A Duck Test for End-to-End Secure Messaging
draft-muffett-end-to-end-secure-messaging-03

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Author Alec Muffett 
Last updated 2021-07-12
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individual submission                                         A. Muffett
Internet-Draft                                       Security Researcher
Intended status: Informational                              12 July 2021
Expires: 13 January 2022

              A Duck Test for End-to-End Secure Messaging
              draft-muffett-end-to-end-secure-messaging-03

Abstract

   This document defines End-to-End Secure Messaging in terms of
   behaviours that MUST be exhibited by software that claims to
   implement it, or which claims to implement that subset which is known
   as End-to-End Encrypted Messaging.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 January 2022.

Copyright Notice

   Copyright (c) 2021 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
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   Please review these documents carefully, as they describe your rights
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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Comments  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Requirements for an End-to-End Secure Messenger . . . . . . .   4
   3.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Message and Platform  . . . . . . . . . . . . . . . . . .   5
     3.2.  Plaintext Content and Sensitive Metadata (PCASM)  . . . .   5
       3.2.1.  Content PCASM . . . . . . . . . . . . . . . . . . . .   5
       3.2.2.  Size PCASM  . . . . . . . . . . . . . . . . . . . . .   5
       3.2.3.  Analytic PCASM  . . . . . . . . . . . . . . . . . . .   5
       3.2.4.  Conversation Metadata (OPTIONAL)  . . . . . . . . . .   5
     3.3.  Entity  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Sender and Recipient  . . . . . . . . . . . . . . . . . .   6
     3.5.  Participants and Non-Participants . . . . . . . . . . . .   6
     3.6.  Conversation, Group, Centralised & Decentralised  . . . .   6
     3.7.  Backdoor  . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Principles  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Transparency of Participation . . . . . . . . . . . . . .   7
     4.2.  Integrity of Participation  . . . . . . . . . . . . . . .   7
       4.2.1.  Retransmission Exception  . . . . . . . . . . . . . .   7
     4.3.  Equality of Participation . . . . . . . . . . . . . . . .   8
     4.4.  Closure of Conversation . . . . . . . . . . . . . . . . .   8
       4.4.1.  Public Conversations and Self-Subscription  . . . . .   8
     4.5.  Management of Participant Clients and Devices . . . . . .   8
   5.  Rationales  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Why: Content PCASM  . . . . . . . . . . . . . . . . . . .   8
     5.2.  Why: Size PCASM . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Why: Analytic PCASM . . . . . . . . . . . . . . . . . . .   9
     5.4.  Why: Conversation Metadata as OPTIONAL PCASM  . . . . . .   9
     5.5.  Why: Entity and Participant . . . . . . . . . . . . . . .   9
     5.6.  Why: Backdoor . . . . . . . . . . . . . . . . . . . . . .  10
     5.7.  Why: Transparency of Participation  . . . . . . . . . . .  11
     5.8.  Why: Integrity of Participation . . . . . . . . . . . . .  11
     5.9.  Why: Equality of Participation  . . . . . . . . . . . . .  11
     5.10. Why: Closure of Conversation  . . . . . . . . . . . . . .  12
     5.11. Why: Management of Participant Clients and Devices  . . .  12
   6.  OPTIONAL Features of E2ESM  . . . . . . . . . . . . . . . . .  12
     6.1.  Disappearing Messages . . . . . . . . . . . . . . . . . .  12
     6.2.  Mutual Identity Verification  . . . . . . . . . . . . . .  13
   7.  Examples of PCASM . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Content PCASM . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  Size PCASM  . . . . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Analytic PCASM  . . . . . . . . . . . . . . . . . . . . .  14
     7.4.  Conversation Metadata as OPTIONAL PCASM . . . . . . . . .  14
     7.5.  Non-PCASM . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Worked Example  . . . . . . . . . . . . . . . . . . . . . . .  15

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   9.  See Also  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   10. Live Drafts . . . . . . . . . . . . . . . . . . . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   13. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   End-to-End Secure Messaging (E2ESM) is a mechanism which offers a
   digital analogue of "closed distribution lists" for sharing message
   content amongst a set of participants, where all participants are
   visible to each other and where non-participants are completely
   excluded from access to message content.

   In client-server-client network models it is common to implement
   E2ESM by means of encryption, in order to obscure content at rest
   upon a central server.  So therefore E2ESM is often narrowly regarded
   in terms of "end-to-end encryption".

