Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Informational                               E. Rescorla
Expires: February 16, 2020                                    RTFM, Inc.
                                                         August 15, 2019

           Issues and Requirements for SNI Encryption in TLS


   This draft describes the general problem of encrypting the Server
   Name Identification (SNI) TLS parameter.  The proposed solutions hide
   a Hidden Service behind a fronting service, only disclosing the SNI
   of the fronting service to external observers.  The draft lists known
   attacks against SNI encryption, discusses the current "co-tenancy
   fronting" solution, and presents requirements for future TLS layer

   In practice, it may well be that no solution can meet every
   requirement, and that practical solutions will have to make some

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 16, 2020.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   ( in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  History of the TLS SNI extension  . . . . . . . . . . . . . .   3
     2.1.  Unanticipated usage of SNI information  . . . . . . . . .   3
     2.2.  SNI encryption timeliness . . . . . . . . . . . . . . . .   4
     2.3.  End-to-end alternatives . . . . . . . . . . . . . . . . .   5
   3.  Security and Privacy Requirements for SNI Encryption  . . . .   5
     3.1.  Mitigate Replay Attacks . . . . . . . . . . . . . . . . .   5
     3.2.  Avoid Widely Shared Secrets . . . . . . . . . . . . . . .   6
     3.3.  Prevent SNI-based Denial of Service Attacks . . . . . . .   6
     3.4.  Do not stick out  . . . . . . . . . . . . . . . . . . . .   6
     3.5.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .   7
     3.6.  Multi-Party Security Contexts . . . . . . . . . . . . . .   7
     3.7.  Supporting multiple protocols . . . . . . . . . . . . . .   8
       3.7.1.  Hiding the Application Layer Protocol Negotiation . .   8
       3.7.2.  Support other transports than TCP . . . . . . . . . .   8
   4.  HTTP Co-Tenancy Fronting  . . . . . . . . . . . . . . . . . .   9
     4.1.  HTTPS Tunnels . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Delegation Control  . . . . . . . . . . . . . . . . . . .  10
     4.3.  Related work  . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Historically, adversaries have been able to monitor the use of web
   services through three primary channels: looking at DNS requests,
   looking at IP addresses in packet headers, and looking at the data
   stream between user and services.  These channels are getting
   progressively closed.  A growing fraction of Internet communication
   is encrypted, mostly using Transport Layer Security (TLS) [RFC5246].
   Progressive deployment of solutions like DNS in TLS [RFC7858] and DNS
   over HTTPS [RFC8484] mitigates the disclosure of DNS information.
   More and more services are colocated on multiplexed servers,
   loosening the relation between IP address and web service.  However,
   multiplexed servers rely on the Service Name Information (SNI) TLS

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   extension to direct connections to the appropriate service
   implementation.  This protocol element is transmitted in clear text.
   As the other methods of monitoring get blocked, monitoring focuses on
   the clear text SNI.  The purpose of SNI encryption and privacy is to
   prevent that.

   In the past, there have been multiple attempts at defining SNI
   encryption.  These attempts have generally floundered, because the
   simple designs fail to mitigate several of the attacks listed in
   Section 3.  In the absence of a TLS-level solution, the most popular
   approach to SNI privacy for web services is HTTP-level fronting,
   which we discuss in Section 4.

2.  History of the TLS SNI extension

   The SNI extension was specified in 2003 in [RFC3546] to facilitate
   management of "colocation servers", in which multiple services shared
   the same IP address.  A typical example would be mutiple web sites
   served by the same web server.  The SNI extension carries the name of
   a specific server, enabling the TLS connection to be established with
   the desired server context.  The current SNI extension specification
   can be found in [RFC6066].

   The SNI specification allowed for different types of server names,
   though only the "hostname" variant was specified and deployed.  In
   that variant, the SNI extension carries the domain name of the target
   server.  The SNI extension is carried in clear text in the TLS
   "ClientHello" message.

