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Versions: 00 01

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
   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 August 6, 2018.






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Copyright Notice

   Copyright (c) 2018 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 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.  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
     9.2.  Client-side enforced ordering . . . . . . . . . . . . . .  26
   10. Message Protection  . . . . . . . . . . . . . . . . . . . . .  26
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  27
     11.1.  Confidentiality of the Group Secrets . . . . . . . . . .  28
     11.2.  Authentication . . . . . . . . . . . . . . . . . . . . .  28



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     11.3.  Forward and post-compromise security . . . . . . . . . .  28
     11.4.  Init Key Reuse . . . . . . . . . . . . . . . . . . . . .  29
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  29
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     14.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   DISCLAIMER: This is a work-in-progress draft of MLS and has not yet
   seen significant security analysis.  It should not be used as a basis
   for building production systems.

   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/ekr/mls-protocol.
   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.

   Groups of agents who want to send each other encrypted messages need
   a way to derive shared symmetric encryption keys.  For two parties,
   this problem has been studied thoroughly, with the Double Ratchet
   emerging as a common solution [doubleratchet] [signal].  Channels
   implementing the Double Ratchet enjoy fine-grained forward secrecy as
   well as post-compromise security, but are nonetheless efficient
   enough for heavy use over low-bandwidth networks.

   For groups of size greater than two, a common strategy is to
   unilaterally broadcast symmetric "sender" keys over existing shared
   symmetric channels, and then for each agent to send messages to the
   group encrypted with their own sender key.  Unfortunately, while this
   improves efficiency over pairwise broadcast of individual messages
   and (with the addition of a hash ratchet) provides forward secrecy,
   it is difficult to achieve post-compromise security with sender keys.
   An adversary who learns a sender key can often indefinitely and
   passively eavesdrop on that sender's messages.  Generating and
   distributing a new sender key provides a form of post-compromise
   security with regard to that sender.  However, it requires
   computation and communications resources that scale linearly as the
   size of the group.

   In this document, we describe a protocol based on tree structures
   that enable asynchronous group keying with forward secrecy and post-
   compromise security.  The use of "asynchronous ratcheting trees"
   [art] allows the members of the group to derive and update shared



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   keys with costs that scale as the log of the group size.  The use of
   Merkle trees to store identity information allows strong
   authentication of group membership, again with logarithmic cost.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   [TODO: The architecture document uses "Client" instead of
   "Participant".  Harmonize terminology.]

   Participant:  An agent that uses this protocol to establish shared
      cryptographic state with other participants.  A participant is
      defined by the cryptographic keys it holds.  An application may
      use one participant per device (keeping keys local to each device)
      or sync keys among a user's devices so that each user appears as a
      single participant.

   Group:  A collection of participants with shared cryptographic state.

   Member:  A participant that is included in the shared state of a
      group, and has access to the group's secrets.

   Initialization Key:  A short-lived Diffie-Hellman key pair used to
      introduce a new member to a group.  Initialization keys can be
      published for both individual participants (UserInitKey) and
      groups (GroupInitKey).

   Leaf Key:  A short-lived Diffie-Hellman key pair that represents a
      group member's contribution to the group secret, so called because
      the participants leaf keys are the leaves in the group's ratchet
      tree.

   Identity Key:  A long-lived signing key pair used to authenticate the
      sender of a message.

   Terminology specific to tree computations is described in Section 5.

   We use the TLS presentation language [I-D.ietf-tls-tls13] to describe
   the structure of protocol messages.

3.  Basic Assumptions

   This protocol is designed to execute in the context of a Messaging
   Service (MS) as described in [I-D.rescorla-mls-architecture].  In
   particular, we assume the MS provides the following services:



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   o  A long-term identity key provider which allows participants to
      authenticate protocol messages in a group.  These keys MUST be
      kept for the lifetime of the group as there is no mechanism in the
      protocol for changing a participant's identity key.

   o  A broadcast channel, for each group, which will relay a message to
      all members of a group.  For the most part, we assume that this
      channel delivers messages in the same order to all participants.
      (See Section 9 for further considerations.)

   o  A directory to which participants can publish initialization keys,
      and from which participant can download initialization keys for
      other participants.

4.  Protocol Overview

   The goal of this protocol is to allow a group of participants to
   exchange confidential and authenticated messages.  It does so by
   deriving a sequence of keys known only to group members.  Keys should
   be secret against an active network adversary and should have both
   forward and post-compromise secrecy with respect to compromise of a
   participant.

   We describe the information stored by each participant as a _state_,
   which includes both public and private data.  An initial state,
   including an initial set of participants, is set up by a group
   creator using the _Init_ algorithm and based on information pre-
   published by the initial members.  The creator sends the _GroupInit_
   message to the participants, who can then set up their own group
   state and derive the same shared key.  Participants then exchange
   messages to produce new shared states which are causally linked to
   their predecessors, forming a logical Directed Acyclic Graph (DAG) of
   states.  Participants can send _Update_ messages for post-compromise
   secrecy and new participants can be added or existing participants
   removed from the group.

   The protocol algorithms we specify here follow.  Each algorithm
   specifies both (i) how a participant performs the operation and (ii)
   how other participants update their state based on it.

   There are four major operations in the lifecycle of a group:

   o  Adding a member, initiated by a current member

   o  Adding a member, initiated by the new member

   o  Key update




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   o  Removal of a member

   Before the initialization of a group, participants publish
   UserInitKey objects to a directory provided to the Messaging Service.

