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Versions: (draft-rescorla-tls-esni) 00 01 02 03 04 05 06

tls                                                          E. Rescorla
Internet-Draft                                                RTFM, Inc.
Intended status: Experimental                                     K. Oku
Expires: 10 September 2020                                        Fastly
                                                             N. Sullivan
                                                              Cloudflare
                                                               C.A. Wood
                                                             Apple, Inc.
                                                            9 March 2020


              Encrypted Server Name Indication for TLS 1.3
                         draft-ietf-tls-esni-06

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 10 September 2020.

Copyright Notice

   Copyright (c) 2020 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




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   as described in Section 4.e of 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.  ClientHello Encryption  . . . . . . . . . . . . . . . . .   5
   4.  Encrypted ClientHello Configuration . . . . . . . . . . . . .   6
   5.  The "encrypted_client_hello" extension  . . . . . . . . . . .   7
   6.  The "echo_nonce" extension  . . . . . . . . . . . . . . . . .   8
     6.1.  Incorporating Outer Extensions  . . . . . . . . . . . . .   9
   7.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Sending an encrypted ClientHello  . . . . . . . . . . . .  10
     7.2.  Handling the server response  . . . . . . . . . . . . . .  11
       7.2.1.  Accepted ECHO . . . . . . . . . . . . . . . . . . . .  12
       7.2.2.  Rejected ECHO . . . . . . . . . . . . . . . . . . . .  12
       7.2.3.  HelloRetryRequest . . . . . . . . . . . . . . . . . .  14
     7.3.  GREASE extensions . . . . . . . . . . . . . . . . . . . .  15
   8.  Client-Facing Server Behavior . . . . . . . . . . . . . . . .  15
   9.  Compatibility Issues  . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Misconfiguration and Deployment Concerns  . . . . . . . .  17
     9.2.  Middleboxes . . . . . . . . . . . . . . . . . . . . . . .  18
   10. TLS and HPKE CipherSuite Mapping  . . . . . . . . . . . . . .  18
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  19
     11.1.  Why is cleartext DNS OK? . . . . . . . . . . . . . . . .  19
     11.2.  Optional Record Digests and Trial Decryption . . . . . .  19
     11.3.  Related Privacy Leaks  . . . . . . . . . . . . . . . . .  20
     11.4.  Comparison Against Criteria  . . . . . . . . . . . . . .  20
       11.4.1.  Mitigate against replay attacks  . . . . . . . . . .  20
       11.4.2.  Avoid widely-deployed shared secrets . . . . . . . .  20
       11.4.3.  Prevent SNI-based DoS attacks  . . . . . . . . . . .  21
       11.4.4.  Do not stick out . . . . . . . . . . . . . . . . . .  21
       11.4.5.  Forward secrecy  . . . . . . . . . . . . . . . . . .  21
       11.4.6.  Proper security context  . . . . . . . . . . . . . .  21
       11.4.7.  Split server spoofing  . . . . . . . . . . . . . . .  21
       11.4.8.  Supporting multiple protocols  . . . . . . . . . . .  22
     11.5.  Misrouting . . . . . . . . . . . . . . . . . . . . . . .  22
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     12.1.  Update of the TLS ExtensionType Registry . . . . . . . .  22
     12.2.  Update of the TLS Alert Registry . . . . . . . . . . . .  22
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     13.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Appendix A.  Alternative SNI Protection Designs . . . . . . . . .  25
     A.1.  TLS-layer . . . . . . . . . . . . . . . . . . . . . . . .  25



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       A.1.1.  TLS in Early Data . . . . . . . . . . . . . . . . . .  25
       A.1.2.  Combined Tickets  . . . . . . . . . . . . . . . . . .  25
     A.2.  Application-layer . . . . . . . . . . . . . . . . . . . .  26
       A.2.1.  HTTP/2 CERTIFICATE Frames . . . . . . . . . . . . . .  26
   Appendix B.  Total Client Hello Encryption  . . . . . . . . . . .  26
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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:

   *  Cleartext client DNS queries.

