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The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-08

Document Type Active Internet-Draft (mls WG)
Authors Benjamin Beurdouche , Eric Rescorla , Emad Omara , Srinivas Inguva , Albert Kwon , Alan Duric
Last updated 2022-06-16
Replaces draft-omara-mls-architecture
Stream Internet Engineering Task Force (IETF)
Intended RFC status (None)
Formats
Stream WG state In WG Last Call
Associated WG milestones
May 2018
Initial working group documents for architecture and key management
Sep 2022
Submit architecture document to IESG as Informational
Document shepherd (None)
IESG IESG state I-D Exists
Consensus boilerplate Yes
Telechat date (None)
Responsible AD (None)
Send notices to Katriel Cohn-Gordon <me@katriel.co.uk>, Cas Cremers <cas.cremers@cs.ox.ac.uk>, Thyla van der Merwe< thyla.van.der@merwe.tech>, Jon Millican <jmillican@fb.com>, Raphael Robert <raphael@wire.com>
draft-ietf-mls-architecture-08
Network Working Group                                      B. Beurdouche
Internet-Draft                                           Inria & Mozilla
Intended status: Informational                               E. Rescorla
Expires: 18 December 2022                                        Mozilla
                                                                E. Omara
                                                                  Google
                                                               S. Inguva
                                                                 Twitter
                                                                 A. Kwon
                                                                     MIT
                                                                A. Duric
                                                                    Wire
                                                            16 June 2022

            The Messaging Layer Security (MLS) Architecture
                     draft-ietf-mls-architecture-08

Abstract

   The Messaging Layer Security (MLS) protocol [I-D.ietf-mls-protocol]
   specification has the role of defining a Group Key Agreement
   protocol, including all the cryptographic operations and
   serialization/deserialization functions necessary for scalable and
   secure group messaging.  The MLS protocol is meant to protect against
   eavesdropping, tampering, message forgery, and provide further
   properties such as Forward Secrecy (FS) and Post-Compromise Security
   (PCS) in the case of past or future device compromises.

   This document describes a general secure group messaging
   infrastructure and its security goals.  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 the MLS Protocol document and left to
   the application or the infrastructure architects to design.

   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 case of active adversaries
   that are able to compromise clients, the delivery service, or the
   authentication service.

Discussion Venues

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   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the MLS Working Group
   mailing list (mls@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/mls/.

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

Status of This Memo

   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 18 December 2022.

Copyright Notice

   Copyright (c) 2022 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.  Group Members and Clients . . . . . . . . . . . . . . . .   7
   3.  Authentication Service  . . . . . . . . . . . . . . . . . . .   8
   4.  Delivery Service  . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Key Storage . . . . . . . . . . . . . . . . . . . . . . .  10

