



Internet Engineering Task Force (IETF)                       E. Rescorla
Request for Comments: 9849                                   Independent
Category: Standards Track                               奥 一穂 (K. Oku)
ISSN: 2070-1721                                                   Fastly
                                                             N. Sullivan
                                             Cryptography Consulting LLC
                                                              C. A. Wood
                                                                   Apple
                                                              March 2026


                       TLS Encrypted Client Hello

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9849.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  Overview
     3.1.  Topologies
     3.2.  Encrypted ClientHello (ECH)
   4.  Encrypted ClientHello Configuration
     4.1.  Configuration Identifiers
     4.2.  Configuration Extensions
   5.  The "encrypted_client_hello" Extension
     5.1.  Encoding the ClientHelloInner
     5.2.  Authenticating the ClientHelloOuter
   6.  Client Behavior
     6.1.  Offering ECH
       6.1.1.  Encrypting the ClientHello
       6.1.2.  GREASE PSK
       6.1.3.  Recommended Padding Scheme
       6.1.4.  Determining ECH Acceptance
       6.1.5.  Handshaking with ClientHelloInner
       6.1.6.  Handshaking with ClientHelloOuter
       6.1.7.  Authenticating for the Public Name
       6.1.8.  Impact of Retry on Future Connections
     6.2.  GREASE ECH
       6.2.1.  Client Greasing
       6.2.2.  Server Greasing
   7.  Server Behavior
     7.1.  Client-Facing Server
       7.1.1.  Processing ClientHello after HelloRetryRequest
     7.2.  Backend Server
       7.2.1.  Sending HelloRetryRequest
   8.  Deployment Considerations
     8.1.  Compatibility Issues
       8.1.1.  Misconfiguration and Deployment Concerns
       8.1.2.  Middleboxes
     8.2.  Deployment Impact
   9.  Compliance Requirements
   10. Security Considerations
     10.1.  Security and Privacy Goals
     10.2.  Unauthenticated and Plaintext DNS
     10.3.  Client Tracking
     10.4.  Ignored Configuration Identifiers and Trial Decryption
     10.5.  Outer ClientHello
     10.6.  Inner ClientHello
     10.7.  Related Privacy Leaks
     10.8.  Cookies
     10.9.  Attacks Exploiting Acceptance Confirmation
     10.10. Comparison Against Criteria
       10.10.1.  Mitigate Cut-and-Paste Attacks
       10.10.2.  Avoid Widely Shared Secrets
       10.10.3.  SNI-Based Denial-of-Service Attacks
       10.10.4.  Do Not Stick Out
       10.10.5.  Maintain Forward Secrecy
       10.10.6.  Enable Multi-party Security Contexts
       10.10.7.  Support Multiple Protocols
     10.11. Padding Policy
     10.12. Active Attack Mitigations
       10.12.1.  Client Reaction Attack Mitigation
       10.12.2.  HelloRetryRequest Hijack Mitigation
       10.12.3.  ClientHello Malleability Mitigation
       10.12.4.  ClientHelloInner Packet Amplification Mitigation
   11. IANA Considerations
     11.1.  Update of the TLS ExtensionType Registry
     11.2.  Update of the TLS Alert Registry
     11.3.  ECH Configuration Extension Registry
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Appendix A.  Linear-Time Outer Extension Processing
   Acknowledgements
   Authors' Addresses

1.  Introduction

   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 information left unencrypted in TLS 1.3.

   This document specifies a new TLS extension called Encrypted Client
   Hello (ECH) that allows clients to encrypt their ClientHello to the
   TLS server.  This protects the SNI and other potentially sensitive
   fields, such as the Application-Layer Protocol Negotiation (ALPN)
   list [RFC7301].  Co-located servers with consistent externally
   visible TLS configurations and behavior, including supported versions
   and cipher suites and how they respond to incoming client
   connections, form an anonymity set.  (Note that implementation-
   specific choices, such as extension ordering within TLS messages or
   division of data into record-layer boundaries, can result in
   different externally visible behavior, even for servers with
   consistent TLS configurations.)  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.
   Deployment implications of this feature are discussed in Section 8.

   ECH is not in itself sufficient to protect the identity of the
   server.  The target domain may also be visible through other
   channels, such as plaintext client DNS queries or visible server IP
   addresses.  However, encrypted DNS mechanisms such as DNS over HTTPS
   [RFC8484], DNS over TLS/DTLS [RFC7858] [RFC8094], and DNS over QUIC
   [RFC9250] provide mechanisms for clients to conceal DNS lookups from
   network inspection, and many TLS servers host multiple domains on the
   same IP address.  Private origins may also be deployed behind a
   common provider, such as a reverse proxy.  In such environments, the
   SNI remains the primary explicit signal available to observers to
   determine the server's identity.

   ECH is supported in TLS 1.3 [RFC8446], DTLS 1.3 [RFC9147], and newer
   versions of the TLS and DTLS protocols.

2.  Conventions and Definitions

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

3.  Overview

   This protocol is designed to operate in one of two topologies
   illustrated below, which we call "Shared Mode" and "Split Mode".
   These modes are described in the following section.

3.1.  Topologies

                   +---------------------+
                   |                     |
                   |   2001:DB8::1111    |
                   |                     |
   Client <----->  | private.example.org |
                   |                     |
                   | public.example.com  |
                   |                     |
                   +---------------------+
                           Server
             (Client-Facing and Backend Combined)

                       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.org |
              |                    |     |                     |
              +--------------------+     +---------------------+
               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, the service provider does not have access to the
   plaintext of the connection beyond the unencrypted portions of the
   handshake.

   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.

   See Section 10 for more discussion about the ECH threat model and how
   it relates to the client, client-facing server, and backend server.

