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Versions: (draft-rescorla-tls-esni) 00 01 02 03 04

tls                                                          E. Rescorla
Internet-Draft                                                RTFM, Inc.
Intended status: Experimental                                     K. Oku
Expires: January 9, 2020                                          Fastly
                                                             N. Sullivan
                                                              Cloudflare
                                                                 C. Wood
                                                             Apple, Inc.
                                                           July 08, 2019


              Encrypted Server Name Indication for TLS 1.3
                         draft-ietf-tls-esni-04

Abstract

   This document defines a simple mechanism for encrypting the Server
   Name Indication for TLS 1.3.

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 January 9, 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
   (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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Topologies  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  SNI Encryption  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Publishing the SNI Encryption Key in the DNS  . . . . . . . .   5
     4.1.  Encrypted SNI Record  . . . . . . . . . . . . . . . . . .   6
     4.2.  Encrypted SNI DNS Resolution  . . . . . . . . . . . . . .   8
       4.2.1.  Address Set Extension . . . . . . . . . . . . . . . .   8
       4.2.2.  Resolution Algorithm  . . . . . . . . . . . . . . . .   9
   5.  The "encrypted_server_name" extension . . . . . . . . . . . .  10
     5.1.  Client Behavior . . . . . . . . . . . . . . . . . . . . .  11
       5.1.1.  Sending an encrypted SNI  . . . . . . . . . . . . . .  11
       5.1.2.  Handling the server response  . . . . . . . . . . . .  14
       5.1.3.  Authenticating for the public name  . . . . . . . . .  15
       5.1.4.  GREASE extensions . . . . . . . . . . . . . . . . . .  16
     5.2.  Client-Facing Server Behavior . . . . . . . . . . . . . .  17
     5.3.  Shared Mode Server Behavior . . . . . . . . . . . . . . .  19
     5.4.  Split Mode Server Behavior  . . . . . . . . . . . . . . .  19
   6.  Compatibility Issues  . . . . . . . . . . . . . . . . . . . .  20
     6.1.  Misconfiguration and Deployment Concerns  . . . . . . . .  20
     6.2.  Middleboxes . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
     7.1.  Why is cleartext DNS OK?  . . . . . . . . . . . . . . . .  21
     7.2.  Optional Record Digests and Trial Decryption  . . . . . .  21
     7.3.  Encrypting other Extensions . . . . . . . . . . . . . . .  22
     7.4.  Related Privacy Leaks . . . . . . . . . . . . . . . . . .  22
     7.5.  Comparison Against Criteria . . . . . . . . . . . . . . .  22
       7.5.1.  Mitigate against replay attacks . . . . . . . . . . .  22
       7.5.2.  Avoid widely-deployed shared secrets  . . . . . . . .  23
       7.5.3.  Prevent SNI-based DoS attacks . . . . . . . . . . . .  23
       7.5.4.  Do not stick out  . . . . . . . . . . . . . . . . . .  23
       7.5.5.  Forward secrecy . . . . . . . . . . . . . . . . . . .  23
       7.5.6.  Proper security context . . . . . . . . . . . . . . .  23
       7.5.7.  Split server spoofing . . . . . . . . . . . . . . . .  24
       7.5.8.  Supporting multiple protocols . . . . . . . . . . . .  24
     7.6.  Misrouting  . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Update of the TLS ExtensionType Registry  . . . . . . . .  24
     8.2.  Update of the TLS Alert Registry  . . . . . . . . . . . .  24
     8.3.  Update of the Resource Record (RR) TYPEs Registry . . . .  25
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  25



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     9.2.  Informative References  . . . . . . . . . . . . . . . . .  26
   Appendix A.  Communicating SNI and Nonce to Backend Server  . . .  27
   Appendix B.  Alternative SNI Protection Designs . . . . . . . . .  27
     B.1.  TLS-layer . . . . . . . . . . . . . . . . . . . . . . . .  27
       B.1.1.  TLS in Early Data . . . . . . . . . . . . . . . . . .  27
       B.1.2.  Combined Tickets  . . . . . . . . . . . . . . . . . .  28
     B.2.  Application-layer . . . . . . . . . . . . . . . . . . . .  28
       B.2.1.  HTTP/2 CERTIFICATE Frames . . . . . . . . . . . . . .  28
   Appendix C.  Total Client Hello Encryption  . . . . . . . . . . .  28
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   DISCLAIMER: This is very early a work-in-progress design and has not
   yet seen significant (or really any) security analysis.  It should
   not be used as a basis for building production systems.

   Although TLS 1.3 [RFC8446] encrypts most of the handshake, including
   the server certificate, there are several other channels that allow
   an on-path attacker to determine the domain name the client is trying
   to connect to, including:

   o  Cleartext client DNS queries.

   o  Visible server IP addresses, assuming the the server is not doing
      domain-based virtual hosting.

   o  Cleartext Server Name Indication (SNI) [RFC6066] in ClientHello
      messages.

   DoH [I-D.ietf-doh-dns-over-https] and DPRIVE [RFC7858] [RFC8094]
   provide mechanisms for clients to conceal DNS lookups from network
   inspection, and many TLS servers host multiple domains on the same IP
   address.  In such environments, SNI is an explicit signal used to
   determine the server's identity.  Indirect mechanisms such as traffic
   analysis also exist.

   The TLS WG has extensively studied the problem of protecting SNI, but
   has been unable to develop a completely generic solution.
   [I-D.ietf-tls-sni-encryption] provides a description of the problem
   space and some of the proposed techniques.  One of the more difficult
   problems is "Do not stick out" ([I-D.ietf-tls-sni-encryption];
   Section 3.4): if only sensitive/private services use SNI encryption,
   then SNI encryption is a signal that a client is going to such a
   service.  For this reason, much recent work has focused on concealing
   the fact that SNI is being protected.  Unfortunately, the result




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   often has undesirable performance consequences, incomplete coverage,
   or both.

   The design in this document takes a different approach: it assumes
   that private origins will co-locate with or hide behind a provider
   (CDN, app server, etc.) which is able to activate encrypted SNI
   (ESNI) for all of the domains it hosts.  Thus, the use of encrypted
   SNI does not indicate that the client is attempting to reach a
   private origin, but only that it is going to a particular service
   provider, which the observer could already tell from the IP address.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.  All TLS notation comes from [RFC8446];
   Section 3.

