can elect to provide such a mechanism
   outside of MLS.  Such mechanisms, if used, may reduce the FS and PCS
   guarantees provided by MLS.

6.8.  Extensibility

   The MLS protocol provides several extension points where additional
   information can be provided.  Extensions to KeyPackages allow clients
   to disclose additional information about their capabilities.  Groups
   can also have extension data associated with them, and the group
   agreement properties of MLS will confirm that all members of the
   group agree on the content of these extensions.

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6.9.  Application Data Framing and Type Advertisements

   Application messages carried by MLS are opaque to the protocol; they
   can contain arbitrary data.  Each application which uses MLS needs to
   define the format of its application_data and any mechanism necessary
   to determine the format of that content over the lifetime of an MLS
   group.  In many applications this means managing format migrations
   for groups with multiple members who may each be offline at
   unpredictable times.

      *RECOMMENDATION:* Use the default content mechanism defined in
      Section 3.3 of [I-D.ietf-mls-extensions], unless the specific
      application defines another mechanism which more appropriately
      addresses the same requirements for that application of MLS.

   The MLS framing for application messages also provides a field where
   clients can send information that is authenticated but not encrypted.
   Such information can be used by servers that handle the message, but
   group members are assured that it has not been tampered with.

6.10.  Federation

   The protocol aims to be compatible with federated environments.
   While this document does not specify all necessary mechanisms
   required for federation, multiple MLS implementations can
   interoperate to form federated systems if they use compatible
   authentication mechanisms, ciphersuites, application content, and
   infrastructure functionalities.  Federation is described in more
   detail in [I-D.ietf-mls-federation].

6.11.  Compatibility with Future Versions of MLS

   It is important that multiple versions of MLS be able to coexist in
   the future.  Thus, MLS offers a version negotiation mechanism; this
   mechanism prevents version downgrade attacks where an attacker would
   actively rewrite messages with a lower protocol version than the ones
   originally offered by the endpoints.  When multiple versions of MLS
   are available, the negotiation protocol guarantees that the version
   agreed upon will be the highest version supported in common by the
   group.

   In MLS 1.0, the creator of the group is responsible for selecting the
   best ciphersuite supported across clients.  Each client is able to
   verify availability of protocol version, ciphersuites and extensions
   at all times once he has at least received the first group operation
   message.

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   Each member of an MLS group advertises the protocol functionality
   they support.  These capability advertisements can be updated over
   time, e.g., if client software is updated while the client is a
   member of a group.  Thus, in addition to preventing downgrade
   attacks, the members of a group can also observe when it is safe to
   upgrade to a new ciphersuite or protocol version.

7.  Operational Requirements

   MLS is a security layer that needs to be integrated with an
   application.  A fully-functional deployment of MLS will have to make
   a number of decisions about how MLS is configured and operated.
   Deployments that wish to interoperate will need to make compatible
   decisions.  This section lists all of the dependencies of an MLS
   deployment that are external to the protocol specification, but would
   still need to be aligned within a given MLS deployment, or for two
   deployments to potentially interoperate.

   The protocol has a built-in ability to negotiate protocol versions,
   ciphersuites, extensions, credential types, and additional proposal
   types.  For two deployments to interoperate, they must have
   overlapping support in each of these categories.  The
   required_capabilities extension (Section 7.2 of [RFC9420]) can
   promote interoperability with a wider set of clients by ensuring that
   certain functionality continues to be supported by a group, even if
   the clients in the group aren't currently relying on it.

   MLS relies on the following network services, that need to be
   compatible in order for two different deployments based on them to
   interoperate.

   *  An *Authentication Service*, described fully in Section 4, defines
      the types of credentials which may be used in a deployment and
      provides methods for:

      1.  Issuing new credentials with a relevant credential lifetime,

      2.  Validating a credential against a reference identifier,

      3.  Validating whether or not two credentials represent the same
          client, and

      4.  Optionally revoking credentials which are no longer
          authorized.

   *  A *Delivery Service*, described fully in Section 5, provides
      methods for:

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      1.  Delivering messages for a group to all members in the group.

      2.  Delivering Welcome messages to new members of a group.

      3.  Uploading new KeyPackages for a user's own clients.

      4.  Downloading KeyPackages for specific clients.  Typically,
          KeyPackages are used once and consumed.

   *  Additional services may or may not be required depending on the
      application design:

      -  In cases where group operations are not encrypted, the DS has
         the ability to observe and maintain a copy of the public group
         state.  In particular, this is useful for clients that do 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
         recover from losing their state.  Such public state can contain
         privacy sensitive information such as group members'
         credentials and related public keys, hence services need to
         carefully evaluate the privacy impact of storing this data on
         the DS.

      -  If external joiners are allowed, there must be a method to
         publish a serialized GroupInfo object (with an external_pub
         extension) that corresponds to a specific group and epoch, and
         keep that object in sync with the state of the group.

      -  If an application chooses not to allow external joining, it may
         instead provide a method for external users to solicit group
         members (or a designated service) to add them to a group.

      -  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
         prove membership in the group in the past, as in Section 6.6)
         there must be a method for distributing these PSKs to group
         members who might not have them, for instance if they joined
         the group after the PSK was generated.

      -  If an application wishes to detect and possibly discipline
         members that send malformed commits with the intention of
         corrupting a group's state, there must be a method for
         reporting and validating malformed commits.

   MLS requires the following parameters to be defined, which must be
   the same for two implementations to interoperate:

   *  The maximum total lifetime that is acceptable for a KeyPackage.

