draft-ietf-mls-protocol-12

draft-ietf-mls-protocol-12
Internet-Draft
MLS
October 2021
Barnes, et al.
Expires 14 April 2022
[Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-ietf-mls-protocol-12
Published:
11 October 2021
Intended Status:
Informational
Expires:
14 April 2022
Authors:
R. Barnes
Cisco
B. Beurdouche
Inria & Mozilla
R. Robert
J. Millican
E. Omara
Google
K. Cohn-Gordon
University of Oxford
The Messaging Layer Security (MLS) Protocol
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
clients 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.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
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
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 14 April 2022.
Copyright Notice
Copyright (c) 2021 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
) 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
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warranty as described in the Simplified BSD License.
Table of Contents
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/mlswg/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.
A group of users who want to send each other encrypted messages needs
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 a group of size greater than two, a common strategy is to
unilaterally broadcast symmetric "sender" keys over existing shared
symmetric channels, and then for each member to send messages to the
group encrypted with their own sender key. Unfortunately, while this
improves efficiency over pairwise broadcast of individual messages and
provides forward secrecy (with the addition of a hash ratchet),
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 member'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 with
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. Based on earlier work on "asynchronous
ratcheting trees"
art
, the protocol presented here uses an
asynchronous key-encapsulation mechanism for tree structures.
This mechanism allows the members of the group to derive and update
shared keys with costs that scale as the log of the group size.
1.1.
Change Log
RFC EDITOR PLEASE DELETE THIS SECTION.
draft-12
Use the GroupContext to derive the joiner_secret (*)
Make PreSharedKeys non optional in GroupSecrets (*)
Update name for this particular key (*)
Truncate tree size on removal (*)
Use HPKE draft-08 (*)
Clarify requirements around identity in MLS groups (*)
Signal the intended wire format for MLS messages (*)
Inject GroupContext as HPKE info instead of AAD (*)
Clarify extension handling and make extension updatable (*)
Improve extensibility of Proposals (*)
Constrain proposal in External Commit (*)
Remove the notion of a 'leaf index' (*)
Add group_context_extensions proposal ID (*)
Add RequiredCapabilities extension (*)
Use cascaded KDF instead of concatenation to consolidate PSKs (*)
Use key package hash to index clients in message structs (*)
Don't require PublicGroupState for external init (*)
Make ratchet tree section clearer.
Handle non-member sender cases in MLSPlaintextTBS
Clarify encoding of signatures with NIST curves
Remove OPEN ISSUEs and TODOs
Normalize the description of the zero vector
draft-11
Include subtree keys in parent hash (*)
Pin HPKE to draft-07 (*)
Move joiner secret to the end of the first key schedule epoch (*)
Add an AppAck proposal
Make initializations of transcript hashes consistent
draft-10
Allow new members to join via an external Commit (*)
Enable proposals to be sent inline in a Commit (*)
Re-enable constant-time Add (*)
Change expiration extension to lifetime extension (*)
Make the tree in the Welcome optional (*)
PSK injection, re-init, sub-group branching (*)
Require the initial init_secret to be a random value (*)
Remove explicit sender data nonce (*)
Do not encrypt to joiners in UpdatePath generation (*)
Move MLSPlaintext signature under the confirmation tag (*)
Explicitly authenticate group membership with MLSPLaintext (*)
Clarify X509Credential structure (*)
Remove uneeded interim transcript hash from GroupInfo (*)
IANA considerations
Derive an authentication secret
Use Extract/Expand from HPKE KDF
Clarify that application messages MUST be encrypted
draft-09
Remove blanking of nodes on Add (*)
Change epoch numbers to uint64 (*)
Add PSK inputs (*)
Add key schedule exporter (*)
Sign the updated direct path on Commit, using "parent hashes" and one
signature per leaf (*)
Use structured types for external senders (*)
Redesign Welcome to include confirmation and use derived keys (*)
Remove ignored proposals (*)
Always include an Update with a Commit (*)
Add per-message entropy to guard against nonce reuse (*)
Use the same hash ratchet construct for both application and handshake keys (*)
Add more ciphersuites
Use HKDF to derive key pairs (*)
Mandate expiration of ClientInitKeys (*)
Add extensions to GroupContext and flesh out the extensibility story (*)
Rename ClientInitKey to KeyPackage
draft-08
Change ClientInitKeys so that they only refer to one ciphersuite (*)
Decompose group operations into Proposals and Commits (*)
Enable Add and Remove proposals from outside the group (*)
Replace Init messages with multi-recipient Welcome message (*)
Add extensions to ClientInitKeys for expiration and downgrade resistance (*)
Allow multiple Proposals and a single Commit in one MLSPlaintext (*)
draft-07
Initial version of the Tree based Application Key Schedule (*)
Initial definition of the Init message for group creation (*)
Fix issue with the transcript used for newcomers (*)
Clarifications on message framing and HPKE contexts (*)
draft-06
Reorder blanking and update in the Remove operation (*)
Rename the GroupState structure to GroupContext (*)
Rename UserInitKey to ClientInitKey
Resolve the circular dependency that draft-05 introduced in the
confirmation MAC calculation (*)
Cover the entire MLSPlaintext in the transcript hash (*)
draft-05
Common framing for handshake and application messages (*)
Handshake message encryption (*)
Convert from literal state to a commitment via the "tree hash" (*)
Add credentials to the tree and remove the "roster" concept (*)
Remove the secret field from tree node values
draft-04
Updating the language to be similar to the Architecture document
ECIES is now renamed in favor of HPKE (*)
Using a KDF instead of a Hash in TreeKEM (*)
draft-03
Added ciphersuites and signature schemes (*)
Re-ordered fields in UserInitKey to make parsing easier (*)
Fixed inconsistencies between Welcome and GroupState (*)
Added encryption of the Welcome message (*)
draft-02
Removed ART (*)
Allowed partial trees to avoid double-joins (*)
Added explicit key confirmation (*)
draft-01
Initial description of the Message Protection mechanism. (*)
Initial specification proposal for the Application Key Schedule
using the per-participant chaining of the Application Secret design. (*)
Initial specification proposal for an encryption mechanism to protect
Application Messages using an AEAD scheme. (*)
Initial specification proposal for an authentication mechanism
of Application Messages using signatures. (*)
Initial specification proposal for a padding mechanism to improving
protection of Application Messages against traffic analysis. (*)
Inversion of the Group Init Add and Application Secret derivations
in the Handshake Key Schedule to be ease chaining in case we switch
design. (*)
Removal of the UserAdd construct and split of GroupAdd into Add
and Welcome messages (*)
Initial proposal for authenticating handshake messages by signing
over group state and including group state in the key schedule (*)
Added an appendix with example code for tree math
Changed the ECIES mechanism used by TreeKEM so that it uses nonces
generated from the shared secret
draft-00
Initial adoption of draft-barnes-mls-protocol-01 as a WG item.
2.
Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14
RFC2119
RFC8174
when, and only when, they appear in all
capitals, as shown here.
Client:
An agent that uses this protocol to establish shared cryptographic
state with other clients. A client is defined by the
cryptographic keys it holds.
Group:
A collection of clients with shared cryptographic state.
Member:
A client that is included in the shared state of a group, hence
has access to the group's secrets.
Key Package:
A signed object describing a client's identity and capabilities, and including
a hybrid public-key encryption (HPKE
I-D.irtf-cfrg-hpke
) public key that
can be used to encrypt to that client.
Initialization Key (InitKey):
A key package that is prepublished by a client, which other clients can use to
introduce the client to a new group.
Signature Key:
A 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
RFC8446
to
describe the structure of protocol messages.
3.
Basic Assumptions
This protocol is designed to execute in the context of a Service Provider (SP)
as described in
I-D.ietf-mls-architecture
. In particular, we assume
the SP provides the following services:
A signature key provider which allows clients to authenticate
protocol messages in a group.
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 13
for further
considerations.)
A directory to which clients can publish key packages and download
key packages for other participants.
4.
Protocol Overview
The goal of this protocol is to allow a group of clients to exchange
confidential and authenticated messages. It does so by deriving a sequence
of secrets and keys known only to members. Those should be secret against an
active network adversary and should have both forward secrecy and
post-compromise security with respect to compromise of any members.
We describe the information stored by each client as
state
, which includes
both public and private data. An initial state is set up by a group creator,
which is a group containing only itself. The creator then sends
Add
proposals for each client in the initial set of members, followed by a
Commit
message which incorporates all of the
Adds
into the group state. Finally, the
group creator generates a
Welcome
message corresponding to the Commit and
sends this directly to all the new members, who can use the information
it contains to set up their own group state and derive a shared
secret. Members exchange Commit messages for post-compromise security, to add new
members, and to remove existing members. These messages produce new shared
secrets which are causally linked to their predecessors, forming a logical
Directed Acyclic Graph (DAG) of states.
The protocol algorithms we specify here follow. Each algorithm specifies
both (i) how a client performs the operation and (ii) how other clients
update their state based on it.
There are three major operations in the lifecycle of a group:
Adding a member, initiated by a current member;
Updating the leaf secret of a member;
Removing a member.
Each of these operations is "proposed" by sending a message of the corresponding
type (Add / Update / Remove). The state of the group is not changed, however,
until a Commit message is sent to provide the group with fresh entropy. In this
section, we show each proposal being committed immediately, but in more advanced
deployment cases an application might gather several proposals before
committing them all at once.
Before the initialization of a group, clients publish InitKeys (as KeyPackage
objects) to a directory provided by the Service Provider.
Group
A B C Directory Channel
| | | | |
| KeyPackageA | | | |
|------------------------------------------------->| |
| | | | |
| | KeyPackageB | | |
| |-------------------------------->| |
| | | | |
| | | KeyPackageC | |
| | |--------------->| |
| | | | |
When a client A wants to establish a group with B and C, it first initializes a
group state containing only itself and downloads KeyPackages for B and C. For
each member, A generates an Add and Commit message adding that member, and
broadcasts them to the group. It also generates a Welcome message and sends this
directly to the new member (there's no need to send it to the group). Only after
A has received its Commit message back from the server does it update its state
to reflect the new member's addition.
Upon receiving the Welcome message, the new member will be able to read and send
new messages to the group. Messages received before the client has joined the
group are ignored.
Group
A B C Directory Channel
| | | | |
| KeyPackageB, KeyPackageC | |
|<-------------------------------------------| |
|state.init() | | | |
| | | | |
| | | | Add(A->AB) |
| | | | Commit(Add) |
|--------------------------------------------------------------->|
| | | | |
| Welcome(B) | | | |
|------------->|state.join() | | |
| | | | |
| | | | Add(A->AB) |
| | | | Commit(Add) |
|<---------------------------------------------------------------|
|state.add(B) | | | |
| | | | |
| | | | |
| | | | Add(AB->ABC) |
| | | | Commit(Add) |
|--------------------------------------------------------------->|
| | | | |
| | Welcome(C) | | |
|---------------------------->|state.join() | |
| | | | |
| | | | Add(AB->ABC) |
| | | | Commit(Add) |
|<---------------------------------------------------------------|
|state.add(C) |<------------------------------------------------|
| |state.add(C) | | |
| | | | |
Subsequent additions of group members proceed in the same way. Any
member of the group can download a KeyPackage for a new client
and broadcast an Add message that the current group can use to update
their state, and a Welcome message that the new client can use to
initialize its state and join the group.
To enforce the forward secrecy and post-compromise security of messages,
each member periodically updates their leaf secret.
Any member can update this information at any time by generating a fresh
KeyPackage and sending an Update message followed by a Commit message.
Once all members have processed both, the group's secrets will be unknown to an
attacker that had compromised the sender's prior leaf secret.
Update messages should be sent at regular intervals of time as long as the group
is active, and members that don't update should eventually be removed from the
group. It's left to the application to determine an appropriate amount of time
between Updates.
Group
A B ... Z Directory Channel
| | | | |
| | Update(B) | | |
| |------------------------------------------->|
| Commit(Upd) | | | |
|---------------------------------------------------------->|
| | | | |
| | | | Update(B) |
| | | | Commit(Upd) |
|<----------------------------------------------------------|
|state.upd(B) |<-------------------------------------------|
| |state.upd(B) |<----------------------------|
| | |state.upd(B) | |
| | | | |
Members are removed from the group in a similar way.
Any member of the group can send a Remove proposal followed by a
Commit message, which adds new entropy to the group state
that's known to all except the removed member.
Note that this does not necessarily imply that any member
is actually allowed to evict other members; groups can
enforce access control policies on top of these
basic mechanism.
Group
A B ... Z Directory Channel
| | | | |
| | | Remove(B) | |
| | | Commit(Rem) | |
| | |---------------------------->|
| | | | |
| | | | Remove(B) |
| | | | Commit(Rem) |
|<----------------------------------------------------------|
|state.rem(B) | |<----------------------------|
| | |state.rem(B) | |
| | | | |
| | | | |
5.
Ratchet Trees
The protocol uses "ratchet trees" for deriving shared secrets among
a group of clients.
5.1.
Tree Computation Terminology
Trees consist of
nodes
. A node is a
leaf
if it has no children, and a
parent
otherwise; note that all
parents in our 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 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.
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 its size is a power of
two and for any parent node in the tree, its left and right subtrees
have the same size.
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.
Given a list of
items, there is a unique left-balanced
binary tree structure with these elements as leaves.
(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
.)
The
direct path
of a root is the empty list, and of any other node
is the concatenation of that node's parent along with the parent's direct path.
The
copath
of a node is the node's sibling concatenated with the list of
siblings of all the nodes in its direct path, excluding the root.
For example, in the below tree:
The direct path of C is (CD, ABCD, ABCDEFG)
The copath of C is (D, AB, EFG)
7 = root
______|______
/ \
3 11
__|__ __|
/ \ / \
1 5 9 |
/ \ / \ / \ |
A B C D E F G

1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2
Each node in the tree is assigned an
index
, starting at zero and
running from left to right. A node is a leaf node if and only if it
has an even index. The node indices for the nodes in the above tree
are as follows:
0 = A
1 = AB
2 = B
3 = ABCD
4 = C
5 = CD
6 = D
7 = ABCDEFG
8 = E
9 = EF
10 = F
11 = EFG
12 = G
A tree with
leaves has
2*n - 1
nodes. For example, the above tree has 7
leaves (A, B, C, D, E, F, G) and 13 nodes. The root of a tree with
leaves
is always the node with index
2^k - 1
, where
is the largest number such
that
2^k < n
5.2.
Ratchet Tree Nodes
A particular instance of a ratchet tree is defined by the same parameters that
define an instance of HPKE, namely:
A Key Encapsulation Mechanism (KEM), including a
DeriveKeyPair
function that
creates a key pair for the KEM from a symmetric secret
A Key Derivation Function (KDF), including
Extract
and
Expand
functions
An AEAD encryption scheme
Each node in a ratchet tree contains up to five values:
A private key (only within the member's direct path, see below)
A public key
An ordered list of node indices for "unmerged" leaves (see
Section 5.3
A credential (only for leaf nodes)
A hash of certain information about the node's parent, as of the last time the
node was changed (see
Section 7.5
).
The conditions under which each of these values must or must not be
present are laid out in
Section 5.3
A node in the tree may also be
blank
, indicating that no value is
present at that node. The
resolution
of a node is an ordered list
of non-blank nodes that collectively cover all non-blank descendants
of the node.
The resolution of a non-blank node comprises the node itself,
followed by its list of unmerged leaves, if any
The resolution of a blank leaf node is the empty list
The resolution of a blank intermediate node is the result of
concatenating the resolution of its left child with the resolution
of its right child, in that order
For example, consider the following tree, where the "_" character
represents a blank node and unmerged leaves are indicated in square
brackets:
__|__
/ \
_ 5[C]
/ \ / \
A _ C D