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

   *  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
   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



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   (CDN, app server, etc.) which is able to activate encrypted SNI, by
   encrypting the entire ClientHello (ECHO), for all of the domains it
   hosts.  As a result, the use of ECHO to protect the 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, and the provider's server 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.

3.2.  ClientHello Encryption

   ECHO works by encrypting the entire ClientHello, including the SNI
   and any additional extensions such as ALPN.  This requires that each
   provider publish a public key and metadata which is used for
   ClientHello encryption for all the domains for which it serves
   directly or indirectly (via Split Mode).  This document defines the
   format of the SNI encryption public key and metadata, referred to as
   an ECHO configuration, and delegates DNS publication details to
   [HTTPSSVC], though other delivery mechanisms are 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 ECHO-supporting provider, it constructs a ClientHello in
   the regular fashion containing the true SNI value (ClientHelloInner)
   and then encrypts it using the public key for the provider.  It then
   constructs a new ClientHello (ClientHelloOuter) with an innocuous SNI
   (and potentially innocuous versions of other extensions such as ALPN
   [RFC7301]) and containing the encrypted ClientHelloInner as an
   extension.  It sends ClientHelloOuter to the server.

   Upon receiving ClientHelloOuter, the server can then decrypt
   ClientHelloInner and either terminate the connection (in Shared Mode)
   or forward it to the backend server (in Split Mode).

   Note that both ClientHelloInner and ClientHelloOuter are both valid,
   complete ClientHello messages.  ClientHelloOuter carries an encrypted
   representation of ClientHelloInner in a "encrypted_client_hello"
   extension, defined in Section 5.



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4.  Encrypted ClientHello Configuration

   ClientHello encryption configuration information is conveyed with the
   following ECHOConfigs structure.

       opaque HpkePublicKey<1..2^16-1>;
       uint16 HkpeKemId; // Defined in I-D.irtf-cfrg-hpke

       struct {
           opaque public_name<1..2^16-1>;

           HpkePublicKey public_key;
           HkpeKemId kem_id;
           CipherSuite cipher_suites<2..2^16-2>;

           uint16 maximum_name_length;
           Extension extensions<0..2^16-1>;
       } ECHOConfigContents;

       struct {
           uint16 version;
           uint16 length;
           select (ECHOConfig.version) {
             case 0xff03: ECHOConfigContents;
           }
       } ECHOConfig;

       ECHOConfig ECHOConfigs<1..2^16-1>;

   The ECHOConfigs structure contains one or more ECHOConfig structures
   in decreasing order of preference.  This allows a server to support
   multiple versions of ECHO and multiple sets of ECHO parameters.

   The ECHOConfig structure contains the following fields:

   version  The version of the structure.  For this specification, that
      value SHALL be 0xff03.  Clients MUST ignore any ECHOConfig
      structure with a version they do not understand.

   contents  An opaque byte string whose contents depend on the version
      of the structure.  For this specification, the contents are an
      ECHOConfigContents structure.

   The ECHOConfigContents structure contains the following fields:

   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 7.2.



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   public_key  The HPKE [I-D.irtf-cfrg-hpke] public key which can be
      used by the client to encrypt the ClientHello.

   kem_id  The HPKE [I-D.irtf-cfrg-hpke] KEM identifier corresponding to
      public_key.  Clients MUST ignore any ECHOConfig structure with a
      key using a KEM they do not support.

   cipher_suites  The list of TLS cipher suites which can be used by the
      client to encrypt the ClientHello.  See Section 10 for information
      on how a cipher suite maps to corresponding HPKE algorithm
      identifiers.

   maximum_name_length  the largest name the server expects to support.
      If the server supports arbitrary wildcard names, it SHOULD set
      this value to 256.  Clients SHOULD reject ESNIConfig as invalid if
      maximum_name_length is greater than 256.

   extensions  A list of extensions that the client can take into
      consideration when generating a Client Hello message.  The purpose
      of the field is to provide room for additional features in the
      future.  The format is defined in [RFC8446]; Section 4.2.  The
      same interpretation rules apply: extensions MAY appear in any
      order, but there MUST NOT be more than one extension of the same
      type in the extensions block.  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 ECHOConfig content.

