Internet Engineering Task Force (IETF) C. Huitema

Internet Engineering Task Force (IETF) C. Huitema
Request for Comments: 8744 Private Octopus Inc.
Category: Informational July 2020
ISSN: 2070-1721

Issues and Requirements for Server Name Identification (SNI) Encryption
in TLS

Abstract

This document 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. This document lists
known attacks against SNI encryption, discusses the current "HTTP co-
tenancy" solution, and presents requirements for future TLS-layer
solutions.

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

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8744.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
2. History of the TLS SNI Extension
2.1. Unanticipated Usage of SNI Information
2.2. SNI Encryption Timeliness
2.3. End-to-End Alternatives
3. Security and Privacy Requirements for SNI Encryption
3.1. Mitigate Cut-and-Paste Attacks
3.2. Avoid Widely Shared Secrets
3.3. Prevent SNI-Based Denial-of-Service Attacks
3.4. Do Not Stick Out
3.5. Maintain Forward Secrecy
3.6. Enable Multi-party Security Contexts
3.7. Support Multiple Protocols
3.7.1. Hiding the Application-Layer Protocol Negotiation
3.7.2. Supporting Other Transports than TCP
4. HTTP Co-tenancy Fronting
4.1. HTTPS Tunnels
4.2. Delegation Control
4.3. Related Work
5. Security Considerations
6. IANA Considerations
7. Informative References
Acknowledgements
Author's Address

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) [RFC8446].
Progressive deployment of solutions like DNS over 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. For
example, in virtual hosting solutions, multiple services can be
hosted as co-tenants on the same server, and the IP address and port
do not uniquely identify a service. In cloud or Content Delivery
Network (CDN) solutions, a given platform hosts the services or
servers of a lot of organizations, and looking up what netblock an IP
address belongs to reveals little. However, multiplexed servers rely
on the Server Name Information (SNI) TLS extension [RFC6066] to
direct connections to the appropriate service implementation. This
protocol element is transmitted in cleartext. As the other methods
of monitoring get blocked, monitoring focuses on the cleartext SNI.
The purpose of SNI encryption is to prevent that and aid privacy.

Replacing cleartext 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. 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.

This document does not present the design of a solution but provides
guidelines for evaluating proposed solutions. (The review of HTTP-
level solutions in Section 4 is not an endorsement of these
solutions.) The need for related work on the encryption of the
Application-Layer Protocol Negotiation (ALPN) parameters of TLS is
discussed in Section 3.7.1.

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 multiple websites
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 cleartext in the TLS
"ClientHello" message.

2.1. Unanticipated Usage of SNI Information

The SNI was defined to facilitate management of servers, but the
developers of middleboxes found out that they could take advantage of
the information. Many examples of such usage are reviewed in
[RFC8404]. Other examples came out during discussions of this
document. They include:

* Filtering or censoring specific services for a variety of reasons

* Content filtering by network operators or ISPs blocking specific
websites, for example, to implement parental controls or to
prevent access to phishing or other fraudulent websites

* ISP assigning different QoS profiles to target services

* Firewalls within enterprise networks blocking websites not deemed
appropriate for work

* Firewalls within enterprise networks exempting specific websites
from man-in-the-middle (MITM) inspection, such as healthcare or
financial sites for which inspection would intrude on the privacy
of employees

The SNI is probably also included in the general collection of
metadata by pervasive surveillance actors [RFC7258], for example, to
identify services used by surveillance targets.

2.2. SNI Encryption Timeliness

The cleartext transmission of the SNI was not flagged as a problem in
the Security Considerations sections of [RFC3546], [RFC4366], or
[RFC6066]. These specifications did not anticipate the alternative
usage 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, such as tracking IP addresses, DNS names, or
server certificates.

Many deployments still allocate different IP addresses to different
services, so that different services can be identified by their IP
addresses. However, CDNs 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) exposes
the same name as the SNI. In TLS versions 1.0 [RFC2246], 1.1
[RFC4346], and 1.2 [RFC5246], servers send certificates in cleartext,
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 certificate transmissions all
contribute to user privacy in the face of an RFC 7258-style adversary
[RFC7258]. Encrypting the SNI complements this push for privacy and
makes it harder to censor or otherwise provide differential treatment
to specific Internet services.

