wdiff rfc9750.original rfc9750.txt

Network Working Group

Internet Engineering Task Force (IETF) B. Beurdouche
Internet-Draft
Request for Comments: 9750 Inria & Mozilla
Intended status:
Category: Informational E. Rescorla
Expires: 4 February 2025
ISSN: 2070-1721 Windy Hill Systems, LLC
E. Omara

S. Inguva

A. Duric
Wire
3 August 2024
February 2025

The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-15

Abstract

The Messaging Layer Security (MLS) protocol (I-D.ietf-mls-protocol) (RFC 9420) provides a
Group Key Agreement protocol for messaging applications. MLS is
meant to protect against eavesdropping, tampering, and message
forgery,
forgery and to provide Forward Secrecy (FS) and Post-Compromise
Security (PCS).

This document describes the architecture for using MLS in a general
secure group messaging infrastructure and defines the security goals
for MLS. It provides guidance on building a group messaging system
and discusses security and privacy tradeoffs trade-offs offered by multiple
security mechanisms that are part of the MLS protocol (e.g.,
frequency of public encryption key rotation). The document also
provides guidance for parts of the infrastructure that are not
standardized by MLS and are instead left to the application.

While the recommendations of this document are not mandatory to
follow in order to interoperate at the protocol level, they affect
the overall security guarantees that are achieved by a messaging
application. This is especially true in the case of active
adversaries that are able to compromise clients, the delivery
service, or the authentication service.

Discussion Venues

Status of This note Memo

This document is to be removed before publishing as not an RFC.

Discussion of this document takes place on the MLS Working Group
mailing list (mls@ietf.org), which Internet Standards Track specification; it is archived at
https://mailarchive.ietf.org/arch/browse/mls/.

Source
published for this draft and an issue tracker can be found at
https://github.com/mlswg/mls-architecture.

Status of informational purposes.

This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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Internet-Drafts are draft the IETF community. It has
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approved by the IESG are candidates for a maximum any level of six months Internet
Standard; see Section 2 of RFC 7841.

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This Internet-Draft will expire on 4 February 2025.
https://www.rfc-editor.org/info/rfc9750.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Setting . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Protocol Overview . . . . . . . . . . . . . . . . . . . . 4
2.2. Abstract Services . . . . . . . . . . . . . . . . . . . . 5
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
3.1. Step 1: Account Creation . . . . . . . . . . . . . . . . 7
3.2. Step 2: Initial Keying Material . . . . . . . . . . . . . 8
3.3. Step 3: Adding Bob to the Group . . . . . . . . . . . . . 8
3.4. Step 4: Adding Charlie to the Group . . . . . . . . . . . 8
3.5. Other Group Operations . . . . . . . . . . . . . . . . . 9
3.6. Proposals and Commits . . . . . . . . . . . . . . . . . . 9
3.7. Users, Clients, and Groups . . . . . . . . . . . . . . . 10
4. Authentication Service . . . . . . . . . . . . . . . . . . . 10
5. Delivery Service . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Key Storage and Retrieval . . . . . . . . . . . . . . . . 12
5.2. Delivery of Messages . . . . . . . . . . . . . . . . . . 14
5.2.1. Strongly Consistent . . . . . . . . . . . . . . . . . 15
5.2.2. Eventually Consistent . . . . . . . . . . . . . . . . 15
5.2.3. Welcome Messages . . . . . . . . . . . . . . . . . . 16
5.3. Invalid Commits . . . . . . . . . . . . . . . . . . . . . 17
6. Functional Requirements . . . . . . . . . . . . . . . . . . . 18
6.1. Membership Changes . . . . . . . . . . . . . . . . . . . 18
6.2. Parallel Groups . . . . . . . . . . . . . . . . . . . . . 19
6.3. Asynchronous Usage . . . . . . . . . . . . . . . . . . . 20
6.4. Access Control . . . . . . . . . . . . . . . . . . . . . 20
6.5. Handling Authentication Failures . . . . . . . . . . . . 21
6.6. Recovery After State Loss . . . . . . . . . . . . . . . . 22
6.7. Support for Multiple Devices . . . . . . . . . . . . . . 22
6.8. Extensibility . . . . . . . . . . . . . . . . . . . . . . 22
6.9. Application Data Framing and Type Advertisements . . . . 23
6.10. Federation . . . . . . . . . . . . . . . . . . . . . . . 23
6.11. Compatibility with Future Versions of MLS . . . . . . . . 23
7. Operational Requirements . . . . . . . . . . . . . . . . . . 24
8. Security and Privacy Considerations . . . . . . . . . . . . . 28
8.1. Assumptions on Transport Security Links . . . . . . . . . 28
8.1.1. Integrity and Authentication of Custom Metadata . . . 29
8.1.2. Metadata Protection for Unencrypted Group Operations . . . . . . . . . . . . . . . . . . . . . 29
8.1.3. DoS protection . . . . . . . . . . . . . . . . . . . 30 Protection
8.1.4. Message Suppression and Error Correction . . . . . . 30
8.2. Intended Security Guarantees . . . . . . . . . . . . . . 31
8.2.1. Message Secrecy and Authentication . . . . . . . . . 31
8.2.2. Forward and Post-Compromise Security . . . . . . . . 31
8.2.3. Non-Repudiation vs vs. Deniability . . . . . . . . . . . 33
8.2.4. Associating a User's Clients . . . . . . . . . . . . 33
8.3. Endpoint Compromise . . . . . . . . . . . . . . . . . . . 34
8.3.1. Compromise of Symmetric Keying Material . . . . . . . 34
8.3.2. Compromise by an active adversary Active Adversary with the ability Ability to
sign messages . . . . . . . . . . . . . . . . . . . . 36
Sign Messages
8.3.3. Compromise of the authentication Authentication with access Access to a
signature key . . . . . . . . . . . . . . . . . . . . 37
Signature Key
8.3.4. Security consideration Considerations in the context Context of a full state
compromise . . . . . . . . . . . . . . . . . . . . . 38 Full State
Compromise
8.4. Service Node Compromise . . . . . . . . . . . . . . . . . 39
8.4.1. General considerations . . . . . . . . . . . . . . . 39 Considerations
8.4.2. Delivery Service Compromise . . . . . . . . . . . . . 40
8.4.3. Authentication Service Compromise . . . . . . . . . . 42
8.5. Considerations for attacks outside Attacks Outside of the threat model . 45 Threat Model
8.6. Cryptographic Analysis of the MLS Protocol . . . . . . . 45
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
10.1. Normative References . . . . . . . . . . . . . . . . . . 46
10.2. Informative References . . . . . . . . . . . . . . . . . 46
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50

1. Introduction

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH

The source for this draft is maintained in GitHub. Suggested changes
should be submitted as pull requests at https://github.com/mlswg/mls-
architecture. Instructions are on that page as well. Editorial
changes can be managed in GitHub, but any substantive change should
be discussed on the MLS mailing list.

End-to-end security is used in the vast majority of instant messaging
systems,
systems and is also deployed in systems for other purposes such as
calling and conferencing. In this context, "end-to-end" captures the
notion that users of the system enjoy some level of security -- with
the precise level depending on the system design -- even in the face
of malicious actions by the operator of the messaging system.

Messaging Layer Security (MLS) specifies an architecture (this
document) and a protocol [I-D.ietf-mls-protocol] [RFC9420] for providing end-
to-end end-to-end security
in this setting. MLS is not intended as a full instant messaging
protocol but rather is intended to be embedded in concrete protocols,
such as XMPP the Extensible Messaging and Presence Protocol (XMPP)
[RFC6120]. Implementations of the MLS protocol will interoperate at
the cryptographic level, though they may have incompatibilities in
terms of how protected messages are delivered, contents of protected
messages, and identity/
authentication identity/authentication infrastructures. The MLS
protocol has been designed to provide the same security guarantees to
all users, for all group sizes, including groups of only two clients.

2. General Setting

2.1. Protocol Overview

MLS provides a way for _clients_ to form _groups_ within which they
can communicate securely. For example, a set of users might use
clients on their phones or laptops to join a group and communicate
with each other. A group may be as small as two clients (e.g., for
simple person to person person-to-person messaging) or as large as hundreds of
thousands. A client that is part of a group is a _member_ of that
group. As groups change membership and group or member properties,
they advance from one _epoch_ to another and the cryptographic state
of the group evolves.

The group is represented as a tree, which represents the members as
the leaves of a tree. It is used to efficiently encrypt to subsets
of the members. Each member has a state called a _LeafNode_ object
holding the client's identity, credentials, and capabilities.

Various messages are used in the evolution from epoch to epoch. A
_Proposal_ message proposes a change to be made in the next epoch,
such as adding or removing a member. A _Commit_ message initiates a
new epoch by instructing members of the group to implement a
collection of proposals. Proposals and Commits are collectively
called _Handshake messages_. A _KeyPackage_ provides keys that can
be used to add the client to a group, including its LeafNode, and
_Signature Key_. A _Welcome_ message provides a new member to the
group with the information to initialize their state for the epoch in
which they were added.

Of course most (but not all) applications use MLS to send encrypted
group messages. An _application message_ is an MLS message with an
arbitrary application payload.

Finally, a _PublicMessage_ contains an integrity-protected MLS
Handshake message, while a _PrivateMessage_ contains a confidential,
integrity-protected Handshake or application message.

For a more detailed explanation of these terms, please consult the
MLS protocol specification [RFC9420].

2.2. Abstract Services

MLS is designed to operate within the context of a messaging service,
which may be a single service provider, a federated system, or some
kind of peer-to-peer system. The service needs to provide two
services that facilitate client communication using MLS:

* An Authentication Service (AS) which is responsible for attesting
to bindings between application-meaningful identifiers and the
public key material used for authentication in the MLS protocol.
The AS must also be able to generate credentials that encode these
bindings and validate credentials provided by MLS clients.

* A Delivery Service (DS) which can receive and distribute messages
between group members. In the case of group messaging, the
delivery service may also be responsible for acting as a
"broadcaster" where the sender sends a single message which is
then forwarded to each recipient in the group by the DS. The DS
is also responsible for storing and delivering initial public key
material required by MLS clients in order to proceed with the
group secret key establishment that is part of the MLS protocol.

For presentation purposes, this document treats the AS and DS as
conventional network services, however services. However, MLS does not require a
specific implementation for the AS or DS. These services may reside
on the same server or different servers, they may be distributed
between server and client components, and they may even involve some
action by users. For example:

* Several secure messaging services today provide a centralized DS, DS
and rely on manual comparison of clients' public keys as the AS.

* MLS clients connected to a peer-to-peer network could instantiate
a decentralized DS by transmitting MLS messages over that network.

