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Versions: (draft-huitema-tls-sni-encryption) 00 01 02 03 04 05 06 07 08 09

Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Informational                               E. Rescorla
Expires: April 30, 2020                                       RTFM, Inc.
                                                        October 28, 2019


           Issues and Requirements for SNI Encryption in TLS
                    draft-ietf-tls-sni-encryption-09

Abstract

   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
   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 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 https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on April 30, 2020.

Copyright Notice

   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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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  History of the TLS SNI extension  . . . . . . . . . . . . . .   3
     2.1.  Unanticipated usage of SNI information  . . . . . . . . .   4
     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 . . . . . . . . . . . . . . . . .   6
     3.2.  Avoid Widely Shared Secrets . . . . . . . . . . . . . . .   6
     3.3.  Prevent SNI-based Denial of Service Attacks . . . . . . .   6
     3.4.  Do not stick out  . . . . . . . . . . . . . . . . . . . .   7
     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 . . . . . . . . . .   9
   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 . . . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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.  For
   example, in virtual hosting solutions, multiple services can be



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   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
   Networks (CDNs) solutions, a given platform hosts the services or
   servers servers of a lot of organization, and looking up to what
   netblock an IP address belongs reveals little.  However, multiplexed
   servers rely on the Service Name Information (SNI) TLS extension
   [RFC6066] 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 is to prevent that
   and aid privacy.

   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.  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 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.







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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 draft.
   They include:

   o  Filtering or censorship of specific services for a variety of
      reasons.

   o  Content filtering by network operators or ISP blocking specific
      web sites in order to implement, for example, parental controls,
      or to prevent access to phishing or other fraudulent 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

   o  Firewalls within enterprise networks exempting specific web sites
      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 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
   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



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   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 certificate transmissions all
   contribute to user privacy in the face of an [RFC7258]-style
   adversary.  Encrypting the SNI complements this push for privacy and
   make 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 middle-boxes 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" 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




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   rejected after security reviews identified plausible attacks.  In
   this section, we collect a list of these known attacks.

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
   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, 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 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 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 Denial of Service
   Attacks (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



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   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 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.

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 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 failed to maintain forward secrecy
   of SNI encryption.

3.6.  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 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 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



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   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 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
   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], IMAP [RFC8314], and 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 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



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   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.  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
   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 refered 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 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.



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   o  The client's browser 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 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 using implementation of other
   applications over HTTP, such as for example DNS over HTTPS [RFC8484].
   The trust issue, however, requires specific developments.

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 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 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
   hidden.example.com, use fake.example.com as a fronting server", when
   "fake.example.com" is under the control of an adversary.




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   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
   fronting and hidden server, or obtaining the relation between 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 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 [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

   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.  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 clear text SNI transmission by an encrypted variant will
   break or reduce the efficacy of the operational practices and
   techniques implemented in middle-boxes as described in Section 2.1.




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   As explained in Section 2.3, alternative solutions will have to be
   developed.

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:
   <https://mailarchive.ietf.org/arch/msg/tls/
   tXvdcqnogZgqmdfCugrV8M90Ftw>.

   Thanks to Daniel Kahn Gillmor for a pretty detailed review of the
   initial draft.  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 publish this document.

8.  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/>.

   [I-D.ietf-httpbis-http2-secondary-certs]
              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.

   [I-D.ietf-quic-tls]
              Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              draft-ietf-quic-tls-23 (work in progress), September 2019.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-33 (work in progress), October
              2019.






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   [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>.

   [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>.




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   [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>.

Authors' Addresses

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor  WA  98250
   U.S.A

   Email: huitema@huitema.net


   Eric Rescorla
   RTFM, Inc.
   U.S.A

   Email: ekr@rtfm.com














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