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     4.2.  Key Retrieval . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Delivery of Messages  . . . . . . . . . . . . . . . . . .  11
     4.4.  Membership knowledge  . . . . . . . . . . . . . . . . . .  12
     4.5.  Membership and offline members  . . . . . . . . . . . . .  13
   5.  Functional Requirements . . . . . . . . . . . . . . . . . . .  13
     5.1.  Membership Changes  . . . . . . . . . . . . . . . . . . .  13
     5.2.  Parallel Groups . . . . . . . . . . . . . . . . . . . . .  14
     5.3.  Asynchronous Usage  . . . . . . . . . . . . . . . . . . .  15
     5.4.  Access Control  . . . . . . . . . . . . . . . . . . . . .  15
     5.5.  Recovery After State Loss . . . . . . . . . . . . . . . .  16
     5.6.  Support for Multiple Devices  . . . . . . . . . . . . . .  16
     5.7.  Extensibility . . . . . . . . . . . . . . . . . . . . . .  17
     5.8.  Application Data Framing and Negotiation  . . . . . . . .  17
     5.9.  Federation  . . . . . . . . . . . . . . . . . . . . . . .  17
     5.10. Compatibility with Future Versions of MLS . . . . . . . .  18
   6.  Operational Requirements  . . . . . . . . . . . . . . . . . .  18
   7.  Security and Privacy Considerations . . . . . . . . . . . . .  21
     7.1.  Assumptions on Transport Security Links . . . . . . . . .  22
       7.1.1.  Metadata Protection for Unencrypted Group
               Operations  . . . . . . . . . . . . . . . . . . . . .  22
       7.1.2.  DoS protection  . . . . . . . . . . . . . . . . . . .  23
       7.1.3.  Message Suppression and Error Correction  . . . . . .  24
     7.2.  Intended Security Guarantees  . . . . . . . . . . . . . .  24
       7.2.1.  Message Secrecy and Authentication  . . . . . . . . .  25
       7.2.2.  Forward and Post-Compromise Security  . . . . . . . .  25
       7.2.3.  Non-Repudiation vs Deniability  . . . . . . . . . . .  26
     7.3.  Endpoint Compromise . . . . . . . . . . . . . . . . . . .  26
       7.3.1.  Compromise of AEAD key material . . . . . . . . . . .  27
       7.3.2.  Compromise of the Group Secrets of a single group for
               one or more group epochs  . . . . . . . . . . . . . .  28
       7.3.3.  Compromise by an active adversary with the ability to
               sign messages . . . . . . . . . . . . . . . . . . . .  29
       7.3.4.  Compromise of the authentication with access to a
               signature key . . . . . . . . . . . . . . . . . . . .  29
       7.3.5.  Security consideration in the context of a full state
               compromise  . . . . . . . . . . . . . . . . . . . . .  30
     7.4.  Service Node Compromise . . . . . . . . . . . . . . . . .  31
       7.4.1.  General considerations  . . . . . . . . . . . . . . .  31
       7.4.2.  Delivery Service Compromise . . . . . . . . . . . . .  32
       7.4.3.  Authentication Service Compromise . . . . . . . . . .  34
     7.5.  Considerations for attacks outside of the threat model  .  36
     7.6.  Cryptographic Analysis of the MLS Protocol  . . . . . . .  37
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  37
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     10.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  39

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

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 a requirement for instant messaging systems
   and is commonly deployed in many such systems.  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, even when it reduces to only two users.

2.  General Setting

   Informally, a group is a set of users who possibly use multiple
   endpoint devices to interact with the Service Provider (SP).  A group
   may be as small as two members (the simple case of person to person
   messaging) or as large as thousands.

   In order to communicate securely, users initially interact with
   services at their disposal to establish the necessary values and
   credentials required for encryption and authentication.

   The Service Provider presents two abstract functionalities that allow
   clients to prepare for sending and receiving messages securely:

   *  An Authentication Service (AS) functionality which is responsible
      for attesting to bindings between application-meaningful
      identifiers and the public key material used for authentication in

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      the MLS protocol.  This functionality must also be able to
      generate credentials that encode these bindings and validate
      credentials provided by MLS clients.

   *  A Delivery Service (DS) functionality 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.

   For convenience, this document adopts the representation of these
   services being standalone servers, however the MLS protocol design is
   made so that this is not necessarily the case.  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 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
      servers.

   It is important to note that the Authentication Service functionality
   can be completely abstract in the case of a Service Provider which
   allows MLS clients to generate, redistribute and validate their
   credentials themselves.

   Similarly to 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.  Note, though, that the MLS
   protocol requires group operation messages to be processed in-order
   by all MLS clients.

   In some sense, a set of MLS clients which can achieve the AS and DS
   functionalities without relying on an external party do not need a
   Service Provider.

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

   In many systems, the AS and the DS are actually operated by the same
   entity and may even be the same server.  However, they are logically
   distinct and, in other systems, may be operated by different
   entities.  Other partitions are also possible, such as having a
   separate directory functionality or service.

   According to this architecture design, a typical group messaging
   scenario might look like this:

   1.  Alice, Bob and Charlie create accounts with a service provider
       and obtain credentials from the AS.

   2.  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.

   3.  When Alice wants to send a message to Bob and Charlie, she
       contacts the DS and looks up their initial keying material.  She
       uses these keys to establish a new set of keys which she can use
       to send encrypted messages to Bob and Charlie.  She then sends
       the encrypted message(s) to the DS, which forwards them to the
       recipients.

