The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-08
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| Authors | Benjamin Beurdouche , Eric Rescorla , Emad Omara , Srinivas Inguva , Albert Kwon , Alan Duric | ||
| Last updated | 2022-07-18 (Latest revision 2022-06-16) | ||
| Replaces | draft-omara-mls-architecture | ||
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draft-ietf-mls-architecture-08
Network Working Group B. Beurdouche
Internet-Draft Inria & Mozilla
Intended status: Informational E. Rescorla
Expires: 18 December 2022 Mozilla
E. Omara
Google
S. Inguva
Twitter
A. Kwon
MIT
A. Duric
Wire
16 June 2022
The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-08
Abstract
The Messaging Layer Security (MLS) protocol [I-D.ietf-mls-protocol]
specification has the role of defining a Group Key Agreement
protocol, including all the cryptographic operations and
serialization/deserialization functions necessary for scalable and
secure group messaging. The MLS protocol is meant to protect against
eavesdropping, tampering, message forgery, and provide further
properties such as Forward Secrecy (FS) and Post-Compromise Security
(PCS) in the case of past or future device compromises.
This document describes a general secure group messaging
infrastructure and its security goals. It provides guidance on
building a group messaging system and discusses security and privacy
tradeoffs 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 the MLS Protocol document and left to
the application or the infrastructure architects to design.
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 case of active adversaries
that are able to compromise clients, the delivery service, or the
authentication service.
Discussion Venues
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This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the MLS Working Group
mailing list (mls@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/mls/.
Source for this draft and an issue tracker can be found at
https://github.com/mlswg/mls-architecture.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 18 December 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Setting . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Group Members and Clients . . . . . . . . . . . . . . . . 7
3. Authentication Service . . . . . . . . . . . . . . . . . . . 8
4. Delivery Service . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Key Storage . . . . . . . . . . . . . . . . . . . . . . . 10
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4.2. Key Retrieval . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Delivery of Messages . . . . . . . . . . . . . . . . . . 11
4.4. Membership knowledge . . . . . . . . . . . . . . . . . . 12
4.5. Membership and offline members . . . . . . . . . . . . . 13
5. Functional Requirements . . . . . . . . . . . . . . . . . . . 13
5.1. Membership Changes . . . . . . . . . . . . . . . . . . . 13
5.2. Parallel Groups . . . . . . . . . . . . . . . . . . . . . 14
5.3. Asynchronous Usage . . . . . . . . . . . . . . . . . . . 15
5.4. Access Control . . . . . . . . . . . . . . . . . . . . . 15
5.5. Recovery After State Loss . . . . . . . . . . . . . . . . 16
5.6. Support for Multiple Devices . . . . . . . . . . . . . . 16
5.7. Extensibility . . . . . . . . . . . . . . . . . . . . . . 17
5.8. Application Data Framing and Negotiation . . . . . . . . 17
5.9. Federation . . . . . . . . . . . . . . . . . . . . . . . 17
5.10. Compatibility with Future Versions of MLS . . . . . . . . 18
6. Operational Requirements . . . . . . . . . . . . . . . . . . 18
7. Security and Privacy Considerations . . . . . . . . . . . . . 21
7.1. Assumptions on Transport Security Links . . . . . . . . . 22
7.1.1. Metadata Protection for Unencrypted Group
Operations . . . . . . . . . . . . . . . . . . . . . 22
7.1.2. DoS protection . . . . . . . . . . . . . . . . . . . 23
7.1.3. Message Suppression and Error Correction . . . . . . 24
7.2. Intended Security Guarantees . . . . . . . . . . . . . . 24
7.2.1. Message Secrecy and Authentication . . . . . . . . . 25
7.2.2. Forward and Post-Compromise Security . . . . . . . . 25
7.2.3. Non-Repudiation vs Deniability . . . . . . . . . . . 26
7.3. Endpoint Compromise . . . . . . . . . . . . . . . . . . . 26
7.3.1. Compromise of AEAD key material . . . . . . . . . . . 27
7.3.2. Compromise of the Group Secrets of a single group for
one or more group epochs . . . . . . . . . . . . . . 28
7.3.3. Compromise by an active adversary with the ability to
sign messages . . . . . . . . . . . . . . . . . . . . 29
7.3.4. Compromise of the authentication with access to a
signature key . . . . . . . . . . . . . . . . . . . . 29
7.3.5. Security consideration in the context of a full state
compromise . . . . . . . . . . . . . . . . . . . . . 30
7.4. Service Node Compromise . . . . . . . . . . . . . . . . . 31
7.4.1. General considerations . . . . . . . . . . . . . . . 31
7.4.2. Delivery Service Compromise . . . . . . . . . . . . . 32
7.4.3. Authentication Service Compromise . . . . . . . . . . 34
7.5. Considerations for attacks outside of the threat model . 36
7.6. Cryptographic Analysis of the MLS Protocol . . . . . . . 37
8. Informative References . . . . . . . . . . . . . . . . . . . 37
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2. Informative References . . . . . . . . . . . . . . . . . 38
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 39
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
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 a requirement for instant messaging systems
and is commonly deployed in many such systems. In this context,
"end-to-end" captures the notion that users of the system enjoy some
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] for providing end-
to-end security in this setting. MLS is not intended as a full
instant messaging protocol but rather is intended to be embedded in
concrete protocols, such as 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 infrastructures. The MLS protocol has been designed
to provide the same security guarantees to all users, for all group
sizes, even when it reduces to only two users.
