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Versions: 00 01 02                                                      
mls                                                            M. Knodel
Internet-Draft                                                       CDT
Intended status: Informational                                  F. Baker
Expires: November 8, 2021
                                                              O. Kolkman
                                                                    ISOC
                                                                 S. Celi
                                                              Cloudflare
                                                               G. Grover
                                         Centre for Internet and Society
                                                            May 07, 2021


                  Definition of End-to-end Encryption
                    draft-knodel-e2ee-definition-01

Abstract

   End-to-end encryption (E2EE) is an application of cryptography in
   communications systems between endpoints.  E2EE systems are unique in
   providing features of confidentiality, integrity and authenticity for
   users.  Improvements to E2EE strive to maximise the system's security
   while balancing usability and availability.  Users of E2EE
   communications expect trustworthy providers of secure implementations
   to respect and protect their right to whisper.

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 November 8, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Formal definition of end-to-end encryption  . . . . . . . . .   3
     2.1.  End point . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  End-to-end principle  . . . . . . . . . . . . . . . . . .   3
     2.3.  Encryption  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  End-to-end encrypted systems design . . . . . . . . . . . . .   5
     3.1.  Features  . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Necessary features  . . . . . . . . . . . . . . . . .   6
       3.1.2.  Optional/desirable features . . . . . . . . . . . . .   6
     3.2.  Challenges  . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  End-user expectations . . . . . . . . . . . . . . . . . . . .   8
     4.1.  A conversation is confidential  . . . . . . . . . . . . .   8
     4.2.  Providers are trustworthy . . . . . . . . . . . . . . . .   9
     4.3.  Access by a third-party is impossible . . . . . . . . . .   9
     4.4.  Pattern inference is minimised  . . . . . . . . . . . . .   9
     4.5.  The e2ee system is not compromised  . . . . . . . . . . .  10
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   This document defines end-to-end encryption (E2EE) using three
   different dimensions that together comprise a full definition of
   E2EE, which can be applied in a variety of contexts.

   The first is a formal definition that draws on the basic
   understanding of end points and cryptography.  The second looks at
   E2EE systems from a design perspective, both its fundamental features
   and the direction of travel towards improving those features.  Lastly
   we consider the expectations of the user of E2EE systems.





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   These dimensions taken as a whole comprise a generally comprehensible
   picture of consensus at the IETF as to what is end-to-end encryption,
   irrespective of application, from messaging to video conferencing,
   and between any number of end points.

2.  Formal definition of end-to-end encryption

   An end-to-end encrypted communications system, irrespective of the
   content or the specific methods employed, relies on two important and
   rigorous technical concepts: The end-to-end principle, defined in the
   IETF because of its importance to internet protocols; and encryption,
   an application of cryptography and the primary means employed by the
   IETF to secure internet protocols.

2.1.  End point

   Intuitively, an "end" either sends messages or receives them, usually
   both; other systems on the path are just that - other systems.

   It is, however, not trivial to establish the definition of an end
   point in isolation, because its existence inherentely depends on at
   least one other entity in a communications system.  That is why we
   will now move directly into an anlaysis of the end-to-end principle,
   which introduces nuance, described in the following sub-section.

   However despite the nuance for engineers, it is now widely accepted
   that the communication system itself begins and ends with the user
   [RFC8890].  We imagine people (through an application's user
   interface, or user agent) as components in a subsystem's design.  An
   important exception to this in E2EE systems might be the use of
   public key infrastructure where a third party is often used in the
   authentication phase to enhance the larger system's trust model.

   We cannot equate user agent and user, yet we also cannot fully
   separate them.  As user-agent computing becomes more complex and
   often more proprietary, the user agent less of an "advocate" for the
   best interests of the user.  This is why we focus in a later section
   on the e2ee system being able to fulfil user expectations.

2.2.  End-to-end principle

   We needed first to answer "what constitutes an end?", which is an
   important question in any review of the End-to-End Principle
   [RFC3724].  However the notion of an end point is more fully defined
   within the principle of end-to-end communications.