   4.  Bob and/or Charlie respond to Alice's message.  In addition, they
       might choose to update their key material which provides post-
       compromise security Section 7.2.2.  As a consequence of that
       change, the group secrets are updated.

   Clients may wish to do the following:

   *  create a group by inviting a set of other clients;

   *  add one or more clients to an existing group;

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   *  remove one or more members from an existing group;

   *  update their own key material

   *  join an existing group;

   *  leave a group;

   *  send a message to everyone in the group;

   *  receive a message from someone in the group.

   At the cryptographic level, clients (and by extension members in
   groups) have equal permissions.  For instance, any member can add or
   remove another client in a group.  This is in contrast to some
   designs in which there is a single group controller who can modify
   the group.  MLS is compatible with having group administration
   restricted to certain users, but we assume that those restrictions
   are enforced by authentication and access control at the application
   layer.

   Thus, for instance, while the MLS protocol allows for any existing
   member of a group to add a new client, applications which use MLS
   might enforce additional restrictions for which only a subset of
   members can qualify, and thus will handle enforcing group policies
   (such as determining if a user is allowed to add new users to the
   group) at the application level.

2.1.  Group Members and Clients

   While informally, a group can be considered to be a set of users
   possibly using multiple endpoint devices to interact with the Service
   Provider, this definition is too simplistic.

   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.  Ownership of a client by a user is
   determined by the fact that the user has knowledge of the associated
   secret values.  When a client is part of a Group, it is called a
   Member.  In some messaging systems, clients belonging to the same
   user must all share the same signature key pair, but MLS does not
   assume this.

   Users will often use multiple devices, e.g., a phone as well as a
   laptop.  Different devices may be represented as different clients,
   with independent cryptographic state, or they may share cryptographic
   state, relying on some application-provided mechanism to sync across
   devices.

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   The formal definition of a Group in MLS is the set of clients that
   have knowledge of the shared group secret established in the group
   key establishment phase of the protocol and have contributed to it.
   Until a Member 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 Member is a member of the group.

3.  Authentication Service

   The Authentication Service (AS) has to provide two functionalities:

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

   2.  Enable a group member to verify that a credential presented by
       another member is valid

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

   The AS is considered an abstract layer by the MLS specification, 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 others' key fingerprints for authentication.  In this
      scenario, the issuance function is simply the generation of a key
      pair (i.e., credential is just an identifier and public key, with
      no information to assist in verification).  The verification
      function is the application functionality that enables users to
      verify keys.

   *  In a system based on Key Transparency (KT) [KeyTransparency], 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 to their identity.

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   By the nature of its roles in MLS authentication, the AS is invested
   with a large amount of trust and the compromise of one of its
   functionalities could allow an adversary to, among other things,
   impersonate group members.  We discuss security considerations
   regarding the compromise of the different AS functionalities in
   detail in Section 7.4.3.

   The association between members' identities and signature keys is
   fairly flexible in MLS.  As noted above, there is no requirement that
   all clients belonging to a given user use the same key pair (in fact,
   such key reuse is forbidden to ensure clients have independent
   cryptographic state).  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
   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.

4.  Delivery Service

   The Delivery Service (DS) is expected to play multiple roles in the
   Service Provider architecture:

   *  Acting 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
      the other client is offline.

   *  Routing MLS messages among clients.

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   Depending on the level of trust given by the group to the Delivery
   Service, the functional and privacy guarantees provided by MLS may
   differ but the authentication and confidentiality guarantees remain
   the same.

   Unlike the Authentication Service which is trusted for authentication
   and secrecy, the Delivery Service is completely untrusted regarding
   this property.  While privacy of group membership might be a problem
   in the case of a Delivery Service server fanout, the Delivery Service
   can be considered as an active, adaptive network attacker from the
   point of view of the security analysis.

4.1.  Key Storage

   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 material.

   All the parameters in the KeyPackage are signed with the signature
   private key corresponding to the credential.

   As noted above, users may own multiple clients, each with their own
   keying material, and thus there may be multiple entries stored by
   each user.

   The Delivery Service is also responsible for allowing users to add,
   remove or update their initial key material, and for ensuring that
   the identifier for these keys are unique across all keys stored on
   the Delivery Service.

