The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-03

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                                                      September 13, 2019

            The Messaging Layer Security (MLS) Architecture
                     draft-ietf-mls-architecture-03

Abstract

   This document describes the architecture and requirements for the
   Messaging Layer Security (MLS) protocol.  MLS provides a security
   layer for group messaging applications with from two to a large
   number of clients.  It is meant to protect against eavesdropping,
   tampering, and message forgery.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 16, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  General Setting . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Group, Members and Clients  . . . . . . . . . . . . . . .   5
     2.2.  Authentication Service  . . . . . . . . . . . . . . . . .   6
     2.3.  Delivery Service  . . . . . . . . . . . . . . . . . . . .   6
       2.3.1.  Key Storage . . . . . . . . . . . . . . . . . . . . .   7
       2.3.2.  Key Retrieval . . . . . . . . . . . . . . . . . . . .   7
       2.3.3.  Delivery of messages and attachments  . . . . . . . .   8
       2.3.4.  Membership knowledge  . . . . . . . . . . . . . . . .   8
       2.3.5.  Membership and offline members  . . . . . . . . . . .   9
   3.  System Requirements . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Functional Requirements . . . . . . . . . . . . . . . . .   9
       3.1.1.  Asynchronous Usage  . . . . . . . . . . . . . . . . .   9
       3.1.2.  Recovery After State Loss . . . . . . . . . . . . . .   9
       3.1.3.  Support for Multiple Devices  . . . . . . . . . . . .   9
       3.1.4.  Extensibility / Pluggability  . . . . . . . . . . . .  10
       3.1.5.  Privacy . . . . . . . . . . . . . . . . . . . . . . .  10
       3.1.6.  Federation  . . . . . . . . . . . . . . . . . . . . .  10
       3.1.7.  Compatibility with future versions of MLS . . . . . .  10
     3.2.  Security Requirements . . . . . . . . . . . . . . . . . .  10
       3.2.1.  Connections between Clients and Servers (one-to-one)   11
       3.2.2.  Message Secrecy and Authentication  . . . . . . . . .  11
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     4.1.  Transport Security Links  . . . . . . . . . . . . . . . .  13
     4.2.  Delivery Service Compromise . . . . . . . . . . . . . . .  14
     4.3.  Authentication Service Compromise . . . . . . . . . . . .  14
     4.4.  Client Compromise . . . . . . . . . . . . . . . . . . . .  14
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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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 when the messaging service they are using performs
   unsatisfactorily.

   Messaging Layer Security (MLS) specifies an architecture (this
   document) and an abstract protocol [MLSPROTO] 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 a
   concrete protocol such as XMPP [RFC6120].  In addition, it does not
   specify a complete wire encoding, but rather a set of abstract data
   structures which can then be mapped onto a variety of concrete
   encodings, such as TLS [I-D.ietf-tls-tls13], CBOR [RFC7049], and JSON
   [RFC7159].  Implementations which adopt compatible encodings will
   have some degree of interoperability at the message level, though
   they may have incompatible identity/authentication infrastructures.

   This document is intended to describe the overall messaging system
   architecture which the MLS protocol fits into, and the requirements
   which it is intended to fulfill.

2.  General Setting

   A Group using a Messaging Service (MS) comprises a set of
   participants called Members where each member is typically expected
   to own multiple devices, called Clients.  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, clients
   initially use services at their disposal to obtain the necessary
   secrets and credentials required for security.

   The Messaging Service (MS) presents as two abstract services that
   allow clients to prepare for sending and receiving messages securely:

   o  An Authentication Service (AS) which is responsible for
      maintaining user long term identities, issuing credentials which

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      allow them to authenticate each other, and potentially allowing
      users to discover each others long-term identity keys.

   o  A Delivery Service (DS) which is responsible for receiving and
      redistributing 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
      to a group 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 clients in order to
      proceed with the group secret key establishment process.

         ----------------      --------------
        | Authentication |    | Delivery     |
        | Service (AS)   |    | Service (DS) |
         ----------------      --------------
                            /        |        \             Group
        *********************************************************
        *                 /          |          \               *
        *                /           |           \              *
        *      ----------       ----------       ----------     *
        *     | Client 0 |     | Client 1 |     | Client N |    *
        *      ----------       ----------       ----------     *
        *     .............................     ............    *
        *     User 0                            User 1          *
        *                                                       *
        *********************************************************

   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, hence we show them as being separate here.  Other
   partitions are also possible, such as having a separate directory
   server.

