Content Negotiation 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, and KeyPackage 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 negotiation 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.)¶

This document describes two extensions to MLS which allow 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 KeyPackage Extension lists the formats a client supports inside application_data, while the required_media_types GroupContext Extension specifies which media types are required for 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 best 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.¶

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 an ASCII string encoded as a TLS vector type containing a single IANA Media Type (including the top-level type and subtype) and any of its parameters. The formal internal structure is defined later in this section. MediaTypeList is an ordered list of MediaType objects.¶

  // Text representation of a single IANA-registered Media Type.
  MediaType media_type<V>;

  struct {
      MediaType media_types<V>;
  } MediaTypeList;

  MediaTypeList accepted_media_types;
  MediaTypeList required_media_types;

The internal format of the MediaType is described in the Augmented Backus-Naur Form (ABNF) [RFC5234] grammar below. The type is the IANA top-level media type (ex: application), subtype is the IANA media subtype, and parameter follows the definition in [RFC2045] Section 5.1. Whitespace inside an MLS MediaType is PROHIBITED. The type, subtype, and parameter attribute names are all case-insensitive.¶

  MediaType := type "/" subtype *(";" parameter)

Example Media Types:¶

  image/png
  text/plain;charset="UTF-8"
  application/json
  application/vnd.example.msgbus+cbor

An MLS client which implements this specification SHOULD include the accepted_media_types extension in its KeyPackages, listing all the media types it can receive.¶

When creating a new MLS group for an application using this specification, the group includes a required_media_type extension in the GroupInfo Extensions. When used in a group, the client MUST include the required_media_types extension 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 Media Types. As an exception, a client could be preconfigured to know that certain clients support the mandatory types.¶

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 the first MediaType specified in required_media_types.¶

5. IANA Considerations

This document proposes registration of two MLS Extension Types.¶

  Template:
  Value: 0x0005
  Name: accepted_media_types

  Message(s): This extension may appear in KeyPackage 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.¶

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