                                                             Group
   A              B              C          Directory       Channel
   |              |              |              |              |
   | UserInitKeyA |              |              |              |
   |------------------------------------------->|              |
   |              |              |              |              |
   |              | UserInitKeyB |              |              |
   |              |---------------------------->|              |
   |              |              |              |              |
   |              |              | UserInitKeyC |              |
   |              |              |------------->|              |
   |              |              |              |              |

   When a participant A wants to establish a group with B and C, it
   first downloads InitKeys for B and C.  It then initializes a group
   state containing only itself and uses the InitKeys to compute
   GroupAdd messages to add B and C, in a sequence chosen by A.  These
   messages are broadcasted to the Group, and processed in sequence by B
   and C.  Messages received before a participant has joined the group
   are ignored.  Only after A has received its GroupAdd messages back
   from the server does it update its state to reflect their addition.

























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                                                                  Group
   A              B              C          Directory            Channel
   |              |              |              |                   |
   |         UserInitKeyB, UserInitKeyC         |                   |
   |<-------------------------------------------|                   |
   |              |              |              |                   |
   |              |              |              | GroupAdd(A->AB)   |
   |--------------------------------------------------------------->|
   |              |              |              |                   |
   |              |              |              | GroupAdd(AB->ABC) |
   |--------------------------------------------------------------->|
   |              |              |              |                   |
   |              |              |              | GroupAdd(A->AB)   |
   |<---------------------------------------------------------------|
   |state.add(B)  |<------------------------------------------------|
   |              |state.init()  |x---------------------------------|
   |              |              |              |                   |
   |              |              |              | GroupAdd(AB->ABC) |
   |<---------------------------------------------------------------|
   |state.add(C)  |<------------------------------------------------|
   |              |state.add(C)  |<---------------------------------|
   |              |              |state.init()  |                   |
   |              |              |              |                   |

   Subsequent additions of group members proceed in the same way.  Any
   member of the group can download an InitKey for a new participant and
   broadcast a GroupAdd which the current group can use to update their
   state and the new participant can use to initialize its state.

   It is sometimes necessary for a new participant to join without an
   explicit invitation from a current member.  For example, if a user
   that is authorized to be in the group logs in on a new device, that
   device will need to join the group as a new participant, but will not
   have been invited.

   In these "user-initiated join" cases, the "InitKey + Add message"
   flow is reversed.  We assume that at some previous point, a group
   member has published a GroupInitKey reflecting the current state of
   the group (A, B, C).  The new participant Z downloads that
   GroupInitKey from the directory, generates a UserAdd message, and
   broadcasts it to the group.  Once current members process this
   message, they will have a shared state that also includes Z.









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                                                             Group
   A              B     ...      Z          Directory       Channel
   | GroupInitKey |              |              |              |
   |------------------------------------------->|              |
   |              |              |              |              |
   ~              ~              ~              ~              ~
   |              |              |              |              |
   |              |              | GroupInitKey |              |
   |              |              |<-------------|              |
   |              |              |              |              |
   |              |              | UserAdd(.->D)|              |
   |              |              |---------------------------->|
   |              |              |              |              |
   |              |              |              | UserAdd(.->D)|
   |<----------------------------------------------------------|
   |state.add(D)  |<-------------------------------------------|
   |              |state.add(D)  |<----------------------------|
   |              |              |state.init()  |              |
   |              |              |              |              |

   To enforce forward secrecy and post-compromise security of messages,
   each participant periodically updates its leaf key, the DH key pair
   that represents its contribution to the group key.  Any member of the
   group can send an Update at any time by generating a fresh leaf key
   pair and sending an Update message that describes how to update the
   group key with that new key pair.  Once all participants have
   processed this message, the group's secrets will be unknown to an
   attacker that had compromised the sender's prior DH leaf private key.

   It is left to the application to determine the interval of time
   between Update messages.  This policy could require a change for each
   message, or it could require sending an update every week or more.

                                                             Group
   A              B     ...      Z          Directory        Channel
   |              |              |              |              |
   | Update(A)    |              |              |              |
   |---------------------------------------------------------->|
   |              |              |              |              |
   |              |              |              | Update(A)    |
   |<----------------------------------------------------------|
   |state.upd(D)  |<-------------------------------------------|
   |              |state.upd(D)  |<----------------------------|
   |              |              |state.upd(A)  |              |
   |              |              |              |              |

   Users are deleted from the group in a similar way, as a key update is
   effectively removing the old leaf from the group.  Any member of the



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   group can generate a Delete message that adds new entropy to the
   group state that is known to all members except the deleted member.
   After other participants have processed this message, the group's
   secrets will be unknown to the deleted participant.  Note that this
   does not necessarily imply that any member is actually allowed to
   evict other members; groups can layer authentication-based access
   control policies on top of these basic mechanism.

                                                             Group
   A              B     ...      Z          Directory       Channel
   |              |              |              |              |
   |              |              | Delete(B)    |              |
   |              |              |---------------------------->|
   |              |              |              |              |
   |              |              |              | Delete(B)    |
   |<----------------------------------------------------------|
   |state.del(B)  |              |<----------------------------|
   |              |              |state.del(B)  |              |
   |              |              |              |              |
   |              |              |              |              |

5.  Binary Trees

   The protocol uses two types of binary tree structures:

   o  Merkle trees for efficiently committing to a set of group
      participants.

   o  Asynchronous ratcheting trees for deriving shared secrets among
      this group of participants.