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   *  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
      delivery:

      -  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 steps that clients will move a secret tree
         ratchet forward in response to a single message before
         rejecting it.

      -  Whether to buffer messages that aren't able to be understood
         yet due to other messages not arriving first, and if so, how
         many and for how long.  For example, Commit messages that
         arrive before a proposal they reference, or application
         messages that arrive before the Commit starting an epoch.

   If implementations differ in these parameters, they will interoperate
   to some extent but may experience unexpected failures in certain
   situations, such as extensive message reordering.

   MLS provides the following locations where an application may store
   arbitrary data.  The format and intention of any data in these
   locations must align for two deployments to interoperate:

   *  Application data, sent as the payload of an encrypted message.

   *  Additional authenticated data, sent unencrypted in an otherwise
      encrypted message.

   *  Group IDs, as decided by group creators and used to uniquely
      identify a group.

   *  Application-level identifiers of public key material (specifically
      the application_id extension as defined in Section 5.3.3 of
      [RFC9420]).

   MLS requires the following policies to be defined, which restrict the
   set of acceptable behavior in a group.  These policies must be
   consistent between deployments for them to interoperate:

   *  A policy on which ciphersuites are acceptable.

   *  A policy on any mandatory or forbidden MLS extensions.

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   *  A policy on when to send proposals and commits in plaintext
      instead of encrypted.

   *  A policy for which proposals are valid to have in a commit,
      including but not limited to:

      -  When a member is allowed to add or remove other members of the
         group.

      -  When, and under what circumstances, a reinitialization proposal
         is allowed.

      -  When proposals from external senders are allowed and how to
         authorize those proposals.

      -  When external joiners are allowed and how to authorize those
         external commits.

      -  Which other proposal types are allowed.

   *  A policy of when members should commit pending proposals in a
      group.

   *  A policy of how to protect and share the GroupInfo objects needed
      for external joins.

   *  A policy for when two credentials represent the same client.  Note
      that many credentials may be issued attesting the same identity
      but for different signature keys, because each credential
      corresponds to a different client owned by the same application
      user.  However, one device may control multiple signature keys --
      for instance if they have keys corresponding to multiple
      overlapping time periods -- but should still only be considered a
      single client.

   *  A policy on how long to allow a member to stay in a group without
      updating its leaf keys before removing them.

   Finally, there are some additional application-defined behaviors that
   are partially an individual application's decision but may overlap
   with interoperability:

   *  When and how to pad messages.

   *  When to send a reinitialization proposal.

   *  How often clients should update their leaf keys.

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   *  Whether to prefer sending full commits or partial/empty commits.

   *  Whether there should be a required_capabilities extension in
      groups.

8.  Security and Privacy Considerations

   MLS adopts the Internet threat model [RFC3552] and therefore assumes
   that the attacker has complete control of the network.  It is
   intended to provide the security services described in Section 8.2 in
   the face of attackers who can:

   *  Monitor the entire network.

   *  Read unprotected messages.

   *  Can generate, inject and delete any message in the unprotected
      transport layer.

   While MLS should be run over a secure transport such as QUIC
   [RFC9000] or TLS [RFC8446], the security guarantees of MLS do not
   depend on the transport.  This departs from the usual design practice
   of trusting the transport because MLS is designed to provide security
   even in the face of compromised network elements, especially the DS.

   Generally, MLS is designed under the assumption that the transport
   layer is present to keep metadata private from network observers,
   while the MLS protocol provides confidentiality, integrity, and
   authentication guarantees for the application data (which could pass
   through multiple systems).  Additional properties such as partial
   anonymity or deniability could also be achieved in specific
   architecture designs.

   In addition, these guarantees are intended to degrade gracefully in
   the presence of compromise of the transport security links as well as
   of both clients and elements of the messaging system, as described in
   the remainder of this section.

8.1.  Assumptions on Transport Security Links

   As discussed above, MLS provides the highest level of security when
   its messages are delivered over an encrypted transport.  The main use
   of the secure transport layer for MLS is to protect the already
   limited amount of metadata.  Very little information is contained in
   the unencrypted header of the MLS protocol message format for group
   operation messages, and application messages are always encrypted in
   MLS.

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      *RECOMMENDATION:* Use transports that provide reliability and
      metadata confidentiality whenever possible, e.g., by transmitting
      MLS messages over a protocol such as TLS [RFC8446] or QUIC
      [RFC9000].

   MLS avoids needing to send the full list of recipients to the server
   for dispatching messages because that list could potentially contain
   tens of thousands of recipients.  Header metadata in MLS messages
   typically consists of an opaque group_id, a numerical value to
   determine the epoch of the group (the number of changes that have
   been made to the group), and whether the message is an application
   message, a proposal, or a commit.

   Even though some of this metadata information does not consist of
   sensitive information, in correlation with other data a network
   observer might be able to reconstruct sensitive information.  Using a
   secure channel to transfer this information will prevent a network
   attacker from accessing this MLS protocol metadata if it cannot
   compromise the secure channel.

8.1.1.  Integrity and Authentication of Custom Metadata

   MLS provides an authenticated "Additional Authenticated Data" (AAD)
   field for applications to make data available outside a
   PrivateMessage, while cryptographically binding it to the message.

      *RECOMMENDATION:* Use the "Additional Authenticated Data" field of
      the PrivateMessage instead of using other unauthenticated means of
      sending metadata throughout the infrastructure.  If the data
      should be kept private, the infrastructure should use encrypted
      Application messages instead.

