The Messaging Layer Security (MLS) Protocol
draft-barnes-mls-protocol-00

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Network Working Group                                          R. Barnes
Internet-Draft                                                     Cisco
Intended status: Informational                               J. Millican
Expires: August 6, 2018                                         Facebook
                                                                E. Omara
                                                                  Google
                                                          K. Cohn-Gordon
                                                    University of Oxford
                                                               R. Robert
                                                                    Wire
                                                       February 02, 2018

              The Messaging Layer Security (MLS) Protocol
                      draft-barnes-mls-protocol-00

Abstract

   Messaging applications are increasingly making use of end-to-end
   security mechanisms to ensure that messages are only accessible to
   the communicating endpoints, and not to any servers involved in
   delivering messages.  Establishing keys to provide such protections
   is challenging for group chat settings, in which more than two
   participants need to agree on a key but may not be online at the same
   time.  In this document, we specify a key establishment protocol that
   provides efficient asynchronous group key establishment with forward
   secrecy and post-compromise security for groups in size ranging from
   two to thousands.

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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   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 August 6, 2018.

Barnes, et al.           Expires August 6, 2018                 [Page 1]
Internet-Draft                     MLS                     February 2018

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Basic Assumptions . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Binary Trees  . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Merkle Trees  . . . . . . . . . . . . . . . . . . . . . .  11
       5.2.1.  Merkle Proofs . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Ratchet Trees . . . . . . . . . . . . . . . . . . . . . .  12
       5.3.1.  Blank Ratchet Tree Nodes  . . . . . . . . . . . . . .  13
   6.  Group State . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Cryptographic Objects . . . . . . . . . . . . . . . . . .  15
       6.1.1.  Curve25519 with SHA-256 . . . . . . . . . . . . . . .  15
       6.1.2.  P-256 with SHA-256  . . . . . . . . . . . . . . . . .  16
     6.2.  Key Schedule  . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Initialization Keys . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  UserInitKey . . . . . . . . . . . . . . . . . . . . . . .  18
     7.2.  GroupInitKey  . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Handshake Messages  . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  Init  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.2.  GroupAdd  . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.3.  UserAdd . . . . . . . . . . . . . . . . . . . . . . . . .  23
     8.4.  Update  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     8.5.  Delete  . . . . . . . . . . . . . . . . . . . . . . . . .  24
   9.  Sequencing of State Changes . . . . . . . . . . . . . . . . .  25
     9.1.  Server-side enforced ordering . . . . . . . . . . . . . .  26