3.  Overview

   This document is designed to operate in one of two primary topologies
   shown below, which we call "Shared Mode" and "Split Mode"

3.1.  Topologies

                   +---------------------+
                   |                     |
                   |   2001:DB8::1111    |
                   |                     |
   Client <----->  | private.example.org |
                   |                     |
                   | public.example.com  |
                   |                     |
                   +---------------------+
                           Server

                      Figure 1: Shared Mode Topology

   In Shared Mode, the provider is the origin server for all the domains
   whose DNS records point to it and clients form a TLS connection
   directly to that provider, which has access to the plaintext of the
   connection.







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                   +--------------------+       +---------------------+
                   |                    |       |                     |
                   |   2001:DB8::1111   |       |   2001:DB8::EEEE    |
   Client <------------------------------------>|                     |
                   | public.example.com |       | private.example.com |
                   |                    |       |                     |
                   +--------------------+       +---------------------+
                     Client-Facing Server            Backend Server

                       Figure 2: Split Mode Topology

   In Split Mode, the provider is _not_ the origin server for private
   domains.  Rather the DNS records for private domains point to the
   provider, but the provider's server just relays the connection back
   to the backend server, which is the true origin server.  The provider
   does not have access to the plaintext of the connection.  In
   principle, the provider might not be the origin for any domains, but
   as a practical matter, it is probably the origin for a large set of
   innocuous domains, but is also providing protection for some private
   domains.  Note that the backend server can be an unmodified TLS 1.3
   server.

3.2.  SNI Encryption

   First, the provider publishes a public key and metadata which is used
   for SNI encryption for all the domains for which it serves directly
   or indirectly (via Split Mode).  This document defines a publication
   mechanism using DNS, but other mechanisms are also possible.  In
   particular, if some of the clients of a private server are
   applications rather than Web browsers, those applications might have
   the public key and metadata preconfigured.

   When a client wants to form a TLS connection to any of the domains
   served by an ESNI-supporting provider, it sends an
   "encrypted_server_name" extension, which contains the true extension
   encrypted under the provider's public key.  The provider can then
   decrypt the extension and either terminate the connection (in Shared
   Mode) or forward it to the backend server (in Split Mode).

4.  Publishing the SNI Encryption Key in the DNS

   Publishing ESNI keys in the DNS requires care to ensure correct
   behavior.  There are deployment environments in which a domain is
   served by multiple server operators who do not manage the ESNI keys.
   Because ESNI and A/AAAA lookups are independent, it is therefore
   possible to obtain an ESNI record which does not match the A/AAAA
   records.  (That is, the host to which an A or AAAA record refers is
   not in possession of the ESNI keys.)  The design of the system must



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   therefore allow clients to detect and recover from this situation
   (see Section 4.2 for more details).

   Content providers operating in Split Mode SHOULD ensure that the A
   and AAAA records for ESNI-enabled server names do not allow
   identifying the server name from the IP address.  This can for
   example be achieved by always returning the same records for all
   ESNI-enabled names, or by having the function that picks addresses
   from a pool not depend on the server name.  This yields an anonymity
   set of cardinality equal to the number of ESNI-enabled server domains
   supported by a given client-facing server.  Thus, even with SNI
   encryption, an attacker which can enumerate the set of ESNI-enabled
   domains supported by a client-facing server can guess the correct SNI
   with probability at least 1/K, where K is the size of this ESNI-
   enabled server anonymity set.  This probability may be increased via
   traffic analysis or other mechanisms.

   The following sections describe a DNS record format that achieve
   these goals.

4.1.  Encrypted SNI Record

   SNI Encryption keys can be published using the following ESNIRecord
   structure.

       // Copied from TLS 1.3
       struct {
           NamedGroup group;
           opaque key_exchange<1..2^16-1>;
       } KeyShareEntry;

       struct {
           uint16 version;
           opaque public_name<1..2^16-1>;
           KeyShareEntry keys<4..2^16-1>;
           CipherSuite cipher_suites<2..2^16-2>;
           uint16 padded_length;
           Extension extensions<0..2^16-1>;
       } ESNIKeys;

       struct {
           ESNIKeys esni_keys;
           Extension dns_extensions<0..2^16-1>;
       } ESNIRecord;

   The outermost ESNIRecord structure contains the following fields:





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   esni_keys  An ESNIKeys structure that contains the actual keys used
      to encrypt the SNI as well as some metadata related to those keys.

   dns_extensions  A list of extensions that the client can take into
      consideration when resolving the target DNS name.  The format is
      defined in [RFC8446]; Section 4.2.  The purpose of the field is to
      provide room for additional features in the future.  An extension
      may be tagged as mandatory by using an extension type codepoint
      with the high order bit set to 1.  A client which receives a
      mandatory extension they do not understand must reject the
      ESNIRecord values.

   The ESNIKeys structure contains the following fields:

   version  The version of the structure.  For this specification, that
      value SHALL be 0xff03.  Clients MUST ignore any ESNIKeys structure
      with a version they do not understand.  [[NOTE: This means that
      the RFC will presumably have a nonzero value.]]

   public_name  The non-empty name of the entity trusted to update these
      encryption keys.  This is used to repair misconfigurations, as
      described in Section 5.1.2.

   keys  The list of keys which can be used by the client to encrypt the
      SNI.  Every key being listed MUST belong to a different group.

   padded_length The length to pad the ServerNameList value to prior to
   encryption.  This value SHOULD be set to the largest ServerNameList
   the server expects to support rounded up the nearest multiple of 16.
   If the server supports arbitrary wildcard names, it SHOULD set this
   value to 260.  Clients SHOULD reject ESNIKeys as invalid if
   padded_length is greater than 260.

   extensions  A list of extensions that the client can take into
      consideration when generating a Client Hello message.  The format
      is defined in [RFC8446]; Section 4.2.  The purpose of the field is
      to provide room for additional features in the future.  An
      extension may be tagged as mandatory by using an extension type
      codepoint with the high order bit set to 1.  A client which
      receives a mandatory extension they do not understand must reject
      the ESNIRecord value.

   Any of the listed keys in the ESNIKeys value may be used to encrypt
   the SNI for the associated domain name.  The cipher suite list is
   orthogonal to the list of keys, so each key may be used with any
   cipher suite.  Clients MUST parse the extension list and check for
   unsupported mandatory extensions.  If an unsupported mandatory
   extension is present, clients MUST reject the ESNIRecord value.



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   The ESNIRecord structure is placed in the RRData section of an ESNI
   record as-is.  Servers MAY supply multiple ESNIRecord values, with
   ESNIKeys either of the same or of different versions.  This allows a
   server to support multiple versions at once.  If the server does not
   supply any ESNIRecord values with an ESNIKeys version known to the
   client, then the client MUST behave as if no ESNI records were found.