8.1.2.  Metadata Protection for Unencrypted Group Operations

   Having no secure channel to exchange MLS messages can have a serious
   impact on privacy when transmitting unencrypted group operation
   messages.  Observing the contents and signatures of the group
   operation messages may lead an adversary to extract information about
   the group membership.

      *RECOMMENDATION:* Never use the unencrypted mode for group
      operations without using a secure channel for the transport layer.

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8.1.3.  DoS protection

   In general we do not consider Denial of Service (DoS) resistance to
   be the responsibility of the protocol.  However, it should not be
   possible for anyone aside from the Delivery Service to perform a
   trivial DoS attack from which it is hard to recover.  This can be
   achieved through the secure transport layer.

   In the centralized setting, DoS protection can typically be performed
   by using tickets or cookies which identify users to a service for a
   certain number of connections.  Such a system helps in preventing
   anonymous clients from sending arbitrary numbers of group operation
   messages to the Delivery Service or the MLS clients.

      *RECOMMENDATION:* Use credentials uncorrellated with specific
      users to help prevent DoS attacks, in a privacy preserving manner.
      Note that the privacy of these mechanisms has to be adjusted in
      accordance with the privacy expected from secure transport links.
      (See more discussion in the next section.)

8.1.4.  Message Suppression and Error Correction

   As noted above, MLS is designed to provide some robustness in the
   face of tampering within the secure transport, i.e., tampering by the
   Delivery Service.  The confidentiality and authenticity properties of
   MLS prevent the DS from reading or writing messages.  MLS also
   provides a few tools for detecting message suppression, with the
   caveat that message suppression cannot always be distinguished from
   transport failure.

   Each encrypted MLS message carries a "generation" number which is a
   per-sender incrementing counter.  If a group member observes a gap in
   the generation sequence for a sender, then they know that they have
   missed a message from that sender.  MLS also provides a facility for
   group members to send authenticated acknowledgments of application
   messages received within a group.

   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 ones sent by group members.  Since only one Commit per epoch is
   meaningful, it's not useful for the DS to transmit multiple Commits
   to clients.  The risk remains that the DS will use the ability
   maliciously.

   While it is difficult or impossible to prevent a network adversary
   from suppressing payloads in transit, in certain infrastructures such
   as banks or governments settings, unidirectional transports can be
   used and be enforced via electronic or physical devices such as

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   diodes.  This can lead to payload corruption which does not affect
   the security or privacy properties of the MLS protocol but does
   affect the reliability of the service.  In that case specific
   measures can be taken to ensure the appropriate level of redundancy
   and quality of service for MLS.

8.2.  Intended Security Guarantees

   MLS aims to provide a number of security guarantees, covering
   authentication, as well as confidentiality guarantees to different
   degrees in different scenarios.

8.2.1.  Message Secrecy and Authentication

   MLS enforces the encryption of application messages and thus
   generally guarantees authentication and confidentiality of
   application messages sent in a group.

   In particular, this means that only other members of a given group
   can decrypt the payload of a given application message, which
   includes information about the sender of the message.

   Similarly, group members receiving a message from another group
   member can authenticate that group member as the sender of the
   message and verify the message's integrity.

   Message content can be deniable if the signature keys are exchanged
   over a deniable channel prior to signing messages.

   Depending on the group settings, handshake messages can be encrypted
   as well.  If that is the case, the same security guarantees apply.

   MLS optionally allows the addition of padding to messages, mitigating
   the amount of information leaked about the length of the plaintext to
   an observer on the network.

8.2.2.  Forward and Post-Compromise Security

   MLS provides additional protection regarding secrecy of past messages
   and future messages.  These cryptographic security properties are
   Forward Secrecy (FS) and Post-Compromise Security (PCS).

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   FS means that access to all encrypted traffic history combined with
   access to all current keying material on clients will not defeat the
   secrecy properties of messages older than the oldest key of the
   compromised client.  Note that this means that clients have to delete
   the appropriate keys as soon as they have been used with the expected
   message, otherwise the secrecy of the messages and the security for
   MLS is considerably weakened.

   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
   t2, then all MLS guarantees apply to messages sent by the member
   after time t2, and by other members after they have processed the
   update.  For example, if an attacker learns all secrets known to
   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,
   then the attacker is unable to violate any of the MLS security
   properties after the updates have been processed.

   Both of these properties are satisfied even against compromised DSs
   and ASs in the case where some other mechanism for verifying keys is
   in use, such as Key Transparency [KT].

   Confidentiality is mainly ensured on the client side.  Because
   Forward Secrecy (FS) and Post-Compromise Security (PCS) rely on the
   active deletion and replacement of keying material, any client which
   is persistently offline may still be holding old keying material and
   thus be a threat to both FS and PCS if it is later compromised.

   MLS partially defends against this problem by active members
   including freshness, however not much can be done on the inactive
   side especially in the case where the client has not processed
   messages.

      *RECOMMENDATION:* Mandate key updates from clients that are not
      otherwise sending messages and evict clients which are idle for
      too long.

   These recommendations will reduce the ability of idle compromised
   clients to decrypt a potentially long set of messages that might have
   followed the point of the compromise.

   The precise details of such mechanisms are a matter of local policy
   and beyond the scope of this document.

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8.2.3.  Non-Repudiation vs Deniability

   MLS provides strong authentication within a group, such that a group
   member cannot send a message that appears to be from another group
   member.  Additionally, some services require that a recipient be able
   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
   cases.  In some deployments, these services are provided by
   mechanisms which allow the receiver to prove a message's origin to a
   third party.  This is often called "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
   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
   in ways that provide certain deniability properties, but defining the
   specific requirements and resulting notions of deniability requires
   further analysis.

