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Network Working Group                                           E. Omara
Internet-Draft                                                    Google
Intended status: Informational                             B. Beurdouche
Expires: August 6, 2018                                            INRIA
                                                             E. Rescorla
                                                               S. Inguva
                                                                 A. Kwon
                                                                A. Duric
                                                       February 02, 2018

                 Messaging Layer Security Architecture


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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on August 6, 2018.

Copyright Notice

   Copyright (c) 2018 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   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  . . . . . . . .   7
       2.3.4.  Membership knowledge  . . . . . . . . . . . . . . . .   8
       2.3.5.  Membership and offline members  . . . . . . . . . . .   8
   3.  System Requirements . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Functional Requirements . . . . . . . . . . . . . . . . .   8
       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  . . . . . . . . . . . .   9
       3.1.5.  Privacy . . . . . . . . . . . . . . . . . . . . . . .   9
       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)   10
       3.2.2.  Message Secrecy and Authentication  . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     4.1.  Transport Security Links  . . . . . . . . . . . . . . . .  13
     4.2.  Delivery Service Compromise . . . . . . . . . . . . . . .  13
     4.3.  Authentication Service Compromise . . . . . . . . . . . .  13
     4.4.  Client Compromise . . . . . . . . . . . . . . . . . . . .  14
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Informative References  . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   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

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   level of security - with the precise level depending on the system
   design - even when the messaging service they are using performs

   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 [RFC3920].  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 should
   be able to have some degree of interoperability at the message level,
   though they may have incompatible identity/authentication

   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, Group Members
   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 Members 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
      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 in order to proceed with the
      group secret key establishment process.

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

   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
       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

   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

   Clients may wish to do the following:

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

   o  add one or more members 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 should be able to add a
   member to a group.  This is in contrast so 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.  Thus, for instance, while it
   might be technically possible for any member to send a message adding
   a new member 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

   In MLS a Group is defined as a set of Members who possibly use
   multiple endpoint devices (Clients) to interact with the Messaging
   Service.  These Clients will typically correspond to end-user devices
   such as phones, web clients or other devices running MLS.

   Each member device owns a long term identity key pair that uniquely
   defines its identity to other Members of the Group.  Because a single
   Member 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 legitimate knowledge of the shared (Encryption)
   Group Key established in the group key establishment phase of the

   In some messaging systems, Clients belonging to the same Member 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.

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

2.2.  Authentication Service

   The basic function of the Authentication Service is to provide a
   trusted mapping from user identities (usernames, phone numbers,
   etc.), which exist 1:1 with Members, to identity keys, which may
   either be one per Client or may be shared amongst the Clients
   attached to a Member.

   o  A certificate 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:

   o  To act as a directory service providing the keying material
      (authentication keys and 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

   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

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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 from each member that will be used in the establishment
   of the shared group key.  This initial keying material MUST be
   authenticated using the Client's identity key.  Thus, the Client

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

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

   As noted above, Members may have multiple Clients, each with their
   own keying material, and thus there may be multiple entries stored by
   each Member.

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 Member, verifies it using the identity key,
   and then is able to form a joint key with each other Client, and from
   those forms the group key, which it can use for the encryption of

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 group members.

   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 reordering
      between Clients.

   o  Consistent ordering: the DS must ensure that all Clients have the
      same view of message ordering.

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   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 should be able to verify these properties.
   For instance, if the DS reorders messages from a Client or provides
   different Clients with inconsistent orderings, then Clients should be
   able to 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 from the Client being actually

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.

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
   doesn't inherently defend against this problem, but MLS-using systems
   should 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.

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 must provide support for conversations
   involving two or more participants, and aim to scale to approximately

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   50,000 clients, typically including many Members using multiple

3.1.1.  Asynchronous Usage

   No operation in MLS should require two distinct users to be online
   simultaneously.  In particular, clients participating in
   conversations protected using MLS must be able to 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 must 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
   must be able to reinitialize their state and continue participating
   in the conversation.  This may entail some level of message loss, but
   should not result in permanent exclusion from the group.

3.1.3.  Support for Multiple Devices

   It is typically expected for Members of the Group to own different

   A new device can join the group and will 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 is already a Member of the
   Group.  Restoring history is typically not allowed at the protocol
   level but applications may elect to provide such a mechanism outside
   of MLS.

3.1.4.  Extensibility / Pluggability

   Messages that don't 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 must only persist 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.

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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 should be able
   to interoperate and to form federated systems.

3.1.7.  Compatibility with future versions of MLS

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

3.2.  Security Requirements

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 should generally survive
   compromise of the transport layer.

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 doesn't have access to the Group's private

   MLS aims to provide Secrecy, Integrity and Authentication for all

   Message Secrecy in the context of MLS means that only intended
   recipients (current group members), should be able to 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
   should only accept a message if it was sent by a group member and
   that one Client must not be able to send a message which other
   Clients accept as being from another Client.

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   A corollary to this statement is that the AS and the DS can't read
   the content of messages sent between Members as they are not Members
   of the Group.  MLS is expected to optionally provide additional
   protections regarding traffic analysis so as to reduce the ability of
   adversaries, or a compromised member of the messaging system, 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.  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
   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 should 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 secret keys and all shared group keys, but
   Alice performs a key update at time t', then the adversary should be
   unable to violate any of the MLS security properties after time t'.

   Both of these properties must be satisfied even against compromised
   DSs and ASs.  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 users.  Others may wish to
   require authorization from the current group members or a subset

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

   Once a Member is part of a Group, the set of devices controlled by
   the member should 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 should not affect the security
   of messages sent after their removal.  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.  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 to perform a trivial Denial of Service (DoS)
   attack from which it is hard to recover.  Deniability

   As described in Section 4.4, MLS aims to provide data origin
   authentication within a group, such that one group member cannot send
   a message that appears to be from another group member.
   Additionally, it is a requirement of some services 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 should support both of
   these use cases.  In some deployments, these services may be 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

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   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

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.  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 doesn't
   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.

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4.4.  Client Compromise

   In general, MLS only provides limited protection against compromised
   Clients.  When the Client is 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  MLS attempts to provide some
   security in the face of client compromise.

   In addition, a Client should not be able to send a message to a group
   which appears to be from another Client with a different identity.
   Note that if Clients from the same Member share keying material, then
   one will be able to impersonate another.

   Finally, Clients should not be able to perform denial of service
   attacks Section

5.  Contributors

   o  Katriel Cohn-Gordon
      University of Oxford

   o  Cas Cremers
      University of Oxford

   o  Thyla van der Merwe
      Royal Holloway, University of London

   o  Jon Millican

   o  Raphael Robert

6.  Informative References

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-23 (work in progress),
              January 2018.

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              Google, ., "Key Transparency", n.d.,

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, DOI 10.17487/RFC3920,
              October 2004, <https://www.rfc-editor.org/info/rfc3920>.

   [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

   Email: emadomara@google.com

   Benjamin Beurdouche

   Email: benjamin.beurdouche@inria.fr

   Eric Rescorla

   Email: ekr@rtfm.com

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   Srinivas Inguva

   Email: singuva@twitter.com

   Albert Kwon

   Email: kwonal@mit.edu

   Alan Duric

   Email: alan@wire.com

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