diff --git a/caw/ech-config-extensions/draft-ietf-tls-esni.html b/caw/ech-config-extensions/draft-ietf-tls-esni.html new file mode 100644 index 00000000..52ee069f --- /dev/null +++ b/caw/ech-config-extensions/draft-ietf-tls-esni.html @@ -0,0 +1,3892 @@ + + + + + + +TLS Encrypted Client Hello + + + + + + + + + + + + + + + + + + + + + + + + + + +
Internet-DraftTLS Encrypted Client HelloOctober 2023
Rescorla, et al.Expires 11 April 2024[Page]
+
+
+
+
Workgroup:
+
tls
+
Internet-Draft:
+
draft-ietf-tls-esni-latest
+
Published:
+
+ +
+
Intended Status:
+
Standards Track
+
Expires:
+
+
Authors:
+
+
+
E. Rescorla
+
RTFM, Inc.
+
+
+
K. Oku
+
Fastly
+
+
+
N. Sullivan
+
Cloudflare
+
+
+
C. A. Wood
+
Cloudflare
+
+
+
+
+

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.

+
+
+

+Discussion Venues +

+

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

+

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

+
+
+
+

+Status of This Memo +

+

+ This Internet-Draft is submitted in full conformance with the + provisions of BCP 78 and BCP 79.

+

+ Internet-Drafts are working documents of the Internet Engineering Task + Force (IETF). Note that other groups may also distribute working + documents as Internet-Drafts. The list of current Internet-Drafts is + at https://datatracker.ietf.org/drafts/current/.

+

+ Internet-Drafts are draft documents valid for a maximum of six months + and may be updated, replaced, or obsoleted by other documents at any + time. It is inappropriate to use Internet-Drafts as reference + material or to cite them other than as "work in progress."

+

+ This Internet-Draft will expire on 11 April 2024.

+
+
+ +
+
+ ▲

+Table of Contents +

+ +
+
+
+
+

+1. Introduction +

+

DISCLAIMER: This draft is work-in-progress and has not yet seen significant (or +really any) security analysis. It should not be used as a basis for building +production systems. This published version of the draft has been designated +an "implementation draft" for testing and interop purposes.

+

Although TLS 1.3 [RFC8446] encrypts most of the handshake, including the +server certificate, there are several ways in which an on-path attacker can +learn private information about the connection. The plaintext Server Name +Indication (SNI) extension in ClientHello messages, which leaks the target +domain for a given connection, is perhaps the most sensitive, unencrypted +information in TLS 1.3.

+

The target domain may also be visible through other channels, such as plaintext +client DNS queries or visible server IP addresses. However, DoH [RFC8484] +and DPRIVE [RFC7858] [RFC8094] provide mechanisms for clients to conceal +DNS lookups from network inspection, and many TLS servers host multiple domains +on the same IP address. 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 used to determine the server's identity.

+

This document specifies a new TLS extension, called Encrypted Client Hello +(ECH), that allows clients to encrypt their ClientHello to such a deployment. +This protects the SNI and other potentially sensitive fields, such as the ALPN +list [RFC7301]. Co-located servers with consistent externally visible TLS +configurations, including supported versions and cipher suites, form an +anonymity set. 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.

+

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".

+
+
+

+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.com |
+           |                    |     |                     |
+           +--------------------+     +---------------------+
+            Client-Facing Server            Backend Server
+
+
+
Figure 2: +Split Mode Topology +
+
+

In Split Mode, the provider is not the origin server for private domains. +Rather, the DNS records for private domains point to the provider, and the +provider's server relays the connection back to the origin server, who +terminates the TLS connection with the client. Importantly, 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.

+
+
+
+
+

+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. The server must publish +this for all the domains it serves via Shared or Split Mode. This document +defines the ECH configuration's format, but delegates DNS publication details +to [HTTPS-RR]. 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. +
  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.