   A typical group messaging scenario might look like this:

   1.  Alice, Bob and Charlie create accounts with a messaging service
       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

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       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.  Their messages
       might trigger a new key derivation step which allows the shared
       group key to be updated to provide post-compromise security
       Section 3.2.2.1.

   Clients may wish to do the following:

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

   o  add one or more clients to an existing group;

   o  remove one or more members from an existing group;

   o  join an existing group;

   o  leave a group;

   o  send a message to everyone in the group;

   o  receive a message from someone in the group.

   At the cryptographic level, clients in groups (and by extension
   Members) are peers.  For instance, any client can add another client
   to 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
   it might be technically possible for any member to send a message
   adding a new client to a group, the group might have the policy that
   only certain members are allowed to make changes and thus other
   members can ignore or reject such a message from an unauthorized
   user.

2.1.  Group, Members and Clients

   Informally, a group is a set of users who possibly use multiple
   endpoint devices to interact with the Messaging Service.  These
   members will typically correspond to end-user devices such as phones,
   web clients or other devices running MLS, which are called clients.

   Each client owns at least one long term identity key pair that
   uniquely defines its identity to other clients or members a the

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   Group.  Because a single user may operate multiple devices
   simultaneously (e.g., a desktop and a phone) or sequentially (e.g.,
   replacing one phone with another), the formal definition of a group
   in MLS is the set of clients that has knowledge of the shared group
   secret established in the group key establishment phase of the
   protocol.  Multiple user devices can be grouped, appearing as one
   virtual client to the rest of the group.

   In some messaging systems, clients belonging to the same user must
   all share the same identity key pair, but MLS does not assume this.
   The MLS architecture considers the more general case and allows for
   important use cases, such as a member adding a new client when all
   their existing clients are offline.

   MLS has been designed to provide similar security guarantees to all
   clients, for all group sizes, even when it reduces to only two
   clients.

2.2.  Authentication Service

   The basic function of the Authentication Service (AS) is to provide a
   trusted mapping from user identities (usernames, phone numbers,
   etc.), to long-term identity keys, which may either be one per client
   or may be shared amongst the clients attached to a user.  It
   typically acts as:

   o  A certification authority, or similar service, which signs some
      sort of portable credential binding an identity to a key;

   o  A directory server which provides the key for a given identity
      (presumably this connection is secured via some form of transport
      security such as TLS).

   By definition, the AS is invested with a large amount of trust.  A
   malicious AS can impersonate - or allow an attacker to impersonate -
   any user of the system.  This risk can be mitigated by publishing the
   binding between identities and keys in a public log such as Key
   Transparency (KT) [KeyTransparency].  It is possible to build a
   functional MLS system without any kind of public key logging, but
   such a system will necessarily be somewhat vulnerable to attack by a
   malicious or untrusted AS.

2.3.  Delivery Service

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

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   o  To act 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.

   o  To route messages between clients and to act as a message
      broadcaster, taking in one message and forwarding it to multiple
      clients (also known as "server side fanout").

   Depending on the level of trust given by the group to the Delivery
   Service, the functional and security guarantees provided by MLS may
   differ.

2.3.1.  Key Storage

   Upon joining the system, each client stores its initial cryptographic
   key material with the DS.  This key material represents the initial
   contribution that will be used in the establishment of the shared
   group secret.  This initial keying material is authenticated using
   the client's identity key.  Thus, the client stores:

   o  A credential from the Authentication service attesting to the
      binding between the user and the client's identity key.

   o  The client's initial keying material signed with the client's
      identity key.

   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 keying material and to ensure that the
   identifier for these keys are unique accross all keys stored on the
   DS.

2.3.2.  Key Retrieval

   When a client wishes to establish a group and send an initial message
   to that group, it contacts the DS and retrieves the initial key
   material for each other client, verifies it using the identity key,
   and from those forms the group secret, which it can use for the
   encryption of messages.

