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Encrypted Client Hello Deployment Considerations

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Andrew Campling , Paul A. Vixie , David Wright , Arnaud Taddei , Simon Edwards
Last updated 2023-03-13
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secdispatch                                               A. J. Campling
Internet-Draft                                    419 Consulting Limited
Intended status: Informational                                  P. Vixie
Expires: 14 September 2023                                      Red Barn
                                                               D. Wright
                                                UK Safer Internet Centre
                                                               A. Taddei
                                                              S. Edwards
                                                           13 March 2023

            Encrypted Client Hello Deployment Considerations


   This document is intended to inform the community about the impact of
   the deployment 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 those providing inline malware detection, parental
   controls, content filtering to prevent access to malware and other
   risky traffic, mandatory security controls etc.

   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 both public and private networks and
   shows how the loss of access to SNI data will cause difficulties in
   the provision of a range of services to end-users, including the
   potential weakening of cybersecurity defences.  Some mitigations are
   identified that may be useful for inclusion by those considering the
   adoption of support for ECH in their software.

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

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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on 14 September 2023.

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   document authors.  All rights reserved.

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Why is the SNI used by middleboxes? . . . . . . . . . . . . .   4
   3.  Encrypted Server Name Indication  . . . . . . . . . . . . . .   5
   4.  The Education Sector  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Context . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Why Content Filtering Matters to Schools  . . . . . . . .   6
     4.3.  Mitigations . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Transparent Proxies . . . . . . . . . . . . . . . . . . . . .   7
   6.  Impact of ECH on Enterprises and Organizations  . . . . . . .   7
     6.1.  The main requirements . . . . . . . . . . . . . . . . . .   8
     6.2.  A degrading threat landscape  . . . . . . . . . . . . . .   8
     6.3.  Examples of regulatory implications . . . . . . . . . . .   9
     6.4.  Impact of ECH deployment on Network Security
           Operations  . . . . . . . . . . . . . . . . . . . . . . .  10
       6.4.1.  Reminders on Network Security . . . . . . . . . . . .  10
       6.4.2.  Implications from loss of Meta Data . . . . . . . . .  10
       6.4.3.  Implications from loss of Selective Decrypt . . . . .  10
   7.  Specific implications for SMBs  . . . . . . . . . . . . . . .  11
   8.  Public Network Service Providers  . . . . . . . . . . . . . .  11
   9.  Threat Detection  . . . . . . . . . . . . . . . . . . . . . .  14
   10. Potential further development of this work  . . . . . . . . .  14
   11. Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   14. Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .  16

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   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     15.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   In order to establish its handshake, the TLS protocol needs to start
   with a first handshake message called the Client Hello.  As this
   handshake message is in clear text, it exposes metadata, e.g. the
   Server Name Indication (SNI) which allow middleboxes on path to make
   policy decisions, in particular but not only for security reasons.
   As part of a wider initiative to achieve pervasive encryption, a
   proposed extension to TLS 1.3 called Encrypted Client Hello (ECH)
   [I-D.draft-ietf-tls-esni] is attempting to encrypt all the remaining
   metadata in the clear.

   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 of or consultation with end users, as
   outlined in [RFC8890].

   There are use cases where encryption of the SNI data may be a useful
   precaution to reduce the risk of pervasive monitoring and offers some
   benefits (e.g Enterprises offering services for their own customers
   will appreciate that their customers privacy be better protected).
   However ECH presents challenges for other use cases (e.g.
   Enterprises in need for network security controls for compliance

   The objective of this document is to list and describe the various
   operational impacts of ECH and not to consider solutions to this
   problem nor to question the development of the ECH proposal.

   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.

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   [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.

   [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 [RFC8404] 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 those providing inline malware detection,
   firewalls, parental controls, content filtering to prevent access to
   malware and other risky traffic, mandatory security controls (e.g.
   Data Loss Prevention) etc.

   This document will focus specifically on the impact of encrypting the
   SNI data by ECH on public and private networks, but it should be
   noted that other elements in the client hello may also be relevant
   for some on-path security methods.

2.  Why is the SNI used by middleboxes?

   (Editor note: this section is experimental).  For middleboxes to be
   able to perform their job they need to identify the destination of
   the requested communication.  Before TLS1.3 a middlebox could rely on
   3 metadata sources: The certificate, the DNS name and the SNI.  A
   middlebox may have used some or all of these metadata to determine
   the destination in the best possible way.  Yet, as part of the
   current initiative to complete pervasive encryption, the certificate
   was encrypted into TLS1.3, then DoH/DoT/DoQ are encrypting the DNS
   flow to its resolver making it harder for middleboxes to use these
   information.  Regardless of the easiness to access the data, the DNS
   could be misleading in some situations (would it point to the real
   destination, or just the site hosting server name, or a proxy?) and
   the SNI was invented precisely to extend on what the DNS name could
   not achieve by design.

