Internet DRAFT - draft-ietf-mls-federation
draft-ietf-mls-federation
Network Working Group E. Omara
Internet-Draft Google
Intended status: Informational R. Robert
Expires: 12 March 2024 Phoenix R&D
9 September 2023
The Messaging Layer Security (MLS) Federation
draft-ietf-mls-federation-03
Abstract
This document describes how the Messaging Layer Security (MLS)
protocol can be used in a federated environment.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/mlswg/mls-federation.
Status of This Memo
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This Internet-Draft will expire on 12 March 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Federated environments . . . . . . . . . . . . . . . . . . . 2
2.1. Delivery Services . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Different client applications . . . . . . . . . . . . 3
2.1.2. Different Delivery Services . . . . . . . . . . . . . 3
2.2. Authentication Service . . . . . . . . . . . . . . . . . 4
3. Further considerations . . . . . . . . . . . . . . . . . . . 4
3.1. Network protocol between different domains . . . . . . . 4
4. Discovery service for clients/users on a specific domain . . 5
5. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Different Delivery Servers . . . . . . . . . . . . . . . 5
5.2. Different client applications . . . . . . . . . . . . . . 5
6. Functional Requirements . . . . . . . . . . . . . . . . . . . 5
6.1. Delivery service . . . . . . . . . . . . . . . . . . . . 5
6.2. Authentication Service . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7.1. Version & ciphersuite negotiation . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
MLS Architecture draft [I-D.ietf-mls-architecture] describes the
overall MLS system architecture, assuming the client and servers
(Delivery Service and Authentication Service) are operated by the
same entity. This is however not a strict requirement, the MLS
protocol does not have an inherent dependency on one single entity
and can instead be used between multiple entities.
The focus of this document is on the different components of the MLS
architecture when used in a federated environment.
2. Federated environments
Federated environments are environments where multiple entities are
operating independent MLS services. In particular, the assumption is
that Delivery Services and Authentication Services are not
necessarily operated by the same entity.
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Entities operating independent MLS services are commonly called
domains. In most cases these domains might correspond to the Domain
Name System, but it is not strictly required. Operating MLS services
in a federated environment can therefore be regarded as federation
between different domains.
Federation is however not limited to the MLS services themselves.
For example, a federated environment could also contain clients that
are provided by different entities. Specifically, different vendors
could provide different applications that differ in scope and
functionality but are interoperable.
2.1. Delivery Services
Depending on the kind of federated environment, the two following
types of federated Delivery Services are possible:
2.1.1. Different client applications
The diagram below shows an MLS group where all clients are provided
by potentially different vendors but operate on the same Delivery
Service:
+------------+
+ Delivery +
+ Service (DS) +
+-----+------+
/ + \ Group
*********************************************************
* / + \ *
* / | \ *
* +--------+ +----+---+ +--------+ *
* + Client 0 + + Client 1 + + Client 3 + *
* +--------+ +--------+ +--------+ *
* ............................. ............ *
* User 0 User 1 *
* *
*********************************************************
2.1.2. Different Delivery Services
The diagram below shows a federated environment in which different or
identical clients applications operate on different Delivery
Services:
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+-----------------+ +-----------------+
+ Deliver Service 1 + + Deliver Service 2 +
+ + + +
+-----------------+ +--------+--------+
| | |
| | | Group
***************|*********|*******************|***********
* | | | *
* | | | *
* +--------+ +--------+ +--------+ *
* + Client 0 + + Client 1 + + Client 3 + *
* +--------+ +--------+ +--------+ *
* ............................. ............ *
* User 0 User 1 *
* *
*********************************************************
2.2. Authentication Service
In a federated environment, authentication becomes more important.
While the specifics of an Authentication Service are out-of-scope for
MLS in general, it is important that strong authentication is
accessible to all clients of a federated environment. As an example,
a shared transparency log like [keytransparency] could be used.
3. Further considerations
The following aspects of federated communication are important for
successful federation but are not considered in scope of the MLS
charter:
## Common format for the content of application messages
The MLS protocol does not impose any format on the content of
application messages. Instead, application messages are considered
to be an opaque sequence of bytes. Applications in a federated
environment are expected to agree on a common format. The
negotiation can be done at the KeyPackage level, or through the MLS
extension mechanism.
3.1. Network protocol between different domains
Cross-domain operations such as sending and receiving messages,
fetching KeyPackages, and querying the Authentication Services have
to be part of a common network protocol that is supported by all
domains in a federated environment.
