TLS Encrypted Client Hello
draft-ietf-tls-esni-12

The information below is for an old version of the document
Document Type Active Internet-Draft (tls WG)
Authors Eric Rescorla  , Kazuho Oku  , Nick Sullivan  , Christopher Wood 
Last updated 2021-07-07
Replaces draft-rescorla-tls-esni
Stream Internet Engineering Task Force (IETF)
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tls                                                          E. Rescorla
Internet-Draft                                                RTFM, Inc.
Intended status: Standards Track                                  K. Oku
Expires: 8 January 2022                                           Fastly
                                                             N. Sullivan
                                                               C.A. Wood
                                                              Cloudflare
                                                             7 July 2021

                       TLS Encrypted Client Hello
                         draft-ietf-tls-esni-12

Abstract

   This document describes a mechanism in Transport Layer Security (TLS)
   for encrypting a ClientHello message under a server public key.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/tlswg/draft-ietf-tls-esni
   (https://github.com/tlswg/draft-ietf-tls-esni).

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 8 January 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 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  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Topologies  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Encrypted ClientHello (ECH) . . . . . . . . . . . . . . .   6
   4.  Encrypted ClientHello Configuration . . . . . . . . . . . . .   7
     4.1.  Configuration Extensions  . . . . . . . . . . . . . . . .   9
   5.  The "encrypted_client_hello" Extension  . . . . . . . . . . .  10
     5.1.  Encoding the ClientHelloInner . . . . . . . . . . . . . .  11
     5.2.  Authenticating the ClientHelloOuter . . . . . . . . . . .  13
   6.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Offering ECH  . . . . . . . . . . . . . . . . . . . . . .  14
       6.1.1.  Encrypting the ClientHello  . . . . . . . . . . . . .  16
       6.1.2.  GREASE PSK  . . . . . . . . . . . . . . . . . . . . .  17
       6.1.3.  Recommended Padding Scheme  . . . . . . . . . . . . .  17
       6.1.4.  Handling the Server Response  . . . . . . . . . . . .  18
       6.1.5.  Handling HelloRetryRequest  . . . . . . . . . . . . .  21
     6.2.  GREASE ECH  . . . . . . . . . . . . . . . . . . . . . . .  21
   7.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  Client-Facing Server  . . . . . . . . . . . . . . . . . .  22
       7.1.1.  Sending HelloRetryRequest . . . . . . . . . . . . . .  24
     7.2.  Backend Server  . . . . . . . . . . . . . . . . . . . . .  25
       7.2.1.  Sending HelloRetryRequest . . . . . . . . . . . . . .  26
   8.  Compatibility Issues  . . . . . . . . . . . . . . . . . . . .  26
     8.1.  Misconfiguration and Deployment Concerns  . . . . . . . .  27
     8.2.  Middleboxes . . . . . . . . . . . . . . . . . . . . . . .  27
   9.  Compliance Requirements . . . . . . . . . . . . . . . . . . .  28
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  28
     10.1.  Security and Privacy Goals . . . . . . . . . . . . . . .  28
     10.2.  Unauthenticated and Plaintext DNS  . . . . . . . . . . .  29
     10.3.  Client Tracking  . . . . . . . . . . . . . . . . . . . .  30
     10.4.  Optional Configuration Identifiers and Trial
             Decryption  . . . . . . . . . . . . . . . . . . . . . .  30
     10.5.  Outer ClientHello  . . . . . . . . . . . . . . . . . . .  30
     10.6.  Related Privacy Leaks  . . . . . . . . . . . . . . . . .  31
     10.7.  Cookies  . . . . . . . . . . . . . . . . . . . . . . . .  32
     10.8.  Attacks Exploiting Acceptance Confirmation . . . . . . .  32

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     10.9.  Comparison Against Criteria  . . . . . . . . . . . . . .  33
       10.9.1.  Mitigate Cut-and-Paste Attacks . . . . . . . . . . .  33
       10.9.2.  Avoid Widely Shared Secrets  . . . . . . . . . . . .  33
       10.9.3.  Prevent SNI-Based Denial-of-Service Attacks  . . . .  33
       10.9.4.  Do Not Stick Out . . . . . . . . . . . . . . . . . .  34
       10.9.5.  Maintain Forward Secrecy . . . . . . . . . . . . . .  35
       10.9.6.  Enable Multi-party Security Contexts . . . . . . . .  35
       10.9.7.  Support Multiple Protocols . . . . . . . . . . . . .  35
     10.10. Padding Policy . . . . . . . . . . . . . . . . . . . . .  35
     10.11. Active Attack Mitigations  . . . . . . . . . . . . . . .  36
       10.11.1.  Client Reaction Attack Mitigation . . . . . . . . .  36
       10.11.2.  HelloRetryRequest Hijack Mitigation . . . . . . . .  37
       10.11.3.  ClientHello Malleability Mitigation . . . . . . . .  38
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
     11.1.  Update of the TLS ExtensionType Registry . . . . . . . .  39
     11.2.  Update of the TLS Alert Registry . . . . . . . . . . . .  40
   12. ECHConfig Extension Guidance  . . . . . . . . . . . . . . . .  40
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  40
     13.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Appendix A.  Alternative SNI Protection Designs . . . . . . . . .  42
     A.1.  TLS-layer . . . . . . . . . . . . . . . . . . . . . . . .  42
       A.1.1.  TLS in Early Data . . . . . . . . . . . . . . . . . .  42
       A.1.2.  Combined Tickets  . . . . . . . . . . . . . . . . . .  43
     A.2.  Application-layer . . . . . . . . . . . . . . . . . . . .  43
       A.2.1.  HTTP/2 CERTIFICATE Frames . . . . . . . . . . . . . .  43
   Appendix B.  Linear-time Outer Extension Processing . . . . . . .  43
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  44
   Appendix D.  Change Log . . . . . . . . . . . . . . . . . . . . .  44
     D.1.  Since draft-ietf-tls-esni-11  . . . . . . . . . . . . . .  44
     D.2.  Since draft-ietf-tls-esni-10  . . . . . . . . . . . . . .  44
     D.3.  Since draft-ietf-tls-esni-09  . . . . . . . . . . . . . .  45
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  45

1.  Introduction

   DISCLAIMER: This draft is work-in-progress 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 ways in which an on-path
   attacker can learn private information about the connection.  The
   plaintext Server Name Indication (SNI) extension in ClientHello
   messages, which leaks the target domain for a given connection, is
   perhaps the most sensitive, unencrypted information in TLS 1.3.

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   The target domain may also be visible through other channels, such as
   plaintext client DNS queries, visible server IP addresses (assuming
   the server does not use domain-based virtual hosting), or other
   indirect mechanisms such as traffic analysis.  DoH [RFC8484] 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, the
   SNI remains the primary explicit signal used to determine the
   server's identity.

   The TLS Working Group has studied the problem of protecting the SNI,
   but has been unable to develop a completely generic solution.
   [RFC8744] provides a description of the problem space and some of the
   proposed techniques.  One of the more difficult problems is "Do not
   stick out" ([RFC8744], Section 3.4): if only sensitive or 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 the SNI is being protected.
   Unfortunately, the result often has undesirable performance
   consequences, incomplete coverage, or both.

   The protocol specified by this document takes a different approach.
   It assumes that private origins will co-locate with or hide behind a
   provider (reverse proxy, application server, etc.) that protects
   sensitive ClientHello parameters, including the SNI, for all of the
   domains it hosts.  These co-located servers form an anonymity set
   wherein all elements have a consistent configuration, e.g., the set
   of supported application protocols, ciphersuites, TLS versions, and
   so on.  Usage of this mechanism reveals that a client is connecting
   to a particular service provider, but does not reveal which server
   from the anonymity set terminates the connection.  Thus, it leaks no
   more than what is already visible from the server IP address.

   This document specifies a new TLS extension, called Encrypted Client
   Hello (ECH), that allows clients to encrypt their ClientHello to a
   supporting server.  This protects the SNI and other potentially
   sensitive fields, such as the ALPN list [RFC7301].  This extension is
   only supported with (D)TLS 1.3 [RFC8446] and newer versions of the
   protocol.

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.

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3.  Overview

   This protocol is designed to operate in one of two topologies
   illustrated 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.  In this mode, the TLS connection is
   terminated by the provider.

              +--------------------+     +---------------------+
              |                    |     |                     |
              |   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
   origin server, who terminates the TLS connection with the client.
   Importantly, service provider does not have access to the plaintext
   of the connection.

   In the remainder of this document, we will refer to the ECH-service
   provider as the "client-facing server" and to the TLS terminator as
   the "backend server".  These are the same entity in Shared Mode, but
   in Split Mode, the client-facing and backend servers are physically
   separated.