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2.3.3.  Delivery of messages and attachments

   The DS's main responsibility is to ensure delivery of messages.
   Specifically, we assume that DSs provide:

   o  Reliable delivery: when a message is provided to the DS, it is
      eventually delivered to all clients.

   o  In-order delivery: messages are delivered to the group in the
      order they are received from a given client and in approximately
      the order in which they are sent by clients.  The latter is an
      approximate guarantee because multiple clients may send messages
      at the same time and so the DS needs some latitude in enforcing
      ordering across clients.

   o  Consistent ordering: the DS must ensure that all clients have the
      same view of message ordering for cryptographically relevant
      operations.  This means that the DS MUST enforce global
      consistency of the ordering of these messages while MLS provides
      causal consistency of the application messages for each sender.

   Note that the DS may provide ordering guarantees by ensuring in-order
   delivery or by providing messages with some kind of sequence
   information and allowing clients to reorder on receipt.

   The MLS protocol itself can verify these properties.  For instance,
   if the DS reorders messages from a client or provides different
   clients with inconsistent orderings, then clients can detect this
   misconduct.  However, MLS need not provide mechanisms to recover from
   a misbehaving DS.

   Note that some forms of DS misbehavior are still possible and
   difficult to detect.  For instance, a DS 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.

2.3.4.  Membership knowledge

   Group membership is itself sensitive information and MLS is designed
   so that neither the DS nor the AS need have static knowledge of which
   clients are in which group.  However, they may learn this information
   through traffic analysis.  For instance, in a server side fanout
   model, the DS learns 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 MS.

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2.3.5.  Membership and offline members

   Because Forward Secrecy (FS) and Post-Compromise Security (PCS) rely
   on the 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
   does not inherently defend against this problem, but MLS-using
   systems can enforce some mechanism for doing so.  Typically this will
   consist of evicting clients which are idle for too long, thus
   containing the threat of compromise.  The precise details of such
   mechanisms are a matter of local policy and beyond the scope of this
   document.

3.  System Requirements

3.1.  Functional Requirements

   MLS is designed as a large scale group messaging protocol and hence
   aims to provide 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 approximately
   50,000 members, typically including many users using multiple
   devices.

3.1.1.  Asynchronous Usage

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

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

3.1.2.  Recovery After State Loss

   Conversation participants whose local MLS state is lost or corrupted
   can reinitialize their state and continue participating in the
   conversation.  This may entail some level of message loss, but does
   not result in permanent exclusion from the group.

3.1.3.  Support for Multiple Devices

   It is typically expected for users within Group to own different
   devices.

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   A new device can be added to a group by sharing of an existing client
   secrets or 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 of MLS.

3.1.4.  Extensibility / Pluggability

   Messages that do not affect the group state can carry an arbitrary
   payload with the purpose of sharing that payload between group
   members.  No assumptions are made about the format of the payload.

3.1.5.  Privacy

   The protocol is designed in a way that limits the server-side (AS and
   DS) metadata footprint.  The DS only persists data required for the
   delivery of messages and avoid Personally Identifiable Information
   (PII) or other sensitive metadata wherever possible.  A Messaging
   Service provider that has control over both the AS and the DS, will
   not be able to correlate encrypted messages forwarded by the DS, with
   the initial public keys signed by the AS.

3.1.6.  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 wire
   encodings.

3.1.7.  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.

3.2.  Security Requirements

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3.2.1.  Connections between Clients and Servers (one-to-one)

   We assume that all transport connections are secured via some
   transport layer security mechanism such as TLS [I-D.ietf-tls-tls13].
   However, as noted above, the security of MLS will generally survive
   compromise of the transport layer, so long as identity keys provided
   by the AS are authenticated at a minimum.

3.2.2.  Message Secrecy and Authentication

   The trust establishment step of the MLS protocol is followed by a
   conversation protection step where encryption is used by clients to
   transmit authenticated messages to other clients through the DS.
   This ensures that the DS does not have access to the group's private
   content.

   MLS aims to provide secrecy, integrity and authentication for all
   messages.

   Message Secrecy in the context of MLS means that only intended
   recipients (current group members), can read any message sent to the
   group, even in the context of an active adversary as described in the
   threat model.

   Message Integrity and Authentication mean that an honest client can
   only accept a message if it was sent by a group member and that a
   client cannot send a message which other clients would accept as
   being from a different client.

   A corollary to this statement is that the AS and the DS cannot read
   the content of messages sent between members as they are not members
   of the group.  MLS optionally provides additional protections
   regarding traffic analysis so as to reduce the ability of
   adversaries, to deduce the content of the messages depending on (for
   example) their size.  One of these protections includes padding
   messages in order to produce ciphertexts of standard length.  While
   this protection is highly recommended it is not mandatory as it can
   be costly in terms of performance for clients and the MS.