3.2.  Encrypted ClientHello (ECH)

   A client-facing server enables ECH by publishing an ECH
   configuration, which is an encryption public key and associated
   metadata.  Domains which wish to use ECH must publish this
   configuration, using the key associated with the client-facing
   server.  This document defines the ECH configuration's format, but
   delegates DNS publication details to [RFC9460].  See [RFC9848] for
   specifics about how ECH configurations are advertised in SVCB and
   HTTPS records.  Other delivery mechanisms are also possible.  For
   example, the client may have the ECH configuration preconfigured.

   When a client wants to establish a TLS session with some backend
   server, it constructs a private ClientHello, referred to as the
   ClientHelloInner.  The client then constructs a public ClientHello,
   referred to as the ClientHelloOuter.  The ClientHelloOuter contains
   innocuous values for sensitive extensions and an
   "encrypted_client_hello" extension (Section 5), which carries the
   encrypted ClientHelloInner.  Finally, the client sends
   ClientHelloOuter to the server.

   The server takes one of the following actions:

   1.  If it does not support ECH or cannot decrypt the extension, it
       completes the handshake with ClientHelloOuter.  This is referred
       to as rejecting ECH.

   2.  If it successfully decrypts the extension, it forwards the
       ClientHelloInner to the backend server, which completes the
       handshake.  This is referred to as accepting ECH.

   Upon receiving the server's response, the client determines whether
   or not ECH was accepted (Section 6.1.4) and proceeds with the
   handshake accordingly.  When ECH is rejected, the resulting
   connection is not usable by the client for application data.
   Instead, ECH rejection allows the client to retry with up-to-date
   configuration (Section 6.1.6).

   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 Hybrid Public Key Encryption (HPKE) for public key
   encryption [HPKE].  The ECH configuration is defined by the following
   ECHConfig structure.

       opaque HpkePublicKey<1..2^16-1>;
       uint16 HpkeKemId;              // Defined in RFC 9180
       uint16 HpkeKdfId;              // Defined in RFC 9180
       uint16 HpkeAeadId;             // Defined in RFC 9180
       uint16 ECHConfigExtensionType; // Defined in Section 11.3

       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 {
           ECHConfigExtensionType type;
           opaque data<0..2^16-1>;
       } ECHConfigExtension;

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

       struct {
           uint16 version;
           uint16 length;
           select (ECHConfig.version) {
             case 0xfe0d: ECHConfigContents contents;
           }
       } ECHConfig;

   The structure contains the following fields:

   version:  The version of ECH for which this configuration is used.
      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.  This length field
      allows implementations to skip over the elements in such a list
      where they cannot parse the specific version of ECHConfig.

   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 (an "ECH key").
      Note that this structure contains the config_id field, which
      applies to the entire ECHConfigContents.

   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 6.1.6.

      See Section 6.1.7 for how the client interprets and validates the
      public_name.

   extensions:  A list of ECHConfigExtension values that the client must
      take into consideration when generating a ClientHello message.
      Each ECHConfigExtension has a 2-octet type and opaque data value,
      where the data value is encoded with a 2-octet integer
      representing the length of the data, in network byte order.
      ECHConfigExtension values are described below (Section 4.2).

   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.  Section 4.1 describes how client-
      facing servers allocate this value.

   kem_id:  The HPKE Key Encapsulation Mechanism (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 Key Derivation Function (KDF) and
      Authenticated Encryption with Associated Data (AEAD) identifier
      pairs clients can use for encrypting ClientHelloInner.  See
      Section 6.1 for how clients choose from this list.

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

       ECHConfig ECHConfigList<4..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 Identifiers

   A client-facing server has a set of known ECHConfig values with
   corresponding private keys.  This set SHOULD contain the currently
   published values, as well as previous values that may still be in
   use, since clients may cache DNS records up to a TTL or longer.

   Section 7.1 describes a trial decryption process for decrypting the
   ClientHello.  This can impact performance when the client-facing
   server maintains many known ECHConfig values.  To avoid this, the
   client-facing server SHOULD allocate distinct config_id values for
   each ECHConfig in its known set.  The RECOMMENDED strategy is via
   rejection sampling, i.e., to randomly select config_id repeatedly
   until it does not match any known ECHConfig.

   It is not necessary for config_id values across different client-
   facing servers to be distinct.  A backend server may be hosted behind
   two different client-facing servers with colliding config_id values
   without any performance impact.  Values may also be reused if the
   previous ECHConfig is no longer in the known set.

4.2.  Configuration Extensions

   ECH configuration extensions are used to provide room for additional
   functionality as needed.  The format is as defined in Section 4 and
   mirrors Section 4.2 of [RFC8446].  However, ECH configuration
   extension types are maintained by IANA as described in Section 11.3.
   ECH configuration extensions follow the same interpretation rules as
   TLS extensions: extensions MAY appear in any order, but there MUST
   NOT be more than one extension of the same type in the extensions
   block.  Unlike TLS extensions, an extension can be tagged as
   mandatory by using an extension type codepoint with the high order
   bit set to 1.

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

   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 an
   enabler for authenticated key mismatch signals (see Section 7).  In
   contrast, the inner ClientHello is the true ClientHello used upon ECH
   negotiation.

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(0xfe0d), (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,
   which is included because TLS servers are not allowed to provide
   extensions in ServerHello which were not included in ClientHello.
   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.key_config.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 EncodedClientHelloInner
      structure, encrypted using HPKE as described in Section 6.1.