   The two trees in the protocol share a common structure, allowing us
   to maintain a direct mapping between their nodes when manipulating
   group membership.  The "nth" leaf in each tree is owned by the "nth"
   group participant.

5.1.  Terminology

   We use a common set of terminology to refer to both types of binary
   tree.

   Trees consist of various different types of _nodes_. A node is a
   _leaf_ if it has no children, and a _parent_ otherwise; note that all
   parents in our Merkle or asynchronous ratcheting trees have precisely
   two children, a _left_ child and a _right_ child.  A node is the
   _root_ of a tree if it has no parents, and _intermediate_ if it has
   both children and parents.  The _descendants_ of a node are that
   node, its children, and the descendants of its children, and we say a



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   tree _contains_ a node if that node is a descendant of the root of
   the tree.  Nodes are _siblings_ if they share the same parent.

   A _subtree_ of a tree is the tree given by the descendants of any
   node, the _head_ of the subtree The _size_ of a tree or subtree is
   the number of leaf nodes it contains.  For a given parent node, its
   _left subtree_ is the subtree with its left child as head
   (respectively _right subtree_).

   All trees used in this protocol are left-balanced binary trees.  A
   binary tree is _full_ (and _balanced_) if it its size is a power of
   two and for any parent node in the tree, its left and right subtrees
   have the same size.  If a subtree is full and it is not a subset of
   any other full subtree, then it is _maximal_.

   A binary tree is _left-balanced_ if for every parent, either the
   parent is balanced, or the left subtree of that parent is the largest
   full subtree that could be constructed from the leaves present in the
   parent's own subtree.  Note that given a list of "n" items, there is
   a unique left-balanced binary tree structure with these elements as
   leaves.  In such a left-balanced tree, the "k-th" leaf node refers to
   the "k-th" leaf node in the tree when counting from the left,
   starting from 0.

   The _direct path_ of a root is the empty list, and of any other node
   is the concatenation of that node with the direct path of its parent.
   The _copath_ of a node is the list of siblings of nodes in its direct
   path, excluding the root, which has no sibling.  The _frontier_ of a
   tree is the list of heads of the maximal full subtrees of the tree,
   ordered from left to right.

   For example, in the below tree:

   o  The direct path of C is (C, CD, ABCD)

   o  The copath of C is (D, AB, EFG)

   o  The frontier of the tree is (ABCD, EF, G)













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               ABCDEFG
              /      \
             /        \
            /          \
        ABCD            EFG
       /    \          /   \
      /      \        /     \
     AB      CD      EF      \
    /  \    /  \    /  \      \
   A    B  C    D  E    F     G

   We extend both types of tree to include a concept of "blank" nodes;
   which are used to replace group members who have been removed.  We
   expand on how these are used and implemented in the sections below.

   (Note that left-balanced binary trees are the same structure that is
   used for the Merkle trees in the Certificate Transparency protocol
   [I-D.ietf-trans-rfc6962-bis].)

5.2.  Merkle Trees

   Merkle trees are used to efficiently commit to a collection of group
   members.  We require a hash function, denoted H, to construct this
   tree.

   Each node in a Merkle tree is the output of the hash function,
   computed as follows:

   o  Leaf nodes: "H( 0x01 || leaf-value )"

   o  Parent nodes: "H( 0x02 || left-value || right-value)"

   o  Blank leaf nodes: "H( 0x00 )"

   The below tree provides an example of a size 2 tree, containing
   identity keys "A" and "B".

                * H(2 || H(1 || A) || H(1 || B))
               / \
              /   \
   H(1 || A) *     * H(1 || B)

   In Merkle trees, blank nodes appear only at the leaves.  In
   computation of intermediate nodes, they are treated in the same way
   as other nodes.






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5.2.1.  Merkle Proofs

   A proof of a given leaf being a member of the Merkle tree consists of
   the value of the leaf node, as well as the values of each node in its
   copath.  From these values, its path to the root can be verified;
   proving the inclusion of the leaf in the Merkle tree.

   In the below tree, we denote with a star the Merkle proof of
   membership for leaf node "A".  For brevity, we notate "Hash(0x02 ||
   A || B)" as "AB".

         ABCD
       /      \
     AB        CD*
    /  \      /  \
   A   B*    C    D

5.3.  Ratchet Trees

   Ratchet trees are used for generating shared group secrets.  These
   are constructed as a series of Diffie-Hellman keys in a binary tree
   arrangement, with each user knowing their direct path, and thus being
   able to compute the shared root secret.

   To construct these trees, we require:

   o  a Diffie-Hellman finite-field group or elliptic curve;

   o  a Derive-Key-Pair function that produces a key pair from an octet
      string, such as the output of a DH computation

   Each node in a ratchet tree contains up to three values:

   o  A secret octet string (optional)

   o  A DH private key (optional)

   o  A DH public key

   To compute the private values (secret and private key) for a given
   node, one must first know the private key from one of its children,
   and the public key from the other child.  Then the value of the
   parent is computed as follows:

   o  secret = DH(L, R)

   o  private, public = Derive-Key-Pair(secret)




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   Ratchet trees are constructed as left-balanced trees, defined such
   that each parent node's key pair is derived from the Diffie-Hellman
   shared secret of its two child nodes.  To compute the root secret and
   private key, a participant must know the public keys of nodes in its
   copath, as well as its own leaf private key.

