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More Instant Messaging Interoperability (MIMI) message content
draft-ietf-mimi-content-04

Document Type Active Internet-Draft (mimi WG)
Author Rohan Mahy
Last updated 2024-06-10
Replaces draft-mahy-mimi-content
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Mar 2025
Forward specification for messaging content format to IESG
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draft-ietf-mimi-content-04
MIMI                                                             R. Mahy
Internet-Draft                            Rohan Mahy Consulting Services
Intended status: Informational                              10 June 2024
Expires: 12 December 2024

     More Instant Messaging Interoperability (MIMI) message content
                       draft-ietf-mimi-content-04

Abstract

   This document describes content semantics common in Instant Messaging
   (IM) systems and describes a profile suitable for instant messaging
   interoperability of messages end-to-end encrypted inside the MLS
   (Message Layer Security) Protocol.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 12 December 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Binary encoding . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Naming schemes  . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Message ID and Accepted Timestamp . . . . . . . . . . . .   6
     3.4.  Message Container . . . . . . . . . . . . . . . . . . . .   6
     3.5.  Message Status Report . . . . . . . . . . . . . . . . . .   6
   4.  MIMI Content Container Message Semantics  . . . . . . . . . .   6
     4.1.  Message Behavior Fields . . . . . . . . . . . . . . . . .   7
     4.2.  Message Ordering  . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Extension Fields  . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Message Bodies  . . . . . . . . . . . . . . . . . . . . .   9
     4.5.  External content  . . . . . . . . . . . . . . . . . . . .  12
     4.6.  Derived Data Values . . . . . . . . . . . . . . . . . . .  13
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Original Message  . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Reply . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.3.  Reaction  . . . . . . . . . . . . . . . . . . . . . . . .  18
     5.4.  Mentions  . . . . . . . . . . . . . . . . . . . . . . . .  20
     5.5.  Edit  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.6.  Delete  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     5.7.  Unlike  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     5.8.  Expiring  . . . . . . . . . . . . . . . . . . . . . . . .  28
     5.9.  Attachments . . . . . . . . . . . . . . . . . . . . . . .  29
     5.10. Conferencing  . . . . . . . . . . . . . . . . . . . . . .  32
     5.11. Topics / Threading  . . . . . . . . . . . . . . . . . . .  34
     5.12. Delivery Reporting and Read Receipts  . . . . . . . . . .  34
       5.12.1.  Delivery Report Example  . . . . . . . . . . . . . .  35
   6.  Support for Specific Media Types  . . . . . . . . . . . . . .  37
     6.1.  MIMI Required and Recommended media types . . . . . . . .  37
     6.2.  Use of proprietary media types  . . . . . . . . . . . . .  37
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
     7.1.  MIME subtype registration of application/mimi-content . .  37
     7.2.  MIME subtype registration of application/
           mimi-message-status . . . . . . . . . . . . . . . . . . .  38
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  39
     8.1.  General handling  . . . . . . . . . . . . . . . . . . . .  39
     8.2.  Validation of timestamp . . . . . . . . . . . . . . . . .  41
     8.3.  Alternate content rendering . . . . . . . . . . . . . . .  41
     8.4.  Link and Mention handling . . . . . . . . . . . . . . . .  41
     8.5.  Delivery and Read Receipts  . . . . . . . . . . . . . . .  42
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  43
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  44
   Appendix A.  CDDL Schemas . . . . . . . . . . . . . . . . . . . .  46

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     A.1.  Complete Message Format Schema  . . . . . . . . . . . . .  46
     A.2.  Implied Message Fields  . . . . . . . . . . . . . . . . .  48
     A.3.  Delivery Report Format  . . . . . . . . . . . . . . . . .  48
   Appendix B.  Multipart examples . . . . . . . . . . . . . . . . .  49
     B.1.  Proprietary and Common formats sent as alternatives . . .  49
     B.2.  Mulitple Reactions Example  . . . . . . . . . . . . . . .  50
     B.3.  Complicated Nested Example  . . . . . . . . . . . . . . .  51
   Appendix C.  Changelog  . . . . . . . . . . . . . . . . . . . . .  54
     C.1.  Changes between draft-mahy-mimi-content-01 and
           draft-mahy-mimi-content-02  . . . . . . . . . . . . . . .  54
     C.2.  Changes between draft-mahy-mimi-content-02 and
           draft-ietf-mimi-content-00  . . . . . . . . . . . . . . .  54
     C.3.  Changes between draft-ietf-mimi-content-00 and
           draft-ietf-mimi-content-01  . . . . . . . . . . . . . . .  55
     C.4.  Changes between draft-ietf-mimi-content-01 and
           draft-ietf-mimi-content-02  . . . . . . . . . . . . . . .  55
     C.5.  Changes between draft-ietf-mimi-content-02 and
           draft-ietf-mimi-content-03  . . . . . . . . . . . . . . .  55
     C.6.  Changes between draft-ietf-mimi-content-03 and
           draft-ietf-mimi-content-04  . . . . . . . . . . . . . . .  55
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  55

1.  Terminology

   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.

   The terms MLS client, MLS group, and KeyPackage have the same
   meanings as in the MLS protocol [RFC9420].  Other relevant
   terminology may be found in [I-D.ietf-mimi-arch] and
   [I-D.ralston-mimi-terminology].

2.  Introduction

   RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH.  The source for
   this draft is maintained in GitHub.  Suggested changes should be
   submitted as pull requests at https://github.com/ietf-wg-mimi/draft-
   ietf-mimi-content (https://github.com/ietf-wg-mimi/draft-ietf-mimi-
   content).  Editorial changes can be managed in GitHub, but any
   substantive change should be discussed on the MIMI mailing list
   (mimi@ietf.org).

   MLS [RFC9420] is a group key establishment protocol motivated by the
   desire for group chat with efficient end-to-end encryption.  While
   one of the motivations of MLS is interoperable standards-based secure

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   messaging, the MLS protocol does not define or prescribe any format
   for the encrypted "application messages" encoded by MLS.  The
   development of MLS was strongly motivated by the needs of a number of
   Instant Messaging (IM) systems, which encrypt messages end-to-end
   using variations of the Double Ratchet protocol [DoubleRatchet].

   End-to-end encrypted instant messaging was also a motivator for the
   Common Protocol for Instant Messaging (CPIM) [RFC3862], however the
   model used at the time assumed standalone encryption of each message
   using a protocol such as S/MIME [RFC8551] or PGP [RFC3156] to
   interoperate between IM protocols such as SIP [RFC3261] and XMPP
   [RFC6120].  For a variety of practical reasons, interoperable end-to-
   end encryption between IM systems was never deployed commercially.

   There are now several instant messaging vendors implementing MLS, and
   the MIMI (More Instant Messaging Interoperability) Working Group is
   chartered to standardize an extensible interoperable messaging format
   for common features to be conveyed "inside" MLS application messages.

   This document assumes that MLS clients advertise media types they
   support and can determine what media types are required to join a
   specific MLS group using the content advertisement extensions in
   Section 2.3 of [I-D.ietf-mls-extensions].  It allows implementations
   to define MLS groups with different media type requirements and
   allows MLS clients to send extended or proprietary messages that
   would be interpreted by some members of the group while assuring that
   an interoperable end-to-end encrypted baseline is available to all
   members, even when the group spans multiple systems or vendors.

   Below is a list of some features commonly found in IM group chat
   systems:

   *  plain text and rich text messaging
   *  mentions
   *  replies
   *  reactions
   *  edit or delete previously sent messages
   *  expiring messages
   *  delivery notifications
   *  read receipts
   *  shared files/audio/videos
   *  calling / conferencing
   *  message threading

3.  Overview

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3.1.  Binary encoding

   The MIMI Content format is encoded in Concise Binary Object
   Representation (CBOR) [RFC8949].  The Working Group chose a binary
   format in part because:

   *  we do not want to scan body parts to check for boundary marker
      collisions.  This rules out using multipart MIME types.
   *  we do not want to base64 encode body parts with binary media types
      (ex: images).  This rules out using JSON to carry the binary data.

   All examples start with an instance document annotated in the CBOR
   Extended Diagnostic Notation (described in [Appendix G of @!RFC8610]
   and more rigorously specified in [I-D.ietf-cbor-edn-literals]), and
   then include a hex dump of the CBOR data in the pretty printed format
   popularized by the CBOR playground website (https://cbor.me
   (https://cbor.me)) with some minor whitespace and comment
   reformatting.  Finally a message ID for the message is included for
   most messages.

   All the instance documents validate using the CDDL schemas in
   Appendix B and are included in the examples directory in the github
   repo for this document.

3.2.  Naming schemes

   IM systems have a number of types of identifiers.  These are
   described in detail in [I-D.mahy-mimi-identity].  A few of these used
   in this document are:

   *  handle identifier (external, friendly representation).  This is
      the type of identifier described later as the senderUserUrl in the
      examples, which is analogous to the From header in email.
   *  client/device identifier (internal representation).  This is the
      type of identifier described as the senderClientUrl in the
      examples.
   *  group or room or conversation or channel name (either internal or
      external representation).  This is the type of identifier
      described as the MLS group URL in the examples.

   This proposal relies on URIs for naming and identifiers.  All the
   example use the im: URI scheme (defined in [RFC3862]), but any
   instant messaging scheme could be used.

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3.3.  Message ID and Accepted Timestamp

   Every MIMI content message has a message ID which is calculated from
   the hash of the ciphertext of the message.  When the content is end-
   to-encrypted with MLS for a specific MLS group, the cipher suite for
   the group specifies a hash algorithm.  The message ID is the first 32
   octets of the hash of the MLSMessage struct using that hash
   algorithm.

   As described in the the MIMI architecture [I-D.ietf-mimi-arch], one
   provider, called the hub, is responsible for ordering messages.  The
   hub is also responsible for recording the time that any application
   message is accepted, and conveying it to any "follower" providers
   which receive messages from the group.  It is represented as the
   whole number of milliseconds since the start of the UNIX epoch
   (01-Jan-1970 00:00:00 UTC).  To the extent that the accepted
   timestamp is available to a MIMI client, the client can use it for
   fine grain sorting of messages into a consistent order.

