The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-06
Network Working Group E. Omara
Internet-Draft Google
Intended status: Informational B. Beurdouche
Expires: 9 September 2021 Inria & Mozilla
E. Rescorla
Mozilla
S. Inguva
Twitter
A. Kwon
MIT
A. Duric
Wire
8 March 2021
The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-06
Abstract
The Messaging Layer Security (MLS) protocol [MLSPROTO] document has
the role of defining a Group Key Agreement, all the necessary
cryptographic operations, and serialization/deserialization functions
necessary to create a scalable and secure group messaging protocol.
The MLS protocol is meant to protect against eavesdropping,
tampering, message forgery, and provide good properties such as
forward-secrecy (FS) and post-compromise security (PCS) in the case
of past or future device compromises.
This document, on the other hand is intended to describe 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 mechanism
that are part of the MLS protocol (ie. frequency of public encryption
key rotation).
The document also extends the guidance to 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, most will
vastly influence the overall security guarantees that are achieved by
the overall messaging system. This is especially true in case of
active adversaries that are able to compromise clients, the delivery
service or the authentication service.
Discussion Venues
Omara, et al. Expires 9 September 2021 [Page 1]
Internet-Draft MLS Architecture March 2021
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 9 September 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Setting . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Group, Members and Clients . . . . . . . . . . . . . . . 7
2.2. Authentication Service . . . . . . . . . . . . . . . . . 8
2.3. Delivery Service . . . . . . . . . . . . . . . . . . . . 9
2.3.1. Key Storage . . . . . . . . . . . . . . . . . . . . . 10
Omara, et al. Expires 9 September 2021 [Page 2]
Internet-Draft MLS Architecture March 2021
2.3.2. Key Retrieval . . . . . . . . . . . . . . . . . . . . 10
2.3.3. Delivery of messages and attachments . . . . . . . . 10
2.3.4. Membership knowledge . . . . . . . . . . . . . . . . 11
2.3.5. Membership and offline members . . . . . . . . . . . 12
2.4. Functional Requirements . . . . . . . . . . . . . . . . . 12
2.4.1. Message Secrecy and Authentication . . . . . . . . . 12
2.4.2. Forward and Post-Compromise Security . . . . . . . . 13
2.4.3. Membership Changes . . . . . . . . . . . . . . . . . 13
2.4.4. Parallel Groups . . . . . . . . . . . . . . . . . . . 14
2.4.5. Security of Attachments . . . . . . . . . . . . . . . 14
2.4.6. Non-Repudiation vs Deniability . . . . . . . . . . . 14
2.4.7. Asynchronous Usage . . . . . . . . . . . . . . . . . 15
2.4.8. Access Control . . . . . . . . . . . . . . . . . . . 15
2.4.9. Recovery After State Loss . . . . . . . . . . . . . . 16
2.4.10. Support for Multiple Devices . . . . . . . . . . . . 16
2.4.11. Extensibility / Pluggability . . . . . . . . . . . . 16
2.4.12. Federation . . . . . . . . . . . . . . . . . . . . . 16
2.4.13. Compatibility with future versions of MLS . . . . . . 17
3. Security and Privacy Considerations . . . . . . . . . . . . . 17
3.1. Considerations for attacks outside of the threat model . 18
3.2. Transport Security Links . . . . . . . . . . . . . . . . 18
3.2.1. Metadata protection for unencrypted group
operations . . . . . . . . . . . . . . . . . . . . . 18
3.2.2. DoS protection . . . . . . . . . . . . . . . . . . . 19
3.2.3. Message suppression and error correction . . . . . . 20
3.3. Delivery Service Compromise . . . . . . . . . . . . . . . 20
3.3.1. Privacy of delivery and push notifications . . . . . 21
3.4. Authentication Service Compromise . . . . . . . . . . . . 22
3.4.1. Authentication compromise: Ghost users and
impersonations . . . . . . . . . . . . . . . . . . . 23
3.4.2. Privacy of the Group Membership . . . . . . . . . . . 24
3.5. Shared considerations regarding adversarial AS or DS
services . . . . . . . . . . . . . . . . . . . . . . . . 24
3.5.1. Privacy of the network connections . . . . . . . . . 25
3.6. Client Compromise . . . . . . . . . . . . . . . . . . . . 25
3.6.1. Compromise of AEAD key material . . . . . . . . . . . 26
3.6.2. Compromise of the Group Secrets of a single group for
one or more group epochs . . . . . . . . . . . . . . 27
3.6.3. Compromise by an active adversary with the ability to
sign messages . . . . . . . . . . . . . . . . . . . . 28
3.6.4. Compromise of the authentication with access to a
signature key . . . . . . . . . . . . . . . . . . . . 28
3.6.5. Security consideration in the context of a full state
compromise . . . . . . . . . . . . . . . . . . . . . 29
3.6.6. More attack scenarios . . . . . . . . . . . . . . . . 31
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 31
6. Informative References . . . . . . . . . . . . . . . . . . . 31
Omara, et al. Expires 9 September 2021 [Page 3]
Internet-Draft MLS Architecture March 2021
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
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.