   In 1984 the "end-to-end argument" was introduced [saltzer] as a
   design principle that helps guide placement of functions among the



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   modules of a distributed computer system.  It suggests that functions
   placed at low levels of a system may be redundant or of little value
   when compared with the cost of providing them at that low level.
   It's used to design around questions about which parts of the system
   should make which decisions, and as such the identity of the actual
   "speaker" or "end" may be less obvious than it appears.  The
   communication described by Saltzer is between communicating
   processes, which may or may not be on the same physical machine, and
   may be implemented in various ways.  For example, a BGP speaker is
   often implemented as a process that manages the Routing Information
   Base (RIB) and communicates with other BGP speakers using an
   operating system service that implements TCP.  The RIB manager might
   find itself searching the RIB for prefixes that should be advertised
   to a peer, and performing "writes" to TCP for each one.  TCP in this
   context often implements a variant of the algorithm described in RFC
   868 (the "Nagle algorithm"), which accumulates writes in a buffer
   until there is no data in flight between the communicants, and then
   sends it - which might happen several times during a single search by
   the RIB manager.  In that sense, the RIB manager might be thought of
   as the "end", because it decides what should be communicated, or TCP
   might be the "end", because it actually sends the TCP Segment,
   detects errors if they occur, retransmits it if necessary, and
   ultimately decides that the segment has been successfully
   transferred.

   Another important question is "what statement exactly summarizes the
   end-to-end principle?".  Saltzer answered this in two ways, the first
   of which is that the service implementing the transaction is most
   correct if it implements the intent of the application that sent it,
   which would be to move the message toward the destination address in
   the relevant IP header.  Salzer's more thorough treatment, however,
   deals with end cases that come up in implementation: "Examples
   discussed in the paper", according to the abstract, "include bit
   error recovery, security using encryption, duplicate message
   suppression, recovery from system crashes, and delivery
   acknowledgement."  It also notes that there is occasionally a
   rationale for ignoring the end-to-end arguments for the purposes of
   optimization.  There may be other user expectations or design
   features, some explained below, which need to be balanced with the
   end-to-end argument.

2.3.  Encryption

   From draft-dkg-hrpc-glossary-00, encryption is fundamental to the
   end-to-end principle.  "End-to-End : The principal of extending
   characteristics of a protocol or system as far as possible within the
   system.  For example, end-to-end instant message encryption would
   conceal communication content from one user's instant messaging



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   application through any intermediate devices and servers all the way
   to the recipient's instant messaging application.  If the message was
   decrypted at any intermediate point-for example at a service
   provider-then the property of end-to-end encryption would not be
   present."[dkg] Note that this only talks about the contents of the
   communication and not the metadata generated from it.

   The way to achieve a truly end-to-end communications system is indeed
   to encrypt the content of the data exchanged between the endpoints,
   eg sender(s) and receiver(s).  The more common end-to-end technique
   for encrypting uses a double-ratchet algorithm with an authenticated
   encryption scheme, present in many modern messenger applications such
   as those considered in the IETF Messaging Layer Security working
   group, whose charter is to create a document that satisfies the need
   for several Internet applications for group key establishment and
   message protection protocols.[mls] OpenPGP, mostly used for email,
   uses a different technique to achieve encryption.  It is also
   chartered in the IETF to create an specification that covers object
   encryption, object signing, and identity certification.[openpgp] Both
   protocols rely on the use of asymmetric and symmetric encryption, and
   have to exchange public keys with amongst end points.

   There are dozens of documents in the RFC Series that fundamentally
   and technically define encryption schemes.  Perhaps interesting work
   to be done would be to survey all existing documents of this kind to
   define, in aggregate, their common features.  The point is, the IETF
   has clear mandate and demonstrated expertise in defining the
   specifics of encrypted communications of the internet.