    +
    3. +
+

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 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 RFC9180
+    uint16 HpkeKdfId;  // Defined in RFC9180
+    uint16 HpkeAeadId; // Defined in RFC9180
+
+    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 ech_config_extension_type;
+        opaque extension_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. Beginning with +draft-08, the version is the same as the code point for the +"encrypted_client_hello" extension. Clients MUST ignore any ECHConfig +structure with a version they do not support.

+
+
+
length
+
+

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

+
+
+
+
+

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

+
+
+
+
+

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

+
+
+
+
+

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

+
+
+
extensions
+
+

A list of ECH configuration extensions that the client must take into +consideration when generating a ClientHello message. These are described below +(Section 4.2).

+
+
+
+

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

+

The HpkeKeyConfig structure contains the following fields:

+
+
config_id
+
+

A one-byte identifier for the given HPKE key configuration. This is used by +clients to indicate the key used for ClientHello encryption. Section 4.1 +describes how client-facing servers allocate this value.

+
+
+
kem_id
+
+

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

+
+
+
public_key
+
+

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

+
+
+
cipher_suites
+
+

The list of HPKE KDF and AEAD identifier pairs clients can use for encrypting +ClientHelloInner. 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<1..2^16-1>;
+
+
+

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

+
+
+

+4.1. Configuration 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. See Section 12 for guidance on +which types of extensions are appropriate for this structure.

+

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. 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.

+
+
+
+
+
+
+

+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. The outer +extension has the following fields:

+
+
config_id
+
+

The ECHConfigContents.key_config.config_id for the chosen ECHConfig.

+
+
+
cipher_suite
+
+

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

+
+
+
enc
+
+

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

+
+
+
payload
+
+

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

+
+
+
+

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 a 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. Note this field +uses the ClientHello structure, defined in Section 4.1.2 of [RFC8446] which +does not include the Handshake structure's four byte header. The zeros field +MUST be all zeroes.

+

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 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, C, while ClientHelloOuter +contains extensions A, D, B, C, E, 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 10.11.4.

+

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

+
+
+
+
+

+5.2. Authenticating the ClientHelloOuter +

+

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

+

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

+

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

+

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

+

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

+
+
+
+
+
+
+

+6. Client Behavior +

+

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

+
+
+

+6.1. Offering ECH +

+

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

+

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

+
    +
  1. +

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

    +
    2. +
  2. +

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

    +
    3. +
  3. +

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

    +
    4. +
  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.)

    +
    5. +
+

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.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. +
  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. +
  3. +

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

    +
    4. +
  4. +

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

    +
    5. +
  5. +

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

    +
    6. +
  6. +

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

    +
    7. +
  7. +

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

    +
    8. +
+

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

+

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 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)
+
+
+

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

+

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

+
+
+
+
+

+6.1.2. GREASE PSK +

+

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

+

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

+

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

+
+
+
+
+ +

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

+

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

+

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

+
    +
  1. +

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

    +
    2. +
  2. +

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

    +
    3. +
+

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. +
  2. +

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

    +
    3. +
+

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, it is either a ServerHello or 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 it has a length other than 8, the client aborts 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.

+

[[OPEN ISSUE: Depending on what we do for issue#450, it may be appropriate to +change the client behavior if the HRR extension is present but with the wrong +value.]]

+

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. +
  2. +

    It constructs EncodedClientHelloInner as described in Section 5.1.

    +
    3. +
  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. +
  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 10.11.2.

    +
    5. +
+

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 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 keys +as securely replaced by the server. It SHOULD retry the handshake with a new +transport connection, using the retry configurations supplied by the +server. The retry configurations may only be applied to the retry +connection. The client MUST NOT use retry configurations for connections beyond +the retry. This avoids introducing pinning concerns or a tracking vector, +should a malicious server present client-specific retry configurations in order +to identify the client in a subsequent ECH handshake.