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4. Discovery service for clients/users on a specific domain
Searching for users and other discovery services are typically part
of messaging systems. In the context of MLS, this functionality
might overlap with the fetching of KeyPackages and message routing.
5. Use cases
5.1. Different Delivery Servers
Different applications operated by different entities can use MLS to
exchange end-to-end encrypted messages. For example, in a messaging
applications, clients of messaging1.tld can encrypt and decrypt end-
to-end encrypted messages from messaging2.tld.
5.2. Different client applications
Different client applications operating on the same server can use
MLS to exchange end-to-end encrypted handshake and application
messages. For example, different browsers can implement the MLS
protocol, and web developers write web applications that use the MLS
implementation in the browser to encrypt and decrypt the messages.
This will require a new standard Web API to allow the client
applications to set the address of the delivery service in the
browser. A more concrete example is using MLS in the browser to
negotiate SRTP keys for multi-party conference calls.
6. Functional Requirements
6.1. Delivery service
While there is no strict requirement regarding the network topology,
there are practical advantages when clients only connect to their own
Delivery Service rather than to the whole federated environment.
This routing strategy can possibly make the design of a cross-domain
network protocol easier in the context of access control.
In such a topology, the client's own Delivery Service relays messages
to the Delivery Service in charge for a specific group.
When several Delivery Services are involved in relaying messages, it
is important that all of them agree on which one is responsible for
ordering handshake messages of a specific group at any given time in
order to enforce the total ordering of handshake messages required by
the MLS protocol.
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Depending on the functional requirements (such high availability and
redundancy), the different Delivery Services can elect a dedicated
Delivery Service to be responsible for ordering handshake messages
for a certain period of time. The election process can be repeated
when the availability of Delivery Services change.
The diagram below shows a federated environment where a client
connects to its own Delivery Service that in turn relays messages to
other Delivery Services.
+-----------------+ +---------+
+--> + Deliver Service B + +---> + Client B1 +
| + + +---------+
| +-----------------+
|
+---+-------------+ +---------+
+---------+ + Deliver Service A + +-------------> + Client A2 +
+ Client A1 + +---> + + +---------+
+---------+ +------+----------+
|
| +-----------------+ +---------+
+--> + Deliver Service C + +---> + Client C1 +
+ + +---------+
+-----------------+
6.2. Authentication Service
The MLS specification only describes how the signatures on the
contents of the leaf nodes of a given group can be verified and that
clients have to support the signature schemes of all other clients in
each group.
The credential (and thus the binding between identity of the group
member and its signature public key) in each (non-blank) leaf node
has to be authenticated by the AS. This becomes relevant in a
federated setting, as the AS, and thus the authentication process of
each member in a given group might differ.
This problem can be solved in a variety of ways, for example, by
having all applications and/or service providers involved in a
federation agree on a shared process, or by having clients advertise
their authentication process in a similar way as their ciphersuite,
with the requirement that all members of a group must support each
others authentication processes.
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Depending on the design of the AS of a given client, other, federated
clients might have to trust that client's service provider to
authenticate its credential. Confidence in authentication provided
by service providers in general can be strengthened by using a scheme
such as [keytransparency], which allows both local and federated
clients to assert a shared view of the authentication information
provided by the service.
In a federated environment, the AS should provide the same degree of
transparency as in a non-federated environment, i.e. end-to-end
authentication should be possible.
7. Security Considerations
7.1. Version & ciphersuite negotiation
In a federated environment, version & ciphersuite negotiation is more
critical, to avoid forcing a downgrade attack by malicious third
party Delivery Services. This is due to the fact that the thread
model is extended to include the following:
* Different entities might have diverging security policies, e.g.
they don't enforce validation of KeyPackages the same way.
* Entities might be malicious and act as a threat actor, e.g. might
generate fake clients controlled by them.
The negotiation of version numbers & ciphersuites can be done at the
KeyPackage level.
8. IANA Considerations
This document makes no requests of IANA.
9. References
9.1. Normative References
[I-D.ietf-mls-architecture]
Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., and
A. Duric, "The Messaging Layer Security (MLS)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-mls-architecture-11, 26 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
architecture-11>.
9.2. Informative References
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[keytransparency]
"Key Transparency", n.d.,
<https://github.com/google/keytransparency>.
Authors' Addresses
Emad Omara
Google
Email: emadomara@google.com
Raphael Robert
Phoenix R&D
Email: ietf@raphaelrobert.com
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