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3.2.  Encrypted ClientHello (ECH)

   ECH allows the client to encrypt sensitive ClientHello extensions,
   e.g., SNI, ALPN, etc., under the public key of the client-facing
   server.  This requires the client-facing server to publish the public
   key and metadata it uses for ECH for all the domains for which it
   serves directly or indirectly (via Split Mode).  This document
   defines the format of the ECH encryption public key and metadata,
   referred to as an ECH configuration, and delegates DNS publication
   details to [HTTPS-RR], 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 establish a TLS session with the backend
   server, it constructs its ClientHello as indicated in Section 6.1.
   We will refer to this as the ClientHelloInner message.  The client
   encrypts this message using the public key of the ECH configuration.
   It then constructs a new ClientHello, the ClientHelloOuter, with
   innocuous values for sensitive extensions, e.g., SNI, ALPN, etc., and
   with an "encrypted_client_hello" extension, which this document
   defines (Section 5).  The extension's payload carries the encrypted
   ClientHelloInner and specifies the ECH configuration used for
   encryption.  Finally, it sends ClientHelloOuter to the server.

   Upon receiving the ClientHelloOuter, a TLS server takes one of the
   following actions:

   1.  If it does not support ECH, it ignores the
       "encrypted_client_hello" extension and proceeds with the
       handshake as usual, per [RFC8446], Section 4.1.2.

   2.  If it is a client-facing server for the ECH protocol, but cannot
       decrypt the extension, then it terminates the handshake using the
       ClientHelloOuter.  This is referred to as "ECH rejection".  When
       ECH is rejected, the client-facing server sends an acceptable ECH
       configuration in its EncryptedExtensions message.

   3.  If it supports ECH and decrypts the extension, it forwards the
       ClientHelloInner to the backend server, who terminates the
       connection.  This is referred to as "ECH acceptance".

   Upon receiving the server's response, the client determines whether
   or not ECH was accepted and proceeds with the handshake accordingly.
   (See Section 6 for details.)

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   The primary goal of ECH is to ensure that connections to servers in
   the same anonymity set are indistinguishable from one another.
   Moreover, it should achieve this goal without affecting any existing
   security properties of TLS 1.3.  See Section 10.1 for more details
   about the ECH security and privacy goals.

4.  Encrypted ClientHello Configuration

   ECH uses draft-08 of HPKE for public key encryption
   [I-D.irtf-cfrg-hpke].  The ECH configuration is defined by the
   following "ECHConfig" structure.

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

       struct {
           HpkeKdfId kdf_id;
           HpkeAeadId aead_id;
       } HpkeSymmetricCipherSuite;

       struct {
           uint8 config_id;
           HpkeKemId kem_id;
           HpkePublicKey public_key;
           HpkeSymmetricCipherSuite cipher_suites<4..2^16-4>;
       } HpkeKeyConfig;

       struct {
           HpkeKeyConfig key_config;
           uint8 maximum_name_length;
           opaque public_name<1..255>;
           Extension extensions<0..2^16-1>;
       } ECHConfigContents;

       struct {
           uint16 version;
           uint16 length;
           select (ECHConfig.version) {
             case 0xfe0c: ECHConfigContents contents;
           }
       } ECHConfig;

   The structure contains the following fields:

   version  The version of ECH for which this configuration is used.

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      Beginning with draft-08, the version is the same as the code point
      for the "encrypted_client_hello" extension.  Clients MUST ignore
      any "ECHConfig" structure with a version they do not support.

   length  The length, in bytes, of the next field.

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

   The "ECHConfigContents" structure contains the following fields:

   key_config  A "HpkeKeyConfig" structure carrying the configuration
      information associated with the HPKE public key.  Note that this
      structure contains the "config_id" field, which applies to the
      entire ECHConfigContents.  Sites MUST NOT publish two different
      "ECHConfigContents" values with the same "HpkeKeyConfig" value.
      The RECOMMENDED technique for choosing "config_id" is to choose a
      random byte.  This process is repeated if this config_id matches
      that of any valid ECHConfig, which could include any ECHConfig
      that has been recently removed from active use.

   maximum_name_length  The longest name of a backend server, if known.
      If not known, this value can be set to zero.  It is used to
      compute padding (Section 6.1.3) and does not constrain server name
      lengths.  Names may exceed this length if, e.g., the server uses
      wildcard names or added new names to the anonymity set.

   public_name  The DNS name of the client-facing server, i.e., the
      entity trusted to update the ECH configuration.  This is used to
      correct misconfigured clients, as described in Section 6.1.4.

      Clients MUST ignore any "ECHConfig" structure whose public_name is
      not parsable as a dot-separated sequence of LDH labels, as defined
      in [RFC5890], Section 2.3.1 or which begins or end with an ASCII
      dot.

      Clients SHOULD ignore the "ECHConfig" if it contains an encoded
      IPv4 address.  To determine if a public_name value is an IPv4
      address, clients can invoke the IPv4 parser algorithm in
      [WHATWG-IPV4].  It returns a value when the input is an IPv4
      address.

      See Section 6.1.4.3 for how the client interprets and validates
      the public_name.

   extensions  A list of extensions that the client must take into

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      consideration when generating a ClientHello message.  These are
      described below (Section 4.1).

   [[OPEN ISSUE: determine if clients should enforce a 63-octet label
   limit for public_name]] [[OPEN ISSUE: fix reference to WHATWG-IPV4]]

   The "HpkeKeyConfig" structure contains the following fields:

   config_id  A one-byte identifier for the given HPKE key
      configuration.  This is used by clients to indicate the key used
      for ClientHello encryption.

   kem_id  The HPKE KEM identifier corresponding to "public_key".
      Clients MUST ignore any "ECHConfig" structure with a key using a
      KEM they do not support.

   public_key  The HPKE public key used by the client to encrypt
      ClientHelloInner.

   cipher_suites  The list of HPKE KDF and AEAD identifier pairs clients
      can use for encrypting ClientHelloInner.

   The client-facing server advertises a sequence of ECH configurations
   to clients, serialized as follows.

       ECHConfig ECHConfigList<1..2^16-1>;

   The "ECHConfigList" structure contains one or more "ECHConfig"
   structures in decreasing order of preference.  This allows a server
   to support multiple versions of ECH and multiple sets of ECH
   parameters.

4.1.  Configuration Extensions

   ECH configuration extensions are used to provide room for additional
   functionality as needed.  See Section 12 for guidance on which types
   of extensions are appropriate for this structure.

   The format is as 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 can be tagged as mandatory by using
   an extension type codepoint with the high order bit set to 1.  A
   client that receives a mandatory extension they do not understand
   MUST reject the "ECHConfig" content.

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   Clients MUST parse the extension list and check for unsupported
   mandatory extensions.  If an unsupported mandatory extension is
   present, clients MUST ignore the "ECHConfig".

5.  The "encrypted_client_hello" Extension

   To offer ECH, the client sends an "encrypted_client_hello" extension
   in the ClientHelloOuter.  When it does, it MUST also send the
   extension in ClientHelloInner.

       enum {
          encrypted_client_hello(0xfe0c), (65535)
       } ExtensionType;

   The payload of the extension has the following structure:

       enum { outer(0), inner(1) } ECHClientHelloType;

       struct {
          ECHClientHelloType type;
          select (ECHClientHello.type) {
              case outer:
                  HpkeSymmetricCipherSuite cipher_suite;
                  uint8 config_id;
                  opaque enc<0..2^16-1>;
                  opaque payload<1..2^16-1>;
              case inner:
                  Empty;
          };
       } ECHClientHello;

   The outer extension uses the "outer" variant and the inner extension
   uses the "inner" variant.  The inner extension has an empty payload.
   The outer extension has the following fields:

   config_id  The ECHConfigContents.key_config.config_id for the chosen
      ECHConfig.

   cipher_suite  The cipher suite used to encrypt ClientHelloInner.
      This MUST match a value provided in the corresponding
      "ECHConfigContents.cipher_suites" list.

   enc  The HPKE encapsulated key, used by servers to decrypt the
      corresponding "payload" field.  This field is empty in a
      ClientHelloOuter sent in response to HelloRetryRequest.

   payload  The serialized and encrypted ClientHelloInner structure,
      encrypted using HPKE as described in Section 6.1.

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   When the client offers the "encrypted_client_hello" extension, if the
   payload is the "outer" variant, then the server MAY include an
   "encrypted_client_hello" extension in its EncryptedExtensions message
   with the following payload:

       struct {
          ECHConfigList retry_configs;
       } ECHEncryptedExtensions;

   The response is valid only when the server used the ClientHelloOuter.
   If the server sent this extension in response to the "inner" variant,
   then the client MUST abort with an "unsupported_extension" alert.

   retry_configs  An ECHConfigList structure containing one or more
      ECHConfig structures, in decreasing order of preference, to be
      used by the client in subsequent connection attempts.  These are
      known as the server's "retry configurations".