8.2.4.  Associating a User's Clients

   When a user has multiple devices, the base MLS protocol only
   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
   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
   time.  Group members would also be able to detect when the user adds
   or removes authorized devices from their account.  For some
   applications, this may be an unacceptable breach of the user's
   privacy.

   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
   to mitigate this risk is by only doing client-level authentication
   within MLS.  If user-level authentication is still desirable, the
   application would have to provide it through some other mechanism.

   It is also possible to maintain user-level authentication while
   hiding information about the clients that a user owns.  This can be
   done by having the clients share cryptographic state, so that they
   appear as a single client within the MLS group.  Appearing as a
   single client has the privacy benefits of no longer leaking which
   device was used to send a particular message, and no longer leaking
   the user's authorized devices.  However, the application would need
   to provide a synchronization mechanism so that the clients' state
   remain consistent across changes to the MLS group.  Flaws in this
   synchronization mechanism may impair the ability of the user to
   recover from a compromise of one of their devices.  In particular,

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   state synchronization may make it easier for an attacker to use one
   compromised device to establish exclusive control of a user's
   account, locking them out entirely and preventing them from
   recovering.

8.3.  Endpoint Compromise

   The MLS protocol adopts a threat model which includes multiple forms
   of endpoint/client compromise.  While adversaries are in a strong
   position if they have compromised an MLS client, there are still
   situations where security guarantees can be recovered thanks to the
   PCS properties achieved by the MLS protocol.

   In this section we will explore the consequences and recommendations
   regarding the following compromise scenarios:

   *  The attacker has access to a symmetric encryption key

   *  The attacker has access to a application ratchet secret

   *  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 the signature key for one group

   *  The attacker has access to all secrets of a user for all groups
      (full state compromise)

8.3.1.  Compromise of Symmetric Keying Material

   As described above, each MLS epoch creates a new Group Secret.

   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
   data (AEAD) [RFC5116] key that is then used to encrypt MLS Plaintext
   messages.  Each time a message is sent, the Ratchet Secret is used to
   create a new Ratchet Secret and a new corresponding AEAD key.
   Because of the properties of the key derivation function, it is not
   possible to compute a Ratchet Secret from its corresponding AEAD key
   or compute Ratchet Secret n-1 from Ratchet Secret n.

   Below, we consider the compromise of each of these pieces of keying
   material in turn, in ascending order of severity.  While this is a
   limited kind of compromise, it can be realistic in cases of
   implementation vulnerabilities where only part of the memory leaks to
   the adversary.

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8.3.1.1.  Compromise of AEAD Keys

   In some circumstances, adversaries may have access to specific AEAD
   keys and nonces which protect an Application or a Group Operation
   message.  Compromise of these keys allows the attacker to decrypt the
   specific message encrypted with that key but no other; because the
   AEAD keys are derived from the Ratchet Secret, it cannot generate the
   next Ratchet Secret and hence not the next AEAD key.

   In the case of an Application message, an AEAD key compromise means
   that the encrypted application message will be leaked as well as the
   signature over that message.  This means that the compromise has both
   confidentiality and privacy implications on the future AEAD
   encryptions of that chain.  In the case of a Group Operation message,
   only the privacy is affected, as the signature is revealed, because
   the secrets themselves are protected by HPKE encryption.  Note that
   under that compromise scenario, authentication is not affected in
   either of these cases.  As every member of the group can compute the
   AEAD keys for all the chains (they have access to the Group Secrets)
   in order to send and receive messages, the authentication provided by
   the AEAD encryption layer of the common framing mechanism is weak.
   Successful decryption of an AEAD 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
   message using that key, but cannot send a message to a group which
   appears to be from any valid client since they cannot forge the
   signature.  This applies to all the forms of symmetric key compromise
   described in Section 8.3.1.

8.3.1.2.  Compromise of Ratchet Secret material

   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
   for that sender in this epoch.  Thus, it can decrypt current and
   future messages by the corresponding sender.  However, because it
   does not have previous Ratchet Secrets, it cannot decrypt past
   messages as long as those secrets and keys have been deleted.

   Because of its Forward Secrecy guarantees, MLS will also retain
   secrecy of all other AEAD keys generated for _other_ MLS clients,
   outside this dedicated chain of AEAD keys and nonces, even within the
   epoch of the compromise.  MLS provides Post-Compromise Security
   against an active adaptive attacker across epochs for AEAD
   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
   able to access any protected messages from future epochs.

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8.3.1.3.  Compromise of the Group Secrets of a single group for one or
          more group epochs

   An adversary who gains access to a set of Group secrets--as when a
   member of the group is compromised--is significantly more powerful.
   In this section, we consider the case where the signature keys are
   not compromised, which can occur if the attacker has access to part
   of the memory containing the group 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
   number of Ratchet Secrets for the epoch and their corresponding AEAD
   encryption keys and thus can encrypt and decrypt all messages for the
   compromised epochs.

   If the adversary is passive, it is expected from the PCS properties
   of the MLS protocol that, as soon as the compromised party remediates
   the compromise and sends an honest Commit message, the next epochs
   will provide message secrecy.

   If the adversary is active, the adversary can engage in the protocol
   itself and perform updates on behalf of the compromised party with no
   ability for an honest group to recover message secrecy.  However, MLS
   provides PCS against active adaptive attackers through its Remove
   group operation.  This means that, as long as other members of the
   group are honest, the protocol will guarantee message secrecy for all
   messages exchanged in the epochs after the compromised party has been
   removed.