DISCLAIMER: A lot of work is still ongoing on the current version of
this draft. Especially, this preliminary writing of the security
considerations has not been reviewed by the working group yet and
might contain errors. Please file an issue on the document's GitHub
if you find errors.
[[TODO: Remove disclaimer.]]
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 when the service provider they are using performs
unsatisfactorily.
Messaging Layer Security (MLS) specifies an architecture (this
document) and an abstract protocol [MLSPROTO] 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]. In addition, it does not specify
a complete wire encoding, but rather a set of abstract data
structures which can then be mapped onto a variety of concrete
encodings, such as TLS [RFC8446], CBOR [RFC7049], and JSON [RFC7159].
Implementations which adopt compatible encodings will have some
degree of interoperability at the message level, though they may have
incompatible 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.
Omara, et al. Expires 9 September 2021 [Page 4]
Internet-Draft MLS Architecture March 2021
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 maintaining a binding between a unique identifier (identity)
and the public key material (credential) used for authentication
in the MLS protocol. This functionality must also be able to
generate these credentials or validate them if they are provided
by MLS clients.
* A Delivery Service (DS) functionality which can receive and
redistributing 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 it is not necessarily the case.
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.
Omara, et al. Expires 9 September 2021 [Page 5]
Internet-Draft MLS Architecture March 2021
---------------- --------------
| Authentication | | Delivery |
| Service (AS) | | Service (DS) |
---------------- --------------
/ | \ Group
/ ************************************
/ * | \ *
---------- * ---------- ---------- *
| Client 0 | * | Member 1 | | Member N | *
---------- * ---------- ---------- *
............ * ............ ............ *
User 0 * User 0 User 1 *
* *
************************************
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 2.4.2. 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;
Omara, et al. Expires 9 September 2021 [Page 6]
Internet-Draft MLS Architecture March 2021
* add one or more clients to an existing group;
* 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 by
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 and its signature key pair uniquely defines its identity to
other clients or members in the Group. In some messaging systems,
clients belonging to the same user must all share the same identity
key pair, but MLS does not assume this.
Omara, et al. Expires 9 September 2021 [Page 7]
Internet-Draft MLS Architecture March 2021
Users will typically own multiple Clients, potentially one or more
per end-user devices (phones, web clients or other devices...) and
may choose to authenticate using the same signature key across
devices, using one signature key per device or even one signature key
per group.
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 contributed to the group secret, other members
cannot assume they are a member of the group.
2.2. Authentication Service
The basic function of the Authentication Service (AS) is to provide a
trusted mapping from user identities (usernames, phone numbers,
etc.), to long-term identity keys, which may either be one per Client
or may be shared amongst the clients attached to a user.
The Authentication Service (AS) is expected to play multiple roles in
the architecture:
* A certification authority or similar service which signs some sort
of portable credential binding an identity to a signature key.
* A directory server which provides the key for a given identity
(presumably this connection is secured via some form of transport
security such as TLS).
The MLS protocol assumes a signature keypair for authentication of
messages. It is important to note that this signature keypair might
be the identity keypair itself, or a different signature keypair for
which the public key has been, for example, signed by the identity
private key. This flexibility allows for multiple infrastructure
considerations and has the benefit of providing ways to use different
signature keys across different groups by using hierarchical
authentication keys. This flexibility also comes at the price of a
security tradeoff, described in the security considerations, between
potential unlinkability of the signature keys across groups and the
amount of time required to reinstate authentication and secrecy of
messages after the compromise of a device.
