The Messaging Layer Security (MLS) Federation

2.1. Delivery Services

Depending on the kind of federated environment, the two following types of federated Delivery Services are possible:¶

                       +------------+
                      + Delivery     +
                      + Service (DS) +
                       +-----+------+
                    /        +        \             Group
*********************************************************
*                 /          +          \               *
*                /           |           \              *
*      +--------+       +----+---+       +--------+     *
*     + Client 0 +     + Client 1 +     + Client 3 +    *
*      +--------+       +--------+       +--------+     *
*     .............................     ............    *
*     User 0                            User 1          *
*                                                       *
*********************************************************

           +-----------------+      +-----------------+
          + 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 sepcifics 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.¶

3.1. 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.¶

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.¶

6.1. 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.¶

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.¶

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.¶

7.1. 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.¶