   The name of each ESNI record MUST match the query domain name or the
   query domain name's canonicalized form.  That is, if a client queries
   example.com, the ESNI Resource Record might be:

   example.com. 60S IN ESNI "..." "..."

   In the event that ESNIKeys is corrupt in transit or otherwise
   invalid, servers will initiate the retry mechanism described in
   Section 5.2 and deliver valid ESNIKeys to clients.

   Note that the length of the ESNIRecord structure MUST NOT exceed 2^16
   - 1, as the RDLENGTH is only 16 bits [RFC1035].

4.2.  Encrypted SNI DNS Resolution

   This section describes a client ESNI resolution algorithm using an
   "address_set" extension for the ESNIRecord structure.  Future
   specifications may introduce new ESNIRecord extensions and
   corresponding resolution algorithms.

4.2.1.  Address Set Extension

   ESNIRecord values MAY indicate one or more IP addresses for the
   host(s) in possession of the private key corresponding to one of the
   keys provided in the ESNIKeys structure, via the following mandatory
   "address_set" extension:

       enum {
           address_set(0x1001), (65535)
       } ExtensionType;

   The body of this extension is encoded using the following structure.












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       enum {
           address_v4(4),
           address_v6(6),
       } AddressType;

       struct {
           AddressType address_type;
           select (address_type) {
               case address_v4: {
                   opaque ipv4Address[4];
               }
               case address_v6: {
                   opaque ipv6Address[16];
               }
           }
       } Address;

       struct {
           Address address_set<1..2^16-1>;
       } AddressSet;

   address_set  A set of Address structures containing IPv4 or IPv6
      addresses to hosts which have the corresponding private ESNI key.

   This extension MUST NOT be placed in the ESNIKeys extensions field,
   but only in the ESNIRecord dns_extensions field.

4.2.2.  Resolution Algorithm

   Clients obtain ESNI records by querying the DNS for ESNI-enabled
   server domains.  In cases where the domain of the A or AAAA records
   being resolved do not match the SNI Server Name, such as when
   [RFC7838] is being used, the alternate domain should be used for
   querying the ESNI record.  (See Section 2.3 of [RFC7838] for more
   details.)

   Clients SHOULD initiate ESNI queries in parallel alongside normal A
   or AAAA queries to obtain address information in a timely manner in
   the event that ESNI is available.  The following algorithm describes
   a procedure by which clients can process ESNI responses as they
   arrive to produce addresses for ESNI-capable hosts.










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1. If an ESNI response containing an ESNIRecord value with an "address_set" extension arrives before an A or
AAAA response, clients SHOULD initiate TLS with ESNI to the provided address(es).

2. If an A or AAAA response arrives before the ESNI response, clients SHOULD wait up
to CD milliseconds before initiating TLS to either address. (Clients may begin
TCP connections in this time. QUIC connections should wait.) If an ESNI
response with an "address_set" extension arrives in this time, clients SHOULD
initiate TLS with ESNI to the provided address(es). If an ESNI response
without an "address_set" extension arrives in this time, clients MAY initiate
TLS with ESNI to the address(es) in the A or AAAA response. If no ESNI response
arrives in this time, clients SHOULD initiate TLS without ESNI to the available address(es).

   CD (Connection Delay) is a configurable parameter.  The recommended
   value is 50 milliseconds, as per the guidance in [RFC8305].

5.  The "encrypted_server_name" extension

   The encrypted SNI is carried in an "encrypted_server_name" extension,
   defined as follows:

      enum {
          encrypted_server_name(0xffce), (65535)
      } ExtensionType;

   For clients (in ClientHello), this extension contains the following
   ClientEncryptedSNI structure:

      struct {
          CipherSuite suite;
          KeyShareEntry key_share;
          opaque record_digest<0..2^16-1>;
          opaque encrypted_sni<0..2^16-1>;
      } ClientEncryptedSNI;

   suite  The cipher suite used to encrypt the SNI.

   key_share  The KeyShareEntry carrying the client's public ephemeral
      key shared used to derive the ESNI key.

   record_digest  A cryptographic hash of the ESNIKeys structure from
      which the ESNI key was obtained, i.e., from the first byte of
      "version" to the end of the structure.  This hash is computed
      using the hash function associated with "suite".

   encrypted_sni  The ClientESNIInner structure, AEAD-encrypted using
      cipher suite "suite" and the key generated as described below.





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   For servers (in EncryptedExtensions), this extension contains the
   following structure:

      enum {
          esni_accept(0),
          esni_retry_request(1),
      } ServerESNIResponseType;

      struct {
          ServerESNIResponseType response_type;
          select (response_type) {
              case esni_accept:        uint8 nonce[16];
              case esni_retry_request: ESNIKeys retry_keys<1..2^16-1>;
          }
      } ServerEncryptedSNI;

   response_type  Indicates whether the server processed the client ESNI
      extension.  (See Section 5.1.2 and Section 5.2.}

   nonce  The contents of ClientESNIInner.nonce.  (See Section 5.1.)

   retry_keys  One or more ESNIKeys structures containing the keys that
      the client should use on subsequent connections to encrypt the
      ClientESNIInner structure.

   This protocol also defines the "esni_required" alert, which is sent
   by the client when it offered an "encrypted_server_name" extension
   which was not accepted by the server.

      enum {
          esni_required(121),
      } AlertDescription;

   Finally, requirements in Section 5.1 and Section 5.2 require
   implementations to track, alongside each PSK established by a
   previous connection, whether the connection negotiated this extension
   with the "esni_accept" response type.  If so, this is referred to as
   an "ESNI PSK".  Otherwise, it is a "non-ESNI PSK".  This may be
   implemented by adding a new field to client and server session
   states.

5.1.  Client Behavior

5.1.1.  Sending an encrypted SNI

   In order to send an encrypted SNI, the client MUST first select one
   of the server ESNIKeyShareEntry values and generate an (EC)DHE share
   in the matching group.  This share will then be sent to the server in



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   the "encrypted_sni" extension and used to derive the SNI encryption
   key.  It does not affect the (EC)DHE shared secret used in the TLS
   key schedule.  It MUST also select an appropriate cipher suite from
   the list of suites offered by the server.  If the client is unable to
   select an appropriate group or suite it SHOULD ignore that ESNIKeys
   value and MAY attempt to use another value provided by the server.
   (Recall that servers might provide multiple ESNIRecord values in
   response to a ESNI record query, each containing an ESNIKeys value.)
   The client MUST NOT send encrypted SNI using groups or cipher suites
   not advertised by the server.