   Other architectural approaches exist - for instance [Ricochet] which
   implements closed distribution by using secure point-to-point
   [RFC7686] networking to literally restrict the distribution of
   content to relevant participants.

   Therefore we describe E2ESM in terms of functional behaviours of the
   software rather than in terms of its implementation technologies and
   architecture.

1.1.  Comments

   Comments are solicited and should be addressed to the working group's
   mailing list and/or the author(s).

1.2.  Notational Conventions

   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.

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2.  Requirements for an End-to-End Secure Messenger

   Software which functions as an End-to-End Secure Messenger MUST
   satisfy the following principles, and MUST satisfy these principles
   in respect of the provided definitions for all forms of communication
   and data-sharing that the software offers.  The E2ESM software MAY
   comprise either a complete application, or a clearly defined subset
   of functionality within a larger application.

   Any software that does not satisfy these requirements is not an End-
   to-End Secure Messenger, and it does not implement End-to-End Secure
   Messaging, nor does it implement End-to-End Encrypted Messaging.

3.  Definitions

   These definitions are drafted in respect of many examples of software
   commonly held to offer (or have offered) end-to-end security; these
   examples include:

   1.  Signal Messenger

   2.  WhatsApp Messenger

   3.  Ricochet Messenger

   4.  PGP-Encrypted Email sent to an ad-hoc list of addressees, or to a
       maillist

   Further context for several of these definitions can also be found in
   the rationales section, below.

   For the avoidance of doubt we define a "messenger" as a software
   solution which enables communication between two or more entities,
   without offering newly-added participants retrospective access to
   content which was previously sent by prior participants.

   This echoes the distinction between a "maillist" versus a "maillist
   archive" or "web forum"; frequently these solutions are integrated
   but we only consider the maillist as a "messenger" per se.

   Use cases of a "messenger" may include sending and receiving any or
   all of:

   1.  UNICODE or ASCII messages

   2.  images, video files or audio files

   3.  one-way streaming video or audio

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   4.  two-way streaming video or audio, as in live calls

3.1.  Message and Platform

   A "message" is information of 0 or more bits, to be communicated.

   Messages possess both plaintext "content", and also "metadata" which
   describes the content.

   A "platform" is a specific instance of software which exists for the
   purpose of routing or exchanging messages.

3.2.  Plaintext Content and Sensitive Metadata (PCASM)

   The "PCASM" of a message is defined as the "plaintext content and
   sensitive metadata" of that message, comprising any or all of:

3.2.1.  Content PCASM

   Content PCASM is any data that can offer better than 50-50 certainty
   regarding the value of any bit of the content.  See "Rationales" for
   more.

3.2.2.  Size PCASM

   For block encryption of content, Size PCASM is the unpadded size of
   the content.

   For stream encryption of content, Size PCASM is currently undefined.
   (TODO, would benefit from broader input.)

   For transport encryption of content, exact Size PCASM SHOULD NOT be
   observable or inferable.

   See "Rationales" for more.

3.2.3.  Analytic PCASM

   Analytic PCASM is data which analyses, describes, reduces, or
   summarises the "content".  See "Rationales" for more.

3.2.4.  Conversation Metadata (OPTIONAL)

   Conversation Metadata MAY exist "outside" of messages and describe
   the conversation context.

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   Whether conversation metadata constitutes PCASM, is an OPTIONAL
   choice for E2ESM software, but that choice MUST be apparent to
   participants.

   See "Rationales" for more.

3.3.  Entity

   An "entity" is a human, machine, software bot, conversation archiver,
   or other, which sends and/or receives messages.

   Entities are bounded by the extent of their Trusted Computing Base
   ("TCB"), including all systems that they control and/or utilise.

3.4.  Sender and Recipient

   A "sender" is an entity which composes and sends messages.

   A "recipient" is an entity which receives messages and MAY be able to
   access the PCASM of those messages.

   For each message there will be one sender and one or more recipients.

3.5.  Participants and Non-Participants

   The union set of sender and recipients for any given message are the
   "participants" in that message.

   It follows that for any given message, all entities that exist
   outside of the participant set will be "non-participants" in respect
   of that message.

3.6.  Conversation, Group, Centralised & Decentralised

   A "conversation" is a sequence of one or more messages, and the
   responses or replies to them, over a period of time, amongst a
   constant or evolving set of participants.

   A given platform MAY distinguish between and support more than one
   conversation at any given time.