8.3.2.  Compromise by an active adversary with the ability to sign
        messages

   If an active adversary has compromised an MLS client and can sign
   messages, two different settings emerge.  In the strongest compromise
   scenario, the attacker has access to the signing key and can forge
   authenticated messages.  In a weaker, yet realistic scenario, the
   attacker has compromised a client but the client signature keys are
   protected with dedicated hardware features which do not allow direct
   access to the value of the private key and instead provide a
   signature API.

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   When considering an active adaptive attacker with access to a
   signature oracle, the compromise scenario implies a significant
   impact on both the secrecy and authentication guarantees of the
   protocol, especially if the attacker also has access to the group
   secrets.  In that case both secrecy and authentication are broken.
   The attacker can generate any message, for the current and future
   epochs, until the compromise is remediated and the formerly
   compromised client sends an honest update.

   Note that under this compromise scenario, the attacker can perform
   all operations which are available to a legitimate client even
   without access to the actual value of the 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
   key and only having access to a signing oracle is not about the
   ability of an active adaptive network attacker to perform different
   operations during the time of the compromise, the attacker can
   perform every operation available to a legitimate client in both
   cases.

   There is a significant difference, however in terms of recovery after
   a compromise.

   Because of the PCS guarantees provided by the MLS protocol, when a
   previously compromised client recovers from compromise and performs
   an honest Commit, both secrecy and authentication of future messages
   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
   cannot authenticate messages on behalf of the compromised party, even
   if they still have control over some group keys by colluding with
   other members of the group.

   This is in contrast with the case where the signature key is leaked.
   In that case the compromised endpoint needs to refresh its
   credentials and invalidate the old credentials before the attacker
   will be unable to authenticate messages.

   Beware that in both oracle and private key access, an active adaptive
   attacker can follow the protocol and request to update its own
   credential.  This in turn induces a signature key rotation which
   could provide the attacker with part or the full value of the private
   key depending on the architecture of the service provider.

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      *RECOMMENDATION:* Signature private keys should be
      compartmentalized from other secrets and preferably protected by
      an HSM or dedicated hardware features to allow recovery of the
      authentication for future messages after a compromise.

      *RECOMMENDATION:* When the credential type supports revocation,
      the users of a group should check for revoked keys.

8.3.4.  Security consideration in the context of a full state compromise

   In real-world compromise scenarios, it is often the case that
   adversaries target specific devices to obtain parts of the memory or
   even the ability to execute arbitrary code in the targeted device.

   Also, recall that in this setting, the application will often retain
   the unencrypted messages.  If so, the adversary does not have to
   break encryption at all to access sent and received messages.
   Messages may also be sent by using the application to instruct the
   protocol implementation.

      *RECOMMENDATION:* If messages are stored on the device, they
      should be protected using encryption at rest, and the keys used
      should be stored securely using dedicated mechanisms on the
      device.

      *RECOMMENDATION:* If the threat model of the system is against an
      adversary which can access the messages on the device without even
      needing to attack MLS, the application should delete plaintext and
      ciphertext messages as soon as practical after encryption or
      decryption.

   Note that this document makes a clear distinction between the way
   signature keys and other group shared secrets must be handled.  In
   particular, a large set of group secrets cannot necessarily be
   assumed to be protected by an HSM or secure enclave features.  This
   is especially true because these keys are frequently used and changed
   with each message received by a client.

   However, the signature private keys are mostly used by clients to
   send a message.  They also provide strong authentication guarantees
   to other clients, hence we consider that their protection by
   additional security mechanisms should be a priority.

   Overall there is no way to detect or prevent these compromises, as
   discussed in the previous sections, performing separation of the
   application secret states can help recovery after compromise, this is
   the case for signature keys but similar concern exists for client's
   encryption private keys.

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      *RECOMMENDATION:* The secret keys used for public key encryption
      should be stored similarly to the way the signature keys are
      stored, as keys can be used to decrypt the group operation
      messages and contain the secret material used to compute all the
      group secrets.

   Even if secure enclaves are not perfectly secure, or even completely
   broken, adopting additional protections for these keys can ease
   recovery of the secrecy and authentication guarantees after a
   compromise where, for instance, an attacker can sign messages without
   having access to the key.  In certain contexts, the rotation of
   credentials might only be triggered by the AS through ACLs, hence be
   outside of the capabilities of the attacker.

8.4.  Service Node Compromise

8.4.1.  General considerations

8.4.1.1.  Privacy of the network connections

   There are many scenarios leading to communication between the
   application on a device and the Delivery Service or the
   Authentication Service.  In particular when:

   *  The application connects to the Authentication Service to generate
      or validate a new credential before distributing it.

   *  The application fetches credentials at the Delivery Service prior
      to creating a messaging group (one-to-one or more than two
      clients).

   *  The application fetches service provider information or messages
      on the Delivery Service.

   *  The application sends service provider information or messages to
      the Delivery Service.

   In all these cases, the application will often connect to the device
   via a secure transport which leaks information about the origin of
   the request such as the IP address and depending on the protocol the
   MAC address of the device.

   Similar concerns exist in the peer-to-peer use cases of MLS.

      *RECOMMENDATION:* In the case where privacy or anonymity is
      important, using adequate protection such as MASQUE
      [I-D.schinazi-masque-proxy], ToR, or a VPN can improve metadata
      protection.

