Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Informational                               E. Rescorla
Expires: November 21, 2018                                    RTFM, Inc.
                                                            May 20, 2018

           Issues and Requirements for SNI Encryption in TLS


   This draft describes the general problem of encryption of the Server
   Name Identification (SNI) 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

Status of This Memo

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   This Internet-Draft will expire on November 21, 2018.

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   include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . .   4
   3.  Security and Privacy Requirements for SNI Encryption  . . . .   5
     3.1.  Mitigate Replay Attacks . . . . . . . . . . . . . . . . .   5
     3.2.  Avoid Widely Shared Secrets . . . . . . . . . . . . . . .   5
     3.3.  Prevent SNI-based Denial of Service Attacks . . . . . . .   6
     3.4.  Do not stick out  . . . . . . . . . . . . . . . . . . . .   6
     3.5.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .   6
     3.6.  Proper Security Context . . . . . . . . . . . . . . . . .   6
     3.7.  Fronting Server Spoofing  . . . . . . . . . . . . . . . .   7
     3.8.  Supporting multiple protocols . . . . . . . . . . . . . .   7
       3.8.1.  Hiding the Application Layer Protocol Negotiation . .   8
       3.8.2.  Support other transports than HTTP  . . . . . . . . .   8
     3.9.  Fail to fronting  . . . . . . . . . . . . . . . . . . . .   8
   4.  HTTP Co-Tenancy Fronting  . . . . . . . . . . . . . . . . . .   9
     4.1.  HTTPS Tunnels . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Delegation Control  . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Historically, adversaries have been able to monitor the use of web
   services through three 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] 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) to direct TLS connections to the appropriate
   service implementation.  This protocol element is transmitted in

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   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.

   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 is HTTP level fronting, which we discuss in
   Section 4.

2.  History of the TLS SNI extension

   The SNI extension was standardized in 2003 in [RFC3546] to facilitate
   management of "colocation servers", in which a 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,
   but only the "hostname" variant was standardized 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
   "Client Hello" message.

2.1.  Unanticipated usage of SNI information

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

   o  Censorship of specific sites by "national firewalls",

   o  Content filtering by ISP blocking specific web sites in order to
      implement "parental controls", or to prevent access to fraudulent
      web sites, such as used for phishing,

   o  ISP assigning different QOS profiles to target services,

   o  Enterprise firewalls blocking web sites not deemed appropriate for

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

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   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 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 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 [I-D.ietf-doh-dns-over-https].

   The common name component of the server certificate generally exposes
   the same name as the SNI.  In TLS versions 1.0 [RFC2246], 1.1
   [RFC4346], and 1.2 [RFC5246], the server send their certificate in
   clear text, ensuring that there would be limited benefits in hiding
   the SNI.  But the transmission of the server certificate is protected
   in TLS 1.3 [I-D.ietf-tls-tls13].

   The decoupling of IP addresses and server names, the deployment of
   DNS privacy, and the protection of server certificates transmissions
   all contribute to user privacy.  Encrypting the SNI now will complete
   this push for privacy and make it much harder to censor specific
   internet services.

2.3.  End-to-end alternatives

   Deploying SNI encryption will help thwarting most 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.  Enterprises can deploy monitoring software to control

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   usage of the enterprises computers.  As SNI encryption becomes
   common, we can expect more deployment of such "end to end" solutions.

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, but were rejected after security reviews pointed 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

   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.

   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.

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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.

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.  Proper Security Context

   We can design solutions in which the multiplexed server or 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
   multiplexed server, or between the client and the fronting service.

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   The master secret is verified by verifying a certificate provided by
   either of these entities, but not by the protected service.

   These solutions expose the client to a Man-In-The-Middle attack by
   the multiplexed server or by the fronting service.  Even if the
   client has some reasonable trust in these services, the possibility
   of MITM attack is troubling.

   The multiplexed server or the fronting services could be pressured by
   adversaries.  By design, they 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 them
   into intercepting or spoofing the communications between client and
   protected service.

3.7.  Fronting Server Spoofing

   Adversaries could 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.

3.8.  Supporting multiple protocols

   The SNI encryption requirement do 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.

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3.8.1.  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.

   It is thus desirable that SNI Encryption mechanisms be also able hide
   the ALPN.

3.8.2.  Support other transports than HTTP

   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 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.

3.9.  Fail to fronting

   It is easy to imagine designs in which the client sends some client
   hello extension that points to a secret shared by client and hidden
   server.  If that secret is incorporated into the handshake secret,
   the exchange will only succeeds if the connection truly ends at the
   hidden server.  The exchange will fail if the extension is stripped
   by an MITM, and the exchange will also fail if an adversary replays
   the extension in a Client Hello.

   The problem with that approach is clear.  Adversaries that replay the
   extension can test whether the client truly wanted to access the
   fronting server, or was simply using that fronting server as an
   access gateway to something else.  The adversaries will not know what
   hidden service the client was trying to reach, but they can guess.

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   They can also start directly interrogate the user, or other
   unpleasant alternatives.

   When designing SNI encryption schemes, we have to take into account
   attacks that strip parameters from the Client Hello, or replay
   attacks.  In both cases, the desired behavior is to fall back to a
   connection with the fronting server, so there is no visble difference
   between a regular connection to that server and an attempt to reach
   the hidden server.

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 "
   some-content", 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 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 protected non
      HTTP protocols such as DNS over TLS [RFC7858] or IMAP over TLS

   The discovery issue is common to pretty much every SNI encryption
   solution.  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

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   DNS over HTTPS [I-D.hoffman-dns-over-https].  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 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 3.7.  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

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   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-".  Most CDN have deployed DNS-based solutions to this

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 provide
   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 if 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.

8.  References

8.1.  Normative References

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              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

8.2.  Informative References

              Hoffman, P. and P. McManus, "DNS Queries over HTTPS",
              draft-hoffman-dns-over-https-01 (work in progress), June

              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DOH)", draft-ietf-doh-dns-over-https-08 (work in
              progress), May 2018.

              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-11 (work in
              progress), April 2018.

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-26 (work in progress), March

              Moriarty, K. and A. Morton, "Effects of Pervasive
              Encryption on Operators", draft-mm-wg-effect-encrypt-25
              (work in progress), March 2018.

   [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,

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   [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,

   [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, <>.

Authors' Addresses

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor  WA  98250


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   Eric Rescorla
   RTFM, Inc.


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