2. General Setting
Informally, a group is a set of users who possibly use multiple
endpoint devices to interact with the Service Provider (SP). A group
may be as small as two members (the simple case of person to person
messaging) or as large as thousands.
In order to communicate securely, users initially interact with
services at their disposal to establish the necessary values and
credentials required for encryption and authentication.
The Service Provider presents two abstract functionalities that allow
clients to prepare for sending and receiving messages securely:
* An Authentication Service (AS) functionality which is responsible
for attesting to bindings between application-meaningful
identifiers and the public key material used for authentication in
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the MLS protocol. This functionality must also be able to
generate credentials that encode these bindings and validate
credentials provided by MLS clients.
* A Delivery Service (DS) functionality 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 convenience, this document adopts the representation of these
services being standalone servers, however the MLS protocol design is
made so that this is not necessarily the case. 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,
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 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
servers.
It is important to note that the Authentication Service functionality
can be completely abstract in the case of a Service Provider which
allows MLS clients to generate, redistribute and validate their
credentials themselves.
Similarly to 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. Note, though, that the MLS
protocol requires group operation messages to be processed in-order
by all MLS clients.
In some sense, a set of MLS clients which can achieve the AS and DS
functionalities without relying on an external party do not need a
Service Provider.
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+----------------+ +--------------+
| Authentication | | Delivery |
| Service (AS) | | Service (DS) |
+----------------+ +-------+------+
/ | \ Group
/ ........|........\................
/ . | \ .
+--------+-+ . +----+-----+ +----------+ .
| Client 1 | . | Client 2 | | Client 3 | .
+----------+ . +----------+ +----------+ .
. Member 1 Member 2 .
. .
..................................
In many systems, the AS and the DS are actually operated by the same
entity and may even be the same server. However, they are logically
distinct and, in other systems, may be operated by different
entities. Other partitions are also possible, such as having a
separate directory functionality or service.
According to this architecture design, a typical group messaging
scenario might look like this:
1. Alice, Bob and Charlie create accounts with a service provider
and obtain credentials from the AS.
2. Alice, Bob and Charlie authenticate to the DS and store some
initial keying material which can be used to send encrypted
messages to them for the first time. This keying material is
authenticated with their long-term credentials.
3. When Alice wants to send a message to Bob and Charlie, she
contacts the DS and looks up their initial keying material. She
uses these keys to establish a new set of keys which she can use
to send encrypted messages to Bob and Charlie. She then sends
the encrypted message(s) to the DS, which forwards them to the
recipients.
4. Bob and/or Charlie respond to Alice's message. In addition, they
might choose to update their key material which provides post-
compromise security Section 7.2.2. As a consequence of that
change, the group secrets are updated.
Clients may wish to do the following:
* create a group by inviting a set of other clients;
* add one or more clients to an existing group;
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* remove one or more members from an existing group;
* update their own key material
* join an existing group;
* leave a group;
* send a message to everyone in the group;
* receive a message from someone in the group.
At the cryptographic level, clients (and by extension members in
groups) have equal permissions. For instance, any member can add or
remove another client in a group. This is in contrast to some
designs in which there is a single group controller who can modify
the group. MLS is compatible with having group administration
restricted to certain users, but we assume that those restrictions
are enforced by authentication and access control at the application
layer.
Thus, for instance, while the MLS protocol allows for any existing
member of a group to add a new client, applications which use MLS
might enforce additional restrictions for which only a subset of
members can qualify, and thus will handle enforcing group policies
(such as determining if a user is allowed to add new users to the
group) at the application level.
2.1. Group Members and Clients
While informally, a group can be considered to be a set of users
possibly using multiple endpoint devices to interact with the Service
Provider, this definition is too simplistic.
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. Ownership of a client by a user is
determined by the fact that the user has knowledge of the associated
secret values. When a client is part of a Group, it is called a
Member. In some messaging systems, clients belonging to the same
user must all share the same signature key pair, but MLS does not
assume this.
Users will often use multiple devices, e.g., a phone as well as a
laptop. Different devices may be represented as different clients,
with independent cryptographic state, or they may share cryptographic
state, relying on some application-provided mechanism to sync across
devices.