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3.  Encrypted Server Name Indication

   [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).

   The text in [RFC8744] 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 certainly does not appear to be the case that any
   such consultation has taken place.

   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
   this "unanticipated usage" of SNI data should not be dismissed as
   lacking in either importance or validity.

   This document is intended to address the above limitations of
   [RFC8744] 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, enterprises and public service providers, building on
   information previously documented in a report from a roundtable
   discussion [ECH_Roundtable] in places.

4.  The Education Sector

4.1.  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].

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   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
   England and Wales have obligations to provide "appropriate filtering
   systems" [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].

4.2.  Why Content Filtering Matters to Schools

   The impact that ineffective content filtering can have on an
   educational institutions 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
   instance, the filtering software installed at the school was either
   faulty or incorrectly configured but the case highlights the harmful
   risks posed if the filtering is bypassed by client software using

4.3.  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 may be 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

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   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.

5.  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 “bring your own device” (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,
   requiring far more intrusive data inspection to be undertaken

6.  Impact of ECH on Enterprises and Organizations

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6.1.  The main requirements

   Enterprises and Organizations need to protect themselves for a vast
   number of reasons, mainly:

   *  Reduce their Risks.  And in particular as part of any Cyber
      Resilience strategy.

   *  Protect their Reputation.  The term Reputation includes many
      aspects way beyond the traditional enterprises and organization
      assets (data, etc.).

   *  Comply to a growing diverse set of Policies, Regulations,
      Certifications, Labeling and Guidelines.  These requirements are
      growing in both scope and complexity as they are added to by
      various bodies in countries and regional authorities around the

6.2.  A degrading threat landscape

   In addition, the general threat landscape which was already very
   large (see [I-D.draft-mcfadden-smart-threat-changes]), has
   significantly increased in three ways:

   *  COVID crisis generally accelerated the overall attack landscape.
      Indeed as the crisis forced many enterprises and organizations to
      accelerate their digital transformation, it increased the
      opportunity for cyber criminals and nation states to launch more
      attacks, leverage innovations to their advantages, better select
      their targets, increase their efficiency and increase their
      rewards, in particular with Ransomware based attacks.

   *  The Supply Chain is under stress as per the [SOLARWIND] attack

   *  Nation State attacks are continuing to evolve, for example as
      noted to those linked to the current Ukraine crisis.

   Attacks are now damaging enterprises and other organizations with
   ransomware being the number 1 issue by a considerable margin.  The
   attacks are increasing in severity, to the extent that this is now
   being measured at macroscopic level in some countries:

   *  EUR1B loss of revenue for French organizations from January to
      August 2022 [LOSSINREVENUE]

   *  Loss in capitalisation between 1-5% [LOSSINCAP]

   *  Degradation by credit notation agencies [LOSSINCREDITSCORE]

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   Another implication from the COVID crisis is the acceleration of BYOD
   with the current reliance on remote working.  This has created two
   side effects for remote employees, contractors and third parties that
   need to connect to one or more enterprise networks on a temporary

   *  need to use a VPN access to the corporate network, which brings
      all the benefits (e.g. protected access to corporate network) and
      risks that VPNs may open (e.g. lateral movement when the end point
      is compromised),

   *  need to access a cloud proxy which requires an agent to be
      installed on the device to steer the traffic to the right place.

   In such circumstances, requiring software or custom configurations to
   be installed on those devices may be problematic (see

   This is why network security solutions are required and this is why
   ECH preventing the access to the SNI makes it impossible for blue
   teams to defend (see the next sections for details).

   Finally there is a major lack of manpower in cybersecurity with a
   lack of professionalization which is not compensated anymore by the
   vocational aspect of cybersecurity so far, so any expansion of
   technical requirements that ECH would cause will exacerbate the

   All the above conditions are weighing on capabilities to defend,

   *  Directly: a lack of visibility on a key meta data like the SNI
      will cause significant issues to enterprises and organizations

   *  Indirectly: should ECH happen and should alternative be provided,
      managing migrations to any alternative not requiring access to the
      SNI, in these conditions, is undesirable from a timing, resources,
      capacities and risks perspectives.