Ultimately, the only requirement is for the applications to be able
to check the credential containing the protocol signing key and the
identity against the Authentication Service at any time.
Omara, et al. Expires 9 September 2021 [Page 8]
Internet-Draft MLS Architecture March 2021
By definition, the Authentication Service is invested with a large
amount of trust. A malicious AS can impersonate -- or allow an
attacker to impersonate -- any user of the system. As a corollary,
by impersonating identities authorized to be members of a group, an
AS can break confidentiality.
This risk can be mitigated by publishing the binding between
identities and keys in a public log such as Key Transparency (KT)
[KeyTransparency]. It is possible to build a functional MLS system
without any kind of public key logging, but such a system will
necessarily be somewhat vulnerable to attack by a malicious or
untrusted AS.
2.3. Delivery Service
The Delivery Service (DS) is expected to play multiple roles in the
Service Provider architecture:
* To act 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.
* To route messages between clients and to act as a message
broadcaster, taking in one message and forwarding it to multiple
clients (also known as "server side fanout").
Because the MLS protocol provides a way for Clients to send and
receive application messages asynchronously, it only provides causal
ordering of application messages from senders while it has to enforce
global ordering of group operations to provide Group Agreement.
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 DS server fanout, the Delivery Service can be
considered as an active adaptative network attacker from the point of
view of the security analysis.
Omara, et al. Expires 9 September 2021 [Page 9]
Internet-Draft MLS Architecture March 2021
2.3.1. Key Storage
Upon joining the system, each client stores its initial cryptographic
key material with the Delivery Service. This key material, called
KeyPackage, advertises the functional abilities of the Client such as
supported protocol versions and 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 signed with
the signature public key associated with 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 keying material and to ensure that the
identifier for these keys are unique across all keys stored on the
DS.
2.3.2. Key Retrieval
When a client wishes to establish a group, it first contacts the DS
to request a KeyPackage for each other client, authenticate it using
the signature keys, and then can use those to form the group.
2.3.3. Delivery of messages and attachments
The main responsibility of the Delivery Service is to ensure delivery
of messages. Specifically, we assume that DSs provide:
* Reliable delivery: when a message is provided to the DS, it is
eventually delivered to all clients.
* In-order delivery: messages are delivered to the group in the
order they are received by the Delivery Service and in
approximately the order in which they are sent by clients. The
latter is an approximate guarantee because multiple clients may
send messages at the same time and so the DS needs some latitude
in enforcing ordering across clients.
* Consistent ordering: the DS must ensure that all clients have the
same view of message ordering for cryptographically relevant
operations. This means that the DS MUST enforce global
consistency of the ordering of group operation messages.
Omara, et al. Expires 9 September 2021 [Page 10]
Internet-Draft MLS Architecture March 2021
Note that the protocol provides three important information within an
MLSCiphertext message in order to provide ordering:
* The Group Identifier (GID) to allow to distinguish the group for
which the message has been sent;
* The Epoch number, which represent the number of changes (version)
of the group associated with a specific GID, and allows for
lexicographical ordering of two messages from the same group;
* The Content Type of the message, which allows the DS to determine
the ordering requirement on the message.
The MLS protocol itself can verify these properties. For instance,
if the DS reorders messages from a Client or provides different
Clients with inconsistent orderings, then Clients can detect this
misconduct. However, the protocol relies on the ordering, and on the
fact that only one honest group operation message is fanned-out to
clients per Epoch, to provide Clients with a consistent view of the
evolving Group State.
Note that some forms of DS misbehavior are still possible and
difficult to detect. For instance, a DS 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.
2.3.4. Membership knowledge
Group membership is itself sensitive information and MLS is designed
to drastically limit the amount of persisted metadata. However,
large groups often require an infrastructure which provides server
fanout. In the case of client fanout, the destinations 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
DS 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 DS.
While this knowledge is not a break of authentication or
confidentiality, 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.