* In an MLS group using a Public Key Infrastructure (PKI) for
authentication, the AS would comprise the certificate issuance and
validation processes, both of which involve logic inside MLS
clients as well as various existing PKI roles (ex: (e.g., Certification
Authorities).

It is important to note that the Authentication Service can be
completely abstract in the case of a Service Provider which that allows MLS
clients to generate, distribute, and validate credentials themselves.
As with the AS, the Delivery Service can be completely abstract if
users are able to distribute credentials and messages without relying
on a central Delivery Service (as in a peer-to-peer system). Note,
though, that in such scenarios, clients will need to implement logic
that assures the delivery properties required of the DS (see
Section 5.2).

Figure 1 shows the relationship of these concepts, with three clients
and one group, and clients 2 and 3 being part of the group and client
1 not being part of any group.

Figure 1: A Simplified Messaging System

Figure 1 shows the relationship of these concepts, with three clients
and one group, and clients 2 and 3 being part of the group and client
1 not being part of any group.

3. Overview of Operation

Figure 2 shows the formation of an example group consisting of Alice,
Bob, and Charlie, with Alice driving the creation of the group.

Alice Bob Charlie AS DS

Initial Keying Material -----------------------------------> |
Initial Keying Material -------------------------> | Step 2
Initial Keying Material ---------------> |

Get Bob Initial Keying Material ---------------------------> |
<------------------------------- Bob Initial Keying Material |
Add Bob to Group ------------------------------------------> | Step 3
Welcome (Bob)----------------------------------------------> (Bob) ---------------------------------------------> |
<-------------------------------- Add Bob to Group |
<----------------------------------- Welcome (Bob) |

Get Charlie Initial Keying Material -----------------------> |
<--------------------------- Charlie Initial Keying Material |
Add Charlie to Group --------------------------------------> |
Welcome (Charlie) -----------------------------------------> | Step 4
<---------------------------- Add Charlie to Group |
<----------------- Add Charlie to Group |
<-------------------- Welcome (Charlie) |

Figure 2: Group Formation Example

This process proceeds as follows.

3.1. Step 1: Account Creation

Alice, Bob, and Charlie create accounts with a service provider and
obtain credentials from the AS. This is a one-time setup phase.

3.2. Step 2: Initial Keying Material

Alice, Bob, and Charlie authenticate to the DS and store some initial
keying material which can be used to send encrypted messages to them
for the first time. This keying material is authenticated with their
long-term credentials. Although in principle this keying material
can be reused for multiple senders, in order to provide forward
secrecy it is better for this material to be regularly refreshed so
that each sender can use a new key.

3.3. Step 3: Adding Bob to the Group

When Alice wants to create a group including Bob, she first uses the
DS to look up his initial keying material. She then generates two
messages:

* A message to the entire group (which at this point is just her and
Bob) that adds Bob to the group.

* A _Welcome_ message just to Bob encrypted with his initial keying
material that includes the secret keying information necessary to
join the group.

She sends both of these messages to the Delivery Services, Service, which is
responsible for sending them to the appropriate people. Note that
the security of MLS does not depend on the DS forwarding the Welcome
message only to Bob, as it is encrypted for him; it is simply not
necessary for other group members to receive it.

3.4. Step 4: Adding Charlie to the Group

If Alice then wants to add Charlie to the group, she follows a
similar procedure as with Bob: she She first uses the DS to look up his
initial keying material and then generates two messages:

* A message to the entire group (consisting of her, Bob, and
Charlie) adding Charlie to the group.

* A _Welcome_ message just to Charlie encrypted with his initial
keying material that includes the secret keying information
necessary to join the group.

At the completion of this process, we have a group with Alice, Bob,
and Charlie, which means that they share a single encryption key
which that
can be used to send messages or to key other protocols.

3.5. Other Group Operations

Once the group has been created, clients can perform other actions,
such as:

* sending a message to everyone in the group

* receiving a message from someone in the group

* adding one or more clients to an existing group

* remove removing one or more members from an existing group

* updating their own key material

* leave leaving a group (by asking to be removed)

Importantly, MLS does not itself enforce any access control on group
operations. For instance, any member of the group can send a message
to add a new member or to evict an existing member. This is in
contrast to some designs in which there is a single group controller
who can modify the group. MLS-using applications are responsible for
setting their own access control policies. For instance, if only the
group administrator is allowed to change group members, then it is
the responsibility of the application to inform members of this
policy and who the administrator is.

3.6. Proposals and Commits

The general pattern for any change in the group state (e.g., to add
or remove a user) is that it consists of two messages:

Proposal

Proposal: This message describes the change to be made (e.g., add
Bob to the group) but does not effect a change.

Commit

Commit: This message changes the group state to include the changes
described in a set of proposals.

The simplest pattern is for a client to just send a Commit which
contains one or more Proposals, Proposals; for instance instance, Alice could send a
Commit with the Proposal Add(Bob) embedded to add Bob to the group.
However, there are situations in which one client might send a
proposal and another might send the commit. For instance, Bob might
wish to remove himself from the group and send a Remove Proposal to
do so (see Section 12.1.3 of [RFC9420]). Because Bob cannot send the
Commit, an existing member must do so. Commits can apply to multiple
valid Proposals, in which case all the listed changes are applied.

It is also possible for a Commit to apply to an empty set of
Proposals
Proposals, in which case it just updates the cryptographic state of
the group without changing its membership.

3.7. Users, Clients, and Groups

While it's natural to think of a messaging system as consisting of
groups of users, possibly using different devices, in MLS the basic
unit of operation is not the user but rather the "client". Formally,
a client is a set of cryptographic objects composed of public values
such as a name (an identity), a public encryption key, and a public
signature key. As usual, a user demonstrates ownership of the client
by demonstrating knowledge of the associated secret values.

In some messaging systems, clients belonging to the same user must
all share the same signature key pair, but MLS does not assume this;
instead
instead, a user may have multiple clients with the same identity and
different keys. In this case, each client will have its own
cryptographic state, and it is up to the application to determine how
to present this situation to users. For instance, it may render
messages to and from a given user identically regardless of which
client they are associated with, or it may choose to distinguish
them.

When a client is part of a Group, it is called a Member. A group in
MLS is defined as the set of clients that have knowledge of the
shared group secret established in the group key establishment phase.
Note that until a client has been added to the group and contributed
to the group secret in a manner verifiable by other members of the
group, other members cannot assume that the client is a member of the
group; for instance, the newly added member might not have received
the Welcome message or might not have been unable able to decrypt it for
some reason.

4. Authentication Service

The Authentication Service (AS) has to provide three services:

1. Issue credentials to clients that attest to bindings between
identities and signature key pairs pairs.

2. Enable a client to verify that a credential presented by another
client is valid with respect to a reference identifier identifier.

3. Enable a group member to verify that a credential represents the
same client as another credential credential.

A member with a valid credential authenticates its MLS messages by
signing them with the private key corresponding to the public key
bound by its credential.

The AS is considered an abstract layer by the MLS specification and specification; part
of this service could be, for instance, running on the members'
devices, while another part is a separate entity entirely. The
following examples illustrate the breadth of this concept:

* A PKI could be used as an AS [RFC5280]. The issuance function
would be provided by the certificate authorities in the PKI, and
the verification function would correspond to certificate
verification by clients.

* Several current messaging applications rely on users verifying
each other's key fingerprints for authentication. In this
scenario, the issuance function is simply the generation of a key
pair (i.e., a credential is just an identifier and public key,
with no information to assist in verification). The verification
function is the application function that enables users to verify
keys.

* In a system based on [CONIKS] end user end-user Key Transparency (KT) [CONIKS] [KT],
the issuance function would correspond to the insertion of a key
in a KT log under a user's identity. The verification function
would correspond to verifying a key's inclusion in the log for a
claimed identity, together with the KT log's mechanisms for a user
to monitor and control which keys are associated with their
identity.

By the nature of its roles in MLS authentication, the AS is invested
with a large amount of trust and the compromise of the AS could allow
an adversary to, among other things, impersonate group members. We
discuss security considerations regarding the compromise of the
different AS functions in detail in Section 8.4.3.

The association between members' identities and their signature keys
is fairly flexible in MLS. As noted above, there is no requirement
that all clients belonging to a given user have the same signature
key (in fact, having duplicate signature keys in a group is
forbidden). A member can also rotate the signature key they use
within a group. These mechanisms allow clients to use different
signature keys in different contexts and at different points in time,
providing unlinkability and post-compromise security benefits. Some
security trade-offs related to this flexibility are discussed in the
security considerations.

In many applications, there are multiple MLS clients that represent a
single entity, entity -- for example example, a human user with a mobile and desktop
version of an application. Often Often, the same set of clients is
represented in exactly the same list of groups. In applications
where this is the intended situation, other clients can check that a
user is consistently represented by the same set of clients. This
would make it more difficult for a malicious AS to issue fake
credentials for a particular user because clients would expect the
credential to appear in all groups of which the user is a member. If
a client credential does not appear in all groups after some
relatively short period of time, clients have an indication that the
credential might have been created without the user's knowledge. Due
to the asynchronous nature of MLS, however, there may be transient
inconsistencies in a user's client set, so correlating users' clients
across groups is more of a detection mechanism than a prevention
mechanism.

5. Delivery Service

The Delivery Service (DS) plays two major roles in MLS:

* As a directory service providing the initial keying material for
clients to use. This allows a client to establish a shared key
and send encrypted messages to other clients even if they're
offline.

* Routing MLS messages among clients.

While MLS depends on correct behavior by the Authentication Service
in order to provide endpoint authentication and hence confidentiality
of the group key, these properties do not depend on correct behavior
by the DS; even a malicious DS cannot add itself to groups or recover
the group key. However, depending precisely on how MLS is used, the
DS may be able to determine group membership or prevent changes to
the group from taking place (e.g., by blocking group change
messages).

5.1. Key Storage and Retrieval

Upon joining the system, each client stores its initial cryptographic
key material with the Delivery Service. This key material, called a
KeyPackage, advertises the functional abilities of the client such as
supported protocol versions, supported extensions, and the following
cryptographic information:

* A credential from the Authentication Service attesting to the
binding between the identity and the client's signature key.

* The client's asymmetric encryption public key.

All the parameters in the KeyPackage are signed with the signature
private key corresponding to the credential. As noted in
Section 3.7, users may own multiple clients, each with their own
keying material. Each KeyPackage is specific to an MLS version and
ciphersuite, but a client may want to offer support for multiple
protocol versions and ciphersuites. As such, there may be multiple
KeyPackages stored by each user for a mix of protocol versions,
ciphersuites, and end-user devices.