6.3.  Examples of regulatory implications

   Regulators are accelerating their lawfare capabilities at accelerated
   pace and new legislations are showing an increased precision on what
   enterprises can and cannot do.  The EU GDPR had ripple effects to
   Financial Institutions to implement Data Loss Prevention which
   requires selective decrypt.  The recent indication that US regulators
   are in the process of levying fines of $200m each on a number of
   institutions because they were unable to track all communications by

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   their employees using WhatsApp or Signal , [Bloomberg], creates new
   auditability constraints.  It is with growing concern that an ECH
   enabled ecosystem may clash with future regulatory requirements.

6.4.  Impact of ECH deployment on Network Security Operations

6.4.1.  Reminders on Network Security

   Network Security is a set of security capabilities which is
   articulated as part of a defense strategy, e.g.  Defense In Depth
   [NIST-DID], Zero Trust, SASE/SSE, etc. and can trigger and enable
   other security capabilities such as sandboxing, Data Loss Prevention,
   Cloud Access Service Broker (CASB), etc.  One constituency is a Web
   Proxy, combining both a TLS proxy and an application level (HTTP)

   In the same way that [I-D.draft-ietf-opsec-ns-impact] showed the
   impact of TLS1.3 on operational security, a loss of visibility of the
   SNI as indicator of compromise (see
   [I-D.draft-ietf-opsec-indicators-of-compromise]) has two main

6.4.2.  Implications from loss of Meta Data

   The loss of visibility of the SNI, at TLS level, will prevent
   transparent proxies from applying corporate policies to manage risk
   and compliancy.  Typical examples:

   *  categories of compromised sites cannot be applied anymore,
      exposing employees and their organisations to potential
      cybersecurity risks; alternative approaches to block access to
      theses sites need to be found

   *  corporate lists of excluded sites for compliance or policy reasons
      need alternatives ways to be blocked.

6.4.3.  Implications from loss of Selective Decrypt

   TLS proxies also have the ability to selectively intercept, avoiding
   any visibility into or modification of the original application
   protocol payload - but such selective intercept relies heavily on
   knowledge of the origin content server hostname, which can be
   extracted in plaintext from the TLS ClientHello SNI (server name)

   This capability allows the application proxy, in particular an HTTPS
   proxy to engage efficiently specific security controls, e.g.  Data
   Loss Prevention, Sandboxing, etc.

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   The loss of SNI visibility will make it more difficult for corporate
   user flows to be intercepted, with it becoming impossible for BYOD
   use cases.

   This will create inefficiencies, will require more resources and will
   increase security risks.  It will also be counter productive for
   privacy as it may require the proxy to decrypt the whole TLS

7.  Specific implications for SMBs

   Small and Medium Business (SMBs) form a particularly vulnerable
   subset of enterprises and organizations and span from Small Office
   Home Office (SOHO, sometimes a one person business) to Medium
   Business with strong variations depending on the country (a 50
   employee company is considered the upper range of SMB business in
   developing countries while it is up to 25'000 in some developed

   Similarly to the above education use case and irrespective of
   definitions, many SMBs have very limited in-house capabilities to
   defend themselves, with security often outsourced to Managed Security
   Service Providers (typically network operators, mid range and small
   service providers).

8.  Public Network Service Providers

   In Public Networks the national, regional and international
   legislator has to balance between freedom of access to the
   information on the one hand, and safety of the internet and the
   protection of other fundamental rights on the other hand.

   There are mainly 2 different approaches:

   *  First, there are countries which do not have any specific
      legislation on the issue of blocking, filtering and takedown of
      illegal internet content: there is no legislative or other
      regulatory system put in place by the state with a view to
      defining the conditions and the procedures to be respected by
      those who engage in the blocking, filtering or takedown of online
      material.  In the absence of a specific or targeted legal
      framework, several countries rely on an existing "general" legal
      framework that is not specific to the internet to conduct - what
      is, generally speaking - limited blocking or takedown of unlawful
      online material. here the approach has been differentiated in
      relying on self regulation from the private sector or limited
      political or legislative intervention to specific areas.

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   *  The other approach has been to set up a legal framework
      specifically aimed at the regulation of the internet and other
      digital media, including the blocking, filtering and removal of
      internet content.  Such legislation typically provides for the
      legal grounds on which blocking or removal may be warranted, the
      administrative or judicial authority which has competence to take
      appropriate action and the procedures to be followed.

   In relation to specific areas where the public interest has to be
   protected more strongly, such as child abuse crimes, terrorism,
   criminality and national security, many states have a framework for
   the urgent removal of internet content regarding the above materials
   without the need of a court order.  In such circumstances,
   administrative authorities, police authorities or public prosecutors
   are given specific powers to order internet access providers to block
   access without advance judicial authority.  It is common to see such
   orders requiring action on the part of the internet access provider
   within 24 hours, and without any notice being given to the content
   provider or host themselves.