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The formal definition of a Group in MLS is the set of clients that
have knowledge of the shared group secret established in the group
key establishment phase of the protocol and have contributed to it.
Until a Member 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 Member is a member of the group.
3. Authentication Service
The Authentication Service (AS) has to provide two functionalities:
1. Issue credentials to clients that attest to bindings between
identities and signature key pairs
2. Enable a group member to verify that a credential presented by
another member is valid
A member with a valid credential authenticates its MLS messages by
signing them with the private key corresponding to the public key in
its credential.
The AS is considered an abstract layer by the MLS 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 others' key fingerprints for authentication. In this
scenario, the issuance function is simply the generation of a key
pair (i.e., credential is just an identifier and public key, with
no information to assist in verification). The verification
function is the application functionality that enables users to
verify keys.
* In a system based on Key Transparency (KT) [KeyTransparency], 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 to their identity.
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By the nature of its roles in MLS authentication, the AS is invested
with a large amount of trust and the compromise of one of its
functionalities could allow an adversary to, among other things,
impersonate group members. We discuss security considerations
regarding the compromise of the different AS functionalities in
detail in Section 7.4.3.
The association between members' identities and signature keys is
fairly flexible in MLS. As noted above, there is no requirement that
all clients belonging to a given user use the same key pair (in fact,
such key reuse is forbidden to ensure clients have independent
cryptographic state). 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, for example a human user with a mobile and desktop
version of an application. 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.
4. Delivery Service
The Delivery Service (DS) is expected to play multiple roles in the
Service Provider architecture:
* Acting 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
the other client is offline.
* Routing MLS messages among clients.
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Depending on the level of trust given by the group to the Delivery
Service, the functional and privacy guarantees provided by MLS may
differ but the authentication and confidentiality guarantees remain
the same.
Unlike the Authentication Service which is trusted for authentication
and secrecy, the Delivery Service is completely untrusted regarding
this property. While privacy of group membership might be a problem
in the case of a Delivery Service server fanout, the Delivery Service
can be considered as an active, adaptive network attacker from the
point of view of the security analysis.
4.1. Key Storage
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 material.
All the parameters in the KeyPackage are signed with the signature
private key corresponding to the credential.
As noted above, users may own multiple clients, each with their own
keying material, and thus there may be multiple entries stored by
each user.
The Delivery Service is also responsible for allowing users to add,
remove or update their initial key material, and for ensuring that
the identifier for these keys are unique across all keys stored on
the Delivery Service.
4.2. Key Retrieval
When a client wishes to establish a group, it first contacts the
Delivery Service to request a KeyPackage for each other client,
authenticates the KeyPackages using the signature keys, and then can
use those to form the group.
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4.3. Delivery of Messages
The main responsibility of the Delivery Service is to ensure delivery
of messages. Some MLS messages need only be delivered to some
members of a group (e.g., the message initializing a new member's
state), while others need to be delivered to all members. The
Delivery Service may enable these delivery patterns 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. The one requirement is
that because an MLS group has a linear history, the members of the
group must agree on the order in which changes are applied.
Concretely, the group must agree on which MLS Commit messages to
apply. There are a variety of ways to achieve this agreement, but
most of them rely on some help from the Delivery Service. For
example, if a Delivery Service provides delivery in the same order to
all group members, then the members can simply apply Commits in the
order in which they appear.
Each Commit is premised on a given state or "epoch" of the group.
The Delivery Service must transmit to the group exactly one Commit
message per epoch.
Much like the Authentication Service, the Delivery Service can be
split between server and client components. Achieving the required
uniqueness property will typically require a combination of client
and server behaviors. For example, all of the following examples
provide a unique Commit per epoch:
* A "filtering server" Delivery Service where a server rejects all
but the first Commit for an epoch and clients apply each Commit
they receive.
* An "ordering server" Delivery Service where a server forwards all
messages but assures that all clients see Commits in the same
order, and clients.
* A "passive server" Delivery Service where a server forwards all
messages without ordering or reliability guarantees, and clients
execute some secondary consensus protocol to choose among the
Commits received in a window.
The MLS protocol provides three important pieces of information
within an MLSCiphertext message in order to provide ordering:
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* The Group Identifier (group ID) to allow for distinguishing the
group for which the message has been sent;
* The Epoch number, which represents the number of changes (version)
of the group associated with a specific group ID, and allows for
lexicographical ordering of messages from different epochs within
the same group;
* The Content Type of the message, which allows the Delivery Service
to determine the ordering requirement on the message, in
particular distinguishing Commit messages from other messages.
The MLS protocol itself can verify these properties. For instance,
if the Delivery Service reorders messages from a client or provides
different clients with inconsistent orderings, then clients can put
messages back in their proper order. The asynchronous nature of MLS
means that within an epoch, messages are only ordered per-sender, not
globally.