   When a client offers the outer version of an "encrypted_client_hello"
   extension, the server MAY include an "encrypted_client_hello"
   extension in its EncryptedExtensions message, as described in
   Section 7.1, 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 as described in Section 6.1.6.  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

   Before encrypting, the client pads and optionally compresses
   ClientHelloInner into an EncodedClientHelloInner structure, defined
   below:

       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.  In TLS, this field uses the ClientHello structure defined in
   Section 4.1.2 of [RFC8446].  In DTLS, it uses the ClientHello
   structure defined in Section 5.3 of [RFC9147].  This does not include
   the Handshake structure's four-byte header in TLS, nor the twelve-
   byte header in DTLS.  The zeros field MUST be all zeros of length
   length_of_padding (see Section 6.1.3).

   Repeating large extensions, such as "key_share" with post-quantum
   algorithms, between ClientHelloInner and ClientHelloOuter can lead to
   excessive size.  To reduce the size impact, the client MAY substitute
   extensions which it knows will be duplicated in ClientHelloOuter.  It
   does so by removing and replacing extensions from
   EncodedClientHelloInner with a single "ech_outer_extensions"
   extension, defined as follows:

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

       ExtensionType OuterExtensions<2..254>;

   OuterExtensions contains a list of the removed ExtensionType values.
   Each value references the matching extension in ClientHelloOuter.
   The values MUST be ordered contiguously in ClientHelloInner, and the
   "ech_outer_extensions" extension MUST be inserted in the
   corresponding position in EncodedClientHelloInner.  Additionally, the
   extensions MUST appear in ClientHelloOuter in the same relative
   order.  However, there is no requirement that they be contiguous.
   For example, OuterExtensions may contain extensions A, B, and C,
   while ClientHelloOuter contains extensions A, D, B, C, E, and F.

   The "ech_outer_extensions" extension can only be included in
   EncodedClientHelloInner and MUST NOT appear in either
   ClientHelloOuter or ClientHelloInner.

   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.

   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.

   *  Any extension is referenced in OuterExtensions more than once.

   *  "encrypted_client_hello" is referenced in OuterExtensions.

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

   These requirements prevent an attacker from performing a packet
   amplification attack by crafting a ClientHelloOuter which
   decompresses to a much larger ClientHelloInner.  This is discussed
   further in Section 10.12.4.

   Receiving implementations SHOULD construct the ClientHelloInner in
   linear time.  Quadratic time implementations (such as may happen via
   naive copying) create a denial-of-service risk.  Appendix A 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 ClientHelloOuter
   while keeping the same encrypted ClientHelloInner (see
   Section 10.12.3), 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] for TLS
   and Section 5.3 of [RFC9147] for DTLS, which matches the
   ClientHelloOuter except that 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
   Handshake structure's four-byte header in TLS nor the twelve-byte
   header in DTLS.

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 Generate Random Extensions And Sustain Extensibility
   (GREASE) [RFC8701] ECH extension, as described in Section 6.2.  The
   client offers ECH if it is in possession of a compatible ECH
   configuration and sends GREASE ECH (see Section 6.2) otherwise.
   Clients of the latter type do not negotiate ECH; instead, they
   generate a dummy ECH extension that is ignored by the server.  (See
   Section 10.10.4 for an explanation.)  It is also possible for clients
   to always send GREASE ECH without implementing the remainder of this
   specification.

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.key_config.kem_id, at least one KDF/AEAD algorithm
   identified by ECHConfig.contents.key_config.cipher_suites, and the
   version of ECH indicated by ECHConfig.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.)

   The client then constructs EncodedClientHelloInner as described in
   Section 5.1.  It also computes an HPKE encryption context and enc
   value as:

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

   Next, it constructs a partial ClientHelloOuterAAD 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.  Note that compatibility mode is not used in DTLS
       1.3, but following this rule will produce the correct results for
       both TLS 1.3 and DTLS 1.3.

   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 10.12.1.)

   5.  It SHOULD place the value of ECHConfig.contents.public_name in
       the "server_name" extension.  Clients that do not follow this
       step, or place a different value in the "server_name" extension,
       risk breaking the retry mechanism described in Section 6.1.6 or
       failing to interoperate with servers that require this step to be
       done; see Section 7.1.

   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 10.12.3.)  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.

   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.

   Finally, the client encrypts the EncodedClientHelloInner with the
   above values, as described in Section 6.1.1, to construct a
   ClientHelloOuter.  It sends this to the server and processes the
   response as described in Section 6.1.4.

6.1.1.  Encrypting the ClientHello

   Given an EncodedClientHelloInner, an HPKE encryption context and enc
   value, and a partial ClientHelloOuterAAD, the client constructs a
   ClientHelloOuter as follows.

   First, 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 then completes the ClientHelloOuterAAD with an
   "encrypted_client_hello" extension.  This extension value contains
   the outer variant of ECHClientHello with the following fields:

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

   *  cipher_suite, the client's chosen cipher suite;

   *  enc, as given above; and

   *  payload, a placeholder byte string containing L zeros.

   If configuration identifiers (see Section 10.4) are to be ignored,
   config_id SHOULD be set to a randomly generated byte in the first
   ClientHelloOuter and, in the event of a HelloRetryRequest (HRR), MUST
   be left unchanged for the second ClientHelloOuter.

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

       final_payload = context.Seal(ClientHelloOuterAAD,
                                    EncodedClientHelloInner)

   Including ClientHelloOuterAAD as the HPKE AAD binds the
   ClientHelloOuter to the ClientHelloInner, thus preventing attackers
   from modifying ClientHelloOuter while keeping the same
   ClientHelloInner, as described in Section 10.12.3.