0 1 2 3 4 5 6
In this tree, we can see all of the above rules in play:
The resolution of node 5 is the list [CD, C]
The resolution of node 2 is the empty list []
The resolution of node 3 is the list [A, CD, C]
Every node, regardless of whether the node is blank or populated, has
a corresponding
hash
that summarizes the contents of the subtree
below that node. The rules for computing these hashes are described
in
Section 7.6
5.3.
Views of a Ratchet Tree
We generally assume that each participant maintains a complete and
up-to-date view of the public state of the group's ratchet tree,
including the public keys for all nodes and the credentials
associated with the leaf nodes.
No participant in an MLS group knows the private key associated with
every node in the tree. Instead, each member is assigned to a leaf of the tree,
which determines the subset of private keys it knows. The
credential stored at that leaf is one provided by the member.
In particular, MLS maintains the members' views of the tree in such
a way as to maintain the
tree invariant
The private key for a node in the tree is known to a member of
the group only if that member's leaf is a descendant of
the node.
In other words, if a node is not blank, then it holds a public key.
The corresponding private key is known only to members occupying
leaves below that node.
The reverse implication is not true: A member may not know the private keys of
all the intermediate nodes they're below. Such a member has an
unmerged
leaf.
Encrypting to an intermediate node requires encrypting to the node's public key,
as well as the public keys of all the unmerged leaves below it. A leaf is
unmerged when it is first added, because the process of adding the leaf does not
give it access to all of the nodes above it in the tree. Leaves are "merged" as
they receive the private keys for nodes, as described in
Section 5.4
5.4.
Ratchet Tree Evolution
A member of an MLS group advances the key schedule to provide forward secrecy
and post-compromise security by providing the group with fresh key material to
be added into the group's shared secret.
To do so, one member of the group generates fresh key
material, applies it to their local tree state, and then sends this key material
to other members in the group via an UpdatePath message (see
Section 7.8
) .
All other group members then apply the key material in the UpdatePath to their
own local tree state to derive the group's now-updated shared secret.
To begin, the generator of the UpdatePath updates its leaf
KeyPackage and its direct path to the root with new secret values. The
HPKE leaf public key within the KeyPackage MUST be derived from a freshly
generated HPKE secret key to provide post-compromise security.
The generator of the UpdatePath starts by sampling a fresh random value called
"leaf_secret", and uses the leaf_secret to generate their leaf HPKE key pair
(see
Section 7
) and to seed a sequence of "path secrets", one for each
ancestor of its leaf. In this setting,
path_secret[0] refers to the node directly above the leaf,
path_secret[1] for its parent, and so on. At each step, the path
secret is used to derive a new secret value for the corresponding
node, from which the node's key pair is derived.
leaf_node_secret = DeriveSecret(leaf_secret, "node")
path_secret[0] = DeriveSecret(leaf_secret, "path")

path_secret[n] = DeriveSecret(path_secret[n-1], "path")
node_secret[n] = DeriveSecret(path_secret[n], "node")

leaf_priv, leaf_pub = KEM.DeriveKeyPair(leaf_node_secret)
node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n])
For example, suppose there is a group with four members, with C an unmerged leaf
at node 5:
__|__
/ \
1 5[C]
/ \ / \
A B C D

0 1 2 3 4 5 6
If member B subsequently generates an UpdatePath based on a secret
"leaf_secret", then it would generate the following sequence
of path secrets:
path_secret[1] --> node_secret[1] --> node_priv[1], node_pub[1]
path_secret[0] --> node_secret[0] --> node_priv[0], node_pub[0]
leaf_secret --> leaf_node_secret --> leaf_priv, leaf_pub
~> leaf_key_package
After applying the UpdatePath, the tree will have the following structure, where
lp
and
np[i]
represent the leaf_priv and node_priv values generated as
described above:
np[1] -> 3
__|__
/ \
np[0] -> 1 5[C]
/ \ / \
A B C D
lp

0 1 2 3 4 5 6
After performing these operations, the generator of the UpdatePath MUST
delete the leaf_secret.
5.5.
Synchronizing Views of the Tree
After generating fresh key material and applying it to ratchet forward their
local tree state as described in the prior section, the generator must broadcast
this update to other members of the group in a Commit message, who
apply it to keep their local views of the tree in
sync with the sender's. More specifically, when a member commits a change to
the tree (e.g., to add or remove a member), it transmits an UpdatePath
containing a set of public keys and encrypted path secrets
for intermediate nodes in the direct path of its leaf. The
other members of the group use these values to update
their view of the tree, aligning their copy of the tree to the
sender's.
An UpdatePath contains
the following information for each node in the direct path of the
sender's leaf, including the root:
The public key for the node
Zero or more encrypted copies of the path secret corresponding to
the node
The path secret value for a given node is encrypted for the subtree
corresponding to the parent's non-updated child, that is, the child
on the copath of the sender's leaf node.
There is one encryption of the path secret to each public key in the resolution
of the non-updated child.
The recipient of an UpdatePath processes it with the following steps:
Compute the updated path secrets.
Identify a node in the direct path for which the local member
is in the subtree of the non-updated child.
Identify a node in the resolution of the copath node for
which this node has a private key.
Decrypt the path secret for the parent of the copath node using
the private key from the resolution node.
Derive path secrets for ancestors of that node using the
algorithm described above.
The recipient SHOULD verify that the received public keys agree
with the public keys derived from the new path_secret values.
Merge the updated path secrets into the tree.
For all updated nodes,
Replace the public key for each node with the received public key.
Set the list of unmerged leaves to the empty list.
Store the updated hash of the node's parent (represented as a ParentNode
struct), going from root to leaf, so that each hash incorporates all the
nodes above it. The root node always has a zero-length hash for this
value.
For nodes where an updated path secret was computed in step 1,
compute the corresponding node key pair and replace the values
stored at the node with the computed values.
For example, in order to communicate the example update described in
the previous section, the sender would transmit the following
values:
Table 1
Public Key
Ciphertext(s)
node_pub[1]
E(pk(5), path_secret[1]), E(pk(C), path_secret[1])
node_pub[0]
E(pk(A), path_secret[0])
In this table, the value pk(ns[X]) represents the public key
derived from the node secret X, whereas pk(X) represents the public leaf key
for user X. The value E(K, S) represents
the public-key encryption of the path secret S to the
public key K (using HPKE).
After processing the update, each recipient MUST delete outdated key material,
specifically:
The path secrets used to derive each updated node key pair.
Each outdated node key pair that was replaced by the update.
6.
Cryptographic Objects
6.1.
Ciphersuites
Each MLS session uses a single ciphersuite that specifies the
following primitives to be used in group key computations:
HPKE parameters:
A Key Encapsulation Mechanism (KEM)
A Key Derivation Function (KDF)
An AEAD encryption algorithm
A hash algorithm
A signature algorithm
MLS uses draft-08 of HPKE
I-D.irtf-cfrg-hpke
for public-key encryption.
The
DeriveKeyPair
function associated to the KEM for the ciphersuite maps
octet strings to HPKE key pairs.
Ciphersuites are represented with the CipherSuite type. HPKE public keys
are opaque values in a format defined by the underlying
protocol (see the Cryptographic Dependencies section of the HPKE specification for more
information).
opaque HPKEPublicKey<1..2^16-1>;
The signature algorithm specified in the ciphersuite is the mandatory algorithm
to be used for signatures in MLSPlaintext and the tree signatures. It MUST be
the same as the signature algorithm specified in the credential field of the
KeyPackage objects in the leaves of the tree (including the InitKeys
used to add new members).
The ciphersuites are defined in section
Section 16.1
6.2.
Credentials
A member of a group authenticates the identities of other participants by means
of credentials issued by some authentication system, like a PKI. Each type of
credential MUST express the following data in the context of the group it is
used with:
The public key of a signature key pair matching the SignatureScheme specified
by the CipherSuite of the group
The identity of the holder of the private key
Credentials MAY also include information that allows a relying party
to verify the identity / signing key binding.
Additionally, Credentials SHOULD specify the signature scheme corresponding to
each contained public key.
// See RFC 8446 and the IANA TLS SignatureScheme registry
uint16 SignatureScheme;

// See IANA registry for registered values
uint16 CredentialType;

struct {
opaque identity<0..2^16-1>;
SignatureScheme signature_scheme;
opaque signature_key<0..2^16-1>;
} BasicCredential;

struct {
opaque cert_data<0..2^16-1>;
} Certificate;

struct {
CredentialType credential_type;
select (Credential.credential_type) {
case basic:
BasicCredential;

case x509:
Certificate chain<1..2^32-1>;
};
} Credential;
A BasicCredential is a raw, unauthenticated assertion of an identity/key
binding. The format of the key in the
public_key
field is defined by the
relevant ciphersuite: the group ciphersuite for a credential in a ratchet tree,
the KeyPackage ciphersuite for a credential in a KeyPackage object.
For X509Credential, each entry in the chain represents a single DER-encoded
X509 certificate. The chain is ordered such that the first entry (chain[0])
is the end-entity certificate and each subsequent certificate in the chain
MUST be the issuer of the previous certificate. The algorithm for the
public_key
in the end-entity certificate MUST match the relevant
ciphersuite.
For ciphersuites using Ed25519 or Ed448 signature schemes, the public key is in
the format specified
RFC8032
. For ciphersuites using ECDSA with the NIST
curves P-256 or P-521, the public key is the output of the uncompressed
Elliptic-Curve-Point-to-Octet-String conversion according to
SECG
The signatures used throughout this document are encoded as specified in
RFC8446
. In particular, ECDSA signatures are DER-encoded and EdDSA signatures
are defined as the concatenation of
and
as specified in
RFC8032
Note that each new credential that has not already been validated
by the application MUST be validated against the Authentication
Service.
7.
Key Packages
In order to facilitate asynchronous addition of clients to a
group, it is possible to pre-publish key packages that
provide some public information about a user. KeyPackage
structures provide information about a client that any existing
member can use to add this client to the group asynchronously.
A KeyPackage object specifies a ciphersuite that the client supports, as well as
providing a public key that others can use for key agreement.
The
identity
arising from the credential, together with the
endpoint_id
in
the KeyPackage serve to uniquely identify a client in a group.
When used as InitKeys, KeyPackages are intended to be used only once and SHOULD NOT
be reused except in case of last resort. (See
Section 15.4
).
Clients MAY generate and publish multiple InitKeys to
support multiple ciphersuites.
KeyPackages contain a public key chosen by the client, which the
client MUST ensure uniquely identifies a given KeyPackage object
among the set of KeyPackages created by this client.
The value for hpke_init_key MUST be a public key for the asymmetric
encryption scheme defined by cipher_suite. The whole structure
is signed using the client's signature key. A KeyPackage object
with an invalid signature field MUST be considered malformed.
The input to the signature computation comprises all of the fields
except for the signature field.
enum {
reserved(0),
mls10(1),
(255)
} ProtocolVersion;

// See IANA registry for registered values
uint16 ExtensionType;

struct {
ExtensionType extension_type;
opaque extension_data<0..2^32-1>;
} Extension;

struct {
ProtocolVersion version;
CipherSuite cipher_suite;
HPKEPublicKey hpke_init_key;
opaque endpoint_id<0..255>;
Credential credential;
Extension extensions<8..2^32-1>;
opaque signature<0..2^16-1>;
} KeyPackage;
KeyPackage objects MUST contain at least two extensions, one of type
capabilities
, and one of
type
lifetime
. The
capabilities
extension
allow MLS session establishment to be safe from downgrade attacks on the
parameters described (as discussed in
Section 10
), while still only advertising
one version / ciphersuite per KeyPackage.
As the
KeyPackage
is a structure which is stored in the Ratchet
Tree and updated depending on the evolution of this tree, each
modification of its content MUST be reflected by a change of its
signature. This allow other members to control the validity of the KeyPackage
at any time and in particular in the case of a newcomer joining the group.
7.1.
Key Package IDs
When it is necessary to refer to a specific KeyPackage, protocol messages
incorporate a KeyPackageID:
struct {
opaque key_package_hash<0..255>;
} KeyPackageID
This value is the hash of the KeyPackage, using the hash indicated by the
cipher_suite
field. KeyPackage hashes are used in a Welcome message to
indicate which KeyPackage is being used to include the new member. Since members
of a group are uniquely identified by their leaf KeyPackages, messages within a
group use the hash of this key package to refer to group members, e.g., to
specify the target of a Remove proposal or the signer of an MLSPlaintext.
7.2.
Client Capabilities
The
capabilities
extension indicates what protocol versions, ciphersuites,
protocol extensions, and non-default proposal types are supported by a client.
Proposal types defined in this document are considered "default" and thus need
not be listed.
struct {
ProtocolVersion versions<0..255>;
CipherSuite ciphersuites<0..255>;
ExtensionType extensions<0..255>;
ProposalType proposals<0..255>;
} Capabilities;
This extension MUST be always present in a KeyPackage. Extensions that appear
in the
extensions
field of a KeyPackage MUST be included in the
extensions
field of the
capabilities
extension.
7.3.
Lifetime
The
lifetime
extension represents the times between which clients will
consider a KeyPackage valid. This time is represented as an absolute time,
measured in seconds since the Unix epoch (1970-01-01T00:00:00Z). A client MUST
NOT use the data in a KeyPackage for any processing before the
not_before
date, or after the
not_after
date.
uint64 not_before;
uint64 not_after;
Applications MUST define a maximum total lifetime that is acceptable for a
KeyPackage, and reject any KeyPackage where the total lifetime is longer than
this duration.
This extension MUST always be present in a KeyPackage.
7.4.
KeyPackage Identifiers
Within MLS, a KeyPackage is identified by its hash (see, e.g.,
Section 11.2.2
). The
external_key_id
extension allows applications to add
an explicit, application-defined identifier to a KeyPackage.
opaque external_key_id<0..2^16-1>;
7.5.
Parent Hash
The
parent_hash
extension carries information to authenticate the structure of
the tree, as described below.
opaque parent_hash<0..255>;
Consider a ratchet tree with a parent node P and children V and S. The parent hash
of P changes whenever an
UpdatePath
object is applied to the ratchet tree along
a path traversing node V (and hence also P). The new "Parent Hash of P (with Co-Path
Child S)" is obtained by hashing P's
ParentHashInput
struct using the resolution
of S to populate the
original_child_resolution
field. This way, P's Parent Hash
fixes the new HPKE public keys of all nodes on the path from P to the root.
Furthermore, for each such key PK the hash also binds the set of HPKE public keys
to which PK's secret key was encrypted in the commit packet that anounced the
UpdatePath
object.
struct {
HPKEPublicKey public_key;
opaque parent_hash<0..255>;
HPKEPublicKey original_child_resolution<0..2^32-1>;
} ParentHashInput;
The Parent Hash of P with Co-Path Child S is the hash of a
ParentHashInput
object
populated as follows. The field
public_key
contains the HPKE public key of P. If P
is the root, then
parent_hash
is set to a zero-length octet string.
Otherwise
parent_hash
is the Parent Hash of P's parent with P's sibling as the
co-path child.
Finally,
original_child_resolution
is the array of
HPKEPublicKey
values of the
nodes in the resolution of S but with the
unmerged_leaves
of P omitted. For
example, in the ratchet tree depicted in
Section 5.2
the
ParentHashInput
of node 5 with co-path child 4 would contain an empty
original_child_resolution
since 4's resolution includes only itself but 4 is also
an unmerged leaf of 5. Meanwhile, the
ParentHashInput
of node 5 with co-path child
6 has an array with one element in it: the HPKE public key of 6.
7.5.1.
Using Parent Hashes
The Parent Hash of P appears in three types of structs. If V is itself a parent node
then P's Parent Hash is stored in the
parent_hash
fields of both V's
ParentHashInput
struct and V's
ParentNode
struct. (The
ParentNode
struct is
used to encapsulate all public information about V that must be conveyed to a new
member joining the group as well as to define the Tree Hash of node V.)
If, on the other hand, V is a leaf and its KeyPackage contains the
parent_hash
extension then the Parent Hash of P (with V's sibling as co-path child) is stored in
that field. In particular, the extension MUST be present in the
leaf_key_package
field of an
UpdatePath
object. (This way, the signature of such a KeyPackage also
serves to attest to which keys the group member introduced into the ratchet tree and
to whom the corresponding secret keys were sent. This helps prevent malicious insiders
from constructing artificial ratchet trees with a node V whose HPKE secret key is
known to the insider yet where the insider isn't assigned a leaf in the subtree rooted
at V. Indeed, such a ratchet tree would violate the tree invariant.)
7.5.2.
Verifying Parent Hashes
To this end, when processing a Commit message clients MUST recompute the
expected value of
parent_hash
for the committer's new leaf and verify that it
matches the
parent_hash
value in the supplied
leaf_key_package
. Moreover, when
joining a group, new members MUST authenticate each non-blank parent node P. A parent
node P is authenticated by performing the following check:
Let L and R be the left and right children of P, respectively
If L.parent_hash is equal to the Parent Hash of P with Co-Path Child R, the check passes
If R is blank, replace R with its left child until R is either non-blank or a leaf node
If R is a blank leaf node, the check fails
If R.parent_hash is equal to the Parent Hash of P with Co-Path Child L, the check passes
Otherwise, the check fails
The left-child recursion under the right child of P is necessary because the expansion of
the tree to the right due to Add proposals can cause blank nodes to be interposed
between a parent node and its right child.
7.6.
Tree Hashes
To allow group members to verify that they agree on the public cryptographic state
of the group, this section defines a scheme for generating a hash value (called
the "tree hash") that represents the contents of the group's ratchet tree and the
members' KeyPackages. The tree hash of a tree is the tree hash of its root node,
which we define recursively, starting with the leaves.
As some nodes may be blank while others contain data we use the following struct
to include data if present.
struct {
uint8 present;
select (present) {
case 0: struct{};
case 1: T value;
} optional;
The tree hash of a leaf node is the hash of leaf's
LeafNodeHashInput
object which
might include a Key Package depending on whether or not it is blank.
struct {
uint32 node_index;
optional key_package;
} LeafNodeHashInput;
Now the tree hash of any non-leaf node is recursively defined to be the hash of
its
ParentNodeTreeHashInput
. This includes an optional
ParentNode
object depending on whether the node is blank or not.
struct {
HPKEPublicKey public_key;
opaque parent_hash<0..255>;
uint32 unmerged_leaves<0..2^32-1>;
} ParentNode;