Note that some forms of Delivery Service misbehavior are still
possible and difficult 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
distinguish this form of Denial of Service (DoS) attack.
4.4. Membership knowledge
Group membership is itself sensitive information and MLS is designed
to limit the amount of persistent metadata. However, large groups
often require an infrastructure which provides server fanout. In the
case of client fanout, the destination of a message is known by all
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. In
the case where metadata has to be persisted for functionality, it
SHOULD be stored encrypted at rest. Applications should also
consider anonymous systems for server fanout such as Loopix [Loopix].
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4.5. Membership and offline members
Because Forward Secrecy (FS) and Post-Compromise Security (PCS) rely
on the active deletion and replacement of keying material, any client
which is persistently offline may still be holding old keying
material and thus be a threat to both FS and PCS if it is later
compromised.
MLS cannot inherently defend against this problem, especially in the
case where the client has not processed messages, but MLS-using
systems can enforce some mechanism to try to retain these properties.
Typically this will consist of evicting clients which are idle for
too long, or mandating a key update from clients that are not
otherwise sending messages. The precise details of such mechanisms
are a matter of local policy and beyond the scope of this document.
5. Functional Requirements
MLS is designed as a large-scale group messaging protocol and hence
aims to provide both performance and safety to its users. Messaging
systems that implement MLS provide support for conversations
involving two or more members, and aim to scale to groups with tens
of thousands of members, typically including many users using
multiple devices.
5.1. 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 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 be
represented in a group by multiple clients. 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 consistently.
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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. External joins do
not allow for more granular authorization checks to be done before
the new member is added to the group, so if an application wishes to
both allow external joins and enforce such checks, then the
application will need to do such checks when a member joins, and
remove them if checks fail.
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 sign off on device signature keys, then revocation
by the owner adds an alternative flag 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.
5.2. 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 not provide PCS healing in other groups; each group
must be updated separately to achieve internal goals. This also
applies to future groups that a member has yet to join, that are
likewise unaffected by updates performed in current groups.
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Applications may strengthen connectivity among parallel groups by
requiring periodic key updates from a user across all groups in which
they have membership.
Applications may use the PSK mechanism 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.
5.3. 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.
5.4. Access Control
The MLS protocol allows each member of the messaging group to perform
operations equally. This is because all clients within a group
(members) have access to the shared cryptographic material. 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.
The MLS protocol can, in certain modes, exchange unencrypted group
operation messages. This flexibility is to allow services to perform
access control tasks on behalf of the group.
While the Application messages will always be encrypted, having the
handshake messages in plaintext has inconveniences in terms of
privacy as someone could collect the signatures on the handshake
messages and use them for tracking.
*RECOMMENDATION:* Prefer using encrypted group operation messages
to avoid privacy issues related to non-encrypted signatures.
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Note that in the default case of encrypted handshake messages, any
access control policies will 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 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 as above.
** RECOMMENDATION:** Have an explicit group policy setting the
conditions under which external joins are allowed.
5.5. Recovery After State Loss
Group members whose local MLS state is lost or corrupted can
reinitialize their state by re-joining 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.
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.
Reinitializing in this way does not provide the member with access to
group messages from during the state loss window, but 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.
5.6. Support for Multiple Devices
It is typically expected for users within a group to 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. Restoring history is typically not allowed at the protocol
level but applications can elect to provide such a mechanism outside
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of MLS. Such mechanisms, if used, may reduce the FS and PCS
guarantees provided by MLS.
5.7. 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.
5.8. Application Data Framing and Negotiation
Application messages carried by MLS are opaque to the protocol; they
can contain arbitrary data. Each application which uses MLS needs to
define the format of its application_data and any mechanism necessary
to negotiate the format of that content over the lifetime of an MLS
group. In many 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
[I-D.mahy-mls-content-neg], unless the specific application
defines another mechanism which 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.
5.9. 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, and infrastructure
functionalities.
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5.10. 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
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 and extensions
at all times once he 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.
6. Operational Requirements
MLS is a security layer that needs to be integrated with an
application. A 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. A
required_capabilities extension can help maintain 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. These network services
would need to be compatible in order for two different deployments
based on them to interoperate.
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* An *Authentication Service*, described fully in Section 3, defines
the types of credentials which may be used in a deployment and
provides methods for:
1. Issuing new credentials,
2. Validating a credential against a reference identifier, and
3. Validating whether or not two credentials represent the same
user.
* A *Delivery Service*, described fully in Section 4, provides
methods for:
1. Delivering messages sent to a group to all members in the
group.
2. Delivering Welcome messages to new members of a group.
3. Downloading KeyPackages for specific clients, and uploading
new KeyPackages for a user's own clients.