2.1.  Unanticipated usage of SNI information

   The SNI was defined to facilitate management of servers, though the
   developers of middleboxes soon found out that they could take
   advantage of the information.  Many examples of such usage are
   reviewed in [RFC8404].  They include:

   o  Monitoring and identification of specific sites,

   o  Content filtering by ISP blocking specific web sites in order to
      implement "parental controls", or to prevent access to phishing or
      other fradulent web sites.

   o  ISP assigning different QoS profiles to target services,

   o  Firewalls within enterprise networks blocking web sites not deemed
      appropriate for work, or

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   o  Firewalls within enterprise networks exempting specific web sites
      from MITM inspection, such as healthcare or financial sites for
      which inspection would intrude with the privacy of employees.

   The SNI is probably also included in the general collection of
   metadata by pervasive surveillance actors.

2.2.  SNI encryption timeliness

   The clear-text transmission of the SNI was not flagged as a problem
   in the security consideration sections of [RFC3546], [RFC4366], or
   [RFC6066].  These specifications did not anticipate the alternative
   uses and abuses described in Section 2.1.  One reason may be that,
   when these RFCs were written, the SNI information was available
   through a variety of other means.

   Many deployments still allocate different IP addresses to different
   services, so that different services can be identified by their IP
   addresses.  However, content distribution networks (CDN) commonly
   serve a large number of services through a comparatively small number
   of addresses.

   The SNI carries the domain name of the server, which is also sent as
   part of the DNS queries.  Most of the SNI usage described in
   Section 2.1 could also be implemented by monitoring DNS traffic or
   controlling DNS usage.  But this is changing with the advent of DNS
   resolvers providing services like DNS over TLS [RFC7858] or DNS over
   HTTPS [RFC8484].

   The subjectAltName extension of type dNSName of the server
   certificate, or in its absence the common name component, expose the
   same name as the SNI.  In TLS versions 1.0 [RFC2246], 1.1 [RFC4346],
   and 1.2 [RFC5246], servers send certificates in clear text, ensuring
   that there would be limited benefits in hiding the SNI.  However, in
   TLS 1.3 [RFC8446], server certificates are encrypted in transit.
   Note that encryption alone is insufficient to protect server
   certificates; see Section 3.1 for details.

   The decoupling of IP addresses and server names, deployment of DNS
   privacy, and protection of server certificates transmissions all
   contribute to user privacy in the face of an [RFC3552]-style
   adversary.  Encrypting the SNI now will complete this push for
   privacy and make it harder to censor or otherwise provide
   differential treatment to specific internet services.

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2.3.  End-to-end alternatives

   Deploying SNI encryption will help thwarting most of the
   "unanticipated" SNI usages described in Section 2.1, including
   censorship and pervasive surveillance.  It will also thwart functions
   that are sometimes described as legitimate.  Most of these functions
   can however be realized by other means.  For example, some DNS
   service providers offer customers the provision to "opt in" filtering
   services for parental control and phishing protection.  Per-stream
   QoS can be provided by a combination of packet marking and end-to-end
   agreements.  As SNI encryption becomes common, we can expect more
   deployment of such "end-to-end" solutions.

   At the moment, enterprises have the option of installing a firewall
   performing SNI filtering to prevent connections to certain websites.
   With SNI encryption this becomes ineffective.  Obviously, managers
   could block usage of SNI encryption in enterprise computers, but this
   wide-scale blocking would diminish the privacy protection of traffic
   leaving the enterprise, which may not be desirable.  Enterprise
   managers could rely instead on filtering software and management
   software deployed on the enterprise's computers.

3.  Security and Privacy Requirements for SNI Encryption

   Over the past years, there have been multiple proposals to add an SNI
   encryption option in TLS.  Many of these proposals appeared
   promising, though were rejected after security reviews identified
   plausible attacks.  In this section, we collect a list of these known

3.1.  Mitigate Replay Attacks

   The simplest SNI encryption designs replace in the initial TLS
   exchange the clear text SNI with an encrypted value, using a key
   known to the multiplexed server.  Regardless of the encryption used,
   these designs can be broken by a simple replay attack, which works as

   1- The user starts a TLS connection to the multiplexed server,
   including an encrypted SNI value.

   2- The adversary observes the exchange and copies the encrypted SNI

   3- The adversary starts its own connection to the multiplexed server,
   including in its connection parameters the encrypted SNI copied from
   the observed exchange.

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   4- The multiplexed server establishes the connection to the protected
   service, thus revealing the identity of the service.