3.4.  Message Container

   Most common instant messaging features are expressed as individual
   messages.  A plain or rich text message is obviously a message, but a
   reaction (ex: like), a reply, editing a previous message, deleting an
   earlier message, and read receipts are all typically modeled as
   another message with different properties.

   This document describes the semantics of a message container, which
   can represent most of these previously mentioned message types.  The
   container typically carries one or more body parts with the actual
   message content (for example, an emoji used in a reaction, a plain
   text or rich text message or reply, a link, or an inline image).

3.5.  Message Status Report

   This document also describes the semantics of a status report of
   other messages.  Because some messaging systems deliver messages in
   batches and allow a user to mark several messages read at a time, the
   report format allows a single report to convey the read/delivered
   status of multiple messages (by message ID) within the same MLS group
   at a time.

4.  MIMI Content Container Message Semantics

   Each MIMI Content message is a container format with two categories
   of information:

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   *  the message behavior fields (which can have default or empty
      values), and
   *  the body part(s) and associated parameters

   The object fields in the structure defined below are numbered in
   curly braces for reference in the text.

   The subsections that follow contain snippets of Concise Data
   Definition Language (CDDL) [RFC8610] schemas for the MIMI Content
   Container.  The complete collected CDDL schema for MIMI Content
   Container is available in Appendix A.1.

4.1.  Message Behavior Fields

   mimiContent = [
     replaces: null / MessageId,       ; {1}
     topicId: bstr,                    ; {2}
     expires: uint .size 4,            ; {3}
     inReplyTo: null / InReplyTo,      ; {4}
     lastSeen: [* MessageId],          ; {5}
     extensions: {* name => value },   ; {6}
     nestedPart: NestedPart            ; {7}
   ]

   MessageId = bstr .size 32          ; MessageId is derived from SHA256 hash

   The replaces {1} data field indicates that the current message is a
   replacement or update to a previous message whose message ID is in
   the replaces data field.  It is used to edit previously-sent
   messages, delete previously-sent messages, and adjust reactions to
   messages to which the client previously reacted.  If the replaces
   field is absent, the receiver assumes that the current message has
   not identified any special relationship with another previous
   message.

   The topicId {2} data field indicates that the current message is part
   of a logical grouping of messages which all share the same value in
   the topicId data field.  If the topicId is zero length, there is no
   such grouping.

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   The expires {3} data field is a hint from the sender to the receiver
   that the message should be locally deleted and disregarded at a
   specific timestamp in the future.  Indicate a message with no
   specific expiration time with the value zero.  The data field is an
   unsigned integer number of seconds after the start of the UNIX epoch.
   Using an 32-bit unsigned integer allows expiration dates until the
   year 2106.  Note that specifying an expiration time provides no
   assurance that the client actually honors or can honor the expiration
   time, nor that the end user didn't otherwise save the expiring
   message (ex: via a screenshot).

   The inReplyTo {4} data field indicates that the current message is a
   related continuation of another message sent in the same MLS group.
   If present, it contains the message ID of the referenced message and
   the SHA-256 hash [RFC6234] of its MimiContent structure.  Otherwise,
   the receiver assumes that the current message has not identified any
   special relationship with another previous message.

   The inReplyTo hash is a message digest from the IANA Named
   Information Hash Algorithm Registry
   (https://www.iana.org/assignments/named-information/named-
   information.xhtml#hash-alg), used to make sure that a MIMI message
   cannot refer to a sequence of referred messages which refers back to
   itself.  When replying, a client MUST NOT knowingly create a sequence
   of replies which create a loop.

   When receiving a message, the client verifies that the hash is
   correct.  Next it checks if the referenced message is itself a Reply.
   If so, it continues following the referenced messages, checking that
   neither the messageId nor the hash of any of referenced messages
   indicates a Reply which "loops" back to a message later in the
   inReplyTo chain.

   InReplyTo = [
     message: MessageId,
     hashAlg: uint .size 8,   ; a value from the IANA Named Information Hash
                              ; Algorithm Registry. Default: SHA-256 = 1
     hash: bstr
   ]

   Note that a inReplyTo always references a specific message ID.  Even
   if the original message was edited several times, a reply always
   refers to a specific version of that message, and SHOULD refer to the
   most current version at the time the reply is sent.

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4.2.  Message Ordering

   The lastSeen {5} data field indicates the latest message the sender
   was aware of in the group.  It is a list of message ids.

   If the sender recently joined the group and has not yet seen any
   messages, the list is empty.

   If the sender identifies a single message as unambiguously the latest
   message in the group, the lastSeen list contains a single message id
   from that message.

   Imagine however that two users (Bob and Cathy) see a message from
   Alice offering free Hawaiian pizza, and reply at the same time.  Bob
   and Cathy both send messages with their lastSeen including a single
   message id (Alice's) message about pizza.  Their messages don't need
   to be replies or reactions.  Bob might just send a message saying he
   doesn't like pineapple on pizza.  Now Doug receives all these
   messages and replies as well.  Doug's message contains a lastSeen
   including the message id list of both Bob's and Cathy's replies,
   effectively "merging" the order of messages.

   The next message after Doug's message contains a lastSeen containing
   only the message id of Doug's message.

4.3.  Extension Fields

   In order to add additional functionality to MIMI, senders can include
   extension fields in the message format {6}. Each extension has a
   name, which contains between 1 and 255 octets of UTF-8, and an opaque
   value.  The value of each extension can be between 0 and 65535
   octets.  The message content extensions field MUST NOT include more
   than one extension field with the same name.

   name = tstr .size (1..255)
   value = bstr .size (0..4095)

4.4.  Message Bodies

   Every MIMI content message has a body {7} which can have multiple,
   possibly nested parts.  A body with zero parts is permitted when
   deleting or unliking.  External body parts Section 4.5 are also
   supported.  When there is a single (inline) part or a (single)
   externally reference part, its IANA media type, subtype, and
   parameters are included in the contentType field {8}.

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   NestedPart = [
     disposition: baseDispos / $extDispos / unknownDispos,  ; {10}
     language: tstr,                                        ; {11}
     partIndex: uint .size 2,                               ; {12}
     ( NullPart // SinglePart // ExternalPart // MultiPart)
   ]

   NullPart = ( cardinality: nullpart )

   SinglePart = (
       cardinality: single,
       contentType: tstr,        ; {8}
       content: bstr
   )

   ExternalPart = (
       cardinality: external,
       contentType: tstr,
       url: uri,
       expires: uint .size 4,
       size: uint .size 8,
       encAlg: uint .size 2,
       key: bstr,
       nonce: bstr,
       aad: bstr,
       hashAlg: uint .size 1,
       contentHash: bstr,
       description: tstr
   )

   MultiPart = (
       cardinality: multi,
       partSemantics: chooseOne / singleUnit / processAll,
       parts: [2* NestedPart]
   )

   ; cardinality
   nullpart = 0
   single   = 1
   external = 2
   multi    = 3

   ; part semantics {9}
   chooseOne  = 0  ; receiver picks exactly one part to process
   singleUnit = 1  ; receiver processes all parts as single unit
   processAll = 2  ; receiver processes all parts individually

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   With some types of message content, there are multiple media types
   associated with the same message which need to be rendered together,
   for example a rich-text message with an inline image.  With other
   messages, there are multiple choices available for the same content,
   for example a choice among multiple languages, or between two
   different image formats.  The relationship semantics among the parts
   is specified as an enumeration {9}.

   The chooseOne part semantic is roughly analogous to the semantics of
   the multipart/alternative media type, except that the ordering of the
   nested body parts is merely a preference of the sender.  The receiver
   can choose the body part among those provided according to its own
   policy.

   The singleUnit part semantic is roughly analogous to the semantics of
   the multipart/related media type, in that all the nested body parts
   at this level are part of a single entity (for example, a rich text
   message with an inline image).  If the receiver does not understand
   even one of the nested parts at this level, the receiver should not
   process any of them.

   The processAll part semantic is roughly analogous to the semantics of
   the multipart/mixed media type.  The receiver should process as many
   of the nested parts at this level as possible.  For example, a rich
   text document with a link, and a preview image of the link target
   could be expressed using this semantic.  Processing the preview image
   is not strictly necessary for the correct rendering of the rich text
   part.

   The disposition {10} and language {11} of each part can be specified
   for any part, including for nested parts.  The disposition represents
   the intended semantics of the body part or a set of nested parts.  It
   is inspired by the values in the Content-Disposition MIME header
   [RFC2183].

   baseDispos = &(
       unspecified: 0,
       render: 1,
       reaction: 2,
       profile: 3,
       inline: 4,
       icon: 5,
       attachment: 6,
       session: 7,
       preview: 8
   )
   unknownDispos = &( unknown: 9..255 ) ; Note: any ext_dispos take precedence

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   The render disposition means that the content should be rendered
   according to local policy.  The inline dispositions means that the
   content should be rendered "inline" directly in the chat interface.
   The attachment disposition means that the content is intended to be
   downloaded by the receiver instead of being rendered immediately.
   The reaction disposition means that the content is a single reaction
   to another message, typically an emoji, but which could be an image,
   sound, or video.  The reaction disposition was originally published
   in [RFC9078], but was incorrectly placed in the Content Disposition
   Parameters IANA registry instead of in the Content Disposition Values
   registry.  The session disposition means that the content is a
   description of a multimedia session, or a URI used to join one.  The
   preview disposition means that the content is a sender-generated
   preview of something, such as the contents of a link.

   The value of the language data field is an empty string or a comma-
   separated list of one or more Language-tags as defined in [RFC5646].

   Each part also has an part index {12}, which is a zero-indexed,
   depth-first integer.  It is used to efficiently refer to a specific
   body part (for example, an inline image) within another part.  See
   {Nested body examples} for an example of how the part index is
   calculated.