4.2.  Key Retrieval

   When a client wishes to establish a group, it first contacts the
   Delivery Service to request a KeyPackage for each other client,
   authenticates the KeyPackages using the signature keys, and then can
   use those to form the group.

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4.3.  Delivery of Messages

   The main responsibility of the Delivery Service is to ensure delivery
   of messages.  Some MLS messages need only be delivered to some
   members of a group (e.g., the message initializing a new member's
   state), while others need to be delivered to all members.  The
   Delivery Service may enable these delivery patterns 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.  The one requirement is
   that because an MLS group has a linear history, the members of the
   group must agree on the order in which changes are applied.
   Concretely, the group must agree on which MLS Commit messages to
   apply.  There are a variety of ways to achieve this agreement, but
   most of them rely on some help from the Delivery Service.  For
   example, if a Delivery Service provides delivery in the same order to
   all group members, then the members can simply apply Commits in the
   order in which they appear.

   Each Commit is premised on a given state or "epoch" of the group.
   The Delivery Service must transmit to the group exactly one Commit
   message per epoch.

   Much like the Authentication Service, the Delivery Service can be
   split between server and client components.  Achieving the required
   uniqueness property will typically require a combination of client
   and server behaviors.  For example, all of the following examples
   provide a unique Commit per epoch:

   *  A "filtering server" Delivery Service where a server rejects all
      but the first Commit for an epoch and clients apply each Commit
      they receive.

   *  An "ordering server" Delivery Service where a server forwards all
      messages but assures that all clients see Commits in the same
      order, and clients.

   *  A "passive server" Delivery Service where a server forwards all
      messages without ordering or reliability guarantees, and clients
      execute some secondary consensus protocol to choose among the
      Commits received in a window.

   The MLS protocol provides three important pieces of information
   within an MLSCiphertext message in order to provide ordering:

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   *  The Group Identifier (group ID) to allow for distinguishing the
      group for which the message has been sent;

   *  The Epoch number, which represents the number of changes (version)
      of the group associated with a specific group ID, and allows for
      lexicographical ordering of messages from different epochs within
      the same group;

   *  The Content Type of the message, which allows the Delivery Service
      to determine the ordering requirement on the message, in
      particular distinguishing Commit messages from other messages.

   The MLS protocol itself can verify these properties.  For instance,
   if the Delivery Service reorders messages from a client or provides
   different clients with inconsistent orderings, then clients can put
   messages back in their proper order.  The asynchronous nature of MLS
   means that within an epoch, messages are only ordered per-sender, not
   globally.

   Note that some forms of Delivery Service misbehavior are still
   possible and difficult 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
   distinguish this form of Denial of Service (DoS) attack.

4.4.  Membership knowledge

   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.  In
   the case where metadata has to be persisted for functionality, it
   SHOULD be stored encrypted at rest.  Applications should also
   consider anonymous systems for server fanout such as Loopix [Loopix].

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4.5.  Membership and offline members

   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 cannot inherently defend against this problem, especially in the
   case where the client has not processed messages, but MLS-using
   systems can enforce some mechanism to try to retain these properties.
   Typically this will consist of evicting clients which are idle for
   too long, or mandating a key update from clients that are not
   otherwise sending messages.  The precise details of such mechanisms
   are a matter of local policy and beyond the scope of this document.

5.  Functional Requirements

   MLS is designed as a large-scale group messaging protocol and hence
   aims to provide both performance and safety 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.

5.1.  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 be
   represented in a group by multiple clients.  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.

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   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.)

   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.  External joins do
   not allow for more granular authorization checks to be done before
   the new member is added to the group, so if an application wishes to
   both allow external joins and enforce such checks, then the
   application will need to do such checks when a member joins, and
   remove them if checks fail.

   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 sign off on device signature keys, then revocation
   by the owner adds an alternative flag 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.

5.2.  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 internal goals.  This also
   applies to future groups that a member has yet to join, that are
   likewise unaffected by updates performed in current groups.

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

   Applications may use the PSK mechanism 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.

5.3.  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.

