Functional Definition of End-to-End Secure Messaging
draft-muffett-end-to-end-secure-messaging-01
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draft-muffett-end-to-end-secure-messaging-01
individual submission A. Muffett
Internet-Draft Security Researcher
Intended status: Informational 24 May 2021
Expires: 25 November 2021
Functional Definition of End-to-End Secure Messaging
draft-muffett-end-to-end-secure-messaging-01
Abstract
This document defines End-to-End Secure Messaging in terms of the
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
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 25 November 2021.
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
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 Simplified BSD License text
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 . . . . . . . . . . . . . . . . . . 4
3.2. Plaintext Content and Sensitive Metadata (PCASM) . . . . 4
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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Sender and Recipient . . . . . . . . . . . . . . . . . . 5
3.5. Participants and Non-Participants . . . . . . . . . . . . 6
3.6. Conversation, Group, De-/Centralised . . . . . . . . . . 6
3.7. Backdoor . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Principles . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Transparency of Participation . . . . . . . . . . . . . . 6
4.2. Integrity of Participation . . . . . . . . . . . . . . . 7
4.2.1. Retransmission Exception . . . . . . . . . . . . . . 7
4.3. Equality of Participation . . . . . . . . . . . . . . . . 7
4.4. Closure of Conversation . . . . . . . . . . . . . . . . . 7
4.4.1. Public Conversations and Self-Subscription . . . . . 7
4.5. Management of Participant Clients and Devices . . . . . . 7
5. Rationales . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Why: Content PCASM . . . . . . . . . . . . . . . . . . . 8
5.2. Why: Size PCASM . . . . . . . . . . . . . . . . . . . . . 8
5.3. Why: Analytic PCASM . . . . . . . . . . . . . . . . . . . 8
5.4. Why: Conversation Metadata as OPTIONAL PCASM . . . . . . 8
5.5. Why: Entity and Participant . . . . . . . . . . . . . . . 8
5.6. Why: Backdoor . . . . . . . . . . . . . . . . . . . . . . 9
5.7. Why: Transparency of Participation . . . . . . . . . . . 10
5.8. Why: Integrity of Participation . . . . . . . . . . . . . 10
5.9. Why: Equality of Participation . . . . . . . . . . . . . 11
5.10. Why: Closure of Conversation . . . . . . . . . . . . . . 11
5.11. Why: Management of Participant Clients and Devices . . . 11
6. OPTIONAL Features of E2ESM . . . . . . . . . . . . . . . . . 12
6.1. Disappearing Messages . . . . . . . . . . . . . . . . . . 12
6.2. Mutual Identity Verification . . . . . . . . . . . . . . 12
7. Examples of PCASM . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Content PCASM . . . . . . . . . . . . . . . . . . . . . . 12
7.2. Size PCASM . . . . . . . . . . . . . . . . . . . . . . . 13
7.3. Analytic PCASM . . . . . . . . . . . . . . . . . . . . . 13
7.4. Conversation Metadata as OPTIONAL PCASM . . . . . . . . . 13
7.5. Non-PCASM . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Worked Example . . . . . . . . . . . . . . . . . . . . . . . 14
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9. See Also . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Live Drafts . . . . . . . . . . . . . . . . . . . . . . . . . 15
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
12. Security Considerations . . . . . . . . . . . . . . . . . . . 15
13. Informative References . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
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 excluded from
access to message content.
In client-server 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 [RicochetRefresh]
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 goals and
technologies.
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 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 (in limited circumstances)
Further context for several of these definitions can also be found in
the rationales section, below.
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 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:
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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.
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
(TBD)
For transport encryption of content, exact Size PCASM SHOULD NOT be
observable or inferable.
3.2.3. Analytic PCASM
Analytic PCASM is data which analyses, describes, reduces, or
summarises the "content".
3.2.4. Conversation Metadata (OPTIONAL)
Conversation Metadata MAY exist "outside" of messages and describe
the conversation context.
Whether conversation metadata constitutes PCASM, is an OPTIONAL
choice for E2ESM software, but that choice MUST be apparent to
participants.
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.
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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, De-/Centralised
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.
In "decentralised" E2ESM such as PGP-Encrypted Email or (somewhat)
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 set of 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.
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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.
4.3. Equality of Participation
All participants MUST be peers who MUST have equal means of 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.
4.4.1. Public Conversations and Self-Subscription
Existing participants MAY publicly share links, data, 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 clients or devices that can
access future PCASM.
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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.
5.2. Why: Size PCASM
Exact size PCASM MUST be protected as it MAY offer insight into
Content PCASM.
5.3. Why: Analytic PCASM
Analytic PCASM MUST be protected as it MAY offer insight into Content
PCASM.
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 irrelevant.
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].
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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].
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" (TLS, then Logjam), "side channel attacks" (Spectre and
Meltdown), "law enforcement access fields" (Clipper), and "key
escrow" (Crypto Wars).
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".
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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".
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 "virtual peer-to-peer" E2ESM solutions such as "Email with PGP"
or (to a limited extent) "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.
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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.
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.
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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.
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" clients which
are bound to 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
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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.
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
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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
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)
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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].
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
[DualUse] Wikipedia, "Dual-use technology", 2021,
<https://en.wikipedia.org/wiki/Dual-use_technology>.
[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://tools.ietf.org/html/draft-
knodel-e2ee-definition-00>.
[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>.
Muffett Expires 25 November 2021 [Page 15]
Internet-Draft e2esm May 2021
[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>.
[RicochetRefresh]
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>.
[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
Muffett Expires 25 November 2021 [Page 16]