Skip to main content

Encrypted Client Hello Deployment Considerations

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Andrew Campling , Paul A. Vixie , David Wright
Last updated 2022-03-07
RFC stream (None)
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
Secdispatch                                                A.J. Campling
Internet-Draft                                    419 Consulting Limited
Intended status: Informational                                  P. Vixie
Expires: 8 September 2022                                       Red Barn
                                                               D. Wright
                                                UK Safer Internet Centre
                                                            7 March 2022

            Encrypted Client Hello Deployment Considerations


   This document is intended to inform the development of the proposed
   Encrypted Client Hello (ECH) standard that encrypts Server Name
   Indication (SNI) and other data.  Data encapsulated by ECH (ie data
   included in the encrypted ClientHelloInner) is of legitimate interest
   to on-path security actors including anti-virus software, parental
   controls and consumer and enterprise firewalls.

   The document includes observations on current use cases for SNI data
   in a variety of contexts.  It highlights how the use of that data is
   important to the operators of private networks and shows how the loss
   of access to SNI data will cause difficulties in the provision of a
   range of services to many millions of end-users.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 September 2022.

Copyright Notice

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

Campling, et al.        Expires 8 September 2022                [Page 1]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Encrypted Server Name Indication  . . . . . . . . . . . . . .   3
   3.  The Education Sector  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Context . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Why Content Filtering Matters to Schools  . . . . . . . .   5
     3.3.  Mitigations . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Transparent Proxies . . . . . . . . . . . . . . . . . . . . .   6
   5.  Enterprises . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Threat Detection  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Mitigations . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   As noted above, this document includes observations on current use
   cases for SNI data in a variety of contexts.  It highlights how the
   use of that data is important to the operators of private networks
   and shows how the loss of access to SNI data will cause difficulties
   in the provision of a range of services to many millions of end-

   The Internet was envisaged as a network of networks, each able to
   determine what data to transmit and receive from their peers.
   Developments like ECH mark a fundamental change in the architecture
   of the Internet, allowing opaque paths to be established from
   endpoints to commercial services, some potentially without the
   knowledge or permission of the device owners.  This change should not
   be undertaken lightly given both the architectural impact on the
   Internet and potentially adverse security implications for end users.
   Given these implications, it certainly should not be undertaken
   without either the knowledge or consultation of end users, as
   outlined in RFC 8890 [RFC8890].

Campling, et al.        Expires 8 September 2022                [Page 2]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

   NB Whilst it is reasonable to counter that VPNs also establish opaque
   paths, a primary difference is that the use of a VPN is a deliberate
   act by the user, rather than a choice made by client software,
   potentially without either the knowledge and/or consent of the end-
   user or device owner.

   RFC 7258 [RFC7258] discusses the critical need to protect users'
   privacy when developing IETF specifications and also recognises that
   making networks unmanageable to mitigate pervasive monitoring is not
   an acceptable outcome.

   RFC 8404 [RFC8404] discusses current security and network operations
   as well as management practices that may be impacted by the shift to
   increased use of encryption to help guide protocol development in
   support of manageable and secure networks.  As RFC 8404 notes, "the
   implications for enterprises that own the data on their networks or
   that have explicit agreements that permit the monitoring of user
   traffic are very different from those for service providers who may
   be accessing content in a way that violates privacy considerations".

   This document considers the implications of ECH for private network
   operators including enterprises and education establishments.  The
   data encapsulated by ECH is of legitimate interest to on-path
   security actors including anti-virus software, parental controls and
   consumer and enterprise firewalls.  This document will focus
   specifically on the impact of encrypting the SNI data by ECH on
   private networks, but it should be noted that other elements will be
   relevant for some on-path security methods.

2.  Encrypted Server Name Indication

   RFC 8744 [RFC8744] describes the general problem of encrypting the
   Server Name Identification (SNI) TLS extension.  The document
   includes a brief description of what it characterises as
   "unanticipated" usage of SNI information (section 2.1) as well as a
   brief (two paragraph) assessment of alternative options in the event
   that the SNI data is encrypted (section 2.3).

Campling, et al.        Expires 8 September 2022                [Page 3]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

   The text in RFC 8744 suggests that most of the unanticipated SNI
   usage "could also be implemented by monitoring DNS traffic or
   controlling DNS usage", although it does then acknowledge the
   difficulties posed by encrypted DNS protocols.  It asserts, with
   limited evidence, that "most of [the unanticipated usage] functions
   can, however, be realized by other means", although without
   considering or quantifying the affordability, operational complexity,
   technical capability of affected parties or privacy implications that
   might be involved.  It is unclear from the document whether any
   stakeholders that may be impacted by the encryption of SNI data have
   been consulted; it does not appear to be the case.

   The characterisation of "unanticipated usage" of SNI data could be
   taken to imply that such usage was not approved and therefore
   inappropriate in some manner.  The reality is that the development of
   the Internet has many examples of permissionless innovation and so
   these should not be dismissed as lacking in importance.