struct {
uint32 node_index;
optional parent_node;
opaque left_hash<0..255>;
opaque right_hash<0..255>;
} ParentNodeTreeHashInput;
The
left_hash
and
right_hash
fields hold the tree hashes of the node's
left and right children, respectively.
7.7.
Group State
Each member of the group maintains a GroupContext object that
summarizes the state of the group:
struct {
opaque group_id<0..255>;
uint64 epoch;
opaque tree_hash<0..255>;
opaque confirmed_transcript_hash<0..255>;
Extension extensions<0..2^32-1>;
} GroupContext;
The fields in this state have the following semantics:
The
group_id
field is an application-defined identifier for the
group.
The
epoch
field represents the current version of the group key.
The
tree_hash
field contains a commitment to the contents of the
group's ratchet tree and the credentials for the members of the
group, as described in
Section 7.6
The
confirmed_transcript_hash
field contains a running hash over
the messages that led to this state.
When a new member is added to the group, an existing member of the
group provides the new member with a Welcome message. The Welcome
message provides the information the new member needs to initialize
its GroupContext.
Different changes to the group will have different effects on the group state.
These effects are described in their respective subsections of
Section 11.1
The following general rules apply:
The
group_id
field is constant
The
epoch
field increments by one for each Commit message that
is processed
The
tree_hash
is updated to represent the current tree and
credentials
The
confirmed_transcript_hash
is updated with the data for an
MLSPlaintext message encoding a Commit message in two parts:
struct {
WireFormat wire_format;
opaque group_id<0..255>;
uint64 epoch;
Sender sender;
opaque authenticated_data<0..2^32-1>;
ContentType content_type = commit;
Commit commit;
opaque signature<0..2^16-1>;
} MLSPlaintextCommitContent;

struct {
optional confirmation_tag;
} MLSPlaintextCommitAuthData;

interim_transcript_hash_[0] = ""; // zero-length octet string

confirmed_transcript_hash_[n] =
Hash(interim_transcript_hash_[n] ||
MLSPlaintextCommitContent_[n]);

interim_transcript_hash_[n+1] =
Hash(confirmed_transcript_hash_[n] ||
MLSPlaintextCommitAuthData_[n]);
Thus the
confirmed_transcript_hash
field in a GroupContext object represents a
transcript over the whole history of MLSPlaintext Commit messages, up to the
confirmation tag field in the current MLSPlaintext message. The confirmation
tag is then included in the transcript for the next epoch. The interim
transcript hash is computed by new members using the confirmation tag in the
GroupInfo struct, and enables existing members to incorporate a Commit message
into the transcript without having to store the whole MLSPlaintextCommitAuthData
structure.
As shown above, when a new group is created, the
interim_transcript_hash
field
is set to the zero-length octet string.
7.8.
Update Paths
As described in
Section 11.2
, each MLS Commit message may optionally
transmit a KeyPackage leaf and node values along its direct path.
The path contains a public key and encrypted secret value for all
intermediate nodes in the path above the leaf. The path is ordered
from the closest node to the leaf to the root; each node MUST be the
parent of its predecessor.
struct {
opaque kem_output<0..2^16-1>;
opaque ciphertext<0..2^16-1>;
} HPKECiphertext;

struct {
HPKEPublicKey public_key;
HPKECiphertext encrypted_path_secret<0..2^32-1>;
} UpdatePathNode;

struct {
KeyPackage leaf_key_package;
UpdatePathNode nodes<0..2^32-1>;
} UpdatePath;
For each
UpdatePathNode
, the resolution of the corresponding copath node MUST
be filtered by removing all new leaf nodes added as part of this MLS Commit
message. The number of ciphertexts in the
encrypted_path_secret
vector MUST be
equal to the length of the filtered resolution, with each ciphertext being the
encryption to the respective resolution node.
The HPKECiphertext values are computed as
kem_output, context = SetupBaseS(node_public_key, group_context)
ciphertext = context.Seal("", path_secret)
where
node_public_key
is the public key of the node that the path
secret is being encrypted for, group_context is the current GroupContext object
for the group, and the functions
SetupBaseS
and
Seal
are defined according to
I-D.irtf-cfrg-hpke
Decryption is performed in the corresponding way, using the private
key of the resolution node and the ephemeral public key
transmitted in the message.
8.
Key Schedule
Group keys are derived using the
Extract
and
Expand
functions from the KDF
for the group's ciphersuite, as well as the functions defined below:
ExpandWithLabel(Secret, Label, Context, Length) =
KDF.Expand(Secret, KDFLabel, Length)

Where KDFLabel is specified as:

struct {
uint16 length = Length;
opaque label<7..255> = "mls10 " + Label;
opaque context<0..2^32-1> = Context;
} KDFLabel;

DeriveSecret(Secret, Label) =
ExpandWithLabel(Secret, Label, "", KDF.Nh)
The value
KDF.Nh
is the size of an output from
KDF.Extract
, in bytes. In
the below diagram:
KDF.Extract takes its salt argument from the top and its IKM
argument from the left
DeriveSecret takes its Secret argument from the incoming arrow
represents an all-zero byte string of length
KDF.Nh
When processing a handshake message, a client combines the
following information to derive new epoch secrets:
The init secret from the previous epoch
The commit secret for the current epoch
The GroupContext object for current epoch
Given these inputs, the derivation of secrets for an epoch
proceeds as shown in the following diagram:
init_secret_[n-1]
commit_secret -> KDF.Extract
ExpandWithLabel(., "joiner", GroupContext_[n], KDF.Nh)
joiner_secret
psk_secret (or 0) -> KDF.Extract
+--> DeriveSecret(., "welcome")
| = welcome_secret
ExpandWithLabel(., "epoch", GroupContext_[n], KDF.Nh)
epoch_secret
+--> DeriveSecret(., )
| =
DeriveSecret(., "init")
init_secret_[n]
A number of secrets are derived from the epoch secret for different purposes:
Table 2
Secret
Label
sender_data_secret
"sender data"
encryption_secret
"encryption"
exporter_secret
"exporter"
authentication_secret
"authentication"
external_secret
"external"
confirmation_key
"confirm"
membership_key
"membership"
resumption_secret
"resumption"
The "external secret" is used to derive an HPKE key pair whose private key is
held by the entire group:
external_priv, external_pub = KEM.DeriveKeyPair(external_secret)
The public key
external_pub
can be published as part of the
PublicGroupState
struct in order to allow non-members to join the group using an external commit.
8.1.
External Initialization
In addition to initializing a new epoch via KDF invocations as described above,
an MLS group can also initialize a new epoch via an asymmetric interaction using
the external key pair for the previous epoch. This is done when an new member
is joining via an external commit.
In this process, the joiner sends a new
init_secret
value to the group using
the HPKE export method. The joiner then uses that
init_secret
with
information provided in the PublicGroupState and an external Commit to initialize
their copy of the key schedule for the new epoch.
kem_output, context = SetupBaseS(external_pub, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
Members of the group receive the
kem_output
in an ExternalInit proposal and
preform the corresponding calculation to retrieve the
init_secret
value.
context = SetupBaseR(kem_output, external_priv, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
In both cases, the
info
input to HPKE is set to the PublicGroupState for the
previous epoch, encoded using the TLS serialization.
8.2.
Pre-Shared Keys
Groups which already have an out-of-band mechanism to generate
shared group secrets can inject those into the MLS key schedule to seed
the MLS group secrets computations by this external entropy.
Injecting an external PSK can improve security in the case
where having a full run of updates across members is too expensive, or if
the external group key establishment mechanism provides
stronger security against classical or quantum adversaries.
Note that, as a PSK may have a different lifetime than an update, it does not
necessarily provide the same Forward Secrecy (FS) or Post-Compromise Security
(PCS) guarantees as a Commit message. Unlike the key pairs populated in the
tree by an Update or Commit, which always freshly generated, PSKs may be
pre-distributed and stored. This creates the risk that a PSK may be compromised
in the process of distribution and storage. The security that the group gets
from injecting a PSK thus depends on both the entropy of the PSK and the risk of
compromise. These factors are outside of the scope of this document, but should
be considered by application designers relying on PSKs.
Each PSK in MLS has a type that designates how it was provisioned.
External PSKs are provided by the application, while recovery and re-init PSKs
are derived from the MLS key schedule and used in cases where it is
necessary to authenticate a member's participation in a prior group state.
In particular, in addition to external PSK types, a PSK derived from within MLS
may be used in the following cases:
Re-Initialization: If during the lifetime of a group, the group members
decide to switch to a more secure ciphersuite or newer protocol version,
a PSK can be used to carry entropy from the old group forward into a new
group with the desired parameters.
Branching: A PSK may be used to bootstrap a subset of current group
members into a new group. This applies if a subset of current group
members wish to branch based on the current group state.
The injection of one or more PSKs into the key schedule is signaled in two ways:
1) as a
PreSharedKey
proposal, and 2) in the
GroupSecrets
object of a
Welcome message sent to new members added in that epoch.
enum {
reserved(0),
external(1),
reinit(2),
branch(3)
(255)
} PSKType;

struct {
PSKType psktype;
select (PreSharedKeyID.psktype) {
case external:
opaque psk_id<0..255>;

case reinit:
opaque psk_group_id<0..255>;
uint64 psk_epoch;

case branch:
opaque psk_group_id<0..255>;
uint64 psk_epoch;
opaque psk_nonce<0..255>;
} PreSharedKeyID;

struct {
PreSharedKeyID psks<0..2^16-1>;
} PreSharedKeys;
On receiving a Commit with a
PreSharedKey
proposal or a GroupSecrets object
with the
psks
field set, the receiving Client includes them in the key
schedule in the order listed in the Commit, or in the
psks
field respectively.
For resumption PSKs, the PSK is defined as the
resumption_secret
of the group and
epoch specified in the
PreSharedKeyID
object. Specifically,
psk_secret
is
computed as follows:
struct {
PreSharedKeyID id;
uint16 index;
uint16 count;
} PSKLabel;

psk_extracted_[i] = KDF.Extract(0, psk_[i])
psk_input_[i] = ExpandWithLabel(psk_extracted_[i], "derived psk", PSKLabel, KDF.Nh)

psk_secret_[0] = 0
psk_secret_[i] = KDF.Extract(psk_input[i-1], psk_secret_[i-1])
psk_secret = psk_secret[n]
Here
represents the all-zero vector of length
KDF.Nh
. The
index
field in
PSKLabel
corresponds to the index of the PSK in the
psk
array, while the
count
field contains the total number of PSKs. In other words, the PSKs are
chained together with KDF.Extract invocations, as follows:
0 0 = psk_secret_[0]
| |
V V
psk_[0] --> KDF.Extract --> ExpandWithLabel --> KDF.Extract = psk_secret_[1]
0 |
| |
V V
psk_[1] --> KDF.Extract --> ExpandWithLabel --> KDF.Extract = psk_secret_[1]
0 ...
| |
V V
psk_[n] --> KDF.Extract --> ExpandWithLabel --> KDF.Extract = psk_secret_[n]
In particular, if there are no PreSharedKey proposals in a given Commit, then
the resulting
psk_secret
is
psk_secret_[0]
, the all-zero vector.
8.3.
Secret Tree
For the generation of encryption keys and nonces, the key schedule begins with
the
encryption_secret
at the root and derives a tree of secrets with the same
structure as the group's ratchet tree. Each leaf in the Secret Tree is
associated with the same group member as the corresponding leaf in the ratchet
tree. Nodes are also assigned an index according to their position in the array
representation of the tree (described in
Appendix A
). If N is a node index in
the Secret Tree then left(N) and right(N) denote the children of N (if they
exist).
The secret of any other node in the tree is derived from its parent's secret
using a call to DeriveTreeSecret:
DeriveTreeSecret(Secret, Label, Node, Generation, Length) =
ExpandWithLabel(Secret, Label, TreeContext, Length)