* Additional services may or may not be required depending on the
application design:
- If assisted joining is desired (meaning that the ratchet tree
is not provided in Welcome messages), there must be a method to
download the ratchet tree corresponding to a group.
- If assisted joining is desired and the Delivery Service is not
able to compute the ratchet tree itself (because some proposals
or commits are sent encrypted), there must be a method for
group members to publish the updated ratchet tree after each
commit.
- If external joiners are allowed, there must be a method to
publish a serialized GroupInfo object (with an external_pub
extension) that corresponds to a specific group and epoch, and
keep that object in sync with the state of the group.
- If an application chooses not to allow assisted or 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 external PSKs, or uses resumption PSKs
that all members of a group may not have access to, there must
be a method for distributing these PSKs to group members.
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- 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 secret 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
- 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.
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.
* The application_id extension of a LeafNode.
MLS requires the following policies to be defined, which restrict the
set of acceptable behavior in a group. These policies must be
consistent between deployments for them to interoperate:
* 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:
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- 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.
- When external joiners are allowed.
* A policy for when two credentials represent the same client. Note
that many credentials may be issued authenticating the same
identity but for different signature keys, because each credential
corresponds to a different device (client) owned by the same
application user. However, one device may control many signature
keys 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:
* If there's any policy on how or when to pad messages.
* If there is any policy for 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.
7. 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 the face of
such attackers.
* The attacker can monitor the entire network.
* The attacker can read unprotected messages.
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* The attacker can generate, inject and delete any message in the
unprotected transport layer.
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.
Generally, MLS is designed under the assumption that the transport
layer is present to protect metadata and privacy in general, while
the MLS protocol is providing stronger guarantees such as
confidentiality, integrity and authentication guarantees. Stronger
properties such as deniability can also be achieved in specific
architecture designs.
7.1. Assumptions on Transport Security Links
Any secure channel can be used as a transport layer to protect MLS
messages such as QUIC, TLS, WireGuard or TOR. However, the MLS
protocol is designed to consider the following threat-model:
* The attacker can read, write, and delete arbitrary messages inside
the secure transport channel.
This departs from most threat models where we consider that the
secure channel used for transport always provides secrecy. The
reason for this consideration is that in the group setting, active
malicious insiders or adversarial services are to be considered.
7.1.1. Metadata Protection for Unencrypted Group Operations
The main use of the secure transport layer for MLS is to protect the
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.
MLS avoids needing to send the full list of recipients to the server
for dispatching messages because that list is potentially extremely
large in MLS. Therefore, the metadata typically consists of a
pseudo-random Group Identifier (GID), a numerical value to determine
the epoch of the group (the number of changes that have been made to
the group), and another numerical value referring to the specific key
needed to decrypt the ciphertext content.
The MLS protocol provides an authenticated "Additional Authenticated
Data" field for applications to make data available outside the
MLSCiphertext.
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*RECOMMENDATION:* Use the "Additional Authenticated Data" field of
the MLSCiphertext message instead of using other unauthenticated
means of sending metadata throughout the infrastructure. If the
data is private, the infrastructure should use encrypted
Application messages instead.
Even though some of this metadata information does not consist of
secret payloads, 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.
More importantly, there is one specific case where having no secure
channel to exchange the MLS messages can have a serious impact on
privacy. In the case of unencrypted group operation messages,
observing the signatures of the group operation messages may lead an
adversary to extract information about the group memberships.
*RECOMMENDATION:* Never use the unencrypted mode for group
operations without using a secure channel for the transport layer.
7.1.2. DoS protection
In general we do not consider Denial of Service (DoS) resistance to
be the responsibility of the protocol. However, it should not be
possible for anyone 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:* Anonymous credentials can be used in order to
help DoS attacks prevention, in a privacy preserving manner. Note
that the privacy of these mechanisms has to be adjusted in
accordance with the privacy expected from the secure transport
links. (See more discussion further down.)
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7.1.3. 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 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 acknowledgements of application
messages received within a group.
As discussed in Section 4, the Delivery Service is trusted to select
the single Commit message that is applied in each epoch from among
the ones 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 settings, unidirectional transports can be
used and be enforced via electronic or physical devices such as
diodes. This can lead to payload 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 specific
measures can be taken to ensure the appropriate level of redundancy
and quality of service for MLS.
*RECOMMENDATION:* If unidirectional transport is used for the
secure transport channel, prefer using a protocol which provides
Forward Error Correction.
7.2. 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.
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7.2.1. 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.
7.2.2. 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
an 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 the
extremely important role of deleting appropriate keys as soon as they
have been used with the expected message, otherwise the secrecy of
the messages and the security for MLS is considerably weakened.