   When offering an encrypted SNI, the client MUST NOT offer to resume
   any non-ESNI PSKs.  It additionally MUST NOT offer to resume any
   sessions for TLS 1.2 or below.

   Let Z be the DH shared secret derived from a key share in ESNIKeys
   and the corresponding client share in ClientEncryptedSNI.key_share.
   The SNI encryption key is computed from Z as follows:

   Zx = HKDF-Extract(0, Z)
   key = HKDF-Expand-Label(Zx, KeyLabel, Hash(ESNIContents), key_length)
   iv = HKDF-Expand-Label(Zx, IVLabel, Hash(ESNIContents), iv_length)

   where ESNIContents is as specified below and Hash is the hash
   function associated with the HKDF instantiation.  The salt argument
   for HKDF-Extract is a string consisting of Hash.length bytes set to
   zeros.  For a client's first ClientHello, KeyLabel = "esni key" and
   IVLabel = "esni iv", whereas for a client's second ClientHello, sent
   in response to a HelloRetryRequest, KeyLabel = "hrr esni key" and
   IVLabel = "hrr esni iv".  (This label variance is done to prevent
   nonce re-use since the client's ESNI key share, and thus the value of
   Zx, does not change across ClientHello retries.)

   [[TODO: label swapping fixes a bug in the spec, though this may not
   be the best way to deal with HRR.  See https://github.com/tlswg/
   draft-ietf-tls-esni/issues/121 and https://github.com/tlswg/draft-
   ietf-tls-esni/pull/170 for more details.]]

      struct {
          opaque record_digest<0..2^16-1>;
          KeyShareEntry esni_key_share;
          Random client_hello_random;
      } ESNIContents;

   The client then creates a ClientESNIInner structure:






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      struct {
          opaque dns_name<1..2^16-1>;
          opaque zeros[ESNIKeys.padded_length - length(sni)];
      } PaddedServerNameList;

      struct {
          uint8 nonce[16];
          PaddedServerNameList realSNI;
      } ClientESNIInner;

   nonce  A random 16-octet value to be echoed by the server in the
      "encrypted_server_name" extension.

   dns_name  The true SNI DNS name, that is, the HostName value that
      would have been sent in the plaintext "server_name" extension.
      (NameType values other than "host_name" are unsupported since SNI
      extensibility failed [SNIExtensibilityFailed]).

   zeros  Zero padding whose length makes the serialized
      PaddedServerNameList struct have a length equal to
      ESNIKeys.padded_length.

   This value consists of the serialized ServerNameList from the
   "server_name" extension, padded with enough zeroes to make the total
   structure ESNIKeys.padded_length bytes long.  The purpose of the
   padding is to prevent attackers from using the length of the
   "encrypted_server_name" extension to determine the true SNI.  If the
   serialized ServerNameList is longer than ESNIKeys.padded_length, the
   client MUST NOT use the "encrypted_server_name" extension.

   The ClientEncryptedSNI.encrypted_sni value is then computed using the
   usual TLS 1.3 AEAD:

    encrypted_sni = AEAD-Encrypt(key, iv, KeyShareClientHello, ClientESNIInner)

   Where KeyShareClientHello is the "extension_data" field of the
   "key_share" extension in a Client Hello (Section 4.2.8 of
   [RFC8446])).  Including KeyShareClientHello in the AAD of AEAD-
   Encrypt binds the ClientEncryptedSNI value to the ClientHello and
   prevents cut-and-paste attacks.

   Note: future extensions may end up reusing the server's
   ESNIKeyShareEntry for other purposes within the same message (e.g.,
   encrypting other values).  Those usages MUST have their own HKDF
   labels to avoid reuse.

   [[OPEN ISSUE: If in the future you were to reuse these keys for 0-RTT
   priming, then you would have to worry about potentially expanding



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   twice of Z_extracted.  We should think about how to harmonize these
   to make sure that we maintain key separation.]]

   This value is placed in an "encrypted_server_name" extension.

   The client MUST place the value of ESNIKeys.public_name in the
   "server_name" extension.  (This is required for technical conformance
   with [RFC7540]; Section 9.2.)  The client MUST NOT send a
   "cached_info" extension [RFC7924] with a CachedObject entry whose
   CachedInformationType is "cert".

5.1.2.  Handling the server response

   If the server negotiates TLS 1.3 or above and provides an
   "encrypted_server_name" extension in EncryptedExtensions, the client
   then processes the extension's "response_type" field:

   o  If the value is "esni_accept", the client MUST check that the
      extension's "nonce" field matches ClientESNIInner.nonce and
      otherwise abort the connection with an "illegal_parameter" alert.
      The client then proceeds with the connection as usual,
      authenticating the connection for the origin server.

   o  If the value is "esni_retry_request", the client proceeds with the
      handshake, authenticating for ESNIKeys.public_name as described in
      Section 5.1.3.  If authentication or the handshake fails, the
      client MUST return a failure to the calling application.  It MUST
      NOT use the retry keys.

      Otherwise, when the handshake completes successfully with the
      public name authenticated, the client MUST abort the connection
      with an "esni_required" alert.  It then processes the "retry_keys"
      field from the server's "encrypted_server_name" extension.

      If one of the values contains a version supported by the client,
      it can regard the ESNI keys as securely replaced by the server.
      It SHOULD retry the handshake with a new transport connection,
      using that value to encrypt the SNI.  The value may only be
      applied to the retry connection.  The client MUST continue to use
      the previously-advertised keys for subsequent connections.  This
      avoids introducing pinning concerns or a tracking vector, should a
      malicious server present client-specific retry keys to identify
      clients.

      If none of the values provided in "retry_keys" contains a
      supported version, the client can regard ESNI as securely disabled
      by the server.  As below, it SHOULD then retry the handshake with
      a new transport connection and ESNI disabled.



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   o  If the field contains any other value, the client MUST abort the
      connection with an "illegal_parameter" alert.

   If the server negotiates an earlier version of TLS, or if it does not
   provide an "encrypted_server_name" extension in EncryptedExtensions,
   the client proceeds with the handshake, authenticating for
   ESNIKeys.public_name as described in Section 5.1.3.  If an earlier
   version was negotiated, the client MUST NOT enable the False Start
   optimization [RFC7918] for this handshake.  If authentication or the
   handshake fails, the client MUST return a failure to the calling
   application.  It MUST NOT treat this as a secure signal to disable
   ESNI.