When a client wishes to establish a group or add clients to a group,
it first contacts the Delivery Service to request KeyPackages for
each other client, authenticates the KeyPackages using the signature
keys, includes the KeyPackages in Add Proposals, and encrypts the
information needed to join the group (the _GroupInfo_ object) with an
ephemeral key, key; it then separately encrypts the ephemeral key with the
init_key from each KeyPackage. When a client requests a KeyPackage
in order to add a user to a group, the Delivery Service should
provide the minimum number of KeyPackages necessary to satisfy the
request. For example, if the request specifies the MLS version, the
DS might provide one KeyPackage per supported ciphersuite, even if it
has multiple such KeyPackages to enable the corresponding client to
be added to multiple groups before needing to upload more fresh
KeyPackages.

In order to avoid replay attacks and provide forward secrecy for
messages sent using the initial keying material, KeyPackages are
intended to be used only once. The Delivery Service is responsible
for ensuring that each KeyPackage is only used to add its client to a
single group, with the possible exception of a "last resort"
KeyPackage that is specially designated by the client to be used
multiple times. Clients are responsible for providing new
KeyPackages as necessary in order to minimize the chance that the
"last resort" KeyPackage will be used.

*RECOMMENDATION:* Ensure that "last resort" KeyPackages don't get
used by provisioning enough standard KeyPackages.

*RECOMMENDATION:* Rotate "last resort" KeyPackages as soon as
possible after being used or if they have been stored for a
prolonged period of time. Overall, avoid reusing last resort "last resort"
KeyPackages as much as possible.

*RECOMMENDATION:* Ensure that the client for which a last resort "last resort"
KeyPackage has been used is updating leaf keys as early as
possible.

Overall, it needs to be noted that key packages need to be updated
when signature keys are changed.

5.2. Delivery of Messages

The main responsibility of the Delivery Service is to ensure delivery
of messages. Some MLS messages need only be delivered to specific
clients (e.g., a Welcome message initializing a new member's state),
while others need to be delivered to all the members of a group. The
Delivery Service may enable the latter delivery pattern via unicast
channels (sometimes known as "client fanout"), broadcast channels
("server fanout"), or a mix of both.

For the most part, MLS does not require the Delivery Service to
deliver messages in any particular order. Applications can set
policies that control their tolerance for out-of-order messages (see
Section 7), and messages that arrive significantly out-of-order out of order can
be dropped without otherwise affecting the protocol. There are two
exceptions to this. First, Proposal messages should all arrive
before the Commit that references them. Second, because an MLS group
has a linear history of epochs, the members of the group must agree
on the order in which changes are applied. Concretely, the group
must agree on a single MLS Commit message that ends each epoch and
begins the next one.

In practice, there's a realistic risk of two members generating
Commit messages at the same time, based on the same epoch, and both
attempting to send them to the group at the same time. The extent to
which this is a problem, and the appropriate solution, depends depend on the
design of the Delivery Service. Per the CAP theorem [CAPBR], there
are two general classes of distributed system systems that the Delivery
Service might fall into:

* Consistent and Partition-tolerant, or Strongly Consistent, systems
systems, which can provide a globally consistent view of data but has
have the inconvenience of clients needing to handle rejected messages;
messages.

* Available and Partition-tolerant, or Eventually Consistent,
systems
systems, which continue working despite network issues but may
return different views of data to different users.

Strategies for sequencing messages in strongly and eventually
consistent systems are described in the next two subsections. Most
Delivery Services will use the Strongly Consistent paradigm paradigm, but this
remains a choice that can be handled in coordination with the client
and advertized advertised in the KeyPackages.

However, note that a malicious Delivery Service could also reorder
messages or provide an inconsistent view to different users. The
"generation" counter in MLS messages provides per-sender loss
detection and ordering that cannot be manipulated by the DS, but this
does not provide complete protection against partitioning. A DS can
cause a partition in the group by partitioning key exchange messages;
this can be detected only by out-of-band comparison (e.g., confirming
that all clients have the same epoch_authenticator value). A
mechanism for more robust protections is discussed in
[I-D.ietf-mls-extensions]. [EXTENSIONS].

Other forms of Delivery Service misbehavior are still possible that
are not easy to detect. For instance, a Delivery Service can simply
refuse to relay messages to and from a given client. Without some
sort of side information, other clients cannot generally detect this
form of Denial of Service Denial-of-Service (DoS) attack.

5.2.1. Strongly Consistent

With this approach, the Delivery Service ensures that some types of
incoming messages have a linear order and all members agree on that
order. The Delivery Service is trusted to break ties when two
members send a Commit message at the same time.

As an example, there could be an "ordering server" Delivery Service
that broadcasts all messages received to all users and ensures that
all clients see messages in the same order. This would allow clients
to only apply the first valid Commit for an epoch and ignore
subsequent ones. Commits. Clients that send a Commit would then wait to
apply it until it is broadcast back to them by the Delivery Service,
assuming that they do not receive another Commit first.

Alternatively, the Delivery Service can rely on the epoch and
content_type fields of an MLSMessage to provide an order only to
handshake messages, and possibly even filter or reject redundant
Commit messages proactively to prevent them from being broadcast.
There is some risk associated with filtering, which filtering; this is discussed
further in Section 5.3.

5.2.2. Eventually Consistent

With this approach, the Delivery Service is built in a way that may
be significantly more available or performant than a strongly
consistent system, but offers weaker consistency guarantees.
Messages may arrive to different clients in different orders and with
varying amounts of latency, which means clients are responsible for
reconciliation.

This type of Delivery Service might arise, for example, when group
members are sending each message to each other member individually, individually or
when a distributed peer-to-peer network is used to broadcast
messages.

Upon receiving a Commit from the Delivery Service, clients can
either: do
either of the following:

1. Pause sending new messages for a short amount of time to account
for a reasonable degree of network latency and see if any other
Commits are received for the same epoch. If multiple Commits are
received, the clients can use a deterministic tie-breaking policy
to decide which to accept, and then resume sending messages as
normal.

2. Accept the Commit immediately but keep a copy of the previous
group state for a short period of time. If another Commit for a
past epoch is received, clients use a deterministic tie-breaking
policy to decide if they should continue using the Commit they
originally accepted or revert and use the later one. Note that
any copies of previous or forked group states must be deleted
within a reasonable amount of time to ensure that the protocol
provides forward-secrecy. forward secrecy.

If the Commit references an unknown proposal, group members may need
to solicit the Delivery Service or other group members individually
for the contents of the proposal.

5.2.3. Welcome Messages

Whenever a commit adds new members to a group, MLS requires the
committer to send a Welcome message to the new members. Applications
should ensure that Welcome messages are coupled with the tie-breaking
logic for commits, discussed in Section commits (see Sections 5.2.1 and Section 5.2.2. 5.2.2). That is, when
multiple commits are sent for the same epoch, applications need to
ensure that only Welcome messages corresponding to the commit that
"succeeded" are processed by new members.

This is particularly important when groups are being reinitialized.
When a group is reinitialized, it is restarted with a different
protocol version and/or ciphersuite but identical membership.
Whenever an authorized member sends and commits a ReInit proposal,
this immediately freezes the existing group and triggers the creation
of a new group with a new group_id.

Ideally, the new group would be created by the same member that
committed the ReInit proposal (including sending Welcome messages for
the new group to all of the previous group's members). However However, this
operation is not always atomic, so it's possible for a member to go
offline after committing a ReInit proposal but before creating the
new group. If this happens, it's necessary for another member to
continue the reinitialization by creating the new group and sending
out Welcome messages.

This has the potential to create a race condition, where multiple
members try to continue the reinitialization at the same time, and
members receive multiple Welcome messages for each attempt at
reinitializing the same group. Ensuring that all members agree on
which reinitialization attempt is "correct" is key to prevent this
from causing forks.

5.3. Invalid Commits

Situations can arise where a malicious or buggy client sends a Commit
that is not accepted by all members of the group, and the DS is not
able to detect this and reject the Commit. For example, a buggy
client might send an encrypted Commit with an invalid set of
proposals, or a malicious client might send a malformed Commit of the
form described in Section 16.12 of [RFC9420].

In situations where the DS is attempting to filter redundant Commits,
the DS might update its internal state under the assumption that a
Commit has succeeded and thus end up in a state inconsistent with the
members of the group. For example, the DS might think that the
current epoch is now n+1 and reject any commits from other epochs,
while the members think the epoch is n, and as a result, the group is
stuck -- no member can send a Commit that the DS will accept.

Such “desynchronization” "desynchronization" problems can arise even when the Delivery
Service takes no stance on which Commit is "correct" for an epoch.
The DS can enable clients to choose between Commits, for example by
providing Commits in the order received and allow clients to reject
any Commits that violate their view of the group's policies. As
such, all honest and correctly-implemented correctly implemented clients will arrive at the
same "first valid Commit" and choose to process it. Malicious or
buggy clients that process a different Commit will end up in a forked
view of the group.

When these desynchronizations happen, the application may choose to
take action to restore the functionality of the group. These actions
themselves can have security implications. For example, a client
developer might have a client automatically rejoin a group, using an
external join, when it processes an invalid Commit. In this
operation, however, the client trusts that the GroupInfo provided by
the DS faithfully represents the state of the group, and not, say, an
earlier state containing a compromised leaf node. In addition, the
DS may be able to trigger this condition by deliberately sending the
victim an invalid Commit. In certain scenarios, this trust can
enable the DS or a malicious insider to undermine the post-compromise
security guarantees provided by MLS.

Actions to recover from desynchronization can also have availability
and DoS implications. For example, if a recovery mechanism relies on
external joins, a malicious member that deliberately posts an invalid
Commit could also post a corrupted GroupInfo object in order to
prevent victims from rejoining the group. Thus, careful analysis of
security implications should be made for any system for recovering
from desynchronization.

6. Functional Requirements

MLS is designed as a large-scale group messaging protocol and hence
aims to provide both performance and security (e.g. (e.g., integrity and
confidentiality) to its users. Messaging systems that implement MLS
provide support for conversations involving two or more members, and
they aim to scale to groups with tens of thousands of members,
typically including many users using multiple devices.

6.1. Membership Changes

MLS aims to provide agreement on group membership, meaning that all
group members have agreed on the list of current group members.

Some applications may wish to enforce ACLs Access Control Lists (ACLs) to
limit addition or removal of group members, to privileged clients or
users. Others may wish to require authorization from the current
group members or a subset thereof. Such policies can be implemented
at the application layer, on top of MLS. Regardless, MLS does not
allow for or support addition or removal of group members without
informing all other members.

Membership of an MLS group is managed at the level of individual
clients. In most cases, a client corresponds to a specific device
used by a user. If a user has multiple devices, the user will
generally be represented in a group by multiple clients (although
applications could choose to have devices share keying material). If
an application wishes to implement operations at the level of users,
it is up to the application to track which clients belong to a given
user and ensure that they are added / removed added/removed consistently.