   Finally, when the client offers the "encrypted_client_hello", if the
   payload is the "inner" variant and the server responds with
   HelloRetryRequest, it MUST include an "encrypted_client_hello"
   extension with the following payload:

       struct {
          opaque confirmation[8];
       } ECHHelloRetryRequest;

   The value of ECHHelloRetryRequest.confirmation is set to
   "hrr_accept_confirmation" as described in Section 7.2.1.

   This document also defines the "ech_required" alert, which the client
   MUST send when it offered an "encrypted_client_hello" extension that
   was not accepted by the server.  (See Section 11.2.)

5.1.  Encoding the ClientHelloInner

   Some TLS 1.3 extensions can be quite large, thus repeating them in
   the ClientHelloInner and ClientHelloOuter can lead to an excessive
   overall size.  One pathological example is "key_share" with post-
   quantum algorithms.  To reduce the impact of duplicated extensions,
   the client may use the "ech_outer_extensions" extension.

       enum {
          ech_outer_extensions(0xfd00), (65535)
       } ExtensionType;

       ExtensionType OuterExtensions<2..254>;

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   OuterExtensions consists of one or more ExtensionType values, each of
   which reference an extension in ClientHelloOuter.  The extensions in
   OuterExtensions MUST appear in ClientHelloOuter in the same relative
   order, however, there is no requirement that they be continguous.
   For example, OuterExtensions may contain extensions A, B, C, while
   ClientHelloOuter contains extensions A, D, B, C, E, F.

   The "ech_outer_extensions" extension is only used for compressing the
   ClientHelloInner.  It can only be included in
   EncodedClientHelloInner, and MUST NOT be sent in either
   ClientHelloOuter or ClientHelloInner.

   When sending ClientHello, the client first computes ClientHelloInner,
   including any PSK binders.  It then computes a new value, the
   EncodedClientHelloInner, which is the following structure:

       struct {
           ClientHello client_hello;
           uint8 zeros[length_of_padding];
       } EncodedClientHelloInner;

   The "client_hello" field is computed by first making a copy of
   ClientHelloInner and setting the "legacy_session_id" field to the
   empty string.  Note this field uses the ClientHello structure,
   defined in Section 4.1.2 of [RFC8446] which does not include the
   Handshake structure's four byte header.  The "zeros" field MUST be
   all zeroes.

   The client then MAY substitute extensions which it knows will be
   duplicated in ClientHelloOuter.  To do so, the client removes and
   replaces extensions from EncodedClientHelloInner with a single
   "ech_outer_extensions" extension.  Removed extensions MUST be ordered
   consecutively in ClientHelloInner.  The list of outer extensions,
   OuterExtensions, includes those which were removed from
   EncodedClientHelloInner, in the order in which they were removed.

   Finally, the client pads the message by setting the "zeros" field to
   a byte string whose contents are all zeros and whose length is the
   amount of padding to add.  Section 6.1.3 describes a recommended
   padding scheme.

   The client-facing server computes ClientHelloInner by reversing this
   process.  First it parses EncodedClientHelloInner, interpreting all
   bytes after "client_hello" as padding.  If any padding byte is non-
   zero, the server MUST abort the connection with an
   "illegal_parameter" alert.

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   Next it makes a copy of the "client_hello" field and copies the
   "legacy_session_id" field from ClientHelloOuter.  It then looks for
   an "ech_outer_extensions" extension.  If found, it replaces the
   extension with the corresponding sequence of extensions in the
   ClientHelloOuter.  The server MUST abort the connection with an
   "illegal_parameter" alert if any of the following are true:

   *  Any referenced extension is missing in ClientHelloOuter.

   *  "encrypted_client_hello" appears in OuterExtensions.

   *  The extensions in ClientHelloOuter corresponding to those in
      OuterExtensions do not occur in the same order.

   Implementations SHOULD bound the time to compute a ClientHelloInner
   proportionally to the ClientHelloOuter size.  If the cost are
   disproportionately large, a malicious client could exploit this in a
   denial of service attack.  Appendix B describes a linear-time
   procedure that may be used for this purpose.

5.2.  Authenticating the ClientHelloOuter

   To prevent a network attacker from modifying the reconstructed
   ClientHelloInner (see Section 10.11.3), ECH authenticates
   ClientHelloOuter by passing ClientHelloOuterAAD as the associated
   data for HPKE sealing and opening operations.  The
   ClientHelloOuterAAD is a serialized ClientHello structure, defined in
   Section 4.1.2 of [RFC8446], which matches the ClientHelloOuter except
   the "payload" field of the "encrypted_client_hello" is replaced with
   a byte string of the same length but whose contents are zeros.  This
   value does not include the four-byte header from the Handshake
   structure.

   The client follows the procedure in Section 6.1.1 to first construct
   ClientHelloOuterAAD with a placeholder "payload" field, then replace
   the field with the encrypted value to compute ClientHelloOuter.

   The server then receives ClientHelloOuter and computes
   ClientHelloOuterAAD by making a copy and replacing the portion
   corresponding to the "payload" field with zeros.

   The payload and the placeholder strings have the same length, so it
   is not necessary for either side to recompute length prefixes when
   applying the above transformations.

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   The decompression process in Section 5.1 forbids
   "encrypted_client_hello" in OuterExtensions.  This ensures the
   unauthenticated portion of ClientHelloOuter is not incorporated into
   ClientHelloInner.

6.  Client Behavior

   Clients that implement the ECH extension behave in one of two ways:
   either they offer a real ECH extension, as described in Section 6.1;
   or they send a GREASE ECH extension, as described in Section 6.2.
   Clients of the latter type do not negotiate ECH.  Instead, they
   generate a dummy ECH extension that is ignored by the server.  (See
   Section 10.9.4 for an explanation.)  The client offers ECH if it is
   in possession of a compatible ECH configuration and sends GREASE ECH
   otherwise.

6.1.  Offering ECH

   To offer ECH, the client first chooses a suitable ECHConfig from the
   server's ECHConfigList.  To determine if a given "ECHConfig" is
   suitable, it checks that it supports the KEM algorithm identified by
   "ECHConfig.contents.kem_id", at least one KDF/AEAD algorithm
   identified by "ECHConfig.contents.cipher_suites", and the version of
   ECH indicated by "ECHConfig.contents.version".  Once a suitable
   configuration is found, the client selects the cipher suite it will
   use for encryption.  It MUST NOT choose a cipher suite or version not
   advertised by the configuration.  If no compatible configuration is
   found, then the client SHOULD proceed as described in Section 6.2.

   Next, the client constructs the ClientHelloInner message just as it
   does a standard ClientHello, with the exception of the following
   rules:

   1.  It MUST NOT offer to negotiate TLS 1.2 or below.  This is
       necessary to ensure the backend server does not negotiate a TLS
       version that is incompatible with ECH.

   2.  It MUST NOT offer to resume any session for TLS 1.2 and below.

   3.  If it intends to compress any extensions (see Section 5.1), it
       MUST order those extensions consecutively.

   4.  It MUST include the "encrypted_client_hello" extension of type
       "inner" as described in Section 5.  (This requirement is not
       applicable when the "encrypted_client_hello" extension is
       generated as described in Section 6.2.)

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   The client then constructs EncodedClientHelloInner as described in
   Section 5.1.  Finally, it constructs the ClientHelloOuter message
   just as it does a standard ClientHello, with the exception of the
   following rules:

   1.  It MUST offer to negotiate TLS 1.3 or above.

   2.  If it compressed any extensions in EncodedClientHelloInner, it
       MUST copy the corresponding extensions from ClientHelloInner.
       The copied extensions additionally MUST be in the same relative
       order as in ClientHelloInner.

   3.  It MUST copy the legacy_session_id field from ClientHelloInner.
       This allows the server to echo the correct session ID for TLS
       1.3's compatibility mode (see Appendix D.4 of [RFC8446]) when ECH
       is negotiated.

   4.  It MAY copy any other field from the ClientHelloInner except
       ClientHelloInner.random.  Instead, It MUST generate a fresh
       ClientHelloOuter.random using a secure random number generator.
       (See Section 10.11.1.)

   5.  The value of "ECHConfig.contents.public_name" MUST be placed in
       the "server_name" extension.

   6.  When the client offers the "pre_shared_key" extension in
       ClientHelloInner, it SHOULD also include a GREASE
       "pre_shared_key" extension in ClientHelloOuter, generated in the
       manner described in Section 6.1.2.  The client MUST NOT use this
       extension to advertise a PSK to the client-facing server.  (See
       Section 10.11.3.)  When the client includes a GREASE
       "pre_shared_key" extension, it MUST also copy the
       "psk_key_exchange_modes" from the ClientHelloInner into the
       ClientHelloOuter.