   This document is intended to address the above limitations of RFC
   8744 by providing more information about the issues posed by the
   introduction of ECH due to the loss of visibility of SNI data on
   private networks.  To do so it considers the situation within schools
   and enterprises, building on information previously documented in a
   report from a roundtable discussion [ECH_Roundtable].

3.  The Education Sector

3.1.  Context

   Focusing specifically on the education sector, the primary issue
   caused by ECH is that it is likely to circumvent the safeguards
   applied to protect children through content filtering, whether in the
   school or home environments, adding to adverse impacts already
   introduced through the use of encrypted DNS protocols such as DNS
   over HTTPS [RFC8484].

   Content filtering that leverages SNI information is used by education
   establishments to protect children from exposure to malicious, adult,
   extremist and other content that is deemed either age-inappropriate
   or unsuitable for other reasons.  Any bypassing of content filtering
   by client software on devices will be problematic and may compromise
   duties placed on education establishments: for example, schools in
   the England and Wales have obligations to provide "appropriate
   filtering systems in place" [KCSE]; schools in the US use Internet
   filters and implement other measures to protect children from harmful
   online content as a condition for the receipt of certain federal
   funding, especially E-rate funds [CIPA].

Campling, et al.        Expires 8 September 2022                [Page 4]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

3.2.  Why Content Filtering Matters to Schools

   The impact that ineffective content filtering can have on an
   educational institution should not be underestimated.  For example, a
   coroner in the UK in 2021 ruled that a school's failure to prevent a
   pupil from accessing harmful material online on its equipment
   contributed to her taking her own life [Coroner].  In this particular
   case, the filtering software installed at the school was either
   faulty or incorrectly configured but it highlights the harmful risks
   posed if the filtering is bypassed by client software using ECH.

3.3.  Mitigations

   Whilst it may be possible for schools to overcome some of the issues
   ECH raises by adopting similar controls to those used by enterprises,
   it should be noted that most schools have a very different budget for
   IT compared to enterprises and usually have very limited technical
   support capabilities.  Therefore, even where technical solutions
   exist that may allow them to continue to meet their compliance
   obligations, affordability and operational expertise will present
   them with significant difficulties.

   Absent funding and technical expertise, schools will need to consider
   the best way forward that allows them to remain compliant.  If client
   software does not allow ECH to be disabled, any such software that
   implements support for ECH may need to be removed from school devices
   and replaced, assuming that suitable alternatives are available.
   This will have a negative impact on budgets and maybe operationally
   challenging if institutions have made a significant investment in the
   deployment and use of particular applications and technologies.

   There are instances where policies in education establishments allow
   for the use of equipment not owned by the institution, including
   personal devices and the devices of contractors and site visitors.
   These devices are unlikely to be configured to use the institution's
   proxy but can nevertheless connect to the school network using a
   transparent proxy (see below).  Transparent proxies used for
   filtering will typically use SNI data to understand whether a user is
   accessing inappropriate data, so encrypting the SNI field will
   disrupt the use of these transparent proxies.

   In the event that transparent proxies are no longer effective,
   institutions will either have to require more invasive software to be
   installed on third party devices before they can be used along with
   ensuring they have the capability to comprehend and adequately manage
   these technologies or will have to prevent those devices from
   operating.  Neither option is desirable.

Campling, et al.        Expires 8 September 2022                [Page 5]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

4.  Transparent Proxies

   A proxy server is a server application that acts as an intermediary
   between a client requesting a resource and the server providing that
   resource.  Instead of connecting directly, the client directs the
   request to the proxy server which evaluates the request before
   performing the required network activity.  Proxies are used for
   various purposes including load balancing, privacy and security.

   Traditionally, proxies are accessed by configuring a user's
   application or network settings, with traffic diverted to the proxy
   rather than the target destination.  With "transparent" proxying, the
   proxy intercepts packets directed to the destination, making it seem
   as though the request is handled by the target destination itself.

   A key advantage of transparent proxies is that they work without
   requiring the configuration of user devices or software.  They are
   commonly used by organisations to provide content filtering for
   devices that they don't own that are connected to their networks.
   For example, some education environments use transparent proxies to
   implement support for BYOD without needing to load software on third-
   party devices.

   Transparent proxies use SNI data to understand whether a user is
   accessing inappropriate content without the need to inspect data
   beyond the SNI field.  Because of this, encryption of the SNI field,
   as is the case with ECH, will disrupt the use of transparent proxies.

5.  Enterprises

   Filtering is an important tool within many enterprises, with uses
   including the prevention of accidental access to malicious content
   due to phishing etc.  In the enterprise market, a number of vendors
   use transparent proxy solutions, often combined with DNS filtering,
   to give stronger protections, with the proxy capability requiring
   unencrypted SNI information.  BYOD is arguably even more important
   with the current reliance on remote working, which is another area
   where the use of transparent proxies can help.  Alternative solutions
   are available but will require the use of more invasive software to
   be installed onto the guest device.

Campling, et al.        Expires 8 September 2022                [Page 6]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

   Any restrictions on the use of BYOD will also affect contractors and
   other third parties that need to connect to one or more enterprise
   networks on a temporary basis.  In such circumstances, requiring
   software or custom configurations to be installed on those devices
   may be problematic, especially for contractors that work across
   multiple organisations.  One solution could be for dedicated
   equipment for each client, however this will have potentially
   significant cost considerations.