PCS means that if a group member's state is compromised at some time
t but the group member subsequently performs an update at some time
t', then all MLS guarantees apply to messages sent by the member
after time t', and by other members after they have processed the
update. For example, if an attacker learns all secrets known to
Alice at time t, including both Alice's long-term secret keys and all
shared group keys, but Alice performs a key update at time t', then
the attacker is unable to violate any of the MLS security properties
after the updates have been processed.
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Both of these properties are satisfied even against compromised DSs
and ASs.
7.2.3. Non-Repudiation 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 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.
7.3. Endpoint Compromise
The MLS protocol adopts a threat model which includes multiple forms
of endpoint/client compromise. While adversaries are in a very
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 we will explore the consequences and recommendations
regarding the following compromise scenarios:
* The attacker has access to a specific symmetric encryption key
* The attacker has access to the group secrets for one group
* The attacker has access to a signature oracle for any group
* The attacker has access to the signature key for one group
* The attacker has access to all secrets of a user for all groups
(full state compromise)
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Recall that the MLS protocol provides chains of AEAD keys, per sender
that are generated from Group Secrets. These keys are used to
protect MLS Plaintext messages which can be Group Operation or
Application messages. The Group Operation messages offer an
additional protection as the secret exchanged within the TreeKEM
group key agreement are public-key encrypted to subgroups with HPKE.
7.3.1. Compromise of AEAD key material
In some circumstances, adversaries may have access to specific AEAD
keys and nonces which protect an Application or a Group Operation
message. While this is a very weak kind of compromise, it can be
realistic in cases of implementation vulnerabilities where only part
of the memory leaks to the adversary.
When an AEAD key is compromised, the adversary has access to a set of
AEAD keys for the same chain and the same epoch, hence can decrypt
messages sent using keys of this chain. An adversary cannot send a
message to a group which appears to be from any valid client since
they cannot forge the signature.
The MLS protocol will ensure that an adversary cannot compute any
previous AEAD keys for the same epoch, or any other epochs. 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. However the MLS protocol does not provide Post
Compromise Secrecy for AEAD encryption within an epoch. This means
that if the AEAD key of a chain is compromised, the adversary can
compute an arbitrary number of subsequent AEAD keys for that chain.
These guarantees are ensured by the structure of the MLS key schedule
which provides Forward Secrecy for these AEAD encryptions, across the
messages within the epoch and also across previous epochs. Those
chains are completely disjoint and compromising keys across the
chains would mean that some Group Secrets have been compromised,
which is not the case in this attack scenario (we explore stronger
compromise scenarios as part of the following sections).
MLS provides Post-Compromise Secrecy 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.
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
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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.
Note that under that compromise scenario, authentication is not
affected in neither 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 very weak. Successful decryption of an AEAD
encrypted message only guarantees that a member of the group sent the
message.
7.3.2. Compromise of the Group Secrets of a single group for one or
more group epochs
The attack scenario considering an adversary gaining access to a set
of Group secrets is significantly stronger. This can typically be
the case when a member of the group is compromised. For this
scenario, we consider that the signature keys are not compromised.
This can be the case for instance if the adversary 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 AEAD encryption keys for any AEAD chains and 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, as soon as an honest Commit message is sent
by the compromised party, the next epochs will provide message
secrecy.
If the adversary is active, the adversary can follow the protocol and
perform updates on behalf of the compromised party with no ability to
an honest group to recover message secrecy. However, MLS provides
PCS against active adaptive attackers through its Remove group
operation. This means 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.
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7.3.3. Compromise by an active adversary with the ability to sign
messages
Under such a scenario, where an active adversary has compromised an
MLS client, two different settings emerge. In the strongest
compromise scenario, the attacker has access to the signing key and
can forge authenticated messages. In a weaker, yet realistic
scenario, the attacker has compromised a client but the client
signature keys are protected with dedicated hardware features which
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 both secrecy and authentication are broken.
The attacker can generate any message, for the current and future
epochs until an honest update from the compromised client happens.
Note that under this compromise scenario, the attacker can perform
all operations which are available to an legitimate client even
without access to the actual value of the signature key.
Without access to the group secrets, the adversary will not have the
ability to generate messages which look valid to other members of the
group and to the infrastructure as they need to have access to group
secrets to compute the encryption keys or the membership tag.
7.3.4. Compromise of the authentication with access to a 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, the attacker can
perform every operation available to a legitimate client in both
cases.
There is a significant difference, however in terms of recovery after
a compromise.
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Because of the PCS guarantees provided by the MLS protocol, when a
previously compromised client performs an honest Commit which is not
under the control of the adversary, both secrecy and authentication
of messages can be recovered in the case where the attacker didn't
get access to the key. Because the adversary doesn't have the key
and has lost the ability to sign messages, 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 PCS of the MLS protocol will eventually allow recovery
of the authentication of messages for future epochs but only after
compromised parties refresh their credentials securely.