2.3. End-to-End Alternatives

Deploying SNI encryption helps thwart most of the unanticipated SNI
usages, including censorship and pervasive surveillance, but it also
will break or reduce the efficacy of the operational practices and
techniques implemented in middleboxes, as described in Section 2.1.
Most of these functions can, however, be realized by other means.
For example, some DNS service providers offer customers the provision
to "opt in" to filtering services for parental control and phishing
protection. Per-stream QoS could 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 time of this writing, 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. A review of the TLS mailing list archives
shows that many of these proposals appeared promising but were
rejected after security reviews identified plausible attacks. In
this section, we collect a list of these known attacks.

3.1. Mitigate Cut-and-Paste Attacks

The simplest SNI encryption designs replace the cleartext SNI in the
initial TLS exchange 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 cut-and-paste attack, which works
as follows:

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
parameter.

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

4. The multiplexed server establishes the connection to the
protected service, which sends its certificate, 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 cut-
and-paste attacks break that goal by allowing adversaries to discover
that service.

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 user 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 (DoS) 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 avenue for DoS
attacks, 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 ClientHello packets or specific
values of the cleartext 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 were 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.

This requirement to not stick out may be difficult to meet in
practice, as noted in Section 5.

3.5. Maintain Forward Secrecy

TLS 1.3 [RFC8446] is designed to provide forward secrecy, so that
(for example) keys used in past sessions will not be compromised even
if the private key of the server is compromised. The general
concerns about forward secrecy apply to SNI encryption as well. 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 should be compromised, the adversaries
would be able to process archival records of past connections and
retrieve the protected SNI used in these connections. These designs
fail to maintain forward secrecy of SNI encryption.

3.6. Enable Multi-party Security Contexts

We can design solutions in which a fronting service acts 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 MITM attack by the fronting service. Even if
the client has some reasonable trust in this service, the possibility
of a 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 MITM attack
by the fronting service. The downside is that the client will not
verify the identity of the fronting service, which enables fronting
server spoofing attacks, such as the "honeypot" attack discussed
below. Overall, end-to-end TLS to the protected service is
preferable, but it is important to also provide a way to authenticate
the fronting service.

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 a MITM attack 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
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. Support Multiple Protocols

The SNI encryption requirement does not stop with HTTP over TLS.
Multiple other applications currently use TLS, including, for
example, SMTP [RFC3207], DNS [RFC7858], IMAP [RFC8314], and the
Extensible Messaging and Presence Protocol (XMPP) [RFC7590]. These
applications, too, will benefit from 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
[RFC7540]. This points to the need for an application-agnostic
solution, which 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
cleartext 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 networks. 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 were
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
independently.

3.7.2. Supporting Other Transports than TCP

The TLS handshake is also used over other transports, such as UDP
with both DTLS [DTLS-1.3] and QUIC [QUIC]. The requirement to
encrypt the SNI applies 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, often referred to as "domain fronting"
[DOMFRONT]. 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
"fronting.example.com" (the fronting server), and HTTP requests sent
over that connection could be directed to "hidden.example.com"
(accessing the hidden service). This solution works well in practice
when the fronting server and the hidden server are "co-tenants" of
the same multiplexed server.

The HTTP domain 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:

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

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

* 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. Thus, the solution does not
mitigate the context sharing issues described in Section 3.6.
Note that this is already the case for co-tenanted sites.

* Since this is an HTTP-level solution, it does not protect non-HTTP
protocols, as discussed in Section 3.7.

The discovery issue is common to most SNI encryption solutions. The
browser issue was solved in [DOMFRONT] by implementing domain
fronting as a pluggable transport for the Tor browser. The multi-
protocol issue can be mitigated by implementing other applications
over HTTP, for example, DNS over HTTPS [RFC8484]. The trust issue,
however, requires specific developments.

4.1. HTTPS Tunnels

The HTTP domain fronting solution places a lot of trust in the
fronting server. This required trust can be reduced by tunneling
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 decrypt 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 server is acceptable by the
hidden service.

This problem is similar to the "word of mouth" variant of the
"fronting server spoofing" attack described in Section 3.6. The
spoofing would be performed by distributing fake advice, such as "to
reach hidden.example.com, use fake.example.com as a fronting server",
when "fake.example.com" 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 harder to mitigate when the hidden service has to be
accessed through general-purpose web browsers.