   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 server were sending this extension
   without solicitation, which would violate the extension rules
   described in [RFC8446].  When offering a PSK in ClientHelloInner,
   clients SHOULD send a GREASE "pre_shared_key" extension in the
   ClientHelloOuter to make 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

   If the ClientHelloInner is encrypted without padding, then the length
   of the ClientHelloOuter.payload can leak information about
   ClientHelloInner.  In order to prevent this, the
   EncodedClientHelloInner structure has a padding field.  This section
   describes a deterministic mechanism for computing the required amount
   of padding 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.

   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 M is
   the maximum_name_length value.

   1.  If the ClientHelloInner contained a "server_name" extension with
       a name of length D, add max(0, M - 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 M + 9
       bytes of padding.  This is the length of a "server_name"
       extension with an M-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.  Determining ECH Acceptance

   As described in Section 7, the server may either accept ECH and use
   ClientHelloInner or reject it and use ClientHelloOuter.  This is
   determined by the server's initial message.

   If the message does not negotiate TLS 1.3 or higher, the server has
   rejected ECH.  Otherwise, the message will be either a ServerHello or
   a HelloRetryRequest.

   If the message is a ServerHello, the client computes
   accept_confirmation as described in Section 7.2.  If this value
   matches the last 8 bytes of ServerHello.random, the server has
   accepted ECH.  Otherwise, it has rejected ECH.

   If the message is a HelloRetryRequest, the client checks for the
   "encrypted_client_hello" extension.  If none is found, the server has
   rejected ECH.  Otherwise, if the extension has a length other than 8,
   the client MUST abort the handshake with a "decode_error" alert.
   Otherwise, the client computes hrr_accept_confirmation as described
   in Section 7.2.1.  If this value matches the extension payload, the
   server has accepted ECH.  Otherwise, it has rejected ECH.

   If the server accepts ECH, the client handshakes with
   ClientHelloInner as described in Section 6.1.5.  Otherwise, the
   client handshakes with ClientHelloOuter as described in
   Section 6.1.6.

6.1.5.  Handshaking with ClientHelloInner

   If the server accepts ECH, the client proceeds with the connection as
   in [RFC8446], with the following modifications:

   The client behaves as if it had sent ClientHelloInner as the
   ClientHello.  That is, it evaluates the handshake using the
   ClientHelloInner's preferences, and, when computing the transcript
   hash (Section 4.4.1 of [RFC8446]), it uses ClientHelloInner as the
   first ClientHello.

   If the server responds with a HelloRetryRequest, the client computes
   the updated ClientHello message as follows:

   1.  It computes a second ClientHelloInner based on the first
       ClientHelloInner, as in Section 4.1.4 of [RFC8446].  The
       ClientHelloInner's "encrypted_client_hello" extension is left
       unmodified.

   2.  It constructs EncodedClientHelloInner as described in
       Section 5.1.

   3.  It constructs a second partial ClientHelloOuterAAD message.  This
       message MUST be syntactically valid.  The extensions MAY be
       copied from the original ClientHelloOuter unmodified or omitted.
       If not sensitive, the client MAY copy updated extensions from the
       second ClientHelloInner for compression.

   4.  It encrypts EncodedClientHelloInner as described in
       Section 6.1.1, using the second partial ClientHelloOuterAAD, to
       obtain a second ClientHelloOuter.  It reuses the original HPKE
       encryption context computed in Section 6.1 and uses the empty
       string for enc.

       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 10.12.2.

   The client then sends the second ClientHelloOuter to the server.
   However, as above, it uses the second ClientHelloInner for
   preferences, and both the ClientHelloInner messages for the
   transcript hash.  Additionally, it checks the resulting ServerHello
   for ECH acceptance as in Section 6.1.4.  If the ServerHello does not
   also indicate ECH acceptance, the client MUST terminate the
   connection with an "illegal_parameter" alert.

6.1.6.  Handshaking with ClientHelloOuter

   If the server rejects ECH, the client proceeds with the handshake,
   authenticating for ECHConfig.contents.public_name as described in
   Section 6.1.7.  If authentication or the handshake fails, the client
   MUST return a failure to the calling application.  It MUST NOT use
   the retry configurations.  It MUST NOT treat this as a secure signal
   to disable ECH.

   If the server supplied an "encrypted_client_hello" extension in its
   EncryptedExtensions message, the client MUST check that it is
   syntactically valid and the client MUST abort the connection with a
   "decode_error" alert otherwise.  If an earlier TLS version was
   negotiated, the client MUST NOT enable the False Start optimization
   [RFC7918] for this handshake.  If both authentication and the
   handshake complete successfully, the client MUST perform the
   processing described below and then abort the connection with an
   "ech_required" alert before sending any application data to the
   server.

   If the server provided "retry_configs" and if at least one of the
   values contains a version supported by the client, the client can
   regard the ECH configuration as securely replaced by the server.  It
   SHOULD retry the handshake with a new transport connection using the
   retry configurations supplied by the server.

   Because the new ECH configuration replaces the old ECH configuration,
   clients can implement a new transport connection in any way that is
   consistent with the previous ECH configuration.  For example, clients
   can reuse the same server IP address when establishing the new
   transport connection or they can choose to use a different IP address
   if DNS provided other IP addresses for the previous configuration.
   However, it is not safe to use IP addresses discovered with a new DNS
   query, as those may correspond to a different ECH server
   configuration, for instance associated with a different ECH server
   with a different public_name.

   The retry configurations are meant to be used for retried
   connections.  Further use of retry configurations could yield a
   tracking vector.  In settings where the client will otherwise already
   let the server track the client, e.g., because the client will send
   cookies to the server in parallel connections, using the retry
   configurations for these parallel connections does not introduce a
   new tracking vector.