   7.  When the client offers the "early_data" extension in
       ClientHelloInner, it MUST also include the "early_data" extension
       in ClientHelloOuter.  This allows servers that reject ECH and use
       ClientHelloOuter to safely ignore any early data sent by the
       client per [RFC8446], Section 4.2.10.

   8.  It MUST include an "encrypted_client_hello" extension with a
       payload constructed as described in Section 6.1.1.

   Note that these rules may change in the presence of an application
   profile specifying otherwise.

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   The client might duplicate non-sensitive extensions in both messages.
   However, implementations need to take care to ensure that sensitive
   extensions are not offered in the ClientHelloOuter.  See Section 10.5
   for additional guidance.

6.1.1.  Encrypting the ClientHello

   To construct the "encrypted_client_hello", the client first
   determines the encapsulated key and HPKE encryption context.  If
   constructing the first ClientHelloOuter, it computes them as:

       pkR = DeserializePublicKey(ECHConfig.contents.public_key)
       enc, context = SetupBaseS(pkR,
                                 "tls ech" || 0x00 || ECHConfig)

   If constructing the second ClientHelloOuter (Section 6.1.5), it
   reuses the encryption context computed for the first
   ClientHelloOuter, and sets "enc" to the empty string.  Note that the
   HPKE context maintains a sequence number, so this operation
   internally uses a fresh nonce for each AEAD operation.  Reusing the
   HPKE context avoids an attack described in Section 10.11.2.

   The client then computes ClientHelloOuterAAD (Section 5.2) by
   constructing a ClientHello with all other extensions determined as in
   Section 6.1.

   Next, the client determines the length L of encrypting
   EncodedClientHelloInner with the selected HPKE AEAD.  This is
   typically the sum of the plaintext length and the AEAD tag length.
   The client fills in an "encrypted_client_hello" extension with the
   outer variant of ECHClientHello with the following values:

   *  "config_id", the identifier corresponding to the chosen ECHConfig
      structure;

   *  "cipher_suite", the client's chosen cipher suite;

   *  "enc", as computed above; and

   *  "payload", a placeholder byte string containing L zeros.

   If optional configuration identifiers (see Section 10.4) are used,
   "config_id" SHOULD be set to a randomly generated byte in the first
   ClientHelloOuter and MUST be left unchanged for the second
   ClientHelloOuter.

   The client serializes this structure to construct the
   ClientHelloOuterAAD.  It then computes the payload as:

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       final_payload = context.Seal(ClientHelloOuterAAD,
                                    EncodedClientHelloInner)

   Finally, the client replaces "payload" with "final_payload" to obtain
   ClientHelloOuter.  The two values have the same length, so it is not
   necessary to recompute length prefixes in the serialized structure.

   Note this construction requires the "encrypted_client_hello" be
   computed after all other extensions.  This is possible because the
   ClientHelloOuter's "pre_shared_key" extension is either omitted, or
   uses a random binder (Section 6.1.2).

6.1.2.  GREASE PSK

   When offering ECH, the client is not permitted to advertise PSK
   identities in the ClientHelloOuter.  However, the client can send a
   "pre_shared_key" extension in the ClientHelloInner.  In this case,
   when resuming a session with the client, the backend server sends a
   "pre_shared_key" extension in its ServerHello.  This would appear to
   a network observer as if the the server were sending this extension
   without solicitation, which would violate the extension rules
   described in [RFC8446].  Sending a GREASE "pre_shared_key" extension
   in the ClientHelloOuter makes it appear to the network as if the
   extension were negotiated properly.

   The client generates the extension payload by constructing an
   "OfferedPsks" structure (see [RFC8446], Section 4.2.11) as follows.
   For each PSK identity advertised in the ClientHelloInner, the client
   generates a random PSK identity with the same length.  It also
   generates a random, 32-bit, unsigned integer to use as the
   "obfuscated_ticket_age".  Likewise, for each inner PSK binder, the
   client generates a random string of the same length.

   Per the rules of Section 6.1, the server is not permitted to resume a
   connection in the outer handshake.  If ECH is rejected and the
   client-facing server replies with a "pre_shared_key" extension in its
   ServerHello, then the client MUST abort the handshake with an
   "illegal_parameter" alert.

6.1.3.  Recommended Padding Scheme

   This section describes a deterministic padding mechanism based on the
   following observation: individual extensions can reveal sensitive
   information through their length.  Thus, each extension in the inner
   ClientHello may require different amounts of padding.  This padding
   may be fully determined by the client's configuration or may require
   server input.

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   By way of example, clients typically support a small number of
   application profiles.  For instance, a browser might support HTTP
   with ALPN values ["http/1.1, "h2"] and WebRTC media with ALPNs
   ["webrtc", "c-webrtc"].  Clients SHOULD pad this extension by
   rounding up to the total size of the longest ALPN extension across
   all application profiles.  The target padding length of most
   ClientHello extensions can be computed in this way.

   In contrast, clients do not know the longest SNI value in the client-
   facing server's anonymity set without server input.  Clients SHOULD
   use the ECHConfig's "maximum_name_length" field as follows, where L
   is the "maximum_name_length" value.

   1.  If the ClientHelloInner contained a "server_name" extension with
       a name of length D, add max(0, L - D) bytes of padding.

   2.  If the ClientHelloInner did not contain a "server_name" extension
       (e.g., if the client is connecting to an IP address), add L + 9
       bytes of padding.  This is the length of a "server_name"
       extension with an L-byte name.

   Finally, the client SHOULD pad the entire message as follows:

   1.  Let L be the length of the EncodedClientHelloInner with all the
       padding computed so far.

   2.  Let N = 31 - ((L - 1) % 32) and add N bytes of padding.

   This rounds the length of EncodedClientHelloInner up to a multiple of
   32 bytes, reducing the set of possible lengths across all clients.

   In addition to padding ClientHelloInner, clients and servers will
   also need to pad all other handshake messages that have sensitive-
   length fields.  For example, if a client proposes ALPN values in
   ClientHelloInner, the server-selected value will be returned in an
   EncryptedExtension, so that handshake message also needs to be padded
   using TLS record layer padding.

6.1.4.  Handling the Server Response

   As described in Section 7, the server MAY either accept ECH and use
   ClientHelloInner or reject it and use ClientHelloOuter.  In handling
   the server's response, the client's first step is to determine which
   value was used.

   If the server replied with a HelloRetryRequest, then the client
   proceeds as described in Section 6.1.5.  Otherwise, if the server
   replied with a ServerHello, then the client checks if the last 8

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   bytes of "ServerHello.random" are equal to "accept_confirmation" as
   defined in Section 7.2.  If so, then it presumes acceptance.
   Otherwise, the client presumes rejection.

6.1.4.1.  Accepted ECH

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

6.1.4.2.  Rejected ECH

   If the server used ClientHelloOuter, the client proceeds with the
   handshake, authenticating for ECHConfig.contents.public_name as
   described in Section 6.1.4.3.  If authentication or the handshake
   fails, the client MUST return a failure to the calling application.
   It MUST NOT use the retry configurations.

   Otherwise, if both authentication and the handshake complete
   successfully, the client MUST abort the connection with an
   "ech_required" alert.  It then processes the "retry_configs" field
   from the server's "encrypted_client_hello" extension.

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

   If none of the values provided in "retry_configs" contains a
   supported version, the client can regard ECH as securely disabled by
   the server.  As below, it SHOULD then retry the handshake with a new
   transport connection and ECH 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_client_hello" extension in EncryptedExtensions,
   the client proceeds with the handshake, authenticating for
   ECHConfig.contents.public_name as described in Section 6.1.4.3.  If
   an earlier version was negotiated, the client MUST NOT enable the
   False Start optimization [RFC7918] for this handshake.  If

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   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 ECH.

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

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

6.1.4.3.  Authenticating for the Public Name

   When the server rejects ECH or otherwise ignores
   "encrypted_client_hello" extension, it continues with the handshake
   using the plaintext "server_name" extension instead (see Section 7).
   Clients that offer ECH then authenticate the connection with the
   public name, as follows:

   *  The client MUST verify that the certificate is valid for
      ECHConfig.contents.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.

   In verifying the client-facing server certificate, the client MUST
   interpret the public name as a DNS-based reference identity.  Clients
   that incorporate DNS names and IP addresses into the same syntax
   (e.g.  [RFC3986], Section 7.4 and [WHATWG-IPV4]) MUST reject names
   that would be interpreted as IPv4 addresses.  Clients that enforce
   this by checking and rejecting encoded IPv4 addresses in
   ECHConfig.contents.public_name do not need to repeat the check at
   this layer.

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

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6.1.5.  Handling HelloRetryRequest

   When the server sends a HelloRetryRequest, the client checks for the
   presence of an "encrypted_client_hello" extension.  If none is found,
   then the client presumes rejection and handles the HelloRetryRequest
   using ClientHelloOuter.  (Note that the client-facing server does not
   send this extension in its HelloRetryRequest.  [[NOTE: This may
   change, depending on the outcome of issue#450.]]) Otherwise it
   proceeds as follows.