4.5.  External content

   It is common in Instant Messaging systems to reference external
   content via URI that will be processed automatically, either to store
   bulky content (ex: videos, images, recorded sounds) outside the
   messaging infrastructure, or to access a specific service URI, for
   example, a media forwarding service for conferencing.

   An ExternalPart is a convenient way to reference this content.  It
   provides a similar function to the message/external-body media type.
   It optionally includes the size of the data in octets (or zero if the
   length is not provided).  It also includes an optional timestamp
   after which the external content is invalid, expressed as seconds
   since the start of the UNIX epoch (01-Jan-1970), or zero if the
   content does not expire.

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   ExternalPart = (
       cardinality: external,
       contentType: tstr,     ; An IANA media type {8}
       url: uri,              ; A URL where the content can be fetched
       expires: uint .size 4, ; expiration in seconds since UNIX epoch
       size: uint .size 8,    ; size of content in octets
       encAlg: uint .size 2,  ; An IANA AEAD Algorithm number, or zero
       key: bstr,             ; AEAD key
       nonce: bstr,           ; AEAD nonce
       aad: bstr,             ; AEAD additional authentiation data
       hashAlg: uint .size 1, ; An IANA Named Information Hash Algorithm
       contentHash: bstr,     ; hash of the content at the target url
       description: tstr      ; an optional text description
   )

   Typically, external content is encrypted with an ephemeral symmetric
   key before it is uploaded, and whatever is necessary for decryption
   is shared over the message channel.

   It is a matter of local policy to where the content is uploaded.
   Often in federated messaging systems, the sender of the content
   stores the external content in their own domain, but in some systems
   the content is stored in the "owning" or "hub" domain of the MLS
   group.

   Before being uploaded, private external content is encrypted with an
   IANA-registered Authenticated Encryption with Additional Data (AEAD)
   algorithm as described in [RFC5116].  The key, nonce, and additional
   authenticated data (aad) values are set to the values used during the
   encryption.  Unless modified by an extension, the default value of
   the aad is empty.

   If the external URL is a service, or the external content is not
   considered private, the encAlg is set to zero, and the key, nonce,
   and aad fields are zero length.

   Implementations of this specification MUST implement the AES-128-GCM
   algorithm.

4.6.  Derived Data Values

   In addition to fields which are contained in a MIMI content message,
   there are also two fields which the implementation can definitely
   derive (the MLS group ID {13}, and the leaf index of the sender
   {14}).  Many implementations could also determine one or more of: the
   sender's client identifier URL {15}, the user identifier URL of the
   credential associated with the sender {16}, and the identifier URL
   for the MIMI room {17}.

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   MessageDerivedValues = [
       messageId: MessageId,              ; sha256 hash of message ciphertext
       hubAcceptedTimestamp: Timestamp,
       mlsGroupId: bstr,                  ; value always available {13}
       senderLeafIndex: uint .size 4,     ; value always available {14}
       senderClientUrl: uri               ; {15},
       senderUserUrl: uri,                ; "From" {16}
       roomUrl: uri                       ; "To"   {17}
   ]

   MessageId = bstr .size 32
   Timestamp = #6.62(uint .size 8)    ; milliseconds since start of UNIX epoch

5.  Examples

   In the following examples, we assume that an MLS group is already
   established and that either out-of-band or using the MLS protocol or
   MLS extensions, or their client to provider protocol that the
   following is known to every member of the group:

   *  The membership of the group (via MLS).
   *  The identity of any MLS client which sends an application message
      (via MLS).
   *  The MLS group ID (via MLS)
   *  The human readable name(s) of the MIMI room, if any (out-of-band
      or extension).
   *  Which media types are mandatory to implement (MLS content
      advertisement extensions).
   *  For each member, the media types each supports (MLS content
      advertisement extensions).

   Messages sent to an MLS group are delivered to every member of the
   group active during the epoch in which the message was sent.

   All the examples start with a CBOR instance document annotated in the
   Extended Diagnostic Format (described in [Appendix G of @!RFC8610]
   and more rigorously specified in [I-D.ietf-cbor-edn-literals]), and
   then include a hex dump of the CBOR data in the pretty printed format
   popularized by the CBOR playground website (https://cbor.me
   (https://cbor.me)) with some minor whitespace and comment
   reformatting.  Finally a message ID for the message is included for
   most messages.

   All the instance documents validate using the CDDL schemas in
   Appendix B and are included in the examples directory in the github
   repo for this document.

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5.1.  Original Message

   In this example, Alice Smith sends a rich-text (Markdown) [RFC7763]
   message to the Engineering Team MLS group.  The following values are
   derived from the client, except for the hub received timestamp, which
   might be available for the client from its provider:

   *  Sender leaf index: 4
   *  Sender client ID URL: im:3b52249d-68f9-45ce-
      8bf5-c799f3cad7ec/0003@example.com
   *  Sender user handle URL: im:%40alice-smith@example.com
   *  MLS group ID: 7u4NEqe1tbeBFa0aHdsTgRyD/XOHxD5meZpZS+7aJr8=
   *  The MIMI room URL: im:#engineering_team@example.com
   *  The MIMI room name: "Engineering Team"
   *  Message ID: 0xd3c14744d1791d02548232c23d35efa9
      7668174ba385af066011e43bd7e51501
   *  Timestamp: 1644387225019 = 2022-02-08T22:13:45.019-00:00

   Below is the message in annotated Extended Diagnostic Notation, and
   pretty printed CBOR.

   [
     null,                             / replaces          /
     h'',                              / topicId           /
     0,                                / expires           /
     null,                             / inReplyTo         /
     [],                               / lastSeen          /
     {},                               / extensions        /
     [                                 / body (NestedPart) /
       1,                                / dispostion = render       /
       "",                               / language                  /
       0,                                / partIndex = 1st part      /
       1,                                / cardinality = single part /
       "text/markdown;variant=GFM",      / contentType               /
                                         / content                   /
       'Hi everyone, we just shipped release 2.0. __Good  work__!'
     ]
   ]

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   87                                      # array(7)
      f6                                   # primitive(22)
      40                                   # bytes(0)
                                           # ""
      00                                   # unsigned(0)
      f6                                   # primitive(22)
      80                                   # array(0)
      a0                                   # map(0)
      86                                   # array(6)
         01                                # unsigned(1)
         60                                # text(0)
                                           # ""
         00                                # unsigned(0)
         01                                # unsigned(1)
         78 1b                             # text(27)
            746578742f6d61726b646f776e3b636861727365743d7574662d38
            # "text/markdown;charset=utf-8"
         58 38                             # bytes(56)
            48692065766572796f6e652c207765206a757374207368697070656420
            72656c6561736520322e302e205f5f476f6f6420776f726b5f5f21
            # "Hi everyone, we just shipped release 2.0. __Good work__!"

   Below are the rest of the implied values for this message:

   [
     / messageId  (Original message) /
     h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501',

     / hubAcceptedTimestamp = 2022-02-08T22:13:45.019-00:00 /
     62(1644387225019),

     / mlsGroupId /
     h'eeee0d12a7b5b5b78115ad1a1ddb13811c83fd7387c43e66799a594beeda26bf',

     / senderLeadIndex /
     4,

     / senderClientUrl /
     32("mimi://example.com/d/3b52249d-68f9-45ce-8bf5-c799f3cad7ec/0003"),

     / senderUserUrl /
     32("mimi://example.com/u/alice-smith"),

     / roomUrl /
     32("mimi://example.com/r/engineering_team")
   ]

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5.2.  Reply

   A reply message looks similar, but contains the message ID of the
   original message in the inReplyTo data field.  The derived MLS group
   ID, URL, and name do not change in this example.  The derived
   senderClientId and senderLeafIndex are not especially relevant so all
   but the user handle URL, message ID, and hub received timestamp will
   be omitted.

   *  Sender user handle URL: im:%40bob-jones@example.com
   *  MessageId: 0xe701beee59f9376282f39092e1041b2a
      c2e3aad1776570c1a28de244979c71ed
   *  Timestamp = 1644387237492 = 2022-02-08T22:13:57.492-00:00

   Below is the annotated message in EDN and pretty printed CBOR:

   [
     null,                                / replaces             /
     h'',                                 / topicId              /
     0,                                   / expires = never      /
     [                                    / InReplyTo            /
        /  message = Original message                            /
        h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501',
        1,                                  /  hashAlg = sha256  /
        /  hash  /
        h'6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5'
     ],
     [                                      / lastSeen (1 item)   /
        /  Original message                                       /
        h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501'
     ],
     {},                                  / extensions             /
     [                                    / body (NestedPart)      /
       1,                                   / dispostion = render  /
       "",                                  / language             /
       0,                                   / partIndex = 1st part /
       1,                                   / cardinality = single /
       "text/markdown;variant=GFM",         / contentType          /
                                            / content              /
       'Right on! _Congratulations_ \'all!'
     ]
   ]

Mahy                    Expires 12 December 2024               [Page 17]
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87                                      # array(7)
   f6                                   # primitive(22)
   40                                   # bytes(0)
                                        # ""
   00                                   # unsigned(0)
   83                                   # array(3)
      58 20                             # bytes(32)
         d3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501
      01                                # unsigned(1)
      58 20                             # bytes(32)
         6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5
   81                                   # array(1)
      58 20                             # bytes(32)
         d3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501
   a0                                   # map(0)
   86                                   # array(6)
      01                                # unsigned(1)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      01                                # unsigned(1)
      78 1b                             # text(27)
         746578742f6d61726b646f776e3b636861727365743d7574662d38
         # "text/markdown;charset=utf-8"
      58 21                             # bytes(33)
         5269676874206f6e21205f436f6e67726174756c6174696f6e735f2027616c6c21
         # "Right on! _Congratulations_ 'all!"