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

3.2.2.1.  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
   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 is compromised at some time T but
   subsequently performs an update at some time T', then all MLS
   guarantees apply to messages sent after time T'.  For example, if an
   adversary learns all secrets known to Alice at time T, including both
   Alice's secrets and all shared group secrets, but Alice performs a
   key update at time T', which is not under the control of the
   adversary, then the adversary is unable to violate any of the MLS
   security properties after time T'.

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

3.2.2.2.  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.  Regardless, MLS does not allow addition or removal
   of group members without informing all other members.

   Once a client is part of a group, the set of devices controlled by
   the user can only be altered by an authorized member of the group.
   This authorization could depend on the application: some applications
   might want to allow certain other members of the group to add or
   remove devices on behalf of another member, while other applications
   might want a more strict policy and allow only the owner of the
   devices to add or remove them at the potential cost of weaker PCS
   guarantees.

   Members who are removed from a group do not enjoy special privileges:
   compromise of a removed group member does not affect the security of
   messages sent after their removal but might affect previous messages
   if the group secrets have not been deleted properly.

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3.2.2.3.  Security of Attachments

   The security properties expected for attachments in the MLS protocol
   are very similar to the ones expected from messages.  The distinction
   between messages and attachments stems from the fact that the typical
   average time between the download of a message and the one from the
   attachments may be different.  For many reasons (a typical reason
   being the lack of high bandwidth network connectivity), the lifetime
   of the cryptographic keys for attachments is usually higher than for
   messages, hence slightly weakening the PCS guarantees for
   attachments.

3.2.2.4.  Denial of Service

   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 DS to perform a trivial DoS attack
   from which it is hard to recover.

3.2.2.5.  Non-Repudiation vs Deniability

   As described in Section 4.4, 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 messaging service
   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 if often called "non-
   repudiation"), but it should also be possible to operate MLS in a
   "deniable" mode where such proof is not possible.  [[OPEN ISSUE:
   Exactly how to supply this is still a protocol question.]]

4.  Security 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 in the face of
   such attackers.  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.

4.1.  Transport Security Links

   [TODO: Mostly DoS, message suppression, and leakage of group
   membership.]

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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 2.3.3,
   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.

4.3.  Authentication Service Compromise

   A compromised AS is a serious matter, as the AS can provide incorrect
   or adversarial identities to clients.  As noted in Section 2.2,
   mitigating this form of attack requires some sort of transparency/
   logging mechanism.  Without such a mechanism, MLS will only provide
   limited security against a compromised AS.

4.4.  Client Compromise

   In general, MLS only provides limited protection against compromised
   clients.  When the client secrets are compromised, then the attacker
   will obviously be able to decrypt any messages for groups in which
   the client is a member.  It will also be able to send messages
   impersonating the compromised client until the client updates its
   keying material (see Section 3.2.2.1).  MLS attempts to provide some
   security in the face of client compromise.

   In addition, a client cannot send a message to a group which appears
   to be from another client with a different identity.  Note that if
   devices from the same user share keying material, then one will be
   able to impersonate another.

   Finally, clients should not be able to perform DoS attacks
   Section 3.2.2.4.

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5.  IANA Considerations

   This document makes no requests of IANA.

6.  Contributors

   o  Katriel Cohn-Gordon
      University of Oxford
      me@katriel.co.uk

   o  Cas Cremers
      University of Oxford
      cas.cremers@cs.ox.ac.uk

   o  Thyla van der Merwe
      Royal Holloway, University of London
      thyla.van.der@merwe.tech

   o  Jon Millican
      Facebook
      jmillican@fb.com

   o  Raphael Robert
      Wire
      raphael@wire.com

7.  Informative References

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

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

   [MLSPROTO]
              Barnes, R., Millican, J., Omara, E., Cohn-Gordon, K., and
              R. Robert, "Messaging Layer Security Protocol", 2018.

   [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/info/rfc3552>.

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

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

Authors' Addresses

   Emad Omara
   Google

   Email: emadomara@google.com

   Benjamin Beurdouche
   INRIA

   Email: benjamin.beurdouche@inria.fr

   Eric Rescorla
   Mozilla

   Email: ekr@rtfm.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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