Definition of End-to-end Encryption
draft-knodel-e2ee-definition-05

sec                                                            M. Knodel
Internet-Draft                                                       CDT
Intended status: Informational                                  F. Baker
Expires: 28 January 2023                                                
                                                              O. Kolkman
                                                                    ISOC
                                                                 S. Celi
                                                                   Brave
                                                               G. Grover
                                         Centre for Internet and Society
                                                            27 July 2022

                  Definition of End-to-end Encryption
                    draft-knodel-e2ee-definition-05

Abstract

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

Status of This Memo

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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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  . . . . . . . . . . . . . . . . . .   4
     2.3.  Encryption  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Concise definition of end-to-end security . . . . . . . .   7
   3.  End-to-end encrypted systems design . . . . . . . . . . . . .   7
     3.1.  Features  . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.1.1.  Necessary features  . . . . . . . . . . . . . . . . .   8
       3.1.2.  Optional/desirable features . . . . . . . . . . . . .   8
     3.2.  Challenges  . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  End-user expectations . . . . . . . . . . . . . . . . . . . .  10
     4.1.  A conversation is confidential  . . . . . . . . . . . . .  11
     4.2.  Providers are trustworthy . . . . . . . . . . . . . . . .  11
     4.3.  Access by a third-party is impossible . . . . . . . . . .  11
     4.4.  Pattern inference is minimised  . . . . . . . . . . . . .  12
     4.5.  The end-to-end encrypted system is not compromised  . . .  12
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document defines end-to-end encryption using three different
   dimensions that together comprise a full definition of end-to-end
   encryption, 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
   end-to-end encrypted systems from a design perspective, both its
   fundamental features and proposed improvements on those features.
   Lastly this document considers the expectations of the user of end-
   to-end encrypted 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 and what
   defines an end, according to 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.  In the field of cryptography it is also possible to
   achieve a concise definition of end-to-end encrypted security.

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 inherently depends on at
   least one other entity in a communications system.  Instead the end-
   to-end principle, which is well established in internet standards and
   introduces nuance, is 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].  Imagine people (through an application's user interface,
   or user agent) as components in a subsystem's design.  An important
   exception to this in end-to-end encrypted 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.
   Responsible use of of public key infrastructure is required in such
   cases, such that the end-to-end encrypted system does not admit third
   parties under the user's identity.

   User agent and user cannot be equated, yet they cannot be fully
   separated.  As user-agent computing becomes more complex and often
   more proprietary, the user agent becomes less of an "advocate" for
   the best interests of the user.  This is why this document introduces
   a later section on the end-to-end encrypted system being able to
   fulfill user expectations.

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2.2.  End-to-end principle

   We need 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
   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
   is 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 Border Gateway
   Protocol (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.

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   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.

   More concisely, suppose that an end user is the end identity.  An
   end-to-end encrypted system may run between potential end points at
   different network layers within the end identity's possession.  These
   end points may then be considered acceptable sub-identities provided
   that no path between the end identity and sub-identity is accessible
   by any third party.  There are quite a number of examples of common
   situations where tunnels are used and this does not apply.  For
   instance, the examples below all provide encryption by which data is
   turned into clear text in locations that are not under control of the
   end user:

   *  The common VPN business model whereby a TLS or an IPsec tunnel
      terminates at the service provider's server and is subsequently
      forwarded to its destination elsewhere in unencrypted form;

   *  Email transport whereby an unencrypted message traverses from
      sending mail user agent, between various mail transfer agents, and
      finally to a receiving mail user agent, all over TLS protected
      connections;

   *  The encrypted connection of last mile connections such as those in
      4G LTE;

   This definition of end points accounts for potentially several
   devices owned by a user, and various application-specific forwarding
   or delivery options among them.  It also accounts for end-to-end
   encrypted systems running at different network layers.  Regardless of
   the sub-identities allowed, the definition is contingent on that all
   end sub-identities are under the end identity's control and no third
   party (or their sub-identities, e.g. system components under third-
   party control) can access the end sub-identities nor links between
   the sub-identity and end identity.  This creates a tree hierarchy

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   with the end user as the root at the top, and all potential end
   points being under their direct control, without third party access.
   As an example, decryption at organizational network router before
   message forwarding (encrypted or unencrypted) to the end identity
   does not constitute end-to-end encryption.  However, end-to-end
   encryption to a user's personal device and subsequent end-to-end
   encrypted message forwarding to another one of the user's personal
   devices (without access available to any third party at any link or
   on device) maintains end-to-end encrypted data possession for the
   user.

