Semi-Private Messages in the Messaging Layer Security (MLS) Protocol
draft-mahy-mls-semiprivatemessage-00

MLS WG                                                           R. Mahy
Internet-Draft                            Rohan Mahy Consulting Services
Intended status: Informational                               8 July 2024
Expires: 9 January 2025

  Semi-Private Messages in the Messaging Layer Security (MLS) Protocol
                  draft-mahy-mls-semiprivatemessage-00

Abstract

   This document defines a SemiPrivateMessage for the Messaging Layer
   Security (MLS) protocol.  It allows members to share otherwise
   private commits and proposals with a designated list of external
   receivers rather than send these handshakes in a PublicMessage.

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-semiprivate/draft-mahy-
   semiprivatemessage.html.  Status information for this document may be
   found at https://datatracker.ietf.org/doc/draft-mahy-mls-
   semiprivatemessage/.

   Discussion of this document takes place on the MLS WG 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-semiprivatemessage.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 9 January 2025.

Copyright Notice

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   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Syntax and Usage  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  SemiPrivateMessage Wire Format  . . . . . . . . . . . . .   6
     5.2.  External Receivers Extension Type . . . . . . . . . . . .   6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   This document defines two extensions of MLS [RFC9420].  The first is
   the SemiPrivateMessage wire format Safe Extension (see
   Section 2.1.7.1 of [I-D.ietf-mls-extensions], which allows an
   otherwise PrivateMessage to be shared with a predefined list of
   external receivers.  It is restricted for use only with commits or
   proposals.  The second is the external_receivers GroupContext
   extension that contains the list of external receivers and allows
   members to agree on that list.

   SemiPrivateMessages are expected to be useful in federated
   environments where messages routinely cross multiple administrative
   domains, but the MLS Distribution Service needs to see the content of
   commits and proposals where group members would otherwise send
   handshakes using PublicMessage.

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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 terminology extensively from MLS [RFC9420] and the
   Safe Extensions framework, defined in Section 2 of
   [I-D.ietf-mls-extensions].

3.  Syntax and Usage

   The external_receivers GroupContext extension is used for all members
   to agree on the list of external receivers in the current epoch.  Its
   construction mirrors the syntax of the external_senders extension in
   [RFC9420].

   struct {
     HPKEPublicKey external_receiver_public_key;
     Credential credential;
   } ExternalReceiver;

   The SemiPrivateMessage wire format Safe Extension also has an
   extension type which is carried in the GroupContext to indicate use
   of the wire format in a group (and in the Capabilities of LeafNodes).
   SemiPrivateMessage substantially reuses the construction of
   PrivateMessage, but like a Welcome message also contains information
   (key_and_nonce) necessary to decrypt the ciphertext and
   encrypted_sender_data, encrypted once for each external receiver in
   the external_receivers extension.

   The snippet below shows the syntax and encryption and decryption
   construction of key_and_nonce into encrypted_key_and_nonce

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   struct {
     opaque key<V>;
     opaque nonce<V>;
   } MessageKeyAndNonce;

   MessageKeyAndNonce key_and_nonce;

   encrypted_key_and_nonce = EncryptWithLabel(
     external_receiver_pulic_key,
     "SemiPrivateMessageReceiver",
     private_message, key_and_nonce)

   key_and_nonce = DecryptWithLabel(
     external_receiver_private_key,
     "SemiPrivateMessageReceiver",
     private_message,
     encrypted_key_and_nonce.kem_output,
     encrypted_key_and_nonce.ciphertext)

   The KeyForExternalReceiver structure contains a hash of the
   ExternalReceiver as a reference and the encrypted_key_and_nonce.

   /* Using the hash function of the group ciphersuite */
   ExternalReceiverRef = hash(ExternalReceiver)

   struct {
     ExternalReceiverRef external_receiver_ref;
     HPKECiphertext encrypted_key_and_nonce;
   } KeyForExternalReceiver;

   The SemiPrivateMessage and SemiPrivateContentAAD structs mirror the
   PrivateMessage and PrivateContentAAD structs and add the
   keys_for_external_receivers list.  The SemiPrivateMessageContent
   struct is the same as PrivateMessageContent except for the addition
   of keys_for_external_receivers, and that application messages are not
   included.

   Encryption of the ciphertext and encrypted_sender_data proceed in the
   same way for SemiPrivateMessage as for PrivateMessage.  Finally, the
   SemiPrivateMessage is wrapped in an ExtensionContent struct.

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   struct {
       opaque group_id<V>;
       uint64 epoch;
       ContentType content_type;
       opaque authenticated_data<V>;
       KeyForExternalReceiver keys_for_external_receivers<V>;
       opaque encrypted_sender_data<V>;
       opaque ciphertext<V>;
   } SemiPrivateMessage;

   struct {
       select (PrivateMessage.content_type) {
           case proposal:
             Proposal proposal;
           case commit:
             Commit commit;
       };
       KeyForExternalReceiver keys_for_external_receivers<V>;
       FramedContentAuthData auth;
       opaque padding[length_of_padding];
   } SemiPrivateMessageContent;

   struct {
       opaque group_id<V>;
       uint64 epoch;
       ContentType content_type;
       opaque authenticated_data<V>;
       KeyForExternalReceiver keys_for_external_receivers<V>;
   } SemiPrivateContentAAD;

   /* IANA-registered value for semi_private_message */
   extension_type = TBD2
   SemiPrivateMessage extension_data;

4.  Security Considerations

   These two extensions provide a privacy improvement over sending
   handshake messages using PublicMessage.  The handshake is shared with
   a specific list of receivers, and that list is visible as part of the
   GroupContext.

   TODO More Security.

5.  IANA Considerations

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5.1.  SemiPrivateMessage Wire Format

   The semi_private_message MLS Extension Type is used to signal support
   for the SemiPrivateMessage Wire Format (a Safe Extension).

   *  Value: TBD1 (to be assigned by IANA)

   *  Name: semi_private_message

   *  Recommended: Y

   *  Reference: RFC XXXX

5.2.  External Receivers Extension Type

   The external_receivers extension contains a list of external
   receivers targeted in a SemiPrivateMessage.

   *  Value: TBD2 (to be assigned by IANA)

   *  Name: external_receivers

   *  Message(s): GC.  This extension may appear in GroupContext
      objects.

   *  Recommended: Y

   *  Reference: RFC XXXX

6.  Normative References

   [I-D.ietf-mls-extensions]
              Robert, R., "The Messaging Layer Security (MLS)
              Extensions", Work in Progress, Internet-Draft, draft-ietf-
              mls-extensions-04, 24 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
              extensions-04>.

   [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 acknowledge.

Author's Address

   Rohan Mahy
   Rohan Mahy Consulting Services
   Email: rohan.ietf@gmail.com

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