Messaging Layer Security R. Barnes
Internet-Draft Cisco
Intended status: Informational R. Mahy
Expires: 24 April 2025 Rohan Mahy Consulting Services
21 October 2024
Efficient Updates to Messaging Layer Security GroupContext Extension
draft-mahy-mls-gce-diff-00
Abstract
The Messaging Layer Security (MLS) protocol allows the members of the
group to agree on a set of GroupContext extensions. MLS includes a
mechanism to do a wholesale replacements of all GroupContext
extensions, but not to modify individual extensions. In this
document, we define a mechanism that allows implementations to add,
update, and remove each element of the GroupContext individually.
This also makes it practical for applications using MLS to exploit
this feature of MLS to ensure that the group members are in agreement
on the state of the application in addition to MLS-related state.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://rohanmahy.github.io/mls-gce-diff/draft-mahy-mls-gce-
diff.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-mahy-mls-gce-diff/.
Discussion of this document takes place on the Messaging Layer
Security Working Group mailing list (mailto:mls@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/mls/. Subscribe
at https://www.ietf.org/mailman/listinfo/mls/.
Source for this draft and an issue tracker can be found at
https://github.com/rohanmahy/mls-gce-diff.
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provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. GroupContextDiff . . . . . . . . . . . . . . . . . . . . . . 4
4. Diff Formats . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Map Diff Format . . . . . . . . . . . . . . . . . . . . . 6
4.2. List Diff Format . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6.1. GroupContextExtensionDiff Proposal . . . . . . . . . . . 7
6.2. Change to the MLS Extension Types registry . . . . . . . 7
6.3. GroupContext Diff Types registry . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Messaging Layer Security (MLS) allows a group of clients to
authenticate each other and establish shared secret state [RFC9420].
One of the primary security benefits of MLS is that the MLS key
schedule confirms that the group agrees on certain metadata, such as
the membership of the group. Members that disagree on the relevant
metadata will arrive at different keys and be unable to communicate.
Applications based on MLS can integrate their state into this
metadata in order to confirm that the members of an MLS group agree
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on application state as well as MLS metadata.
Unfortunately this state can only be modified using the
GroupContextExtensions Proposal, which needs to include the entire
GroupContext. This may include dozens of individual extensions and
may be quite large. MLS clients should be able to modify orthogonal
aspects of the GroupContext in separate Proposals and should not need
to send large amounts of data for a small change.
Here, we define a new MLS proposal type GroupContextExtensionsDiff.
This proposal type allows modification on a per extension basis. It
also allows GroupContext extensions to define simple diff formats
(for example, to add, remove, and update elements in a list or items
in a map).
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses terms from [RFC9420]. Some are similar or may be
confusing, so they are summarized here:
* GroupContext: The state of the group agreed on in a given epoch
* GroupContext extension: A (type, value) tuple included in the
GroupContext
* ExtensionType: A two-byte identifier for the type of a
GroupContext extension
* GroupContextExtensions proposal: A proposal to remove all of the
extensions in the group's GroupContext and replace them with a new
set of GroupContext extensions
Currently, the GroupContextExtensions proposal is the only mechanism
defined for updating a group's GroupContext extensions.
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3. GroupContextDiff
This document defines a new GroupContextExtensionsDiff proposal. It
is not a Safe Extension as defined in Section 2 of
[I-D.ietf-mls-extensions], because it can modify any GroupContext
extension, including those defined in [RFC9420]. The
GroupContextExtensionsDiff proposal requires an UpdatePath, and may
be sent by an authorized external sender.
enum {
remove(0),
add(1),
replace(2),
diff(3),
(255)
} OperationType;
uint8 DiffType;
struct {
/* MUST be a GroupContext extension */
ExtensionType extension_type;
OperationType operation;
select (operation) {
case remove:
struct {};
case add:
opaque extension_data<V>;
case replace:
opaque extension_data<V>;
case diff:
opaque diff_data<V>;
};
} ExtensionDiff;
struct {
ExtensionDiff group_context_extension_diffs<V>;
} GroupContextExtensionsDiff;
The semantics of OperationType are as follows:
* remove means that the GroupContext extension was present, and will
be completely absent from the GroupContext.
* add means that the GroupContext extension was absent, and will be
present with the value provided in extension_data.
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* replace means that the GroupContext extension was present and will
be completely replaced with the new value in extension_data
* diff means that the GroupContext extension will be modified
according to the diff format defined by that extension. (Two
concrete diff formats are defined in Section 4 which extension
designers are free to use or not use.)
A single GroupContextExtensionsDiff proposal can contain changes to
multiple extensions. Likewise a single Commit can include multiple
GroupContextExtensionsDiff proposals, however a single Commit MUST
NOT include multiple GroupContextExtensionsDiff proposals which refer
to the same ExtensionType. A GroupContextExtensions proposal MUST
NOT appear in a commit with any GroupContextExtensionsDiff proposal.
