Internet-Draft e2esm May 2021
Muffett Expires 8 November 2021 [Page]
Workgroup:
individual submission
Internet-Draft:
draft-muffett-end-to-end-secure-messaging-00
Published:
Intended Status:
Informational
Expires:
Author:
A. Muffett
Security Researcher

Functional Definition of End-to-End Secure Messaging

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 8 November 2021.

Table of Contents

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 plaintext content to relevant participants.

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

1.1. Comments

Comments are solicited and should be addressed to the working group's mailing list TODO 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.

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.

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. 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:

3.1. Equality of Participation

All participants MUST be peers who MUST have equal access to any given message's PCASM.

3.2. Transparency of Participation

The existence of all current conversation participants MUST be visible at the current time to all current conversation participants.

3.3. 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.

3.3.1. 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.

3.3.2. Non-Participation

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

3.4. 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.

3.4.1. 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.

3.5. Management and Visibility of Participant Clients and Devices

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

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.

4. Definitions

These principles MUST be measured with respect to the following definitions:

4.1. Participant

A participant is any entity - human, machine, software bot, conversation archiver, or other, that is bounded by the extent of that entity's [TrustedComputingBase].

4.2. Conversation

A conversation is a sequence of one or more messages over a period of time amongst a constant or evolving set of participants.

4.3. Plaintext Content and Sensitive Metadata (PCASM)

The PCASM of a message is defined as any of:

4.3.1. Content PCASM

Content PCASM is any data that can offer better than 50-50 certainty regarding the value of any given bit of the plaintext message content. ("content")

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 in ASCII encoding, is zero
  4. The MD5 hash of the content is 080aef839b95facf73ec599375e92d47
  5. The Salted-MD5 Hash of the content is : ...

4.3.2. 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.

For transport encryption of content, precise Size PCASM SHOULD NOT be observable.

4.3.3. Descriptive PCASM

Descriptive PCASM is data that describes the "content".

Descriptive 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: ...

4.3.4. Conversation Metadata (OPTIONAL)

Whether per-conversation "group" metadata, such as "group titles", "group topics", "group icons", or "group participant lists" constitute PCASM, is an OPTIONAL choice for the E2ESM software, but that choice MUST be made apparent to participants.

4.3.5. 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: ...

4.4. 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.

4.4.1. Why call this a "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 have 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".

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. Scope of a Participant in E2ESM

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

Participants are defined in terms of an entity's [TrustedComputingBase] to acknowledge that an entity MAY legitimately store, forward, or access messages by means that are outside of the E2ESM software.

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].

6. Rationale

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 privileged access to PCASM, for instance via direct database access, but it MAY have "normal" access to PCASM of conversations where it is a participant. (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 particpant 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.

7. See Also

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

8. IANA Considerations

This document has no IANA actions.

9. Security Considerations

This document is entirely composed of security considerations.

10. Informative References

[DualUse]
Wikipedia, "Dual-use technology", , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8174>.
[RicochetRefresh]
BlueprintForFreeSpeech, "Ricochet Refresh", , <https://www.ricochetrefresh.net>.
[TrustedComputingBase]
Wikipedia, "Trusted Computing Base", , <https://en.wikipedia.org/wiki/Trusted_computing_base>.

Author's Address

Alec Muffett
Security Researcher