MLS provides two mechanisms for changing the membership of a group.
The primary mechanism is for an authorized member of the group to
send a Commit that adds or removes other members. The second
mechanism is an "external join": A member of the group publishes
certain information about the group, which a new member can use to
construct an "external" Commit message that adds the new member to
the group. (There is no similarly unilateral way for a member to
leave the group; they must be removed by a remaining member.)

With both mechanisms, changes to the membership are initiated from
inside the group. When members perform changes directly, this is
clearly the case. External joins are authorized indirectly, in the
sense that a member publishing a GroupInfo object authorizes anyone
to join who has access to the GroupInfo object, subject to whatever
access control policies the application applies for external joins.

Both types of joins are done via a Commit message, which could be
blocked by the DS or rejected by clients if the join is not
authorized. The former approach requires that Commits be visible to
the DS; the latter approach requires that clients all share a
consistent policy. In the unfortunate event that an unauthorized
member is able to join, MLS enables any member to remove them.

Application setup may also determine other criteria for membership
validity. For example, per-device signature keys can be signed by an
identity key recognized by other participants. If a certificate
chain is used to authenticate device signature keys, then revocation
by the owner adds an alternative mechanism to prompt membership
removal.

An MLS group's secrets change on every change of membership, so each
client only has access to the secrets used by the group while they
are a member. Messages sent before a client joins or after they are
removed are protected with keys that are not accessible to the
client. Compromise of a member removed from a group does not affect
the security of messages sent after their removal. Messages sent
during the client's membership are also secure as long as the client
has properly implemented the MLS deletion schedule, which calls for
the secrets used to encrypt or decrypt a message to be deleted after
use, along with any secrets that could be used to derive them.

6.2. Parallel Groups

Any user or client may have membership in several groups
simultaneously. The set of members of any group may or may not form
a subset of the members of another group. MLS guarantees that the FS
and PCS goals within a given group are maintained and not weakened by
user membership in multiple groups. However, actions in other groups
likewise do not strengthen the FS and PCS guarantees within a given
group, e.g., key updates within a given group following a device
compromise does do not provide PCS healing in other groups; each group
must be updated separately to achieve these security objectives.
This also applies to future groups that a member has yet to join,
which are likewise unaffected by updates performed in current groups.

Applications can strengthen connectivity among parallel groups by
requiring periodic key updates from a user across all groups in which
they have membership.

MLS provides a pre-shared key (PSK) that can be used to link healing
properties among parallel groups. For example, suppose a common
member M of two groups A and B has performed a key update in group A
but not in group B. The key update provides PCS with regard to M in
group A. If a PSK is exported from group A and injected into group
B, then some of these PCS properties carry over to group B, since the
PSK and secrets derived from it are only known to the new, updated
version of M, not to the old, possibly compromised version of M.

6.3. Asynchronous Usage

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

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

6.4. Access Control

Because all clients within a group (members) have access to the
shared cryptographic material, the MLS protocol allows each member of
the messaging group to perform operations. However, every service/
infrastructure has control over policies applied to its own clients.
Applications managing MLS clients can be configured to allow for
specific group operations. On the one hand, an application could
decide that a group administrator will be the only member to perform
add and remove operations. On the other hand, in many settings such
as open discussion forums, joining can be allowed for anyone.

While MLS Application messages are always encrypted, MLS handshake
messages can be sent either encrypted (in an MLS PrivateMessage) or
unencrypted (in an MLS PublicMessage). Applications may be designed
such that intermediaries need to see handshake messages, for example
to enforce policy on which commits are allowed, or to provide MLS
ratchet tree data in a central location. If handshake messages are
unencrypted, it is especially important that they be sent over a
channel with strong transport encryption (see Section 8) in order to
prevent external attackers from monitoring the status of the group.
Applications that use unencrypted handshake messages may take
additional steps to reduce the amount of metadata that is exposed to
the intermediary. Everything else being equal, using encrypted
handshake messages provides stronger privacy properties than using
unencrypted handshake messages, as it prevents intermediaries from
learning about the structure of the group.

If handshake messages are encrypted, any access control policies must
be applied at the client, so the application must ensure that the
access control policies are consistent across all clients to make
sure that they remain in sync. If two different policies were
applied, the clients might not accept or reject a group operation and
end-up
end up in different cryptographic states, breaking their ability to
communicate.

*RECOMMENDATION:* Avoid using inconsistent access control policies
in the case of encrypted group operations.

MLS allows actors outside the group to influence the group in two
ways: External signers can submit proposals for changes to the group,
and new joiners can use an external join to add themselves to the
group. The external_senders extension ensures that all members agree
on which signers are allowed to send proposals, but any other
policies must be assured to be consistent consistent, as noted above.

*RECOMMENDATION:* Have an explicit group policy setting the
conditions under which external joins are allowed.

6.5. Handling Authentication Failures

Within an MLS group, every member is authenticated to every other
member by means of credentials issued and verified by the
Authentication Service. MLS does not prescribe what actions, if any,
an application should take in the event that a group member presents
an invalid credential. For example, an application may require such
a member to be immediately evicted, evicted or may allow some grace period for
the problem to be remediated. To avoid operational problems, it is
important for all clients in a group to have a consistent view of
which credentials in a group are valid, and how to respond to invalid
credentials.

*RECOMMENDATION:* Have a uniform credential validation process to
ensure that all group members evaluate other members' credentials
in the same way.

*RECOMMENDATION:* Have a uniform policy for how invalid
credentials are handled.

In some authentication systems, it is possible for a previously-valid previously valid
credential to become invalid over time. For example, in a system
based on X.509 certificates, credentials can expire or be revoked.
The MLS update mechanisms allow a client to replace an old credential
with a new one. This is best done before the old credential becomes
invalid.

*RECOMMENDATION:* Proactively rotate credentials, especially if a
credential is about to become invalid.

6.6. Recovery After State Loss

Group members whose local MLS state is lost or corrupted can
reinitialize their state by re-joining rejoining the group as a new member and
removing the member representing their earlier state. An application
can require that a client performing such a reinitialization prove
its prior membership with a PSK that was exported from the prevoius previous
state.

There are a few practical challenges to this approach. For example,
the application will need to ensure that all members have the
required PSK, including any new members that have joined the group
since the epoch in which the PSK was issued. And of course, if the
PSK is lost or corrupted along with the member's other state, then it
cannot be used to recover.

Reinitializing in this way does not provide the member with access to
group messages from exchanged during the state loss window, but it enables
proof of prior membership in the group. Applications may choose
various configurations for providing lost messages to valid group
members that are able to prove prior membership.

6.7. Support for Multiple Devices

It is typically expected for that users within a group to will own various
devices. A new device can be added to a group and be considered as a
new client by the protocol. This client will not gain access to the
history even if it is owned by someone who owns another member of the
group. MLS does not provide direct support for restoring history in
this case, but applications can elect to provide such a mechanism
outside of MLS. Such mechanisms, if used, may reduce the FS and PCS
guarantees provided by MLS.

6.8. Extensibility

The MLS protocol provides several extension points where additional
information can be provided. Extensions to KeyPackages allow clients
to disclose additional information about their capabilities. Groups
can also have extension data associated with them, and the group
agreement properties of MLS will confirm that all members of the
group agree on the content of these extensions.

6.9. Application Data Framing and Type Advertisements

Application messages carried by MLS are opaque to the protocol; they
can contain arbitrary data. Each application which that uses MLS needs to
define the format of its application_data and any mechanism necessary
to determine the format of that content over the lifetime of an MLS
group. In many applications applications, this means managing format migrations
for groups with multiple members who may each be offline at
unpredictable times.

*RECOMMENDATION:* Use the default content mechanism defined in
Section 3.3 of [I-D.ietf-mls-extensions], [EXTENSIONS], unless the specific application
defines another mechanism which that more appropriately addresses the
same requirements for that application of MLS.

The MLS framing for application messages also provides a field where
clients can send information that is authenticated but not encrypted.
Such information can be used by servers that handle the message, but
group members are assured that it has not been tampered with.

6.10. Federation

The protocol aims to be compatible with federated environments.
While this document does not specify all necessary mechanisms
required for federation, multiple MLS implementations can
interoperate to form federated systems if they use compatible
authentication mechanisms, ciphersuites, application content, and
infrastructure functionalities. Federation is described in more
detail in [I-D.ietf-mls-federation]. [FEDERATION].

6.11. Compatibility with Future Versions of MLS

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

In MLS 1.0, the creator of the group is responsible for selecting the
best ciphersuite supported across clients. Each client is able to
verify availability of protocol version, ciphersuites ciphersuites, and extensions
at all times once he it has at least received the first group operation
message.

Each member of an MLS group advertises the protocol functionality
they support. These capability advertisements can be updated over
time, e.g., if client software is updated while the client is a
member of a group. Thus, in addition to preventing downgrade
attacks, the members of a group can also observe when it is safe to
upgrade to a new ciphersuite or protocol version.

7. Operational Requirements

MLS is a security layer that needs to be integrated with an
application. A fully-functional fully functional deployment of MLS will have to make
a number of decisions about how MLS is configured and operated.
Deployments that wish to interoperate will need to make compatible
decisions. This section lists all of the dependencies of an MLS
deployment that are external to the protocol specification, but would
still need to be aligned within a given MLS deployment, or for two
deployments to potentially interoperate.

The protocol has a built-in ability to negotiate protocol versions,
ciphersuites, extensions, credential types, and additional proposal
types. For two deployments to interoperate, they must have
overlapping support in each of these categories. The
required_capabilities extension (Section 7.2 of [RFC9420]) can
promote interoperability with a wider set of clients by ensuring that
certain functionality continues to be supported by a group, even if
the clients in the group aren't currently relying on it.

MLS relies on the following network services, that which need to be
compatible in order for two different deployments based on them to
interoperate.

* An *Authentication Service*, described fully in Section 4, defines
the types of credentials which that may be used in a deployment and
provides methods for:

1. Issuing new credentials with a relevant credential lifetime,

2. Validating a credential against a reference identifier,

3. Validating whether or not two credentials represent the same
client, and

4. Optionally revoking credentials which that are no longer authorized.

* A *Delivery Service*, described fully in Section 5, provides
methods for:

1. Delivering messages for a group to all members in the group.

2. Delivering Welcome messages to new members of a group.

3. Uploading new KeyPackages for a user's own clients.

4. Downloading KeyPackages for specific clients. Typically,
KeyPackages are used once and consumed.

* Additional services may or may not be required required, depending on the
application design:

- In cases where group operations are not encrypted, the DS has
the ability to observe and maintain a copy of the public group
state. In particular, this is useful for (1) clients that do
not have the ability to send the full public state in a Welcome
message when inviting a user, user or for (2) a client that needs to
recover from losing their state. Such public state can contain
privacy sensitive
privacy-sensitive information such as group members'
credentials and related public keys, hence keys; hence, services need to
carefully evaluate the privacy impact of storing this data on
the DS.