   If none of the values provided in "retry_configs" contains a
   supported version, the server did not supply an
   "encrypted_client_hello" extension in its EncryptedExtensions
   message, or an earlier TLS version was negotiated, the client can
   regard ECH as securely disabled by the server, and it SHOULD retry
   the handshake with a new transport connection and ECH disabled.

   Clients SHOULD NOT accept "retry_config" in response to a connection
   initiated in response to a "retry_config".  Sending a "retry_config"
   in this situation is a signal that the server is misconfigured, e.g.,
   the server might have multiple inconsistent configurations so that
   the client reached a node with configuration A in the first
   connection and a node with configuration B in the second.  Note that
   this guidance does not apply to the cases in the previous paragraph
   where the server has securely disabled ECH.

   If a client does not retry, it MUST report an error to the calling
   application.

6.1.7.  Authenticating for the Public Name

   When the server rejects ECH, 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
   [RFC9525].  Clients that incorporate DNS names and IP addresses into
   the same syntax (e.g. Section 7.4 of [RFC3986] and [WHATWG-IPV4])
   MUST reject names that would be interpreted as IPv4 addresses.
   Clients that enforce this by checking ECHConfig.contents.public_name
   do not need to repeat the check when processing ECH rejection.

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

   Prior to attempting a connection, a client SHOULD validate the
   ECHConfig.contents.public_name.  Clients SHOULD ignore any ECHConfig
   structure with a public_name that is not a valid host name in
   preferred name syntax (see Section 2 of [DNS-TERMS]).  That is, to be
   valid, the public_name needs to be a dot-separated sequence of LDH
   labels, as defined in Section 2.3.1 of [RFC5890], where:

   *  the sequence does not begin or end with an ASCII dot, and

   *  all labels are at most 63 octets.

   Clients additionally SHOULD ignore the structure if the final LDH
   label either consists of all ASCII digits (i.e., '0' through '9') or
   is "0x" or "0X" followed by some, possibly empty, sequence of ASCII
   hexadecimal digits (i.e., '0' through '9', 'a' through 'f', and 'A'
   through 'F').  This avoids public_name values that may be interpreted
   as IPv4 literals.

6.1.8.  Impact of Retry on Future Connections

   Clients MAY use information learned from a rejected ECH for future
   connections to avoid repeatedly connecting to the same server and
   being forced to retry.  However, they MUST handle ECH rejection for
   those connections as if it were a fresh connection, rather than
   enforcing the single retry limit from Section 6.1.6.  The reason for
   this requirement is that if the server sends a "retry_config" and
   then immediately rejects the resulting connection, it is most likely
   misconfigured.  However, if the server sends a "retry_config" and
   then the client tries to use that to connect some time later, it is
   possible that the server has changed its configuration again and is
   now trying to recover.

   Any persisted information MUST be associated with the ECHConfig
   source used to bootstrap the connection, such as a DNS SVCB
   ServiceMode record [RFC9848].  Clients MUST limit any sharing of
   persisted ECH-related state to connections that use the same
   ECHConfig source.  Otherwise, it might become possible for the client
   to have the wrong public name for the server, making recovery
   impossible.

   ECHConfigs learned from ECH rejection can be used as a tracking
   vector.  Clients SHOULD impose the same lifetime and scope
   restrictions that they apply to other server-based tracking vectors
   such as PSKs.

   In general, the safest way for clients to minimize ECH retries is to
   comply with any freshness rules (e.g., DNS TTLs) imposed by the ECH
   configuration.

6.2.  GREASE ECH

   The GREASE ECH mechanism allows a connection between an ECH-capable
   client and a non-ECH server to appear to use ECH, thus reducing the
   extent to which ECH connections stick out (see Section 10.10.4).

6.2.1.  Client Greasing

   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, so that all
      plausible configurations are exercised, but MAY be held constant
      for successive connections to the same server in the same session.
      Note: A "plausible" configuration is one that an observer might
      expect to see.  A client that fully supports ECH will have a set
      of supported HPKE cipher suites to select from.  A client that
      only supports GREASE ECH has no such list, so it should select
      from a set of values that are in common usage.

   *  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.

   If sending a second ClientHello in response to a HelloRetryRequest,
   the client copies the entire "encrypted_client_hello" extension from
   the first ClientHello.  The identical value will reveal to an
   observer that the value of "encrypted_client_hello" was fake, but
   this only occurs if there is a HelloRetryRequest.

   If the server sends an "encrypted_client_hello" extension in either
   HelloRetryRequest or EncryptedExtensions, 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.  It MUST NOT save the "retry_configs" value in
   EncryptedExtensions.

   Offering a GREASE extension is not considered offering an encrypted
   ClientHello for purposes of requirements in Section 6.1.  In
   particular, the client MAY offer to resume sessions established
   without ECH.

6.2.2.  Server Greasing

   Section 11.3 describes a set of Reserved extensions which will never
   be registered.  These can be used by servers to "grease" the contents
   of the ECH configuration, as inspired by [RFC8701].  This helps
   ensure clients process ECH extensions correctly.  When constructing
   ECH configurations, servers SHOULD randomly select from reserved
   values with the high-order bit clear.  Correctly implemented clients
   will ignore those extensions.

   The reserved values with the high-order bit set are mandatory, as
   defined in Section 4.2.  Servers SHOULD randomly select from these
   values and include them in extraneous ECH configurations.  Correctly
   implemented clients will ignore these configurations because they do
   not recognize the mandatory extension.  Servers SHOULD ensure that
   any client using these configurations encounters a warning or error
   message.  This can be accomplished in several ways, including:

   *  By giving the extraneous configurations distinctive config IDs or
      public names, and rejecting the TLS connection or inserting an
      application-level warning message when these are observed.