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 depending on the architecture of the service provider.
*RECOMMENDATION:* Signature private keys should be
compartmentalized from other secrets and preferably protected by
an HSM or dedicated hardware features to allow recovery of the
authentication for future messages after a compromise.
7.3.5. Security consideration in the context of a 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 can access the messages on the device without even
needing to attack MLS, the application should delete plaintext
messages and ciphertexts immediately after encryption or
decryption.
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Even though, from the strict point of view of the security
formalization, a ciphertext is always public and will forever be,
there is no loss in trying to erase ciphertexts as much as possible.
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 extremely 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 are providing the strong authentication
guarantees to other clients, hence we consider that their protection
by additional security mechanism should be a priority.
Overall there is no way to detect or prevent these compromise, as
discussed in the previous sections, performing separation of the
application secret states can help recovery after compromise, this is
the case for signature keys but similar concern exists for the
encryption private key used in the TreeKEM Group Key Agreement.
*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, or 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, hence be
outside of the capabilities of the attacker.
7.4. Service Node Compromise
7.4.1. General considerations
7.4.1.1. Privacy of the 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 when:
* The application connects to the Authentication Service to generate
or validate a new credential before distributing it.
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* 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 leaks information about the origin of
the request such as the IP address and depending on the protocol the
MAC address of the device.
Similar concerns exist in the peer-to-peer use cases of MLS.
*RECOMMENDATION:* In the case where privacy or anonymity is
important, using adequate protection such as TOR or a VPN can
improve metadata protection.
More generally, using anonymous credentials in an MLS based
architecture might not be enough to provide strong privacy or
anonymity properties.
7.4.2. Delivery Service Compromise
MLS is intended to provide strong guarantees in the face of
compromise of the DS. Even a totally compromised DS should not be
able to read messages or inject messages that will be acceptable to
legitimate clients. It should also not be able to undetectably
remove, reorder or replay messages.
However, a DS can mount a variety of DoS attacks on the system,
including total DoS attacks (where it simply refuses to forward any
messages) and partial DoS attacks (where it refuses to forward
messages to and from specific clients). As noted in Section 4.3,
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 does allow
it to attack forward security to a limited extent. This threat can
be mitigated by having initial keys expire.
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7.4.2.1. Privacy of delivery and push notifications
An important mechanism that is often ignored from the privacy
considerations are the push-tokens. 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 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 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.
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 notification are often sent real-
time as it is not acceptable to create artificial delays for message
retrieval.
*RECOMMENDATION:* If real time notifications are not necessary and
that specific steps must be taken to improve privacy, one can
delay notifications randomly across recipient devices using a
mixnet or other techniques.
Note that it is quite easy for legal requests to ask the service
provider for the push-token associated to an identifier and perform 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 or other
information.
*RECOMMENDATION:* If stronger privacy guarantees are needed vis-
a-vis the push notification provider, the client can choose to
periodically connect to the Delivery Service without the need of a
dedicated push notification infrastructure.
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7.4.3. 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
* The attacker can generate new credentials
* The attacker can sign new credentials
* The attacker can publish or distribute credentials
Infrastructures that provide cryptographic material or credentials in
place of the MLS client (which is under the control of the user) have
often the ability to use the associated secrets to perform operations
on behalf of the user, which is unacceptable in many situations.
Other mechanisms can be used to prevent this issue, such as the
service blessing cryptographic material used by an MLS client.
*RECOMMENDATION:* Make clients submit signature public keys to the
AS, this is usually better than the AS generating public key pairs
because the AS cannot sign on behalf of the client. This is a
benefit of a Public Key Infrastructure in the style of the
Internet PKI.
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:* Using HSMs to store the root signature keys to
limit the ability of an adversary with no physical access to
extract the top-level signature key.
7.4.3.1. Authentication compromise: Ghost users and impersonations
One thing for which the MLS Protocol is designed for is to make sure
that all clients know who is in the group at all times. This means
that - if all Members of the group and the Authentication Service are
honest - no other parties than the members of the current group can
read and write messages protected by the protocol for that Group.
Beware though, the link between the cryptographic identity of the
Client and the real identity of the User is important. With some
Authentication Service designs, a private or centralized authority
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can be trusted to generate or validate signature keypairs used in the
MLS protocol. This is typically the case in some of the biggest
messaging infrastructures.
While this service is often very well protected from external
attackers, it might be the case that this service is compromised. In
such infrastructure, the AS could generate or validate a signature
keypair for an identity which is not the expected one. Because a
user can have many MLS clients running the MLS protocol, it possibly
has many signature keypairs for multiple devices.
