Leaf Operation Intents

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Table of Contents

1. Introduction

To leave an MLS group, a member cannot create a commit, but rather has to propose its own removal. This can create difficulties, some of which have been solved by the introduction of the SelfRemove proposal, which may be included in external commits.¶

One drawback of Remove and SelfRemove proposals is that they (like other proposals) are bound to a specific group epoch. This means that authors of such proposals must have an up-to-date group state to send such a proposal and continue to keep track of that group state both to re-send the proposal if necessary.¶

This can be a problem if an application wants to cleanly leave a group and immediately delete the associated group state, e.g., to erase the assocaited metadata. The deletion of the group state makes it impossible to for the client to re-send the proposal in case it's not covered by the next commit. Similarly, the client might be offline at the time and the group state might not be up-to-date.¶

The LeafOperationIntent specified in this document allow a client to bind an intent to leave a group or update its own leaf to the leaf's current state. That intent can then be proposed by any party (e.g. an external sender) in an arbitrary epoch as long as the leaf doesn't change its state due to an update.¶

2. LeafOperationIntent

A LeafOperationIntent can be created by clients and distributed either to other group members or to one or more external senders.¶

HashReference LeafNodeRef;

MakeLeafNodeRef(value)
  = RefHash("MLS 1.0 LeafNode Reference", value)

enum {
  reserved(0),
  update(1)
  remove(2),
  (255)
} IntentType

struct {
  IntentType intent_type;
  select (Intent.intent_type) {
    case remove:
      {}
    case update:
      LeafNode leaf_node;
  }
} Intent

struct {
  opaque group_id<V>;
  uint32 sender_index;
  LeafNodeRef leaf_ref;
  Intent intent;
} LeafOperationIntentTBS

struct {
  opaque group_id<V>;
  uint32 sender_index;
  LeafNodeRef leaf_ref;
  Intent intent;
  /* SignWithLabel(., "LeafOperationIntentTBS", LeafOperationIntentTBS) */
  opaque signature<V>;
} LeafOperationIntent

struct LeafOperationProposal {
  LeafOperationIntent intent;
}

LeafNode, RefHash and SignWithLabel are as defined in [RFC9420].¶

• group_id: The ID of the group in which context the LeafOperationIntent was sent¶

• sender_index: The index of the sender's leaf in the group¶

• leaf_ref: A hash computed over the LeafNode of the sender using the MakeLeafNodeRef function¶

• intent: The intent and a potential payload¶

• signature: A signature over all fields except the signature itself using the sender's leaf signature key¶

3. Security Considerations

In contrast to proposals, LeafOperationIntents are not bound to an epoch and thus remain valid as long as the creator's leaf doesn't change its state.¶

Each LeafOperationIntent can thus be proposed and committed to until the sender is either removed from the group or updates its own leaf.¶

This allows scenarios, where, for example, members get added to or removed from a group in the time between the creation and the proposal of the intent.¶

If a tighter bound to the epoch, i.e. the current group state is required, clients should use regular Update, Remove or SelfRemove proposals instead.¶

4. IANA Considerations

This document requests the addition of a new Proposal Type under the heading of "Messaging Layer Security".¶

The leaf_operation_intent MLS Proposal Type is used to allow members or external sender to convey the intent of a leaf owner to perform an operation on their leaf.¶

• Value: 0x000c (suggested)¶

• Name: leaf_operation_intent¶

• Recommended: Y¶

• External: Y¶

• Path Required: Y¶

5. Normative References

[RFC9420]

Barnes, R., Beurdouche, B., Robert, R., Millican, J., Omara, E., and K. Cohn-Gordon, "The Messaging Layer Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420, July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.

Acknowledgments

TODO acknowledge.¶

Contributors

Author's Address