Where TreeContext is specified as:

struct {
uint32 node = Node;
uint32 generation = Generation;
} TreeContext;
If N is a node index in the Secret Tree then the secrets of the children
of N are defined to be:
tree_node_[N]_secret
+--> DeriveTreeSecret(., "tree", left(N), 0, KDF.Nh)
| = tree_node_[left(N)]_secret
+--> DeriveTreeSecret(., "tree", right(N), 0, KDF.Nh)
= tree_node_[right(N)]_secret
The secret in the leaf of the Secret Tree is used to initiate two symmetric hash
ratchets, from which a sequence of single-use keys and nonces are derived, as
described in
Section 8.4
. The root of each ratchet is computed as:
tree_node_[N]_secret
+--> DeriveTreeSecret(., "handshake", N, 0, KDF.Nh)
| = handshake_ratchet_secret_[N]_[0]
+--> DeriveTreeSecret(., "application", N, 0, KDF.Nh)
= application_ratchet_secret_[N]_[0]
8.4.
Encryption Keys
As described in
Section 9
, MLS encrypts three different
types of information:
Metadata (sender information)
Handshake messages (Proposal and Commit)
Application messages
The sender information used to look up the key for content encryption is
encrypted with an AEAD where the key and nonce are derived from both
sender_data_secret
and a sample of the encrypted message content.
For handshake and application messages, a sequence of keys is derived via a
"sender ratchet". Each sender has their own sender ratchet, and each step along
the ratchet is called a "generation".
A sender ratchet starts from a per-sender base secret derived from a Secret
Tree, as described in
Section 8.3
. The base secret initiates a symmetric
hash ratchet which generates a sequence of keys and nonces. The sender uses the
j-th key/nonce pair in the sequence to encrypt (using the AEAD) the j-th message
they send during that epoch. Each key/nonce pair MUST NOT be used to encrypt
more than one message.
Keys, nonces, and the secrets in ratchets are derived using
DeriveTreeSecret. The context in a given call consists of the index
of the sender's leaf in the ratchet tree and the current position in
the ratchet. In particular, the node index of the sender's leaf in the
ratchet tree is the same as the node index of the leaf in the Secret Tree
used to initialize the sender's ratchet.
ratchet_secret_[N]_[j]
+--> DeriveTreeSecret(., "nonce", N, j, AEAD.Nn)
| = ratchet_nonce_[N]_[j]
+--> DeriveTreeSecret(., "key", N, j, AEAD.Nk)
| = ratchet_key_[N]_[j]
DeriveTreeSecret(., "secret", N, j, KDF.Nh)
= ratchet_secret_[N]_[j+1]
Here, AEAD.Nn and AEAD.Nk denote the lengths
in bytes of the nonce and key for the AEAD scheme defined by
the ciphersuite.
8.5.
Deletion Schedule
It is important to delete all security-sensitive values as soon as they are
consumed
. A sensitive value S is said to be
consumed
if
S was used to encrypt or (successfully) decrypt a message, or if
a key, nonce, or secret derived from S has been consumed. (This goes for
values derived via DeriveSecret as well as ExpandWithLabel.)
Here, S may be the
init_secret
commit_secret
epoch_secret
encryption_secret
as well as any secret in a Secret Tree or one of the
ratchets.
As soon as a group member consumes a value they MUST immediately delete
(all representations of) that value. This is crucial to ensuring
forward secrecy for past messages. Members MAY keep unconsumed values around
for some reasonable amount of time to handle out-of-order message delivery.
For example, suppose a group member encrypts or (successfully) decrypts an
application message using the j-th key and nonce in the ratchet of node
index N in some epoch n. Then, for that member, at least the following
values have been consumed and MUST be deleted:
the
commit_secret
joiner_secret
epoch_secret
encryption_secret
of
that epoch n as well as the
init_secret
of the previous epoch n-1,
all node secrets in the Secret Tree on the path from the root to the leaf with
node index N,
the first j secrets in the application data ratchet of node index N and
application_ratchet_nonce_[N]_[j]
and
application_ratchet_key_[N]_[j]
Concretely, suppose we have the following Secret Tree and ratchet for
participant D:
/ \
/ \
E F
/ \ / \
A B C D
/ \
HR0 AR0 -+- K0
| |
| +- N0
AR1 -+- K1
| |
| +- N1
AR2
Then if a client uses key K1 and nonce N1 during epoch n then it must consume
(at least) values G, F, D, AR0, AR1, K1, N1 as well as the key schedule secrets
used to derive G (the
encryption_secret
), namely
init_secret
of epoch n-1
and
commit_secret
joiner_secret
epoch_secret
of epoch n. The client MAY
retain (not consume) the values K0 and N0 to allow for out-of-order delivery,
and SHOULD retain AR2 for processing future messages.
8.6.
Exporters
The main MLS key schedule provides an
exporter_secret
which can
be used by an application as the basis to derive new secrets called
exported_value
outside the MLS layer.
MLS-Exporter(Label, Context, key_length) =
ExpandWithLabel(DeriveSecret(exporter_secret, Label),
"exporter", Hash(Context), key_length)
Each application SHOULD provide a unique label to
MLS-Exporter
that
identifies its use case. This is to prevent two
exported outputs from being generated with the same values and used
for different functionalities.
The exported values are bound to the group epoch from which the
exporter_secret
is derived, hence reflects a particular state of
the group.
It is RECOMMENDED for the application generating exported values
to refresh those values after a Commit is processed.
8.7.
Resumption Secret
The main MLS key schedule provides a
resumption_secret
which can provide extra
security in some cross-group operations.
The application SHOULD specify an upper limit on the number of past
epochs for which the
resumption_secret
may be stored.
There are two ways in which a
resumption_secret
can be used: to re-initialize
the group with different parameters, or to create a
sub-group of an existing group as detailed in
Section 8.2
Resumption keys are distinguished from exporter keys in that they have specific
use inside the MLS protocol, whereas the use of exporter secrets may be
decided by an external application. They are thus derived separately to avoid
key material reuse.
8.8.
State Authentication Keys
The main MLS key schedule provides a per-epoch
authentication_secret
If one of the parties is being actively impersonated by an attacker, their
authentication_secret
will differ from that of the other group members.
Thus, members of a group MAY use their
authentication_secrets
within
an out-of-band authentication protocol to ensure that they
share the same view of the group.
9.
Message Framing
Handshake and application messages use a common framing structure.
This framing provides encryption to ensure confidentiality within the
group, as well as signing to authenticate the sender within the group.
The two main structures involved are MLSPlaintext and MLSCiphertext.
MLSCiphertext represents a signed and encrypted message, with
protections for both the content of the message and related
metadata. MLSPlaintext represents a message that is only signed,
and not encrypted. Applications MUST use MLSCiphertext to encrypt
application messages and SHOULD use MLSCiphertext to encode
handshake messages, but MAY transmit handshake messages encoded
as MLSPlaintext objects in cases where it is necessary for the
Delivery Service to examine such messages.
enum {
reserved(0),
application(1),
proposal(2),
commit(3),
(255)
} ContentType;

enum {
reserved(0),
member(1),
preconfigured(2),
new_member(3),
(255)
} SenderType;

struct {
SenderType sender_type;
switch (sender_type) {
case member: KeyPackageID member;
case preconfigured: opaque external_key_id<0..255>;
case new_member: struct{};
} Sender;

struct {
opaque mac_value<0..255>;
} MAC;

enum {
reserved(0),
mls_plaintext(1),
mls_ciphertext(2),
(255)
} WireFormat;

struct {
WireFormat wire_format;
opaque group_id<0..255>;
uint64 epoch;
Sender sender;
opaque authenticated_data<0..2^32-1>;

ContentType content_type;
select (MLSPlaintext.content_type) {
case application:
opaque application_data<0..2^32-1>;

case proposal:
Proposal proposal;

case commit:
Commit commit;

opaque signature<0..2^16-1>;
optional confirmation_tag;
optional membership_tag;
} MLSPlaintext;

struct {
WireFormat wire_format = mls_ciphertext;
opaque group_id<0..255>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<0..2^32-1>;
opaque encrypted_sender_data<0..255>;
opaque ciphertext<0..2^32-1>;
} MLSCiphertext;
The field
confirmation_tag
MUST be present if
content_type
equals commit.
Otherwise, it MUST NOT be present.
External sender types are sent as MLSPlaintext, see
Section 11.1.9
for their use.
The remainder of this section describes how to compute the signature of an
MLSPlaintext object and how to convert it to an MLSCiphertext object for
member
sender types. The steps are:
Set
group_id
epoch
content_type
and
authenticated_data
fields from the
MLSPlaintext object directly
Identify the key and key generation depending on the content type
Encrypt an MLSCiphertextContent for the ciphertext field using the key
identified and MLSPlaintext object
Encrypt the sender data using a key and nonce derived from the
sender_data_secret
for the epoch and a sample of the encrypted
MLSCiphertextContent.
Decryption is done by decrypting the sender data, then the message, and then
verifying the content signature.
The following sections describe the encryption and signing processes in detail.
9.1.
Content Authentication
The
signature
field in an MLSPlaintext object is computed using the signing
private key corresponding to the public key, which was authenticated by the
credential at the leaf of the tree indicated by the sender field. The signature
covers the plaintext metadata and message content, which is all of MLSPlaintext
except for the
signature
, the
confirmation_tag
and
membership_tag
fields.
If the sender is a member of the group, the signature also covers the
GroupContext for the current epoch, so that signatures are specific to a given
group and epoch.
struct {
select (MLSPlaintextTBS.sender.sender_type) {
case member:
GroupContext context;

case preconfigured:
case new_member:
struct{};

WireFormat wire_format;
opaque group_id<0..255>;
uint64 epoch;
Sender sender;
opaque authenticated_data<0..2^32-1>;

ContentType content_type;
select (MLSPlaintextTBS.content_type) {
case application:
opaque application_data<0..2^32-1>;

case proposal:
Proposal proposal;

case commit:
Commit commit;
} MLSPlaintextTBS;
The
membership_tag
field in the MLSPlaintext object authenticates the sender's
membership in the group. For an MLSPlaintext with a sender type other than
member
, this field MUST be omitted. For messages sent by members, it MUST be
present and set to the following value:
struct {
MLSPlaintextTBS tbs;
opaque signature<0..2^16-1>;
optional confirmation_tag;
} MLSPlaintextTBM;

membership_tag = MAC(membership_key, MLSPlaintextTBM);
Note that the
membership_tag
only needs to be computed for MLSPlaintext
messages that will be sent over the wire (
wire_format == mls_plaintext
). It
isn't needed for messages that will be encrypted and transmitted as
MLSCiphertext messages (
wire_format == mls_ciphertext
).
9.2.
Content Encryption
The
ciphertext
field of the MLSCiphertext object is produced by supplying the
inputs described below to the AEAD function specified by the ciphersuite in use.
The plaintext input contains the content and signature of the MLSPlaintext, plus
optional padding. These values are encoded in the following form:
struct {
select (MLSCiphertext.content_type) {
case application:
opaque application_data<0..2^32-1>;

case proposal:
Proposal proposal;

case commit:
Commit commit;

opaque signature<0..2^16-1>;
optional confirmation_tag;
opaque padding<0..2^16-1>;
} MLSCiphertextContent;
In the MLS key schedule, the sender creates two distinct key ratchets for
handshake and application messages for each member of the group. When encrypting
a message, the sender looks at the ratchets it derived for its own member and
chooses an unused generation from either the handshake or application ratchet
depending on the content type of the message. This generation of the ratchet is
used to derive a provisional nonce and key.
Before use in the encryption operation, the nonce is XORed with a fresh random
value to guard against reuse. Because the key schedule generates nonces
deterministically, a client must keep persistent state as to where in the key
schedule it is; if this persistent state is lost or corrupted, a client might
reuse a generation that has already been used, causing reuse of a key/nonce pair.
To avoid this situation, the sender of a message MUST generate a fresh random
4-byte "reuse guard" value and XOR it with the first four bytes of the nonce
from the key schedule before using the nonce for encryption. The sender MUST
include the reuse guard in the
reuse_guard
field of the sender data object, so
that the recipient of the message can use it to compute the nonce to be used for
decryption.
+-+-+-+-+---------...---+
| Key Schedule Nonce |
+-+-+-+-+---------...---+
XOR
+-+-+-+-+---------...---+
| Guard | 0 |
+-+-+-+-+---------...---+
===
+-+-+-+-+---------...---+
| Encrypt/Decrypt Nonce |
+-+-+-+-+---------...---+
The Additional Authenticated Data (AAD) input to the encryption
contains an object of the following form, with the values used to
identify the key and nonce:
struct {
opaque group_id<0..255>;
uint64 epoch;
ContentType content_type;
opaque authenticated_data<0..2^32-1>;
} MLSCiphertextContentAAD;
9.3.
Sender Data Encryption
The "sender data" used to look up the key for the content encryption is
encrypted with the ciphersuite's AEAD with a key and nonce derived from both the
sender_data_secret
and a sample of the encrypted content. Before being
encrypted, the sender data is encoded as an object of the following form:
struct {
KeyPackageID sender;
uint32 generation;
opaque reuse_guard[4];
} MLSSenderData;
MLSSenderData.sender is assumed to be a
member
sender type. When constructing
an MLSSenderData from a Sender object, the sender MUST verify Sender.sender_type
is
member
and use Sender.sender for MLSSenderData.sender.
The
reuse_guard
field contains a fresh random value used to avoid nonce reuse
in the case of state loss or corruption, as described in
Section 9.2
The key and nonce provided to the AEAD are computed as the KDF of the first
KDF.Nh
bytes of the ciphertext generated in the previous section. If the
length of the ciphertext is less than
KDF.Nh
, the whole ciphertext is used
without padding. In pseudocode, the key and nonce are derived as:
ciphertext_sample = ciphertext[0..KDF.Nh-1]

sender_data_key = ExpandWithLabel(sender_data_secret, "key", ciphertext_sample, AEAD.Nk)
sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce", ciphertext_sample, AEAD.Nn)
The Additional Authenticated Data (AAD) for the SenderData ciphertext is all the
fields of MLSCiphertext excluding
encrypted_sender_data
struct {
opaque group_id<0..255>;
uint64 epoch;
ContentType content_type;
} MLSSenderDataAAD;
When parsing a SenderData struct as part of message decryption, the recipient
MUST verify that the KeyPackageID indicated in the
sender
field identifies a
member of the group.
10.
Group Creation
A group is always created with a single member, the "creator". The other
members are added when the creator effectively sends itself an Add proposal and
commits it, then sends the corresponding Welcome message to the new
participants. These processes are described in detail in
Section 11.1.1
Section 11.2
and
Section 11.2.2
The creator of a group MUST take the following steps to initialize the group:
Fetch KeyPackages for the members to be added, and select a version and
ciphersuite according to the capabilities of the members. To protect against
downgrade attacks, the creator MUST use the
capabilities
extensions
in these KeyPackages to verify that the
chosen version and ciphersuite is the best option supported by all members.
Initialize a one-member group with the following initial values:
Ratchet tree: A tree with a single node, a leaf containing an HPKE public
key and credential for the creator
Group ID: A value set by the creator
Epoch: 0
Tree hash: The root hash of the above ratchet tree
Confirmed transcript hash: The zero-length octet string
Interim transcript hash: The zero-length octet string
Init secret: A fresh random value of size
KDF.Nh
Extensions: Any values of the creator's choosing
For each member, construct an Add proposal from the KeyPackage for that
member (see
Section 11.1.1
Construct a Commit message that commits all of the Add proposals, in any order
chosen by the creator (see
Section 11.2
Process the Commit message to obtain a new group state (for the epoch in which
the new members are added) and a Welcome message
Transmit the Welcome message to the other new members
The recipient of a Welcome message processes it as described in
Section 11.2.2
In principle, the above process could be streamlined by having the
creator directly create a tree and choose a random value for first
epoch's epoch secret. We follow the steps above because it removes
unnecessary choices, by which, for example, bad randomness could be
introduced. The only choices the creator makes here are its own
KeyPackage, the leaf secret from which the Commit is built, and the
intermediate key pairs along the direct path to the root.
10.1.
Required Capabilities
The configuration of a group imposes certain requirements on clients in the
group. At a minimum, all members of the group need to support the ciphersuite
and protocol version in use. Additional requirements can be imposed by
including a
required_capabilities
extension in the GroupContext.
struct {
ExtensionType extensions<0..255>;
ProposalType proposals<0..255>;
} RequiredCapabilities;
This extension lists the extensions and proposal types that must be supported by
all members of the group. For new members, it is enforced by existing members during the
application of Add commits. Existing members should of course be in compliance
already. In order to ensure this continues to be the case even as the group's
extensions can be updated, a GroupContextExtensions proposal is invalid if it
contains a
required_capabilities
extension that requires capabililities not
supported by all current members.
10.2.
Linking a New Group to an Existing Group
A new group may be tied to an already existing group for the purpose of
re-initializing the existing group, or to branch into a sub-group.
Re-initializing an existing group may be used, for example, to restart the group
with a different ciphersuite or protocol version. Branching may be used to
bootstrap a new group consisting of a subset of current group members, based on
the current group state.
In both cases, the
psk_nonce
included in the
PreSharedKeyID
object must be a
randomly sampled nonce of length
KDF.Nh
to avoid key re-use.
10.2.1.
Sub-group Branching
If a client wants to create a subgroup of an existing group, they MAY choose to
include a
PreSharedKeyID
in the
GroupSecrets
object of the Welcome message choosing
the
psktype
branch
, the
group_id
of the group from which a subgroup is to
be branched, as well as an epoch within the number of epochs for which a
resumption_secret
is kept.
11.
Group Evolution
Over the lifetime of a group, its membership can change, and existing members
might want to change their keys in order to achieve post-compromise security. In
MLS, each such change is accomplished by a two-step process:
A proposal to make the change is broadcast to the group in a Proposal message
A member of the group or a new member broadcasts a Commit message that causes one or more
proposed changes to enter into effect
The group thus evolves from one cryptographic state to another each time a
Commit message is sent and processed. These states are referred to as "epochs"
and are uniquely identified among states of the group by eight-octet epoch values.
When a new group is initialized, its initial state epoch is 0x0000000000000000. Each time
a state transition occurs, the epoch number is incremented by one.
11.1.
Proposals
Proposals are included in an MLSPlaintext by way of a Proposal structure that
indicates their type:
// See IANA registry for registered values
uint16 ProposalType;