   For example, the ratchet tree consisting of the private keys (A, B,
   C, D) is constructed as follows:

   DH(DH(AB), DH(CD))
       /      \
    DH(AB)    DH(CD)
    /  \      /  \
   A    B    C    D

   Ratchet trees constructed this way provide the property that one must
   hold at least one private key from the tree to compute the secret
   root key.  With all participants holding one leaf private key; this
   allows any individual to update their own key and change the shared
   root key, such that only group members can compute the new key.

5.3.1.  Blank Ratchet Tree Nodes

   Nodes in a ratchet tree can have a special value "_", used to
   indicate that the node should be ignored during path computations.
   Such nodes are used to replace leaves when participants are deleted
   from the group.

   If any node in the copath of a leaf is _, it should be ignored during
   the computation of the path.  For example, the tree consisting of the
   private keys (A, _, C, D) is constructed as follows:

     DH(A, DH(CD))
      /      \
     A       DH(CD)
    / \      /  \
   A   _    C    D

   If two sibling nodes are both _, their parent value also becomes _.

   Blank nodes effectively result in an unbalanced tree, but allow the
   tree management to behave as for a balanced tree for programming
   simplicity.








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6.  Group State

   The state of an MLS group at a given time comprises:

   o  A group identifier (GID)

   o  A ciphersuite used for cryptographic computations

   o  A Merkle tree over the participants' identity keys

   o  A ratchet tree over the participants' leaf key pairs

   o  A message master secret (known only to participants)

   o  An add key pair (private key known only to participants)

   o  An init secret (known only to participants)

   Since a group can evolve over time, a session logically comprises a
   sequence of states.  The time in which each individual state is used
   is called an "epoch", and each state is assigned an epoch number that
   increments when the state changes.

   MLS handshake messages provide each node with enough information
   about the trees to authenticate messages within the group and compute
   the group secrets.

   Thus, each participant will need to store the following information
   about each state of the group:

   1.   The participant's index in the identity/ratchet trees

   2.   The private key associated with the participant's leaf public
        key

   3.   The private key associated with the participant's identity
        public key

   4.   The current epoch number

   5.   The group identifier (GID)

   6.   A subset of the identity tree comprising at least the copath for
        the participant's leaf

   7.   A subset of the ratchet tree comprising at least the copath for
        the participant's leaf




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   8.   The current message encryption shared secret, called the master
        secret

   9.   The current add key pair

   10.  The current init secret

6.1.  Cryptographic Objects

   Each MLS session uses a single ciphersuite that specifies the
   following primitives to be used in group key computations:

   o  A hash function

   o  A Diffie-Hellman finite-field group or elliptic curve

   The ciphersuite must also specify an algorithm "Derive-Key-Pair" that
   maps octet strings with the same length as the output of the hash
   function to key pairs for the Diffie-Hellman group.

   Public keys and Merkle tree nodes used in the protocol are opaque
   values in a format defined by the ciphersuite, using the following
   four types:

   uint16 CipherSuite;
   opaque DHPublicKey<1..2^16-1>;
   opaque SignaturePublicKey<1..2^16-1>;
   opaque MerkleNode<1..255>

   [[OPEN ISSUE: In some cases we will want to include a raw key when we
   sign and in others we may want to include an identity or a
   certificate containing the key.  This type needs to be extended to
   accommodate that.]]

6.1.1.  Curve25519 with SHA-256

   This ciphersuite uses the following primitives:

   o  Hash function: SHA-256

   o  Diffie-Hellman group: Curve25519 [RFC7748]

   Given an octet string X, the private key produced by the Derive-Key-
   Pair operation is SHA-256(X).  (Recall that any 32-octet string is a
   valid Curve25519 private key.)  The corresponding public key is
   X25519(SHA-256(X), 9).





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   Implementations SHOULD use the approach specified in [RFC7748] to
   calculate the Diffie-Hellman shared secret.  Implementations MUST
   check whether the computed Diffie-Hellman shared secret is the all-
   zero value and abort if so, as described in Section 6 of [RFC7748].
   If implementers use an alternative implementation of these elliptic
   curves, they SHOULD perform the additional checks specified in
   Section 7 of {{RFC7748]}

6.1.2.  P-256 with SHA-256

   This ciphersuite uses the following primitives:

   o  Hash function: SHA-256

   o  Diffie-Hellman group: secp256r1 (NIST P-256)

   Given an octet string X, the private key produced by the Derive-Key-
   Pair operation is SHA-256(X), interpreted as a big-endian integer.
   The corresponding public key is the result of multiplying the
   standard P-256 base point by this integer.

   P-256 ECDH calculations (including parameter and key generation as
   well as the shared secret calculation) are performed according to
   [IEEE1363] using the ECKAS-DH1 scheme with the identity map as key
   derivation function (KDF), so that the shared secret is the
   x-coordinate of the ECDH shared secret elliptic curve point
   represented as an octet string.  Note that this octet string (Z in
   IEEE 1363 terminology) as output by FE2OSP, the Field Element to
   Octet String Conversion Primitive, has constant length for any given
   field; leading zeros found in this octet string MUST NOT be
   truncated.

   (Note that this use of the identity KDF is a technicality.  The
   complete picture is that ECDH is employed with a non-trivial KDF
   because MLS does not directly use this secret for anything other than
   for computing other secrets.)

   Clients MUST validate remote public values by ensuring that the point
   is a valid point on the elliptic curve.  The appropriate validation
   procedures are defined in Section 4.3.7 of [X962] and alternatively
   in Section 5.6.2.3 of [keyagreement].  This process consists of three
   steps: (1) verify that the value is not the point at infinity (O),
   (2) verify that for Y = (x, y) both integers are in the correct
   interval, (3) ensure that (x, y) is a correct solution to the
   elliptic curve equation.  For these curves, implementers do not need
   to verify membership in the correct subgroup.