   In "centralised" E2ESM such as WhatsApp or Signal, the software MAY
   offer collective "group" conversation contexts that provide
   prefabricated sets of recipients for the client to utilise when a
   message is composed or sent.

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   In "decentralised" E2ESM such as PGP-Encrypted Email or Ricochet the
   recipients of each message are individually determined by each sender
   at the point of composition; however "group" metadata may also exist,
   in terms of (e.g.) email addressees or subject lines.

3.7.  Backdoor

   A "backdoor" is any intentional or unintentional mechanism, in
   respect of a given message and that message's participants, where
   some PCASM of that message MAY become available to a non-participant
   without the intentional action of a participant.

4.  Principles

   For a series of one or more "messages" each which are composed of
   "plaintext content and sensitive metadata" (PCASM) and which
   constitute a "conversation" amongst a set of "participants", to
   provide E2ESM will require:

4.1.  Transparency of Participation

   In the nature of "closed distribution lists", the participants in a
   message MUST be frozen into an immutable set at the moment when the
   message is composed or sent.

   The complete set of all recipients MUST be visible to the sender at
   the moment of message composition or sending.

   The complete set of participants in a message MUST be visible to all
   other participants.

4.2.  Integrity of Participation

   Excusing the "retransmission exception", PCASM of any given message
   MUST only be available to the fixed set of conversation participants
   from whom, to whom, and at the time when it was sent.

4.2.1.  Retransmission Exception

   If a participant that can access an "original" message intentionally
   "retransmits" (e.g. quotes, forwards) that message to create a new
   message within the E2ESM software, then the original message's PCASM
   MAY become available to a new, additional, and possibly different set
   of conversation participants, via that new message.

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4.3.  Equality of Participation

   All participants MUST be peers, i.e. they MUST have equal access to
   the PCASM of any message; see also "Integrity of Participation".

4.4.  Closure of Conversation

   The set of participants in a conversation SHALL NOT be increased
   except by the intentional action of one or more existing
   participants.

   Per "Transparency of Participation" that action (introducing a new
   participant) MUST be visible to all other participants

4.4.1.  Public Conversations and Self-Subscription

   Existing participants MAY publicly share links to the conversation,
   identifying data to assist discovery of the conversation, or other
   mechanisms to enable non-participant entities to subscribe themselves
   as conversation participants.  This MAY be considered legitimate
   "intentional action" to increase the set of participants in the
   group.

4.5.  Management of Participant Clients and Devices

   Where there exists centralised E2ESM software that hosts
   participants:

   1.  The E2ESM software MUST provide each participant entity with
       means to review or revoke access for that participant's clients
       or devices that can access future PCASM.

   2.  The E2ESM software MUST provide each participant entity with
       notifications and/or complete logs of changes to the set of
       clients or devices that can or could access message PCASM.

5.  Rationales

   This explanatory section regarding the principles has been broken out
   for clarity and argumentation purposes.

5.1.  Why: Content PCASM

   Content PCASM MUST be protected as it comprises that which is
   "closed" from general distribution.

   The test for measuring this is (intended to be) modeled upon
   ciphertext indistinguishability [CipherInd]

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5.2.  Why: Size PCASM

   Exact size PCASM MUST be protected as it MAY offer insight into
   Content PCASM.

   The test for measuring this is (intended) to address risk of content
   becoming evident via plaintext length.

5.3.  Why: Analytic PCASM

   Analytic PCASM MUST be protected as it MAY offer insight into Content
   PCASM, for instance that the content shares features with other,
   specimen, or known plaintext content.

5.4.  Why: Conversation Metadata as OPTIONAL PCASM

   Conversational Metadata MAY offer insight into Content PCASM, however
   the abstractions of transport mechanism, group management, or
   platform choice, MAY render this moot.

   For example an PGP-Encrypted email distribution list named
   "blockchain-fans@example.com" would leak its implicit topic and
   participant identities to capable observers.

5.5.  Why: Entity and Participant

   The term "participant" in this document exists to supersede the more
   vague notion of "end" in the phrase "end-to-end".

   Entities, and thus participants, are defined in terms of their
   [TrustedComputingBase] to acknowledge that an entity MAY legitimately
   store, forward, or access messages by means that are outside of the
   E2ESM software.

   It is important to note that the concept of "entity" as defined by
   their TCB, is the foundation for all other trust in E2ESM.  This
   develops from the basic definitions of a [TrustedComputingBase] and
   from the concepts of "trust-to-trust" discussed in [RoleOfTrust].
   Failure of a participant to maintain integrity or control over their
   TCB should not be considered a failure of an E2ESM that connects it
   to other participants.