   One of the goals of SNI encryption is to prevent adversaries from
   knowing which Hidden Service the client is using.  Successful replay
   attacks breaks that goal by allowing adversaries to discover that

3.2.  Avoid Widely Shared Secrets

   It is easy to think of simple schemes in which the SNI is encrypted
   or hashed using a shared secret.  This symmetric key must be known by
   the multiplexed server, and by every users of the protected services.
   Such schemes are thus very fragile, since the compromise of a single
   user would compromise the entire set of users and protected services.

3.3.  Prevent SNI-based Denial of Service Attacks

   Encrypting the SNI may create extra load for the multiplexed server.
   Adversaries may mount denial of service attacks by generating random
   encrypted SNI values and forcing the multiplexed server to spend
   resources in useless decryption attempts.

   It may be argued that this is not an important DOS avenue, as regular
   TLS connection attempts also require the server to perform a number
   of cryptographic operations.  However, in many cases, the SNI
   decryption will have to be performed by a front-end component with
   limited resources, while the TLS operations are performed by the
   component dedicated to their respective services.  SNI-based DOS
   attacks could target the front-end component.

3.4.  Do not stick out

   In some designs, handshakes using SNI encryption can be easily
   differentiated from "regular" handshakes.  For example, some designs
   require specific extensions in the Client Hello packets, or specific
   values of the clear text SNI parameter.  If adversaries can easily
   detect the use of SNI encryption, they could block it, or they could
   flag the users of SNI encryption for special treatment.

   In the future, it might be possible to assume that a large fraction
   of TLS handshakes use SNI encryption.  If that was the case, the
   detection of SNI encryption would be a lesser concern.  However, we
   have to assume that in the near future, only a small fraction of TLS
   connections will use SNI encryption.

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3.5.  Forward Secrecy

   The general concerns about forward secrecy apply to SNI encryption
   just as well as to regular TLS sessions.  For example, some proposed
   designs rely on a public key of the multiplexed server to define the
   SNI encryption key.  If the corresponding private key was
   compromised, the adversaries would be able to process archival
   records of past connections, and retrieve the protected SNI used in
   these connections.  These designs failed to maintain forward secrecy
   of SNI encryption.

3.6.  Multi-Party Security Contexts

   We can design solutions in which a fronting service act as a relay to
   reach the protected service.  Some of those solutions involve just
   one TLS handshake between the client and the fronting service.  The
   master secret is verified by verifying a certificate provided by the
   fronting service, but not by the protected service.  These solutions
   expose the client to a Man-In-The-Middle attack by the fronting
   service.  Even if the client has some reasonable trust in this
   service, the possibility of MITM attack is troubling.

   There are other classes of solutions in which the master secret is
   verified by verifying a certificate provided by the protected
   service.  These solutions offer more protection against a Man-In-The-
   Middle attack by the fronting service.  The downside is the the
   client will not verify the identity of the fronting service with
   risks discussed in , but solutions will have to mitigate this risks.
   Overall, end-to-end TLS to the protected service is preferable.

   The fronting service could be pressured by adversaries.  By design,
   it could be forced to deny access to the protected service, or to
   divulge which client accessed it.  But if MITM is possible, the
   adversaries would also be able to pressure the fronting service into
   intercepting or spoofing the communications between client and
   protected service.

   Adversaries could also mount an attack by spoofing the fronting
   service.  A spoofed fronting service could act as a "honeypot" for
   users of hidden services.  At a minimum, the fake server could record
   the IP addresses of these users.  If the SNI encryption solution
   places too much trust on the fronting server, the fake server could
   also serve fake content of its own choosing, including various forms
   of malware.

   There are two main channels by which adversaries can conduct this
   attack.  Adversaries can simply try to mislead users into believing
   that the honeypot is a valid fronting server, especially if that

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   information is carried by word of mouth or in unprotected DNS
   records.  Adversaries can also attempt to hijack the traffic to the
   regular fronting server, using for example spoofed DNS responses or
   spoofed IP level routing, combined with a spoofed certificate.