- If external joiners are allowed, there must be a method to
publish for
publishing a serialized GroupInfo object (with an external_pub
extension) that corresponds to a specific group and epoch, and
keep
for keeping that object in sync with the state of the group.

- If an application chooses not to allow external joining, it may
instead provide a method for external users to solicit group
members (or a designated service) to add them to a group.

- If the application uses PSKs that members of a group may not
have access to (e.g., to control entry into the group or to
prove membership in the group in the past, as discussed in
Section 6.6) 6.6), there must be a method for distributing these
PSKs to group members who might not have them, them -- for instance instance,
if they joined the group after the PSK was generated.

- If an application wishes to detect and possibly discipline
members that send malformed commits with the intention of
corrupting a group's state, there must be a method for
reporting and validating malformed commits.

MLS requires the following parameters to be defined, which must be
the same for two implementations to interoperate:

* The maximum total lifetime that is acceptable for a KeyPackage.

* How long to store the resumption PSK for past epochs of a group.

* The degree of tolerance that's allowed for out-of-order message
delivery:

- How long to keep unused nonce and key pairs for a sender sender.

- A maximum number of unused key pairs to keep.

- A maximum number of steps that clients will move a secret tree
ratchet forward in response to a single message before
rejecting it.

- Whether to buffer messages that aren't yet able to be
understood
yet due to other messages not arriving first, and first and, if so,
how many and for how long. For long -- for example, Commit messages that
arrive before a proposal they reference, reference or application messages
that arrive before the Commit starting an epoch.

If implementations differ in these parameters, they will interoperate
to some extent but may experience unexpected failures in certain
situations, such as extensive message reordering.

MLS provides the following locations where an application may store
arbitrary data. The format and intention of any data in these
locations must align for two deployments to interoperate:

* Application data, sent as the payload of an encrypted message.

* Additional authenticated data, sent unencrypted in an otherwise
encrypted message.

* Group IDs, as decided by group creators and used to uniquely
identify a group.

* Application-level identifiers of public key material (specifically
(specifically, the application_id extension as defined in
Section 5.3.3 of [RFC9420]).

MLS requires the following policies to be defined, which restrict the
set of acceptable behavior behaviors in a group. These policies must be
consistent between deployments for them to interoperate:

* A policy on which ciphersuites are acceptable.

* A policy on any mandatory or forbidden MLS extensions.

* A policy on when to send proposals and commits in plaintext
instead of encrypted.

* A policy for which proposals are valid to have in a commit,
including but not limited to:

- When a member is allowed to add or remove other members of the
group.

- When, and under what circumstances, a reinitialization proposal
is allowed.

- When proposals from external senders are allowed and how to
authorize those proposals.

- When external joiners are allowed and how to authorize those
external commits.

- Which other proposal types are allowed.

* A policy of when members should commit pending proposals in a
group.

* A policy of how to protect and share the GroupInfo objects needed
for external joins.

* A policy for when two credentials represent the same client. Note
that many credentials may be issued attesting the same identity
but for different signature keys, because each credential
corresponds to a different client owned by the same application
user. However, one device may control multiple signature keys --
for instance instance, if they have the device has keys corresponding to multiple
overlapping time periods -- but should still only be considered a
single client.

* A policy on how long to allow a member to stay in a group without
updating its leaf keys before removing them.

Finally, there are some additional application-defined behaviors that
are partially an individual application's decision but may overlap
with interoperability:

* When and how to pad messages.

* When to send a reinitialization proposal.

* How often clients should update their leaf keys.

* Whether to prefer sending full commits or partial/empty commits.

* Whether there should be a required_capabilities extension in
groups.

8. Security and Privacy Considerations

MLS adopts the Internet threat model [RFC3552] and therefore assumes
that the attacker has complete control of the network. It is
intended to provide the security services described in Section 8.2 in
the face of attackers who can:

* Monitor the entire network.

* Read unprotected messages.

* Can generate, inject Generate, inject, and delete any message in the unprotected
transport layer.

While MLS should be run over a secure transport such as QUIC
[RFC9000] or TLS [RFC8446], the security guarantees of MLS do not
depend on the transport. This departs from the usual design practice
of trusting the transport because MLS is designed to provide security
even in the face of compromised network elements, especially the DS.

Generally, MLS is designed under the assumption that the transport
layer is present to keep metadata private from network observers,
while the MLS protocol provides confidentiality, integrity, and
authentication guarantees for the application data (which could pass
through multiple systems). Additional properties such as partial
anonymity or deniability could also be achieved in specific
architecture designs.

In addition, these guarantees are intended to degrade gracefully in
the presence of compromise of the transport security links as well as
of both clients and elements of the messaging system, as described in
the remainder of this section.

8.1. Assumptions on Transport Security Links

As discussed above, MLS provides the highest level of security when
its messages are delivered over an encrypted transport. The main use
of the secure transport layer for MLS is to protect the already already-
limited amount of metadata. Very little information is contained in
the unencrypted header of the MLS protocol message format for group
operation messages, and application messages are always encrypted in
MLS.

*RECOMMENDATION:* Use transports that provide reliability and
metadata confidentiality whenever possible, e.g., by transmitting
MLS messages over a protocol such as TLS [RFC8446] or QUIC
[RFC9000].

MLS avoids needing to send the full list of recipients to the server
for dispatching messages because that list could potentially contain
tens of thousands of recipients. Header metadata in MLS messages
typically consists of an opaque group_id, a numerical value to
determine the epoch of the group (the number of changes that have
been made to the group), and whether the message is an application
message, a proposal, or a commit.

Even though some of this metadata information does not consist of
sensitive information, in correlation with other data a network
observer might be able to reconstruct sensitive information. Using a
secure channel to transfer this information will prevent a network
attacker from accessing this MLS protocol metadata if it cannot
compromise the secure channel.

8.1.1. Integrity and Authentication of Custom Metadata

MLS provides an authenticated "Additional Authenticated Data" (AAD)
field for applications to make data available outside a
PrivateMessage, while cryptographically binding it to the message.

*RECOMMENDATION:* Use the "Additional Authenticated Data" field of
the PrivateMessage instead of using other unauthenticated means of
sending metadata throughout the infrastructure. If the data
should be kept private, the infrastructure should use encrypted
Application messages instead.

8.1.2. Metadata Protection for Unencrypted Group Operations

Having no secure channel to exchange MLS messages can have a serious
impact on privacy when transmitting unencrypted group operation
messages. Observing the contents and signatures of the group
operation messages may lead an adversary to extract information about
the group membership.

*RECOMMENDATION:* Never use the unencrypted mode for group
operations without using a secure channel for the transport layer.

8.1.3. DoS protection Protection

In general general, we do not consider Denial of Service (DoS) DoS resistance to be the
responsibility of the protocol. However, it should not be possible
for anyone aside from the Delivery Service to perform a trivial DoS
attack from which it is hard to recover. This can be achieved
through the secure transport layer.

In the centralized setting, DoS protection can typically be performed
by using tickets or cookies which identify users to a service for a
certain number of connections. Such a system helps in preventing
anonymous clients from sending arbitrary numbers of group operation
messages to the Delivery Service or the MLS clients.

*RECOMMENDATION:* Use credentials uncorrellated uncorrelated with specific users
to help prevent DoS attacks, in a privacy preserving manner. manner that preserves privacy.
Note that the privacy of these mechanisms has to be adjusted in
accordance with the privacy expected from secure transport links.
(See more discussion in the next section.)

8.1.4. Message Suppression and Error Correction

As noted above, MLS is designed to provide some robustness in the
face of tampering within the secure transport, i.e., tampering by the
Delivery Service. The confidentiality and authenticity properties of
MLS prevent the DS from reading or writing messages. MLS also
provides a few tools for detecting message suppression, with the
caveat that message suppression cannot always be distinguished from
transport failure.

Each encrypted MLS message carries a "generation" number which is a
per-sender incrementing counter. If a group member observes a gap in
the generation sequence for a sender, then they know that they have
missed a message from that sender. MLS also provides a facility for
group members to send authenticated acknowledgments of application
messages received within a group.

As discussed in Section 5, the Delivery Service is trusted to select
the single Commit message that is applied in each epoch from among
the ones Commits sent by group members. Since only one Commit per epoch
is meaningful, it's not useful for the DS to transmit multiple
Commits to clients. The risk remains that the DS will use the
ability maliciously.

While it is difficult or impossible to prevent a network adversary
from suppressing payloads in transit, in certain infrastructures such
as banks or governments government settings, unidirectional transports can be
used and be enforced via electronic or physical devices such as
diodes. This can lead to payload corruption corruption, which does not affect
the security or privacy properties of the MLS protocol but does
affect the reliability of the service. In that case case, specific
measures can be taken to ensure the appropriate level of redundancy
and quality of service for MLS.

8.2. Intended Security Guarantees

MLS aims to provide a number of security guarantees, covering
authentication, as well as confidentiality guarantees to different
degrees in different scenarios.

8.2.1. Message Secrecy and Authentication

MLS enforces the encryption of application messages and thus
generally guarantees authentication and confidentiality of
application messages sent in a group.

In particular, this means that only other members of a given group
can decrypt the payload of a given application message, which
includes information about the sender of the message.

Similarly, group members receiving a message from another group
member can authenticate that group member as the sender of the
message and verify the message's integrity.

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

Depending on the group settings, handshake messages can be encrypted
as well. If that is the case, the same security guarantees apply.

MLS optionally allows the addition of padding to messages, mitigating
the amount of information leaked about the length of the plaintext to
an observer on the network.

8.2.2. Forward and Post-Compromise Security

MLS provides additional protection regarding secrecy of past messages
and future messages. These cryptographic security properties are
Forward Secrecy (FS) and Post-Compromise Security (PCS).

FS means that access to all encrypted traffic history combined with
access to all current keying material on clients will not defeat the
secrecy properties of messages older than the oldest key of the
compromised client. Note that this means that clients have to delete
the appropriate keys as soon as they have been used with the expected
message, otherwise
message; otherwise, the secrecy of the messages and the security for of
MLS is are considerably weakened.

