MIMI Delivery Service

Introduction The MLS protocol document specifies a protocol between two or more clients. The MLS architecture document introduces an abstract concept of a "Delivery Service" that is specifically responsible for ordering handshake messages and more generally for delivering messages to the intended recipients. This document describes a Delivery Service that performs the mandated ordering of handshake messages but also offers the basic functionality that is common to several messaging services. In particular, it offers the following features:

A protocol between clients and the Delivery Service that allows clients to interact with the Delivery Service, including the specific wire format of the messages. The protocol is specifically designed to be operated in a decentralized, federated architecture but can be used in single instances as well.
Assistance for new joiners of a group: The Delivery Service can keep and update state such as the public ratchet tree, group extensions, etc.
Message validation: The Delivery Service can inspect and validate handshake messages and reject malformed, invalid or malicious messages.
Built-in authentication of group members: The Delivery Service can authenticate group members, reject messages from non-members and enforce group administration policies. Additionally, the Delivery Service can be linked to the Authentication Service to enforce more policies and validation rules.
Privacy-preserving message delivery: The Delivery Service can deliver messages to the intended recipients without learning the identity of group members, in particular the sender or the recipients of messages. This feature is optional and is compatible with most other features, such as assistance and validation.
Scalability: The protocol makes no assumption about whether the Delivery Service runs as a single instance or as a cluster of instances.
Network fluid: While the Delivery Service would typically run as a server-side component, the only requirement is that it is accesible by all clients.
Transport agnostic: Messages between clients and the Delivery Service can be sent via an arbitrary transport protocol. Additionally, in the federated case, client messages to a guest Delivery Service can be forwarded by a client's local instance of this service.
Multi-device capable: The Delivery Service can be used by multiple devices of the same users and supports adding and removing devices of a user.
A Queueing Service subcomponent that can be used to implement a queueing service for messages that are delivered asynchronously, particularly in federated environments, as well as the protocol between the Delivery Service and the Queueing Service.

Protocol overview The Delivery Service is designed around an MLS group as a basic entity. The Delivery Service can be used with multiple groups but does not require a link between the groups. All operations of the Delivery Service are relative to a specific group.

High level functions of the Delivery Service

Authentication of group members
Validation of handshake messages
Ordering of handshake messages
Delivery of messages to the Queueing Service
Assistance for clients who (re)join a group

Flow The protocol is designed in a request/response pattern, whereby the client sends a DSRequest message to the Delivery Service and the Delivery Service responds with a DSResponse message. This pattern can easily be used over e.g. RESTful APIs.

Delivery Service Request/Response scheme | | | | DSResponse | |<---------------+ | | ]]> Depending on the client's request, the Delivery Service sends a message to the Queueing Service. This happens whenever a message needs to be fanned out to all other members of a group.

High level overview of the operations

Group creation/deletion
Updating client queue information
Assistance to join a group from a Welcome message as a new member
Assistance to join a group through an External Commit message as a new member or client of an existing member
Adding and removing users to/from a group
Adding and removing clients to/from a member of the group
Client updates (MLS leaf updates)
Assistance to resync a client with a group in case of state loss
Sending application messages

Client removals induced by the Delivery Service The Delivery Service can remove clients from a group by issuing remove proposals in the following cases:

A user has removed a client from its account
A user has been deleted
The client is removed from the group because it has been inactive for too long

Authentication Clients authenticate to the Delivery Service using the signature key from their respective leaf node in the group and a client-specific identifier. Depending on the operation, one or more kind of client identifier can be used:

A leaf index: The client identifier is the leaf index of the client in the group.
KeyPackageRef: The client identifier is the KeyPackageRef of the client's KeyPackage.
User key hash: The client identifier is the hash of the user's public key.