5.3.  Reaction

   A reaction looks like a reply, but uses the Disposition token of
   reaction.  It is modeled on the reaction Content-Disposition token
   defined in [RFC9078].  Both indicate that the intended disposition of
   the contents of the message is a reaction.

   The content in the sample message is a single Unicode heart character
   (U+2665) which is expressed in UTF-8 as 0xe299a5.  Discovering the
   range of characters each implementation could render as a reaction
   can occur out-of-band and is not within the scope of this proposal.
   However, an implementation which receives a reaction character string
   it does not recognize could render the reaction as a reply, possibly
   prefixing with a localized string such as "Reaction: ".  Note that a
   reaction could theoretically even be another media type (ex: image,
   audio, or video), although not currently implemented in major instant
   messaging systems.  Note that many systems allow mutiple independent
   reactions per sender.

   *  Sender user handle URL: im:cathy-washington@example.com

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   *  Message ID: 0x4dcab7711a77ea1dd025a6a1a7fe01ab
      3b0d690f82417663cb752dfcc37779a1
   *  Timestamp: 1644387237728 = 2022-02-08T22:13:57.728-00:00

   Below is the annotated message in EDN and pretty printed CBOR:

   [
     null,                                / replaces             /
     h'',                                 / topicId              /
     0,                                   / expires = never      /
     [                                    / InReplyTo            /
        /  message = Original message                            /
        h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501',
        1,                                  /  hashAlg = sha256  /
        /  hash  /
        h'6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5'
     ],
     [                                      / lastSeen (1 item)   /
        /  Reply message                                          /
        h'e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed'
     ],
     {},                                  / extensions              /
     [                                    / body (NestedPart)       /
       2,                                   / dispostion = reaction /
       "",                                  / language              /
       0,                                   / partIndex = 1st part  /
       1,                                   / cardinality = single  /
       "text/plain;charset=utf-8",          / contentType           /
       '❤'                                  / content = U+2665 (heart) /
     ]
   ]

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87                                      # array(7)
   f6                                   # primitive(22)
   40                                   # bytes(0)
                                        # ""
   00                                   # unsigned(0)
   83                                   # array(3)
      58 20                             # bytes(32)
         d3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501
      01                                # unsigned(1)
      58 20                             # bytes(32)
         6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5
   81                                   # array(1)
      58 20                             # bytes(32)
         e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed
   a0                                   # map(0)
   86                                   # array(6)
      02                                # unsigned(2)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      01                                # unsigned(1)
      78 18                             # text(24)
         746578742f706c61696e3b636861727365743d7574662d38
         # "text/plain;charset=utf-8"
      43                                # bytes(3)
         e299a5                         # "♥"

5.4.  Mentions

   In instant messaging systems and social media, a mention allows
   special formatting and behavior when a name, handle, or tag
   associated with a known group is encountered, often when prefixed
   with a commercial-at "@" character for mentions of users or a hash
   "#" character for groups or tags.  A message which contains a mention
   may trigger distinct notifications on the IM client.

   We can convey a mention by linking the user handle URI, or group URI
   in Markdown or HTML rich content.  For example, a mention using
   Markdown is indicated below.

   *  Sender user handle URL: im:cathy-washington@example.com
   *  Message ID: 0x6b50bfdd71edc83554ae21380080f4a3
      ba77985da34528a515fac3c38e4998b8
   *  Timestamp: 1644387243008 = 2022-02-08T22:14:03.008-00:00

   Below is the annotated message in EDN and pretty printed CBOR:

Mahy                    Expires 12 December 2024               [Page 20]
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   [
     null,                                / replaces             /
     h'',                                 / topicId              /
     0,                                   / expires = never      /
     null,                                / InReplyTo            /
     [                                      / lastSeen (1 item)   /
        /  Reply message                                          /
        h'e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed'
     ],
     {},                                  / extensions              /
     [                                    / body (NestedPart)       /
       1,                                   / dispostion = render   /
       "",                                  / language              /
       0,                                   / partIndex = 1st part  /
       1,                                   / cardinality = single  /
       "text/markdown;variant=GFM",         / contentType           /
                                            / content               /
       'Kudos to [@Alice Smith](im:alice-smith@example.com)'
       ' for making the release happen!'
     ]
   ]

87                                      # array(7)
   f6                                   # primitive(22)
   40                                   # bytes(0)
                                        # ""
   00                                   # unsigned(0)
   f6                                   # primitive(22)
   81                                   # array(1)
      58 20                             # bytes(32)
         e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed
   a0                                   # map(0)
   86                                   # array(6)
      01                                # unsigned(1)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      01                                # unsigned(1)
      78 1b                             # text(27)
         746578742f6d61726b646f776e3b636861727365743d7574662d38
         # "text/markdown;charset=utf-8"
      58 52                             # bytes(82)
         4b75646f7320746f205b40416c69636520536d6974685d28696d3a
         616c6963652d736d697468406578616d706c652e636f6d2920666f
         72206d616b696e67207468652072656c656173652068617070656e21
         # "Kudos to [@Alice Smith](im:alice-smith@example.com)
         # for making the release happen!"

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   The same mention using HTML [W3C.CR-html52-20170808] would instead
   replace in the EDN the contentType and content indicated below.

     / ... /
     "text/html;charset=utf-8",
     '<p>Kudos to <a href="im:alice-smith@example.com">@Alice Smith</a> for making the release happen!</p>'

5.5.  Edit

   Unlike with email messages, it is common in IM systems to allow the
   sender of a message to edit or delete the message after the fact.
   Typically the message is replaced in the user interface of the
   receivers (even after the original message is read) but shows a
   visual indication that it has been edited.

   The replaces data field includes the message ID of the message to
   edit/replace.  The message included in the body is a replacement for
   the message with the replaced message ID.

   Here Bob Jones corrects a typo in his original message:

   *  Sender user handle URL: im:%40bob-jones@example.com
   *  Message ID:0x89d3472622a4d9de526742bcd00b09dc
      78fa4edceaf2720e17b730c6dfba8be4
   *  Timestamp: 1644387248621 = 2022-02-08T22:14:08.621-00:00

Mahy                    Expires 12 December 2024               [Page 22]
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   [
     / replaces = Reply message                                  /
     h'e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed',
     h'',                                 / topicId              /
     0,                                   / expires = never      /
     [                                      / InReplyTo          /
        /  message = Original message                            /
        h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501',
        1,                                  /  hashAlg = sha256  /
        /  hash                /
        h'6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5'
     ],
     [                                      / lastSeen (2 items)   /
       /  Reaction message /
       h'4dcab7711a77ea1dd025a6a1a7fe01ab
         3b0d690f82417663cb752dfcc37779a1',
       /  Mention message  /
       h'6b50bfdd71edc83554ae21380080f4a3
         ba77985da34528a515fac3c38e4998b8'
     ],
     {},                                  / extensions             /
     [                                    / body (NestedPart)      /
       1,                                   / dispostion = render  /
       "",                                  / language             /
       0,                                   / partIndex = 1st part /
       1,                                   / cardinality = single /
       "text/markdown;variant=GFM",         / contentType          /
                                            / content              /
       'Right on! _Congratulations_ y\'all'
     ]
   ]

Mahy                    Expires 12 December 2024               [Page 23]
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87                                      # array(7)
   58 20                                # bytes(32)
      e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed
   40                                   # bytes(0)
                                        # ""
   00                                   # unsigned(0)
   83                                   # array(3)
      58 20                             # bytes(32)
         d3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501
      01                                # unsigned(1)
      58 20                             # bytes(32)
         6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5
   82                                   # array(2)
      58 20                             # bytes(32)
         4dcab7711a77ea1dd025a6a1a7fe01ab3b0d690f82417663cb752dfcc37779a1
      58 20                             # bytes(32)
         6b50bfdd71edc83554ae21380080f4a3ba77985da34528a515fac3c38e4998b8
   a0                                   # map(0)
   86                                   # array(6)
      01                                # unsigned(1)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      01                                # unsigned(1)
      78 1b                             # text(27)
         746578742f6d61726b646f776e3b636861727365743d7574662d38
         # "text/markdown;charset=utf-8"
      58 22                             # bytes(34)
         5269676874206f6e21205f436f6e67726174756c6174696f6e735f
         207927616c6c21
         # "Right on! _Congratulations_ y'all!"

   Note that replies and reactions always refer to a specific message
   id, and therefore a specific "version" of a message, which could have
   been edited before and/or after the message id referenced in the
   reply or reaction.  It is a matter of local policy how to render (if
   at all) a reaction to a subsequently edited message.

5.6.  Delete

   In IM systems, a delete means that the author of a specific message
   has retracted the message, regardless if other users have read the
   message or not.  Typically a placeholder remains in the user
   interface showing that a message was deleted.  Replies which
   reference a deleted message typically hide the quoted portion and
   reflect that the original message was deleted.

Mahy                    Expires 12 December 2024               [Page 24]
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   If Bob deleted his message instead of modifying it, we would
   represent it using the replaces data field, and using an empty body
   (NullPart), as shown below.