   If the extension's payload has a length other than 8, then the client
   aborts the handshake with an "decode_error" alert.  If the payload
   length is equal to 8, then the client checks if the payload is equal
   to "hrr_accept_confirmation" as defined in Section 7.2.  If so, then
   it presumes acceptance and handles handles the HelloRetryRequest
   using ClientHelloInner.  Otherwise, it presumes rejection.

   [[OPEN ISSUE: Depending on what we do for issue#450, it may be
   appropriate to change the client behavior if the HRR payload is
   missing or malformed.]]

   The client encodes the second ClientHelloInner as in Section 5.1,
   using the second ClientHelloOuter for any referenced extensions.  It
   then encrypts the new EncodedClientHelloInner value as a second
   message with the previous HPKE context as described in Section 6.1.1.

6.2.  GREASE ECH

   If the client attempts to connect to a server and does not have an
   ECHConfig structure available for the server, it SHOULD send a GREASE
   [RFC8701] "encrypted_client_hello" extension in the first ClientHello
   as follows:

   *  Set the "config_id" field to a random byte.

   *  Set the "cipher_suite" field to a supported
      HpkeSymmetricCipherSuite.  The selection 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 "payload" field to a randomly-generated string of L+C
      bytes, where C is the ciphertext expansion of the selected AEAD
      scheme and L is the size of the EncodedClientHelloInner the client
      would compute when offering ECH, padded according to
      Section 6.1.3.

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   When sending a second ClientHello in response to a HelloRetryRequest,
   the client copies the entire "encrypted_client_hello" extension from
   the first ClientHello.

   [[OPEN ISSUE: The above doesn't match HRR handling for either ECH
   acceptance or rejection.  See issue https://github.com/tlswg/draft-
   ietf-tls-esni/issues/358.]]

   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.  It otherwise ignores the
   extension and MUST NOT use the retry keys.

   Offering a GREASE extension is not considered offering an encrypted
   ClientHello for purposes of requirements in Section 6.  In
   particular, the client MAY offer to resume sessions established
   without ECH.

7.  Server Behavior

   Servers that support ECH play one of two roles, depending on the
   payload of the "encrypted_client_hello" extension in the initial
   ClientHello:

   *  If "ECHClientHello.type" is "outer", then the server acts as a
      client-facing server and proceeds as described in Section 7.1 to
      extract a ClientHelloInner, if available.

   *  If "ECHClientHello.type" is "inner", then the server acts as a
      backend server and proceeds as described in Section 7.2.

   *  Otherwise, if "ECHClientHello.type" is not a valid
      "ECHClientHelloType", then the server MUST abort with an
      "illegal_parameter" alert.

   If the "encrypted_client_hello" is not present, then the server
   completes the handshake normally, as described in [RFC8446].

7.1.  Client-Facing Server

   Upon receiving an "encrypted_client_hello" extension in an initial
   ClientHello, the client-facing server determines if it will accept
   ECH, prior to negotiating any other TLS parameters.  Note that
   successfully decrypting the extension will result in a new
   ClientHello to process, so even the client's TLS version preferences
   may have changed.

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   First, the server collects a set of candidate ECHConfig values.  This
   list is determined by one of the two following methods:

   1.  Compare ECHClientHello.config_id against identifiers of each
       known ECHConfig and select the ones that match, if any, as
       candidates.

   2.  Collect all known ECHConfig values as candidates, with trial
       decryption below determining the final selection.

   Some uses of ECH, such as local discovery mode, may randomize the
   ECHClientHello.config_id since it can be used as a tracking vector.
   In such cases, the second method should be used for matching the
   ECHClientHello to a known ECHConfig.  See Section 10.4.  Unless
   specified by the application using (D)TLS or externally configured on
   both sides, implementations MUST use the first method.

   The server then iterates over the candidate ECHConfig values,
   attempting to decrypt the "encrypted_client_hello" extension:

   The server verifies that the ECHConfig supports the cipher suite
   indicated by the ECHClientHello.cipher_suite and that the version of
   ECH indicated by the client matches the ECHConfig.version.  If not,
   the server continues to the next candidate ECHConfig.

   Next, the server decrypts ECHClientHello.payload, using the private
   key skR corresponding to ECHConfig, as follows:

       context = SetupBaseR(ECHClientHello.enc, skR,
                            "tls ech" || 0x00 || ECHConfig)
       EncodedClientHelloInner = context.Open(ClientHelloOuterAAD,
                                            ECHClientHello.payload)

   ClientHelloOuterAAD is computed from ClientHelloOuter as described in
   Section 5.2.  The "info" parameter to SetupBaseR is the concatenation
   "tls ech", a zero byte, and the serialized ECHConfig.  If decryption
   fails, the server continues to the next candidate ECHConfig.
   Otherwise, the server reconstructs ClientHelloInner from
   EncodedClientHelloInner, as described in Section 5.1.  It then stops
   iterating over the candidate ECHConfig values.

   Upon determining the ClientHelloInner, the client-facing server
   checks that the message includes a well-formed
   "encrypted_client_hello" extension of type "inner" and that it does
   not offer TLS 1.2 or below.  If either of these checks fails, the
   client-facing server MUST abort with an "illegal_parameter" alert.

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   If these checks succeed, the client-facing server then forwards the
   ClientHelloInner to the appropriate backend server, which proceeds as
   in Section 7.2.  If the backend server responds with a
   HelloRetryRequest, the client-facing server forwards it, decrypts the
   client's second ClientHelloOuter using the procedure in
   Section 7.1.1, and forwards the resulting second ClientHelloInner.
   The client-facing server forwards all other TLS messages between the
   client and backend server unmodified.

   Otherwise, if all candidate ECHConfig values fail to decrypt the
   extension, the client-facing server MUST ignore the extension and
   proceed with the connection using ClientHelloOuter.  This connection
   proceeds as usual, except the server MUST include the
   "encrypted_client_hello" extension in its EncryptedExtensions with
   the "retry_configs" field set to one or more ECHConfig structures
   with up-to-date keys.  Servers MAY supply multiple ECHConfig values
   of different versions.  This allows a server to support multiple
   versions at once.

   Note that decryption failure could indicate a GREASE ECH extension
   (see Section 6.2), so it is necessary for servers to proceed with the
   connection and rely on the client to abort if ECH was required.  In
   particular, the unrecognized value alone does not indicate a
   misconfigured ECH advertisement (Section 8.1).  Instead, servers can
   measure occurrences of the "ech_required" alert to detect this case.

7.1.1.  Sending HelloRetryRequest

   After sending or forwarding a HelloRetryRequest, the client-facing
   server does not repeat the steps in Section 7.1 with the second
   ClientHelloOuter.  Instead, it continues with the ECHConfig selection
   from the first ClientHelloOuter as follows:

   If the client-facing server accepted ECH, it checks the second
   ClientHelloOuter also contains the "encrypted_client_hello"
   extension.  If not, it MUST abort the handshake with a
   "missing_extension" alert.  Otherwise, it checks that
   ECHClientHello.cipher_suite and ECHClientHello.config_id are
   unchanged, and that ECHClientHello.enc is empty.  If not, it MUST
   abort the handshake with an "illegal_parameter" alert.

   Finally, it decrypts the new ECHClientHello.payload as a second
   message with the previous HPKE context:

       EncodedClientHelloInner = context.Open(ClientHelloOuterAAD,
                                            ECHClientHello.payload)

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   ClientHelloOuterAAD is computed as described in Section 5.2, but
   using the second ClientHelloOuter.  If decryption fails, the client-
   facing server MUST abort the handshake with a "decrypt_error" alert.
   Otherwise, it reconstructs the second ClientHelloInner from the new
   EncodedClientHelloInner as described in Section 5.1, using the second
   ClientHelloOuter for any referenced extensions.

   The client-facing server then forwards the resulting ClientHelloInner
   to the backend server.  It forwards all subsequent TLS messages
   between the client and backend server unmodified.

   If the client-facing server rejected ECH, or if the first ClientHello
   did not include an "encrypted_client_hello" extension, the client-
   facing server proceeds with the connection as usual.  The server does
   not decrypt the second ClientHello's ECHClientHello.payload value, if
   there is one.

   Note that a client-facing server that forwards the first ClientHello
   cannot include its own "cookie" extension if the backend server sends
   a HelloRetryRequest.  This means that the client-facing server either
   needs to maintain state for such a connection or it needs to
   coordinate with the backend server to include any information it
   requires to process the second ClientHello.

7.2.  Backend Server

   Upon receipt of an "encrypted_client_hello" extension of type "inner"
   in a ClientHello, if the backend server negotiates TLS 1.3 or higher,
   then it MUST confirm ECH acceptance to the client by computing its
   ServerHello as described here.