PCS means that if a group member's state is compromised at some time
t1 but the group member subsequently performs an update at some time
t2, then all MLS guarantees apply to messages sent by the member
after time t2, t2 and sent by other members after they have processed the
update. For example, if an attacker learns all secrets known to
Alice at time t1, including both Alice's long-term secret keys and
all shared group keys, but Alice performs a key update at time t2,
then the attacker is unable to violate any of the MLS security
properties after the updates have been processed.

Both of these properties are satisfied even against compromised DSs
and ASs ASes in the case where some other mechanism for verifying keys is
in use, such as Key Transparency [KT].

Confidentiality is mainly ensured on the client side. Because
Forward Secrecy (FS) FS and Post-Compromise Security (PCS)
PCS rely on the active deletion and replacement of keying material,
any client which that is persistently offline may still be holding old
keying material and thus be a threat to both FS and PCS if it is
later compromised.

MLS partially defends against this problem by active members
including freshness, however freshness. However, not much can be done on the inactive
side especially in the case where the client has not processed
messages.

*RECOMMENDATION:* Mandate key updates from clients that are not
otherwise sending messages and evict clients which that are idle for too
long.

These recommendations will reduce the ability of idle compromised
clients to decrypt a potentially long set of messages that might have
followed
been sent after the point of the compromise.

The precise details of such mechanisms are a matter of local policy
and beyond the scope of this document.

8.2.3. Non-Repudiation vs vs. Deniability

MLS provides strong authentication within a group, such that a group
member cannot send a message that appears to be from another group
member. Additionally, some services require that a recipient be able
to prove to the service provider that a message was sent by a given
client, in order to report abuse. MLS supports both of these use
cases. In some deployments, these services are provided by
mechanisms which that allow the receiver to prove a message's origin to a
third party. This is often called "non-repudiation".

Roughly speaking, "deniability" is the opposite of "non-repudiation",
i.e., the property that it is impossible to prove to a third party
that a message was sent by a given sender. MLS does not make any
claims with regard to deniability. It may be possible to operate MLS
in ways that provide certain deniability properties, but defining the
specific requirements and resulting notions of deniability requires
further analysis.

8.2.4. Associating a User's Clients

When a user has multiple devices, the base MLS protocol only
describes how to operate each device as a distinct client in the MLS
groups that the user is a member of. As a result, the other members
of the group will be able to identify which of a user's devices sent
each message, and therefore message and, therefore, which device the user was using at the
time. Group members would also be able to detect when the user adds
or removes authorized devices from their account. For some
applications, this may be an unacceptable breach of the user's
privacy.

This risk only arises when the leaf nodes for the clients in question
provide data that can be used to correlate the clients. So So, one way
to mitigate this risk is by only doing client-level authentication
within MLS. If user-level authentication is still desirable, the
application would have to provide it through some other mechanism.

It is also possible to maintain user-level authentication while
hiding information about the clients that a user owns. This can be
done by having the clients share cryptographic state, so that they
appear as a single client within the MLS group. Appearing as a
single client has the privacy benefits of no longer leaking which
device was used to send a particular message, message and no longer leaking
the user's authorized devices. However, the application would need
to provide a synchronization mechanism so that the clients' state
remain consistent across changes to the MLS group. Flaws in this
synchronization mechanism may impair the ability of the user to
recover from a compromise of one of their devices. In particular,
state synchronization may make it easier for an attacker to use one
compromised device to establish exclusive control of a user's
account, locking them out entirely and preventing them from
recovering.

8.3. Endpoint Compromise

The MLS protocol adopts a threat model which that includes multiple forms
of endpoint/client compromise. While adversaries are in a strong
position if they have compromised an MLS client, there are still
situations where security guarantees can be recovered thanks to the
PCS properties achieved by the MLS protocol.

In this section section, we will explore the consequences and recommendations
regarding the following compromise scenarios:

* The attacker has access to a symmetric encryption key key.

* The attacker has access to a an application ratchet secret secret.

* The attacker has access to the group secrets for one group group.

* The attacker has access to a signature oracle for any group group.

* The attacker has access to the signature key for one group group.

* The attacker has access to all secrets of a user for all groups
(full state compromise) compromise).

8.3.1. Compromise of Symmetric Keying Material

As described above, each MLS epoch creates a new Group Secret.

These group secrets are then used to create a per-sender Ratchet
Secret, which in turn is used to create a per-sender with an
additional data (AEAD) [RFC5116] (Authenticated Encryption with Associated Data
(AEAD)) key [RFC5116] that is then used to encrypt MLS Plaintext
messages. Each time a message is sent, the Ratchet Secret is used to
create a new Ratchet Secret and a new corresponding AEAD key.
Because of the properties of the key derivation function, it is not
possible to compute a Ratchet Secret from its corresponding AEAD key
or compute Ratchet Secret n-1 from Ratchet Secret n.

Below, we consider the compromise of each of these pieces of keying
material in turn, in ascending order of severity. While this is a
limited kind of compromise, it can be realistic in cases of
implementation vulnerabilities where only part of the memory leaks to
the adversary.

8.3.1.1. Compromise of AEAD Keys

In some circumstances, adversaries may have access to specific AEAD
keys and nonces which that protect an Application message or a Group
Operation message. Compromise of these keys allows the attacker to
decrypt the specific message encrypted with that key but no other;
because the AEAD keys are derived from the Ratchet Secret, it cannot
generate the next Ratchet Secret and hence not the next AEAD key.

In the case of an Application message, an AEAD key compromise means
that the encrypted application message will be leaked as well as the
signature over that message. This means that the compromise has both
confidentiality and privacy implications on the future AEAD
encryptions of that chain. In the case of a Group Operation message,
only the privacy is affected, as the signature is revealed, because
the secrets themselves are protected by HPKE encryption. Hybrid Public Key Encryption
(HPKE). Note that under that compromise scenario, authentication is
not affected in either of these cases. As every member of the group
can compute the AEAD keys for all the chains (they have access to the
Group Secrets) in order to send and receive messages, the
authentication provided by the AEAD encryption layer of the common
framing mechanism is weak. Successful decryption of an AEAD
encrypted message only guarantees that some member of the group sent
the message.

Compromise of the AEAD keys allows the attacker to send an encrypted
message using that key, but the attacker cannot send a message to a
group which that appears to be from any valid client since they because the attacker
cannot forge the signature. This applies to all the forms of
symmetric key compromise described in Section 8.3.1.

8.3.1.2. Compromise of Ratchet Secret material Material

When a Ratchet Secret is compromised, the adversary can compute both
the current AEAD keys for a given sender as well as any future keys
for that sender in this epoch. Thus, it can decrypt current and
future messages by the corresponding sender. However, because it
does not have previous Ratchet Secrets, it cannot decrypt past
messages as long as those secrets and keys have been deleted.

Because of its Forward Secrecy guarantees, MLS will also retain
secrecy of all other AEAD keys generated for _other_ MLS clients,
outside this dedicated chain of AEAD keys and nonces, even within the
epoch of the compromise. MLS provides Post-Compromise Security
against an active adaptive attacker across epochs for AEAD
encryption, which means that as soon as the epoch is changed, if the
attacker does not have access to more secret material they won't be
able to access any protected messages from future epochs.

8.3.1.3. Compromise of the Group Secrets of a single group Single Group for one One or
more group epochs
More Group Epochs

An adversary who gains access to a set of Group secrets--as secrets -- for
example, when a member of the group is compromised--is compromised -- is
significantly more powerful. In this section, we consider the case
where the signature keys are not compromised, which can occur if the
attacker has access to part of the memory containing the group
secrets but not to the signature keys which might be stored in a
secure enclave.

In this scenario, the adversary gains the ability to compute any
number of Ratchet Secrets for the epoch and their corresponding AEAD
encryption keys and thus can encrypt and decrypt all messages for the
compromised epochs.

If the adversary is passive, it is expected from the PCS properties
of the MLS protocol that, that as soon as the compromised party remediates
the compromise and sends an honest Commit message, the next epochs
will provide message secrecy.

If the adversary is active, the adversary can engage in the protocol
itself and perform updates on behalf of the compromised party with no
ability for an honest group to recover message secrecy. However, MLS
provides PCS against active adaptive attackers through its Remove
group operation. This means that, that as long as other members of the
group are honest, the protocol will guarantee message secrecy for all
messages exchanged in the epochs after the compromised party has been
removed.

8.3.2. Compromise by an active adversary Active Adversary with the ability Ability to sign
messages Sign
Messages

If an active adversary has compromised an MLS client and can sign
messages, two different settings scenarios emerge. In the strongest
compromise scenario, the attacker has access to the signing key and
can forge authenticated messages. In a weaker, yet weaker but realistic
scenario, the attacker has compromised a client but the client
signature keys are protected with dedicated hardware features which that do
not allow direct access to the value of the private key and instead
provide a signature API.

When considering an active adaptive attacker with access to a
signature oracle, the compromise scenario implies a significant
impact on both the secrecy and authentication guarantees of the
protocol, especially if the attacker also has access to the group
secrets. In that case case, both secrecy and authentication are broken.
The attacker can generate any message, for the current and future
epochs, until the compromise is remediated and the formerly
compromised client sends an honest update.

Note that under this compromise scenario, the attacker can perform
all operations which that are available to a legitimate client even without
access to the actual value of the signature key.

8.3.3. Compromise of the authentication Authentication with access Access to a signature key Signature Key

The difference between having access to the value of the signature
key and only having access to a signing oracle is not about the
ability of an active adaptive network attacker to perform different
operations during the time of the compromise, compromise; the attacker can
perform every operation available to a legitimate client in both
cases.

There is a significant difference, however however, in terms of recovery
after a compromise.

Because of the PCS guarantees provided by the MLS protocol, when a
previously compromised client recovers from compromise and performs
an honest Commit, both secrecy and authentication of future messages
can be recovered as long as the attacker doesn't otherwise get access
to the key. Because the adversary doesn't have the signing key, they
cannot authenticate messages on behalf of the compromised party, even
if they still have control over some group keys by colluding with
other members of the group.

This is in contrast with the case where the signature key is leaked.
In that case case, the compromised endpoint needs to refresh its
credentials and invalidate the old credentials before the attacker
will be unable to authenticate messages.

Beware that in both oracle and private key access, an active adaptive
attacker can follow the protocol and request to update its own
credential. This in turn induces a signature key rotation which
could provide the attacker with part or the full value of the private
key
key, depending on the architecture of the service provider.

*RECOMMENDATION:* Signature private keys should be
compartmentalized from other secrets and preferably protected by
an HSM a
Hardware Security Module (HSM) or dedicated hardware features to
allow recovery of the authentication for future messages after a
compromise.

*RECOMMENDATION:* When the credential type supports revocation,
the users of a group should check for revoked keys.

8.3.4. Security consideration Considerations in the context Context of a full state compromise Full State
Compromise

In real-world compromise scenarios, it is often the case that
adversaries target specific devices to obtain parts of the memory or
even the ability to execute arbitrary code in the targeted device.