   *  Sender user handle URL: im:%40bob-jones@example.com
   *  Message ID: 0x89d3472622a40d6ceeb27c42490fdc64
      c0e9c20c598f9d7c8e81640dae8db0fb
   *  Timestamp: 1644387248621 = 2022-02-08T22:14:08.621-00:00

   [
     / replaces = Reply message                                  /
     h'e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed',
     h'',                                 / topicId              /
     0,                                   / expires = never      /
     [                                      / InReplyTo          /
        /  message = Original message                            /
        h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501',
        1,                                  /  hashAlg = sha256  /
        /  hash                /
        h'6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5'
     ],
     [                                      / lastSeen (1 items)   /
       /  Edit message /
       h'89d3472622a4d9de526742bcd00b09dc78fa4edceaf2720e17b730c6dfba8be4'
     ],
     {},                                  / extensions             /
     [                                    / body (NestedPart)      /
       1,                                   / dispostion = render  /
       "",                                  / language             /
       0,                                   / partIndex = 1st part /
       0                                    / cardinality = zero parts /
     ]
   ]

Mahy                    Expires 12 December 2024               [Page 25]
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87                                      # array(7)
   58 20                                # bytes(32)
      4dcab7711a77ea1dd025a6a1a7fe01ab3b0d690f82417663cb752dfcc37779a1
   40                                   # bytes(0)
                                        # ""
   00                                   # unsigned(0)
   83                                   # array(3)
      58 20                             # bytes(32)
         d3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501
      01                                # unsigned(1)
      58 20                             # bytes(32)
         6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5
   81                                   # array(1)
      58 20                             # bytes(32)
         89d3472622a40d6ceeb27c42490fdc64c0e9c20c598f9d7c8e81640dae8db0fb
   a0                                   # map(0)
   84                                   # array(4)
      02                                # unsigned(2)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      00                                # unsigned(0)

5.7.  Unlike

   In most IM systems, not only is it possible to react to a message
   ("Like"), but it is possible to remove a previous reaction
   ("Unlike").  This can be accomplished by deleting the message which
   creates the original reaction

   If Cathy removes her reaction, we would represent the removal using a
   replaces data field with an empty body, referring to the message
   which created the reaction, as shown below.

   *  Sender user handle URL: im:cathy-washington@example.com
   *  Message ID: 0x1a771ca1d84f8fda4184a1e02a549e20
      1bf434c6bfcf1237fa45463c6861853b
   *  Timestamp: 1644387250389 = 2022-02-08T22:14:10.389-00:00

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   [
     / replaces = Reaction message                               /
     h'4dcab7711a77ea1dd025a6a1a7fe01ab3b0d690f82417663cb752dfcc37779a1',
     h'',                                 / topicId              /
     0,                                   / expires = never      /
     [                                      / InReplyTo          /
        /  message = Original message                            /
        h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501',
        1,                                  /  hashAlg = sha256  /
        /  hash                /
        h'6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5'
     ],
     [                                      / lastSeen (1 items)   /
       /  Delete message /
       h'89d3472622a40d6ceeb27c42490fdc64c0e9c20c598f9d7c8e81640dae8db0fb'
     ],
     {},                                  / extensions              /
     [                                    / body (NestedPart)       /
       2,                                   / dispostion = reaction /
       "",                                  / language              /
       0,                                   / partIndex = 1st part  /
       0                                    / cardinality = zero parts /
     ]
   ]

87                                      # array(7)
   58 20                                # bytes(32)
      e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed
   40                                   # bytes(0)
                                        # ""
   00                                   # unsigned(0)
   83                                   # array(3)
      58 20                             # bytes(32)
         d3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501
      01                                # unsigned(1)
      58 20                             # bytes(32)
         6b44053cb68e3f0cdd219da8d7104afc2ae5ffff782154524cef093de39345a5
   81                                   # array(1)
      58 20                             # bytes(32)
         89d3472622a4d9de526742bcd00b09dc78fa4edceaf2720e17b730c6dfba8be4
   a0                                   # map(0)
   84                                   # array(4)
      01                                # unsigned(1)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      00                                # unsigned(0)

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5.8.  Expiring

   Expiring messages are designed to be deleted automatically by the
   receiving client at a certain time whether they have been read or
   not.  As with manually deleted messages, there is no guarantee that
   an uncooperative client or a determined user will not save the
   content of the message, however most clients respect the convention.

   The expires data field contains the timestamp when the message can be
   deleted.  The semantics of the header are that the message is
   automatically deleted by the receiving clients at the indicated time
   without user interaction or network connectivity necessary.

   *  Sender user handle URL: im:alice-smith@example.com
   *  Message ID: 0x5c95a4dfddab84348bcc265a479299fb
      d3a2eecfa3d490985da5113e5480c7f1
   *  Timestamp: 1644389403227 = 2022-02-08T22:49:06.227-00:00

   [
     null,                             / replaces                 /
     h'',                              / topicId                  /
     1644390004,                       / expires = in 10 minutes  /
     null,                             / inReplyTo                /
     [                                 / lastSeen (1 item)        /
        /  Unlike message                                          /
        h'1a771ca1d84f8fda4184a1e02a549e201bf434c6bfcf1237fa45463c6861853b'
     ],
     {},                               / extensions        /
     [                                 / body (NestedPart) /
       1,                                / dispostion = render       /
       "",                               / language                  /
       0,                                / partIndex = 1st part      /
       1,                                / cardinality = single part /
       "text/markdown;variant=GFM",      / contentType               /
                                         / content                   /
       '__*VPN GOING DOWN*__ I\'m rebootinging the VPN in ten minutes'
       ' unless anyone objects.'
     ]
   ]

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87                                      # array(7)
   f6                                   # primitive(22)
   40                                   # bytes(0)
                                        # ""
   1a 62036674                          # unsigned(1644390004)
   f6                                   # primitive(22)
   81                                   # array(1)
      58 20                             # bytes(32)
         1a771ca1d84f8fda4184a1e02a549e201bf434c6bfcf1237fa45463c6861853b
   a0                                   # map(0)
   86                                   # array(6)
      01                                # unsigned(1)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      01                                # unsigned(1)
      78 1b                             # text(27)
         746578742f6d61726b646f776e3b636861727365743d7574662d38
         # "text/markdown;charset=utf-8"
      58 50                             # bytes(80)
         5f5f2a56504e20474f494e4720444f574e2a5f5f0a49276d207265
         626f6f74696e67207468652056504e20696e2074656e206d696e75
         74657320756e6c65737320616e796f6e65206f626a656374732e
         # "__*VPN GOING DOWN*__\nI'm rebooting the VPN in ten
         #  minutes unless anyone objects."

5.9.  Attachments

   An ExternalPart is a convenient way to present both "attachments" and
   (possibly inline rendered) content which is too large to be included
   in an MLS application message.  The disposition data field is set to
   inline if the sender recommends inline rendering, or attachment if
   the sender intends the content to be downloaded or rendered
   separately.

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   [
     null,                             / replaces                 /
     h'',                              / topicId                  /
     0,                                / expires = never          /
     null,                             / inReplyTo                /
     [                                 / lastSeen (1 item)        /
        /  Expiring message                                          /
        h'5c95a4dfddab84348bcc265a479299fbd3a2eecfa3d490985da5113e5480c7f1'
     ],
     {},                               / extensions        /
     [                                 / body (NestedPart) /
       6,                                / dispostion = attachment   /
       "en",                             / language = en             /
       0,                                / partIndex = 1st part      /
       2,                                / cardinality = external part /
       "video/mp4",                      / contentType               /
       32("https://example.com/storage/bigfile.mp4"), / url          /
       0,                                / expires                   /
       708234961,                        / size                      /
       1,                                / encAlg = AES-128-GCM      /
       h'21399320958a6f4c745dde670d95e0d8',   / key                  /
       h'c86cf2c33f21527d1dd76f5b',      / nonce                     /
       h'',                              / aad                       /
       1,                                / hashAlg = sha256          /
       /  content hash                                               /
       h'9ab17a8cf0890baaae7ee016c7312fcc080ba46498389458ee44f0276e783163',
       "2 hours of key signing video"    / description               /
     ]
   ]

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87                                      # array(7)
   f6                                   # primitive(22)
   40                                   # bytes(0)
                                        # ""
   00                                   # unsigned(0)
   f6                                   # primitive(22)
   81                                   # array(1)
      58 20                             # bytes(32)
         5c95a4dfddab84348bcc265a479299fbd3a2eecfa3d490985da5113e5480c7f1
   a0                                   # map(0)
   8f                                   # array(15)
      06                                # unsigned(6)
      62                                # text(2)
         656e                           # "en"
      00                                # unsigned(0)
      02                                # unsigned(2)
      69                                # text(9)
         766964656f2f6d7034             # "video/mp4"
      d8 20                             # tag(32)
         78 1c                          # text(28)
            68747470733a6578616d706c652e636f6d62696766696c652e6d7034
            # "https:example.combigfile.mp4"
      00                                # unsigned(0)
      1a 2a36ced1                       # unsigned(708234961)
      01                                # unsigned(1)
      50                                # bytes(16)
         21399320958a6f4c745dde670d95e0d8
      4c                                # bytes(12)
         c86cf2c33f21527d1dd76f5b
      40                                # bytes(0)
                                        # ""
      01                                # unsigned(1)
      58 20                             # bytes(32)
         9ab17a8cf0890baaae7ee016c7312fcc080ba46498389458ee44f0276e783163
      78 1c                             # text(28)
         3220686f757273206f66206b6579207369676e696e6720766964656f
         # "2 hours of key signing video"

   message ID
     0xb267614d43e7676d28ef5b15e8676f23
       679fe365c78849d83e2ba0ae8196ec4e

   Other dispositions of external content are also possible, for example
   an external GIF animation of a rocket ship could be used with a
   reaction disposition.

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5.10.  Conferencing

   Joining a conference via an external URL is possible.  The link could
   be rendered to the user, requiring a click.  Alternatively the URL
   could be rendered the disposition could be specified as session which
   could be processed differently by the client (for example, alerting
   the user or presenting a dialog box).  Further discussion of calling
   and conferencing functionality is out-of-scope of this document.