5.4.  Access Control

   The MLS protocol allows each member of the messaging group to perform
   operations equally.  This is because all clients within a group
   (members) have access to the shared cryptographic material.  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.

   The MLS protocol can, in certain modes, exchange unencrypted group
   operation messages.  This flexibility is to allow services to perform
   access control tasks on behalf of the group.

   While the Application messages will always be encrypted, having the
   handshake messages in plaintext has inconveniences in terms of
   privacy as someone could collect the signatures on the handshake
   messages and use them for tracking.

      *RECOMMENDATION:* Prefer using encrypted group operation messages
      to avoid privacy issues related to non-encrypted signatures.

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   Note that in the default case of encrypted handshake messages, any
   access control policies will 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.

5.5.  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.

   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.

   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.

5.6.  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.  Restoring history is typically not allowed at the protocol
   level but applications can elect to provide such a mechanism outside

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   of MLS.  Such mechanisms, if used, may reduce the FS and PCS
   guarantees provided by MLS.

5.7.  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.

5.8.  Application Data Framing and Negotiation

   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 negotiate 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
      [I-D.mahy-mls-content-neg], 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.

5.9.  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, and infrastructure
   functionalities.

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5.10.  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.

   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.

6.  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.  A
   required_capabilities extension can help maintain 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.  These network services
   would need to be compatible in order for two different deployments
   based on them to interoperate.

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   *  An *Authentication Service*, described fully in Section 3, defines
      the types of credentials which may be used in a deployment and
      provides methods for:

      1.  Issuing new credentials,

      2.  Validating a credential against a reference identifier, and

      3.  Validating whether or not two credentials represent the same
          user.

   *  A *Delivery Service*, described fully in Section 4, provides
      methods for:

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

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

      3.  Downloading KeyPackages for specific clients, and uploading
          new KeyPackages for a user's own clients.

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

      -  If assisted joining is desired (meaning that the ratchet tree
         is not provided in Welcome messages), there must be a method to
         download the ratchet tree corresponding to a group.

      -  If assisted joining is desired and the Delivery Service is not
         able to compute the ratchet tree itself (because some proposals
         or commits are sent encrypted), there must be a method for
         group members to publish the updated ratchet tree after each
         commit.

      -  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 assisted or 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 external PSKs, or uses resumption PSKs
         that all members of a group may not have access to, there must
         be a method for distributing these PSKs to group members.

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      -  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.

   *  How long to store the resumption secret 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.

   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.

   *  The application_id extension of a LeafNode.

   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 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:

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      -  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.

      -  When external joiners are allowed.

   *  A policy for when two credentials represent the same client.  Note
      that many credentials may be issued authenticating the same
      identity but for different signature keys, because each credential
      corresponds to a different device (client) owned by the same
      application user.  However, one device may control many signature
      keys 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:

   *  If there's any policy on how or when to pad messages.

   *  If there is any policy for when to send a reinitialization
      proposal.

   *  How often clients should update their leaf keys.

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

   *  Whether there should be a required_capabilities extension in
      groups.

7.  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 the face of
   such attackers.

   *  The attacker can monitor the entire network.

   *  The attacker can read unprotected messages.

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   *  The attacker can generate, inject and delete any message in the
      unprotected transport layer.

   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.

   Generally, MLS is designed under the assumption that the transport
   layer is present to protect metadata and privacy in general, while
   the MLS protocol is providing stronger guarantees such as
   confidentiality, integrity and authentication guarantees.  Stronger
   properties such as deniability can also be achieved in specific
   architecture designs.

7.1.  Assumptions on Transport Security Links

   Any secure channel can be used as a transport layer to protect MLS
   messages such as QUIC, TLS, WireGuard or TOR.  However, the MLS
   protocol is designed to consider the following threat-model:

   *  The attacker can read, write, and delete arbitrary messages inside
      the secure transport channel.

   This departs from most threat models where we consider that the
   secure channel used for transport always provides secrecy.  The
   reason for this consideration is that in the group setting, active
   malicious insiders or adversarial services are to be considered.

7.1.1.  Metadata Protection for Unencrypted Group Operations

   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.