In the case where an adversarial keypair is generated for a specific
identity, an infrastructure without any transparency mechanism or
out-of-band authentication mechanism could inject a malicious client
into a group by impersonating a user. This is especially the case in
large groups where the UI might not reflect all the changes back to
the users.
*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, a
public log should be maintained in the state of the device so that
the user can examine it.
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,
authentication secret which can be exchanged between users to prove
their identity to each other. This can be done for instance using a
QR code that can be scanned by the other parties.
Another way to improve the security for the users is to provide a
transparency mechanism which allows each user to check if credentials
used in groups have been published in the transparency log. Another
benefit of this mechanism is for revocation. The users of a group
could check for revoked keys (in case of compromise detection) using
a mechanism such as CRLite or some more advanced privacy preserving
technology.
*RECOMMENDATION:* Provide a Key Transparency and Out-of-Band
authentication mechanisms to limit the impact of an Authentication
Service compromise.
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We note, again, that as described prior to that section, the
Authentication Service is facultative to design a working
infrastructure and can be replaced by many mechanisms such as
establishing prior one-to-one deniable channels, gossiping, or using
TOFU for credentials used by the MLS Protocol.
Another important consideration is the ease of redistributing new
keys on client compromise, which helps recovering security faster in
various cases.
7.4.3.2. Privacy of the Group Membership
Often, expectation from users is that the infrastructure will not
retain the ability to constantly map the user identity to signature
public keys of the MLS protocol. Some infrastructures will keep a
mapping between signature public keys of clients and user identities.
This can benefit an adversary that has compromised the AS (or
required access according to regulation) the ability of monitoring
unencrypted traffic and correlating the messages exchanged within the
same group.
*RECOMMENDATION:* Always use encrypted group operation messages to
reduce issues related to privacy.
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:* Do not use the same signature keypair across
groups.
*RECOMMENDATION:* Separate the service binding the identities and
the public keys from the service which generates or validates the
credentials or cryptographic material of the Clients.
7.5. Considerations for attacks outside of the 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 in a software adversary, which
can maintain active adaptive compromise and arbitrarily change the
behavior of the client or service.
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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.
7.6. 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 7), [ACDT21] (draft 11)
and [AJM20] (draft 12).
Individual components of various drafts of the MLS protocol have been
analyzed in isolation and with differing adversarial models, for
example, [BBR18], [ACDT19], [ACCKKMPPWY19], [AJM20] and [ACJM20]
analyze the ratcheting tree as the sub-protocol of MLS that
facilitates key agreement, while [BCK21] analyzes the key derivation
paths in the ratchet tree and key schedule. Finally, [CHK19]
analyzes the authentication and cross-group healing guarantees
provided by MLS.
8. Informative References
* ACDT19: https://eprint.iacr.org/2019/1189
* ACCKKMPPWY19: https://eprint.iacr.org/2019/1489
* ACJM20: https://eprint.iacr.org/2020/752
* AJM20: https://eprint.iacr.org/2020/1327
* ACDT21: https://eprint.iacr.org/2021/1083
* AHKM21: https://eprint.iacr.org/2021/1456
* CHK19: https://eprint.iacr.org/2021/137
* BCK21: https://eprint.iacr.org/2021/137
* BBR18: https://hal.inria.fr/hal-02425247
* BBN19: https://hal.laas.fr/INRIA/hal-02425229
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9. IANA Considerations
This document makes no requests of IANA.
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-14, 3 May 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
protocol-14>.
10.2. Informative References
[I-D.mahy-mls-content-neg]
Mahy, R., "Content Negotiation for Message Layer Security
(MLS)", Work in Progress, Internet-Draft, draft-mahy-mls-
content-neg-00, 31 March 2022,
<https://datatracker.ietf.org/doc/html/draft-mahy-mls-
content-neg-00>.
[KeyTransparency]
Google, "Key Transparency", 2017,
<https://KeyTransparency.org>.
[Loopix] Piotrowska, A. M., Hayes, J., Elahi, T., Meiser, S., and
G. Danezis, "The Loopix Anonymity System", 2017.
[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>.
[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>.
[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>.
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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
Konrad Kohbrok
Email: konrad.kohbrok@datashrine.de
Brendan McMillion
Email: brendanmcmillion@gmail.com
Thyla van der Merwe
Email: tjvdmerwe@gmail.com
Jon Millican
Meta Platforms
Email: jmillican@fb.com
Raphael Robert
Email: ietf@raphaelrobert.com
Authors' Addresses
Benjamin Beurdouche
Inria & Mozilla
Email: ietf@beurdouche.com
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Eric Rescorla
Mozilla
Email: ekr@rtfm.com
Emad Omara
Google
Email: emadomara@google.com
Srinivas Inguva
Twitter
Email: singuva@twitter.com
Albert Kwon
MIT
Email: kwonal@mit.edu
Alan Duric
Wire
Email: alan@wire.com
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