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6.2.  Key Schedule

   Group keys are derived using the HKDF-Extract and HKDF-Expand
   functions as defined in [RFC5869], as well as the functions defined
   below:

   Derive-Secret(Secret, Label, ID, Epoch, Msg) =
        HKDF-Expand(Secret, HkdfLabel, Length)

   Where HkdfLabel is specified as:

   struct {
       uint16 length = Length;
       opaque label<7..255> = "mls10 " + Label;
       opaque group_id<0..2^16-1> = ID;
       uint32 epoch = Epoch;
       opaque message<1..2^16-1> = Msg
   } HkdfLabel;

   The Hash function used by HKDF is the ciphersuite hash algorithm.
   Hash.length is its output length in bytes.  In the below diagram:

   o  HKDF-Extract takes its Salt argument form the top and its IKM
      argument from the left

   o  Derive-Secret takes its Secret argument from the incoming arrow

   When processing a handshake message, a participant combines the
   following information to derive new epoch secrets:

   o  The init secret from the previous epoch

   o  The update secret for the current epoch

   o  The handshake message that caused the epoch change

   o  The current group identifier (GID) and epoch

   The derivation of the update secret depends on the change being made,
   as described below.

   For UserAdd or GroupAdd, the new user does not know the prior epoch
   init secret.  Instead, entropy from the prior epoch is added via the
   update secret, and an all-zero vector with the same length as a hash
   output is used in the place of the init secret.

   Given these inputs, the derivation of secrets for an epoch proceeds
   as shown in the following diagram:



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                  Init Secret [n-1] (or 0)
                        |
                        V
   Update Secret -> HKDF-Extract = Epoch Secret
                        |
                        |
                        +--> Derive-Secret(., "msg", ID, Epoch, Msg)
                        |       = message_master_secret
                        |
                        +--> Derive-Secret(., "add", ID, Epoch, Msg)
                        |       |
                        |       V
                        |    Derive-Key-Pair(.) = Add Key Pair
                        |
                        V
                  Derive-Secret(., "init", ID, Epoch, Msg)
                        |
                        V
                  Init Secret [n]

7.  Initialization Keys

   In order to facilitate asynchronous addition of participants to a
   group, it is possible to pre-publish initialization keys that provide
   some public information about a user or group.  UserInitKey messages
   provide information about a potential group member, that a group
   member can use to add this user to a group without asynchronously.
   GroupInitKey messages provide information about a group that a new
   user can use to join the group without any of the existing members of
   the group being online.

7.1.  UserInitKey

   A UserInitKey object specifies what ciphersuites a client supports,
   as well as providing public keys that the client can use for key
   derivation and signing.  The client's identity key is intended to be
   stable throughout the lifetime of the group; there is no mechanism to
   change it.  Init keys are intended to be used a very limited number
   of times, potentially once. (see Section 11.4).

   The init_keys array MUST have the same length as the cipher_suites
   array, and each entry in the init_keys array MUST be a public key for
   the DH group defined by the corresponding entry in the cipher_suites
   array.

   The whole structure is signed using the client's identity key.  A
   UserInitKey object with an invalid signature field MUST be considered




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   malformed.  The input to the signature computation comprises all of
   the fields except for the signature field.

   struct {
       CipherSuite cipher_suites<0..255>;
       DHPublicKey init_keys<1..2^16-1>;
       SignaturePublicKey identity_key;
       SignatureScheme algorithm;
       opaque signature<0..2^16-1>;
   } UserInitKey;

7.2.  GroupInitKey

   A GroupInitKey object specifies the aspects of a group's state that a
   new member needs to initialize its state (together with an identity
   key and a fresh leaf key pair).

   o  The current epoch number

   o  The number of participants currently in the group

   o  The group ID

   o  The cipher suite used by the group

   o  The public key of the current update key pair for the group

   o  The frontier of the identity tree, as a sequence of hash values

   o  The frontier of the ratchet tree, as a sequence of public keys

   GroupInitKey messages are not themselves signed.  A GroupInitKey
   should not be published "bare"; instead, it should be published by
   constructing a handshake message with type "none", which will include
   a signature by a member of the group and a proof of membership in the
   group.

   struct {
       uint32 epoch;
       uint32 group_size;
       opaque group_id<0..2^16-1>;
       CipherSuite cipher_suite;
       DHPublicKey add_key;
       MerkleNode identity_frontier<0..2^16-1>;
       DHPublicKey ratchet_frontier<0..2^16-1>;
   } GroupInitKey;





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8.  Handshake Messages

   Over the lifetime of a group, its state will change for:

   o  Group initialization

   o  A current member adding a new participant

   o  A new participant adding themselves

   o  A current participant updating its leaf key

   o  A current member deleting another current member

   In MLS, these changes are accomplished by broadcasting "handshake"
   messages to the group.  Note that unlike TLS and DTLS, there is not a
   consolidated handshake phase to the protocol.  Rather, handshake
   messages are exchanged throughout the lifetime of a group, whenever a
   change is made to the group state.

   An MLS handshake message encapsulates a specific message that
   accomplishes a change to the group state.  It also includes two other
   important features:

   o  A GroupInitKey so that a new participant can observe the latest
      state of the handshake and initialize itself

   o  A signature by a member of the group, together with a Merkle
      inclusion proof that demonstrates that the signer is a legitimate
      member of the group.