The motivation for allowing multiple extensions in a single
GroupContextExtensionsDiff proposal is to allow a client to modify
multiple extensions in a single "transaction". A common example of
this is to add a GroupContext extension while also adding that
extension to the required_capabilities GroupContext extension.
A proposal which removes a GroupContext extension that is present in
the required_capabilities list is invalid. Adding a required
capability that is not supported by all group members is already
forbidden by [RFC9420].
*TODO*: Fix the below
There's nothing in MLS that says that there can only be one
extension per ExtensionType in the GroupContext. If there were
duplicates, this would be ambiguous. Could fix this with
something dumb like (a) defining a noop OperationType and (b)
saying that the n-th ExtensionDiff of a given type affects the
n-th entry. That has the benefit of degrading to the normal thing
in the sane case.
4. Diff Formats
The following two subsections define two Diff Formats for very common
data structures.
Both of the Diff Formats defined below use the following struct for
an opaque byte string.
struct {
opaque element<V>;
} OpaqueElement;
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4.1. Map Diff Format
This Diff Format logically represents a map or dictionary that is not
allowed to have duplicate map keys.
struct {
opaque elementName<V>;
opaque elementValue<V>;
} OpaqueMapElement;
struct {
OpaqueElement removedKeys<V>;
OpaqueMapElement newOrUpdatedElements<V>;
} MapDiff;
A diff using this format first removes all the keys in removedKeys
and than replaces or adds the elements in newOrUpdatedElements.
Removing a non-existant map key renders the diff and its proposal
invalid.
4.2. List Diff Format
This Diff Format logically represents changes to an ordered list.
struct {
uint32 index;
opaque element<V>;
} ElementWithIndex;
struct {
ElementWithIndex replacedElements<V>;
uint32 removedIndices<V>;
ElementWithIndex insertedElements<V>;
OpaqueElement appendedEntries<V>;
} ListDiff;
A diff using this format first replaces all the elements (index-by-
index) in replacedElements, then the removes the elements in
removedIndices according to the then order of the array, then inserts
all the elements in insertedElements according to the then order of
the array, then finally appends the appendedEntries (in order). All
indices are zero-based.
For the avoidance of doubt, two insertedElements with the same index
will cause the second inserted element to be _before_ the first
inserted element.
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Attempting to insert or replace at a non-existing index renders the
diff and its proposal invalid.
5. Security Considerations
The mechanism defined in this document provides strong authenticity,
integrity, and change control properties to the state information it
manages. No unauthorized parties can make changes to the
GroupContext, and the identity of the group member making each change
is authenticated.
The data synchronized via this mechanism may or may not be
confidential to the group, depending on whether the
GroupContextExtensionsDiff proposal is sent as an MLS PublicMessage
or PrivateMessage. As with application data, applications should
generally prefer the use of Private Message. There may be cases,
however, where it is useful for intermediaries to inspect application
state updates, e.g., to enforce policy.
6. IANA Considerations
6.1. GroupContextExtensionDiff Proposal
The gce_diff MLS Proposal Type is used to update Group Context
Extensions in a group more efficiently than using a
group_context_extensions proposal type. The gce_diff type is
updating rather than replacing the extensions.
* Value: 0x000d
* Name: gce_diff
* Recommended: Y
* External: Y
* Path Required: Y
6.2. Change to the MLS Extension Types registry
This document adds a "Diff" column to the MLS Extension Types
registry. In the registration template this will be referred to as
"Diff Type". The value value can be "-" indicating no Diff Types are
supported, or a value present in the GroupContext Diff Types
registry.
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6.3. GroupContext Diff Types registry
The "MLS GroupContext Diff Types" registry lists identifiers for
types of difference algorithms to be applied to GroupContext
extension data. The diff type field is one byte wide, so valid
values are in the range 0x00 to 0xFF.
Template:
* Value: The numeric value of the diff type
* Name: The name of the diff type
* Recommended: Same as in Section 17.1 of [RFC9420]
* Reference: The document where this extensions is defined
Initial contents:
+=======+==========+===+==========+
| Value | Name | R | Ref |
+=======+==========+===+==========+
| 0x00 | reserved | - | RFC XXXX |
+-------+----------+---+----------+
| 0x01 | list | Y | RFC XXXX |
+-------+----------+---+----------+
| 0x02 | map | Y | RFC XXXX |
+-------+----------+---+----------+
Table 1
7. Normative References
[I-D.ietf-mls-extensions]
Robert, R., "The Messaging Layer Security (MLS)
Extensions", Work in Progress, Internet-Draft, draft-ietf-
mls-extensions-05, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
extensions-05>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
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[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:* Acknowledgements.
Authors' Addresses
Richard Barnes
Cisco
Email: rlb@ipv.sx
Rohan Mahy
Rohan Mahy Consulting Services
Email: rohan.ietf@gmail.com
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