   Clients MUST parse the extension list and check for unsupported
   mandatory extensions.  If an unsupported mandatory extension is
   present, clients MUST reject the ECHOConfig value.

5.  The "encrypted_client_hello" extension

   The encrypted ClientHelloInner is carried in an
   "encrypted_client_hello" extension, defined as follows:

      enum {
          encrypted_client_hello(TBD), (65535)
      } ExtensionType;

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








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      struct {
          CipherSuite suite;
          opaque record_digest<0..2^16-1>;
          opaque enc<1..2^16-1>;
          opaque encrypted_ch<1..2^16-1>;
      } ClientEncryptedCH;

   suite  The cipher suite used to encrypt ClientHelloInner.

   record_digest  A cryptographic hash of the ECHOConfig structure from
      which the ECHO 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".

   enc  The HPKE encapsulated key, used by servers to decrypt the
      corresponding encrypted_ch field.

   encrypted_ch  The serialized and encrypted ClientHelloInner
      structure, AEAD-encrypted using cipher suite "suite" and the key
      generated as described below.

   If the server accepts ECHO, it does not send this extension.  If it
   rejects ECHO, then it sends the following structure in
   EncryptedExtensions:

      struct {
          ECHOConfigs retry_configs;
      } ServerEncryptedCH;

   retry_configs  An ESNIConfigs structure containing one or more
      ECHOConfig structures in decreasing order of preference that the
      client should use on subsequent connections to encrypt the
      ClientHelloInner structure.

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

6.  The "echo_nonce" extension

   When using ECHO, the client MUST also add an extension of type
   "echo_nonce" to the ClientHelloInner (but not to the outer
   ClientHello).  This nonce ensures that the server's encrypted
   Certificate can only be read by the entity which sent this
   ClientHello.  [[TODO: Describe HRR cut-and-paste 1 in Security
   Considerations.]] This extension is defined as follows:





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

      struct {
          uint8 nonce[16];
      } ECHONonce;

   nonce  A 16-byte nonce exported from the HPKE encryption context.
      See Section 7.1 for details about its computation.

   Finally, requirements in Section 7 and Section 8 require
   implementations to track, alongside each PSK established by a
   previous connection, whether the connection negotiated this extension
   with the "echo_accept" response type.  If so, this is referred to as
   an "ECHO PSK".  Otherwise, it is a "non-ECHO PSK".  This may be
   implemented by adding a new field to client and server session
   states.

6.1.  Incorporating Outer Extensions

   Some TLS 1.3 extensions can be quite large and having them both in
   the inner and outer ClientHello will lead to a very large overall
   size.  One particularly pathological example is "key_share" with
   post-quantum algorithms.  In order to reduce the impact of duplicated
   extensions, the client may use the "outer_extensions" extension.

      enum {
          outer_extension(TBD), (65535)
      } ExtensionType;

      struct {
          ExtensionType outer_extensions<2..254>;
          uint8 hash<32..255>;
      } OuterExtensions;

   OuterExtensions MUST only be used in ClientHelloInner.  It consists
   of one or more ExtensionType values, each of which reference an
   extension in ClientHelloOuter, and a digest of the complete
   ClientHelloInner.

