Internet DRAFT - draft-mahy-mls-content-adv
draft-mahy-mls-content-adv
MLS R. Mahy
Internet-Draft Wire
Intended status: Informational 23 October 2022
Expires: 26 April 2023
Content Type Advertisement for Message Layer Security (MLS)
draft-mahy-mls-content-adv-00
Abstract
This document describes a default mechanism for the advertisement of
content types and content type capabilities inside the Message Layer
Security (MLS). It defines two new extensions and a minimal framing
format.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 26 April 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Extension Description . . . . . . . . . . . . . . . . . . . . 3
4. Framing of application_data . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5.1. accepted_media_types MLS Extension Type . . . . . . . . . 5
5.2. required_media_types MLS Extension Type . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
8. Informative References . . . . . . . . . . . . . . . . . . . 7
Appendix A. TLS Presentation Language multipart container
format . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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, and RequiredCapabilities have the
same meanings as in the MLS protocol [I-D.ietf-mls-protocol].
2. Introduction
MLS is a group key establishment protocol which has several
applications. As described in the MLS architecture document
[I-D.ietf-mls-architecture], applications need to define specific
behavior of the MLS Distribution Service, the MLS Authentication
Service, and the format and negotiation of application data. This
document describes a default content advertisement mechanism
recommended by the MLS architecture specification. (The MLS protocol
specification does not define or prescribe any format for the
encrypted application_data encoded by MLS.)
MLS includes a framework for advertising extension capabilities in
LeafNodes which are used to represent each member in an MLS group and
also included in KeyPackages. There is also an existing mechanism in
which an MLS group specifies which MLS extensions are mandatory
within the group. When the membership of a group changes, or when
the policy of the group changes, it is responsibility of the
committer to insure that the membership and policies are compatible.
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This document describes two extensions to MLS. The first allows MLS
clients to advertise their support for specific formats inside MLS
application_data. These are expressed using the extensive IANA Media
Types registry (formerly called MIME Types). The
accepted_media_types LeafNode extension lists the formats a client
supports inside application_data. The second, the
required_media_types GroupContext extension specifies which media
types need to be supported by all members of a particular MLS group.
These allow clients to confirm that all members of a group can
communicate. Finally, this document defines a minimal framing format
so MLS clients can signal which media type is being sent when
multiple formats are permitted in the same group. As clients are
upgraded to support new formats they can use these extensions to
detect when all members support a new or more efficient encoding, or
select the relevant format or formats to send.
Note that the usage of IANA media types in general does not imply the
usage of MIME Headers [RFC2045] for framing. Vendor-specific media
subtypes starting with vnd. can be registered with IANA without
standards action as described in [RFC6838]. Implementations which
wish to send multiple formats in a single application message, may be
interested in the multipart/alternative media type defined in
[RFC2046] or may use or define another type with similar semantics
(see Appendix A for a container format defined using TLS Presentation
Language syntax [RFC8446]).
3. Extension Description
This document specifies two MLS extensions of type MediaTypeList:
accepted_media_types, and required_media_types. The syntax is
described using the TLS Presentation Language [RFC8446].
MediaType is a TLS encoding of a single IANA media type (including
top-level type and subtype) and any of its parameters. Even if the
parameter_value would have required formatting as a quoted-string in
a text encoding, only the contents inside the quoted-string are
included in parameter_value. MediaTypeList is an ordered list of
MediaType objects.
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struct {
opaque parameter_name<V>;
/* Note: parameter_value never includes the quotation marks of an
* RFC 2045 quoted-string */
opaque parameter_value<V>;
} Parameter;
struct {
/* media_type is an IANA top-level media type, a "/" character,
* and the IANA media subtype */
opaque media_type<V>;
/* a list of zero or more parameters defined for the subtype */
Parameter parameters<V>;
} MediaType;
struct {
MediaType media_types<V>;
} MediaTypeList;
MediaTypeList accepted_media_types;
MediaTypeList required_media_types;
Example IANA media types with optional parameters:
image/png
text/plain ;charset="UTF-8"
application/json
application/vnd.example.msgbus+cbor
For the example media type for text/plain, the media_type field would
be text/plain, parameters would contain a single Parameter with a
parameter_name of charset and a parameter_value of UTF-8.
An MLS client which implements this specification SHOULD include the
accepted_media_types extension in its LeafNodes, listing all the
media types it can receive. As with all other extensions, the client
also includes accepted_media_types in its capabilities field in its
LeafNodes (including LeafNodes inside its KeyPackages).
When creating a new MLS group for an application using this
specification, the group MAY include a required_media_type extension
in the GroupContext Extensions. As with all other extensions, the
client also includes required_media_types in its capabilities field
in its LeafNodes (including LeafNodes inside its KeyPackages). When
used in a group, the client MUST include the required_media_types and
accepted_media_types extensions in the list of extensions in
RequiredCapabilities.
