The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-04
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Katriel Cohn-Gordon <me@katriel.co.uk>, Cas Cremers <cas.cremers@cs.ox.ac.uk>, Thyla van der Merwe< thyla.van.der@merwe.tech>, Jon Millican <jmillican@fb.com>, Raphael Robert <raphael@wire.com>
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Network Working Group E. Omara
Internet-Draft Google
Intended status: Informational B. Beurdouche
Expires: July 29, 2020 INRIA
E. Rescorla
Mozilla
S. Inguva
Twitter
A. Kwon
MIT
A. Duric
Wire
January 26, 2020
The Messaging Layer Security (MLS) Architecture
draft-ietf-mls-architecture-04
Abstract
This document describes the reference architecture, functional and
security requirements for the Messaging Layer Security (MLS)
protocol. MLS provides a security layer for group messaging
applications with from two to a large number of clients. It is meant
to protect against eavesdropping, tampering, and message forgery.
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 July 29, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. General Setting . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Group, Members and Clients . . . . . . . . . . . . . . . 5
2.2. Authentication Service . . . . . . . . . . . . . . . . . 6
2.3. Delivery Service . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Key Storage . . . . . . . . . . . . . . . . . . . . . 8
2.3.2. Key Retrieval . . . . . . . . . . . . . . . . . . . . 8
2.3.3. Delivery of messages and attachments . . . . . . . . 8
2.3.4. Membership knowledge . . . . . . . . . . . . . . . . 9
2.3.5. Membership and offline members . . . . . . . . . . . 10
3. System Requirements . . . . . . . . . . . . . . . . . . . . . 10
3.1. Functional Requirements . . . . . . . . . . . . . . . . . 10
3.1.1. Asynchronous Usage . . . . . . . . . . . . . . . . . 10
3.1.2. Recovery After State Loss . . . . . . . . . . . . . . 10
3.1.3. Support for Multiple Devices . . . . . . . . . . . . 11
3.1.4. Extensibility / Pluggability . . . . . . . . . . . . 11
3.1.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.6. Federation . . . . . . . . . . . . . . . . . . . . . 11
3.1.7. Compatibility with future versions of MLS . . . . . . 12
3.2. Security Requirements . . . . . . . . . . . . . . . . . . 12
3.2.1. Connections between Clients and Servers (one-to-one) 12
3.2.2. Message Secrecy and Authentication . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
4.1. Transport Security Links . . . . . . . . . . . . . . . . 15
4.2. Delivery Service Compromise . . . . . . . . . . . . . . . 15
4.3. Authentication Service Compromise . . . . . . . . . . . . 16
4.4. Client Compromise . . . . . . . . . . . . . . . . . . . . 16
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
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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 when the messaging service 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 a
concrete protocol 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 [I-D.ietf-tls-tls13], 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.
This document is intended to describe the overall messaging system
architecture which the MLS protocol fits into, including the
operational requirements needed to achieve a functional system, and
to describe the security goals it is intended to fulfill.
2. General Setting
Informally, a group is a set of users who possibly use multiple
endpoint devices to interact with the Messaging Service (MS). 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.
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The Messaging Service (MS) presents as two abstract services that
allow clients to prepare for sending and receiving messages securely:
o An Authentication Service (AS) which is responsible for
maintaining user long term identities, issuing credentials which
allow them to authenticate each other, and potentially allowing
users to discover each others long-term identity keys.
o A Delivery Service (DS) which is responsible for receiving 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
to a group 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 clients in order to
proceed with the group secret key establishment process.
---------------- --------------
| 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, hence we show them as being separate here. Other
partitions are also possible, such as having a separate directory
server.
A typical group messaging scenario might look like this:
1. Alice, Bob and Charlie create accounts with a messaging service
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.
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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 addtion, they
might choose to update their key material which provides post-
compromise security Section 3.2.2.1. As a consequence of that
change, the group secrets are updated
Clients may wish to do the following:
o create a group by inviting a set of other clients;
o add one or more clients to an existing group;
o remove one or more members from an existing group;
o update their own key material
o join an existing group;
o leave a group;
o send a message to everyone in the group;
o 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 it might be cryptographically
possible for any member to send a message adding a new client to a
group, the group might have the policy that only certain members are
allowed to make changes and thus other members can ignore or reject
such a message from an unauthorized user.
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
Messaging Service, this definition is too simplistic.
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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 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 a 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.
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 she is 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:
o A certification authority or similar service which signs some sort
of portable credential binding an identity to a signature key.
o 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 directly, or a different signature keypair
for which the 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
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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.
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 corrolary, 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
Messaging Service architecture:
o 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.
o 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
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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.