3.7.  Supporting multiple protocols

   The SNI encryption requirement does not stop with HTTP over TLS.
   Multiple other applications currently use TLS, including for example
   SMTP [RFC5246], DNS [RFC7858], or XMPP [RFC7590].  These applications
   too will benefit of SNI encryption.  HTTP only methods like those
   described in Section 4.1 would not apply there.  In fact, even for
   the HTTPS case, the HTTPS tunneling service described in Section 4.1
   is compatible with HTTP 1.0 and HTTP 1.1, but interacts awkwardly
   with the multiple streams feature of HTTP 2.0 [RFC7540].  This points
   to the need of an application-agnostic solution, that would be
   implemented fully in the TLS layer.

3.7.1.  Hiding the Application Layer Protocol Negotiation

   The Application Layer Protocol Negotiation (ALPN) parameters of TLS
   allow implementations to negotiate the application layer protocol
   used on a given connection.  TLS provides the ALPN values in clear
   text during the initial handshake.  While exposing the ALPN does not
   create the same privacy issues as exposing the SNI, there is still a
   risk.  For example, some networks may attempt to block applications
   that they do not understand, or that they wish users would not use.

   In a sense, ALPN filtering could be very similar to the filtering of
   specific port numbers exposed in some network.  This filtering by
   ports has given rise to evasion tactics in which various protocols
   are tunneled over HTTP in order to use open ports 80 or 443.
   Filtering by ALPN would probably beget the same responses, in which
   the applications just move over HTTP, and only the HTTP ALPN values
   are used.  Applications would not need to do that if the ALPN was
   hidden in the same way as the SNI.

   In addition to hiding the SNI, it is thus desirable to also hide the
   ALPN.  Of course, this implies engineering trade-offs.  Using the
   same technique for hiding the ALPN and encrypting the SNI may result
   in excess complexity.  It might be preferable to encrypt these

3.7.2.  Support other transports than TCP

   The TLS handshake is also used over other transports such as UDP with
   both DTLS [I-D.ietf-tls-dtls13] and QUIC [I-D.ietf-quic-tls].  The

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   requirement to encrypt the SNI apply just as well for these
   transports as for TLS over TCP.

   This points to a requirement for SNI Encryption mechanisms to also be
   applicable to non-TCP transports such as DTLS or QUIC.

4.  HTTP Co-Tenancy Fronting

   In the absence of TLS-level SNI encryption, many sites rely on an
   "HTTP Co-Tenancy" solution.  The TLS connection is established with
   the fronting server, and HTTP requests are then sent over that
   connection to the hidden service.  For example, the TLS SNI could be
   set to "", the fronting server, and HTTP requests
   sent over that connection could be directed to "",
   accessing the hidden service.  This solution works well in practice
   when the fronting server and the hidden server are "co-tenant" of the
   same multiplexed server.

   The HTTP fronting solution can be deployed without modification to
   the TLS protocol, and does not require using any specific version of
   TLS.  There are however a few issues regarding discovery, client
   implementations, trust, and applicability:

   o  The client has to discover that the hidden service can be accessed
      through the fronting server.

   o  The client browser's has to be directed to access the hidden
      service through the fronting service.

   o  Since the TLS connection is established with the fronting service,
      the client has no cryptographic proof that the content does in
      fact come from the hidden service.  The solution does thus not
      mitigate the context sharing issues described in Section 3.6.

   o  Since this is an HTTP-level solution, it would not protect non-
      HTTP protocols such as DNS over TLS [RFC7858] or IMAP over TLS

   The discovery issue is common to most SNI encryption solutions.  The
   browser issue may be solved by developing a browser extension that
   support HTTP Fronting, and manages the list of fronting services
   associated with the hidden services that the client uses.  The multi-
   protocol issue can be mitigated by using implementation of other
   applications over HTTP, such as for example DNS over HTTPS [RFC8484].
   The trust issue, however, requires specific developments.

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4.1.  HTTPS Tunnels

   The HTTP Fronting solution places a lot of trust in the Fronting
   Server.  This required trust can be reduced by tunnelling HTTPS in
   HTTPS, which effectively treats the Fronting Server as an HTTP Proxy.
   In this solution, the client establishes a TLS connection to the
   Fronting Server, and then issues an HTTP Connect request to the
   Hidden Server.  This will establish an end-to-end HTTPS over TLS
   connection between the client and the Hidden Server, mitigating the
   issues described in Section 3.6.