   [
     null,                             / replaces                 /
     h'466f6f20313138',                / topicId = Foo 118        /
     0,                                / expires                  /
     null,                             / inReplyTo                /
     [                                 / lastSeen (1 item)        /
        /  Attachment message                                        /
        h'b267614d43e7676d28ef5b15e8676f23679fe365c78849d83e2ba0ae8196ec4e'
     ],
     {},                               / extensions               /
     [                                 / body (NestedPart)        /
       7,                                / dispostion = session      /
       "",                               / language                  /
       0,                                / partIndex = 1st part      /
       2,                                / cardinality = external part /
       "",                               / contentType               /
       32("https://example.com/join/12345"),  / url                  /
       0,                                / expires                   /
       0,                                / size                      /
       0,                                / encAlg = none             /
       h'',                              / key                       /
       h'',                              / nonce                     /
       h'',                              / aad                       /
       0,                                / hashAlg = none            /
       h'',                              / content hash              /
       "Join the Foo 118 conference"     / description               /
     ]
   ]

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87                                      # array(7)
   f6                                   # primitive(22)
   47                                   # bytes(7)
      466f6f20313138                    # "Foo 118"
   00                                   # unsigned(0)
   f6                                   # primitive(22)
   81                                   # array(1)
      58 20                             # bytes(32)
         b267614d43e7676d28ef5b15e8676f23679fe365c78849d83e2ba0ae8196ec4e
   a0                                   # map(0)
   8f                                   # array(15)
      07                                # unsigned(7)
      60                                # text(0)
                                        # ""
      00                                # unsigned(0)
      02                                # unsigned(2)
      60                                # text(0)
                                        # ""
      d8 20                             # tag(32)
         76                             # text(22)
            68747470733a6578616d706c652e636f6d3132333435
            # "https:example.com12345"
      00                                # unsigned(0)
      00                                # unsigned(0)
      00                                # unsigned(0)
      40                                # bytes(0)
                                        # ""
      40                                # bytes(0)
                                        # ""
      40                                # bytes(0)
                                        # ""
      00                                # unsigned(0)
      40                                # bytes(0)
                                        # ""
      78 1b                             # text(27)
         4a6f696e2074686520466f6f2031313820636f6e666572656e6365
         # "Join the Foo 118 conference"

   message ID
     0xb267614d43e7676d28ef5b15e8676f23
       679fe365c78849d83e2ba0ae8196ec4e

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5.11.  Topics / Threading

   As popularized by the messaging application Slack, some messaging
   applications have a notion of a Topic or message Thread (not to be
   confused with message threading as used in email).  Clients beginning
   a new "topic" populate the topicId with a unique opaque octet string.
   This could be the message ID of the first message sent related to the
   topic.  Subsequent messages may include the same topicId for those
   messages to be associated with the same topic.  The sort order for
   messages within a thread uses the timestamp field.  If more than one
   message has the same timestamp, the lexically lowest message ID sorts
   earlier.

5.12.  Delivery Reporting and Read Receipts

   In instant messaging systems, read receipts typically generate a
   distinct indicator for each message.  In some systems, the number of
   users in a group who have read the message is subtly displayed and
   the list of users who read the message is available on further
   inspection.

   Of course, Internet mail has support for read receipts as well, but
   the existing message disposition notification mechanism defined for
   email in [RFC8098] is completely inappropriate in this context:

   *  notifications can be sent by intermediaries
   *  only one notification can be sent about a single message per
      recipient
   *  a human-readable version of the notification is expected
   *  each notification can refer to only one message
   *  it is extremely verbose

   Instead we would like to be able to include status changes about
   multiple messages in each report, the ability to mark a message
   delivered, then read, then unread, then expired for example.

   The format below, application/mimi-message-status is sent by one
   member of an MLS group to the entire group and can refer to multiple
   messages in that group.  The format contains its own timestamp, and a
   list of message ID / status pairs.  As the status at the recipient
   changes, the status can be updated in a subsequent notification.
   Below is the CDDL schema for message status.

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   MessageStatusReport = [
       timestamp: Timestamp,
       statuses: [ * PerMessageStatus ]
   ]

   PerMessageStatus = [
       messageId: MessageId,
       status: baseStatus / $extStatus / unknownStatus
   ]

   baseStatus = &(
       unread: 0,
       delivered: 1,
       read: 2,
       expired: 3,
       deleted: 4,
       hidden: 5,
       error: 6
   )
   unknownStatus = &( unknown: 7..255 )

   MessageId = bstr .size 32
   Timestamp = #6.62(uint .size 8)    ; milliseconds since start of UNIX epoch

   *  Sender user handle URL: im:bob-jones@example.com

5.12.1.  Delivery Report Example

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   [
     62(1644284703227),   / Timestamp of the report /
     [
       [
         /  Original message     /
         h'd3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501',
         2    / status = read    /
       ],
       [
         /  Reply message        /
         h'e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed',
         2    / status = read    /
       ],
       [
         /  Mention message      /
         h'6b50bfdd71edc83554ae21380080f4a3ba77985da34528a515fac3c38e4998b8',
         0    / status = unread  /
       ],
       [
         /  Expiring message     /
         h'5c95a4dfddab84348bcc265a479299fbd3a2eecfa3d490985da5113e5480c7f1',
         3    / status = expired /
       ]
     ]
   ]

82                                      # array(2)
   d8 3e                                # tag(62)
      1b 0000017ed70171fb               # unsigned(1644284703227)
   84                                   # array(4)
      82                                # array(2)
         58 20                          # bytes(32)
            d3c14744d1791d02548232c23d35efa97668174ba385af066011e43bd7e51501
         02                             # unsigned(2)
      82                                # array(2)
         58 20                          # bytes(32)
            e701beee59f9376282f39092e1041b2ac2e3aad1776570c1a28de244979c71ed
         02                             # unsigned(2)
      82                                # array(2)
         58 20                          # bytes(32)
            6b50bfdd71edc83554ae21380080f4a3ba77985da34528a515fac3c38e4998b8
         00                             # unsigned(0)
      82                                # array(2)
         58 20                          # bytes(32)
            5c95a4dfddab84348bcc265a479299fbd3a2eecfa3d490985da5113e5480c7f1
         03                             # unsigned(3)

Mahy                    Expires 12 December 2024               [Page 36]
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6.  Support for Specific Media Types

6.1.  MIMI Required and Recommended media types

   As the MIMI Content container is just a container, the plain text or
   rich text messages sent inside, and any image or other formats needs
   to be specified.  Clients compliant with MIMI MUST be able to receive
   the following media types:

   *  application/mimi-content -- the MIMI Content container format
      (described in this document)
   *  text/plain;charset=utf-8
   *  text/markdown;variant=GFM -- Github Flavored Markdown [GFM])

   Note that it is acceptable to render the contents of a received
   markdown document as plain text.

   The following MIME types are RECOMMENDED:

   *  text/markdown;variant=CommonMark -- CommonMark
      (https://spec.commonmark.org/0.30)
   *  text/html
   *  application/mimi-message-status -- (described in this document)
   *  image/jpeg
   *  image/png

   Clients compliant with this specification must be able to download
   ExternalParts with http and https URLs, and decrypt downloaded
   content encrypted with the AES-128-GCM AEAD algorithm.

6.2.  Use of proprietary media types

   As most messaging systems are proprietary, standalone systems, it is
   useful to allow clients to send and receive proprietary formats among
   themselves.  Using the functionality in the MIMI Content container,
   clients can send a message using the basic functionality described in
   this document AND a proprietary format for same-vendor clients
   simultaneously over the same group with end-to-end encryption.  An
   example is given in the Appendix.

7.  IANA Considerations

7.1.  MIME subtype registration of application/mimi-content

   This document proposes registration of a media subtype with IANA.

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   Type name: application

   Subtype name: mimi-content

   Required parameters: none

   Optional parameters: none

   Encoding considerations:
      This message type should be encoded as binary data

   Security considerations:
      See Section A of RFC XXXX

   Interoperability considerations:
      See Section Y.Z of RFC XXXX

   Published specification: RFC XXXX

   Applications that use this media type:
      Instant Messaging Applications

   Fragment identifier considerations: N/A

   Additional information:

      Deprecated alias names for this type: N/A
      Magic number(s): N/A
      File extension(s): N/A
      Macintosh file type code(s): N/A

   Person & email address to contact for further information:
      IETF MIMI Working Group mimi@ietf.org

7.2.  MIME subtype registration of application/mimi-message-status

   This document proposes registration of a media subtype with IANA.

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   Type name: application

   Subtype name: mimi-message-status

   Required parameters: none

   Optional parameters: none

   Encoding considerations:
      This message type should be encoded as binary data

   Security considerations:
      See Section A of RFC XXXX

   Interoperability considerations:
      See Section Y.Z of RFC XXXX

   Published specification: RFC XXXX

   Applications that use this media type:
      Instant Messaging Applications

   Fragment identifier considerations: N/A

   Additional information:

      Deprecated alias names for this type: N/A
      Magic number(s): N/A
      File extension(s): N/A
      Macintosh file type code(s): N/A

   Person & email address to contact for further information:
      IETF MIMI Working Group mimi@ietf.org

8.  Security Considerations

8.1.  General handling

   The following cases are examples of nonsensical values that most
   likely represent malicious messages.  These should be logged and
   discarded.

   *  sender of the message
      -  where the apparent sender is not a member of the target MLS
         group
   *  message IDs
      -  which duplicate another message ID already encountered
   *  timestamps

Mahy                    Expires 12 December 2024               [Page 39]
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      -  received more than a few minutes in the future, or
      -  before the first concrete syntax of this document is published
      -  before the room containing them was created
   *  inReplyTo
      -  inReplyTo.hash-alg is none even when the inReplyTo.message is
         present
      -  inReplyTo.hash-alg is an unknown value
      -  the length of inReplyTo.replyToHash does not correspond to the
         algorithm specified in inReplyTo.hash-alg
   *  topicId
      -  the topicId is very long (greater than 4096 octets)
      -  a topic is specified, but an inReplyTo or replaces field refers
         to a message outside of the topic
   *  expires
      -  refers to a date more than a year in the past
      -  refers to a date more than a year in the future
   *  lastSeen
      -  is empty, but the sender has previously sent messages in the
         room
      -  results in a loop
      -  refers to an excessive number of lastSeen messages
         simultaneously (contains more than 65535 message IDs).  (Note
         that a popular message sent in a large group can result in
         thousands of reactions in a few hundred milliseconds.)
   *  body
      -  has too many body parts (more than 1024)
      -  is nested too deeply (more than 4 levels deep)
      -  is too large (according to local policy)
      -  has an unknown PartSemantics value
      -  contains partIndex values which are not continuous from zero

   For the avoidance of doubt, the following cases may be examples of
   legitimate use cases, and should not be considered the result of a
   malicious sender.