   For example: if a participant accesses their E2ESM software via
   remote desktop software, and their RDP session is hijacked by a third
   party; of if they back-up their messages in cleartext to cloud
   storage leading somehow to data exfiltration, neither of these would
   be a failure of E2ESM.  This would instead be a failure of the
   participant's [TrustedComputingBase].

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   Further: it would be obviously possible to burden an E2ESM with
   surfacing potential integrity issues of any given participant to
   other participants, e.g. "patch compliance".  But to require such in
   this standard would risk harming the privacy of the participant
   entity.  See also: "Mutual Identity Verification" in "OPTIONAL
   Features of E2ESM"

5.6.  Why: Backdoor

   In software engineering there is a perpetual tension between the
   concepts of "feature" versus "bug" - and occasionally "misfeature"
   versus "misbug".  These tensions arise from the problem of [DualUse]
   - that it is not feasible to firmly and completely ascribe
   "intention" to any hardware or software mechanism.

   The information security community has experienced a historical
   spectrum of mechanisms which have assisted non-participant access to
   PCASM.  These have variously been named as "export-grade key
   restrictions" ([ExportControl], then [Logjam]), "side channel
   attacks" ([Spectre] and [Meltdown]), "law enforcement access fields"
   [Clipper], and "key escrow" [CryptoWars].

   All of these terms combine an "access facilitation mechanism" with an
   "intention or opportunity" - and for all of them an access
   facilitation mechanism is first REQUIRED.

   An access facilitation mechanism is a "door", and is inherently
   [DualUse].  Because the goal of E2ESM is to limit access to PCASM
   exclusively to a defined set of participants, then the intended means
   of access is clearly the "front door"; and any other access mechanism
   is a "back door".

   If the term "back door" is considered innately pejorative,
   alternative, uncertain constructions such as "illegitimate access
   feature", "potentially intentional data-access weakness", "legally-
   obligated exceptional access mechanism", or any other phrase, all
   MUST combine both notions of an access mechanism (e.g. "door") and a
   definite or suspected intention (e.g. "legal obligation").

   So the phrase "back door" is brief, clear, and widely understood to
   mean "a secondary means of access".  In the above definition we
   already allow for the term to be prefixed with "intentional" or
   "unintentional".

   Thus it seems appropriate to use this term in this context, not least
   because it is also not far removed from the similar and established
   term "side channel".

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5.7.  Why: Transparency of Participation

   The "ends" of "end to end" are the participants; for a message to be
   composed to be exclusively accessible to that set of participants,
   all participants must be visible.

   For decentralised "virtual point-to-point" E2ESM solutions such as
   PGP-Encrypted Email or Ricochet, the set of participants is fixed by
   the author at the time of individual message composition, and MUST be
   visible to all participants.

   For "centralised" E2ESM solutions such as Signal or WhatsApp, the set
   of participants is a "group context" shared amongst all participants
   and at the time of individual message composition it MUST be
   inherited into a set of "fixed" per-participant access capabilities
   by the author.

5.8.  Why: Integrity of Participation

   Inherent in the term "end to end secure messenger" is the intention
   that PCASM will only be available to the participants ("ends") at the
   time the message was composed.

   If this was not the intention we would deduce that an E2ESM would
   automatically make past content available to newly-added conversation
   participants, thereby breaking forward secrecy.  This is not a
   characteristic of any E2ESM, but it is characteristic of several non-
   E2ESM.  Therefore the converse is true.

   As a concrete example this means that participants who are newly
   added to a "group" MUST NOT be able to read messages that were sent
   before they joined that group - unless (for instance) one pre-
   existing participant is explicitly intended to provide a "searchable
   archive" or similar function.  The function of such a participant is
   considered to be out of scope for the messenger.

5.9.  Why: Equality of Participation

   Without equality of participation it would be allowed for a person to
   deploy a standalone cleartext chat server, available solely over TLS-
   encrypted links, declare themselves to be "participants" in every
   conversation from its outset, access all message PCASM on that basis,
   and yet call themselves an E2ESM.

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   So this is an "anti-cheating" clause: all participant access to PCASM
   MUST be via the same mechanisms for all participants without favour
   or privilege, and in particular PCASM MUST NOT be available via other
   means, e.g. raw block-device access, raw filestore, raw database
   access, or network sniffing.