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
ClientInitKey, advertizes the functional abilities of the Client such
as supported protocol versions and extensions and the following
cryptographic information:
o A credential from the Authentication Service attesting to the
binding between the identity and the client's signature key.
o The client's asymmetric encryption key material signed with the
signature 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 accross 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 ClientInitKey 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:
o Reliable delivery: when a message is provided to the DS, it is
eventually delivered to all clients.
o 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.
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o 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.
Note that the protocol provides three important information within an
MLSCiphertext message in order to provide ordering:
o The Group Identifier (GID) to allow to distinguish the group for
which the message has been sent;
o 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;
o 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 faned-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
so 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 MS.
While this knowledge is not a break of authentication or
confidentiality, it is a serious issue for privacy. In the case
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where metadata has to be persisted for functionality, it SHOULD be
stored encrypted at rest.
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.
3. System Requirements
3.1. 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.
3.1.1. 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.
3.1.2. Recovery After State Loss
Conversation participants whose local MLS state is lost or corrupted
can reinitialize their state and continue participating in the
conversation.
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[[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."]]
3.1.3. 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.
3.1.4. 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.
3.1.5. Privacy
The protocol is designed in a way that limits the server-side (AS and
DS) metadata footprint. The DS only persists data required for the
delivery of messages and avoid Personally Identifiable Information
(PII) or other sensitive metadata wherever possible. A Messaging
Service provider that has control over both the AS and the DS, will
not be able to correlate encrypted messages forwarded by the DS, with
the initial public keys signed by the AS.
[[OPEN ISSUE: These privacy statements seem very strong. BB. I
would be willing to keep them as requirements since we have example
solutions in the Server-Assist draft.]]
3.1.6. 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.
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3.1.7. 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.
3.2. Security Requirements
3.2.1. Connections between Clients and Servers (one-to-one)
We assume that all transport connections are secured via some
transport layer security mechanism such as TLS [I-D.ietf-tls-tls13].
However, as noted above, the security of MLS will generally survive
compromise of the transport layer, so long as identity keys provided
by the AS are authenticated at a minimum. However, MLS ciphertext
contain the Group Identifier, Epoch number and Content Type that may
be use to improve attacks on the privacy of the group.
3.2.2. 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
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Client can send a message which other Clients accept as being from
another Client.
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 MS.
Message content can be deniable if the signature keys are exchanged
over a deniable channel prior to signing messages.
3.2.2.1. 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.
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3.2.2.2. 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. 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.
3.2.2.3. 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.
3.2.2.4. 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.
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3.2.2.5. Denial of Service
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.
3.2.2.6. Non-Repudiation vs Deniability
As described in Section 4.4, 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 messaging service
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 if often called "non-
repudiation"), but it should also be possible to operate MLS in a
"deniable" mode where such proof is not possible. [[OPEN ISSUE:
Exactly how to supply this is still a protocol question.]]
4. Security 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. 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.
4.1. Transport Security Links
[TODO: Mostly DoS, message suppression, and leakage of group
membership.]
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 2.3.3,
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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.
4.3. Authentication Service Compromise
A compromised AS is a serious matter, as the AS can provide incorrect
or attacker-provided identities to clients. As noted in Section 2.2,
detecting this form of attack requires some sort of transparency/
logging mechanism. Without such a mechanism, MLS cannot detect a
compromised AS.
4.4. Client Compromise
MLS provides a limited form of protection against compromised Clients
through PCS. When the Client is fully compromised, then the attacker
will be able to decrypt any messages for groups in which the Client
is a member, and will be able to send messages impersonating the
compromised Client. However, if the Client afterwards updates its
keying material (see Section 3.2.2.1) (using fresh randomness that
the attacker does not know) then the PCS property enables the Client
to recover.
In addition, a client cannot send a message to a group which appears
to be from another client with a different identity. Note that if
devices from the same user share keying material, then one will be
able to impersonate another.
Finally, clients should not be able to perform DoS attacks
Section 3.2.2.5.
5. IANA Considerations
This document makes no requests of IANA.
6. Contributors
o Katriel Cohn-Gordon
University of Oxford
me@katriel.co.uk
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o Cas Cremers
University of Oxford
cas.cremers@cs.ox.ac.uk
o Thyla van der Merwe
Royal Holloway, University of London
thyla.van.der@merwe.tech
o Jon Millican
Facebook
jmillican@fb.com
o Raphael Robert
Wire
raphael@wire.com
7. Informative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[KeyTransparency]
Google, ., "Key Transparency", n.d.,
<https://KeyTransparency.org>.
[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/info/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/info/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/info/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/info/rfc7159>.
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Authors' Addresses
Emad Omara
Google
Email: emadomara@google.com
Benjamin Beurdouche
INRIA
Email: benjamin.beurdouche@inria.fr
Eric Rescorla
Mozilla
Email: ekr@rtfm.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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