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   More generally, using anonymous credentials in an MLS based
   architecture might not be enough to provide strong privacy or
   anonymity properties.

8.4.1.2.  Storage of Metadata and Ecryption at rest on the Servers

   In the case where private data or metadata has to be persisted on the
   servers for functionality (mappings between identities and push
   tokens, group metadata...), it should be stored encrypted at rest and
   only decrypted upon need during the execution.  Honest Service
   Providers can rely on such encryption at rest mechanism to be able to
   prevent access to the data when not using it.

      *RECOMMENDATION:* Store cryptographic material used for server-
      side decryption of sensitive meta-data on the clients and only
      send it when needed.  The server can use the secret to open and
      update encrypted data containers after which they can delete these
      keys until the next time they need it, in which case those can be
      provided by the client.

      *RECOMMENDATION:* Rely on group secrets exported from the MLS
      session for server-side encryption at rest and update the key
      after each removal from the group.  Rotate those keys on a regular
      basis otherwise.

8.4.2.  Delivery Service Compromise

   MLS is intended to provide strong guarantees in the face of
   compromise of the DS.  Even a totally compromised DS should not be
   able to read messages or inject messages that will be acceptable to
   legitimate clients.  It should also not be able to undetectably
   remove, reorder or replay messages.

   However, a malicious DS can mount a variety of DoS attacks on the
   system, including total DoS attacks (where it simply refuses to
   forward any messages) and partial DoS attacks (where it refuses to
   forward messages to and from specific clients).  As noted in
   Section 5.2, these attacks are only partially detectable by clients
   without an out-of-band channel.  Ultimately, failure of the DS to
   provide reasonable service must be dealt with as a customer service
   matter, not via technology.

   Because the DS is responsible for providing the initial keying
   material to clients, it can provide stale keys.  This does not
   inherently lead to compromise of the message stream, but does allow
   it to attack forward security to a limited extent.  This threat can
   be mitigated by having initial keys expire.

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   Initial keying material (KeyPackages) using the basic Credential type
   is more vulnerable to replacement by a malicious or compromised DS,
   as there is no built-in cryptographic binding between the identity
   and the public key of the client.

      *RECOMMENDATION:* Prefer a Credential type in KeyPackages which
      includes a strong cryptographic binding between the identity and
      its key (for example the x509 Credential type).  When using the
      basic Credential type take extra care to verify the identity
      (typically out-of-band).

8.4.2.1.  Privacy of delivery and push notifications

   An important mechanism that is often ignored from the privacy
   considerations are the push-tokens.  In many modern messaging
   architectures, applications are using push notification mechanisms
   typically provided by OS vendors.  This is to make sure that when
   messages are available at the Delivery Service (or by other
   mechanisms if the DS is not a central server), the recipient
   application on a device knows about it.  Sometimes the push
   notification can contain the application message itself which saves a
   round trip with the DS.

   To "push" this information to the device, the service provider and
   the OS infrastructures use unique per-device, per-application
   identifiers called push-tokens.  This means that the push
   notification provider and the service provider have information on
   which devices receive information and at which point in time.
   Alternatively, non-mobile applications could use a websocket or
   persistent connection for notifications directly from the DS.

   Even though they can't necessarily access the content, which is
   typically encrypted MLS messages, the service provider and the push
   notification provider have to be trusted to avoid making correlation
   on which devices are recipients of the same message.

   For secure messaging systems, push notifications are often sent real-
   time as it is not acceptable to create artificial delays for message
   retrieval.

      *RECOMMENDATION:* If real time notifications are not necessary,
      one can delay notifications randomly across recipient devices
      using a mixnet or other techniques.

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   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
   token with a second request to the company operating the push-
   notification system to get information about the device, which is
   often linked with a real identity via a cloud account, a credit card
   or other information.

      *RECOMMENDATION:* If stronger privacy guarantees are needed with
      regard to the push notification provider, the client can choose to
      periodically connect to the Delivery Service without the need of a
      dedicated push notification infrastructure.

   Applications can also consider anonymous systems for server fanout
   (for example [Loopix]).

8.4.3.  Authentication Service Compromise

   The Authentication Service design is left to the infrastructure
   designers.  In most designs, a compromised AS is a serious matter, as
   the AS can serve incorrect or attacker-provided identities to
   clients.

   *  The attacker can link an identity to a credential

   *  The attacker can generate new credentials

   *  The attacker can sign new credentials

   *  The attacker can publish or distribute credentials

   An attacker that can generate or sign new credentials may or may not
   have access to the underlying cryptographic material necessary to
   perform such operations.  In that last case, it results in windows of
   time for which all emitted credentials might be compromised.

      *RECOMMENDATION:* Use HSMs to store the root signature keys to
      limit the ability of an adversary with no physical access to
      extract the top-level signature private key.

   Note that historically some systems generate signature keys on the
   Authentication Service and distribute the private keys to clients
   along with their credential.  This is a dangerous practice because it
   allows the AS or an attacker who has compromised the AS to silently
   impersonate the client.

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8.4.3.1.  Authentication compromise: Ghost users and impersonations

   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
   and the Authentication Service are honest, no parties other than the
   members of the current group can read and write messages protected by
   the protocol for that Group.

   This guarantee applies to the cryptographic identities of the
   members.  Details about how to verify the identity of a client depend
   on the MLS Credential type used.  For example, cryptographic
   verification of credentials can be largely performed autonomously
   (e.g., without user interaction) by the clients themselves for the
   x509 Credential type.