3.  End-to-end encrypted systems design

   When looking at E2EE systems from a design perspective, the first
   consideration is the list of fundamental features that distinguish an
   E2EE system from one that does not employ E2EE.  Secondly one must
   consider the direction of travel for improving the features of E2EE
   systems.  In other words, what challenges are the designers,
   developers and implementers of E2EE systems facing?

   The features and challenges listed below are framed holistically
   rather than from the perspective of their design, development,
   implementation or use.

3.1.  Features

   Defining a technology can also be done by inspecting what it does, or
   is meant to do, in the form of features.  The features of end-to-end
   encryption from an implementation perspective can be inspected across
   several important categories: 1) the necessary features of E2EE of



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   authenticity, confidentiality and integrity, whereas features of 2)
   availability, deniability, forward secrecy and post-compromise
   security are enhancements to E2EE systems.

3.1.1.  Necessary features

   Authenticity  A system provides message authenticity if the recipient
      is certain who sent the message and the sender is certain who
      received it.

   Confidentiality  A system provides message confidentiality if only
      the sender and intended recipient(s) can read the message
      plaintext, i.e. messages are encrypted by the sender such that
      only the intended recipient(s) can decrypt them.

   Integrity  A system provides message integrity when it guarantees
      that messages has not been modified in transit, i.e. a recipient
      is assured that the message they have received is exactly what the
      sender intented to sent.

3.1.2.  Optional/desirable features

   Availability  A system provides high availability if the user is able
      to get to the message when they so desire and potentially from
      more than one device, i.e. a message arrives to a recipient even
      if they have been offline for a long time.

   Deniability  Deniability ensures that anyone with a record of the
      transcript, including message recipients, cannot cryptographically
      prove to others that a particular participant of a communication
      authored the message.  As demonstrated by the Signal and OTR
      protocols, this property has to exist in conjunction with message
      authenticity, i.e. participants in a communication have to be
      assured that they are communicating with the intended parties but
      this assurance cannot be proof to any other parties.

   Forward secrecy  Forward secrecy is a security property that prevents
      attackers from decrypting all encrypted data they have previously
      captured over a communication channel, even if they have
      compromised one of the endpoints.  Forward secrecy is usually
      achieved by updating the encryption/decryption keys, and older
      ones are deleted periodically.

   Post-compromise security  Post-compromise security is a security
      property that seeks to guarantee a way to recover from an end-
      point compromise (and consequently that communication sent post-
      compromise is protected with the same security properties that
      existed before the compromise).  It is usually achieved by adding



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      ephemeral key exchanges to the derivation of encryption/decryption
      keys.

3.2.  Challenges

   Earlier we defined end-to-end encryption using formal definitions
   assumed by internet protocol implementations.  And because "the IETF
   is a place for state-of-the-art producing high quality, relevant
   technical documents that influence the way people design, use, and
   manage the Internet" we can be confident that current deployments of
   end-to-end encrypted technologies in the IETF indicate the cutting
   edge of their developments, yet another way to define what is, or
   ideally should be, how a technology is defined.

   Below is an exhaustive, yet vaguely summarised, list of the
   challenges currently faced by protocol designers of end-to-end
   encrypted systems.  In other words, in order to realise the goals of
   end-to-end encrypted systems, both for users and implementers (see
   previous section), these problems must be tackled.  Problems that
   fall outside of this list are likely 1) unnecessary feature requests
   that negligibly, or do nothing to, achieve the aims of end-to-end
   encrypted systems or 2) in some way antithetical to the goals of end-
   to-end encrypted systems.

   Public key verification is very difficult for users to manage.
   Authentication of the two ends is required for confidential
   conversations.  Therefore solving the problem of verification of
   public keys is a major concern for any end-to-end encrypted system
   design.  Some applications bind together the account identity and the
   key, and leaves users to establish a trust relationship between them,
   assisted by public key fingerprint information.

   Users want to smoothly switch application use between devices, but
   this comes at a cost to the security of user data.  Thus there is a
   problem of availability in end-to-end encrypted systems because the
   account identity's private key is generated by and stored on the end-
   user's original device and to move the private key to another device
   compromises the security of one of the end-points of the system.