+

If none of the values provided in "retry_configs" contains a supported version, +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 implement a limit on retries caused by receipt of "retry_configs" +or servers which do not acknowledge the "encrypted_client_hello" extension. If +the client does not retry in either scenario, it MUST report an error to the +calling application.

+
+
+
+
+

+6.1.7. 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. Clients that incorporate DNS +names and IP addresses into the same syntax (e.g. [RFC3986], Section 7.4 and +[WHATWG-IPV4]) MUST reject names that would be interpreted as IPv4 addresses. +Clients that enforce this by checking and rejecting encoded IPv4 addresses +in ECHConfig.contents.public_name do not need to repeat the check at this layer.

+

Note that authenticating a connection for the public name does not authenticate +it for the origin. The TLS implementation MUST NOT report such connections as +successful to the application. It additionally MUST ignore all session tickets +and session IDs presented by the server. These connections are only used to +trigger retries, as described in Section 6.1.6. This may be implemented, for +instance, by reporting a failed connection with a dedicated error code.

+
+
+
+
+
+
+

+6.2. GREASE ECH +

+

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

+
    +
  • +

    Set the config_id field to a random byte.

    +
    • +
  • +

    Set the cipher_suite field to a supported HpkeSymmetricCipherSuite. The +selection SHOULD vary to exercise all supported configurations, but MAY be +held constant for successive connections to the same server in the same +session.

    +
    • +
  • +

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

    +
    • +
  • +

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

    +
    • +
+

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.

+
+
+
+
+
+
+

+7. Server Behavior +

+

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

+ +

If 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. +
  2. +

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

    +
    3. +
+

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:

+

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

+

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

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

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

+

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

+

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 10.9.4 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). Instead, servers can measure occurrences of the +"ech_required" alert to detect this case.

+
+
+

+7.1.1. Sending HelloRetryRequest +

+

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

+

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

+

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

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

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.

+

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

+
+
+

+7.2.1. Sending HelloRetryRequest +

+

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

+

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

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

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

+
+
+
+
+
+
+
+
+

+8. Compatibility Issues +

+

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

+

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

+
+
+

+8.1. Misconfiguration and Deployment Concerns +

+

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

+

The retry mechanism repairs inconsistencies, provided the server is +authoritative for the public name. If server and advertised keys mismatch, the +server will 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.

+
+
+
+
+

+8.2. Middleboxes +

+

When connecting through a TLS-terminating proxy that does not support this +extension, [RFC8446], Section 9.3 requires the proxy still act as a +conforming TLS client and server. 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 public name, 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.

+
+
+
+
+
+
+

+9. Compliance Requirements +

+

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

+ +
+
+
+
+

+10. Security Considerations +

+
+
+

+10.1. Security and Privacy Goals +

+

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

+

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

+
    +
  1. +

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

    +
    2. +
  2. +

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

    +
    3. +
  3. +

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

    +
    4. +
+

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 host, 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 hosts as possible. An +attacker can distinguish two hosts that have different ECHConfig values based +on the ECHClientHello.config_id value. This also means public information in a +TLS handshake should be consistent across hosts. For example, if a +client-facing server services many backend origin hosts, only one of which +supports some cipher suite, it may be possible to identify that host based on +the contents of unencrypted handshake messages.

+

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

+
+
+
+
+

+10.2. Unauthenticated and Plaintext DNS +

+

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

+

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

+
+
+
+
+

+10.3. Client Tracking +

+

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

+

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

+
+
+
+
+

+10.4. 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, +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 a different ALPN lists to different servers or in different +application contexts. A client that treats this context as sensitive SHOULD NOT +send context-specific values in ClientHelloOuter.