Also, recall that in this setting, the application will often retain
the unencrypted messages. If so, the adversary does not have to
break encryption at all to access sent and received messages.
Messages may also be sent by using the application to instruct the
protocol implementation.

*RECOMMENDATION:* If messages are stored on the device, they
should be protected using encryption at rest, and the keys used
should be stored securely using dedicated mechanisms on the
device.

*RECOMMENDATION:* If the threat model of the system is against an
adversary which that can access the messages on the device without even
needing to attack MLS, the application should delete plaintext and
ciphertext messages as soon as practical after encryption or
decryption.

Note that this document makes a clear distinction between the way
signature keys and other group shared secrets must be handled. In
particular, a large set of group secrets cannot necessarily be
assumed to be protected by an HSM or secure enclave features. This
is especially true because these keys are frequently used and changed
with each message received by a client.

However, the signature private keys are mostly used by clients to
send a message. They also provide strong authentication guarantees
to other clients, hence clients; hence, we consider that their protection by
additional security mechanisms should be a priority.

Overall

Overall, there is no way to detect or prevent these compromises, as
discussed in the previous sections, performing sections: Performing separation of the
application secret states can help recovery after compromise, compromise; this is
the case for signature keys keys, but similar concern exists concerns exist for a
client's encryption private keys.

*RECOMMENDATION:* The secret keys used for public key encryption
should be stored similarly to the way the signature keys are
stored, as keys can be used to decrypt the group operation
messages and contain the secret material used to compute all the
group secrets.

Even if secure enclaves are not perfectly secure, secure or are even
completely broken, adopting additional protections for these keys can
ease recovery of the secrecy and authentication guarantees after a
compromise where, for instance, an attacker can sign messages without
having access to the key. In certain contexts, the rotation of
credentials might only be triggered by the AS through ACLs, ACLs and hence
be
outside of beyond the capabilities of the attacker.

8.4. Service Node Compromise

8.4.1. General considerations Considerations

8.4.1.1. Privacy of the network connections Network Connections

There are many scenarios leading to communication between the
application on a device and the Delivery Service or the
Authentication Service. In particular Service -- in particular, when:

* The application connects to the Authentication Service to generate
or validate a new credential before distributing it.

* The application fetches credentials at the Delivery Service prior
to creating a messaging group (one-to-one or more than two
clients).

* The application fetches service provider information or messages
on the Delivery Service.

* The application sends service provider information or messages to
the Delivery Service.

In all these cases, the application will often connect to the device
via a secure transport which that leaks information about the origin of the request
request, such as the IP address and -- depending on the protocol --
the
MAC Media Access Control (MAC) address of the device.

Similar concerns exist in the peer-to-peer use cases of for MLS.

*RECOMMENDATION:* In the case where privacy or anonymity is
important, using adequate protection such as MASQUE
[I-D.schinazi-masque-proxy], ToR, Multiplexed
Application Substrate over QUIC Encryption (MASQUE)
[MASQUE-PROXY], Top-of-Rack (ToR) switches, or a VPN can improve
metadata protection.

More generally, using anonymous credentials in an MLS based MLS-based
architecture might not be enough to provide strong privacy or
anonymity properties.

8.4.1.2. Storage of Metadata and Ecryption Encryption at rest Rest on the Servers

In the case where private data or metadata has to be persisted on the
servers for functionality (mappings between identities and push
tokens, group metadata...), metadata, etc.), it should be stored encrypted at rest
and only decrypted upon need during the execution. Honest Service
Providers can rely on such encryption an "encryption at rest rest" mechanism to be
able to prevent access to the data when not using it.

*RECOMMENDATION:* Store cryptographic material used for server-
side decryption of sensitive meta-data metadata on the clients and only send
it when needed. The server can use the secret to open and update
encrypted data containers after which they can delete these keys
until the next time they need it, in which case those can be
provided by the client.

*RECOMMENDATION:* Rely on group secrets exported from the MLS
session for server-side encryption at rest and update the key
after each removal from the group. Rotate Otherwise, rotate those keys
on a regular
basis otherwise. basis.

8.4.2. Delivery Service Compromise

MLS is intended to provide strong guarantees in the face of
compromise of the DS. Even a totally compromised DS should not be
able to read messages or inject messages that will be acceptable to
legitimate clients. It should also not be able to undetectably
remove, reorder reorder, or replay messages.

However, a malicious DS can mount a variety of DoS attacks on the
system, including total DoS attacks (where it simply refuses to
forward any messages) and partial DoS attacks (where it refuses to
forward messages to and from specific clients). As noted in
Section 5.2, these attacks are only partially detectable by clients
without an out-of-band channel. Ultimately, failure of the DS to
provide reasonable service must be dealt with as a customer service
matter, not via technology.

Because the DS is responsible for providing the initial keying
material to clients, it can provide stale keys. This does not
inherently lead to compromise of the message stream, but it does
allow it to attack forward security to a limited extent. This threat
can be mitigated by having initial keys expire.

Initial keying material (KeyPackages) using the basic Credential type
is more vulnerable to replacement by a malicious or compromised DS,
as there is no built-in cryptographic binding between the identity
and the public key of the client.

*RECOMMENDATION:* Prefer a Credential type in KeyPackages which that
includes a strong cryptographic binding between the identity and
its key (for example example, the x509 Credential type). When using the
basic Credential type type, take extra care to verify the identity
(typically out-of-band). out of band).

8.4.2.1. Privacy of delivery Delivery and push notifications

An Push Notifications

Push-tokens provide an important mechanism that is often ignored from
the standpoint of privacy
considerations are the push-tokens. considerations. In many modern messaging
architectures, applications are using push notification mechanisms
typically provided by OS vendors. This is to make sure that when
messages are available at the Delivery Service (or by via other
mechanisms if the DS is not a central server), the recipient
application on a device knows about it. Sometimes the push
notification can contain the application message itself which itself; this saves a
round trip with the DS.

To "push" this information to the device, the service provider and
the OS infrastructures use unique per-device, per-application
identifiers called push-tokens. This means that the push
notification provider and the service provider have information on
which devices receive information and at which point in time.
Alternatively, non-mobile applications could use a websocket WebSocket or
persistent connection for notifications directly from the DS.

Even though they can't necessarily access the content, which is
typically encrypted MLS messages, the service provider and the push
notification provider have to be trusted to avoid making correlation
on which devices are recipients of the same message.

For secure messaging systems, push notifications are often sent real-
time in
real time, as it is not acceptable to create artificial delays for
message retrieval.

*RECOMMENDATION:* If real time real-time notifications are not necessary,
one can delay notifications randomly across recipient devices
using a mixnet or other techniques.

Note that with a legal request to ask the service provider for the
push-token associated with an identifier, it is easy to correlate the
token with a second request to the company operating the push-
notification system to get information about the device, which is
often linked with a real identity via a cloud account, a credit card card,
or other information.

*RECOMMENDATION:* If stronger privacy guarantees are needed with
regard to the push notification provider, the client can choose to
periodically connect to the Delivery Service without the need of a
dedicated push notification infrastructure.

Applications can also consider anonymous systems for server fanout
(for example example, [Loopix]).

8.4.3. Authentication Service Compromise

The Authentication Service design is left to the infrastructure
designers. In most designs, a compromised AS is a serious matter, as
the AS can serve incorrect or attacker-provided identities to
clients.

* The attacker can link an identity to a credential credential.

* The attacker can generate new credentials credentials.

* The attacker can sign new credentials credentials.

* The attacker can publish or distribute credentials credentials.

An attacker that can generate or sign new credentials may or may not
have access to the underlying cryptographic material necessary to
perform such operations. In that last case, it results in windows of
time for which all emitted credentials might be compromised.

*RECOMMENDATION:* Use HSMs to store the root signature keys to
limit the ability of an adversary with no physical access to
extract the top-level signature private key.

Note that historically some systems generate signature keys on the
Authentication Service and distribute the private keys to clients
along with their credential. This is a dangerous practice because it
allows the AS or an attacker who has compromised the AS to silently
impersonate the client.

8.4.3.1. Authentication compromise: Compromise: Ghost users Users and impersonations Impersonations

One important property of MLS is that all Members know which other
members are in the group at all times. If all Members of the group
and the Authentication Service are honest, no parties other than the
members of the current group can read and write messages protected by
the protocol for that Group.

This guarantee applies to the cryptographic identities of the
members. Details about how to verify the identity of a client depend
on the MLS Credential type used. For example, cryptographic
verification of credentials can be largely performed autonomously
(e.g., without user interaction) by the clients themselves for the
x509 Credential type.

In contrast, when MLS clients use the basic Credential type, then some
other mechanism must be used to verify identities. For instance instance, the
Authentication Service could operate some sort of directory server to
provide keys, or users could verify keys via an out-of-band
mechanism.

*RECOMMENDATION:* Select the MLS Credential type with the
strongest security which that is supported by all target members of an
MLS group.

*RECOMMENDATION:* Do not use the same signature keypair across
groups. Update all keys for all groups on a regular basis. Do
not preserve keys in different groups when suspecting a
compromise.

If the AS is compromised, it could validate a signature keypair (or
generate a new)
signature keypair new one) for an attacker. The attacker could then use
this keypair to join a group as if it were another of the user's
clients. Because a user can have many MLS clients running the MLS
protocol, it possibly has many signature keypairs for multiple
devices. These attacks could be very difficult to detect, especially
in large groups where the UI might not reflect all the changes back
to the users. If the application participates in a key transparency
mechanism in which it is possible to determine every key for a given
user, then this would allow for detection of surreptitiously created
false credentials.

*RECOMMENDATION:* Make sure that MLS clients reflect all the
membership changes to the users as they happen. If a choice has
to be made because the number of notifications is too high, the
client should provide a log of state of the device so that the
user can examine it.

*RECOMMENDATION:* Provide a key transparency mechanism for the
Authentication Services Service to allow public verification of the
credentials authenticated by this service.

While the ways to handle MLS credentials are not defined by the
protocol or the architecture documents, the MLS protocol has been
designed with a mechanism that can be used to provide out-of-band
authentication to users. The "authentication_secret" generated for
each user at each epoch of the group is a one-time, per client, per-client
authentication secret which that can be exchanged between users to prove
their identity identities to each other. This can be done done, for instance instance,
using a QR code that can be scanned by the other parties.

*RECOMMENDATION:* Provide one or more out-of-band authentication
mechanisms to limit the impact of an Authentication Service
compromise.

We note, again, that the Authentication Service may not be a
centralized system, system and could be realized by many mechanisms such as
establishing prior one-to-one deniable channels, gossiping, or using
trust on first use (TOFU) for credentials used by the MLS Protocol. protocol.