   *  By giving the extraneous configurations an invalid public key and
      a public name not associated with the server so that the initial
      ClientHelloOuter will not be decryptable and the server cannot
      perform the recovery flow described in Section 6.1.6.

7.  Server Behavior

   As described in Section 3.1, servers can play two roles, either as
   the client-facing server or as the backend server.  Depending on the
   server role, the ECHClientHello will be different:

   *  A client-facing server expects an ECHClientHello.type of outer,
      and proceeds as described in Section 7.1 to extract a
      ClientHelloInner, if available.

   *  A backend server expects an ECHClientHello.type of inner, and
      proceeds as described in Section 7.2.

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

   In split mode, a client-facing server which receives a ClientHello
   with ECHClientHello.type of inner MUST abort with an
   "illegal_parameter" alert.  Similarly, in split mode, a backend
   server which receives a ClientHello with ECHClientHello.type of outer
   MUST abort with an "illegal_parameter" alert.

   In shared mode, a server plays both roles, first decrypting the
   ClientHelloOuter and then using the contents of the ClientHelloInner.
   A shared mode server which receives a ClientHello with
   ECHClientHello.type of inner MUST abort with an "illegal_parameter"
   alert, because such a ClientHello should never be received directly
   from the network.

   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.

   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 profile or otherwise externally
   configured, implementations MUST use the first method.

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

   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.

   Once the server has chosen the correct ECHConfig, it MAY verify that
   the value in the ClientHelloOuter "server_name" extension matches the
   value of ECHConfig.contents.public_name and abort with an
   "illegal_parameter" alert if these do not match.  This optional check
   allows the server to limit ECH connections to only use the public SNI
   values advertised in its ECHConfigs.  The server MUST be careful not
   to unnecessarily reject connections if the same ECHConfig id or
   keypair is used in multiple ECHConfigs with distinct public names.

   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.

   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 with the following
   modifications:

   *  If sending a HelloRetryRequest, the server MAY include an
      "encrypted_client_hello" extension with a payload of 8 random
      bytes; see Section 10.10.4 for details.

   *  If the server is configured with any ECHConfigs, it 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 8.1.1).  Instead, servers
   can measure occurrences of the "ech_required" alert to detect this
   case.

7.1.1.  Processing ClientHello after 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 that 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)

   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.  Moreover, if the server is configured with any
   ECHConfigs, it 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, as described in
   Section 7.1.

   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)

   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.  In DTLS, the
   modified version of HKDF-Expand-Label defined in [RFC9147],
   Section 5.9 is used instead.

   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.  Deployment Considerations

   The design of ECH as specified in this document necessarily requires
   changes to client, client-facing server, and backend server.
   Coordination between client-facing and backend server requires care,
   as deployment mistakes can lead to compatibility issues.  These are
   discussed in Section 8.1.

   Beyond coordination difficulties, ECH deployments may also create
   challenges for uses of information that ECH protects.  In particular,
   use cases which depend on this unencrypted information may no longer
   work as desired.  This is elaborated upon in Section 8.2.

8.1.  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.

   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.

8.1.1.  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 TLS server
   has a certificate for the public name.  If server and advertised keys
   mismatch, the server will reject ECH and respond with
   "retry_configs".  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 6.1.6.

   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.

   In order to ensure that the retry mechanism works successfully,
   servers SHOULD ensure that every endpoint which might receive a TLS
   connection is provisioned with an appropriate certificate for the
   public name.  This is especially important during periods of server
   reconfiguration when different endpoints might have different
   configurations.

8.1.2.  Middleboxes

   The requirements in [RFC8446], Section 9.3 which require proxies to
   act as conforming TLS client and server provide interoperability with
   TLS-terminating proxies even in cases where the server supports ECH
   but the proxy does not, as detailed below.

   The proxy must ignore unknown parameters and generate its own
   ClientHello containing only parameters it understands.  Thus, when
   presenting a certificate to the client or sending a ClientHello to
   the server, the proxy will act as if connecting to the
   ClientHelloOuter server_name, which SHOULD match the public name (see
   Section 6.1) without echoing the "encrypted_client_hello" extension.

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

8.2.  Deployment Impact

   Some use cases which depend on information ECH encrypts may break
   with the deployment of ECH.  The extent of breakage depends on a
   number of external factors, including, for example, whether ECH can
   be disabled, whether or not the party disabling ECH is trusted to do
   so, and whether or not client implementations will fall back to TLS
   without ECH in the event of disablement.

   Depending on implementation details and deployment settings, use
   cases which depend on plaintext TLS information may require
   fundamentally different approaches to continue working.  For example,
   in managed enterprise settings, one approach may be to disable ECH
   entirely via group policy and for client implementations to honor
   this action.  Server deployments which depend on SNI -- e.g., for
   load balancing -- may no longer function properly without updates;
   the nature of those updates is out of scope of this specification.

   In the context of Section 6.1.6, another approach may be to intercept
   and decrypt client TLS connections.  The feasibility of alternative
   solutions is specific to individual deployments.

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 Section 7.1 of [HPKE])

   *  KDF: HKDF-SHA256 (see Section 7.2 of [HPKE])

   *  AEAD: AES-128-GCM (see Section 7.3 of [HPKE])

10.  Security Considerations

   This section contains security considerations for ECH.

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 [RFC3552] for TLS 1.3 [RFC8446].

   Passive and active attackers can exist anywhere in the network,
   including between the client and client-facing server, as well as
   between the client-facing and backend servers when running ECH in
   split mode.  However, for split mode in particular, ECH makes two
   additional assumptions:

   1.  The channel between each client-facing and each backend server is
       authenticated such that the backend server only accepts messages
       from trusted client-facing servers.  The exact mechanism for
       establishing this authenticated channel is out of scope for this
       document.