   When sending ClientHello, the client first computes ClientHelloInner,
   including any PSK binders, and then MAY substitute extensions which
   it knows will be duplicated in ClientHelloOuter.  To do so, the
   client computes a hash H of the entire ClientHelloInner message with
   the same hash as for the KDF used to encrypt ClienHelloInner.  Then,
   the client removes and and replaces extensions from ClientHelloInner
   with a single "outer_extensions" extension.  The list of



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   outer_extensions include those which were removed from
   ClientHelloInner, in the order in which they were removed.  The hash
   contains the full ClientHelloInner hash H computed above.

   This process is reversed by client-facing servers upon receipt.
   Specifically, the server replaces the "outer_extensions" with
   extensions contained in ClientHelloOuter.  The server then computes a
   hash H' of the reconstructed ClientHelloInner.  If H' does not equal
   OuterExtensions.hash, the server aborts the connection with an
   "illegal_parameter" alert.

   Clients SHOULD only use this mechanism for extensions which are
   large.  All other extensions SHOULD appear in both ClientHelloInner
   and ClientHelloOuter even if they have identical values.

7.  Client Behavior

7.1.  Sending an encrypted ClientHello

   In order to send an encrypted ClientHello, the client first
   determines if it supports the server's chosen KEM, as identified by
   ECHOConfig.kem_id.  If one is supported, the client MUST select an
   appropriate cipher suite from the list of suites offered by the
   server.  If the client does not support the corresponding KEM or is
   unable to select an appropriate group or suite, it SHOULD ignore that
   ECHOConfig value and MAY attempt to use another value provided by the
   server.  The client MUST NOT send ECHO using HPKE algorithms not
   advertised by the server.

   Given a compatible ECHOConfig with fields public_key and kem_id,
   carrying the HpkePublicKey and KEM identifier corresponding to the
   server, clients compute an HPKE encryption context as follows:

   pkR = HPKE.KEM.Unmarshal(ECHOConfig.public_key)
   enc, context = SetupBaseS(pkR, "tls13-echo")
   echo_nonce = context.Export("tls13-echo-nonce", 16)
   echo_hrr_key = context.Export("tls13-echo-hrr-key", 16)

   Note that the HPKE algorithm identifiers are those which match the
   client's chosen CipherSuite, according to Section 10.  The client MAY
   replace any large, duplicated extensions in ClientHelloInner with the
   corresponding "outer_extensions" extension, as described in
   Section 6.1.

   The client then generates a ClientHelloInner value.  In addition to
   the normal values, ClientHelloInner MUST also contain:

   *  an "echo_nonce" extension



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   *  TLS padding [RFC7685]

   Padding SHOULD be P = L - D bytes, where

   *  L = ECHOConfig.maximum_name_length, rounded up to the nearest
      multiple of 16

   *  D = len(dns_name), where dns_name is the DNS name in the
      ClientHelloInner "server_name" extension

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

   The encrypted ClientHello value is then computed as:

       encrypted_ch = context.Seal("", ClientHelloInner)

   Finally, the client MUST generate a ClientHelloOuter message
   containing the "encrypted_client_hello" extension with the values as
   indicated above.  In particular,

   *  suite contains the client's chosen ciphersuite;

   *  record_digest contains the digest of the corresponding ECHOConfig
      structure;

   *  enc contains the encapsulated key as output by SetupBaseS; and

   *  encrypted_ch contains the HPKE encapsulated key (enc) and the
      ClientHelloInner ciphertext (encrypted_ch_inner).

   The client MUST place the value of ECHOConfig.public_name in the
   ClientHelloOuter "server_name" extension.  The remaining contents of
   the ClientHelloOuter MAY be identical to those in ClientHelloInner
   but MAY also differ.  The ClientHelloOuter MUST NOT contain a
   "cached_info" extension [RFC7924] with a CachedObject entry whose
   CachedInformationType is "cert", since this indication would divulge
   the true server name.

7.2.  Handling the server response

   As described in Section 8, the server MAY either accept ECHO and use
   ClientHelloInner or reject it and use ClientHelloOuter.  However,
   there is no indication in ServerHello of which one the server has
   done and the client must therefore use trial decryption in order to
   determine this.