struct {
ProposalType msg_type;
select (Proposal.msg_type) {
case add: Add;
case update: Update;
case remove: Remove;
case psk: PreSharedKey;
case reinit: ReInit;
case external_init: ExternalInit;
case app_ack: AppAck;
case group_context_extensions: GroupContextExtensions;
};
} Proposal;
On receiving an MLSPlaintext containing a Proposal, a client MUST verify the
signature on the enclosing MLSPlaintext. If the signature verifies
successfully, then the Proposal should be cached in such a way that it can be
retrieved by hash (as a ProposalOrRef object) in a later Commit message.
11.1.1.
Add
An Add proposal requests that a client with a specified KeyPackage be added
to the group. The proposer of the Add MUST validate the KeyPackage in the same
way as receipients are required to do below.
struct {
KeyPackage key_package;
} Add;
The proposer of the Add does not control where in the group's ratchet tree the
new member is added. Instead, the sender of the Commit message chooses a
location for each added member and states it in the Commit message.
An Add is applied after being included in a Commit message. The position of the
Add in the list of proposals determines the node index
index
of the leaf node
where the new member will be added. For the first Add in the Commit,
index
is
the leftmost empty leaf in the tree, for the second Add, the next empty leaf to
the right, etc.
Validate the KeyPackage:
Verify that the signature on the KeyPackage is valid using the public key
in the KeyPackage's credential
Verify that the following fields in the KeyPackage are unique among the
members of the group (including any other members added in the same
Commit):
(credential.identity, endpoint_id)
tuple
credential.signature_key
hpke_init_key
Verify that the KeyPackage is compatible with the group's parameters. The
ciphersuite and protocol version of the KeyPackage must match those in
use in the group. If the GroupContext has a
required_capabilities
extension, then the required extensions and proposals MUST be listed in
the KeyPackage's
capabilities
extension.
If necessary, extend the tree to the right until it has at least index + 1
leaves
For each non-blank intermediate node along the path from the leaf at position
index
to the root, add
index
to the
unmerged_leaves
list for the node.
Set the leaf node in the tree at position
index
to a new node containing the
public key from the KeyPackage in the Add, as well as the credential under
which the KeyPackage was signed
11.1.2.
Update
An Update proposal is a similar mechanism to Add with the distinction
that it is the sender's leaf KeyPackage in the tree which would be
updated with a new KeyPackage.
struct {
KeyPackage key_package;
} Update;
The values in the following fields of the KeyPackage contained in an
Update
proposal MUST be the same as those of the KeyPackage it replaces in the tree.
version
cipher_suite
credential.identity
endpoint_id
. However, the
value of the
credential.signature_key
field of the new KeyPackage MUST be
different from that of all other KeyPackages in the tree. Furthermore, the value
of the
hpke_init_key
field of the new KeyPackage MUST be different from that
of the KeyPackage it replaces.
A member of the group applies an Update message by taking the following steps:
Replace the sender's leaf KeyPackage with the one contained in
the Update proposal
Blank the intermediate nodes along the path from the sender's leaf to the root
11.1.3.
Remove
A Remove proposal requests that the member with KeyPackageID
removed
be removed
from the group.
struct {
KeyPackageID removed;
} Remove;
A member of the group applies a Remove message by taking the following steps:
Identify a leaf node containing a key package matching
removed
. This
lookup MUST be done on the tree before any non-Remove proposals have
been applied (the "old" tree in the terminology of
Section 11.2
), since
proposals such as Update can change the KeyPackage stored at a leaf.
Let
removed_index
be the node index of this leaf node.
Replace the leaf node at
removed_index
with a blank node
Blank the intermediate nodes along the path from
removed_index
to the root
Truncate the tree by reducing the size of tree until the rightmost non-blank leaf node
11.1.4.
PreSharedKey
A PreSharedKey proposal can be used to request that a pre-shared key be
injected into the key schedule in the process of advancing the epoch.
struct {
PreSharedKeyID psk;
} PreSharedKey;
The
psktype
of the pre-shared key MUST be
external
and the
psk_nonce
MUST
be a randomly sampled nonce of length
KDF.Nh
. When processing a Commit message
that includes one or more PreSharedKey proposals, group members derive
psk_secret
as described in
Section 8.2
, where the order of the PSKs
corresponds to the order of the
PreSharedKey
proposals in the Commit.
11.1.5.
ReInit
A ReInit proposal represents a request to re-initialize the group with different
parameters, for example, to increase the version number or to change the
ciphersuite. The re-initialization is done by creating a completely new group
and shutting down the old one.
struct {
opaque group_id<0..255>;
ProtocolVersion version;
CipherSuite cipher_suite;
Extension extensions<0..2^32-1>;
} ReInit;
A member of the group applies a ReInit proposal by waiting for the committer to
send the Welcome message and by checking that the
group_id
and the parameters
of the new group corresponds to the ones specified in the proposal. The Welcome
message MUST specify exactly one pre-shared key with
psktype = reinit
, and with
psk_group_id
and
psk_epoch
equal to the
group_id
and
epoch
of the
existing group after the Commit containing the
reinit
Proposal was processed.
The Welcome message may specify the inclusion of other pre-shared keys with a
psktype
different from
reinit
If a ReInit proposal is included in a Commit, it MUST be the only proposal
referenced by the Commit. If other non-ReInit proposals have been sent during
the epoch, the committer SHOULD prefer them over the ReInit proposal, allowing
the ReInit to be resent and applied in a subsequent epoch. The
version
field
in the ReInit proposal MUST be no less than the version for the current group.
11.1.6.
ExternalInit
An ExternalInit proposal is used by new members that want to join a group by
using an external commit. This propsal can only be used in that context.
struct {
opaque kem_output<0..2^16-1>;
} ExternalInit;
A member of the group applies an ExternalInit message by initializing the next
epoch using an init secret computed as described in
Section 8.1
The
kem_output
field contains the required KEM output.
11.1.7.
AppAck
An AppAck proposal is used to acknowledge receipt of application messages.
Though this information implies no change to the group, it is structured as a
Proposal message so that it is included in the group's transcript by being
included in Commit messages.
struct {
KeyPackageID sender;
uint32 first_generation;
uint32 last_generation;
} MessageRange;

struct {
MessageRange received_ranges<0..2^32-1>;
} AppAck;
An AppAck proposal represents a set of messages received by the sender in the
current epoch. Messages are represented by the
sender
and
generation
values
in the MLSCiphertext for the message. Each MessageRange represents receipt of a
span of messages whose
generation
values form a continuous range from
first_generation
to
last_generation
, inclusive.
AppAck proposals are sent as a guard against the Delivery Service dropping
application messages. The sequential nature of the
generation
field provides
a degree of loss detection, since gaps in the
generation
sequence indicate
dropped messages. AppAck completes this story by addressing the scenario where
the Delivery Service drops all messages after a certain point, so that a later
generation is never observed. Obviously, there is a risk that AppAck messages
could be suppressed as well, but their inclusion in the transcript means that if
they are suppressed then the group cannot advance at all.
The schedule on which sending AppAck proposals are sent is up to the application,
and determines which cases of loss/suppression are detected. For example:
The application might have the committer include an AppAck proposal whenever a
Commit is sent, so that other members could know when one of their messages
did not reach the committer.
The application could have a client send an AppAck whenever an application
message is sent, covering all messages received since its last AppAck. This
would provide a complete view of any losses experienced by active members.
The application could simply have clients send AppAck proposals on a timer, so
that all participants' state would be known.
An application using AppAck proposals to guard against loss/suppression of
application messages also needs to ensure that AppAck messages and the Commits
that reference them are not dropped. One way to do this is to always encrypt
Proposal and Commit messages, to make it more difficult for the Delivery Service
to recognize which messages conatain AppAcks. The application can also have
clients enforce an AppAck schedule, reporting loss if an AppAck is not received
at the expected time.
11.1.8.
GroupContextExtensions
A GroupContextExtensions proposal is used to update the list of extensions in
the GroupContext for the group.
struct {
Extension extensions<0..2^32-1>;
} GroupContextExtensions;
A member of the group applies a GroupContextExtensions proposal with the
following steps:
If the new extensions include a
required_capabilities
extension, verify that
all members of the group support the required capabilities (including those
added in the same commit, and excluding those removed).
Remove all of the existing extensions from the GroupContext object for the
group and replacing them with the list of extensions in the proposal. (This
is a wholesale replacement, not a merge. An extension is only carried over if
the sender of the proposal includes it in the new list.)
Note that once the GroupContext is updated, its inclusion in the
confirmation_tag by way of the key schedule will confirm that all members of the
group agree on the extensions in use.
11.1.9.
External Proposals
Add and Remove proposals can be constructed and sent to the group by a party
that is outside the group. For example, a Delivery Service might propose to
remove a member of a group who has been inactive for a long time, or propose adding
a newly-hired staff member to a group representing a real-world team. Proposals
originating outside the group are identified by a
preconfigured
or
new_member
SenderType in MLSPlaintext.
ReInit proposals can also be sent to the group by a
preconfigured
sender, for
example to enforce a changed policy regarding MLS version or ciphersuite.
The
new_member
SenderType is used for clients proposing that they themselves
be added. For this ID type the sender value MUST be zero and the Proposal type
MUST be Add. The MLSPlaintext MUST be signed with the private key corresponding
to the KeyPackage in the Add message. Recipients MUST verify that the
MLSPlaintext carrying the Proposal message is validly signed with this key.
The
preconfigured
SenderType is reserved for signers that are pre-provisioned
to the clients within a group. If proposals with these sender IDs are to be
accepted within a group, the members of the group MUST be provisioned by the
application with a mapping between these IDs and authorized signing keys.
Recipients MUST verify that the MLSPlaintext carrying the Proposal message is
validly signed with the corresponding key. To ensure consistent handling of
external proposals, the application MUST ensure that the members of a group
have the same mapping and apply the same policies to external proposals.
An external proposal MUST be sent as an MLSPlaintext
object, since the sender will not have the keys necessary to construct an
MLSCiphertext object.
11.2.
Commit
A Commit message initiates a new epoch for the group, based on a collection of
Proposals. It instructs group members to update their representation of the
state of the group by applying the proposals and advancing the key schedule.
Each proposal covered by the Commit is included by a ProposalOrRef value, which
identifies the proposal to be applied by value or by reference. Proposals
supplied by value are included directly in the Commit object. Proposals
supplied by reference are specified by including the hash of the MLSPlaintext in
which the Proposal was sent, using the hash function from the group's
ciphersuite. For proposals supplied by value, the sender of the proposal is the
same as the sender of the Commit. Conversely, proposals sent by people other
than the committer MUST be included by reference.
enum {
reserved(0),
proposal(1)
reference(2),
(255)
} ProposalOrRefType;

struct {
ProposalOrRefType type;
select (ProposalOrRef.type) {
case proposal: Proposal proposal;
case reference: opaque hash<0..255>;
} ProposalOrRef;

struct {
ProposalOrRef proposals<0..2^32-1>;
optional path;
} Commit;
A group member that has observed one or more proposals within an epoch MUST send
a Commit message before sending application data. This ensures, for example,
that any members whose removal was proposed during the epoch are actually
removed before any application data is transmitted.
The sender of a Commit MUST include all valid proposals that it has received
during the current epoch. Invalid proposals include, for example, proposals with
an invalid signature or proposals that are semantically invalid, such as an Add
when the sender does not have the application-level permission to add new users.
Proposals with a non-default proposal type MUST NOT be included in a commit
unless the proposal type is supported by all the members of the group that will
process the Commit (i.e., not including any members being added or removed by
the Commit).
If there are multiple proposals that apply to the same leaf, the committer
chooses one and includes only that one in the Commit, considering the rest
invalid. The committer MUST prefer any Remove received, or the most recent
Update for the leaf if there are no Removes. If there are multiple Add proposals
containing KeyPackages with the same tuple
(credential.identity, endpoint_id)
the committer again chooses one to include and considers the rest invalid. Add
proposals that contain KeyPackages with an
(credential.identity, endpoint_id)
tuple that matches that of an existing KeyPackage in the group MUST be
considered invalid. The comitter MUST consider invalid any Add or Update
proposal if the Credential in the contained KeyPackage shares the same signature
key with a Credential in any leaf of the group, or indeed if the KeyPackage
shares the same
hpke_init_key
with another KeyPackage in the group.
The Commit MUST NOT combine proposals sent within different epochs. In the event
that a valid proposal is omitted from the next Commit, the sender of the
proposal SHOULD retransmit it in the new epoch.
A member of the group MAY send a Commit that references no proposals at all,
which would thus have an empty
proposals
vector. Such
a Commit resets the sender's leaf and the nodes along its direct path, and
provides forward secrecy and post-compromise security with regard to the sender
of the Commit. An Update proposal can be regarded as a "lazy" version of this
operation, where only the leaf changes and intermediate nodes are blanked out.
The
path
field of a Commit message MUST be populated if the Commit covers at
least one Update or Remove proposal. The
path
field MUST also be populated
if the Commit covers no proposals at all (i.e., if the proposals vector
is empty). The
path
field MAY be omitted if the Commit covers only Add
proposals. In pseudocode, the logic for validating a Commit is as follows:
hasUpdates = false
hasRemoves = false

for i, id in commit.proposals:
proposal = proposalCache[id]
assert(proposal != null)

hasUpdates = hasUpdates || proposal.msg_type == update
hasRemoves = hasRemoves || proposal.msg_type == remove