There are several proposed solutions to this problem, such as
creating a special form of certificate to codify the relation between
the fronting and hidden server or obtaining the relation between the
hidden and fronting service through the DNS, possibly using DNSSEC,
to avoid spoofing. The experiment described in [DOMFRONT] solved the
issue by integrating with the Lantern Internet circumvention tool.

We can observe that CDNs have a similar requirement. They need to
convince the client that "www.example.com" can be accessed through
the seemingly unrelated "cdn-node-xyz.example.net". Most CDNs have
deployed DNS-based solutions to this problem. However, the CDN often
holds the authoritative certificate of the origin. There is,
simultaneously, verification of a relationship between the origin and
the CDN (through the certificate) and a risk that the CDN can spoof
the content from the origin.

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 [HTTP2-SEC-CERTS] can be used to manage authentication
of hidden server content or to perform client authentication before
accessing hidden content.

5. Security Considerations

This document lists a number of attacks against SNI encryption in
Sections 3 and 4.2 and presents a list of requirements to mitigate
these attacks. 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.

Replacing cleartext SNI transmission by an encrypted variant will
break or reduce the efficacy of the operational practices and
techniques implemented in middleboxes, as described in Section 2.1.
As explained in Section 2.3, alternative solutions will have to be
developed.

6. IANA Considerations

This document has no IANA actions.

7. Informative References

[DOMFRONT] Fifield, D., Lan, C., Hynes, R., Wegmann, P., and V.
Paxson, "Blocking-resistant communication through domain
fronting", DOI 10.1515/popets-2015-0009, 2015,
<https://www.bamsoftware.com/papers/fronting/>.

[DTLS-1.3] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-38, 29 May 2020,
<https://tools.ietf.org/html/draft-ietf-tls-dtls13-38>.

[HTTP2-SEC-CERTS]
Bishop, M., Sullivan, N., and M. Thomson, "Secondary
Certificate Authentication in HTTP/2", Work in Progress,
Internet-Draft, draft-ietf-httpbis-http2-secondary-certs-
06, 14 May 2020, <https://tools.ietf.org/html/draft-ietf-
httpbis-http2-secondary-certs-06>.

[QUIC] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
Work in Progress, Internet-Draft, draft-ietf-quic-tls-29,
9 June 2020,
<https://tools.ietf.org/html/draft-ietf-quic-tls-29>.

[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999,
<https://www.rfc-editor.org/info/rfc2246>.

[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>.

[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,
<https://www.rfc-editor.org/info/rfc3546>.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>.

[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,
<https://www.rfc-editor.org/info/rfc4366>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.

[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.

[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, <https://www.rfc-editor.org/info/rfc7590>.

[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, <https://www.rfc-editor.org/info/rfc7858>.

[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:
Use of Transport Layer Security (TLS) for Email Submission
and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
<https://www.rfc-editor.org/info/rfc8314>.

[RFC8336] Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
RFC 8336, DOI 10.17487/RFC8336, March 2018,
<https://www.rfc-editor.org/info/rfc8336>.

[RFC8404] Moriarty, K., Ed. and A. Morton, Ed., "Effects of
Pervasive Encryption on Operators", RFC 8404,
DOI 10.17487/RFC8404, July 2018,
<https://www.rfc-editor.org/info/rfc8404>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.

Acknowledgements

A large part of this document originated 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:
<https://mailarchive.ietf.org/arch/msg/tls/
tXvdcqnogZgqmdfCugrV8M90Ftw>.

Thanks to Eric Rescorla for his multiple suggestions, reviews, and
edits to the successive draft versions of this document.

Thanks to Daniel Kahn Gillmor for a pretty detailed review of the
initial draft of this document. Thanks to Bernard Aboba, Mike
Bishop, Alissa Cooper, Roman Danyliw, Stephen Farrell, Warren Kumari,
Mirja Kuelewind, Barry Leiba, Martin Rex, Adam Roach, Meral
Shirazipour, Martin Thomson, Eric Vyncke, and employees of the UK
National Cyber Security Centre for their reviews. Thanks to Chris
Wood, Ben Kaduk, and Sean Turner for helping move this document
toward publication.

Author's Address

Christian Huitema
Private Octopus Inc.
Friday Harbor, WA 98250
United States of America

Email: [email protected]