   The backend server embeds in ServerHello.random a string derived from
   the inner handshake.  It begins by computing its ServerHello as
   usual, except the last 8 bytes of ServerHello.random are set to zero.
   It then computes the transcript hash for ClientHelloInner up to and
   including the modified ServerHello, as described in [RFC8446],
   Section 4.4.1.  Let transcript_ech_conf denote the output.  Finally,
   the backend server overwrites the last 8 bytes of the
   ServerHello.random with the following string:

      accept_confirmation = HKDF-Expand-Label(
         HKDF-Extract(0, ClientHelloInner.random),
         "ech accept confirmation",
         transcript_ech_conf,
         8)

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   where HKDF-Expand-Label is defined in [RFC8446], Section 7.1, "0"
   indicates a string of Hash.length bytes set to zero, and Hash is the
   hash function used to compute the transcript hash.

   The backend server MUST NOT perform this operation if it negotiated
   TLS 1.2 or below.  Note that doing so would overwrite the downgrade
   signal for TLS 1.3 (see [RFC8446], Section 4.1.3).

7.2.1.  Sending HelloRetryRequest

   When the backend server sends HelloRetryRequest in response to the
   ClientHello, it similarly confirms ECH acceptance by adding a
   confirmation signal to its HelloRetryRequest.  But instead of
   embedding the signal in the HelloRetryRequest.random (the value of
   which is specified by [RFC8446]), it sends the signal in an
   extension.

   The backend server begins by computing HelloRetryRequest as usual,
   except that it also contains an "encrypted_client_hello" extension
   with a payload of 8 zero bytes.  It then computes the transcript hash
   for the first ClientHelloInner, denoted ClientHelloInner1, up to and
   including the modified HelloRetryRequest.  Let
   transcript_hrr_ech_conf denote the output.  Finally, the backend
   server overwrites the payload of the "encrypted_client_hello"
   extension with the following string:

      hrr_accept_confirmation = HKDF-Expand-Label(
         HKDF-Extract(0, ClientHelloInner1.random),
         "hrr ech accept confirmation",
         transcript_hrr_ech_conf,
         8)

   In the subsequent ServerHello message, the backend server sends the
   accept_confirmation value as described in Section 7.2.

8.  Compatibility Issues

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

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   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 ECH keys and
   servers, at the cost of extra round-trips due to a retry.  Two
   specific scenarios are detailed below.

8.1.  Misconfiguration and Deployment Concerns

   It is possible for ECH 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
   ECH keys, or if a deployment of ECH 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 ech_retry_requested.  If the
   server does not understand the "encrypted_client_hello" extension at
   all, it will ignore it as required by Section 4.1.2 of [RFC8446].
   Provided the server can present a certificate valid for the public
   name, the client can safely retry with updated settings, as described
   in Section 6.1.4.

   Unless ECH 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.

8.2.  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_client_hello" 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 6.1.4 or result in a connection
   failure.  A proxy which is not authoritative for the public name
   cannot forge a signal to disable ECH.

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

9.  Compliance Requirements

   In the absence of an application profile standard specifying
   otherwise, a compliant ECH application MUST implement the following
   HPKE cipher suite:

   *  KEM: DHKEM(X25519, HKDF-SHA256) (see [I-D.irtf-cfrg-hpke],
      Section 7.1)

   *  KDF: HKDF-SHA256 (see [I-D.irtf-cfrg-hpke], Section 7.2)

   *  AEAD: AES-128-GCM (see [I-D.irtf-cfrg-hpke], Section 7.3)

10.  Security Considerations

10.1.  Security and Privacy Goals

   ECH considers two types of attackers: passive and active.  Passive
   attackers can read packets from the network, but they cannot perform
   any sort of active behavior such as probing servers or querying DNS.
   A middlebox that filters based on plaintext packet contents is one
   example of a passive attacker.  In contrast, active attackers can
   also write packets into the network for malicious purposes, such as
   interfering with existing connections, probing servers, and querying
   DNS.  In short, an active attacker corresponds to the conventional
   threat model for TLS 1.3 [RFC8446].

   Given these types of attackers, the primary goals of ECH are as
   follows.

   1.  Use of ECH does not weaken the security properties of TLS without
       ECH.

   2.  TLS connection establishment to a host with a specific ECHConfig
       and TLS configuration is indistinguishable from a connection to
       any other host with the same ECHConfig and TLS configuration.
       (The set of hosts which share the same ECHConfig and TLS
       configuration is referred to as the anonymity set.)

   Client-facing server configuration determines the size of the
   anonymity set.  For example, if a client-facing server uses distinct
   ECHConfig values for each host, then each anonymity set has size k =

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   1.  Client-facing servers SHOULD deploy ECH in such a way so as to
   maximize the size of the anonymity set where possible.  This means
   client-facing servers should use the same ECHConfig for as many hosts
   as possible.  An attacker can distinguish two hosts that have
   different ECHConfig values based on the ECHClientHello.config_id
   value.  This also means public information in a TLS handshake is also
   consistent across hosts.  For example, if a client-facing server
   services many backend origin hosts, only one of which supports some
   cipher suite, it may be possible to identify that host based on the
   contents of unencrypted handshake messages.

   Beyond these primary security and privacy goals, ECH also aims to
   hide, to some extent, the fact that it is being used at all.
   Specifically, the GREASE ECH extension described in Section 6.2 does
   not change the security properties of the TLS handshake at all.  Its
   goal is to provide "cover" for the real ECH protocol (Section 6.1),
   as a means of addressing the "do not stick out" requirements of
   [RFC8744].  See Section 10.9.4 for details.

10.2.  Unauthenticated and Plaintext DNS

   In comparison to [I-D.kazuho-protected-sni], wherein DNS Resource
   Records are signed via a server private key, ECH 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 ECH records (so that the client encrypts data to them) or strip
   the ECH 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 ECH 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.

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10.3.  Client Tracking

   A malicious client-facing server could distribute unique, per-client
   ECHConfig structures as a way of tracking clients across subsequent
   connections.  On-path adversaries which know about these unique keys
   could also track clients in this way by observing TLS connection
   attempts.

   The cost of this type of attack scales linearly with the desired
   number of target clients.  Moreover, DNS caching behavior makes
   targeting individual users for extended periods of time, e.g., using
   per-client ECHConfig structures delivered via HTTPS RRs with high
   TTLs, challenging.  Clients can help mitigate this problem by
   flushing any DNS or ECHConfig state upon changing networks.

10.4.  Optional Configuration Identifiers and Trial Decryption

   Optional configuration identifiers may be useful in scenarios where
   clients and client-facing servers do not want to reveal information
   about the client-facing server in the "encrypted_client_hello"
   extension.  In such settings, clients send a randomly generated
   config_id in the ECHClientHello.  Servers in these settings must
   perform trial decryption since they cannot identify the client's
   chosen ECH key using the config_id value.  As a result, support for
   optional configuration identifiers may exacerbate DoS attacks.
   Specifically, an adversary may send malicious ClientHello messages,
   i.e., those which will not decrypt with any known ECH key, in order
   to force wasteful decryption.  Servers that support this feature
   should, for example, implement some form of rate limiting mechanism
   to limit the damage caused by such attacks.

   Unless specified by the application using (D)TLS or externally
   configured on both sides, implementations MUST NOT use this mode.

10.5.  Outer ClientHello

   Any information that the client includes in the ClientHelloOuter is
   visible to passive observers.  The client SHOULD NOT send values in
   the ClientHelloOuter which would reveal a sensitive ClientHelloInner
   property, such as the true server name.  It MAY send values
   associated with the public name in the ClientHelloOuter.

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   In particular, some extensions require the client send a server-name-
   specific value in the ClientHello.  These values may reveal
   information about the true server name.  For example, the
   "cached_info" ClientHello extension [RFC7924] can contain the hash of
   a previously observed server certificate.  The client SHOULD NOT send
   values associated with the true server name in the ClientHelloOuter.
   It MAY send such values in the ClientHelloInner.

   A client may also use different preferences in different contexts.
   For example, it may send a different ALPN lists to different servers
   or in different application contexts.  A client that treats this
   context as sensitive SHOULD NOT send context-specific values in
   ClientHelloOuter.

   Values which are independent of the true server name, or other
   information the client wishes to protect, MAY be included in
   ClientHelloOuter.  If they match the corresponding ClientHelloInner,
   they MAY be compressed as described in Section 5.1.  However, note
   the payload length reveals information about which extensions are
   compressed, so inner extensions which only sometimes match the
   corresponding outer extension SHOULD NOT be compressed.

   Clients MAY include additional extensions in ClientHelloOuter to
   avoid signaling unusual behavior to passive observers, provided the
   choice of value and value itself are not sensitive.  See
   Section 10.9.4.

10.6.  Related Privacy Leaks

   ECH 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.