   2.  The attacker cannot correlate messages between a client and
       client-facing server with messages between client-facing and
       backend server.  Such correlation could allow an attacker to link
       information unique to a backend server, such as their server name
       or IP address, with a client's encrypted ClientHelloInner.
       Correlation could occur through timing analysis of messages
       across the client-facing server, or via examining the contents of
       messages sent between client-facing and backend servers.  The
       exact mechanism for preventing this sort of correlation is out of
       scope for this document.

   Given this threat model, the primary goals of ECH are as follows.

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

   2.  Handshake privacy.  TLS connection establishment to a server name
       within an anonymity set is indistinguishable from a connection to
       any other server name within the anonymity set.  (The anonymity
       set is defined in Section 1.)

   3.  Downgrade resistance.  An attacker cannot downgrade a connection
       that attempts to use ECH to one that does not use ECH.

   These properties were formally proven in [ECH-Analysis].

   With regards to handshake privacy, client-facing server configuration
   determines the size of the anonymity set.  For example, if a client-
   facing server uses distinct ECHConfig values for each server name,
   then each anonymity set has size k = 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 server names as possible.  An attacker can
   distinguish two server names that have different ECHConfig values
   based on the ECHClientHello.config_id value.

   This also means public information in a TLS handshake should be
   consistent across server names.  For example, if a client-facing
   server services many backend origin server names, only one of which
   supports some cipher suite, it may be possible to identify that
   server name based on the contents of the unencrypted handshake
   message.  Similarly, if a backend origin reuses KeyShare values, then
   that provides a unique identifier for that server.

   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 10.10.4 for details.

10.2.  Unauthenticated and Plaintext DNS

   ECH supports delivery of configurations through the DNS using SVCB or
   HTTPS records without requiring any verifiable authenticity or
   provenance information [RFC9848].  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
   configurations (so that the client encrypts data to them) or strip
   the ECH configurations 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 substitute a unique IP address for each DNS name that
   was looked up.  Thus, using DNS records without additional
   authentication 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 encrypted DNS transport is also a
   defense 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.

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 (this may
   not be possible if clients use the operating system resolver rather
   than doing their own resolution).

   ECHConfig rotation rate is also an issue for non-malicious servers,
   which may want to rotate keys frequently to limit exposure if the key
   is compromised.  Rotating too frequently limits the client anonymity
   set.  In practice, servers which service many server names and thus
   have high loads are the best candidates to be client-facing servers
   and so anonymity sets will typically involve many connections even
   with fairly fast rotation intervals.

10.4.  Ignored Configuration Identifiers and Trial Decryption

   Ignoring 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, ignoring
   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
   potential damage caused by such attacks.

   Unless specified by the application using (D)TLS or externally
   configured, client 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.

   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 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
   that 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 10.10.4.

10.6.  Inner ClientHello

   Values which depend on the contents of ClientHelloInner, such as the
   true server name, can influence how client-facing servers process
   this message.  In particular, timing side channels can reveal
   information about the contents of ClientHelloInner.  Implementations
   should take such side channels into consideration when reasoning
   about the privacy properties that ECH provides.

10.7.  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 Online Certificate
   Status Protocol (OCSP) or Certificate Revocation List (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.

   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.

10.8.  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 either by handling key rotation with trial decryption
   or by coordinating to match key names.

10.9.  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
   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.

10.10.  Comparison Against Criteria

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

10.10.1.  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.

10.10.2.  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
   of private keys by publishing different DNS records containing
   ECHConfig values with different public keys using a short TTL.

10.10.3.  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 transport
   connections an attacker can open.

10.10.4.  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, the 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.

   2.  The attacker keeps per-connection state only.  In particular, it
       does not track endpoints across connections.

   Moreover, real ECH and GREASE ECH are designed so that the following
   features do not noticeably vary to the attacker, i.e., they are not
   distinguishers:

   1.  the code points of extensions negotiated in the clear, and their
       order;

   2.  the length of messages; and

   3.  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;

   2.  the value of the outer SNI;

   3.  the TLS version negotiated, which may depend on ECH acceptance;

   4.  client authentication, which may depend on ECH acceptance; and

   5.  HRR issuance, which may depend on ECH acceptance.

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

10.10.5.  Maintain Forward Secrecy

   This design does not provide forward secrecy for the inner
   ClientHello 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 regularly.

10.10.6.  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
   allowing the backend origin server to hide behind the client-facing
   server without the client-facing server decrypting and reencrypting
   the connection.

   Conversely, if the DNS records used for configuration are
   authenticated, e.g., via DNSSEC, 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.

10.10.7.  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.

10.11.  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.

10.12.  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.

10.12.1.  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

   ClientHelloInner.random prevents this attack: because 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.

10.12.2.  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

   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.

10.12.3.  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.

         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.  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.  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.

10.12.4.  ClientHelloInner Packet Amplification Mitigation

   Client-facing servers must decompress EncodedClientHelloInners.  A
   malicious attacker may craft a packet which takes excessive resources
   to decompress or may be much larger than the incoming packet:

   *  If looking up a ClientHelloOuter extension takes time linear in
      the number of extensions, the overall decoding process would take
      O(M*N) time, where M is the number of extensions in
      ClientHelloOuter and N is the size of OuterExtensions.

   *  If the same ClientHelloOuter extension can be copied multiple
      times, an attacker could cause the client-facing server to
      construct a large ClientHelloInner by including a large extension
      in ClientHelloOuter of length L and an OuterExtensions list
      referencing N copies of that extension.  The client-facing server
      would then use O(N*L) memory in response to O(N+L) bandwidth from
      the client.  In split mode, an O(N*L)-sized packet would then be
      transmitted to the backend server.