   Particularly in relation to material concerning child abuse and other
   serious crimes, many countries adopt a “list” system, whereby a
   central list of blocked URLs or domain names are maintained and
   updated by the relevant administrative authority.  This is notified
   to the relevant internet access providers, who are required to ensure
   that blocking is enforced.  Additionally in some states the
   authorities can request the removal of content that infringes
   intellectual property, privacy or defamation rights.  In this case
   the removal need to be requested by a court order.

   Generally speaking, the grounds relied on broadly correspond to the
   interests protected under Article 10(2) of the European Convention of
   Human Rights (ECHR), namely: the protection of national security,
   territorial integrity or public safety, the prevention of disorder or
   crime, the protection of health or morals, the protection of the
   reputation or rights of others, and the prevention of the disclosure
   of information received in confidence.  From the methodology we have
   to distinguish between blocking or takedown of content.

   *  The blocking, filtering or prevention of access to internet
      content are generally technical measures intended to restrict
      access to information or resources typically hosted in another
      jurisdiction.  Such action is normally taken by the internet
      access provider through hardware or software products that block
      specific targeted content from being received or displayed on the
      devices of customers of the internet access provider.

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   *  Takedown or removal of internet content, on the other hand, will
      instead broadly refer to demands or measures aimed at the website
      operator (or "host") to remove or delete the offending website
      content or sub content.

   In these considerations we will refer to blocking only.

   This can be achieved through a number of techniques, including the
   blocking of the Domain Name System (DNS), the analysis of the SNI
   field or the Uniform Resource Locator (URL).  Given the increasing
   adoption of encryption techniques often a mixture of the above
   techniques is needed.

   In particular for the most serious crimes such as child abuse or
   national security many countries adopt a “list” methodology, where a
   central list of blocked Domains or URLs is maintained by the
   authorities and updated on a regular basis (daily or even hourly) and
   shared with Public Network Operators that have to enforce the

   In many jurisdictions there are legal consequences for the Operator
   not complying with the blocking order.

   Technically the blocking can be implemented using some techniques
   that have been adapted during time based on the new technologies

   Historically depending on the content of the list the technique have
   been based on DNS or proxy blocking.

   DNS is effective on Domains (the whole domain is blocked), while
   proxy is effective either on Domain (for encrypted traffic) or URL
   (for unencrypted traffic).

   Given that nowadays the vast majority of traffic is encrypted, the
   capability of blocking based on URL is limited to a small portion of
   traffic and proxy blocking is as effective as that based on the DNS.

   Theoretically DNS blocking would be the preferred option for
   operators given the more limited investments necessary to implement
   blocking of the Domains, but given the increased usage of external
   encrypted DNS services DNS blocking is becoming less effective and
   operators need to use SNI analysis as well in order to fulfil legal

   The adoption of ECH will cause additional problems and limit the
   possibility of implementing operators fulfilling their legal blocking
   obligations, exposing the population to illegal content related to

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   crimes such as Child Sex Abuse Material (CSAM), malware and other
   malicious content, and possibly even content deemed to be detrimental
   to National Security.

9.  Threat Detection

   [RFC8404] 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
   [I-D.draft-ietf-opsec-indicators-of-compromise], 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 pain').

   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.

10.  Potential further development of this work

   This work could consider several potential developments:

   *  If ECH is enforced what are the solutions to all the above
      problems and what are the migration paths?

   *  Elaborate on endpoint security complications as
      [I-D.draft-taddei-smart-cless-introduction] as well as [MAGECART]
      [MITB] [MITB-MITRE] [MALVERTISING] showed that in some cases, the
      only way to detect an attack is through the use of network-based
      security.  The loss of visibility of the SNI data will make it
      much harder to detect attacks.  The endpoints components
      (operating system, applications, browsers, etc.) cannot be judge
      and party.

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   *  There are need for further clarifications from the ECH draft, e.g.
      The link between the Client Facing and the backend servers are not
      clear enough and need further description.  It can't be just 'left
      to the implementation'

   *  Will there be any impact to the DNS by adding so many new RRs?

   *  What happens if Client Facing servers are controlled by malicious

   *  The Client Facing servers are acting as a new category of
      middleboxes.  In this shift left movement, until the attack
      surface is minimal and complexities are removed, you have to rely
      on third parties for inspection.  In these conditions, on which
      basis can they be more trusted than any other middleboxes?  Is
      this creating a concentration problem?