5.10.  Why: Closure of Conversation

   If a conversation is not "only extensible from within" then it would
   be possible for participants to be injected into the conversation
   thereby defeating the closure of message distribution.

   A subtle centralised vs: decentralised edge-case is as follows:
   consider a PGP-encrypted email distribution list.  Would it break
   "closure of conversation" for a non-participant email administrator
   to simply add new users to the maillist?

   Answer: no, because in this case the maillist is functioning as a
   "platform" for multiple "conversation" threads, and mere addition of
   of a new "transport-level" maillist member would not include them as
   a participant in ongoing E2ESM conversations; such inclusion would be
   a future burden upon existing participants.

   However: similar external injection of a new entity into a
   centralised WhatsApp or Signal "group" would be clearly a breach of
   "closure of conversation".

5.11.  Why: Management of Participant Clients and Devices

   There is little benefit in requiring conversations to be closed
   against "participant injection" if a non-participant may obtain PCASM
   access by forcing a platform to silently add extra means of PCASM
   access to an existing participant on behalf of that non-participant.

   Therefore to be an E2ESM the platform MUST provide the described
   management of participant clients and devices.

6.  OPTIONAL Features of E2ESM

6.1.  Disappearing Messages

   "Disappearing", "expiring", "exploding", "ephemeral" or other forms
   of time-limited access to PCASM are strongly RECOMMENDED but not
   obligatory mechanisms for E2ESM, not least because they are
   impossible to implement in a way that cannot be circumvented by e.g.
   screenshots.

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6.2.  Mutual Identity Verification

   Some manner of "shared key" which mutually assures participant
   identity and communications integrity are strongly RECOMMENDED but
   not obligatory mechanisms for E2ESM.

   The benefits of such mechanisms are limited to certain perspectives
   of certain platforms.

   For instance: in Ricochet the identity key of a user is the absolute
   source of truth for their identity, and excusing detection of
   typographic errors there is nothing which can be added to that in
   order to further assure their "identity".

   Similarly WhatsApp provides each participant with a "verifiable
   security QR code" and "security code change notifications", but these
   codes do not "leak" the number of "WhatsApp For Web" connections,
   desktop WhatsApp applications, or other clients which are bound to
   the E2ESM software which executes on that phone.

   Participant-client information of this kind MAY be a highly private
   aspect of that participant's TCB, and SHOULD be treated sensitively
   by platforms.

7.  Examples of PCASM

   For an example message with content ("content") of "Hello, world.",
   for the purposes of this example encoded as an ASCII string of length
   13 bytes without terminator character.

7.1.  Content PCASM

   Examples of Content PCASM would include, non-exclusively:

   1.  The content is "Hello, world."

   2.  The content starts with the word "Hello"

   3.  The top bit of the first byte of the content, is zero

   4.  The MD5 hash of the content is 080aef839b95facf73ec599375e92d47

   5.  The Salted-MD5 Hash of the content is : ...

7.2.  Size PCASM

   Size PCASM is defined in the main text, as it relates to the
   transport and/or content encryption mechanisms.

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7.3.  Analytic PCASM

   Examples of Analytic PCASM would include, non-exclusively:

   1.  The content contains the substring "ello"

   2.  The content does not contain the word "Goodbye"

   3.  The content contains a substring from amongst the following set:
       ...

   4.  The content does not contain a substring from amongst the
       following set: ...

   5.  The hash of the content exists amongst the following set of
       hashes: ...

   6.  The hash of the content does not exist amongst the following set
       of hashes: ...

   7.  The content was matched by a machine-learning classifier with the
       following training set: ...

7.4.  Conversation Metadata as OPTIONAL PCASM

   Examples of Conversation Metadata would include, non-exclusively:

   1.  maillist email addresses

   2.  maillist server names

   3.  group titles

   4.  group topics

   5.  group icons

   6.  group participant lists

7.5.  Non-PCASM

   Information which would not be PCASM would include, non-exclusively:

   1.  The content is sent from Alice

   2.  The content is sent to Bob

   3.  The content is between 1 and 16 bytes long

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   4.  The content was sent at the following date and time: ...

   5.  The content was sent from the following IP address: ...

   6.  The content was sent from the following geolocation: ...

   7.  The content was composed using the following platform: ...

8.  Worked Example

   Consider FooBook, a hypothetical example company which provides
   messaging services for conversations between entities who use it.