if len(commit.proposals) == 0 || hasUpdates || hasRemoves:
assert(commit.path != null)
To summarize, a Commit can have three different configurations, with different
uses:
An "empty" Commit that references no proposals, which updates the committer's
contribution to the group and provides PCS with regard to the committer.
A "partial" Commit that references Add, PreSharedKey, or ReInit proposals but
where the path is empty. Such a commit doesn't provide PCS with regard to the
committer.
A "full" Commit that references proposals of any type, which provides FS with
regard to any removed members and PCS for the committer and any updated
members.
When creating or processing a Commit, three different ratchet trees and
their associated GroupContexts are used:
"Old" refers to the ratchet tree and GroupContext for the epoch before the
commit. The old GroupContext is used when signing the MLSPlainText so that
existing group members can verify the signature before processing the
commit.
"Provisional" refers to the ratchet tree and GroupContext constructed after
applying the proposals that are referenced by the Commit. The provisional
GroupContext uses the epoch number for the new epoch, and the old confirmed
transcript hash. This is used when creating the UpdatePath, if the
UpdatePath is needed.
"New" refers to the ratchet tree and GroupContext constructed after applying
the proposals and the UpdatePath (if any). The new GroupContext uses the
epoch number for the new epoch, and the new confirmed transcript hash. This
is used when deriving the new epoch secrets, and is the only GroupContext
that newly-added members will have.
A member of the group creates a Commit message and the corresponding Welcome
message at the same time, by taking the following steps:
Construct an initial Commit object with the
proposals
field populated from Proposals received during the current epoch, and an empty
path
field.
Generate the provisional ratchet tree and GroupContext by applying the proposals
referenced in the initial Commit object, as described in
Section 11.1
. Update
proposals are applied first, followed by Remove proposals, and then finally
Add proposals. Add proposals are applied in the order listed in the
proposals
vector, and always to the leftmost unoccupied leaf in the tree, or
the right edge of the tree if all leaves are occupied.
Note that the order in which different types of proposals are applied should
be updated by the implementation to include any new proposals added by
negotiated group extensions.
PreSharedKey proposals are processed later when deriving the
psk_secret
for the Key
Schedule.
Decide whether to populate the
path
field: If the
path
field is required
based on the proposals that are in the commit (see above), then it MUST be
populated. Otherwise, the sender MAY omit the
path
field at its discretion.
If populating the
path
field: Create an UpdatePath using the provisional
ratchet tree and GroupContext. Any new member (from an add proposal) MUST be
exluded from the resolution during the computation of the UpdatePath. The
leaf_key_package
for this UpdatePath must have a
parent_hash
extension.
Note that the KeyPackage in the
UpdatePath
effectively updates an existing
KeyPackage in the group and thus MUST adhere to the same restrictions as
KeyPackages used in
Update
proposals.
Assign this UpdatePath to the
path
field in the Commit.
Apply the UpdatePath to the tree, as described in
Section 5.5
, creating the new ratchet tree. Define
commit_secret
as the value
path_secret[n+1]
derived from the
path_secret[n]
value assigned to the root node.
If not populating the
path
field: Set the
path
field in the Commit to the
null optional. Define
commit_secret
as the all-zero vector of length
KDF.Nh
(the same length as a
path_secret
value would be). In this case,
the new ratchet tree is the same as the provisional ratchet tree.
Derive the
psk_secret
as specified in
Section 8.2
, where the order
of PSKs in the derivation corresponds to the order of PreSharedKey proposals
in the
proposals
vector.
Construct an MLSPlaintext object containing the Commit object. Sign the
MLSPlaintext using the old GroupContext as context.
Use the MLSPlaintext to update the confirmed transcript hash and generate
the new GroupContext.
Use the
init_secret
from the previous epoch, the
commit_secret
and the
psk_secret
as defined in the previous steps, and the new GroupContext to
compute the new
joiner_secret
welcome_secret
epoch_secret
, and
derived secrets for the new epoch.
Use the
confirmation_key
for the new epoch to compute the
confirmation_tag
value, and the
membership_key
for the old epoch to
compute the
membership_tag
value in the MLSPlaintext.
Calculate the interim transcript hash using the new confirmed transcript
hash and the
confirmation_tag
from the MLSPlaintext.
Construct a GroupInfo reflecting the new state:
Group ID, epoch, tree, confirmed transcript hash, interim transcript
hash, and group context extensions from the new state
The confirmation_tag from the MLSPlaintext object
Other extensions as defined by the application
Sign the GroupInfo using the member's private signing key
Encrypt the GroupInfo using the key and nonce derived from the
joiner_secret
for the new epoch (see
Section 11.2.2
For each new member in the group:
Identify the lowest common ancestor in the tree of the new member's
leaf node and the member sending the Commit
If the
path
field was populated above: Compute the path secret
corresponding to the common ancestor node
Compute an EncryptedGroupSecrets object that encapsulates the
init_secret
for the current epoch and the path secret (if present).
Construct a Welcome message from the encrypted GroupInfo object, the encrypted
key packages, and any PSKs for which a proposal was included in the Commit. The
order of the
psks
MUST be the same as the order of PreSharedKey proposals in the
proposals
vector.
If a ReInit proposal was part of the Commit, the committer MUST create a new
group with the parameters specified in the ReInit proposal,
and with the same members as the original group.
The Welcome message MUST include a
PreSharedKeyID
with
psktype
reinit
and with
psk_group_id
and
psk_epoch
corresponding to the current
group and the epoch after the commit was processed.
A member of the group applies a Commit message by taking the following steps:
Verify that the
epoch
field of the enclosing MLSPlaintext message is equal
to the
epoch
field of the current GroupContext object
Verify that the signature on the MLSPlaintext message verifies using the
public key from the credential stored at the leaf in the tree indicated by
the
sender
field.
Verify that all PSKs specified in any PreSharedKey proposals in the
proposals
vector
are available.
Generate the provisional ratchet tree and GroupContext by applying the proposals
referenced in the initial Commit object, as described in
Section 11.1
. Update
proposals are applied first, followed by Remove proposals, and then finally
Add proposals. Add proposals are applied in the order listed in the
proposals
vector, and always to the leftmost unoccupied leaf in the tree, or
the right edge of the tree if all leaves are occupied.
Note that the order in which different types of proposals are applied should
be updated by the implementation to include any new proposals added by
negotiated group extensions.
Verify that the
path
value is populated if the
proposals
vector contains
any Update or Remove proposals, or if it's empty. Otherwise, the
path
value
MAY be omitted.
If the
path
value is populated: Process the
path
value using the
provisional ratchet tree and GroupContext, to generate the new ratchet tree
and the
commit_secret
Apply the UpdatePath to the tree, as described in
Section 5.5
, and store
leaf_key_package
at the
Committer's leaf.
Verify that the KeyPackage has a
parent_hash
extension and that its value
matches the new parent of the sender's leaf node.
Define
commit_secret
as the value
path_secret[n+1]
derived from the
path_secret[n]
value assigned to the root node.
If the
path
value is not populated: Define
commit_secret
as the all-zero
vector of length
KDF.Nh
(the same length as a
path_secret
value would be).
Update the confirmed and interim transcript hashes using the new Commit, and
generate the new GroupContext.
Derive the
psk_secret
as specified in
Section 8.2
, where the order
of PSKs in the derivation corresponds to the order of PreSharedKey proposals
in the
proposals
vector.
Use the
init_secret
from the previous epoch, the
commit_secret
and the
psk_secret
as defined in the previous steps, and the new GroupContext to
compute the new
joiner_secret
welcome_secret
epoch_secret
, and
derived secrets for the new epoch.
Use the
confirmation_key
for the new epoch to compute the confirmation tag
for this message, as described below, and verify that it is the same as the
confirmation_tag
field in the MLSPlaintext object.
If the above checks are successful, consider the new GroupContext object
as the current state of the group.
If the Commit included a ReInit proposal, the client MUST NOT use the group to
send messages anymore. Instead, it MUST wait for a Welcome message from the committer
and check that
The
version
cipher_suite
and
extensions
fields of the new group
corresponds to the ones in the
ReInit
proposal, and that the
version
is greater than or equal to that of the original group.
The
psks
field in the Welcome message includes a
PreSharedKeyID
with
psktype
reinit
, and
psk_epoch
and
psk_group_id
equal to the epoch
and group ID of the original group after processing the Commit.
The confirmation tag value confirms that the members of the group have arrived
at the same state of the group:
MLSPlaintext.confirmation_tag =
MAC(confirmation_key, GroupContext.confirmed_transcript_hash)
11.2.1.
External Commits
External Commits are a mechanism for new members (external parties that want to
become members of the group) to add themselves to a group, without requiring
that an existing member has to come online to issue a Commit that references an
Add Proposal.
Whether existing members of the group will accept or reject an External Commit
follows the same rules that are applied to other handshake messages.
New members can create and issue an External Commit if they have access to the
following information for the group's current epoch:
group ID
epoch ID
ciphersuite
public tree hash
interim transcript hash
group extensions
external public key
This information is aggregated in a
PublicGroupState
object as follows:
struct {
CipherSuite cipher_suite;
opaque group_id<0..255>;
uint64 epoch;
opaque tree_hash<0..255>;
opaque interim_transcript_hash<0..255>;
Extension group_context_extensions<0..2^32-1>;
Extension other_extensions<0..2^32-1>;
HPKEPublicKey external_pub;
KeyPackageID signer;
opaque signature<0..2^16-1>;
} PublicGroupState;
Note that the
tree_hash
field is used the same way as in the Welcome message.
The full tree can be included via the
ratchet_tree
extension
Section 11.3
The signature MUST verify using the public key taken from the credential in the
leaf node of the member with KeyPackageID
signer
. The signature covers the
following structure, comprising all the fields in the PublicGroupState above
signature
struct {
opaque group_id<0..255>;
uint64 epoch;
opaque tree_hash<0..255>;
opaque interim_transcript_hash<0..255>;
Extension group_context_extensions<0..2^32-1>;
Extension other_extensions<0..2^32-1>;
HPKEPublicKey external_pub;
KeyPackageID signer;
} PublicGroupStateTBS;
This signature authenticates the HPKE public key, so that the joiner knows that
the public key was provided by a member of the group. The fields that are not
signed are included in the key schedule via the GroupContext object. If the
joiner is provided an inaccurate data for these fields, then its external Commit
will have an incorrect
confirmation_tag
and thus be rejected.
The information in a PublicGroupState is not deemed public in general, but
applications can choose to make it available to new members in order to allow
External Commits.
External Commits work like regular Commits, with a few differences:
The proposals included by value in an External Commit MUST meet the following
conditions:
There MUST be a single Add proposal that adds the new issuing new member to
the group
There MUST be a single ExternalInit proposal
There MUST NOT be any Update proposals
If a Remove proposal is present, then the
credential
and
endpoint_id
of
the removed leaf MUST be the same as the corresponding values in the Add
KeyPackage.
The proposals included by reference in an External Commit MUST meet the following
conditions:
There MUST NOT be any ExternalInit proposals
External Commits MUST contain a
path
field (and is therefore a "full"
Commit)
External Commits MUST be signed by the new member. In particular, the
signature on the enclosing MLSPlaintext MUST verify using the public key for
the credential in the
leaf_key_package
of the
path
field.
When processing a Commit, both existing and new members MUST use the external
init secret as described in
Section 8.1
The sender type for the MLSPlaintext encapsulating the External Commit MUST be
new_member
In other words, External Commits come in two "flavors" -- a "join" commit that
adds the sender to the group or a "resync" commit that replaces a member's prior
appearance with a new one.
Note that the "resync" operation allows an attacker that has compromised a
member's signature private key to introduce themselves into the group and remove the
prior, legitimate member in a single Commit. Without resync, this
can still be done, but requires two operations, the external Commit to join and
a second Commit to remove the old appearance. Applications for whom this
distinction is salient can choose to disallow external commits that contain a
Remove, or to allow such resync commits only if they contain a "reinit" PSK
proposal that demonstrates the joining member's presence in a prior epoch of the
group. With the latter approach, the attacke would need to compromise the PSK
as well as the signing key, but the application will need to ensure that
continuing, non-resync'ing members have the required PSK.
11.2.2.
Welcoming New Members
The sender of a Commit message is responsible for sending a Welcome message to
any new members added via Add proposals. The Welcome message provides the new
members with the current state of the group, after the application of the Commit
message. The new members will not be able to decrypt or verify the Commit
message, but will have the secrets they need to participate in the epoch
initiated by the Commit message.
In order to allow the same Welcome message to be sent to all new members,
information describing the group is encrypted with a symmetric key and nonce
derived from the
joiner_secret
for the new epoch. The
joiner_secret
is
then encrypted to each new member using HPKE. In the same encrypted package,
the committer transmits the path secret for the lowest node contained in the
direct paths of both the committer and the new member. This allows the new
member to compute private keys for nodes in its direct path that are being
reset by the corresponding Commit.
If the sender of the Welcome message wants the receiving member to include a PSK
in the derivation of the
epoch_secret
, they can populate the
psks
field indicating which
PSK to use.
struct {
opaque group_id<0..255>;
uint64 epoch;
opaque tree_hash<0..255>;
opaque confirmed_transcript_hash<0..255>;
Extension group_context_extensions<0..2^32-1>;
Extension other_extensions<0..2^32-1>;
MAC confirmation_tag;
KeyPackageID signer;
opaque signature<0..2^16-1>;
} GroupInfo;

struct {
opaque path_secret<1..255>;
} PathSecret;

struct {
opaque joiner_secret<1..255>;
optional path_secret;
PreSharedKeys psks;
} GroupSecrets;

struct {
KeyPackageID new_member<1..255>;
HPKECiphertext encrypted_group_secrets;
} EncryptedGroupSecrets;

struct {
ProtocolVersion version = mls10;
CipherSuite cipher_suite;
EncryptedGroupSecrets secrets<0..2^32-1>;
opaque encrypted_group_info<1..2^32-1>;
} Welcome;
The client processing a Welcome message will need to have a copy of the group's
ratchet tree. The tree can be provided in the Welcome message, in an extension
of type
ratchet_tree
. If it is sent otherwise (e.g., provided by a caching
service on the Delivery Service), then the client MUST download the tree before
processing the Welcome.
On receiving a Welcome message, a client processes it using the following steps:
Identify an entry in the
secrets
array where the
new_member
value corresponds to one of this client's KeyPackages, using the hash
indicated by the
cipher_suite
field. If no such field exists, or if the
ciphersuite indicated in the KeyPackage does not match the one in the
Welcome message, return an error.
Decrypt the
encrypted_group_secrets
using HPKE with the algorithms indicated
by the ciphersuite and the HPKE private key corresponding to the GroupSecrets.
If a
PreSharedKeyID
is part of the GroupSecrets and the client is not in
possession of the corresponding PSK, return an error.
From the
joiner_secret
in the decrypted GroupSecrets object and the PSKs
specified in the
GroupSecrets
, derive the
welcome_secret
and using that
the
welcome_key
and
welcome_nonce
. Use the key and nonce to decrypt the
encrypted_group_info
field.
welcome_nonce = KDF.Expand(welcome_secret, "nonce", AEAD.Nn)
welcome_key = KDF.Expand(welcome_secret, "key", AEAD.Nk)
Verify the signature on the GroupInfo object. The signature input comprises
all of the fields in the GroupInfo object except the signature field. The
public key and algorithm are taken from the credential in the leaf node of the
member with KeyPackageID
signer
. If there is no matching leaf node, or if
signature verification fails, return an error.
Verify the integrity of the ratchet tree.
Verify that the tree hash of the ratchet tree matches the
tree_hash
field
in the GroupInfo.
For each non-empty parent node, verify that exactly one of the node's
children are non-empty and have the hash of this node set as their
parent_hash
value (if the child is another parent) or has a
parent_hash
extension in the KeyPackage containing the same value (if the child is a
leaf). If either of the node's children is empty, and in particular does not
have a parent hash, then its respective children's
parent_hash
values have
to be considered instead.
For each non-empty leaf node, verify the signature on the KeyPackage.
Identify a leaf in the
tree
array (any even-numbered node) whose
key_package
field is identical to the KeyPackage. If no such field
exists, return an error. Let
index
represent the index of this node in the
tree.
Construct a new group state using the information in the GroupInfo object.
The GroupContext contains the
group_id
epoch
tree_hash
confirmed_transcript_hash
, and
group_context_extensions
fields from
the GroupInfo object.
The new member's position in the tree is
index
, as defined above.
Update the leaf at index
index
with the private key corresponding to the
public key in the node.
If the
path_secret
value is set in the GroupSecrets object: Identify the
lowest common ancestor of the node index
index
and of the node index of
the member with KeyPackageID
GroupInfo.signer
. Set the private key for
this node to the private key derived from the
path_secret
For each parent of the common ancestor, up to the root of the tree, derive
a new path secret and set the private key for the node to the private key
derived from the path secret. The private key MUST be the private key
that corresponds to the public key in the node.
Use the
joiner_secret
from the GroupSecrets object to generate the epoch secret
and other derived secrets for the current epoch.
Set the confirmed transcript hash in the new state to the value of the
confirmed_transcript_hash
in the GroupInfo.
Verify the confirmation tag in the GroupInfo using the derived confirmation
key and the
confirmed_transcript_hash
from the GroupInfo.
Use the confirmed transcript hash and confirmation tag to compute the interim
transcript hash in the new state.
11.3.
Ratchet Tree Extension
By default, a GroupInfo message only provides the joiner with a commitment
to the group's ratchet tree. In order to process or generate handshake
messages, the joiner will need to get a copy of the ratchet tree from some other
source. (For example, the DS might provide a cached copy.) The inclusion of
the tree hash in the GroupInfo message means that the source of the ratchet
tree need not be trusted to maintain the integrity of tree.
In cases where the application does not wish to provide such an external source,
the whole public state of the ratchet tree can be provided in an extension of
type
ratchet_tree
, containing a
ratchet_tree
object of the following form:
enum {
reserved(0),
leaf(1),
parent(2),
(255)
} NodeType;