   MLS avoids needing to send the full list of recipients to the server
   for dispatching messages because that list is potentially extremely
   large in MLS.  Therefore, the metadata typically consists of a
   pseudo-random Group Identifier (GID), a numerical value to determine
   the epoch of the group (the number of changes that have been made to
   the group), and another numerical value referring to the specific key
   needed to decrypt the ciphertext content.

   The MLS protocol provides an authenticated "Additional Authenticated
   Data" field for applications to make data available outside the
   MLSCiphertext.

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

   Even though some of this metadata information does not consist of
   secret payloads, 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.

   More importantly, there is one specific case where having no secure
   channel to exchange the MLS messages can have a serious impact on
   privacy.  In the case of unencrypted group operation messages,
   observing the signatures of the group operation messages may lead an
   adversary to extract information about the group memberships.

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

7.1.2.  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:* Anonymous credentials can be used in order to
      help DoS attacks prevention, in a privacy preserving manner.  Note
      that the privacy of these mechanisms has to be adjusted in
      accordance with the privacy expected from the secure transport
      links.  (See more discussion further down.)

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7.1.3.  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 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 acknowledgements of application
   messages received within a group.

   As discussed in Section 4, 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
   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.

      *RECOMMENDATION:* If unidirectional transport is used for the
      secure transport channel, prefer using a protocol which provides
      Forward Error Correction.

7.2.  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.

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7.2.1.  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.

7.2.2.  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).

   FS means that access to all encrypted traffic history combined with
   an 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 the
   extremely important role of deleting 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
   t but the group member subsequently performs an update at some time
   t', then all MLS guarantees apply to messages sent by the member
   after time t', and by other members after they have processed the
   update.  For example, if an attacker learns all secrets known to
   Alice at time t, including both Alice's long-term secret keys and all
   shared group keys, but Alice performs a key update at time t', then
   the attacker is unable to violate any of the MLS security properties
   after the updates have been processed.

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   Both of these properties are satisfied even against compromised DSs
   and ASs.

7.2.3.  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.

7.3.  Endpoint Compromise

   The MLS protocol adopts a threat model which includes multiple forms
   of endpoint/client compromise.  While adversaries are in a very
   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 specific symmetric encryption key

   *  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)

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   Recall that the MLS protocol provides chains of AEAD keys, per sender
   that are generated from Group Secrets.  These keys are used to
   protect MLS Plaintext messages which can be Group Operation or
   Application messages.  The Group Operation messages offer an
   additional protection as the secret exchanged within the TreeKEM
   group key agreement are public-key encrypted to subgroups with HPKE.

7.3.1.  Compromise of AEAD key material

   In some circumstances, adversaries may have access to specific AEAD
   keys and nonces which protect an Application or a Group Operation
   message.  While this is a very weak kind of compromise, it can be
   realistic in cases of implementation vulnerabilities where only part
   of the memory leaks to the adversary.

   When an AEAD key is compromised, the adversary has access to a set of
   AEAD keys for the same chain and the same epoch, hence can decrypt
   messages sent using keys of this chain.  An adversary cannot send a
   message to a group which appears to be from any valid client since
   they cannot forge the signature.

   The MLS protocol will ensure that an adversary cannot compute any
   previous AEAD keys for the same epoch, or any other epochs.  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.  However the MLS protocol does not provide Post
   Compromise Secrecy for AEAD encryption within an epoch.  This means
   that if the AEAD key of a chain is compromised, the adversary can
   compute an arbitrary number of subsequent AEAD keys for that chain.

   These guarantees are ensured by the structure of the MLS key schedule
   which provides Forward Secrecy for these AEAD encryptions, across the
   messages within the epoch and also across previous epochs.  Those
   chains are completely disjoint and compromising keys across the
   chains would mean that some Group Secrets have been compromised,
   which is not the case in this attack scenario (we explore stronger
   compromise scenarios as part of the following sections).

   MLS provides Post-Compromise Secrecy 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.

   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

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   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 neither 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 very weak.  Successful decryption of an AEAD
   encrypted message only guarantees that a member of the group sent the
   message.