   Existing protocols are vulnerable to meta-data analysis, even though
   meta-data is often much more sensitive than content.  Meta-data is
   plaintext information that travels across the wire and includes
   delivery-relevant details that central servers need such as the
   account identity of end-points, timestamps, message size.  Meta-data
   is difficult to obfuscate efficiently.






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   Users need to communicate in groups, but this presents major problems
   of scale for end-to-end encryption systems that rely on public key
   cryptography.

   The whole of a user's data should remain secure if only one message
   is compromised.  However, for encrypted communication, you must
   currently choose between forward secrecy or the ability to
   communicate asynchronously.  This presents a problem for application
   design that uses end-to-end encryption for asynchronous messaging
   over email, RCS, etc.

   Users of E2EE systems should be able to communicate with any medium
   of their choice, from plain text to large files, however there is
   often a resource problem because there are no open protocols to allow
   users to securely share the same resource in an end-to-end encrypted
   system.  Client-side, eg end-point, activities like URL unfurling
   scanning.

   Usability considerations are sometimes in conflict with security
   considerations, such as message read status, typing indicators, URL/
   link previews.

   Deployment is notoriously challenging for any software application
   where maintenance and updates can be particularly disastrous for
   obsolete cryptographic libraries.

4.  End-user expectations

   While the formal definition and properties of an E2EE system relate
   to communication security, they do not draw from a comprehensive
   threat model or speak to what users expect from E2EE communication.
   It is in this context that some E2EE designs and architectures may
   ultimately run contrary to user expectations of E2EE systems.
   [GEC-EU] Although some system designs do not directly violate "the
   math" of encryption algorithms, they do so by implicating and
   weakening other important aspects of an E2EE _system_.

4.1.  A conversation is confidential

   Users talking to one another in an E2EE system should be the only
   ones that know what they are talking about [RFC7624].  People have
   the right to privacy as defined in international human rights law and
   within the right to free expression and to hold opinions is inferred
   the right to whisper, whether or not they are using digital
   communications or walking through a field.






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4.2.  Providers are trustworthy

   While trustworth can be rigourously defined from an engineering
   perspective, for the purposes of this document we choose a definition
   of Trustworthy inspired by an internal workshop by Internet Society
   staff:

   Trustworthy  A system is completely trustworthy if and only if it is
      completely resilient, reliable, accountable, and secure in a way
      that consistently meets users' expectations.  The opposite of
      trustworthy is untrustworthy.

   This definition is complete in its positive and negative aspects:
   what it is, eg "Worthy of confidence" and what it isn't, eg in RFC
   7258: "behavior that subverts the intent of communicating
   partieswithout the agreement of those parties."  [RFC7258]

   Therefore, a trustworthy end-to-end encrypted communication system is
   the set of functions needed by two or more parties to communicate
   among each other in a confidential and authenticated fashion without
   any third party having access to the content of that communication
   where the functions that offer the confidentiallity and authenticity
   are trustworthy.

4.3.  Access by a third-party is impossible

   No matter the specifics, any methods used to access to the content of
   the messages by a third party would violate a user's expectations of
   E2EE messaging.  "[T]hese access methods scan message contents on the
   user's [device]", which are then "scanned for matches against a
   database of prohibited content before, and sometimes after, the
   message is sent to the recipient."  [GEC-EU]

   If a method makes private communication, intended to be sent over an
   encrypted channel between end points, available to parties other than
   the recipient, without formally interfering with channel
   confidentiality, that method violates the understood expectation of
   that security property.

4.4.  Pattern inference is minimised

   Analyses such as traffic fingerprinting or homomorphic encryption
   computations should be considred outside the scope of an E2EE
   system's goals of providing secure communications to end users.