   ECH mitigates this attack by requiring that OuterExtensions be
   referenced in order, that duplicate references be rejected, and by
   recommending that client-facing servers use a linear scan to perform
   decompression.  These requirements are detailed in Section 5.1.

11.  IANA Considerations

11.1.  Update of the TLS ExtensionType Registry

   IANA has created the following entries in the existing "TLS
   ExtensionType Values" registry (defined in [RFC8446]):

   1.  encrypted_client_hello (0xfe0d), with "TLS 1.3" column values set
       to "CH, HRR, EE", "DTLS-Only" column set to "N", and
       "Recommended" column set to "Y".

   2.  ech_outer_extensions (0xfd00), with the "TLS 1.3" column values
       set to "CH", "DTLS-Only" column set to "N", "Recommended" column
       set to "Y", and the "Comment" column set to "Only appears in
       inner CH."

11.2.  Update of the TLS Alert Registry

   IANA has created an entry, ech_required (121) in the existing "TLS
   Alerts" registry (defined in [RFC8446]), with the "DTLS-OK" column
   set to "Y".

11.3.  ECH Configuration Extension Registry

   IANA has created a new "TLS ECHConfig Extension" registry in a new
   "TLS Encrypted Client Hello (ECH) Configuration Extensions" registry
   group.  New registrations will list the following attributes:

   Value:  The two-byte identifier for the ECHConfigExtension, i.e., the
      ECHConfigExtensionType
   Extension Name:  Name of the ECHConfigExtension
   Recommended:  A "Y" or "N" value indicating if the TLS Working Group
      recommends that the extension be supported.  This column is
      assigned a value of "N" unless explicitly requested.  Adding a
      value of "Y" requires Standards Action [RFC8126].
   Reference:  The specification where the ECHConfigExtension is defined
   Notes:  Any notes associated with the entry

   New entries in the "TLS ECHConfig Extension" registry are subject to
   the Specification Required registration policy ([RFC8126],
   Section 4.6), with the policies described in [RFC9847], Section 16.
   IANA has added the following note to the "TLS ECHConfig Extension"
   registry:

   Note: The role of the designated expert is described in RFC 9847.
   The designated expert [RFC8126] ensures that the specification is
   publicly available.  It is sufficient to have an Internet-Draft (that
   is posted and never published as an RFC) or a document from another
   standards body, industry consortium, university site, etc.  The
   expert may provide more in-depth reviews, but their approval should
   not be taken as an endorsement of the extension.

   This document defines several Reserved values for ECH configuration
   extensions to be used for "greasing" as described in Section 6.2.2.

   The initial contents for this registry consists of multiple reserved
   values with the following attributes, which are repeated for each
   registration:

   Value:  0x0000, 0x1A1A, 0x2A2A, 0x3A3A, 0x4A4A, 0x5A5A, 0x6A6A,
      0x7A7A, 0x8A8A, 0x9A9A, 0xAAAA, 0xBABA, 0xCACA, 0xDADA, 0xEAEA,
      0xFAFA
   Extension Name:  RESERVED
   Recommended:  Y
   Reference:  RFC 9849
   Notes:  GREASE entries

12.  References

12.1.  Normative References

   [HPKE]     Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/info/rfc9180>.

   [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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/info/rfc9460>.

   [RFC9525]  Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              RFC 9525, DOI 10.17487/RFC9525, November 2023,
              <https://www.rfc-editor.org/info/rfc9525>.

   [RFC9847]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 9847, DOI 10.17487/RFC9847, December 2025,
              <https://www.rfc-editor.org/info/rfc9847>.

12.2.  Informative References

   [DNS-TERMS]
              Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

   [ECH-Analysis]
              Bhargavan, K., Cheval, V., and C. Wood, "A Symbolic
              Analysis of Privacy for TLS 1.3 with Encrypted Client
              Hello", CCS '22: Proceedings of the 2022 ACM SIGSAC
              Conference on Computer and Communications Security, pp.
              365-379, DOI 10.1145/3548606.3559360, November 2022,
              <https://www.cs.ox.ac.uk/people/vincent.cheval/publis/BCW-
              ccs22.pdf>.

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

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [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>.

   [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>.

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,
              <https://www.rfc-editor.org/info/rfc9250>.

   [RFC9848]  Schwartz, B., Bishop, M., and E. Nygren, "Bootstrapping
              TLS Encrypted ClientHello with DNS Service Bindings",
              RFC 9848, DOI 10.17487/RFC9848, March 2026,
              <https://www.rfc-editor.org/info/rfc9848>.

   [WHATWG-IPV4]
              WHATWG, "URL - IPv4 Parser", WHATWG Living Standard, May
              2021, <https://url.spec.whatwg.org/>.  Commit snapshot:
              <https://url.spec.whatwg.org/commit-
              snapshots/1b8b8c55eb4bed9f139c9a439fb1c1bf5566b619/#concept-
              ipv4-parser>.

Appendix A.  Linear-Time Outer Extension Processing

   The following procedure processes the "ech_outer_extensions"
   extension (see Section 5.1) in linear time, ensuring that each
   referenced extension in the ClientHelloOuter is included at most
   once:

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

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

       *  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.

Acknowledgements

   This document draws extensively from ideas in [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.

Authors' Addresses

   Eric Rescorla
   Independent
   Email: ekr@rtfm.com


   Kazuho Oku
   Fastly
   Email: kazuhooku@gmail.com

   Additional contact information:

      奥 一穂
      Fastly


   Nick Sullivan
   Cryptography Consulting LLC
   Email: nicholas.sullivan+ietf@gmail.com


   Christopher A. Wood
   Apple
   Email: caw@heapingbits.net
