Internet DRAFT - draft-mahy-mls-selfremove
draft-mahy-mls-selfremove
MLS R. Mahy
Internet-Draft Wire
Intended status: Informational 25 June 2023
Expires: 27 December 2023
The SelfRemove Proposal for Message Layer Security (MLS)
draft-mahy-mls-selfremove-00
Abstract
This document describes the SelfRemove Message Layer Security (MLS)
Proposal type, which improves handling of a client removing itself
from an MLS group.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 27 December 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Extension Description . . . . . . . . . . . . . . . . . . . . 3
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4.1. self_remove MLS Proposal Type . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2219].
The terms MLS client, MLS group, LeafNode, GroupContext, KeyPackage,
GroupContextExtensions Proposal, Credential, CredentialType, and
RequiredCapabilities have the same meanings as in the MLS protocol
[I-D.ietf-mls-protocol].
2. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH. The source for
this draft is maintained in GitHub. Suggested changes should be
submitted as pull requests at https://github.com/rohan-wire/ietf-
drafts (https://github.com/rohan-wire/ietf-drafts). Editorial
changes can be managed in GitHub, but any substantive change should
be discussed on the MLS mailing list (mls@ietf.org).
The design of the MLS protocol prevents a member of an MLS group from
removing itself immediately from the group. To cause an immediate
change in the group, a member must send a Commit message. However
the sender of a Commit message knows the keying material of the new
epoch and therefore needs to be part of the group. Instead a member
wishing to remove itself can send a Remove Proposal and wait for
another member to Commit its Proposal.
Unfortunately, MLS clients that join via an External Commit ignore
pending, but otherwise valid, Proposals. The member trying to remove
itself has to monitor the group and send a new Proposal in any new
epoch until the member is removed. In a group with a burst of
external joiners, a member connected over a high-latency link (or one
that is merely unlucky) might have to wait several epochs to remove
itself. A real-world situation in which this happens is a member
trying to remove itself from a conference call as several dozen new
participants are trying to join (often on the hour).
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This document described a new SelfRemove Proposal extension type. It
is designed to be included in External Commits.
3. Extension Description
This document specifies a new MLS Proposal type called SelfRemove.
Its syntax is described using the TLS Presentation Language [RFC8446]
below (its contents is an empty struct). It is allowed in External
Commits and requires an UpdatePath.
struct {} SelfRemove;
struct {
ProposalType msg_type;
select (Proposal.msg_type) {
case add: Add;
case update: Update;
case remove: Remove;
case psk: PreSharedKey;
case reinit: ReInit;
case external_init: ExternalInit;
case group_context_extensions: GroupContextExtensions;
case self_remove: SelfRemove;
};
} Proposal;
The description of behavior below only applies if all the members of
a group support this extension in their capabilities; such a group is
a "self-remove-capable group".
An MLS client which implements this specification can send a
SelfRemove Proposal whenever it would like to remove itself from a
self-remove-capable group. Because the point of a SelfRemove
Proposal is to be available to external joiners (which are not yet
members), these proposals MUST be sent as an MLS PublicMessage.
Whenever a member receives a SelfRemove Proposal, it includes it
along with any other pending Propsals when sending a Commit. It
already MUST send a Commit of pending Proposals before sending new
application messages.
When a member receives a Commit with an embedded SelfRemove Proposal,
it treats the proposal like a Remove Proposal, except the leaf node
to remove is determined by looking in the Sender leaf_index of the
original Proposal. The member is able to verify that the Sender was
a member.
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Whenever a new joiner is about to join a self-remove-capable group
with an External Commit, the new joiner MUST fetch any pending
SelfRemove Proposals along with the GroupInfo object, and include the
SelfRemove Proposals in its External Commit by reference. The new
joiner validates the SelfRemove Proposal before including it by
reference, except that it skips the validation of the membership_tag
because a non-member cannot verify membership.
The MLS Distribution Service (DS) needs to validate SelfRemove
Proposals it receives (except that it cannot validate the
membership_tag). If the DS provides a GroupInfo object to an
external joiner, the DS SHOULD attach any SelfRemove proposals known
to the DS to the GroupInfo object.
As with Remove proposals, clients need to be prepared to receive the
Commit message which removes them from the group via a SelfRemove.
If the DS does not forward a Commit to a removed client, it needs to
inform the removed client out-of-band.
4. IANA Considerations
This document proposes registration of a new MLS Proposal Type.
RFC EDITOR: Please replace XXXX throughout with the RFC number
assigned to this document
4.1. self_remove MLS Proposal Type
The self_remove MLS Proposal Type is used for a member to remove
itself from a group more efficiently than using a remove proposal
type, as the self_remove type is permitted in External Commits.
Template:
Value: 0x0008
Name: self_remove
Recommended: Y
External: Y
Path Required: Y
Reference: RFC XXXX
5. Security Considerations
The Security Considerations of MLS apply.
An external recipient of a SelfRemove Proposal cannot verify the
membership_tag. However, an external joiner also has no way to
completely validate a GroupInfo object that it receives. An insider
can prevent an External Join by providing either an invalid GroupInfo
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object or an invalid SelfRemove Proposal. The security properties of
external joins does not change with the addition of this proposal
type.
6. Normative References
[I-D.ietf-mls-protocol]
Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", Work in Progress, Internet-
Draft, draft-ietf-mls-protocol-20, 27 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
protocol-20>.
[RFC2219] Hamilton, M. and R. Wright, "Use of DNS Aliases for
Network Services", BCP 17, RFC 2219, DOI 10.17487/RFC2219,
October 1997, <https://www.rfc-editor.org/info/rfc2219>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Author's Address
Rohan Mahy
Wire
Email: rohan.mahy@wire.com
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