   Before considering a handshake message valid, the recipient MUST
   verify both that the signature is valid, the Merkle inclusion proof
   is valid, and the sender is authorized to make the change according
   to group policy.  The input to the signature computations comprises
   the entire handshake message except for the signature field.

   The Merkle tree head to be used for validating the inclusion proof
   MUST be one that the recipient trusts to represent the current list
   of participant identity keys.











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   enum {
       none(0),
       init(1),
       user_add(2),
       group_add(3),
       update(4),
       delete(5),
       (255)
   } HandshakeType;

   struct {
       HandshakeType msg_type;
       uint24 inner_length;
       select (Handshake.msg_type) {
           case none:      struct{};
           case init:      Init;
           case user_add:  UserAdd;
           case group_add: GroupAdd;
           case update:    Update;
           case delete:    Delete;
       };

       uint32 prior_epoch;
       GroupInitKey init_key;

       uint32 signer_index;
       MerkleNode identity_proof<1..2^16-1>;
       SignaturePublicKey identity_key;

       SignatureScheme algorithm;
       opaque signature<1..2^16-1>;
   } Handshake;

   [[ OPEN ISSUE: There will be a need to integrate credentials from an
   authentication service that associate identities to the identity keys
   used to sign messages.  This integration will enable meaningful
   authentication (of identities, rather than keys), and will need to be
   done in such a way as to prevent unknown key share attacks. ]]

   [[ OPEN ISSUE: The GroupAdd and Delete operations create a "double-
   join" situation, where a participants leaf key is also known to
   another participant.  When a participant A is double-joined to
   another B, deleting A will not remove them from the conversation,
   since they will still hold the leaf key for B.  These situations are
   resolved by updates, but since operations are asynchronous and
   participants may be offline for a long time, the group will need to
   be able to maintain security in the presence of double-joins. ]]




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   [[ OPEN ISSUE: It is not possible for the recipient of a handshake
   message to verify that ratchet tree information in the message is
   accurate, because each node can only compute the secret and private
   key for nodes in its direct path.  This creates the possibility that
   a malicious participant could cause a denial of service by sending a
   handshake message with invalid values for public keys in the ratchet
   tree. ]]

8.1.  Init

   [[ OPEN ISSUE: Direct initialization is currently undefined.  A
   participant can create a group by initializing its own state to
   reflect a group including only itself, then adding the initial
   participants.  This has computation and communication complexity O(N
   log N) instead of the O(N) complexity of direct initialization. ]]

8.2.  GroupAdd

   A GroupAdd message is sent by a group member to add a new participant
   to the group.  The content of the message is only the UserInitKey for
   the user being added.

   struct {
       UserInitKey init_key;
   } GroupAdd;

   A group member generates such a message by requesting from the
   directory a UserInitKey for the user to be added.  The new
   participant processes the message together with the private key
   corresponding to the UserInitKey to initialize his state as follows:

   o  Compute the participant's leaf key pair by combining the init key
      in the UserInitKey with the prior epoch's add key pair

   o  Use the frontiers in the GroupInitKey of the Handshake message to
      add its keys to the trees

   An existing participant receiving a GroupAdd message first verifies
   the signature on the message, then verifies its identity proof
   against the identity tree held by the participant.  The participant
   then updates its state as follows:

   o  Compute the new participant's leaf key pair by combining the leaf
      key in the UserInitKey with the prior epoch add key pair

   o  Update the group's identity tree and ratchet tree with the new
      participant's information




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   The update secret resulting from this change is the output of a DH
   computation between the private key for the root of the ratchet tree
   and the add public key from the previous epoch.

   [[ ALTERNATIVE: The sender could also generate the new participant's
   leaf using a fresh key pair, as opposed to a key pair derived from
   the prior epoch's secret.  This would reduce the "double-join"
   problem, at the cost of the GroupAdd having to include a new ratchet
   frontier. ]]

8.3.  UserAdd

   A UserAdd message is sent by a new group participant to add
   themselves to the group, based on having already had access to a
   GroupInitKey for the group.

   struct {
       DHPublicKey add_path<1..2^16-1>;
   } UserAdd;

   A new participant generates this message using the following steps:

   o  Fetch a GroupInitKey for the group

   o  Use the frontiers in the GroupInitKey to add its keys to the trees

   o  Compute the direct path from the new participant's leaf in the new
      ratchet tree (the add_path).

   An existing participant receiving a UserAdd first verifies the
   signature on the message, then verifies its identity inclusion proof
   against the updated identity tree expressed in the GroupInitKey of
   the Handshake message (since the signer is not included in the prior
   group state held by the existing participant).  The participant then
   updates its state as follows:

   o  Update trees with the descriptions in the new GroupInitKey

   o  Update the local ratchet tree with the add path in the UserAdd
      message, replacing any common nodes with the values in the add
      path

   The update secret resulting from this change is the output of a DH
   computation between the private key for the root of the ratchet tree
   and the add public key from the previous epoch.






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8.4.  Update

   An Update message is sent by a group participant to update its leaf
   key pair.  This operation provides post-compromise security with
   regard to the participant's prior leaf private key.

   struct {
       DHPublicKey ratchetPath<1..2^16-1>;
   } Update;

   The sender of an Update message creates it in the following way:

   o  Generate a fresh leaf key pair

   o  Compute its direct path in the current ratchet tree

   An existing participant receiving a Update message first verifies the
   signature on the message, then verifies its identity proof against
   the identity tree held by the participant.  The participant then
   updates its state as follows:

   o  Update the cached ratchet tree by replacing nodes in the direct
      path from the updated leaf with the corresponding nodes in the
      Update message

   The update secret resulting from this change is the secret for the
   root node of the ratchet tree.