   Clear audit trails of any communications between parties are required
   in the finance sector amongst others for compliance purposes.  If it
   becomes possible for communications to take place without an audit
   trail or any visibility to the enterprise, then there is increased
   scope for abuse to take place, including insider trading or fraud.

   In addition to concerns about the loss of visibility of deliberate
   activity by users, the loss of visibility of potential command and
   control and other activity by malicious software is of concern to
   enterprises.  In such cases, the lack of visibility from these
   privacy protections could lead to negative impacts on security and
   privacy for the enterprise, its employees, suppliers and customers.

   When considering the operational and cost implications for
   enterprises, it should be remembered that the resources available
   will vary significantly between a multinational organisation and a
   small to medium-sized enterprise.  It should not be assumed that a
   solution that can be absorbed financially and operationally by the
   former is practical for the latter.  The needs of both need to be
   taken into account when evaluating potential solutions.

6.  Threat Detection

   [To be completed, additional input welcome]

   RFC 8404 identifies a number of issues arising from increased
   encryption of data, some of which apply to ECH.  For example, it
   notes that an early trigger for DDoS mitigation involves
   distinguishing attacker traffic from legitimate user traffic.; this
   become more difficult if traffic sources are obscured.

   The various indicators of compromise (IoCs) are documented in draft-
   ietf-opsec-indicators-of-compromise-00, which also describes how they
   are used effectively in cyber defence.  For example, section 4.1.1 of
   the document describes the importance of IoCs as part of a defence-
   in-depth strategy; in this context, SNI is just one of the range of
   indicators that can be used to build up a resilient defence (see
   section 3.1 in the same document on IoC types and the 'pyramid of

Campling, et al.        Expires 8 September 2022                [Page 7]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

   In the same Internet-Draft, section 6.1 expands on the importance of
   the defence in depth strategy.  In particular, it explains the role
   that domains and IP addresses can play, especially where end-point
   defences are compromised or ineffective, or where endpoint security
   isn't possible, such as in BYOD, IoT and legacy environments.  SNI
   data plays a role here, in particular where DNS data is unavailable
   because it has been encrypted; if SNI data is lost too, alongside
   DNS, defences are weakened and the attack surface increased.

7.  Mitigations

   Access to SNI data is sometimes necessary in order for institutions,
   including those in the education and finance sectors, to discharge
   their compliance obligations.  The introduction of ECH in client
   software poses operational challenges that could be overcome on
   devices owned by those institutions if policy settings are supported
   within the software that allows the ECH functionality to be disabled.

   Third-party devices pose an additional challenge, primarily because
   the use of ECH will render transparent proxies inoperable.  The most
   likely solution is that institutions will require the installation of
   full proxies and certificates on those devices before they are
   allowed to be connected to the host networks.  They may alternatively
   determine that such an approach is impractical and instead withdraw
   the ability for network access by third-party devices.

   An additional option that warrants further consideration is the
   development of a standard that allows a network to declare its policy
   regarding ECH and other such developments.  Clients would then have
   the option to continue in setting up a connection if they are happy
   to accept those policies, or to disconnect and try alternative
   network options if not.  Such a standard is outside of the scope of
   this document but may provide a mechanism that allows the interests
   and preferences of client software, end-users and network operators
   to be balanced.

8.  Security Considerations

   In addition to introducing new operational and financial issues, the
   introduction of SNI encryption poses new challenges for threat
   detection which this document outlines.  These do not appear to have
   been considered within either RFC 8744 or the current ECH Internet-
   Draft [draft-ietf-tls-esni-14] and should be addressed fully within
   the latter's security considerations section.

Campling, et al.        Expires 8 September 2022                [Page 8]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

9.  Acknowledgements

   In addition to the authors, this document is the product of an
   informal group of experts including the following people:

10.  Informative References

   [CIPA]     FCC, "Children's Internet Protection Act (CIPA)", 30
              December 2019, <

   [Coroner]  Henderson, "Prevention of future deaths report", 26
              November 2021, <

              419 Consulting, "Encrypted Client Hello - Notes from an
              ECH Roundtable", 18 August 2021,

   [KCSE]     DfE, "Keeping children safe in education 2021", 1
              September 2021,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8744]  Huitema, C., "Issues and Requirements for Server Name
              Identification (SNI) Encryption in TLS", RFC 8744,
              DOI 10.17487/RFC8744, July 2020,

   [RFC8890]  Nottingham, M., "The Internet is for End Users", RFC 8890,
              DOI 10.17487/RFC8890, August 2020,

Campling, et al.        Expires 8 September 2022                [Page 9]
Internet-Draft  Encrypted Client Hello Deployment Consid      March 2022

Authors' Addresses

   Andrew J Campling
   419 Consulting Limited
   Email: Andrew.Campling@419.Consulting
   URI:   https://www.419.Consulting/

   Paul Vixie
   Red Barn

   David Wright
   UK Safer Internet Centre

Campling, et al.        Expires 8 September 2022               [Page 10]