   Otherwise, when the handshake completes successfully with the public
   name authenticated, the client MUST abort the connection with an
   "esni_required" alert.  The client can then regard ESNI as securely
   disabled by the server.  It SHOULD retry the handshake with a new
   transport connection and ESNI disabled.

   [[TODO: Key replacement is significantly less scary than saying that
   ESNI-naive servers bounce ESNI off.  Is it worth defining a strict
   mode toggle in the ESNI keys, for a deployment to indicate it is
   ready for that? ]]

   Clients SHOULD implement a limit on retries caused by
   "esni_retry_request" or servers which do not acknowledge the
   "encrypted_server_name" extension.  If the client does not retry in
   either scenario, it MUST report an error to the calling application.

   If the server sends a HelloRetryRequest in response to the
   ClientHello and the client can send a second updated ClientHello per
   the rules in [RFC8446], the "encrypted_server_name" extension values
   which do not depend on the (possibly updated) KeyShareClientHello,
   i.e,, ClientEncryptedSNI.suite, ClientEncryptedSNI.key_share, and
   ClientEncryptedSNI.record_digest, MUST NOT change across ClientHello
   messages.  Moreover, ClientESNIInner MUST not change across
   ClientHello messages.  Informally, the values of all unencrypted
   extension information, as well as the inner extension plaintext, must
   be consistent between the first and second ClientHello messages.

5.1.3.  Authenticating for the public name

   When the server cannot decrypt or does not process the
   "encrypted_server_name" extension, it continues with the handshake
   using the cleartext "server_name" extension instead (see
   Section 5.2).  Clients that offer ESNI then authenticate the
   connection with the public name, as follows:




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   o  If the server resumed a session or negotiated a session that did
      not use a certificate for authentication, the client MUST abort
      the connection with an "illegal_parameter" alert.  This case is
      invalid because Section 5.1.1 requires the client to only offer
      ESNI-established sessions, and Section 5.2 requires the server to
      decline ESNI-established sessions if it did not accept ESNI.

   o  The client MUST verify that the certificate is valid for
      ESNIKeys.public_name.  If invalid, it MUST abort the connection
      with the appropriate alert.

   o  If the server requests a client certificate, the client MUST
      respond with an empty Certificate message, denoting no client
      certificate.

   Note that authenticating a connection for the public name does not
   authenticate it for the origin.  The TLS implementation MUST NOT
   report such connections as successful to the application.  It
   additionally MUST ignore all session tickets and session IDs
   presented by the server.  These connections are only used to trigger
   retries, as described in Section 5.1.2.  This may be implemented, for
   instance, by reporting a failed connection with a dedicated error
   code.

5.1.4.  GREASE extensions

   If the client attempts to connect to a server and does not have an
   ESNIKeys structure available for the server, it SHOULD send a GREASE
   [I-D.ietf-tls-grease] "encrypted_server_name" extension as follows:

   o  Select a supported cipher suite, named group, and padded_length
      value.  The padded_length value SHOULD be 260 or a multiple of 16
      less than

      1.  Set the "suite" field to the selected cipher suite.  These
          selections SHOULD vary to exercise all supported
          configurations, but MAY be held constant for successive
          connections to the same server in the same session.

   o  Set the "key_share" field to a randomly-generated valid public key
      for the named group.

   o  Set the "record_digest" field to a randomly-generated string of
      hash_length bytes, where hash_length is the length of the hash
      function associated with the chosen cipher suite.






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   o  Set the "encrypted_sni" field to a randomly-generated string of 16
      + padded_length + tag_length bytes, where tag_length is the tag
      length of the chosen cipher suite's associated AEAD.

   If the server sends an "encrypted_server_name" extension, the client
   MUST check the extension syntactically and abort the connection with
   a "decode_error" alert if it is invalid.  If the "response_type"
   field contains "esni_retry_requested", the client MUST ignore the
   extension and proceed with the handshake.  If it contains
   "esni_accept" or any other value, the client MUST abort the
   connection with an "illegal_parameter" alert.

   Offering a GREASE extension is not considered offering an encrypted
   SNI for purposes of requirements in Section 5.1.  In particular, the
   client MAY offer to resume sessions established without ESNI.

5.2.  Client-Facing Server Behavior

   Upon receiving an "encrypted_server_name" extension, the client-
   facing server MUST check that it is able to negotiate TLS 1.3 or
   greater.  If not, it MUST abort the connection with a
   "handshake_failure" alert.

   The ClientEncryptedSNI value is said to match a known ESNIKeys if
   there exists an ESNIKeys that can be used to successfully decrypt
   ClientEncryptedSNI.encrypted_sni.  This matching procedure should be
   done using one of the following two checks:

   1.  Compare ClientEncryptedSNI.record_digest against cryptographic
       hashes of known ESNIKeys and choose the one that matches.

   2.  Use trial decryption of ClientEncryptedSNI.encrypted_sni with
       known ESNIKeys and choose the one that succeeds.

   Some uses of ESNI, such as local discovery mode, may omit the
   ClientEncryptedSNI.record_digest since it can be used as a tracking
   vector.  In such cases, trial decryption should be used for matching
   ClientEncryptedSNI to known ESNIKeys.  Unless specified by the
   application using (D)TLS or externally configured on both sides,
   implementations MUST use the first method.

   If the ClientEncryptedSNI value does not match any known ESNIKeys
   structure, it MUST ignore the extension and proceed with the
   connection, with the following added behavior:

   o  It MUST include the "encrypted_server_name" extension in
      EncryptedExtensions message with the "response_type" field set to
      "esni_retry_requested" and the "retry_keys" field set to one or



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      more ESNIKeys structures with up-to-date keys.  Servers MAY supply
      multiple ESNIKeys values of different versions.  This allows a
      server to support multiple versions at once.

   o  The server MUST ignore all PSK identities in the ClientHello which
      correspond to ESNI PSKs.  ESNI PSKs offered by the client are
      associated with the ESNI name.  The server was unable to decrypt
      then ESNI name, so it should not resume them when using the
      cleartext SNI name.  This restriction allows a client to reject
      resumptions in Section 5.1.3.

   Note that an unrecognized ClientEncryptedSNI.record_digest value may
   be a GREASE ESNI extension (see Section 5.1.4), so it is necessary
   for servers to proceed with the connection and rely on the client to
   abort if ESNI was required.  In particular, the unrecognized value
   alone does not indicate a misconfigured ESNI advertisement
   (Section 6.1).  Instead, servers can measure occurrences of the
   "esni_required" alert to detect this case.