8.5.  Delete

   A delete message is sent by a group member to remove one or more
   participants from the group.

   struct {
       uint32 deleted;
       DHPublicKey path<1..2^16-1>;
   } Delete;

   The sender of a Delete message must know the deleted node's copath.
   Based on this knowledge, it computes a Delete message as follows:

   o  Generate a fresh leaf key pair

   o  Compute the direct path from the deleted node's index with the
      fresh leaf key pair in the current ratchet tree

   An existing participant receiving a Update message first verifies the
   signature on the message, then verifies its identity proof against



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   the identity tree held by the participant.  The participant then
   updates its state as follows:

   o  Update the cached ratchet tree by replacing nodes in the direct
      path from the deleted leaf with the corresponding nodes in the
      Update message

   o  Update the cached ratchet tree and identity tree by replacing the
      deleted node's leaves with blank nodes

   The update secret resulting from this change is the secret for the
   root node of the ratchet tree after both updates.

9.  Sequencing of State Changes

   [[ OPEN ISSUE: This section has an initial set of considerations
   regarding sequencing.  It would be good to have some more detailed
   discussion, and hopefully have a mechanism to deal with this issue.
   ]]

   Each handshake message is premised on a given starting state,
   indicated in its "prior_epoch" field.  If the changes implied by a
   handshake messages are made starting from a different state, the
   results will be incorrect.

   This need for sequencing is not a problem as long as each time a
   group member sends a handshake message, it is based on the most
   current state of the group.  In practice, however, there is a risk
   that two members will generate handshake messages simultaneously,
   based on the same state.

   When this happens, there is a need for the members of the group to
   deconflict the simultaneous handshake messages.  There are two
   general approaches:

   o  Have the delivery service enforce a total order

   o  Have a signal in the message that clients can use to break ties

   In either case, there is a risk of starvation.  In a sufficiently
   busy group, a given member may never be able to send a handshake
   message, because he always loses to other members.  The degree to
   which this is a practical problem will depend on the dynamics of the
   application.

   Regardless of how messages are kept in sequence, implementations MUST
   only update their cryptographic state when valid handshake messages
   are received.  Generation of handshake messages MUST be stateless,



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   since the endpoint cannot know at that time whether the change
   implied by the handshake message will succeed or not.

9.1.  Server-side enforced ordering

   With this approach, the delivery service ensures that incoming
   messages are added to an ordered queue and outgoing messages are
   dispatched in the same order.  The server is trusted to resolve
   conflicts during race-conditions (when two members send a message at
   the same time), as the server doesn't have any additional knowledge
   thanks to the confidentiality of the messages.

   Messages should have a counter field sent in clear-text that can be
   checked by the server and used for tie-breaking.  The counter starts
   at 0 and is incremented for every new incoming message.  If two group
   members send a message with the same counter, the first message to
   arrive will be accepted by the server and the second one will be
   rejected.  The rejected message needs to be sent again with the
   correct counter number.

   To prevent counter manipulation by the server, the counter's
   integrity can be ensured by including the counter in a signed message
   envelope.

   This applies to all messages, not only state changing messages.

9.2.  Client-side enforced ordering

   Order enforcement can be implemented on the client as well, one way
   to achieve it is to use a two step update protocol: the first client
   sends a proposal to update and the proposal is accepted when it gets
   50%+ approval from the rest of the group, then it sends the approved
   update.  Clients which didn't get their proposal accepted, will wait
   for the winner to send their update before retrying new proposals.

   While this seems safer as it doesn't rely on the server, it is more
   complex and harder to implement.  It also could cause starvation for
   some clients if they keep failing to get their proposal accepted.

   [[OPEN ISSUE: Another possibility here is batching + deterministic
   selection.]]

10.  Message Protection

   [[ OPEN ISSUE: This section has initial considerations about message
   protection.  This issue clearly needs more specific recommendations,
   possibly a protocol specification in this document or a separate one.
   ]]



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   The primary purpose of this protocol is to enable an authenticated
   group key exchange among participants.  In order to protect messages
   sent among those participants, an application will need to specify
   how messages are protected.

   For every epoch, the root key of the ratcheting tree can be used to
   derive key material for symmetric operations such as encryption/AEAD
   and MAC; AEAD or MAC MUST be used to ensure that the message
   originated from a member of the group.

   In addition, asymmetric signatures SHOULD be used to authenticate the
   sender of a message.

   In combination with server-side enforced ordering, data from previous
   messages is used (as a salt when hashing) to:

   o  add freshness to derived symmetric keys

   o  cryptographically bind the transcript of all previous messages
      with the current group shared secret

   Possible candidates for that are:

   o  the key used for the previous message (hash ratcheting)

   o  the counter of the previous message (needs to be known to new
      members of the group)

   o  the hash of the previous message (proof that other participants
      saw the same history)

   The requirement for this is that all participants know these values.
   If additional clear-text fields are attached to messages (like the
   counter), those fields MUST be protected by a signed message
   envelope.

   Alternatively, the hash of the previous message can also be included
   as an additional field rather than change the encryption key.  This
   allows for a more flexible approach, because the receiving party can
   choose to ignore it (if the value is not known, or if transcript
   security is not required).