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7.2.1.  Accepted ECHO

   If the server used ClientHelloInner, the client proceeds with the
   connection as usual, authenticating the connection for the origin
   server.

7.2.2.  Rejected ECHO

   If the server used ClientHelloOuter, the client proceeds with the
   handshake, authenticating for ECHOConfig.public_name as described in
   Section 7.2.2.1.  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
   "echo_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 ECHO keys as securely replaced by the server.  It
   SHOULD retry the handshake with a new transport connection, using
   that value to encrypt the ClientHello.  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 ECHO as securely disabled by the
   server.  As below, it SHOULD then retry the handshake with a new
   transport connection and ECHO disabled.

   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
   ECHOConfigContents.public_name as described in Section 7.2.2.1.  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 ECHO.

   Otherwise, when the handshake completes successfully with the public
   name authenticated, the client MUST abort the connection with an



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   "echo_required" alert.  The client can then regard ECHO as securely
   disabled by the server.  It SHOULD retry the handshake with a new
   transport connection and ECHO disabled.

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

   Clients SHOULD implement a limit on retries caused by
   "echo_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.

7.2.2.1.  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 8).
   Clients that offer ECHO then authenticate the connection with the
   public name, as follows:

   *  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 7.1 requires the client to only offer
      ECHO-established sessions, and Section 8 requires the server to
      decline ECHO-established sessions if it did not accept ECHO.

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

   *  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 7.2.  This may be implemented, for
   instance, by reporting a failed connection with a dedicated error
   code.






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7.2.3.  HelloRetryRequest

   If the server sends a HelloRetryRequest in response to the
   ClientHello, the client sends a second updated ClientHello per the
   rules in [RFC8446].  However, at this point, the client does not know
   whether the server processed ClientHelloOuter or ClientHelloInner,
   and MUST regenerate both values to be acceptable.  Note: if the inner
   and outer ClientHellos use different groups for their key shares or
   differ in some other way, then the HelloRetryRequest may actually be
   invalid for one or the other ClientHello, in which case a fresh
   ClientHello MUST be generated, ignoring the instructions in
   HelloRetryRequest.  Otherwise, the usual rules for HelloRetryRequest
   processing apply.

   Clients bind encryption of the second ClientHelloInner to encryption
   of the first ClientHelloInner via the derived echo_hrr_key by
   modifying HPKE setup as follows:

   pkR = HPKE.KEM.Unmarshal(ECHOConfig.public_key)
   enc, context = SetupPSKS(pkR, "tls13-echo-hrr", echo_hrr_key, "")
   echo_nonce = context.Export("tls13-echo-hrr-nonce", 16)

   Clients then encrypt the second ClientHelloInner using this new HPKE
   context.  In doing so, the encrypted value is also authenticated by
   echo_hrr_key.

   Client-facing servers perform the corresponding process when
   decrypting second ClientHelloInner messages.  In particular, upon
   receipt of a second ClientHello message with a ClientEncryptedCH
   value, servers setup their HPKE context and decrypt ClientEncryptedCH
   as follows:

   context = SetupPSKR(ClientEncryptedCH.enc, skR, "tls13-echo-hrr", echo_hrr_key, "")
   ClientHelloInner = context.Open("", ClientEncryptedCH.encrypted_ch)
   echo_nonce = context.Export("tls13-echo-hrr-nonce", 16)

   [[OPEN ISSUE: Should we be using the PSK input or the info input?  On
   the one hand, the requirements on info seem weaker, but maybe
   actually this needs to be secret?  Analysis needed.]]

   [[OPEN ISSUE: This, along with trial decryption is pretty gross.  It
   would just be a lot easier if we were willing to have the server
   indicate whether ECHO had been accepted or not.  Given that the
   server is supposed to only reject ECHO when it doesn't know the key,
   and this is easy to probe for, can we just instead have an extension
   to indicate what has happened.]]