   For each conversation FooBook MUST decide whether to represent itself
   as a conversation participant or as a non-participant.  (Transparency
   of Participation)

   If FooBook decides to represent itself as a non-participant, then it
   MUST NOT have any access to PCASM.  (Integrity of Participation /
   Non-Participation)

   If FooBook decides to represent itself as a participant, then it MUST
   NOT have "exceptional" access to PCASM, despite being the provider of
   the service - for instance via raw database access or network
   sniffing.  However it MAY participate in E2ESM conversations in a
   "normal" way, and thereby have "normal" access to intra-conversation
   PCASM.  (Integrity of Participation, Equality of Participation)

   FooBook MAY retain means to eject reported abusive participants from
   a conversation.  (Decrease in Closure of Participation)

   FooBook MUST NOT retain means to forcibly insert new participants
   into a conversation.  For clarity: this specification does not
   recognise any notion of "atomic" exchange of one participant with
   another, treating it as an ejection, followed by an "illegitimate"
   insertion.  (Increase in Closure of Participation)

   FooBook MUST enable the user to observe and manage the complete state
   of their [TrustedComputingBase] with respect to their FooBook
   messaging services.  (Management and Visibility)

   FooBook MAY treat conversation metadata as PCASM, but it MUST
   communicate to participants whether it does or does not.

9.  See Also

   A different approach to defining (specifically) end-to-end encryption
   is discussed in [I-D.knodel-e2ee-definition].

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10.  Live Drafts

   Live working drafts of this document are at:
   https://github.com/alecmuffett/draft-muffett-end-to-end-secure-
   messaging (https://github.com/alecmuffett/draft-muffett-end-to-end-
   secure-messaging)

11.  IANA Considerations

   This document has no IANA actions.

12.  Security Considerations

   This document is entirely composed of security considerations.

13.  Informative References

   [CipherInd]
              Wikipedia, "Ciphertext indistinguishability", 2021,
              <https://en.wikipedia.org/wiki/
              Ciphertext_indistinguishability>.

   [Clipper]  Wikipedia, "Clipper chip", 2021,
              <https://en.wikipedia.org/wiki/Clipper_chip>.

   [CryptoWars]
              Wikipedia, "Crypto Wars", 2021,
              <https://en.wikipedia.org/wiki/Crypto_Wars>.

   [DualUse]  Wikipedia, "Dual-use technology", 2021,
              <https://en.wikipedia.org/wiki/Dual-use_technology>.

   [ExportControl]
              Wikipedia, "Export of cryptography from the United
              States", 2021, <https://en.wikipedia.org/wiki/Export_of_cr
              yptography_from_the_United_States#Cold_War_era>.

   [I-D.knodel-e2ee-definition]
              Knodel, M., Baker, F., Kolkman, O., Celi, S., and G.
              Grover, "Definition of End-to-end Encryption", Work in
              Progress, Internet-Draft, draft-knodel-e2ee-definition-00,
              22 February 2021, <https://datatracker.ietf.org/doc/html/
              draft-knodel-e2ee-definition-00>.

   [Logjam]   Wikipedia, "Logjam", 2021, <https://en.wikipedia.org/wiki/
              Logjam_(computer_security)>.

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   [Meltdown] Wikipedia, "Meltdown", 2021,
              <https://en.wikipedia.org/wiki/
              Meltdown_(security_vulnerability)>.

   [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>.

   [RFC7686]  Appelbaum, J. and A. Muffett, "The ".onion" Special-Use
              Domain Name", RFC 7686, DOI 10.17487/RFC7686, October
              2015, <https://www.rfc-editor.org/info/rfc7686>.

   [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>.

   [Ricochet] BlueprintForFreeSpeech, "Ricochet Refresh", 2021,
              <https://www.ricochetrefresh.net>.

   [RoleOfTrust]
              Clark, D. D. and M. S. Blumenthal, "The End-to-End
              Argument and Application Design: The Role of Trust", 2011,
              <https://www.repository.law.indiana.edu/fclj/vol63/
              iss2/3>.

   [Spectre]  Wikipedia, "Spectre", 2021,
              <https://en.wikipedia.org/wiki/
              Spectre_(security_vulnerability)>.

   [TrustedComputingBase]
              Wikipedia, "Trusted Computing Base", 2021,
              <https://en.wikipedia.org/wiki/Trusted_computing_base>.

Author's Address

   Alec Muffett
   Security Researcher

   Email: alec.muffett@gmail.com

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