   If the ClientEncryptedSNI value does match a known ESNIKeys, the
   server performs the following checks:

   o  If the ClientEncryptedSNI.key_share group does not match one in
      the ESNIKeys.keys, it MUST abort the connection with an
      "illegal_parameter" alert.

   o  If the length of the "encrypted_server_name" extension is
      inconsistent with the advertised padding length (plus AEAD
      expansion) the server MAY abort the connection with an
      "illegal_parameter" alert without attempting to decrypt.

   Assuming these checks succeed, the server then computes K_sni and
   decrypts the ServerName value.  If decryption fails, the server MUST
   abort the connection with a "decrypt_error" alert.

   If the decrypted value's length is different from the advertised
   ESNIKeys.padded_length or the padding consists of any value other
   than 0, then the server MUST abort the connection with an
   "illegal_parameter" alert.  Otherwise, the server uses the
   PaddedServerNameList.sni value as if it were the "server_name"
   extension.  Any actual "server_name" extension is ignored, which also
   means the server MUST NOT send the "server_name" extension to the
   client.

   Upon determining the true SNI, the client-facing server then either
   serves the connection directly (if in Shared Mode), in which case it
   executes the steps in the following section, or forwards the TLS
   connection to the backend server (if in Split Mode).  In the latter



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   case, it does not make any changes to the TLS messages, but just
   blindly forwards them.

   If the ClientHello is the result of a HelloRetryRequest, servers MUST
   abort the connection with an "illegal_parameter" alert if any of the
   ClientEncryptedSNI.suite, ClientEncryptedSNI.key_share,
   ClientEncryptedSNI.record_digest, or decrypted ClientESNIInner values
   from the second ClientHello do not match that of the first
   ClientHello.

5.3.  Shared Mode Server Behavior

   A server operating in Shared Mode uses PaddedServerNameList.sni as if
   it were the "server_name" extension to finish the handshake.  It
   SHOULD pad the Certificate message, via padding at the record layer,
   such that its length equals the size of the largest possible
   Certificate (message) covered by the same ESNI key.  Moreover, the
   server MUST include the "encrypted_server_name" extension in
   EncryptedExtensions with the "response_type" field set to
   "esni_accept" and the "nonce" field set to the decrypted
   PaddedServerNameList.nonce value from the client
   "encrypted_server_name" extension.

   If the server sends a NewSessionTicket message, the corresponding
   ESNI PSK MUST be ignored by all other servers in the deployment when
   not negotiating ESNI, including servers which do not implement this
   specification.

   This restriction provides robustness for rollbacks (see Section 6.1).

5.4.  Split Mode Server Behavior

   In Split Mode, the backend server must know
   PaddedServerNameList.nonce to echo it back in EncryptedExtensions and
   complete the handshake.  Appendix A describes one mechanism for
   sending both PaddedServerNameList.sni and ClientESNIInner.nonce to
   the backend server.  Thus, backend servers function the same as
   servers operating in Shared Mode.

   As in Shared Mode, if the backend server sends a NewSessionTicket
   message, the corresponding ESNI PSK MUST be ignored by other servers
   in the deployment when not negotiating ESNI, including servers which
   do not implement this specification.








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6.  Compatibility Issues

   Unlike most TLS extensions, placing the SNI value in an ESNI
   extension is not interoperable with existing servers, which expect
   the value in the existing cleartext extension.  Thus server operators
   SHOULD ensure servers understand a given set of ESNI keys before
   advertising them.  Additionally, servers SHOULD retain support for
   any previously-advertised keys for the duration of their validity.

   However, in more complex deployment scenarios, this may be difficult
   to fully guarantee.  Thus this protocol was designed to be robust in
   case of inconsistencies between systems that advertise ESNI keys and
   servers, at the cost of extra round-trips due to a retry.  Two
   specific scenarios are detailed below.

6.1.  Misconfiguration and Deployment Concerns

   It is possible for ESNI advertisements and servers to become
   inconsistent.  This may occur, for instance, from DNS
   misconfiguration, caching issues, or an incomplete rollout in a
   multi-server deployment.  This may also occur if a server loses its
   ESNI keys, or if a deployment of ESNI must be rolled back on the
   server.

   The retry mechanism repairs inconsistencies, provided the server is
   authoritative for the public name.  If server and advertised keys
   mismatch, the server will respond with esni_retry_requested.  If the
   server does not understand the "encrypted_server_name" extension at
   all, it will ignore it as required by [RFC8446]; Section 4.1.2.
   Provided the server can present a certificate valid for the public
   name, the client can safely retry with updated settings, as described
   in Section 5.1.2.

   Unless ESNI is disabled as a result of successfully establishing a
   connection to the public name, the client MUST NOT fall back to
   cleartext SNI, as this allows a network attacker to disclose the SNI.
   It MAY attempt to use another server from the DNS results, if one is
   provided.

6.2.  Middleboxes

   A more serious problem is MITM proxies which do not support this
   extension.  [RFC8446]; Section 9.3 requires that such proxies remove
   any extensions they do not understand.  The handshake will then
   present a certificate based on the public name, without echoing the
   "encrypted_server_name" extension to the client.





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   Depending on whether the client is configured to accept the proxy's
   certificate as authoritative for the public name, this may trigger
   the retry logic described in Section 5.1.2 or result in a connection
   failure.  A proxy which is not authoritative for the public name
   cannot forge a signal to disable ESNI.

   A non-conformant MITM proxy which instead forwards the ESNI
   extension, substituting its own KeyShare value, will result in the
   client-facing server recognizing the key, but failing to decrypt the
   SNI.  This causes a hard failure.  Clients SHOULD NOT attempt to
   repair the connection in this case.

7.  Security Considerations

7.1.  Why is cleartext DNS OK?

   In comparison to [I-D.kazuho-protected-sni], wherein DNS Resource
   Records are signed via a server private key, ESNI records have no
   authenticity or provenance information.  This means that any attacker
   which can inject DNS responses or poison DNS caches, which is a
   common scenario in client access networks, can supply clients with
   fake ESNI records (so that the client encrypts SNI to them) or strip
   the ESNI record from the response.  However, in the face of an
   attacker that controls DNS, no SNI encryption scheme can work because
   the attacker can replace the IP address, thus blocking client
   connections, or substituting a unique IP address which is 1:1 with
   the DNS name that was looked up (modulo DNS wildcards).  Thus,
   allowing the ESNI records in the clear does not make the situation
   significantly worse.