   In contrast, when MLS clients use the basic Credential type, then
   some other mechanism must be used to verify identities.  For instance
   the Authentication Service could operate some sort of directory
   server to provide keys, or users could verify keys via an out-of-band
   mechanism.

      *RECOMMENDATION:* Select the MLS Credential type with the
      strongest security which is supported by all target members of an
      MLS group.

      *RECOMMENDATION:* Do not use the same signature keypair across
      groups.  Update all keys for all groups on a regular basis.  Do
      not preserve keys in different groups when suspecting a
      compromise.

   If the AS is compromised, it could validate a (or generate a new)
   signature keypair for an attacker.  The attacker could then use this
   keypair to join a group as if it were another of the user's clients.
   Because a user can have many MLS clients running the MLS protocol, it
   possibly has many signature keypairs for multiple devices.  These
   attacks could be very difficult to detect, especially in large groups
   where the UI might not reflect all the changes back to the users.  If
   the application participates in a key transparency mechanism in which
   it is possible to determine every key for a given user, then this
   would allow for detection of surreptitiously created false
   credentials.

      *RECOMMENDATION:* Make sure that MLS clients reflect all the
      membership changes to the users as they happen.  If a choice has
      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
      user can examine it.

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      *RECOMMENDATION:* Provide a key transparency mechanism for the
      Authentication Services to allow public verification of the
      credentials authenticated by this service.

   While the ways to handle MLS credentials are not defined by the
   protocol or the architecture documents, the MLS protocol has been
   designed with a mechanism that can be used to provide out-of-band
   authentication to users.  The "authentication_secret" generated for
   each user at each epoch of the group is a one-time, per client,
   authentication secret which can be exchanged between users to prove
   their identity to each other.  This can be done for instance using a
   QR code that can be scanned by the other parties.

      *RECOMMENDATION:* Provide one or more out-of-band authentication
      mechanisms to limit the impact of an Authentication Service
      compromise.

   We note, again, that the Authentication Service may not be a
   centralized system, and could be realized by many mechanisms such as
   establishing prior one-to-one deniable channels, gossiping, or using
   trust on first use (TOFU) for credentials used by the MLS Protocol.

   Another important consideration is the ease of redistributing new
   keys on client compromise, which helps recovering security faster in
   various cases.

8.4.3.2.  Privacy of the Group Membership

   Group membership is itself sensitive information and MLS is designed
   to limit the amount of persistent metadata.  However, large groups
   often require an infrastructure which provides server fanout.  In the
   case of client fanout, the destination of a message is known by all
   clients, hence the server usually does not need this information.
   However, they may learn this information through traffic analysis.
   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
   other clients.  In addition, there may be applications of MLS in
   which the group membership list is stored on some server associated
   with the Delivery Service.

   While this knowledge is not a breach of the protocol's authentication
   or confidentiality guarantees, it is a serious issue for privacy.

   Some infrastructure keep a mapping between keys used in the MLS
   protocol and user identities.  An attacker with access to this
   information due to compromise or regulation can associate unencrypted
   group messages (e.g., Commits and Proposals) with the corresponding
   user identity.

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      *RECOMMENDATION:* Use encrypted group operation messages to limit
      privacy risks whenever possible.

   In certain cases, the adversary can access specific bindings between
   public keys and identities.  If the signature keys are reused across
   groups, the adversary can get more information about the targeted
   user.

      *RECOMMENDATION:* Ensure that linking between public keys and
      identities only happens in expected scenarios.

8.5.  Considerations for attacks outside of the threat model

   Physical attacks on devices storing and executing MLS principals are
   not considered in depth in the threat model of the MLS protocol.
   While non-permanent, non-invasive attacks can sometimes be equivalent
   to software attacks, physical attacks are considered outside of the
   MLS threat model.

   Compromise scenarios typically consist of a software adversary, which
   can maintain active adaptive compromise and arbitrarily change the
   behavior of the client or service.

   On the other hand, security goals consider that honest clients will
   always run the protocol according to its specification.  This relies
   on implementations of the protocol to securely implement the
   specification, which remains non-trivial.

      *RECOMMENDATION:* Additional steps should be taken to protect the
      device and the MLS clients from physical compromise.  In such
      settings, HSMs and secure enclaves can be used to protect
      signature keys.

8.6.  Cryptographic Analysis of the MLS Protocol

   Various academic works have analyzed MLS and the different security
   guarantees it aims to provide.  The security of large parts of the
   protocol has been analyzed by [BBN19] (draft 7), [ACDT21] (draft 11)
   and [AJM20] (draft 12).

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   Individual components of various drafts of the MLS protocol have been
   analyzed in isolation and with differing adversarial models, for
   example, [BBR18], [ACDT19], [ACCKKMPPWY19], [AJM20], [ACJM20], and
   [AHKM21] analyze the ratcheting tree sub-protocol of MLS that
   facilitates key agreement, [WPBB22] analyzes the sub-protocol of MLS
   for group state agreement and authentication, while [BCK21] analyzes
   the key derivation paths in the ratchet tree and key schedule.
   Finally, [CHK21] analyzes the authentication and cross-group healing
   guarantees provided by MLS.

9.  IANA Considerations

   This document makes no requests of IANA.

10.  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
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5116>.

   [RFC9420]  Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
              Omara, E., and K. Cohn-Gordon, "The Messaging Layer
              Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
              July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.