11.  Security Considerations

   The security goals of MLS are described in [[the architecture doc]].
   We describe here how the protocol achieves its goals at a high level,
   though a complete security analysis is outside of the scope of this
   document.



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11.1.  Confidentiality of the Group Secrets

   Group secrets are derived from (i) previous group secrets, and (ii)
   the root key of a ratcheting tree.  Only group members know their
   leaf private key in the group, therefore, the root key of the group's
   ratcheting tree is secret and thus so are all values derived from it.

   Initial leaf keys are known only by their owner and the group
   creator, because they are derived from an authenticated key exchange
   protocol.  Subsequent leaf keys are known only by their owner.
   [[TODO: or by someone who replaced them.]]

   Note that the long-term identity keys used by the protocol MUST be
   distributed by an "honest" authentication service for parties to
   authenticate their legitimate peers.

11.2.  Authentication

   There are two forms of authentication we consider.  The first form
   considers authentication with respect to the group.  That is, the
   group members can verify that a message originated from one of the
   members of the group.  This is implicitly guaranteed by the secrecy
   of the shared key derived from the ratcheting trees: if all members
   of the group are honest, then the shared group key is only known to
   the group members.  By using AEAD or appropriate MAC with this shared
   key, we can guarantee that a participant in the group (who knows the
   shared secret key) has sent a message.

   The second form considers authentication with respect to the sender,
   meaning the group members can verify that a message originated from a
   particular member of the group.  This property is provided by digital
   signatures on the messages under identity keys.

   [[ OPEN ISSUE: Signatures under the identity keys, while simple, have
   the side-effect of preclude deniability.  We may wish to allow other
   options, such as (ii) a key chained off of the identity key, or (iii)
   some other key obtained through a different manner, such as a
   pairwise channel that provides deniability for the message
   contents.]]

11.3.  Forward and post-compromise security

   Message encryption keys are derived via a hash ratchet, which
   provides a form of forward secrecy: learning a message key does not
   reveal previous message or root keys.  Post-compromise security is
   provided by Update operations, in which a new root key is generated
   from the latest ratcheting tree.  If the adversary cannot derive the




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   updated root key after an Update operation, it cannot compute any
   derived secrets.

11.4.  Init Key Reuse

   Initialization keys are intended to be used only once and then
   deleted.  Reuse of init keys is not believed to be inherently
   insecure [dhreuse], although it can complicate protocol analyses.

12.  IANA Considerations

   TODO: Registries for protocol parameters, e.g., ciphersuites

13.  Contributors

   o  Benjamin Beurdouche
      INRIA
      benjamin.beurdouche@ens.fr

   o  Karthikeyan Bhargavan
      INRIA
      karthikeyan.bhargavan@inria.fr

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

   o  Alan Duric
      Wire
      alan@wire.com

   o  Srinivas Inguva
      Twitter
      singuva@twitter.com

   o  Albert Kwon
      MIT
      kwonal@mit.edu

   o  Eric Rescorla
      Mozilla
      ekr@rtfm.com

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





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14.  References

14.1.  Normative References

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

   [IEEE1363]
              "IEEE Standard Specifications for Password-Based Public-
              Key Cryptographic Techniques", IEEE standard,
              DOI 10.1109/ieeestd.2009.4773330, n.d..

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [X962]     ANSI, "Public Key Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62, 1998.

14.2.  Informative References

   [art]      Cohn-Gordon, K., Cremers, C., Garratt, L., Millican, J.,
              and K. Milner, "On Ends-to-Ends Encryption: Asynchronous
              Group Messaging with Strong Security Guarantees", January
              2018, <https://eprint.iacr.org/2017/666.pdf>.

   [dhreuse]  Menezes, A. and B. Ustaoglu, "On reusing ephemeral keys in
              Diffie-Hellman key agreement protocols", International
              Journal of Applied Cryptography Vol. 2, pp. 154,
              DOI 10.1504/ijact.2010.038308, 2010.








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   [doubleratchet]
              Cohn-Gordon, K., Cremers, C., Dowling, B., Garratt, L.,
              and D. Stebila, "A Formal Security Analysis of the Signal
              Messaging Protocol", 2017 IEEE European Symposium on
              Security and Privacy (EuroS&P),
              DOI 10.1109/eurosp.2017.27, April 2017.

   [I-D.ietf-trans-rfc6962-bis]
              Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
              Stradling, "Certificate Transparency Version 2.0", draft-
              ietf-trans-rfc6962-bis-27 (work in progress), October
              2017.

   [keyagreement]
              Barker, E., Chen, L., Roginsky, A., and M. Smid,
              "Recommendation for Pair-Wise Key Establishment Schemes
              Using Discrete Logarithm Cryptography", National Institute
              of Standards and Technology report,
              DOI 10.6028/nist.sp.800-56ar2, May 2013.

   [signal]   (ed), T. and M. Marlinspike, "The Double Ratchet
              Algorithm", n.d.,
              <https://www.signal.org/docs/specifications/
              doubleratchet/>.

Authors' Addresses

   Richard Barnes
   Cisco

   Email: rlb@ipv.sx


   Jon Millican
   Facebook

   Email: jmillican@fb.com


   Emad Omara
   Google

   Email: emadomara@google.com








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   Katriel Cohn-Gordon
   University of Oxford

   Email: me@katriel.co.uk


   Raphael Robert
   Wire

   Email: raphael@wire.com









































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