Internet DRAFT - draft-robert-mimi-delivery-service
draft-robert-mimi-delivery-service
Network Working Group R. Robert
Internet-Draft K. Kohbrok
Intended status: Informational Phoenix R&D
Expires: 10 May 2023 6 November 2022
MIMI Delivery Service
draft-robert-mimi-delivery-service-00
Abstract
This document describes the MIMI Delivery Service.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol overview . . . . . . . . . . . . . . . . . . . . . . 3
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2.1. High level functions of the Delivery Service . . . . . . 3
2.1.1. Flow . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. High level overview of the operations . . . . . . . . . . 4
2.3. Client removals induced by the Delivery Service . . . . . 5
2.4. Authentication . . . . . . . . . . . . . . . . . . . . . 5
2.5. Request/Response scheme . . . . . . . . . . . . . . . . . 6
3. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Create group . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Validation . . . . . . . . . . . . . . . . . . . . . 7
3.2. Delete group . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Update queue information . . . . . . . . . . . . . . . . 8
3.4. Further operations . . . . . . . . . . . . . . . . . . . 8
4. Delivery Service state . . . . . . . . . . . . . . . . . . . 8
5. Queueing Service . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Queueing Service protocol . . . . . . . . . . . . . . . . 9
5.2. Queueing service in federated environments . . . . . . . 10
6. Privacy preserving message delivery . . . . . . . . . . . . . 10
7. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The MLS protocol document specifies a protocol between two or more
clients. The MLS architecture document introduce 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.
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* 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.
* 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.
2. 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.
2.1. 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
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2.1.1. 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.
Client Delivery Service
| |
| DSRequest |
+--------------->|
| |
| DSResponse |
|<---------------+
| |
Figure 1: Delivery Service Request/Response scheme
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.
Client Delivery Service Queueing Service
| | |
| DSRequest | |
+--------------->| |
| | |
| DSResponse | |
|<---------------+ |
| | Fanout |
| +-------------------->|
| | |
Figure 2: Client/Delivery Service communication with fanout
2.2. 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
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* 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
2.3. 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
2.4. 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:
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enum {
leaf_index(0),
key_package_ref(1),
user_key_hash(2),
} DSSenderType;
struct {
DSSenderType sender_type;
select (DSSenderID.sender_type) {
case leaf_index
uint32 leaf_index;
case key_package_ref
opaque key_package_ref<V>;
case user_key_hash
opaque user_key_hash<V>;
}
} DSSenderID;
To authenticate, the client sends the following token to the Delivery
Service:
struct {
opaque group_id<V>;
uint32 timestamp;
DSSenderID sender_id;
// Signature over DSAuthTokenTBS
opaque signature<0..255>;
} DSAuthToken;
struct {
opaque group_id<V>;
uint32 timestamp;
DSSenderID sender_id;
} DSAuthTokenTBS;
The Delivery Service can mandate additional authentication
mechanisms, such as user-based authentication.
2.5. Request/Response scheme
The various request types, each corresponding to an operation, are
combined as follows:
enum {
ds_create_group(0),
ds_delete_group(1),
...
} DSRequestType;
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The request type is encoded in the DSRequest message as follows:
struct {
DSRequestType request_type;
select (DSRequest.request_type) {
case ds_create_group:
CreateGroupRequest create_group_request;
case ds_delete_group:
DeleteGroupRequest delete_group_request;
...
}
} DSRequest;
3. Operations
3.1. 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:
struct {
opaque group_id<V>;
KeyPackage key_package;
ClientQueueConfig client_queue_config;
GroupInfo group_info;
} CreateGroupRequest;
The Delivery Service responds with a CreateGroupResponse:
enum {
invalid_group_id(0),
invalid_key_package(1),
invalid_group_info(2),
} CreateGroupResponse;
3.1.1. 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.
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* The GroupInfo is valid, according to (I-D.ietf-mls-protocol) 11.1.
KeyPackage validation.
3.2. Delete group
A request from the client to delete a group from the Delivery
Service.
3.3. 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.
3.4. 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
4. 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
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* 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)
5. 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.
5.1. Queueing Service protocol
The Queueing Service protocol is a simple request/response protocol
used between the Delivery Service and the Queueing Service.
Delivery Service Queueing Service
| |
| QueueingServiceRequest |
+------------------------->|
| |
| QueueingServiceResponse |
|<-------------------------+
| |
Figure 3: Queueing Service Request/Response scheme
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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:
struct {
opaque client_queue_information<V>;
MLSMessage message;
} QueueingServiceRequest;
The Queueing Service responds with a QueueingServiceResponse:
enum {
ok(0),
invalid_client_queue_information(1),
} 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.
5.2. 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.
6. 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 pseudonimity at the Delivery
Service level
* Encrypting the group state with a key that is only known to the
group members, providing encryption at rest
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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.
7. Extensions
The Delivery Service can make use of the following MLS extensions:
* Example: Group administration extension
* TBD
Authors' Addresses
Raphael Robert
Phoenix R&D
Email: ietf@raphaelrobert.com
Konrad Kohbrok
Phoenix R&D
Email: konrad.kohbrok@datashrine.de
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