Omara, et al. Expires 9 September 2021 [Page 11]
Internet-Draft MLS Architecture March 2021
2.3.5. 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 hasn't processed messages but MLS-using systems
can enforce some mechanism to try retaining these properties.
Typically this will consist of evicting clients which are idle for
too long, thus containing the threat of compromise. The precise
details of such mechanisms are a matter of local policy and beyond
the scope of this document.
2.4. Functional Requirements
MLS is designed as a large scale group messaging protocol and hence
aims to provide 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 as large as
50,000 members, typically including many users using multiple
devices.
2.4.1. Message Secrecy and Authentication
The trust establishment step of the MLS protocol is followed by a
conversation protection step where encryption is used by clients to
transmit authenticated messages to other clients through the DS.
This ensures that the DS does not have access to the group's private
content.
MLS aims to provide secrecy, integrity and authentication for all
messages.
Message Secrecy in the context of MLS means that only intended
recipients (current group members), can read any message sent to the
group, even in the context of an active attacker as described in the
threat model.
Message Integrity and Authentication mean that an honest Client can
only accept a message if it was sent by a group member and that no
Client can send a message which other Clients accept as being from
another Client.
Omara, et al. Expires 9 September 2021 [Page 12]
Internet-Draft MLS Architecture March 2021
A corollary to this statement is that the AS and the DS cannot read
the content of messages sent between Members as they are not Members
of the Group. MLS optionally provides additional protections
regarding traffic analysis so as to reduce the ability of attackers,
or a compromised member of the messaging system, to deduce the
content of the messages depending on (for example) their size. One
of these protections includes padding messages in order to produce
ciphertexts of standard length. While this protection is highly
recommended it is not mandatory as it can be costly in terms of
performance for clients and the SP.
Message content can be deniable if the signature keys are exchanged
over a deniable channel prior to signing messages.
2.4.2. 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.
Both of these properties are satisfied even against compromised DSs
and ASs.
2.4.3. Membership Changes
MLS aims to provide agreement on group membership, meaning that all
group members have agreed on the list of current group members.
Omara, et al. Expires 9 September 2021 [Page 13]
Internet-Draft MLS Architecture March 2021
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. Regardless, MLS does not allow addition or removal
of group members without informing all other members.
Once a client is part of a group, the set of devices controlled by
the user can only be altered by an authorized member of the group.
This authorization could depend on the application: some applications
might want to allow certain other members of the group to add or
remove devices on behalf of another member, while other applications
might want a more strict policy and allow only the owner of the
devices to add or remove them at the potential cost of weaker PCS
guarantees.
Members who are removed from a group do not enjoy special privileges:
compromise of a removed group member does not affect the security of
messages sent after their removal but might affect previous messages
if the group secrets have not been deleted properly.
2.4.4. Parallel Groups
Any user 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
are maintained and not weakened by user membership in multiple
groups.
2.4.5. Security of Attachments
The security properties expected for attachments in the MLS protocol
are very similar to the ones expected from messages. The distinction
between messages and attachments stems from the fact that the typical
average time between the download of a message and the one from the
attachments may be different. For many reasons (a typical reason
being the lack of high bandwidth network connectivity), the lifetime
of the cryptographic keys for attachments is usually higher than for
messages, hence slightly weakening the PCS guarantees for
attachments.
2.4.6. Non-Repudiation vs Deniability
As described in Section 3.6, 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
Omara, et al. Expires 9 September 2021 [Page 14]
Internet-Draft MLS Architecture March 2021
services are provided by mechanisms which allow the receiver to prove
a message's origin to a third party (this if often called "non-
repudiation"), but it should also be possible to operate MLS in a
"deniable" mode where such proof is not possible.
2.4.7. 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 shared keys, add or
remove new members, and send messages and attachments without waiting
for another user's reply.
Messaging systems that implement MLS have to provide a transport
layer for delivering messages asynchronously and reliably.
2.4.8. 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 have control over policies applied to
their own clients. Applications managing MLS clients can be
configured to allow for specific Group operations. An application
can, for example, decide to provide specific permissions to a group
administrator that will be the one to perform add and remove
operations, but the flexibility is immense here. 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 can 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 it for tracking.