struct {
NodeType node_type;
select (Node.node_type) {
case leaf: KeyPackage key_package;
case parent: ParentNode node;
};
} Node;

optional ratchet_tree<1..2^32-1>;
The presence of a
ratchet_tree
extension in a GroupInfo message does not
result in any changes to the GroupContext extensions for the group. The ratchet
tree provided is simply stored by the client and used for MLS operations.
If this extension is not provided in a Welcome message, then the client will
need to fetch the ratchet tree over some other channel before it can generate or
process Commit messages. Applications should ensure that this out-of-band
channel is provided with security protections equivalent to the protections that
are afforded to Proposal and Commit messages. For example, an application that
encrypts Proposal and Commit messages might distribute ratchet trees encrypted
using a key exchanged over the MLS channel.
12.
Extensibility
This protocol includes a mechanism for negotiating extension parameters similar
to the one in TLS
RFC8446
. In TLS, extension negotiation is one-to-one: The
client offers extensions in its ClientHello message, and the server expresses
its choices for the session with extensions in its ServerHello and
EncryptedExtensions messages. In MLS, extensions appear in the following
places:
In KeyPackages, to describe client capabilities and aspects of their
participation in the group (once in the ratchet tree)
In the Welcome message, to tell new members of a group what parameters are
being used by the group, and to provide any additional details required to
join the group
In the GroupContext object, to ensure that all members of the group have the
same view of the parameters in use
In other words, an application can use GroupContext extensions to ensure that
all members of the group agree on a set of parameters. Clients indicate
their support for parameters in KeyPackage extensions. New members of a
group are informed of the group's GroupContext extensions via the
group_context_extensions
field in the GroupInfo or PublicGroupState object.
The
other_extensions
field in a GroupInfo object can be used to provide
additional parameters to new joiners that are used to join the group.
This extension mechanism is designed to allow for secure and forward-compatible
negotiation of extensions. For this to work, implementations MUST correctly
handle extensible fields:
A client that posts a KeyPackage MUST support all parameters advertised in
it. Otherwise, another client might fail to interoperate by selecting one of
those parameters.
A client initiating a group MUST ignore all unrecognized ciphersuites,
extensions, and other parameters. Otherwise, it may fail to interoperate with
newer clients.
A client adding a new member to a group MUST verify that the KeyPackage
for the new member contains extensions that are consistent with the group's
extensions. For each extension in the GroupContext, the KeyPackage MUST
have an extension of the same type, and the contents of the extension MUST be
consistent with the value of the extension in the GroupContext, according to
the semantics of the specific extension.
If any extension in a GroupInfo message is unrecognized (i.e., not contained
in the corresponding KeyPackage), then the client MUST reject the Welcome
message and not join the group.
The extensions populated into a GroupContext object are drawn from those in
the GroupInfo object, according to the definitions of those extensions.
Note that the latter two requirements mean that all MLS extensions are
mandatory, in the sense that an extension in use by the group MUST be supported
by all members of the group.
This document does not define any way for the parameters of the group to change
once it has been created; such a behavior could be implemented as an extension.
13.
Sequencing of State Changes
Each Commit message is premised on a given starting state,
indicated by the
epoch
field of the enclosing MLSPlaintext
message. If the changes implied by a Commit 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 Commit message, it is based on the most
current state of the group. In practice, however, there is a risk
that two members will generate Commit messages simultaneously,
based on the same state.
When this happens, there is a need for the members of the group to
deconflict the simultaneous Commit messages. There are two
general approaches:
Have the Delivery Service enforce a total order
Have a signal in the message that clients can use to break ties
As long as Commit messages cannot be merged, there is a risk of
starvation. In a sufficiently busy group, a given member may never
be able to send a Commit 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.
It might be possible, because of the non-contributivity of intermediate
nodes, that Commit messages could be applied one after the other
without the Delivery Service having to reject any Commit message,
which would make MLS more resilient regarding the concurrency of
Commit messages.
The Messaging system can decide to choose the order for applying
the state changes. Note that there are certain cases (if no total
ordering is applied by the Delivery Service) where the ordering is
important for security, ie. all updates must be executed before
removes.
Regardless of how messages are kept in sequence, implementations
MUST only update their cryptographic state when valid Commit
messages are received.
Generation of Commit messages MUST NOT modify a client's state, since the
endpoint doesn't know at that time whether the changes implied by
the Commit message will succeed or not.
13.1.
Server-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 break ties
when two members send a Commit message at the same time.
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.
13.2.
Client-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.
14.
Application Messages
The primary purpose of the Handshake protocol is to provide an
authenticated group key exchange to clients. In order to protect
Application messages sent among the members of a group, the Application
secret provided by the Handshake key schedule is used to derive nonces
and encryption keys for the Message Protection Layer according to
the Application Key Schedule. That is, each epoch is equipped with
a fresh Application Key Schedule which consist of a tree of Application
Secrets as well as one symmetric ratchet per group member.
Each client maintains their own local copy of the Application Key
Schedule for each epoch during which they are a group member. They
derive new keys, nonces and secrets as needed while deleting old
ones as soon as they have been used.
Application messages MUST be protected with the Authenticated-Encryption
with Associated-Data (AEAD) encryption scheme associated with the
MLS ciphersuite using the common framing mechanism.
Note that "Authenticated" in this context does not mean messages are
known to be sent by a specific client but only from a legitimate
member of the group.
To authenticate a message from a particular member, signatures are
required. Handshake messages MUST use asymmetric signatures to strongly
authenticate the sender of a message.
14.1.
Message Encryption and Decryption
The group members MUST use the AEAD algorithm associated with
the negotiated MLS ciphersuite to AEAD encrypt and decrypt their
Application messages according to the Message Framing section.
The group identifier and epoch allow a recipient to know which group secrets
should be used and from which Epoch secret to start computing other secrets
and keys. The sender identifier is used to identify the member's
symmetric ratchet from the initial group Application secret. The application
generation field is used to determine how far into the ratchet to iterate in
order to reproduce the required AEAD keys and nonce for performing decryption.
Application messages SHOULD be padded to provide some resistance
against traffic analysis techniques over encrypted traffic.
CLINIC
HCJ16
While MLS might deliver the same payload less frequently across
a lot of ciphertexts than traditional web servers, it might still provide
the attacker enough information to mount an attack. If Alice asks Bob:
"When are we going to the movie ?" the answer "Wednesday" might be leaked
to an adversary by the ciphertext length. An attacker expecting Alice to
answer Bob with a day of the week might find out the plaintext by
correlation between the question and the length.
Similarly to TLS 1.3, if padding is used, the MLS messages MUST be
padded with zero-valued bytes before AEAD encryption. Upon AEAD decryption,
the length field of the plaintext is used to compute the number of bytes
to be removed from the plaintext to get the correct data.
As the padding mechanism is used to improve protection against traffic
analysis, removal of the padding SHOULD be implemented in a "constant-time"
manner at the MLS layer and above layers to prevent timing side-channels that
would provide attackers with information on the size of the plaintext.
The padding length length_of_padding can be chosen at the time of the message
encryption by the sender. Recipients can calculate the padding size from knowing
the total size of the ApplicationPlaintext and the length of the content.
14.2.
Restrictions
During each epoch senders MUST NOT encrypt more data than permitted by the
security bounds of the AEAD scheme used.
Note that each change to the Group through a Handshake message will also set a
new
encryption_secret
. Hence this change MUST be applied before encrypting
any new application message. This is required both to ensure that any users
removed from the group can no longer receive messages and to (potentially)
recover confidentiality and authenticity for future messages despite a past
state compromise.
14.3.
Delayed and Reordered Application messages
Since each Application message contains the group identifier, the epoch and a
message counter, a client can receive messages out of order.
If they are able to retrieve or recompute the correct AEAD decryption key
from currently stored cryptographic material clients can decrypt
these messages.
For usability, MLS clients might be required to keep the AEAD key
and nonce for a certain amount of time to retain the ability to decrypt
delayed or out of order messages, possibly still in transit while a
decryption is being done.
15.
Security Considerations
The security goals of MLS are described in
I-D.ietf-mls-architecture
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.
15.1.
Confidentiality of the Group Secrets
Group secrets are partly derived from the output of a ratchet tree. Ratchet
trees work by assigning each member of the group to a leaf in the tree and
maintaining the following property: the private key of a node in the tree is
known only to members of the group that are assigned a leaf in the node's
subtree. This is called the
ratchet tree invariant
and it makes it possible to
encrypt to all group members except one, with a number of ciphertexts that's
logarithmic in the number of group members.
The ability to efficiently encrypt to all members except one allows members to
be securely removed from a group. It also allows a member to rotate their
keypair such that the old private key can no longer be used to decrypt new
messages.
15.2.
Authentication
The first form of authentication we provide is that group members can verify a
message originated from one of the members of the group. For encrypted messages,
this is guaranteed because messages are encrypted with an AEAD under a key
derived from the group secrets. For plaintext messages, this is guaranteed by
the use of a
membership_tag
which constitutes a MAC over the message, under a
key derived from the group secrets.
The second form of authentication is that group members can verify a message
originated from a particular member of the group. This is guaranteed by a
digital signature on each message from the sender's signature key.
The signature keys held by group members are critical to the security of MLS
against active attacks. If a member's signature key is compromised, then an
attacker can create KeyPackages impersonating the member; depending on the
application, this can then allow the attacker to join the group with the
compromised member's identity. For example, if a group has enabled external
parties to join via external commits, then an attacker that has compromised a
member's signature key could use an external commit to insert themselves into
the group -- even using a "resync"-style external commit to replace the
compromised member in the group.
Applications can mitigate the risks of signature key compromise using pre-shared
keys. If a group requires joiners to know a PSK in addition to authenticating
with a credential, then in order to mount an impersonation attack, the attacker
would need to compromise the relevant PSK as well as the victim's signature key.
The cost of this mitigation is that the application needs some external
arrangement that ensures that the legitimate members of the group to have the
required PSKs.
15.3.
Forward Secrecy and Post-Compromise Security
Post-compromise security is provided between epochs by members regularly
updating their leaf key in the ratchet tree. Updating their leaf key prevents
group secrets from continuing to be encrypted to previously compromised public
keys.
Forward-secrecy between epochs is provided by deleting private keys from past
version of the ratchet tree, as this prevents old group secrets from being
re-derived. Forward secrecy
within
an epoch is provided by deleting message
encryption keys once they've been used to encrypt or decrypt a message.
Post-compromise security is also provided for new groups by members regularly
generating new InitKeys and uploading them to the Delivery Service, such that
compromised key material won't be used when the member is added to a new group.
15.4.
InitKey Reuse
InitKeys are intended to be used only once. That is, once an InitKey has been
used to introduce the corresponding client to a group, it SHOULD be deleted from
the InitKey publication system. Reuse of InitKeys can lead to replay attacks.
An application MAY allow for reuse of a "last resort" InitKey in order to
prevent denial of service attacks. Since an InitKey is needed to add a client
to a new group, an attacker could prevent a client being added to new groups by
exhausting all available InitKeys.
15.5.
Group Fragmentation by Malicious Insiders
It is possible for a malicious member of a group to "fragment" the group by
crafting an invalid UpdatePath. Recall that an UpdatePath encrypts a sequence
of path secrets to different subtrees of the group's ratchet trees. These path
secrets should be derived in a sequence as described in
Section 5.4
, but the UpdatePath syntax allows the sender to
encrypt arbitrary, unrelated secrets. The syntax also does not guarantee that
the encrypted path secret encrypted for a given node corresponds to the public
key provided for that node.
Both of these types of corruption will cause processing of a Commit to fail for
some members of the group. If the public key for a node does not match the path
secret, then the members that decrypt that path secret will reject the commit
based on this mismatch. If the path secret sequence is incorrect at some point,
then members that can decrypt nodes before that point will compute a different
public key for the mismatched node than the one in the UpdatePath, which also
causes the Commit to fail. Applications SHOULD provide mechanisms for failed
commits to be reported, so that group members who were not able to recognize the
error themselves can reject the commit and roll back to a previous state if
necessary.
Even with such an error reporting mechanism in place, however, it is still
possible for members to get locked out of the group by a malformed commit.
Since malformed Commits can only be recognized by certain members of the group,
in an asynchronous application, it may be the case that all members that could
detect a fault in a Commit are offline. In such a case, the Commit will be
accepted by the group, and the resulting state possibly used as the basis for
further Commits. When the affected members come back online, they will reject
the first commit, and thus be unable to catch up with the group.
Applications can address this risk by requiring certain members of the group to
acknowledge successful processing of a Commit before the group regards the
Commit as accepted. The minimum set of acknowledgements necessary to verify
that a Commit is well-formed comprises an acknowledgement from one member per
node in the UpdatePath, that is, one member from each subtree rooted in the
copath node corresponding to the node in the UpdatePath.
16.
IANA Considerations
This document requests the creation of the following new IANA registries:
MLS Ciphersuites (
Section 16.1
MLS Extension Types (
Section 16.2
MLS Proposal Types (
Section 16.3
MLS Credential Types (
Section 16.4
All of these registries should be under a heading of "Messaging Layer Security",
and assignments are made via the Specification Required policy
RFC8126
. See
Section 16.5
for additional information about the MLS Designated Experts (DEs).
RFC EDITOR: Please replace XXXX throughout with the RFC number assigned to
this document
16.1.
MLS Ciphersuites
A ciphersuite is a combination of a protocol version and the set of
cryptographic algorithms that should be used.
Ciphersuite names follow the naming convention:
CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE;
Where VALUE is represented as a sixteen-bit integer:
uint16 CipherSuite;
Table 3
Component
Contents
MLS
The string "MLS" followed by the major and minor version, e.g. "MLS10"
LVL
The security level
KEM
The KEM algorithm used for HPKE in TreeKEM group operations
AEAD
The AEAD algorithm used for HPKE and message protection
HASH
The hash algorithm used for HPKE and the MLS transcript hash
SIG
The Signature algorithm used for message authentication
The columns in the registry are as follows:
Value: The numeric value of the ciphersuite
Name: The name of the ciphersuite
Recommended: Whether support for this ciphersuite is recommended by the IETF MLS
WG. Valid values are "Y" and "N". The "Recommended" column is assigned a
value of "N" unless explicitly requested, and adding a value with a
"Recommended" value of "Y" requires Standards Action
RFC8126
. IESG Approval
is REQUIRED for a Y->N transition.
Reference: The document where this ciphersuite is defined
Initial contents:
Table 4
Value
Name
Recommended
Reference
0x0000
RESERVED
N/A
RFC XXXX
0x0001
MLS10_128_DHKEMX25519_AES128GCM_SHA256_Ed25519
RFC XXXX
0x0002
MLS10_128_DHKEMP256_AES128GCM_SHA256_P256
RFC XXXX
0x0003
MLS10_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519
RFC XXXX
0x0004
MLS10_256_DHKEMX448_AES256GCM_SHA512_Ed448
RFC XXXX
0x0005
MLS10_256_DHKEMP521_AES256GCM_SHA512_P521
RFC XXXX
0x0006
MLS10_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448
RFC XXXX
0xff00 - 0xffff
Reserved for Private Use
N/A
RFC XXXX
All of these ciphersuites use HMAC
RFC2104
as their MAC function, with
different hashes per ciphersuite. The mapping of ciphersuites to HPKE
primitives, HMAC hash functions, and TLS signature schemes is as follows
I-D.irtf-cfrg-hpke
RFC8446
Table 5
Value
KEM
KDF
AEAD
Hash
Signature
0x0001
0x0020
0x0001
0x0001
SHA256
ed25519
0x0002
0x0010
0x0001
0x0001
SHA256
ecdsa_secp256r1_sha256
0x0003
0x0020
0x0001
0x0003
SHA256
ed25519
0x0004
0x0021
0x0003
0x0002
SHA512
ed448
0x0005
0x0012
0x0003
0x0002
SHA512
ecdsa_secp521r1_sha512
0x0006
0x0021
0x0003
0x0003
SHA512
ed448
The hash used for the MLS transcript hash is the one referenced in the
ciphersuite name. In the ciphersuites defined above, "SHA256" and "SHA512"
refer to the SHA-256 and SHA-512 functions defined in
SHS
It is advisable to keep the number of ciphersuites low to increase the chances
clients can interoperate in a federated environment, therefore the ciphersuites
only inlcude modern, yet well-established algorithms. Depending on their
requirements, clients can choose between two security levels (roughly 128-bit
and 256-bit). Within the security levels clients can choose between faster
X25519/X448 curves and FIPS 140-2 compliant curves for Diffie-Hellman key
negotiations. Additionally clients that run predominantly on mobile processors
can choose ChaCha20Poly1305 over AES-GCM for performance reasons. Since
ChaCha20Poly1305 is not listed by FIPS 140-2 it is not paired with FIPS 140-2
compliant curves. The security level of symmetric encryption algorithms and hash
functions is paired with the security level of the curves.
The mandatory-to-implement ciphersuite for MLS 1.0 is
MLS10_128_DHKEMX25519_AES128GCM_SHA256_Ed25519
which uses
Curve25519 for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and
Ed25519 for signatures.
Values with the first byte 255 (decimal) are reserved for Private Use.
New ciphersuite values are assigned by IANA as described in
Section 16
16.2.
MLS Extension Types
This registry lists identifiers for extensions to the MLS protocol. The
extension type field is two bytes wide, so valid extension type values are in
the range 0x0000 to 0xffff.
Template:
Value: The numeric value of the extension type
Name: The name of the extension type
Message(s): The messages in which the extension may appear, drawn from the following
list:
KP: KeyPackage messages
GC: GroupContext objects (and the
group_context_extensions
field of
GroupInfo objects)
GI: The
other_extensions
field of GroupInfo objects
Recommended: Whether support for this extension is recommended by the IETF MLS
WG. Valid values are "Y" and "N". The "Recommended" column is assigned a
value of "N" unless explicitly requested, and adding a value with a
"Recommended" value of "Y" requires Standards Action
RFC8126
. IESG Approval
is REQUIRED for a Y->N transition.
Reference: The document where this extension is defined
Initial contents:
Table 6
Value
Name
Message(s)
Recommended
Reference
0x0000
RESERVED
N/A
N/A
RFC XXXX
0x0001
capabilities
KP
RFC XXXX
0x0002
lifetime
KP
RFC XXXX
0x0003
external_key_id
KP
RFC XXXX
0x0004
parent_hash
KP
RFC XXXX
0x0005
ratchet_tree
GI
RFC XXXX
0xff00 - 0xffff
Reserved for Private Use
N/A
N/A
RFC XXXX
16.3.
MLS Proposal Types
This registry lists identifiers for types of proposals that can be made for
changes to an MLS group. The extension type field is two bytes wide, so valid
extension type values are in the range 0x0000 to 0xffff.
Template:
Value: The numeric value of the proposal type
Name: The name of the proposal type
Recommended: Whether support for this extension is recommended by the IETF MLS
WG. Valid values are "Y" and "N". The "Recommended" column is assigned a
value of "N" unless explicitly requested, and adding a value with a
"Recommended" value of "Y" requires Standards Action
RFC8126
. IESG Approval
is REQUIRED for a Y->N transition.
Reference: The document where this extension is defined
Initial contents:
Table 7
Value
Name
Recommended
Reference
0x0000
RESERVED
N/A
RFC XXXX
0x0001
add
RFC XXXX
0x0002
update
RFC XXXX
0x0003
remove
RFC XXXX
0x0004
psk
RFC XXXX
0x0005
reinit
RFC XXXX
0x0006
external_init
RFC XXXX
0x0007
app_ack
RFC XXXX
0x0008
group_context_extensions
RFC XXXX
0xff00 - 0xffff
Reserved for Private Use
N/A
RFC XXXX
16.4.
MLS Credential Types
This registry lists identifiers for types of credentials that can be used for
authentication in the MLS protocol. The credential type field is two bytes wide,
so valid credential type values are in the range 0x0000 to 0xffff.
Template:
Value: The numeric value of the credential type
Name: The name of the credential type
Recommended: Whether support for this credential is recommended by the IETF MLS
WG. Valid values are "Y" and "N". The "Recommended" column is assigned a
value of "N" unless explicitly requested, and adding a value with a
"Recommended" value of "Y" requires Standards Action
RFC8126
. IESG Approval
is REQUIRED for a Y->N transition.
Reference: The document where this credential is defined
Initial contents:
Table 8
Value
Name
Recommended
Reference
0x0000
RESERVED
N/A
RFC XXXX
0x0001
basic
RFC XXXX
0x0002
x509
RFC XXXX
0xff00 - 0xffff
Reserved for Private Use
N/A
RFC XXXX
16.5.
MLS Designated Expert Pool
Specification Required
RFC8126
registry requests are registered
after a three-week review period on the MLS DEs' mailing list:
mls-reg-review@ietf.org
, on the advice of one or more of the MLS DEs. However,
to allow for the allocation of values prior to publication, the MLS
DEs may approve registration once they are satisfied that such a
specification will be published.
Registration requests sent to the MLS DEs mailing list for review
SHOULD use an appropriate subject (e.g., "Request to register value
in MLS Bar registry").
Within the review period, the MLS DEs will either approve or deny
the registration request, communicating this decision to the MLS DEs
mailing list and IANA. Denials SHOULD include an explanation and, if
applicable, suggestions as to how to make the request successful.
Registration requests that are undetermined for a period longer than
21 days can be brought to the IESG's attention for resolution using
the
iesg@ietf.org
mailing list.
Criteria that SHOULD be applied by the MLS DEs includes determining
whether the proposed registration duplicates existing functionality,
whether it is likely to be of general applicability or useful only
for a single application, and whether the registration description
is clear. For example, the MLS DEs will apply the ciphersuite-related
advisory found in
Section 6.1
IANA MUST only accept registry updates from the MLS DEs and SHOULD
direct all requests for registration to the MLS DEs' mailing list.
It is suggested that multiple MLS DEs be appointed who are able to
represent the perspectives of different applications using this
specification, in order to enable broadly informed review of
registration decisions. In cases where a registration decision could
be perceived as creating a conflict of interest for a particular
MLS DE, that MLS DE SHOULD defer to the judgment of the other MLS DEs.
17.
Contributors
Joel Alwen
Wickr
joel.alwen@wickr.com
Karthikeyan Bhargavan
INRIA
karthikeyan.bhargavan@inria.fr
Cas Cremers
University of Oxford
cremers@cispa.de
Alan Duric
Wire
alan@wire.com
Britta Hale
Naval Postgraduate School
britta.hale@nps.edu
Srinivas Inguva
singuva@twitter.com
Konrad Kohbrok
Aalto University
konrad.kohbrok@datashrine.de
Albert Kwon
MIT
kwonal@mit.edu
Brendan McMillion
Cloudflare
brendan@cloudflare.com
Eric Rescorla
Mozilla
ekr@rtfm.com
Michael Rosenberg
Trail of Bits
michael.rosenberg@trailofbits.com
Thyla van der Merwe
Royal Holloway, University of London
thyla.van.der@merwe.tech
18.
References
18.1.
Normative References
[I-D.irtf-cfrg-hpke]
Barnes, R. L.
Bhargavan, K.
Lipp, B.
, and
C. A. Wood
"Hybrid Public Key Encryption"
Work in Progress
Internet-Draft, draft-irtf-cfrg-hpke-12
2 September 2021
[RFC2104]
Krawczyk, H.
Bellare, M.
, and
R. Canetti
"HMAC: Keyed-Hashing for Message Authentication"
RFC 2104
DOI 10.17487/RFC2104
February 1997
[RFC2119]
Bradner, S.
"Key words for use in RFCs to Indicate Requirement Levels"
BCP 14
RFC 2119
DOI 10.17487/RFC2119
March 1997
[RFC8126]
Cotton, M.
Leiba, B.
, and
T. Narten
"Guidelines for Writing an IANA Considerations Section in RFCs"
BCP 26
RFC 8126
DOI 10.17487/RFC8126
June 2017
[RFC8174]
Leiba, B.
"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"
BCP 14
RFC 8174
DOI 10.17487/RFC8174
May 2017
[RFC8446]
Rescorla, E.
"The Transport Layer Security (TLS) Protocol Version 1.3"
RFC 8446
DOI 10.17487/RFC8446
August 2018
18.2.
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"
18 January 2018
[CLINIC]
Miller, B.
Huang, L.
Joseph, A.
, and
J. Tygar
"I Know Why You Went to the Clinic: Risks and Realization of HTTPS Traffic Analysis"
Privacy Enhancing Technologies pp. 143-163
DOI 10.1007/978-3-319-08506-7_8
2014
[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
[HCJ16]
Husák, M.
Čermák, M.
Jirsík, T.
, and
P. Čeleda
"HTTPS traffic analysis and client identification using passive SSL/TLS fingerprinting"
EURASIP Journal on Information Security Vol. 2016
DOI 10.1186/s13635-016-0030-7
February 2016
[I-D.ietf-mls-architecture]
Beurdouche, B.
Rescorla, E.
Omara, E.
Inguva, S.
Kwon, A.
, and
A. Duric
"The Messaging Layer Security (MLS) Architecture"
Work in Progress
Internet-Draft, draft-ietf-mls-architecture-07
4 October 2021
[I-D.ietf-trans-rfc6962-bis]
Laurie, B.
Langley, A.
Kasper, E.
Messeri, E.
, and
R. Stradling
"Certificate Transparency Version 2.0"
Work in Progress
Internet-Draft, draft-ietf-trans-rfc6962-bis-42
31 August 2021
[RFC8032]
Josefsson, S.
and
I. Liusvaara
"Edwards-Curve Digital Signature Algorithm (EdDSA)"
RFC 8032
DOI 10.17487/RFC8032
January 2017
[SECG]
"Elliptic Curve Cryptography, Standards for Efficient Cryptography Group, ver. 2"
2009
[SHS]
Dang, Q.
"Secure Hash Standard"
National Institute of Standards and Technology report
DOI 10.6028/nist.fips.180-4
July 2015
[signal]
Perrin(ed), T.
and
M. Marlinspike
"The Double Ratchet Algorithm"
20 November 2016
Appendix A.
Tree Math
One benefit of using left-balanced trees is that they admit a simple
flat array representation. In this representation, leaf nodes are
even-numbered nodes, with the n-th leaf at 2*n. Intermediate nodes
are held in odd-numbered nodes. For example, an 11-element tree has
the following structure:
X X X
X X X X X
X X X X X X X X X X X
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
This allows us to compute relationships between tree nodes simply by
manipulating indices, rather than having to maintain complicated
structures in memory, even for partial trees. The basic
rule is that the high-order bits of parent and child nodes have the
following relation (where
is an arbitrary bit string):
parent=01x => left=00x, right=10x
The following python code demonstrates the tree computations
necessary for MLS. Test vectors can be derived from the diagram
above.
# The exponent of the largest power of 2 less than x. Equivalent to:
# int(math.floor(math.log(x, 2)))
def log2(x):
if x == 0:
return 0

