MLS                                                              R. Mahy
Internet-Draft                                                      Wire
Intended status: Informational                             31 March 2022
Expires: 2 October 2022

          Content Negotiation for Message Layer Security (MLS)


   This document describes a default mechanism for the negotiation of
   content inside an MLS group.  It defines two new extensions to the
   MLS (Messaging Layer Security) Protocol to allow for negotiation of
   media types exchanged among members of an MLS group, and a minimal
   framing format.

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   This Internet-Draft will expire on 2 October 2022.

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Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Extension Description . . . . . . . . . . . . . . . . . . . .   3
   4.  Framing of application_data . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  accepted_media_types MLS Extension Type . . . . . . . . .   5
     5.2.  required_media_types GroupContext extension . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.)

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   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

     // 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

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   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-

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

   Example Media Types:


   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

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

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5.  IANA Considerations

   This document proposes registration of two MLS Extension Types.

5.1.  accepted_media_types MLS Extension Type

   The accepted_media_types MLS Extension Type is used inside KeyPackage
   objects.  It contains a mediaTypeList representing all the media
   Types supported by the MLS client publishing the KeyPackage.

     Value: 0x0005
     Name: accepted_media_types

     Message(s): This extension may appear in KeyPackage objects
     Recommended: Y
     Reference: RFC XXXX

   Description: list of media types supported by the MLS client
   advertising the KeyPackage

5.2.  required_media_types GroupContext extension

   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

     Value: 0x0006
     Name: required_media_types

     Message(s): This extension may appear in GroupContext objects
     Recommended: Y
     Reference: RFC XXXX

   Description: list of media types which every member of the MLS group
   is required to support.

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.

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   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

              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-07, 4 October 2021,

              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-13, 7 March 2022,

   [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,

   [RFC2219]  Hamilton, M. and R. Wright, "Use of DNS Aliases for
              Network Services", BCP 17, RFC 2219, DOI 10.17487/RFC2219,
              October 1997, <>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

8.  Informative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,

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   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

Author's Address

   Rohan Mahy

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