   *  message IDs
      -  where inReplyTo.message or replaces refer to an unknown
         message.  Such a message could have been sent before the local
         client joined.
   *  lastSeen
      -  refers to an unknown message
      -  is empty for the sender's first message sent in the room
   *  body
      -  where a body part contains an unrecognized Disposition value.
         The unknown value should be treated as if it where render.
      -  where a contentType is unrecognized or unsupported.
      -  where a language tag is unrecognized or unsupported.

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8.2.  Validation of timestamp

   The timestamp is the time a message is accepted by the hub provider.
   As such, the hub provider can manipulate the timestamp, and the
   sending provider can delay sending messages selectively to cause the
   timestamp on a hub to be later.

   |  *TODO*: Discuss how to sanity check lastSeen, timestamp and the
   |  MLS epoch and generation, and the limitations of this approach.

8.3.  Alternate content rendering

   This document includes a mechanism where the sender can offer
   alternate versions of content in a single message.  For example, the
   sender could send:

   *  an plain text and an HTML version of a text message
   *  a thumbnail preview and link to a high-resolution image or video
   *  versions of the same message in multiple languages
   *  an PNG image and a scalable vector graphics version of a line
      drawing

   A malicious client could use this mechanism to send content that will
   appear different to a subset of the members of a group and possibly
   elicit an incorrect or misleading response.

   Message as seen by Alice (manager)
   Xavier: Do you want me to reserve a room for the review meeting?

   Message as seen by Bob (Alice's assistant)
   Xavier: @Bob I need to pickup Alice's Ferarri keys. She'll confirm

   Reply sent by Alice
   Alice: Yes please.

8.4.  Link and Mention handling

   Both Markdown and HTML support links.  Using the example of an https
   link, if the rendered text and the link target match exactly or are
   canonically equivalent, there is no need for confirmation if the end
   user selects the link.

   [example.com/foobar](https://example.com/foobar)
   [https://example.com/foobar](https://example.com/foobar)
   [https://example.com:443/foobar](https://example.com/foobar)

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   However, if the link text is different, or the scheme is downgraded
   from https to http, the user should be presented with an alert
   warning that the text is not the same.

   [https://example.com/foobar](https://spearphishers.example/foobar)
   [https://example.com/foobar](http://example.com/foobar)

   An IM URI link to a user who has a member client in the MLS group in
   which the message was sent is considered a mention.  Clients may
   support special rendering of mentions instead of treating them like
   any other type of link.  In Markdown and HTML, the display text
   portion of a link is considered a rendering hint from the sender to
   the receiver of the message.  The receiver should use local policy to
   decide if the hint is an acceptable local representation of the user
   represented by the link itself.  If the hint is not an acceptable
   representation, the client should instead display its canonical
   representation for the user.

   For example, in the first examples, the sender expresses no
   preference about how to render the mention.  In the second example,
   the sender requests that the mention is rendered as the literal URI.
   In the third example, the sender requests the canonical handle for
   Alice.  In the fourth example, the sender requests Alice's first
   name.

   <im:alice-smith@example.com>
   [im:alice-smith@example.com](im:alice-smith@example.com)
   [@AliceSmith](im:alice-smith@example.com)
   [Alice](im:alice-smith@example.com)

   Note that in some clients, presence of a mention for the local user
   may result in a different notification policy.

   If the client does not support special rendering of mentions, the
   application, should render the text like any other link.

8.5.  Delivery and Read Receipts

   Delivery and Read Receipts can provide useful information inside a
   group, or they can reveal sensitive private information.  In many IM
   systems there is are per-group policies for and/or delivery read
   receipts:

   *  they are required
   *  they are permitted, but optional
   *  they are forbidden

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   In the first case, everyone in the group would have to claim to
   support read receipts to be in the group and agree to the policy of
   sending them whenever a message was read.  A user who did not wish to
   send read receipts could review the policy (automatically or
   manually) and choose not to join the group.  Of course, requiring
   read receipts is a cooperative effort just like using self-deleting
   messages.  A malicious client could obviously read a message and not
   send a read receipt, or send a read receipt for a message that was
   never rendered.  However, cooperating clients have a way to agree
   that they will send read receipts when a message is read in a
   specific group.

   In the second case, sending a read receipt would be at the discretion
   of each receiver of the message (via local preferences).

9.  References

9.1.  Normative References

   [GFM]      GitHub, "GitHub Flavored Markdown Spec, Version 0.29-gfm",
              6 March 2019, <https://github.github.com/gfm/>.

   [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/info/rfc2119>.

   [RFC3862]  Klyne, G. and D. Atkins, "Common Presence and Instant
              Messaging (CPIM): Message Format", RFC 3862,
              DOI 10.17487/RFC3862, August 2004,
              <https://www.rfc-editor.org/info/rfc3862>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

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   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC7763]  Leonard, S., "The text/markdown Media Type", RFC 7763,
              DOI 10.17487/RFC7763, March 2016,
              <https://www.rfc-editor.org/info/rfc7763>.

   [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/info/rfc8174>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [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/info/rfc9420>.

   [W3C.CR-html52-20170808]
              Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and
              S. Moon, "HTML 5.2", World Wide Web Consortium CR CR-
              html52-20170808, 8 August 2017,
              <https://www.w3.org/TR/2017/CR-html52-20170808>.

9.2.  Informative References

   [DoubleRatchet]
              Perrin, T. and M. Marlinspike, "The Double Ratchet
              Algorithm", 20 November 2016,
              <https://signal.org/docs/specifications/doubleratchet/>.

   [I-D.ietf-cbor-edn-literals]
              Bormann, C., "CBOR Extended Diagnostic Notation (EDN):
              Application-Oriented Literals, ABNF, and Media Type", Work
              in Progress, Internet-Draft, draft-ietf-cbor-edn-literals-
              09, 18 May 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-cbor-edn-literals-09>.

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   [I-D.ietf-mimi-arch]
              Barnes, R., "An Architecture for More Instant Messaging
              Interoperability (MIMI)", Work in Progress, Internet-
              Draft, draft-ietf-mimi-arch-00, 2 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mimi-
              arch-00>.

   [I-D.mahy-mimi-identity]
              Mahy, R., "More Instant Messaging Interoperability (MIMI)
              Identity Concepts", Work in Progress, Internet-Draft,
              draft-mahy-mimi-identity-02, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-mahy-mimi-
              identity-02>.

   [I-D.ralston-mimi-terminology]
              Ralston, T., "MIMI Terminology", Work in Progress,
              Internet-Draft, draft-ralston-mimi-terminology-03, 23
              October 2023, <https://datatracker.ietf.org/doc/html/
              draft-ralston-mimi-terminology-03>.

   [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>.

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183,
              DOI 10.17487/RFC2183, August 1997,
              <https://www.rfc-editor.org/info/rfc2183>.

   [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
              "MIME Security with OpenPGP", RFC 3156,
              DOI 10.17487/RFC3156, August 2001,
              <https://www.rfc-editor.org/info/rfc3156>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

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   [RFC8098]  Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition
              Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098,
              February 2017, <https://www.rfc-editor.org/info/rfc8098>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/info/rfc8551>.

   [RFC9078]  Crocker, D., Signes, R., and N. Freed, "Reaction:
              Indicating Summary Reaction to a Message", RFC 9078,
              DOI 10.17487/RFC9078, August 2021,
              <https://www.rfc-editor.org/info/rfc9078>.

Appendix A.  CDDL Schemas

   Below are Concise Data Definition Language (CDDL) [RFC8610] schemas
   for the formats described in the body of the document.

A.1.  Complete Message Format Schema

   mimiContent = [
     replaces: null / MessageId,
     topicId: bstr,
     expires: uint .size 4,
     inReplyTo: null / InReplyTo,
     lastSeen: [* MessageId],
     extensions: {* name => value },
     nestedPart: NestedPart
   ]

   NestedPart = [
     disposition: baseDispos / $extDispos / unknownDispos,
     language: tstr,
     partIndex: uint .size 2,
     ( NullPart // SinglePart // ExternalPart // MultiPart)
   ]

   NullPart = ( cardinality: nullpart )

   SinglePart = (
       cardinality: single,
       contentType: tstr,
       content: bstr
   )

   ExternalPart = (
       cardinality: external,

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       contentType: tstr,
       url: uri,
       expires: uint .size 4,
       size: uint .size 8,
       encAlg: uint .size 2,
       key: bstr,
       nonce: bstr,
       aad: bstr,
       hashAlg: uint .size 1,
       contentHash: bstr,
       description: tstr
   )

   MultiPart = (
       cardinality: multi,
       partSemantics: chooseOne / singleUnit / processAll,
       parts: [2* NestedPart]
   )

   InReplyTo = [
     message: MessageId,
     hashAlg: uint .size 8,
     hash: bstr
   ]

   baseDispos = &(
       unspecified: 0,
       render: 1,
       reaction: 2,
       profile: 3,
       inline: 4,
       icon: 5,
       attachment: 6,
       session: 7,
       preview: 8
   )
   unknownDispos = &( unknown: 9..255 ) ; Note: any ext_dispos take precedence

   MessageId = bstr .size 32          ; MessageId is derived from SHA256 hash
   name = tstr .size (1..255)
   value = bstr .size (0..4095)

   nullpart = 0
   single   = 1
   external = 2
   multi    = 3

   chooseOne  = 0

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   singleUnit = 1
   processAll = 2

A.2.  Implied Message Fields

   Below is a CDDL schema for the implied message fields.