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   Attacks that rely on non-ECH traffic to infer server identity in an
   ECH connection are out of scope for this document.  For example, a
   client that connects to a particular host prior to ECH deployment may
   later resume a connection to that same host after ECH deployment.  An
   adversary that observes this can deduce that the ECH-enabled
   connection was made to a host that the client previously connected to
   and which is within the same anonymity set.

10.7.  Cookies

   Section 4.2.2 of [RFC8446] defines a cookie value that servers may
   send in HelloRetryRequest for clients to echo in the second
   ClientHello.  While ECH encrypts the cookie in the second
   ClientHelloInner, the backend server's HelloRetryRequest is
   unencrypted.This means differences in cookies between backend
   servers, such as lengths or cleartext components, may leak
   information about the server identity.

   Backend servers in an anonymity set SHOULD NOT reveal information in
   the cookie which identifies the server.  This may be done by handling
   HelloRetryRequest statefully, thus not sending cookies, or by using
   the same cookie construction for all backend servers.

   Note that, if the cookie includes a key name, analogous to Section 4
   of [RFC5077], this may leak information if different backend servers
   issue cookies with different key names at the time of the connection.
   In particular, if the deployment operates in Split Mode, the backend
   servers may not share cookie encryption keys.  Backend servers may
   mitigate this by either handling key rotation with trial decryption,
   or coordinating to match key names.

10.8.  Attacks Exploiting Acceptance Confirmation

   To signal acceptance, the backend server overwrites 8 bytes of its
   ServerHello.random with a value derived from the
   ClientHelloInner.random.  (See Section 7.2 for details.)  This
   behavior increases the likelihood of the ServerHello.random colliding
   with the ServerHello.random of a previous session, potentially
   reducing the overall security of the protocol.  However, the
   remaining 24 bytes provide enough entropy to ensure this is not a
   practical avenue of attack.

   On the other hand, the probability that two 8-byte strings are the
   same is non-negligible.  This poses a modest operational risk.
   Suppose the client-facing server terminates the connection (i.e., ECH
   is rejected or bypassed): if the last 8 bytes of its
   ServerHello.random coincide with the confirmation signal, then the
   client will incorrectly presume acceptance and proceed as if the

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   backend server terminated the connection.  However, the probability
   of a false positive occurring for a given connection is only 1 in
   2^64.  This value is smaller than the probability of network
   connection failures in practice.

   Note that the same bytes of the ServerHello.random are used to
   implement downgrade protection for TLS 1.3 (see [RFC8446],
   Section 4.1.3).  These mechanisms do not interfere because the
   backend server only signals ECH acceptance in TLS 1.3 or higher.

10.9.  Comparison Against Criteria

   [RFC8744] lists several requirements for SNI encryption.  In this
   section, we re-iterate these requirements and assess the ECH design
   against them.

10.9.1.  Mitigate Cut-and-Paste Attacks

   Since servers process either ClientHelloInner or ClientHelloOuter,
   and because ClientHelloInner.random is encrypted, it is not possible
   for an attacker to "cut and paste" the ECH value in a different
   Client Hello and learn information from ClientHelloInner.

10.9.2.  Avoid Widely 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 ECH
   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 ECHConfig values with
   different keys using a short TTL.

10.9.3.  Prevent SNI-Based Denial-of-Service Attacks

   This design requires servers to decrypt ClientHello messages with
   ECHClientHello 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.

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10.9.4.  Do Not Stick Out

   As a means of reducing the impact of network ossification, [RFC8744]
   recommends SNI-protection mechanisms be designed in such a way that
   network operators do not differentiate connections using the
   mechanism from connections not using the mechanism.  To that end, ECH
   is designed to resemble a standard TLS handshake as much as possible.
   The most obvious difference is the extension itself: as long as
   middleboxes ignore it, as required by [RFC8446], the rest of the
   handshake is designed to look very much as usual.

   The GREASE ECH protocol described in Section 6.2 provides a low-risk
   way to evaluate the deployability of ECH.  It is designed to mimic
   the real ECH protocol (Section 6.1) without changing the security
   properties of the handshake.  The underlying theory is that if GREASE
   ECH is deployable without triggering middlebox misbehavior, and real
   ECH looks enough like GREASE ECH, then ECH should be deployable as
   well.  Thus, our strategy for mitigating network ossification is to
   deploy GREASE ECH widely enough to disincentivize differential
   treatment of the real ECH protocol by the network.

   Ensuring that networks do not differentiate between real ECH and
   GREASE ECH may not be feasible for all implementations.  While most
   middleboxes will not treat them differently, some operators may wish
   to block real ECH usage but allow GREASE ECH.  This specification
   aims to provide a baseline security level that most deployments can
   achieve easily, while providing implementations enough flexibility to
   achieve stronger security where possible.  Minimally, real ECH is
   designed to be indifferentiable from GREASE ECH for passive
   adversaries with following capabilities: 1.  The attacker does not
   know the ECHConfigList used by the server. 1.  The attacker keeps
   per-connection state only.  In particular, it does not track
   endpoints across connections. 1.  ECH and GREASE ECH are designed so
   that the following features do not vary: the code points of
   extensions negotiated in the clear; the length of messages; and the
   values of plaintext alert messages.

   This leaves a variety of practical differentiators out-of-scope.
   including, though not limited to, the following: 1. the value of the
   configuration identifier; 1. the value of the outer SNI; 1. use of
   the "pre_shared_key" extension in the ClientHelloOuter, which is
   permitted in GREASE ECH but not real ECH; [[TODO: Remove this
   differentiator if issue #384 is resolved by a spec change.]] 1. the
   TLS version negotiated, which may depend on ECH acceptance; 1. client
   authentication, which may depend on ECH acceptance; and 1.  HRR
   issuance, which may depend on ECH acceptance.

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   These can be addressed with more sophisticated implementations, but
   some mitigations require coordination between the client and server.
   These mitigations are out-of-scope for this specification.

10.9.5.  Maintain Forward Secrecy

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

10.9.6.  Enable Multi-party Security Contexts

   This design permits servers operating in Split Mode to forward
   connections directly to backend origin servers.  The client
   authenticates the identity of the backend origin server, thereby
   avoiding unnecessary MiTM attacks.

   Conversely, assuming ECH records retrieved from DNS are
   authenticated, e.g., via DNSSEC or fetched from a trusted Recursive
   Resolver, spoofing a client-facing server operating in Split Mode is
   not possible.  See Section 10.2 for more details regarding plaintext
   DNS.

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

10.9.7.  Support 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 application and transport protocols.  By encrypting the
   entire ClientHello, this design additionally supports encrypting the
   ALPN extension.

10.10.  Padding Policy

   Variations in the length of the ClientHelloInner ciphertext could
   leak information about the corresponding plaintext.  Section 6.1.3
   describes a RECOMMENDED padding mechanism for clients aimed at
   reducing potential information leakage.

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10.11.  Active Attack Mitigations

   This section describes the rationale for ECH properties and mechanics
   as defenses against active attacks.  In all the attacks below, the
   attacker is on-path between the target client and server.  The goal
   of the attacker is to learn private information about the inner
   ClientHello, such as the true SNI value.

10.11.1.  Client Reaction Attack Mitigation

   This attack uses the client's reaction to an incorrect certificate as
   an oracle.  The attacker intercepts a legitimate ClientHello and
   replies with a ServerHello, Certificate, CertificateVerify, and
   Finished messages, wherein the Certificate message contains a "test"
   certificate for the domain name it wishes to query.  If the client
   decrypted the Certificate and failed verification (or leaked
   information about its verification process by a timing side channel),
   the attacker learns that its test certificate name was incorrect.  As
   an example, suppose the client's SNI value in its inner ClientHello
   is "example.com," and the attacker replied with a Certificate for
   "test.com".  If the client produces a verification failure alert
   because of the mismatch faster than it would due to the Certificate
   signature validation, information about the name leaks.  Note that
   the attacker can also withhold the CertificateVerify message.  In
   that scenario, a client which first verifies the Certificate would
   then respond similarly and leak the same information.

    Client                         Attacker               Server
      ClientHello
      + key_share
      + ech         ------>      (intercept)     -----> X (drop)

                                ServerHello
                                + key_share
                      {EncryptedExtensions}
                      {CertificateRequest*}
                             {Certificate*}
                       {CertificateVerify*}
                    <------
      Alert
                    ------>

                      Figure 3: Client reaction attack

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   ClientHelloInner.random prevents this attack.  In particular, since
   the attacker does not have access to this value, it cannot produce
   the right transcript and handshake keys needed for encrypting the
   Certificate message.  Thus, the client will fail to decrypt the
   Certificate and abort the connection.