The identifier is encoded as follows: ; case user_key_hash opaque user_key_hash; } } DSSenderID; ]]> To authenticate, the client signs the corresponding request. The key used to sign/verify a given request depends on the sender_id. ; } DSMessage; struct { DSSenderID sender_id; DSRequest request; } DSMessageTBS; ]]>

Request/Response scheme The various request types, each corresponding to an operation, are combined as follows: The request type is encoded in the DSRequest message as follows:

Operations

Create group A request from the client to create a new group on the Delivery Service. The client sends the following CreateGroupRequest to the Delivery Service: ; KeyPackage key_package; ClientQueueConfig client_queue_config; GroupInfo group_info; } CreateGroupRequest; ]]> The Delivery Service responds with a CreateGroupResponse:

Validation The Delivery Service validates the request as follows:

The group ID is not empty and is not already in use.
The KeyPackage is valid, according to (I-D.ietf-mls-protocol) 13.4.3.2. Joining via External Commits.
The GroupInfo is valid, according to (I-D.ietf-mls-protocol) 11.1. KeyPackage validation.

Delete group A request from the client to delete a group from the Delivery Service.

Update queue information A request from the client to update the queue information for a group. Clients can provision a queue information object to indicate to the Delivery Service how to transmit messages to the client during fanout.

Further operations The following operations follow the same pattern as the create group operation but are not fully specified in this version of the document:

Add user to group
Remove user from group
Add client to group
Remove client from group
Update client
Resync client
Update client queue information
Join from a Welcome message
Join from an External Commit message
Send message

Delivery Service state For each group hostend on a particular Delivery Service instance, the Delivery Service keeps the following state:

The public MLS ratchet tree of the group
The GroupInfo of the current epoch
Past group states (for members who join through a Welcome message)
Client queue information (for message fanout via the Queueing Service)
Proposal store that stores standalone proposals of the current epoch
Extensions (TBD)

Queueing Service The Queueing Service is a service that is used by the Delivery Service to fan out messages to clients. Each client has a queue that is used to store messages for later retrieval by the client. The group state on the Delivery Service contains client queue information for each client of a group. This information is forwarded to the Queueing Service along with the message during message fanout. The information is opaque to the Delivery Service and is only used by the Queueing Service to deliver the message to the client. Note that this document uses the term "client queue" to refer to an abstract mechanism to forward messages to clients and does not necessarily imply that an actual queueing mechanism is used. This document only specifies the interface between the Delivery Service and the Queueing Service. Other aspects of the Queueing Service are not specified in this document.

Queueing Service protocol The Queueing Service protocol is a simple request/response protocol used between the Delivery Service and the Queueing Service.

Queueing Service Request/Response scheme | | | | QueueingServiceResponse | |<-------------------------+ | | ]]> Whenever the Delivery Service has validated an incoming message from a client and starts to fan out the message, it sends the following request to the Queueing Service for every client in the group: ; MLSMessage message; } QueueingServiceRequest; ]]> The Queueing Service responds with a QueueingServiceResponse: In case the Queueing Service considers the client queue information to be in invalid (e.g. because the corresponding user/client has been deleted), it responds with invalid_client_queue_information. Otherwise, it responds with ok. The Delivery Service can use rejected messages to subsequentially issue remove proposals to remove the client from the group. The Delivery Service rejects Commit messages and application messages until a client has sent a Commit that covers the proposals.

Queueing service in federated environments In federated environments, the Queueing Service does not necessarily have to be part of the same domain as the Delivery Service. In this case, the Delivery Service sends requests over an inter-domain transport protocol to the Queueing Service.

Privacy preserving message delivery If the desired mode of operation is for the Delivery Service to learn as little as possible about the groups it serves and their members, the protocol can be extended to protect the group state on the Delivery Service. This can happen through two complementary mechanisms:

Unlinking member credentials from credentials issued by the Authentication Service, providing pseudonymity at the Delivery Service level
Encrypting the group state with a key that is only known to the group members, providing encryption at rest

In practice, the requests from clients to the Delivery Server are extended with additional parameters, such as decryption keys for the group state and additional pseudonymous user-level authentication.

Extensions The Delivery Service can make use of the following MLS extensions:

Example: Group administration extension
TBD