10.2.  Informative References

   [ACCKKMPPWY19]
              Alwen, J., Capretto, M., Cueto, M., Kamath, C., Klein, K.,
              Markov, I., Pascual-Perez, G., Pietrzak, K., Walter, M.,
              and M. Yeo, "Keep the Dirt: Tainted TreeKEM, Adaptively
              and Actively Secure Continuous Group Key Agreement", 2019,
              <https://eprint.iacr.org/2019/1489>.

   [ACDT19]   Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis,
              "Security Analysis and Improvements for the IETF MLS
              Standard for Group Messaging", 2019,
              <https://eprint.iacr.org/2019/1189.pdf>.

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   [ACDT21]   Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis,
              "Modular Design of Secure Group Messaging Protocols and
              the Security of MLS", 2021,
              <https://eprint.iacr.org/2021/1083.pdf>.

   [ACJM20]   Alwen, J., Coretti, S., Jost, D., and M. Mularczyk,
              "Continuous Group Key Agreement with Active Security",
              2020, <https://eprint.iacr.org/2020/752.pdf>.

   [AHKM21]   Alwen, J., Hartmann, D., Kiltz, E., and M. Mularczyk,
              "Server-Aided Continuous Group Key Agreement", 2021,
              <https://eprint.iacr.org/2021/1456.pdf>.

   [AJM20]    Alwen, J., Jost, D., and M. Mularczyk, "On The Insider
              Security of MLS", 2020,
              <https://eprint.iacr.org/2020/1327.pdf>.

   [BBN19]    Bhargavan, K., Beurdouche, B., and P. Naldurg, "Formal
              Models and Verified Protocols for Group Messaging: Attacks
              and Proofs for IETF MLS", 2019,
              <https://inria.hal.science/hal-02425229/document>.

   [BBR18]    Bhargavan, K., Barnes, R., and E. Rescorla, "TreeKEM:
              Asynchronous Decentralized Key Management for Large
              Dynamic Groups A protocol proposal for Messaging Layer
              Security (MLS)", 2018, <https://hal.inria.fr/hal-
              02425247/file/treekem+%281%29.pdf>.

   [BCK21]    Brzuska, C., Cornelissen, E., and K. Kohbrok,
              "Cryptographic Security of the MLS RFC, Draft 11", 2021,
              <https://eprint.iacr.org/2021/137.pdf>.

   [CAPBR]    Brewer, E., "Towards robust distributed systems
              (abstract)", ACM, Proceedings of the nineteenth annual ACM
              symposium on Principles of distributed computing,
              DOI 10.1145/343477.343502, July 2000,
              <https://doi.org/10.1145/343477.343502>.

   [CHK21]    Cremers, C., Hale, B., and K. Kohbrok, "The Complexities
              of Healing in Secure Group Messaging: Why Cross-Group
              Effects Matter", 2021,
              <https://www.usenix.org/system/files/sec21-cremers.pdf>.

   [CONIKS]   Melara, M., Blankstein, A., Bonneau, J., Felten, E., and
              M. Freedman, "CONIKS: Bringing Key Transparency to End
              Users", 2015,
              <https://www.usenix.org/system/files/conference/
              usenixsecurity15/sec15-paper-melara.pdf>.

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   [I-D.ietf-mls-extensions]
              Robert, R., "The Messaging Layer Security (MLS)
              Extensions", Work in Progress, Internet-Draft, draft-ietf-
              mls-extensions-04, 24 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
              extensions-04>.

   [I-D.ietf-mls-federation]
              Omara, E. and R. Robert, "The Messaging Layer Security
              (MLS) Federation", Work in Progress, Internet-Draft,
              draft-ietf-mls-federation-03, 9 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
              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
              Progress, Internet-Draft, draft-ietf-keytrans-
              architecture-01, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              keytrans-architecture-01>.

   [Loopix]   Piotrowska, A. M., Hayes, J., Elahi, T., Meiser, S., and
              G. Danezis, "The Loopix Anonymity System", 2017.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/rfc/rfc3552>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/rfc/rfc6120>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

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   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [WPBB22]   Wallez, T., Protzenko, J., Beurdouche, B., and K.
              Bhargavan, "TreeSync: Authenticated Group Management for
              Messaging Layer Security", 2022,
              <https://eprint.iacr.org/2022/1732.pdf>.

Contributors

   Richard Barnes
   Cisco
   Email: rlb@ipv.sx

   Katriel Cohn-Gordon
   Meta Platforms
   Email: me@katriel.co.uk

   Cas Cremers
   CISPA Helmholtz Center for Information Security
   Email: cremers@cispa.de

   Britta Hale
   Naval Postgraduate School
   Email: britta.hale@nps.edu

   Albert Kwon
   Badge Inc.
   Email: kwonalbert@badgeinc.com

   Konrad Kohbrok
   Phoenix R&D
   Email: konrad.kohbrok@datashrine.de

   Rohan Mahy
   Wire
   Email: rohan.mahy@wire.com

   Brendan McMillion

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   Email: brendanmcmillion@gmail.com

   Thyla van der Merwe
   Email: tjvdmerwe@gmail.com

   Jon Millican
   Meta Platforms
   Email: jmillican@meta.com

   Raphael Robert
   Phoenix R&D
   Email: ietf@raphaelrobert.com

Authors' Addresses

   Benjamin Beurdouche
   Inria & Mozilla
   Email: ietf@beurdouche.com

   Eric Rescorla
   Windy Hill Systems, LLC
   Email: ekr@rtfm.com

   Emad Omara
   Email: emad.omara@gmail.com

   Srinivas Inguva
   Email: singuva@yahoo.com

   Alan Duric
   Wire
   Email: alan@wire.com

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