10.11.2.  HelloRetryRequest Hijack Mitigation

   This attack aims to exploit server HRR state management to recover
   information about a legitimate ClientHello using its own attacker-
   controlled ClientHello.  To begin, the attacker intercepts and
   forwards a legitimate ClientHello with an "encrypted_client_hello"
   (ech) extension to the server, which triggers a legitimate
   HelloRetryRequest in return.  Rather than forward the retry to the
   client, the attacker, attempts to generate its own ClientHello in
   response based on the contents of the first ClientHello and
   HelloRetryRequest exchange with the result that the server encrypts
   the Certificate to the attacker.  If the server used the SNI from the
   first ClientHello and the key share from the second (attacker-
   controlled) ClientHello, the Certificate produced would leak the
   client's chosen SNI to the attacker.

    Client                         Attacker                   Server
      ClientHello
      + key_share
      + ech         ------>       (forward)        ------->
                                                 HelloRetryRequest
                                                       + key_share
                                 (intercept)       <-------

                                 ClientHello
                                 + key_share'
                                 + ech'           ------->
                                                       ServerHello
                                                       + key_share
                                             {EncryptedExtensions}
                                             {CertificateRequest*}
                                                    {Certificate*}
                                              {CertificateVerify*}
                                                        {Finished}
                                                   <-------
                            (process server flight)

                 Figure 4: HelloRetryRequest hijack attack

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   This attack is mitigated by using the same HPKE context for both
   ClientHello messages.  The attacker does not possess the context's
   keys, so it cannot generate a valid encryption of the second inner
   ClientHello.

   If the attacker could manipulate the second ClientHello, it might be
   possible for the server to act as an oracle if it required parameters
   from the first ClientHello to match that of the second ClientHello.
   For example, imagine the client's original SNI value in the inner
   ClientHello is "example.com", and the attacker's hijacked SNI value
   in its inner ClientHello is "test.com".  A server which checks these
   for equality and changes behavior based on the result can be used as
   an oracle to learn the client's SNI.

10.11.3.  ClientHello Malleability Mitigation

   This attack aims to leak information about secret parts of the
   encrypted ClientHello by adding attacker-controlled parameters and
   observing the server's response.  In particular, the compression
   mechanism described in Section 5.1 references parts of a potentially
   attacker-controlled ClientHelloOuter to construct ClientHelloInner,
   or a buggy server may incorrectly apply parameters from
   ClientHelloOuter to the handshake.

   To begin, the attacker first interacts with a server to obtain a
   resumption ticket for a given test domain, such as "example.com".
   Later, upon receipt of a ClientHelloOuter, it modifies it such that
   the server will process the resumption ticket with ClientHelloInner.
   If the server only accepts resumption PSKs that match the server
   name, it will fail the PSK binder check with an alert when
   ClientHelloInner is for "example.com" but silently ignore the PSK and
   continue when ClientHelloInner is for any other name.  This
   introduces an oracle for testing encrypted SNI values.

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         Client              Attacker                       Server

                                       handshake and ticket
                                          for "example.com"
                                          <-------->

         ClientHello
         + key_share
         + ech
         + ech_outer_extensions(pre_shared_key)
         + pre_shared_key
                     -------->
                           (intercept)
                           ClientHello
                           + key_share
                           + ech
                              + ech_outer_extensions(pre_shared_key)
                           + pre_shared_key'
                                             -------->
                                                            Alert
                                                            -or-
                                                      ServerHello
                                                               ...
                                                         Finished
                                             <--------

              Figure 5: Message flow for malleable ClientHello

   This attack may be generalized to any parameter which the server
   varies by server name, such as ALPN preferences.

   ECH mitigates this attack by only negotiating TLS parameters from
   ClientHelloInner and authenticating all inputs to the
   ClientHelloInner (EncodedClientHelloInner and ClientHelloOuter) with
   the HPKE AEAD.  See Section 5.2.  An earlier iteration of this
   specification only encrypted and authenticated the "server_name"
   extension, which left the overall ClientHello vulnerable to an
   analogue of this attack.

11.  IANA Considerations

11.1.  Update of the TLS ExtensionType Registry

   IANA is requested to create the following three entries in the
   existing registry for ExtensionType (defined in [RFC8446]):

   1.  encrypted_client_hello(0xfe0c), with "TLS 1.3" column values set
       to "CH, HRR, EE", and "Recommended" column set to "Yes".

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   2.  ech_outer_extensions(0xfd00), with the "TLS 1.3" column values
       set to "", and "Recommended" column set to "Yes".

11.2.  Update of the TLS Alert Registry

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

12.  ECHConfig Extension Guidance

   Any future information or hints that influence ClientHelloOuter
   SHOULD be specified as ECHConfig extensions.  This is primarily
   because the outer ClientHello exists only in support of ECH.  Namely,
   it is both an envelope for the encrypted inner ClientHello and
   enabler for authenticated key mismatch signals (see Section 7).  In
   contrast, the inner ClientHello is the true ClientHello used upon ECH
   negotiation.

13.  References

13.1.  Normative References

   [HTTPS-RR] Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-https-06, 16 June 2021,
              <https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
              https-06.txt>.

   [I-D.ietf-tls-exported-authenticator]
              Sullivan, N., "Exported Authenticators in TLS", Work in
              Progress, Internet-Draft, draft-ietf-tls-exported-
              authenticator-14, 25 January 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tls-exported-
              authenticator-14.txt>.

   [I-D.irtf-cfrg-hpke]
              Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood,
              "Hybrid Public Key Encryption", Work in Progress,
              Internet-Draft, draft-irtf-cfrg-hpke-10, 7 July 2021,
              <https://www.ietf.org/archive/id/draft-irtf-cfrg-hpke-
              10.txt>.

   [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>.

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   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [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>.

   [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>.

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <https://www.rfc-editor.org/info/rfc5077>.

   [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>.

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   [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>.

   [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>.

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

   [RFC8701]  Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,
              <https://www.rfc-editor.org/info/rfc8701>.

   [RFC8744]  Huitema, C., "Issues and Requirements for Server Name
              Identification (SNI) Encryption in TLS", RFC 8744,
              DOI 10.17487/RFC8744, July 2020,
              <https://www.rfc-editor.org/info/rfc8744>.

   [WHATWG-IPV4]
              "URL Living Standard - IPv4 Parser", May 2021,
              <https://url.spec.whatwg.org/#concept-ipv4-parser>.

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.

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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.

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.

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.  Linear-time Outer Extension Processing

   The following procedure processes the "ech_outer_extensions"
   extension (see Section 5.1) in linear time:

   1.  Let I be zero and N be the number of extensions in
       ClientHelloOuter.

   2.  For each extension type, E, in OuterExtensions:

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       *  If E is "encrypted_client_hello", abort the connection with an
          "illegal_parameter" alert and terminate this procedure.

       *  While I is less than N and the I-th extension of
          ClientHelloOuter does not have type E, increment I.

       *  If I is equal to N, abort the connection with an
          "illegal_parameter" alert and terminate this procedure.

       *  Otherwise, the I-th extension of ClientHelloOuter has type E.
          Copy it to the EncodedClientHelloInner and increment I.

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 ECH key.
   Richard Barnes, Christian Huitema, Patrick McManus, Matthew Prince,
   Nick Sullivan, Martin Thomson, and David Benjamin also provided
   important ideas and contributions.

Appendix D.  Change Log

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

   Issue and pull request numbers are listed with a leading octothorp.

D.1.  Since draft-ietf-tls-esni-11

   *  Move ClientHello padding to the encoding (#443)

   *  Align codepoints (#464)

   *  Relax OuterExtensions checks for alignment with RFC8446 (#467)

   *  Clarify HRR acceptance and rejection logic (#470)

   *  Editorial improvements (#468, #465, #462, #461)

D.2.  Since draft-ietf-tls-esni-10

   *  Make HRR confirmation and ECH acceptance explicit (#422, #423)

   *  Relax computation of the acceptance signal (#420, #449)

   *  Simplify ClientHelloOuterAAD generation (#438, #442)

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   *  Allow empty enc in ECHClientHello (#444)

   *  Authenticate ECHClientHello extensions position in
      ClientHelloOuterAAD (#410)

   *  Allow clients to send a dummy PSK and early_data in
      ClientHelloOuter when applicable (#414, #415)

   *  Compress ECHConfigContents (#409)

   *  Validate ECHConfig.contents.public_name (#413, #456)

   *  Validate ClientHelloInner contents (#411)

   *  Note split-mode challenges for HRR (#418)

   *  Editorial improvements (#428, #432, #439, #445, #458, #455)

D.3.  Since draft-ietf-tls-esni-09

   *  Finalize HPKE dependency (#390)

   *  Move from client-computed to server-chosen, one-byte config
      identifier (#376, #381)

   *  Rename ECHConfigs to ECHConfigList (#391)

   *  Clarify some security and privacy properties (#385, #383)

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
   Cloudflare

   Email: caw@heapingbits.net

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