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

   The attack scenario considering an adversary gaining access to a set
   of Group secrets is significantly stronger.  This can typically be
   the case when a member of the group is compromised.  For this
   scenario, we consider that the signature keys are not compromised.
   This can be the case for instance if the adversary 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 AEAD encryption keys for any AEAD chains and 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 an honest Commit message is sent
   by the compromised party, the next epochs will provide message
   secrecy.

   If the adversary is active, the adversary can follow the protocol and
   perform updates on behalf of the compromised party with no ability to
   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.

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

   Under such a scenario, where an active adversary has compromised an
   MLS client, 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.

   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 an honest update from the compromised client happens.

   Note that under this compromise scenario, the attacker can perform
   all operations which are available to an legitimate client even
   without access to the actual value of the signature key.

   Without access to the group secrets, the adversary will not have the
   ability to generate messages which look valid to other members of the
   group and to the infrastructure as they need to have access to group
   secrets to compute the encryption keys or the membership tag.

7.3.4.  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.

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   Because of the PCS guarantees provided by the MLS protocol, when a
   previously compromised client performs an honest Commit which is not
   under the control of the adversary, both secrecy and authentication
   of messages can be recovered in the case where the attacker didn't
   get access to the key.  Because the adversary doesn't have the key
   and has lost the ability to sign messages, 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 PCS of the MLS protocol will eventually allow recovery
   of the authentication of messages for future epochs but only after
   compromised parties refresh their credentials securely.

   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.

      *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.

7.3.5.  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
      messages and ciphertexts immediately after encryption or
      decryption.

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   Even though, from the strict point of view of the security
   formalization, a ciphertext is always public and will forever be,
   there is no loss in trying to erase ciphertexts as much as possible.

   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 extremely 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 are providing the strong authentication
   guarantees to other clients, hence we consider that their protection
   by additional security mechanism should be a priority.

   Overall there is no way to detect or prevent these compromise, 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 the
   encryption private key used in the TreeKEM Group Key Agreement.

      *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.

7.4.  Service Node Compromise

7.4.1.  General considerations

7.4.1.1.  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.

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   *  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 TOR or a VPN can
      improve metadata protection.

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

7.4.2.  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 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 4.3,
   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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7.4.2.1.  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.

   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 notification 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 and
      that specific steps must be taken to improve privacy, one can
      delay notifications randomly across recipient devices using a
      mixnet or other techniques.

   Note that it is quite easy for legal requests to ask the service
   provider for the push-token associated to an identifier and perform 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 vis-
      a-vis the push notification provider, the client can choose to
      periodically connect to the Delivery Service without the need of a
      dedicated push notification infrastructure.

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7.4.3.  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

   Infrastructures that provide cryptographic material or credentials in
   place of the MLS client (which is under the control of the user) have
   often the ability to use the associated secrets to perform operations
   on behalf of the user, which is unacceptable in many situations.
   Other mechanisms can be used to prevent this issue, such as the
   service blessing cryptographic material used by an MLS client.

      *RECOMMENDATION:* Make clients submit signature public keys to the
      AS, this is usually better than the AS generating public key pairs
      because the AS cannot sign on behalf of the client.  This is a
      benefit of a Public Key Infrastructure in the style of the
      Internet PKI.

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

7.4.3.1.  Authentication compromise: Ghost users and impersonations

   One thing for which the MLS Protocol is designed for is to make sure
   that all clients know who is in the group at all times.  This means
   that - if all Members of the group and the Authentication Service are
   honest - no other parties than the members of the current group can
   read and write messages protected by the protocol for that Group.

   Beware though, the link between the cryptographic identity of the
   Client and the real identity of the User is important.  With some
   Authentication Service designs, a private or centralized authority

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   can be trusted to generate or validate signature keypairs used in the
   MLS protocol.  This is typically the case in some of the biggest
   messaging infrastructures.

   While this service is often very well protected from external
   attackers, it might be the case that this service is compromised.  In
   such infrastructure, the AS could generate or validate a signature
   keypair for an identity which is not the expected one.  Because a
   user can have many MLS clients running the MLS protocol, it possibly
   has many signature keypairs for multiple devices.