Another important consideration is the ease of redistributing new
keys on client compromise, which helps recovering security faster in
various cases.

8.4.3.2. Privacy of the Group Membership

Group membership is itself sensitive information information, and MLS is designed
to limit the amount of persistent metadata. However, large groups
often require an infrastructure which that provides server fanout. In the
case of client fanout, the destination of a message is known by all
clients, hence
clients; hence, the server usually does not need this information.
However, they may learn this information through traffic analysis.
Unfortunately, in a server-side fanout model, the Delivery Service
can learn that a given client is sending the same message to a set of
other clients. In addition, there may be applications of MLS in
which the group membership list is stored on some server associated
with the Delivery Service.

While this knowledge is not a breach of the protocol's authentication
or confidentiality guarantees, it is a serious issue for privacy.

Some infrastructure infrastructures keep a mapping between keys used in the MLS
protocol and user identities. An attacker with access to this
information due to compromise or regulation can associate unencrypted
group messages (e.g., Commits and Proposals) with the corresponding
user identity.

*RECOMMENDATION:* Use encrypted group operation messages to limit
privacy risks whenever possible.

In certain cases, the adversary can access specific bindings between
public keys and identities. If the signature keys are reused across
groups, the adversary can get more information about the targeted
user.

*RECOMMENDATION:* Ensure that linking between public keys and
identities only happens in expected scenarios.

8.5. Considerations for attacks outside Attacks Outside of the threat model Threat Model

Physical attacks on devices storing and executing MLS principals are
not considered in depth in the threat model of the MLS protocol.
While non-permanent, non-invasive attacks can sometimes be equivalent
to software attacks, physical attacks are considered outside of the
MLS threat model.

Compromise scenarios typically consist of a software adversary, which
can maintain active adaptive compromise and arbitrarily change the
behavior of the client or service.

On the other hand, security goals consider that honest clients will
always run the protocol according to its specification. This relies
on implementations of the protocol to securely implement the
specification, which remains non-trivial.

*RECOMMENDATION:* Additional steps should be taken to protect the
device and the MLS clients from physical compromise. In such
settings, HSMs and secure enclaves can be used to protect
signature keys.

8.6. Cryptographic Analysis of the MLS Protocol

Various academic works have analyzed MLS and the different security
guarantees it aims to provide. The security of large parts of the
protocol has been analyzed by [BBN19] (draft (regarding MLS Draft 7),
[ACDT21] (draft 11) (regarding MLS Draft 11), and [AJM20] (draft (regarding MLS Draft
12).

Individual components of various drafts of the MLS protocol have been
analyzed in isolation and with differing adversarial models, for models. For
example, [BBR18], [ACDT19], [ACCKKMPPWY19], [AJM20], [ACJM20], and
[AHKM21] analyze the ratcheting tree sub-protocol of MLS that
facilitates key agreement, agreement; [WPBB22] analyzes the sub-protocol of MLS
for group state agreement and authentication, while authentication; and [BCK21] analyzes
the key derivation paths in the ratchet tree and key schedule.
Finally, [CHK21] analyzes the authentication and cross-group healing
guarantees provided by MLS.

9. IANA Considerations

This document makes has no requests of IANA. IANA actions.

10. References

10.1. Normative References

[I-D.ietf-mls-protocol]
Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", Work in Progress, Internet-
Draft, draft-ietf-mls-protocol-20, 27 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
protocol-20>.

[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/rfc/rfc5116>.
<https://www.rfc-editor.org/info/rfc5116>.

[RFC9420] Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
July 2023, <https://www.rfc-editor.org/rfc/rfc9420>. <https://www.rfc-editor.org/info/rfc9420>.

10.2. Informative References

[ACCKKMPPWY19]
Alwen, J., Capretto, M., Cueto, M., Kamath, C., Klein, K.,
Markov, I., Pascual-Perez, G., Pietrzak, K., Walter, M.,
and M. Yeo, "Keep the Dirt: Tainted TreeKEM, Adaptively
and Actively Secure Continuous Group Key Agreement",
Cryptology ePrint Archive, 2019,
<https://eprint.iacr.org/2019/1489>.

[ACDT19] Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis,
"Security Analysis and Improvements for the IETF MLS
Standard for Group Messaging", Cryptology ePrint Archive,
2019, <https://eprint.iacr.org/2019/1189.pdf>.

[ACDT21] Alwen, J., Coretti, S., Dodis, Y., and Y. Tselekounis,
"Modular Design of Secure Group Messaging Protocols and
the Security of MLS", Cryptology ePrint Archive, 2021,
<https://eprint.iacr.org/2021/1083.pdf>.

[ACJM20] Alwen, J., Coretti, S., Jost, D., and M. Mularczyk,
"Continuous Group Key Agreement with Active Security",
Cryptology ePrint Archive, 2020,
<https://eprint.iacr.org/2020/752.pdf>.

[AHKM21] Alwen, J., Hartmann, D., Kiltz, E., and M. Mularczyk,
"Server-Aided Continuous Group Key Agreement", Cryptology
ePrint Archive, 2021,
<https://eprint.iacr.org/2021/1456.pdf>.

[AJM20] Alwen, J., Jost, D., and M. Mularczyk, "On The Insider
Security of MLS", Cryptology ePrint Archive, 2020,
<https://eprint.iacr.org/2020/1327.pdf>.

[BBN19] Bhargavan, K., Beurdouche, B., and P. Naldurg, "Formal
Models and Verified Protocols for Group Messaging: Attacks
and Proofs for IETF MLS", HAL ID hal-02425229, 2019,
<https://inria.hal.science/hal-02425229/document>.

[BBR18] Bhargavan, K., Barnes, R., and E. Rescorla, "TreeKEM:
Asynchronous Decentralized Key Management for Large
Dynamic Groups - A protocol proposal for Messaging Layer
Security (MLS)", HAL ID hal-02425247, 2018, <https://hal.inria.fr/hal-
02425247/file/treekem+%281%29.pdf>.
<https://hal.inria.fr/hal-02425247/file/
treekem+%281%29.pdf>.

[BCK21] Brzuska, C., Cornelissen, E., and K. Kohbrok,
"Cryptographic Security "Security
Analysis of the MLS RFC, Draft 11", Key Distribution", Cryptology ePrint
Archive, 2021, <https://eprint.iacr.org/2021/137.pdf>.

[CAPBR] Brewer, E., E. A., "Towards robust distributed systems
(abstract)", ACM, Proceedings of the nineteenth annual ACM
symposium on Principles of distributed computing, p. 7,
DOI 10.1145/343477.343502, July 2000,
<https://doi.org/10.1145/343477.343502>.
<https://dl.acm.org/doi/10.1145/343477.343502>.

[CHK21] Cremers, C., Hale, B., and K. Kohbrok, "The Complexities
of Healing in Secure Group Messaging: Why Cross-Group
Effects Matter", Proceedings of the 30th USENIX Security
Symposium, August 2021,
<https://www.usenix.org/system/files/sec21-cremers.pdf>.

[CONIKS] Melara, M., M. S., Blankstein, A., Bonneau, J., Felten, E., E. W.,
and M. J. Freedman, "CONIKS: Bringing Key Transparency to
End Users", Proceedings of the 24th USENIX Security
Symposium, August 2015,
<https://www.usenix.org/system/files/conference/
usenixsecurity15/sec15-paper-melara.pdf>.

[I-D.ietf-mls-extensions]

[EXTENSIONS]
Robert, R., "The Messaging Layer Security (MLS)
Extensions", Work in Progress, Internet-Draft, draft-ietf-
mls-extensions-04, 24 April 2024,
mls-extensions-06, 19 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
extensions-04>.

[I-D.ietf-mls-federation]
extensions-06>.

[FEDERATION]
Omara, E. and R. Robert, "The Messaging Layer Security
(MLS) Federation", Work in Progress, Internet-Draft,
draft-ietf-mls-federation-03, 9 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
federation-03>.

[I-D.schinazi-masque-proxy]
Schinazi, D., "The MASQUE Proxy", Work in Progress,
February 2024, <https://datatracker.ietf.org/doc/html/
draft-schinazi-masque-proxy-02>.

[KT] McMillion, B., "Key Transparency Architecture", Work in
Progress, Internet-Draft, draft-ietf-keytrans-
architecture-01, 4 March
architecture-02, 26 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-
keytrans-architecture-01>.
keytrans-architecture-02>.

[Loopix] Piotrowska, A. M., Hayes, J., Elahi, T., Meiser, S., and
G. Danezis, "The Loopix Anonymity System", Proceedings of
the 26th USENIX Security Symposium, August 2017.

[MASQUE-PROXY]
Schinazi, D., "The MASQUE Proxy", Work in Progress,
February 2025, <https://datatracker.ietf.org/doc/html/
draft-schinazi-masque-proxy-05>.

[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/rfc/rfc3552>.
<https://www.rfc-editor.org/info/rfc3552>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
<https://www.rfc-editor.org/info/rfc5280>.

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

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
<https://www.rfc-editor.org/info/rfc8446>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
<https://www.rfc-editor.org/info/rfc9000>.

[WPBB22] Wallez, T., Protzenko, J., Beurdouche, B., and K.
Bhargavan, "TreeSync: Authenticated Group Management for
Messaging Layer Security", Cryptology ePrint Archive,
2022, <https://eprint.iacr.org/2022/1732.pdf>.

Contributors

Richard Barnes
Cisco
Email: rlb@ipv.sx

Katriel Cohn-Gordon
Meta Platforms
Email: me@katriel.co.uk

Cas Cremers
CISPA Helmholtz Center for Information Security
Email: cremers@cispa.de

Britta Hale
Naval Postgraduate School
Email: britta.hale@nps.edu

Albert Kwon
Badge Inc.
Email: kwonalbert@badgeinc.com

Konrad Kohbrok
Phoenix R&D
Email: konrad.kohbrok@datashrine.de

Rohan Mahy
Wire
Email: rohan.mahy@wire.com

Brendan McMillion
Email: brendanmcmillion@gmail.com

Thyla van der Merwe
Email: tjvdmerwe@gmail.com

Jon Millican
Meta Platforms
Email: jmillican@meta.com

Raphael Robert
Phoenix R&D
Email: ietf@raphaelrobert.com

Authors' Addresses

Benjamin Beurdouche
Inria & Mozilla
Email: ietf@beurdouche.com

Eric Rescorla
Windy Hill Systems, LLC
Email: ekr@rtfm.com

Emad Omara
Email: emad.omara@gmail.com

Srinivas Inguva
Email: singuva@yahoo.com

Alan Duric
Wire
Email: alan@wire.com