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7.3.  GREASE extensions

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

   *  Select a supported cipher suite, named group, and padded_length
      value.  The padded_length value SHOULD be 260 (sum of the maximum
      DNS name length and TLS encoding overhead) or a multiple of 16
      less than 260.  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.

   *  Set the "enc" field to a randomly-generated valid encapsulated
      public key output by the HPKE KEM.

   *  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.

   *  Set the "encrypted_client_hello" field to a randomly-generated
      string of [TODO] bytes.

   If the server sends an "encrypted_client_hello" extension, the client
   MUST check the extension syntactically and abort the connection with
   a "decode_error" alert if it is invalid.

   Offering a GREASE extension is not considered offering an encrypted
   ClientHello for purposes of requirements in Section 7.  In
   particular, the client MAY offer to resume sessions established
   without ECHO.

8.  Client-Facing Server Behavior

   Upon receiving an "encrypted_client_hello" 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 ClientEncryptedCH value is said to match a known ECHOConfig if
   there exists an ECHOConfig that can be used to successfully decrypt
   ClientEncryptedCH.encrypted_ch.  This matching procedure should be
   done using one of the following two checks:

   1.  Compare ClientEncryptedCH.record_digest against cryptographic
       hashes of known ECHOConfig and choose the one that matches.



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   2.  Use trial decryption of ClientEncryptedCH.encrypted_ch with known
       ECHOConfig and choose the one that succeeds.

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

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

   *  It MUST include the "encrypted_client_hello" extension with the
      "retry_keys" field set to one or more ECHOConfig structures with
      up-to-date keys.  Servers MAY supply multiple ECHOConfig values of
      different versions.  This allows a server to support multiple
      versions at once.

   *  The server MUST ignore all PSK identities in the ClientHello which
      correspond to ECHO PSKs.  ECHO PSKs offered by the client are
      associated with the ECHO name.  The server was unable to decrypt
      then ECHO name, so it should not resume them when using the
      cleartext SNI name.  This restriction allows a client to reject
      resumptions in Section 7.2.2.1.

   Note that an unrecognized ClientEncryptedCH.record_digest value may
   be a GREASE ECHO extension (see Section 7.3), so it is necessary for
   servers to proceed with the connection and rely on the client to
   abort if ECHO was required.  In particular, the unrecognized value
   alone does not indicate a misconfigured ECHO advertisement
   (Section 9.1).  Instead, servers can measure occurrences of the
   "echo_required" alert to detect this case.

   If the ClientEncryptedCH value does match a known ECHOConfig, the
   server then decrypts ClientEncryptedCH.encrypted_ch, using the
   private key skR corresponding to ESNIConfig, as follows:

   context = SetupBaseR(ClientEncryptedCH.enc, skR, "tls13-echo")
   ClientHelloInner = context.Open("", ClientEncryptedCH.encrypted_ch)
   echo_nonce = context.Export("tls13-echo-nonce", 16)
   echo_hrr_key = context.Export("tls13-echo-hrr-key", 16)

   If decryption fails, the server MUST abort the connection with a
   "decrypt_error" alert.  Moreover, if there is no "echo_nonce"
   extension, or if its value does not match the derived echo_nonce, the
   server MUST abort the connection with a "decrypt_error" alert.  Next,



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   the server MUST scan ClientHelloInner for any "outer_extension"
   extensions and substitute their values with the values in
   ClientHelloOuter.  It MUST first verify that the hash found in the
   extension matches the hash of the extension to be interpolated in and
   if it does not, abort the connections with a "decrypt_error" alert.

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

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

9.  Compatibility Issues

   Unlike most TLS extensions, placing the SNI value in an ECHO
   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 ECHO 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 ECHO keys and
   servers, at the cost of extra round-trips due to a retry.  Two
   specific scenarios are detailed below.