k = 0
while (x >> k) > 0:
k += 1
return k-1

# The level of a node in the tree. Leaves are level 0, their parents are
# level 1, etc. If a node's children are at different levels, then its
# level is the max level of its children plus one.
def level(x):
if x & 0x01 == 0:
return 0

k = 0
while ((x >> k) & 0x01) == 1:
k += 1
return k

# The number of nodes needed to represent a tree with n leaves.
def node_width(n):
if n == 0:
return 0
else:
return 2*(n - 1) + 1

# The index of the root node of a tree with n leaves.
def root(n):
w = node_width(n)
return (1 << log2(w)) - 1

# The left child of an intermediate node. Note that because the tree is
# left-balanced, there is no dependency on the size of the tree.
def left(x):
k = level(x)
if k == 0:
raise Exception('leaf node has no children')

return x ^ (0x01 << (k - 1))

# The right child of an intermediate node. Depends on the number of
# leaves because the straightforward calculation can take you beyond the
# edge of the tree.
def right(x, n):
k = level(x)
if k == 0:
raise Exception('leaf node has no children')

r = x ^ (0x03 << (k - 1))
while r >= node_width(n):
r = left(r)
return r

# The immediate parent of a node. May be beyond the right edge of the
# tree.
def parent_step(x):
k = level(x)
b = (x >> (k + 1)) & 0x01
return (x | (1 << k)) ^ (b << (k + 1))

# The parent of a node. As with the right child calculation, we have to
# walk back until the parent is within the range of the tree.
def parent(x, n):
if x == root(n):
raise Exception('root node has no parent')

p = parent_step(x)
while p >= node_width(n):
p = parent_step(p)
return p

# The other child of the node's parent.
def sibling(x, n):
p = parent(x, n)
if x < p:
return right(p, n)
else:
return left(p)

# The direct path of a node, ordered from leaf to root.
def direct_path(x, n):
r = root(n)
if x == r:
return []

d = []
while x != r:
x = parent(x, n)
d.append(x)
return d

# The copath of a node, ordered from leaf to root.
def copath(x, n):
if x == root(n):
return []

d = direct_path(x, n)
d.insert(0, x)
d.pop()
return [sibling(y, n) for y in d]

# The common ancestor of two nodes is the lowest node that is in the
# direct paths of both leaves.
def common_ancestor_semantic(x, y, n):
dx = set([x]) | set(direct_path(x, n))
dy = set([y]) | set(direct_path(y, n))
dxy = dx & dy
if len(dxy) == 0:
raise Exception('failed to find common ancestor')

return min(dxy, key=level)

# The common ancestor of two nodes is the lowest node that is in the
# direct paths of both leaves.
def common_ancestor_direct(x, y, _):
# Handle cases where one is an ancestor of the other
lx, ly = level(x)+1, level(y)+1
if (lx <= ly) and (x>>ly == y>>ly):
return y
elif (ly <= lx) and (x>>lx == y>>lx):
return x

# Handle other cases
xn, yn = x, y
k = 0
while xn != yn:
xn, yn = xn >> 1, yn >> 1
k += 1
return (xn << k) + (1 << (k-1)) - 1
Authors' Addresses
Richard Barnes
Cisco
Email:
rlb@ipv.sx
Benjamin Beurdouche
Inria & Mozilla
Email:
ietf@beurdouche.com
Raphael Robert
Email:
ietf@raphaelrobert.com
Jon Millican
Email:
jmillican@fb.com
Emad Omara
Google
Email:
emadomara@google.com
Katriel Cohn-Gordon
University of Oxford
Email:
me@katriel.co.uk
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Richard Barnes
Benjamin Beurdouche
Raphael Robert
Jon Millican
Emad Omara
Katriel Cohn-Gordon
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