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
   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,
   e.g. sender(s) and receiver(s).  The more common end-to-end technique
   for encrypting uses the 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 a
   specification that covers object encryption, object signing, and
   identity certification [openpgp].  Both protocols rely on the use of
   asymmetric and symmetric encryption, and exchange public keys 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.

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   While encryption is fundamental to the end-to-end principle, it does
   not stand alone.  As in the history of all security, authentication
   and data integrity properties are also linked, and contributed to the
   end-to-end nature of end-to-end encryption.  Permission of data
   manipulation or pseudo-identities for third parties to allow access
   under the user's identity are against the intention of end-to-end
   encryption.  Thus, end point authenticity must be established as
   (sub-)identities of the end user, and end-to-end integrity must also
   be maintained by the system.  There is considerable system design
   flexibility available in entity authentication mechanisms and data
   authentication that still meet this requirement.

2.4.  Concise definition of end-to-end security

   A concise definition for end-to-end security can describe the
   security of the system by the probability of an adversary's success
   in breaking the system.  Example snippet:

   The adversary successfully subverts an end-to-end encrypted system if
   it can succeed in either of the following: 1) the adversary can
   produce the participant's local state (meaning the adversary has
   learned the contents of participant's messages), or 2) the states of
   conversation participants do not match (meaning that the adversary
   has influenced their communication in some way).  To prevent the
   adversary from trivially winning, the adversary is not allowed to
   compromise the participants' local state.

   We can say that a system is end-to-end secure if the adversary has
   negligible probability of success in either of these two scenarios
   [komlo].

3.  End-to-end encrypted systems design

   When looking at end-to-end encrypted systems from a design
   perspective, the first consideration is the list of fundamental
   features that distinguish an end-to-end encrypted system from one
   that does not employ end-to-end encryption.  Secondly one must
   consider the development goals for improving the features of end-to-
   end encrypted systems, in other words, the challenges defined by the
   designers, developers and implementers of end-to-end encrypted
   systems.

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

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

3.1.1.  Necessary features

   Authenticity  A system provides message authenticity if the recipient
      and sender attest to each other's identities in relation to the
      contents of their communications.

   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 recipient(s) shared and agreed by all recipients can
      decrypt them.

   Integrity  A system provides message integrity when it guarantees
      that messages that have been modified in transit can be detected
      reliably, i.e. a recipient is assured that a message cannot be
      undetectably modified in any way.

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 able to decrypt 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 widely
      implemented protocols, this optional property must exist in
      conjunction with the necessary property of message authenticity,
      i.e. participants in a communication must be assured that they are
      communicating with the intended parties but this assurance cannot
      be transmitted to any other parties.

   Forward secrecy  Forward secrecy is a security property that prevents
      attackers from decrypting encrypted data they have previously
      captured over a communication channel before the time of
      compromise, even if they have compromised one of the endpoints.

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      Forward secrecy is usually achieved by deriving new encryption/
      decryption keys, and old keys no longer required to encrypt or
      decrypt any new messages are immediately destroyed.

   Post-compromise security  Post-compromise security is a security
      property that seeks to guarantee future confidentiality and
      integrity even on the face of 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 new ephemeral key
      exchanges to the derivation of encryption/decryption keys.

   Metadata obfuscation  Digital communication inevitably generates data
      other than the content of the communication itself, such as IP
      addresses, date and time.  To enhance the confidentiality and
      security of end-to-end encrypted systems, steps should be taken to
      minimize metadata leakage such as user obfuscating IP addresses,
      reducing non-routing metadata, and avoiding extraneous message
      headers.