*RECOMMENDATION:* Prefer using encrypted group operation messages
to avoid privacy issues related to non-encrypted signatures.
Note that in the default case of encrypted handshake messages, the
application level must make sure 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.
Omara, et al. Expires 9 September 2021 [Page 15]
Internet-Draft MLS Architecture March 2021
*RECOMMENDATION:* Avoid using inconsistent access control policies
in the case of encrypted group operations.
2.4.9. Recovery After State Loss
Conversation participants whose local MLS state is lost or corrupted
can reinitialize their state and continue participating in the
conversation.
[[OPEN ISSUE: The previous statement seems too strong, establish what
exact functional requirement we have regarding state recovery.
Previously: "This may entail some level of message loss, but does not
result in permanent exclusion from the group."]]
2.4.10. Support for Multiple Devices
It is typically expected for users within a Group to own different
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
of MLS. Such mechanisms, if used, may undermine the FS and PCS
guarantees provided by MLS.
2.4.11. Extensibility / Pluggability
Messages that do not affect the group state can carry an arbitrary
payload with the purpose of sharing that payload between group
members. No assumptions are made about the format of the payload.
2.4.12. 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 and infrastructure functionalities.
Omara, et al. Expires 9 September 2021 [Page 16]
Internet-Draft MLS Architecture March 2021
2.4.13. 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 proposed 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.
3. 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
-- The attacker can generate and inject 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.
Omara, et al. Expires 9 September 2021 [Page 17]
Internet-Draft MLS Architecture March 2021
3.1. 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 sometime 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 adaptative 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
setting, HSMs and secure enclaves can be used to protect signature
keys.
More information will be available in the Server-Assist draft.
[[TODO: Reference to server assist when the draft is available.]]
3.2. Transport Security Links
Any secure channel can be used as a transport layer to protect MLS
messages such as QUIC, TLS, WireGuard or TOR. Though the MLS
protocol is designed to consider the following threat-model:
-- The attacker can read and write 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 be considered.
3.2.1. 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.
Omara, et al. Expires 9 September 2021 [Page 18]
Internet-Draft MLS Architecture March 2021
Contrary to popular messaging services, the full list of recipients
cannot be sent to the server for dispatching messages because that
list is potentially extremely large in MLS. So, the metadata
typically consists of a pseudo-random Group Identifier (GID), an
numerical index referring to the key needed to decrypt the ciphertext
content and another numerical value to determine the epoch of the
group (the number of group operations that have been performed).
MLS protocol provides an authenticated "Authenticated Additional
Data" field for application to make data available outside the
MLSCiphertext.
*RECOMMENDATION:* Use the "Authenticated Additional 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 these metadata information are not 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 to access
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.
3.2.2. 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 DS 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 preventing
anonymous clients to send arbitrary numbers of Group Operation
messages to the Delivery Service or the MLS clients.
Omara, et al. Expires 9 September 2021 [Page 19]
Internet-Draft MLS Architecture March 2021
*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.)
3.2.3. Message suppression and error correction
The MLS protocol is particularly sensitive about Group Operation
message loss and reordering. This is because in the default setting,
MLS clients have to process those specific messages in order to have
a synchronized group state, after what the MLS protocol efficiently
generates keys for application messages.
The Delivery Service can have the role of helping with reliability,
but is mainly useful for reliability in the asynchronous aspect of
the communication between MLS clients.
While it is difficult or impossible to prevent a network adversary to
suppress 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.
3.3. 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 2.3.3,
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.
Omara, et al. Expires 9 September 2021 [Page 20]
Internet-Draft MLS Architecture March 2021
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.
3.3.1. 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 notification 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.
Omara, et al. Expires 9 September 2021 [Page 21]
Internet-Draft MLS Architecture March 2021
*RECOMMENDATION:* If stronger privacy guarantees are needed vis-
a-vis of the push notification provider, the client can choose to
periodically connect to the Delivery Service without the need of a
dedicated push notification infrastructure.
3.4. 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 credential 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.
Omara, et al. Expires 9 September 2021 [Page 22]
Internet-Draft MLS Architecture March 2021
3.4.1. 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
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 the
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
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
Omara, et al. Expires 9 September 2021 [Page 23]
Internet-Draft MLS Architecture March 2021
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.