   MessageDerivedValues = [
       messageId: MessageId,               ; sha256 hash of message ciphertext
       hubAcceptedTimestamp: Timestamp,
       mlsGroupId: bstr,
       senderLeafIndex: uint .size 4,
       senderClientUrl: uri,
       senderUserUrl: uri,
       roomUrl: uri
   ]

   MessageId = bstr .size 32
   Timestamp = #6.62(uint .size 8)    ; milliseconds since start of UNIX epoch

A.3.  Delivery Report Format

   MessageStatusReport = [
       timestamp: Timestamp,
       statuses: [ * PerMessageStatus ]
   ]

   PerMessageStatus = [
       messageId: MessageId,
       status: baseStatus / $extStatus / unknownStatus
   ]

   baseStatus = &(
       unread: 0,
       delivered: 1,
       read: 2,
       expired: 3,
       deleted: 4,
       hidden: 5,
       error: 6
   )
   unknownStatus = &( unknown: 7..255 )

   MessageId = bstr .size 32
   Timestamp = #6.62(uint .size 8)    ; milliseconds since start of UNIX epoch

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Appendix B.  Multipart examples

   In a heterogenous group of IM clients, it is often desirable to send
   more than one media type as alternatives, such that IM clients have a
   choice of which media type to render.  For example, imagine an IM
   group containing a set of clients which support a common video format
   and a subset which only support animated GIFs.  The sender could use
   a MultiPart NestablePart with chooseOne semantics containing both
   media types.  Every client in the group chat could render something
   resembling the media sent.  This is analogous to the multipart/
   alternative [RFC2046] media type.

   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 a
   MultiPart NestablePart with processAll semantics.  This is analogous
   to the multipart/mixed [RFC2046] media type.

   Note that there is a minor semantic difference between multipart/
   alternative and MultiPart with chooseOne semantics.  In multipart/
   alternative, the parts are presented in preference order by the
   sender.  With MultiPart the receiver chooses its "best" format to
   render according to its own preferences.

B.1.  Proprietary and Common formats sent as alternatives

   This shows sending a message containing both this profile and a
   proprietary messaging format simultaneously.

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   [
     null,                             / replaces          /
     h'',                              / topicId           /
     0,                                / expires           /
     null,                             / inReplyTo         /
     [],                               / lastSeen          /
     {},                               / extensions        /
     [
       1,                          / disposition = render      /
       "",                         / language                  /
       0,                          / partIndex = 0 (1st part)  /
       3,                          / cardinality = multi       /
       0,                          / partSemantics = chooseOne /
       [
         [
           1,                           / disposition = render /
           "",                          / language             /
           1,                           / partIndex = 1        /
           1,                           / cardinality = single /
           "text/markdown;variant=GFM", / contentType          /
           '# Welcome!'                 / content              /
         ],
         [
           1,                           / disposition = render /
           "",                          / language             /
           2,                           / partIndex = 1        /
           1,                           / cardinality = single /
                                        / contentType          /
           "application/vnd.examplevendor-fancy-im-message",
           h'dc861ebaa718fd7c3ca159f71a2001'       / content   /
         ]
       ]
     ]
   ]

B.2.  Mulitple Reactions Example

   This example shows sending a reaction with multiple separate emojis.

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   [
     null,                             / replaces          /
     h'',                              / topicId           /
     0,                                / expires           /
     null,                             / inReplyTo         /
     [],                               / lastSeen          /
     {},                               / extensions        /
     [
       1,                           / disposition = render       /
       "",                          / language                   /
       0,                           / partIndex = 0 (1st part)   /
       3,                           / cardinality = multi        /
       2,                           / partSemantics = processAll /
       [
         [
           1,                             / disposition = render /
           "",                            / language             /
           1,                             / partIndex = 1        /
           1,                             / cardinality = single /
           "text/plain;charset=utf-8",    / contentType          /
           h'E2 9D A4'               / content = Unicode "heart" /
         ],
         [
           1,                             / disposition = render /
           "",                            / language             /
           2,                             / partIndex = 2        /
           1,                             / cardinality = single /
           "text/plain;charset=utf-8",    / contentType          /
           h'F0 9F A5 B3'       / content = Unicode "party face" /
         ],
         [
           1,                             / disposition = render /
           "",                            / language             /
           3,                             / partIndex = 3        /
           1,                             / cardinality = single /
           "text/plain;charset=utf-8",    / contentType          /
           h'F0 9F A4 9E'  / content = Unicode "fingers crossed" /
         ]
       ]
     ]
   ]

B.3.  Complicated Nested Example

   This example shows separate GIF and PNG inline images with English
   and French versions of an HTML message.  A summary of the 11 parts
   are shown below.

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   Part  Description
    0    choose either GIF or PNG
    1      (with GIF) process all
    2        choose either English or French
    3          English
    4          French
    5        GIF
    6      (with PNG) process all
    7        choose either English or French
    8          English
    9          French
   10        PNG

   [
     null,                             / replaces          /
     h'',                              / topicId           /
     0,                                / expires           /
     null,                             / inReplyTo         /
     [],                               / lastSeen          /
     {},                               / extensions        /
     [
       1,     / disposition = render /
       "",    / language /
       0,     / partIndex = 0 (1st part)  /
       3,     / cardinality = multi       /
       0,     / partSemantics = chooseOne /
       [
         [
           1,   / disposition = render /
           "",  / language /
           1,   / partIndex = 1 /
           3,   / cardinality = multi /
           2,   / partSemantics = processAll /
           [
             [
               1,   / disposition = render /
               "",  / language /
               2,   / partIndex = 2 /
               3,   / cardinality = multi /
               0,   / partSemantics = chooseOne /
               [
                 [
                   1,   / disposition = render /
                   "en",  / language /
                   3,   / partIndex = 3 /
                   1,   / cardinality = single /
                   "text/html;charset=utf-8",   / contentType /
                   / content /

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                   '<html><body><h1>Welcome!</h1>'
                   '<img src="cid:5" alt="Welcome image"/>'
                   '</body></html>'
                 ],          / English HTML /
                 [
                   1,   / disposition = render /
                   "fr",  / language /
                   4,   / partIndex = 4 /
                   1,   / cardinality = single /
                   "text/html;charset=utf-8",   / contentType /
                   / content /
                   '<html><body><h1>Bienvenue!</h1>'
                   '<img src="cid:5" alt="Image bienvenue"/>'
                     '</body></html>'
                 ]           / French HTML /
               ]
             ],          / English or French HTML (refers to GIF)/
             [
               4,   / disposition = inline /
               "",  / language /
               5,   / partIndex = 5 /
               1,   / cardinality = single /
               "image/gif",   / contentType /
               h'028f83c894ca744f'       / content /
             ]               / GIF /
           ]
         ],                  / GIF with English or French HTML /
         [
           1,   / disposition = render /
           "",  / language /
           6,   / partIndex = 6 /
           3,   / cardinality = multi /
           2,   / partSemantics = processAll /
           [
             [
               1,   / disposition = render /
               "",  / language /
               7,   / partIndex = 7 /
               3,   / cardinality = multi /
               0,   / partSemantics = chooseOne /
               [
                 [
                   1,   / disposition = render /
                   "en",  / language /
                   8,   / partIndex = 8 /
                   1,   / cardinality = single /
                   "text/html;charset=utf-8",   / contentType /
                   / content /

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                   '<html><body><h1>Welcome!</h1>'
                   '<img src="cid:10" alt="Welcome image"/>'
                   '</body></html>'
                 ],          / English HTML /
                 [
                   1,   / disposition = render /
                   "fr",  / language /
                   9,   / partIndex = 9 /
                   1,   / cardinality = single /
                   "text/html;charset=utf-8",   / contentType /
                   / content /
                   '<html><body><h1>Bienvenue!</h1>'
                   '<img src="cid:10" alt="Image bienvenue"/>'
                   '</body></html>'
                 ]           / French HTML /
               ]
             ],          / English or French HTML (refers to PNG)/
             [
               4,   / disposition = inline /
               "",  / language /
               10,   / partIndex = 10 /
               1,   / cardinality = single /
               "image/png",
               h'6963cff36275fdb8'       / content /
             ]               / PNG /
           ]
         ]                  / PNG with English or French HTML /
       ]           / GIF or PNG (with English or French HTML) /
     ]
   ]

Appendix C.  Changelog

   RFC Editor, please remove this entire section.

C.1.  Changes between draft-mahy-mimi-content-01 and draft-mahy-mimi-
      content-02

   *  made semantics abstract (C++ structs) instead of using CPIM or
      MIME headers

C.2.  Changes between draft-mahy-mimi-content-02 and draft-ietf-mimi-
      content-00

   *  replaced threadId with topicId
   *  inReplyTo now has a hash of the referenced message
   *  clarified that replies are always to a specific version of a
      modified message

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   *  changed timestamp to a whole number of milliseconds since the
      epoch to avoid confusion
   *  added Security Considerations section
   *  added IANA Considerations section
   *  added change log

C.3.  Changes between draft-ietf-mimi-content-00 and draft-ietf-mimi-
      content-01

   *  created new abstract format for attachment information, instead of
      using message/external-body
   *  added discussion of encrypting external content
   *  clarified the difference between render and inline dispositions
   *  created a way for the messageId and timestamp to be shared in the
      MLS additional authenticated data field
   *  expanded discussion of what can and should be rendered when a
      mention is encountered; discussed how to prevent confusion attacks
      with mentions.
   *  added a lastSeen field used to ensure a more consistent sort order
      of messages in a room.

C.4.  Changes between draft-ietf-mimi-content-01 and draft-ietf-mimi-
      content-02

   *  consensus at IETF 118 was to use a hash of the ciphertext in lieu
      of the message ID
   *  consensus at IETF 118 was to use the hub accepted timestamp for
      protocol actions like sorting
   *  Updated author's address

C.5.  Changes between draft-ietf-mimi-content-02 and draft-ietf-mimi-
      content-03

   *  added hash of content to external content
   *  replaced abstract syntax with concrete TLS Presentation Language
      and CBOR syntaxes

C.6.  Changes between draft-ietf-mimi-content-03 and draft-ietf-mimi-
      content-04

   *  use CBOR as the binary encoding
   *  add multipart examples

Author's Address

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

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