   *  What prevents a Client Facing server providing security solutions
      to protect the data path?

   *  Consolidation considerations - the use of ECH may accelerate the
      move of content away from standalone servers and on to CDNs,
      reducing infrastructure resilience.

   *  Find missing sources to illustrate a number of points, e.g. show
      how adversaries use digital transformation to accelerate their
      attacks, how ECH will increase security risks.

   *  Keep streamlining, clarifying the text e.g. the 2 approaches in
      the public network service providers section, "Technically the
      blocking ...".

11.  Conclusion

   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.

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   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.

12.  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 [RFC8744] or the current ECH Internet-
   Draft [I-D.draft-ietf-tls-esni] and should be addressed fully within
   the latter's security considerations section.

   This I-D should help improve security in deployments of ECH.

13.  IANA Considerations

   This document has no IANA actions.

14.  Acknowledgment

   In memory of Simon Edwards who passed away in the night of 8th-9th of
   January 2023.

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

15.  References

15.1.  Normative References

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

15.2.  Informative References

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              Spezzati, S., Robinson, M., and L. Beyoud, "Wall Street's
              Record Fines Over WhatsApp Use Were Years in the Making",
              16 August 2022, <

   [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,

              Paine, K., Whitehouse, O., Sellwood, J., and A. S,
              "Indicators of Compromise (IoCs) and Their Role in Attack
              Defence", Work in Progress, Internet-Draft, draft-ietf-
              opsec-indicators-of-compromise-04, 3 February 2023,

              Cam-Winget, N., Wang, E., Danyliw, R., and R. DuToit,
              "Impact of TLS 1.3 to Operational Network Security
              Practices", Work in Progress, Internet-Draft, draft-ietf-
              opsec-ns-impact-04, 26 January 2021,

              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-15, 3 October 2022,

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              McFadden, M., "BCP72 - A Problem Statement", Work in
              Progress, Internet-Draft, draft-mcfadden-smart-threat-
              changes-04, 22 January 2022,

              Taddei, A., Wueest, C., Roundy, K. A., and D. Lazanski,
              "Capabilities and Limitations of an Endpoint-only Security
              Solution", Work in Progress, Internet-Draft, draft-taddei-
              smart-cless-introduction-03, 13 July 2020,

   [KCSE]     DfE, "Keeping children safe in education 2021", 1 November
              2021, <>.

              Neyret, A. and Autorité des Marchés Financiers, "La
              cybercriminalité boursière – définition, cas et
              perspectives", 10 October 2019, <https://www.amf-

              Deloitte, "Beneath the surface of a cyberattack – A deeper
              look at business impacts", 2016,

              September 2022, <

   [MAGECART] Wikipedia, "Magecart", 3 April 2022,

              Wikipedia, "Malvertising", 2 June 2022,

   [MITB]     OWASP, "Man-in-the-browser attack", n.d.,

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              MITRE, "Browser Session Hijacking - T1185", 25 February
              2022, <>.

   [NIST-DID] NIST, "Glossary - defense-in-depth", n.d.,

   [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,

   [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,

              Symantec, a Division of Broadcom Software Group,
              "SolarWinds (Sunburst) Attack What You Need to Know",
              December 2020, <


   Eric Chien

   Eric contributed to the analysis of the Man in the Browser attacks.

   Gianpaolo Scalone

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   Contributed the research on the conflicts of ECH with local
   legislations to block.

   Daniel Engberg
   Skandinaviska Enskilda Banken AB (SEB)

   Validate the issues for his organization.

   Celine Leroy
   Eight Advisory

   Thank you to Céline for her work on cybersecurity financial impacts
   on enterprises.

   Daniel Engberg
   Skandinaviska Enskilda Banken AB (SEB)

   Validate the issues for his organization.

   Gianpiero Tavano

   Review the text, provided feedback and reminded us on the budgetary

   Roelof duToit

   Roelof contributed many things including research, former I-D, text,
   the newly setup github, etc.

   Diego Lopez

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   Diego contributed in several aspects including MCPs.

   Gary Tomic

   Gary contributed many things including research, keep us on scope,
   critique for when issues where not impacted by ECH as we initially

Authors' Addresses

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

   Paul Vixie
   Red Barn

   David Wright
   UK Safer Internet Centre

   Arnaud Taddei
   1320 Ridder Park Dr
   San Jose, CA 95131
   United States of America
   Phone: 41795061129

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   Simon Edwards
   1320 Ridder Park Dr
   San Jose, CA 95131
   United States of America

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