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.
3.4.2. 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 correlate 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 to 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.
3.5. Shared considerations regarding adversarial AS or DS services
Omara, et al. Expires 9 September 2021 [Page 24]
Internet-Draft MLS Architecture March 2021
3.5.1. 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.
* 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 concern 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 credential in an MLS based
architecture might not be enough to provide strong privacy or
anonymity properties.
3.6. Client Compromise
The MLS protocol adopts a threat model which includes multiple forms
of Client compromises. 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
Omara, et al. Expires 9 September 2021 [Page 25]
Internet-Draft MLS Architecture March 2021
-- 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)
[[TODO: Cite the research papers in the context of these compromise
models]]
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.
3.6.1. 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).
Omara, et al. Expires 9 September 2021 [Page 26]
Internet-Draft MLS Architecture March 2021
MLS provides Post-Compromise Secrecy against an active adaptative
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 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.
3.6.2. 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 adaptative attackers through its Remove group
operation. This means that, as long as other members of the group
Omara, et al. Expires 9 September 2021 [Page 27]
Internet-Draft MLS Architecture March 2021
are honest, the protocol will guarantee message secrecy for all
messages exchanged in the epochs after the compromised party has been
removed.
3.6.3. 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 adaptative 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.
3.6.4. Compromise of the authentication with access to a signature key
DISCLAIMER: Significant work remains in this section. [[TODO: Remove
disclaimer.]]
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 adaptative network attacker to perform different
operations during the time of the compromise, the attacker can
perform every operations available to a legitimate client in both
cases.
There is a significant difference, however in terms of recovery after
a compromise.
Omara, et al. Expires 9 September 2021 [Page 28]
Internet-Draft MLS Architecture March 2021
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
adaptative attacker, can follow the protocol and request to update
its own credential. This in turn induce 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 compromised.
Even if the dedicated hardware approach is used, ideally, neither the
Client or the Authentication service alone should provide the
signature private key. Both should contribute to the key and it
should be stored securely by the client with no direct access.
3.6.5. 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 send 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.
Omara, et al. Expires 9 September 2021 [Page 29]
Internet-Draft MLS Architecture March 2021
*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.
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 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 key 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.
[[TODO: Considerations for Signature keys being reused or not across
groups]]
Omara, et al. Expires 9 September 2021 [Page 30]
Internet-Draft MLS Architecture March 2021
3.6.6. More attack scenarios
[[TODO: Make examples for more complex attacks, cross groups, multi
collusions...]]
[[TODO: Do we discuss PCFS in this document? If yes, where?]]
4. IANA Considerations
This document makes no requests of IANA.
5. Contributors
* Katriel Cohn-Gordon
University of Oxford
me@katriel.co.uk
* Cas Cremers
University of Oxford
cremers@cispa.de
* Thyla van der Merwe
Royal Holloway, University of London
thyla.van.der@merwe.tech
* Jon Millican
Facebook
jmillican@fb.com
* Raphael Robert
Wire
raphael@wire.com
6. Informative References
[KeyTransparency]
Google, ., "Key Transparency", 2017,
<https://KeyTransparency.org>.
Omara, et al. Expires 9 September 2021 [Page 31]
Internet-Draft MLS Architecture March 2021
[MLSPROTO] Barnes, R., Beurdouche, B., Millican, J., Omara, E., Cohn-
Gordon, K., and R. Robert, "Messaging Layer Security
Protocol", 2018.
[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>.
[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>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/rfc/rfc7049>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <https://www.rfc-editor.org/rfc/rfc7159>.
[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>.
Authors' Addresses
Emad Omara
Google
Email: emadomara@google.com
Benjamin Beurdouche
Inria & Mozilla
Email: benjamin.beurdouche@inria.fr
Eric Rescorla
Mozilla
Email: ekr@rtfm.com
Srinivas Inguva
Twitter
Omara, et al. Expires 9 September 2021 [Page 32]
Internet-Draft MLS Architecture March 2021
Email: singuva@twitter.com
Albert Kwon
MIT
Email: kwonal@mit.edu
Alan Duric
Wire
Email: alan@wire.com
Omara, et al. Expires 9 September 2021 [Page 33]