9.1.  Misconfiguration and Deployment Concerns

   It is possible for ECHO 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
   ECHO keys, or if a deployment of ECHO 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 echo_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



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   name, the client can safely retry with updated settings, as described
   in Section 7.2.

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

   Client-facing servers with non-uniform cryptographic configurations
   across backend origin servers segment the ECHO anonymity set based on
   these configurations.  For example, if a client-facing server hosts k
   backend origin servers, and exactly one of those backend origin
   servers supports a different set of cryptographic algorithms than the
   other (k - 1) servers, it may be possible to identify this single
   server based on the contents of the ServerHello as this message is
   not encrypted.

9.2.  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.

   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 7.2 or result in a connection
   failure.  A proxy which is not authoritative for the public name
   cannot forge a signal to disable ECHO.

   A non-conformant MITM proxy which instead forwards the ECHO
   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.

10.  TLS and HPKE CipherSuite Mapping

   Per {{RFC8446}, TLS ciphersuites define an AEAD and hash algorithm.
   In contrast, HPKE defines separate AEAD algorithms and key derivation
   functions.  The table below lists the mapping between ciphersuites
   and HPKE identifiers.  TLS_AES_128_CCM_SHA256 and
   TLS_AES_128_CCM_8_SHA256 are not supported ECHO ciphersuites as they
   have no HPKE equivalent.




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     +------------------------------+------------------+-------------+
     | TLS CipherSuite              | HPKE AEAD        | HPKE KDF    |
     +==============================+==================+=============+
     | TLS_AES_128_GCM_SHA256       | AES-GCM-128      | HKDF-SHA256 |
     +------------------------------+------------------+-------------+
     | TLS_AES_256_GCM_SHA384       | AES-GCM-256      | HKDF-SHA256 |
     +------------------------------+------------------+-------------+
     | TLS_CHACHA20_POLY1305_SHA256 | ChaCha20Poly1305 | HKDF-SHA256 |
     +------------------------------+------------------+-------------+

                                  Table 1

11.  Security Considerations

11.1.  Why is cleartext DNS OK?

   In comparison to [I-D.kazuho-protected-sni], wherein DNS Resource
   Records are signed via a server private key, ECHO 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 ECHO records (so that the client encrypts data to them) or strip
   the ECHO record from the response.  However, in the face of an
   attacker that controls DNS, no 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 ECHO 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 ClientHello and SNI encryption are desired.  Moreover, as noted
   in the introduction, SNI encryption is less useful without encryption
   of DNS queries in transit via DoH or DPRIVE mechanisms.

11.2.  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 ECHO 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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11.3.  Related Privacy Leaks

   ECHO 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.

11.4.  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 ECHO design against them.

11.4.1.  Mitigate against replay attacks

   Since servers process either ClientHelloInner or ClientHelloOuter,
   and ClientHelloInner contains an HPKE-derived nonce, it is not
   possible for an attacker to "cut and paste" the ECHO value in a
   different Client Hello and learn information from ClientHelloInner.
   This is because the attacker lacks access to the HPKE-derived nonce
   used to derive the handshake secrets.

11.4.2.  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 ECHO
   key is provided, sharing is optimally bound by the number of hosts
   that share an IP address.  Server operators may further limit sharing
   by publishing different DNS records containing ECHOConfig values with
   different keys using a short TTL.




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11.4.3.  Prevent SNI-based DoS attacks

   This design requires servers to decrypt ClientHello messages with
   ClientEncryptedCH 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.

11.4.4.  Do not stick out

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

   Additionally, this specification allows for clients to send GREASE
   ECHO extensions (see Section 7.3), which helps ensure the ecosystem
   handles the values correctly.

11.4.5.  Forward secrecy

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

11.4.6.  Proper security context

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

11.4.7.  Split server spoofing

   Assuming ECHO records retrieved from DNS are authenticated, e.g., via
   DNSSEC or fetched from a trusted Recursive Resolver, spoofing a
   server operating in Split Mode is not possible.  See Section 11.1 for
   more details regarding cleartext DNS.