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MLS clients SHOULD NOT add an MLS client to an MLS group with
required_media_types unless the MLS client advertises it can support
all of the required MediaTypes. As an exception, a client could be
preconfigured to know that certain clients support the requried
types. Likewise, an MLS client is already forbidden from issuing or
committing a GroupContextExtensions Proposal which introduces
required extensions which are not supported by all members in the
resulting epoch.
4. Framing of application_data
When an MLS group contains the required_media_types GroupContext
extension, the application_data sent in that group is interpreted as
ApplicationFraming as defined below:
struct {
MediaType media_type;
opaque<V> application_content;
} ApplicationFraming;
The media_type MAY be zero length, in which case, the media type of
the application_content is interpreted as the first MediaType
specified in required_media_types.
5. IANA Considerations
This document proposes registration of two MLS Extension Types.
RFC EDITOR: Please replace XXXX throughout with the RFC number
assigned to this document
5.1. accepted_media_types MLS Extension Type
The accepted_media_types MLS Extension Type is used inside LeafNode
objects. It contains a MediaTypeList representing all the media
types supported by the MLS client referred to by the LeafNode.
Template:
Value: 0x0005
Name: accepted_media_types
Message(s): This extension may appear in LeafNode objects
Recommended: Y
Reference: RFC XXXX
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5.2. required_media_types MLS Extension Type
The required_media_types MLS Extension Type is used inside
GroupContext objects. It contains a MediaTypeList representing the
media types which are mandatory for all MLS members of the group to
support.
Template:
Value: 0x0006
Name: required_media_types
Message(s): This extension may appear in GroupContext objects
Recommended: Y
Reference: RFC XXXX
6. Security Considerations
The Security Considerations of MLS apply.
Use of the extensions in this document could leak some private
information both in KeyPackages and inside an MLS group. They could
be used to infer a specific implementation, platform, or even
version. Clients should consider carefully the implications in their
environment of making a list of acceptable media types available.
A client which can take over group administration could prevent
members from joining or sending messages in an established group, by
requiring a list of required media types which the attacker knows is
unsupported. This attack is not especially helpful, as taking over
group administration can have more disruptive effects.
7. Normative References
[I-D.ietf-mls-architecture]
Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., Kwon,
A., and A. Duric, "The Messaging Layer Security (MLS)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-mls-architecture-09, 19 August 2022,
<https://www.ietf.org/archive/id/draft-ietf-mls-
architecture-09.txt>.
[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-16, 11 July 2022,
<https://www.ietf.org/archive/id/draft-ietf-mls-protocol-
16.txt>.
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[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>.
8. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
Appendix A. TLS Presentation Language multipart container format
In a heterogenous group of MLS clients, it is often desirable to send
more than one media type as alternatives, such that MLS clients have
a choice of which media type to render. For example, imagine an MLS
group containing a set of clients which support a common video format
and a subset which only support animated GIFs. The sender could send
a multipart/alternative [RFC2046] container containing both media
types. Every client in the group could render something resembling
the media sent.
Likewise it is often desirable to send more than one media type
intended to be rendered together as in (for example a rich text
document with embedded images), which can be represented using the
multipart/mixed [RFC2046] media type.
Some implementors complain that the multipart types are unnatural to
use inside a binary protocol which requires explicit lengths.
Concretely, an implementation has to scan through the entire content
to construct a boundary token which is not contained in the content.
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The author does not care one whit about the specific syntax used, but
presents a multipart container format using the TLS presentation
language syntax.
Note that there is a minor semantic difference between multipart/
alternative and the proposal below. In multipart/alternative, the
parts are presented in preference order by the sender. The receiver
is support to render the first type which it supports. This
container includes an ordering flag. As well, even if the flag is
ordered, it is up to the IETF community to decide if it is acceptable
for the receiver to choose its "best" format to render among an
ordered preference list provided by the sender, or if the receiver
must respect the ordered preference of the sender.
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struct {
/* a valid "Language-tag" as defined in RFC 5646 */
opaque language_tag<1..52>;
} LanguageTag;
struct {
ContentType content_type;
LanguageTag content_languages<V>;
opaque<V> body;
} Part;
enum {
reserved(0),
multipart_container_v1(1),
(255)
} MultipartVersion;
enum {
reserved(0),
mixed(1),
alternative(2),
(255)
} MultipartSemantics;
enum {
reserved(0),
unordered(1),
ordered(2),
(255)
} MultipartOrdering;
struct {
uint8 container_version;
uint16 number_of_parts;
MultipartSemantics semantics;
MultipartOrdering ordering;
Part parts<V>;
} MultipartContainer;
Author's Address
Rohan Mahy
Wire
Email: rohan.mahy@wire.com
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