3.2.  Challenges

   Earlier in this document end-to-end encryption was defined using
   formal definitions assumed by internet protocol implementations.
   Also because the IETF is the place for "producing high quality,
   relevant technical documents that influence the way people design,
   use, and manage the Internet" [RFC3935] the reader can be confident
   that current deployments of end-to-end encrypted technologies in the
   IETF indicate the cutting edge of their developments, which is yet
   another way to define what is, or ideally should be, a particular
   kind of technology.

   Below is best effort list of the challenges currently faced by
   protocol designers of end-to-end encrypted systems.  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 are 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 leave users to establish a trust relationship between
      them, assisted by public key fingerprint information.

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   *  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 eliminate or obfuscate entirely.

   *  Users need to communicate in groups, but this presents 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 end-to-end encrypted systems should be able to
      communicate with any medium of their choice, from 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.

   *  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 end-to-end encrypted
   system relate to communication security, they do not draw from a
   comprehensive threat model or speak to what users expect from end-to-
   end encrypted communication.  It is in this context that some designs
   and architectures of end-to-end encryption may ultimately run
   contrary to user expectations of end-to-end encrypted systems

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   [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 end-to-end encrypted
   _system_.

4.1.  A conversation is confidential

   Users talking to one another in an end-to-end encrypted system should
   be the only ones that know what they are talking about [RFC7624].
   People have the right to data 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.

4.2.  Providers are trustworthy

   While "trustworthy" can be rigourously defined from an engineering
   perspective, for the purposes of this document a definition inspired
   by an internal workshop for Internet Society staff is sufficient:

   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, e.g.  "Worthy of confidence" and what it is not, e.g. in
   RFC 7258: "behavior that subverts the intent of communicating parties
   without the agreement of those parties" [RFC7258].

   Therefore, a trustworthy end-to-end encrypted communication system is
   the provider of 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 confidentiality 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
   end-to-end encrypted 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].  Third party access
   also covers cases without scanning -- namely, it should not be
   possible for any third-party end point to access the data regardless
   of reason.

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   If a method makes private communication, intended to be sent over an
   encrypted channel between end points, available to parties other than
   the sender and intended recipient(s), 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 other (encrypted or
   unencrypted) data analysis techniques should be considered outside
   the scope of an end-to-end encrypted system's goals of providing
   secure communications to end users.

   Such methods of analyses, outside of or as part of end-to-end
   encrypted system design, allow third parties to draw inferences from
   communication that was intended to be confidential.  "By allowing
   private user data 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 end-to-end encrypted system value user data
   privacy by not enabling 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 end-to-end encrypted 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 end-to-end encrypted systems, to not be intentionally
   compromised [RFC3552].  Intentional compromises of end-to-end
   encrypted systems are often referred to as "backdoors" but are often
   presented as additional design features under terms like "key
   escrow."  Users of end-to-end encrypted 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 end-to-end encrypted 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.  End-to-end
   encryption is a technology that is constantly improving to achieve
   the ideal as defined in this document.

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   Features and functionalities of end-to-end encrypted 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.

   Adrian Farrel, Paul Wouters, __ are acknowleded for their review,
   comments, or questions that lead to improvement of this document.

   Chelsea Komlo and Britta Hale have contributed their deep expertise
   and consice and rigourous writing to this draft.

7.  Security Considerations

   This document does not specify new protocols and therefore does not
   bring up technical security considerations.

   Because some policy decicions may affect the security of the
   Internet, a clear and shared definition of end to end encrypted
   communication is important in policy related discussions.  This
   document aims to provide that clarity.

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>.

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   [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/>.

   [komlo]    Chelsea Komlo, "Defining end-to-end security", 2021,
              <https://github.com/chelseakomlo/e2ee/blob/master/
              e2ee_definition.pdf>.

   [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>.

   [RFC3935]  Alvestrand, H., "A Mission Statement for the IETF",
              BCP 95, RFC 3935, DOI 10.17487/RFC3935, October 2004,
              <https://www.rfc-editor.org/info/rfc3935>.

   [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>.

   [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>.

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   [saltzer]  Saltzer, et al, J. H., "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

   Sofía Celi
   Brave
   Email: cherenkov@riseup.net

   Gurshabad Grover
   Centre for Internet and Society
   Email: gurshabad@cis-india.org

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