   Such methods of analyses, outside of or as part of E2EE system
   design, allows third parties to draw inferences from communication
   that was intended to be confidential.  "By allowing private user data



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   to be scanned via direct access by servers and their providers," the
   use of these methods should be considered an affront to "the privacy
   expectations of users of end-to-end encrypted communication systems."
   [GEC-EU]

   Not only should an E2EE system value user privacy by not allowing
   pattern inference, it should actively be attempting to solve issues
   of metadata and traceability (enhanced metadata) through further
   innovation that stays ahead of advances in these techniques.

4.5.  The e2ee system is not compromised

   RFC 3552 talks about the Internet Threat model such as the assumption
   that the user can expect any communications systems, but perhaps
   especially E2EE systems, not being compromised.[RFC3552] Compromises
   to E2EE systems are often referred to as "backdoors" but are often
   presented as additional design features like "key escrow."  Users of
   E2EE systems would not expect a front, back or side door entrance
   into their confidential conversations and would expect a provider to
   actively resist- technically and legally- compromise through these
   means.

5.  Conclusions

   From messaging to video conferencing, there are many competing
   features in an E2EE system that is secure and usable.  The most well
   designed system cannot meet the expectations of every user, nor does
   an ideal system exist from any dimension.  E2EE is a technology that
   is constantly improving to achieve the ideal as defined in this
   document.

   Features and functionalities of e2ee systems should be developed and
   improved in service of end user expectations for privacy preserving
   communications.

6.  Acknowledgements

   Fred Baker, Stephen Farrell, Richard Barnes, Olaf Kolkman all
   contributed to the early strategic thinking of this document and
   whether it would be useful to the IETF community.

   The folks at Riseup and the LEAP Encryption Access Project have
   articulated brilliantly the hardest parts of end-to-end encryption
   systems that serve the end users' right to whisper.

   Ryan Polk at the Internet Society has energy to spare when it comes
   to organising meaningful contributions, like this one, for the
   technical advisors of the Global Encryption Coalition.



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7.  Security Considerations

   As this draft concerns an informational document, there are no
   security considerations.

8.  IANA Considerations

   This document has no actions for IANA.

9.  Informative References

   [dkg]      Gillmor, D., "Human Rights Protocol Considerations
              Glossary", 2015,
              <https://tools.ietf.org/html/draft-dkg-hrpc-glossary-00>.

   [GEC-EU]   Global Encryption Coaliation, ., "Breaking encryption
              myths: What the European Commission's leaked report got
              wrong about online security", 2020,
              <https://www.globalencryption.org/2020/11/breaking-
              encryption-myths/>.

   [mls]      IETF, ., "Messaging Layer Security", 2018,
              <https://datatracker.ietf.org/doc/charter-ietf-mls>.

   [openpgp]  IETF, ., "Open Specification for Pretty Good Privacy",
              2020,
              <https://datatracker.ietf.org/doc/charter-ietf-openpgp>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC3724]  Kempf, J., Ed., Austein, R., Ed., and IAB, "The Rise of
              the Middle and the Future of End-to-End: Reflections on
              the Evolution of the Internet Architecture", RFC 3724,
              DOI 10.17487/RFC3724, March 2004,
              <https://www.rfc-editor.org/info/rfc3724>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.









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   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.

   [RFC8890]  Nottingham, M., "The Internet is for End Users", RFC 8890,
              DOI 10.17487/RFC8890, August 2020,
              <https://www.rfc-editor.org/info/rfc8890>.

   [saltzer]  Saltzer, et al, J., "End-to-end arguments in system
              design", 1984,
              <https://web.mit.edu/Saltzer/www/publications/endtoend/
              endtoend.pdf>.

Authors' Addresses

   Mallory Knodel
   CDT

   Email: mknodel@cdt.org


   Fred Baker

   Email: fredbaker.IETF@gmail.com


   Olaf Kolkman
   ISOC

   Email: kolkman@isoc.org


   Sofia Celi
   Cloudflare

   Email: cherenkov@riseup.net


   Gurshabad Grover
   Centre for Internet and Society

   Email: gurshabad@cis-india.org






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