   In the case where an adversarial keypair is generated for a specific
   identity, an infrastructure without any transparency mechanism or
   out-of-band authentication mechanism could inject a malicious client
   into a group by impersonating a user.  This is especially the case in
   large groups where the UI might not reflect all the changes back to
   the users.

      *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, a
      public log should be maintained in the state of the device so that
      the user can examine it.

   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.

   Another way to improve the security for the users is to provide a
   transparency mechanism which allows each user to check if credentials
   used in groups have been published in the transparency log.  Another
   benefit of this mechanism is for revocation.  The users of a group
   could check for revoked keys (in case of compromise detection) using
   a mechanism such as CRLite or some more advanced privacy preserving
   technology.

      *RECOMMENDATION:* Provide a Key Transparency and Out-of-Band
      authentication mechanisms to limit the impact of an Authentication
      Service compromise.

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   We note, again, that as described prior to that section, the
   Authentication Service is facultative to design a working
   infrastructure and can be replaced by many mechanisms such as
   establishing prior one-to-one deniable channels, gossiping, or using
   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.

7.4.3.2.  Privacy of the Group Membership

   Often, expectation from users is that the infrastructure will not
   retain the ability to constantly map the user identity to signature
   public keys of the MLS protocol.  Some infrastructures will keep a
   mapping between signature public keys of clients and user identities.
   This can benefit an adversary that has compromised the AS (or
   required access according to regulation) the ability of monitoring
   unencrypted traffic and correlating the messages exchanged within the
   same group.

      *RECOMMENDATION:* Always use encrypted group operation messages to
      reduce issues related to privacy.

   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:* Do not use the same signature keypair across
      groups.

      *RECOMMENDATION:* Separate the service binding the identities and
      the public keys from the service which generates or validates the
      credentials or cryptographic material of the Clients.

7.5.  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 in a software adversary, which
   can maintain active adaptive compromise and arbitrarily change the
   behavior of the client or service.

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   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.

7.6.  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).

   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] and [ACJM20]
   analyze the ratcheting tree as the sub-protocol of MLS that
   facilitates key agreement, while [BCK21] analyzes the key derivation
   paths in the ratchet tree and key schedule.  Finally, [CHK19]
   analyzes the authentication and cross-group healing guarantees
   provided by MLS.

8.  Informative References

   *  ACDT19: https://eprint.iacr.org/2019/1189

   *  ACCKKMPPWY19: https://eprint.iacr.org/2019/1489

   *  ACJM20: https://eprint.iacr.org/2020/752

   *  AJM20: https://eprint.iacr.org/2020/1327

   *  ACDT21: https://eprint.iacr.org/2021/1083

   *  AHKM21: https://eprint.iacr.org/2021/1456

   *  CHK19: https://eprint.iacr.org/2021/137

   *  BCK21: https://eprint.iacr.org/2021/137

   *  BBR18: https://hal.inria.fr/hal-02425247

   *  BBN19: https://hal.laas.fr/INRIA/hal-02425229

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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-14, 3 May 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
              protocol-14>.

10.2.  Informative References

   [I-D.mahy-mls-content-neg]
              Mahy, R., "Content Negotiation for Message Layer Security
              (MLS)", Work in Progress, Internet-Draft, draft-mahy-mls-
              content-neg-00, 31 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-mahy-mls-
              content-neg-00>.

   [KeyTransparency]
              Google, "Key Transparency", 2017,
              <https://KeyTransparency.org>.

   [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>.

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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

   Konrad Kohbrok
   Email: konrad.kohbrok@datashrine.de

   Brendan McMillion
   Email: brendanmcmillion@gmail.com

   Thyla van der Merwe
   Email: tjvdmerwe@gmail.com

   Jon Millican
   Meta Platforms
   Email: jmillican@fb.com

   Raphael Robert
   Email: ietf@raphaelrobert.com

Authors' Addresses

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

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   Eric Rescorla
   Mozilla
   Email: ekr@rtfm.com

   Emad Omara
   Google
   Email: emadomara@google.com

   Srinivas Inguva
   Twitter
   Email: singuva@twitter.com

   Albert Kwon
   MIT
   Email: kwonal@mit.edu

   Alan Duric
   Wire
   Email: alan@wire.com

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