   Clearly, DNSSEC (if the client validates and hard fails) is a defense
   against this form of attack, but DoH/DPRIVE are also defenses against
   DNS attacks by attackers on the local network, which is a common case
   where SNI is desired.  Moreover, as noted in the introduction, SNI
   encryption is less useful without encryption of DNS queries in
   transit via DoH or DPRIVE mechanisms.

7.2.  Optional Record Digests and Trial Decryption

   Supporting optional record digests and trial decryption opens oneself
   up to DoS attacks.  Specifically, an adversary may send malicious
   ClientHello messages, i.e., those which will not decrypt with any
   known ESNI key, in order to force decryption.  Servers that support
   this feature should, for example, implement some form of rate
   limiting mechanism to limit the damage caused by such attacks.






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7.3.  Encrypting other Extensions

   ESNI protects only the SNI in transit.  Other ClientHello extensions,
   such as ALPN, might also reveal privacy-sensitive information to the
   network.  As such, it might be desirable to encrypt other extensions
   alongside the SNI.  However, the SNI extension is unique in that non-
   TLS-terminating servers or load balancers may act on its contents.
   Thus, using keys specifically for SNI encryption promotes key
   separation between client-facing servers and endpoints party to TLS
   connections.  Moreover, the ESNI design described herein does not
   preclude a mechanism for generic ClientHello extension encryption.

7.4.  Related Privacy Leaks

   ESNI requires encrypted DNS to be an effective privacy protection
   mechanism.  However, verifying the server's identity from the
   Certificate message, particularly when using the X509
   CertificateType, may result in additional network traffic that may
   reveal the server identity.  Examples of this traffic may include
   requests for revocation information, such as OCSP or CRL traffic, or
   requests for repository information, such as
   authorityInformationAccess.  It may also include implementation-
   specific traffic for additional information sources as part of
   verification.

   Implementations SHOULD avoid leaking information that may identify
   the server.  Even when sent over an encrypted transport, such
   requests may result in indirect exposure of the server's identity,
   such as indicating a specific CA or service being used.  To mitigate
   this risk, servers SHOULD deliver such information in-band when
   possible, such as through the use of OCSP stapling, and clients
   SHOULD take steps to minimize or protect such requests during
   certificate validation.

7.5.  Comparison Against Criteria

   [I-D.ietf-tls-sni-encryption] lists several requirements for SNI
   encryption.  In this section, we re-iterate these requirements and
   assess the ESNI design against them.

7.5.1.  Mitigate against replay attacks

   Since the SNI encryption key is derived from a (EC)DH operation
   between the client's ephemeral and server's semi-static ESNI key, the
   ESNI encryption is bound to the Client Hello.  It is not possible for
   an attacker to "cut and paste" the ESNI value in a different Client
   Hello, with a different ephemeral key share, as the terminating
   server will fail to decrypt and verify the ESNI value.



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7.5.2.  Avoid widely-deployed shared secrets

   This design depends upon DNS as a vehicle for semi-static public key
   distribution.  Server operators may partition their private keys
   however they see fit provided each server behind an IP address has
   the corresponding private key to decrypt a key.  Thus, when one ESNI
   key is provided, sharing is optimally bound by the number of hosts
   that share an IP address.  Server operators may further limit sharing
   by sending different Resource Records containing ESNIRecord and
   ESNIKeys values with different keys using a short TTL.

7.5.3.  Prevent SNI-based DoS attacks

   This design requires servers to decrypt ClientHello messages with
   ClientEncryptedSNI extensions carrying valid digests.  Thus, it is
   possible for an attacker to force decryption operations on the
   server.  This attack is bound by the number of valid TCP connections
   an attacker can open.

7.5.4.  Do not stick out

   As more clients enable ESNI support, e.g., as normal part of Web
   browser functionality, with keys supplied by shared hosting
   providers, the presence of ESNI extensions becomes less suspicious
   and part of common or predictable client behavior.  In other words,
   if all Web browsers start using ESNI, the presence of this value does
   not signal suspicious behavior to passive eavesdroppers.

   Additionally, this specification allows for clients to send GREASE
   ESNI extensions (see Section 5.1.4), which helps ensure the ecosystem
   handles the values correctly.

7.5.5.  Forward secrecy

   This design is not forward secret because the server's ESNI key is
   static.  However, the window of exposure is bound by the key
   lifetime.  It is RECOMMENDED that servers rotate keys frequently.

7.5.6.  Proper security context

   This design permits servers operating in Split Mode to forward
   connections directly to backend origin servers, thereby avoiding
   unnecessary MiTM attacks.








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7.5.7.  Split server spoofing

   Assuming ESNI records retrieved from DNS are validated, e.g., via
   DNSSEC or fetched from a trusted Recursive Resolver, spoofing a
   server operating in Split Mode is not possible.  See Section 7.1 for
   more details regarding cleartext DNS.

   Validating the ESNIKeys structure additionally validates the public
   name.  This validates any retry signals from the server because the
   client validates the server certificate against the public name
   before retrying.

7.5.8.  Supporting multiple protocols

   This design has no impact on application layer protocol negotiation.
   It may affect connection routing, server certificate selection, and
   client certificate verification.  Thus, it is compatible with
   multiple protocols.

7.6.  Misrouting

   Note that the backend server has no way of knowing what the SNI was,
   but that does not lead to additional privacy exposure because the
   backend server also only has one identity.  This does, however,
   change the situation slightly in that the backend server might
   previously have checked SNI and now cannot (and an attacker can route
   a connection with an encrypted SNI to any backend server and the TLS
   connection will still complete).  However, the client is still
   responsible for verifying the server's identity in its certificate.

   [[TODO: Some more analysis needed in this case, as it is a little
   odd, and probably some precise rules about handling ESNI and no SNI
   uniformly?]]

8.  IANA Considerations

8.1.  Update of the TLS ExtensionType Registry

   IANA is requested to create an entry, encrypted_server_name(0xffce),
   in the existing registry for ExtensionType (defined in [RFC8446]),
   with "TLS 1.3" column values being set to "CH, EE", and "Recommended"
   column being set to "Yes".

8.2.  Update of the TLS Alert Registry

   IANA is requested to create an entry, esni_required(121) in the
   existing registry for Alerts (defined in [RFC8446]), with the "DTLS-
   OK" column being set to "Y".



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8.3.  Update of the Resource Record (RR) TYPEs Registry

   IANA is requested to create an entry, ESNI(0xff9f), in the existing
   registry for Resource Record (RR) TYPEs (defined in [RFC6895]) with
   "Meaning" column value being set to "Encrypted SNI".