   Authenticating the ECHOConfigs structure naturally authenticates the
   included public name.  This also authenticates any retry signals from
   the server because the client validates the server certificate
   against the public name before retrying.







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11.4.8.  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.

11.5.  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 ECHO and no SNI
   uniformly?]]

12.  IANA Considerations

12.1.  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".

12.2.  Update of the TLS Alert Registry

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

13.  References

13.1.  Normative References

   [HTTPSSVC] Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPSSVC)", Work in Progress, Internet-Draft, draft-
              nygren-dnsop-svcb-httpssvc-00, 23 September 2019,
              <http://www.ietf.org/internet-drafts/draft-nygren-dnsop-
              svcb-httpssvc-00.txt>.




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   [I-D.ietf-tls-exported-authenticator]
              Sullivan, N., "Exported Authenticators in TLS", Work in
              Progress, Internet-Draft, draft-ietf-tls-exported-
              authenticator-11, 18 December 2019, <http://www.ietf.org/
              internet-drafts/draft-ietf-tls-exported-authenticator-
              11.txt>.

   [I-D.irtf-cfrg-hpke]
              Barnes, R. and K. Bhargavan, "Hybrid Public Key
              Encryption", Work in Progress, Internet-Draft, draft-irtf-
              cfrg-hpke-02, 4 November 2019, <http://www.ietf.org/
              internet-drafts/draft-irtf-cfrg-hpke-02.txt>.

   [RFC1035]  Mockapetris, P.V., "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>.

   [RFC7685]  Langley, A., "A Transport Layer Security (TLS) ClientHello
              Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
              October 2015, <https://www.rfc-editor.org/info/rfc7685>.

   [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




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              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

13.2.  Informative References

   [I-D.ietf-doh-dns-over-https]
              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", Work in Progress, Internet-Draft, draft-ietf-doh-
              dns-over-https-14, 16 August 2018, <http://www.ietf.org/
              internet-drafts/draft-ietf-doh-dns-over-https-14.txt>.

   [I-D.ietf-tls-grease]
              Benjamin, D., "Applying GREASE to TLS Extensibility", Work
              in Progress, Internet-Draft, draft-ietf-tls-grease-04, 22
              August 2019, <http://www.ietf.org/internet-drafts/draft-
              ietf-tls-grease-04.txt>.

   [I-D.ietf-tls-sni-encryption]
              Huitema, C. and E. Rescorla, "Issues and Requirements for
              SNI Encryption in TLS", Work in Progress, Internet-Draft,
              draft-ietf-tls-sni-encryption-09, 28 October 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-tls-sni-
              encryption-09.txt>.

   [I-D.kazuho-protected-sni]
              Oku, K., "TLS Extensions for Protecting SNI", Work in
              Progress, Internet-Draft, draft-kazuho-protected-sni-00,
              18 July 2017, <http://www.ietf.org/internet-drafts/draft-
              kazuho-protected-sni-00.txt>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [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>.

   [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>.



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   [SNIExtensibilityFailed]
              "Accepting that other SNI name types will never work",
              March 2016, <https://mailarchive.ietf.org/arch/msg/
              tls/1t79gzNItZd71DwwoaqcQQ_4Yxc>.

Appendix A.  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.

A.1.  TLS-layer

A.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.

   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.

A.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.




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A.2.  Application-layer

A.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 B.  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 ECHO.  That design has the following
   advantages:

   *  It protects all the extensions from ordinary eavesdroppers

   *  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:

   *  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.

   *  It requires a mechanism for the client-facing server to provide
      the extension-encryption key to the backend server and thus cannot
      be used with an unmodified backend server.

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






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Appendix C.  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 ECHO 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


   Christopher A. Wood
   Apple, Inc.

   Email: cawood@apple.com


















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