Content Type Advertisement for Message Layer Security (MLS)

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Table of Contents

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.¶

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.¶

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.¶

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¶

  Template:
  Value: 0x0005
  Name: accepted_media_types
  Message(s): This extension may appear in LeafNode objects
  Recommended: Y
  Reference: RFC XXXX

  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.¶

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.¶

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.¶

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;

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