9.  References

9.1.  Normative References

   [I-D.ietf-tls-exported-authenticator]
              Sullivan, N., "Exported Authenticators in TLS", draft-
              ietf-tls-exported-authenticator-09 (work in progress), May
              2019.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

   [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>.

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/info/rfc7838>.





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   [RFC7918]  Langley, A., Modadugu, N., and B. Moeller, "Transport
              Layer Security (TLS) False Start", RFC 7918,
              DOI 10.17487/RFC7918, August 2016,
              <https://www.rfc-editor.org/info/rfc7918>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <https://www.rfc-editor.org/info/rfc7924>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [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>.

9.2.  Informative References

   [I-D.ietf-doh-dns-over-https]
              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", draft-ietf-doh-dns-over-https-14 (work in
              progress), August 2018.

   [I-D.ietf-tls-grease]
              Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-02 (work in progress), January 2019.

   [I-D.ietf-tls-sni-encryption]
              Huitema, C. and E. Rescorla, "Issues and Requirements for
              SNI Encryption in TLS", draft-ietf-tls-sni-encryption-04
              (work in progress), November 2018.

   [I-D.kazuho-protected-sni]
              Oku, K., "TLS Extensions for Protecting SNI", draft-
              kazuho-protected-sni-00 (work in progress), July 2017.

   [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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   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/info/rfc8094>.

   [SNIExtensibilityFailed]
              "Accepting that other SNI name types will never work",
              n.d., <https://mailarchive.ietf.org/arch/msg/
              tls/1t79gzNItZd71DwwoaqcQQ_4Yxc>.

Appendix A.  Communicating SNI and Nonce to Backend Server

   When operating in Split Mode, backend servers will not have access to
   PaddedServerNameList.sni or ClientESNIInner.nonce without access to
   the ESNI keys or a way to decrypt ClientEncryptedSNI.encrypted_sni.

   One way to address this for a single connection, at the cost of
   having communication not be unmodified TLS 1.3, is as follows.
   Assume there is a shared (symmetric) key between the client-facing
   server and the backend server and use it to AEAD-encrypt Z and send
   the encrypted blob at the beginning of the connection before the
   ClientHello.  The backend server can then decrypt ESNI to recover the
   true SNI and nonce.

   Another way for backend servers to access the true SNI and nonce is
   by the client-facing server sharing the ESNI keys.

Appendix B.  Alternative SNI Protection Designs

   Alternative approaches to encrypted SNI may be implemented at the TLS
   or application layer.  In this section we describe several
   alternatives and discuss drawbacks in comparison to the design in
   this document.

B.1.  TLS-layer

B.1.1.  TLS in Early Data

   In this variant, TLS Client Hellos are tunneled within early data
   payloads belonging to outer TLS connections established with the
   client-facing server.  This requires clients to have established a
   previous session --- and obtained PSKs --- with the server.  The
   client-facing server decrypts early data payloads to uncover Client
   Hellos destined for the backend server, and forwards them onwards as
   necessary.  Afterwards, all records to and from backend servers are
   forwarded by the client-facing server - unmodified.  This avoids
   double encryption of TLS records.




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   Problems with this approach are: (1) servers may not always be able
   to distinguish inner Client Hellos from legitimate application data,
   (2) nested 0-RTT data may not function correctly, (3) 0-RTT data may
   not be supported - especially under DoS - leading to availability
   concerns, and (4) clients must bootstrap tunnels (sessions), costing
   an additional round trip and potentially revealing the SNI during the
   initial connection.  In contrast, encrypted SNI protects the SNI in a
   distinct Client Hello extension and neither abuses early data nor
   requires a bootstrapping connection.

B.1.2.  Combined Tickets

   In this variant, client-facing and backend servers coordinate to
   produce "combined tickets" that are consumable by both.  Clients
   offer combined tickets to client-facing servers.  The latter parse
   them to determine the correct backend server to which the Client
   Hello should be forwarded.  This approach is problematic due to non-
   trivial coordination between client-facing and backend servers for
   ticket construction and consumption.  Moreover, it requires a
   bootstrapping step similar to that of the previous variant.  In
   contrast, encrypted SNI requires no such coordination.

B.2.  Application-layer

B.2.1.  HTTP/2 CERTIFICATE Frames

   In this variant, clients request secondary certificates with
   CERTIFICATE_REQUEST HTTP/2 frames after TLS connection completion.
   In response, servers supply certificates via TLS exported
   authenticators [I-D.ietf-tls-exported-authenticator] in CERTIFICATE
   frames.  Clients use a generic SNI for the underlying client-facing
   server TLS connection.  Problems with this approach include: (1) one
   additional round trip before peer authentication, (2) non-trivial
   application-layer dependencies and interaction, and (3) obtaining the
   generic SNI to bootstrap the connection.  In contrast, encrypted SNI
   induces no additional round trip and operates below the application
   layer.

Appendix C.  Total Client Hello Encryption

   The design described here only provides encryption for the SNI, but
   not for other extensions, such as ALPN.  Another potential design
   would be to encrypt all of the extensions using the same basic
   structure as we use here for ESNI.  That design has the following
   advantages:

   o  It protects all the extensions from ordinary eavesdroppers




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   o  If the encrypted block has its own KeyShare, it does not
      necessarily require the client to use a single KeyShare, because
      the client's share is bound to the SNI by the AEAD (analysis
      needed).

   It also has the following disadvantages:

   o  The client-facing server can still see the other extensions.  By
      contrast we could introduce another EncryptedExtensions block that
      was encrypted to the backend server and not the client-facing
      server.

   o  It requires a mechanism for the client-facing server to provide
      the extension-encryption key to the backend server (as in
      Appendix A and thus cannot be used with an unmodified backend
      server.

   o  A conformant middlebox will strip every extension, which might
      result in a ClientHello which is just unacceptable to the server
      (more analysis needed).

Appendix D.  Acknowledgements

   This document draws extensively from ideas in
   [I-D.kazuho-protected-sni], but is a much more limited mechanism
   because it depends on the DNS for the protection of the ESNI key.
   Richard Barnes, Christian Huitema, Patrick McManus, Matthew Prince,
   Nick Sullivan, Martin Thomson, and David Benjamin also provided
   important ideas and contributions.

Authors' Addresses

   Eric Rescorla
   RTFM, Inc.

   Email: ekr@rtfm.com


   Kazuho Oku
   Fastly

   Email: kazuhooku@gmail.com


   Nick Sullivan
   Cloudflare

   Email: nick@cloudflare.com



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   Christopher A. Wood
   Apple, Inc.

   Email: cawood@apple.com















































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