   The HTTPS in HTTPS solution requires double encryption of every
   packet.  It also requires that the fronting server decrypts and relay
   messages to the hidden server.  Both of these requirements make the
   implementation onerous.

4.2.  Delegation Control

   Clients would see their privacy compromised if they contacted the
   wrong fronting server to access the hidden service, since this wrong
   server could disclose their access to adversaries.  This requires a
   controlled way to indicate which fronting ferver is acceptable by the
   hidden service.

   This problem is both similar and different from the "fronting server
   spoofing" attack described in Section 3.6.  Here, the spoofing would
   be performed by distributing fake advice, such as "to reach example, use as a fronting server", when
   "" is under the control of an adversary.

   In practice, this attack is well mitigated when the hidden service is
   accessed through a specialized application.  The name of the fronting
   server can then be programmed in the code of the application.  But
   the attack is much harder to mitigate when the hidden service has to
   be accessed through general purpose web browsers.  The browsers will
   need a mechanism to obtain the fronting server indication in a secure

   There are several proposed solutions to this problem, such as
   creating a special form of certificate to codify the relation between
   fronting and hidden server, or obtaining the relation between hidden
   and fronting service through the DNS, possibly using DNSSEC to avoid

   We can observe that content distribution network have a similar
   requirement.  They need to convince the client that ""
   can be accessed through the seemingly unrelated "cdn-node-

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   this problem.

4.3.  Related work

   The ORIGIN frame defined for HTTP/2 [RFC8336] can be used to flag
   content provided by the hidden server.  Secondary certificate
   authentication [I-D.ietf-httpbis-http2-secondary-certs] can be used
   to manage authentication of hidden server content, or to perform
   client authentication before accessing hidden content.

5.  Security Considerations

   Replacing clear text SNI transmission by an encrypted variant will
   improve the privacy and reliability of TLS connections, but the
   design of proper SNI encryption solutions is difficult.  This
   document does not present the design of a solution, but provides
   guidelines for evaluating proposed solutions.

   This document lists a number of attacks against SNI encryption in
   Section 3, and also in Section 4.2, and presents a list of
   requirements to mitigate these attacks.  The current HTTP based
   solutions described in Section 4 only meet some of these
   requirements.  In practice, it may well be that no solution can meet
   every requirement, and that practical solutions will have to make
   some compromises.

   In particular, the requirement to not stick out presented in
   Section 3.4 may have to be lifted, especially for proposed solutions
   that could quickly reach large scale deployments.

6.  IANA Considerations

   This draft does not require any IANA action.

7.  Acknowledgements

   A large part of this draft originates in discussion of SNI encryption
   on the TLS WG mailing list, including comments after the tunneling
   approach was first proposed in a message to that list:

   Thanks to Daniel Kahn Gillmor for a pretty detailed review of the
   initial draft.  Thanks to Stephen Farrell, Martin Rex Martin Thomson
   and employees of the UK National Cyber Security Centre for their

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8.  Informative References

              Bishop, M., Sullivan, N., and M. Thomson, "Secondary
              Certificate Authentication in HTTP/2", draft-ietf-httpbis-
              http2-secondary-certs-04 (work in progress), April 2019.

              Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              draft-ietf-quic-tls-22 (work in progress), July 2019.

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-32 (work in progress), July

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, DOI 10.17487/RFC2246, January 1999,

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,

   [RFC3546]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 3546, DOI 10.17487/RFC3546, June 2003,

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,

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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC7590]  Saint-Andre, P. and T. Alkemade, "Use of Transport Layer
              Security (TLS) in the Extensible Messaging and Presence
              Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June
              2015, <>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <>.

   [RFC8336]  Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
              RFC 8336, DOI 10.17487/RFC8336, March 2018,

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

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

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

Authors' Addresses

Huitema & Rescorla      Expires February 16, 2020              [Page 13]

Internet-Draft       TLS-SNI Encryption Requirements         August 2019

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor  WA  98250


   Eric Rescorla
   RTFM, Inc.


Huitema & Rescorla      Expires February 16, 2020              [Page 14]