Definition of End-to-end Encryption
draft-knodel-e2ee-definition-03
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Mallory Knodel , Fred Baker , Olaf Kolkman , Sofia Celi , Gurshabad Grover | ||
| Last updated | 2022-03-07 | ||
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draft-knodel-e2ee-definition-03
mls M. Knodel
Internet-Draft CDT
Intended status: Informational F. Baker
Expires: 8 September 2022
O. Kolkman
ISOC
S. Celi
Cloudflare
G. Grover
Centre for Internet and Society
7 March 2022
Definition of End-to-end Encryption
draft-knodel-e2ee-definition-03
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2022.
Copyright Notice
Copyright (c) 2022 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
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provided without warranty as described in the Revised 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 . . . . . . . . . . . . . . . . . . 4
2.3. Encryption . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Succinct definition of end-to-end security . . . . . . . 7
3. End-to-end encrypted systems design . . . . . . . . . . . . . 7
3.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 7
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 E2EE 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 . . . . . . . . . . . . . . . . . . . . . . . 14
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 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 tradition of cryptography it's also possible to
achieve a succinct 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. That is why we
will now move directly into an analysis 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.
Responsible use of of public key infrastructure is required in such
cases, such that the E2EE system does not admit third parties under
the user's identity.
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 becomes 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 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 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
E2EE 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 VPNS 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 is traverses from
sending mail user agent, between various mail transfer agents, and
finally to the 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 E2EE 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 with the
end user as the root at the top, and all potential end points being
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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 E2EE. However, E2EE to a user's personal device and
subsequent E2EE 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 E2EE 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 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 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 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.
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 E2EE. Permission of data manipulation or
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pseudo-identities for third parties to allow access under the user's
identity are against the intention of E2EE. 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. Succinct definition of end-to-end security
A succinct 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, we do not allow the adversary 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 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
authenticity, confidentiality, and integrity, whereas features of 2)
availability, deniability, forward secrecy, and post-compromise
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security are enhancements to E2EE systems.
3.1.1. Necessary features
Authenticity A system provides message authenticity if the recipient
and sender agree on each other's identities and 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 intended recipient(s) 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 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 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 proof 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.
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
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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
ephemeral key exchanges to the derivation of encryption/decryption
keys.
Metadata obfuscation Steps should be taken to minimize metadata such
as user obfuscating IP addresses, reducing non-routing metadata,
and avoiding extraneous message headers can enhance the
confidentiality and security features of E2EE systems.
3.2. Challenges
Earlier we defined end-to-end encryption using formal definitions
assumed by internet protocol implementations. Also 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 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.
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.
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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.
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 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, e.g. 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_.
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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 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 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, 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 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
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]. 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 E2EE system's goals of providing secure
communications to end users.
Such methods of analyses, outside of or as part of E2EE 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 E2EE 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 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, to not be intentionally compromised
[RFC3552]. Intentional compromises of E2EE systems are often
referred to as "backdoors" but are often presented as additional
design features under terms 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.
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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.
Adrian Farrel, are acknowleded for their review, comments, or
questions that lead to improvement of this document.
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>.
[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/>.
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[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>.
[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>.
[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
Knodel, et al. Expires 8 September 2022 [Page 14]
Internet-Draft e2ee March 2022
Fred Baker
Email: fredbaker.IETF@gmail.com
Olaf Kolkman
ISOC
Email: kolkman@isoc.org
Sofía Celi
Cloudflare
Email: cherenkov@riseup.net
Gurshabad Grover
Centre for Internet and Society
Email: gurshabad@cis-india.org
Knodel, et al. Expires 8 September 2022 [Page 15]