+

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

+

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

+
+
+ +
+
+

+10.7. Cookies +

+

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

+

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

+

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

+
+
+
+
+

+10.8. Attacks Exploiting Acceptance Confirmation +

+

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

+

On the other hand, the probability that two 8-byte strings are the same is +non-negligible. This poses a modest operational risk. Suppose the client-facing +server terminates the connection (i.e., ECH is rejected or bypassed): if the +last 8 bytes of its ServerHello.random coincide with the confirmation signal, +then the client will incorrectly presume acceptance and proceed as if the +backend server terminated the connection. However, the probability of a false +positive occurring for a given connection is only 1 in 2^64. This value is +smaller than the probability of network connection failures in practice.

+

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

+
+
+
+
+

+10.9. Comparison Against Criteria +

+

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

+
+
+

+10.9.1. Mitigate Cut-and-Paste Attacks +

+

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

+
+
+
+
+

+10.9.2. Avoid Widely Shared Secrets +

+

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

+
+
+
+
+

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

+

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

+
+
+
+
+

+10.9.4. Do Not Stick Out +

+

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

+

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

+

Ensuring that networks do not differentiate between real ECH and GREASE ECH may +not be feasible for all implementations. While most middleboxes will not treat +them differently, some operators may wish to block real ECH usage but allow +GREASE ECH. This specification aims to provide a baseline security level that +most deployments can achieve easily, while providing implementations enough +flexibility to achieve stronger security where possible. Minimally, real ECH is +designed to be indifferentiable from GREASE ECH for passive adversaries with +following capabilities:

+
    +
  1. +

    The attacker does not know the ECHConfigList used by the server.

    +
    2. +
  2. +

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

    +
    3. +
  3. +

    ECH and GREASE ECH are designed so that the following features do not vary: +the code points of extensions negotiated in the clear; the length of +messages; and the values of plaintext alert messages.

    +
    4. +
+

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. +
  2. +

    the value of the outer SNI;

    +
    3. +
  3. +

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

    +
    4. +
  4. +

    client authentication, which may depend on ECH acceptance; and

    +
    5. +
  5. +

    HRR issuance, which may depend on ECH acceptance.

    +
    6. +
+

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

+
+
+
+
+

+10.9.5. Maintain Forward Secrecy +

+

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

+
+
+
+
+

+10.9.6. Enable Multi-party Security Contexts +

+

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

+

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

+

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

+
+
+
+
+

+10.9.7. Support Multiple Protocols +

+

This design has no impact on application layer protocol negotiation. It may +affect connection routing, server certificate selection, and client certificate +verification. Thus, it is compatible with multiple application and transport +protocols. By encrypting the entire ClientHello, this design additionally +supports encrypting the ALPN extension.

+
+
+
+
+
+
+

+10.10. Padding Policy +

+

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

+
+
+
+
+

+10.11. Active Attack Mitigations +

+

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

+
+
+

+10.11.1. Client Reaction Attack Mitigation +

+

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

+
+
+
+
+ Client                         Attacker               Server
+   ClientHello
+   + key_share
+   + ech         ------>      (intercept)     -----> X (drop)
+
+                             ServerHello
+                             + key_share
+                   {EncryptedExtensions}
+                   {CertificateRequest*}
+                          {Certificate*}
+                    {CertificateVerify*}
+                 <------
+   Alert
+                 ------>
+
+
+
Figure 3: +Client reaction attack +
+
+

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

+
+
+
+
+

+10.11.2. HelloRetryRequest Hijack Mitigation +

+

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

+
+
+
+
+ Client                         Attacker                   Server
+   ClientHello
+   + key_share
+   + ech         ------>       (forward)        ------->
+                                              HelloRetryRequest
+                                                    + key_share
+                              (intercept)       <-------
+
+                              ClientHello
+                              + key_share'
+                              + ech'           ------->
+                                                    ServerHello
+                                                    + key_share
+                                          {EncryptedExtensions}
+                                          {CertificateRequest*}
+                                                 {Certificate*}
+                                           {CertificateVerify*}
+                                                     {Finished}
+                                                <-------
+                         (process server flight)
+
+
+
Figure 4: +HelloRetryRequest hijack attack +
+
+

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

+

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

+
+
+
+
+

+10.11.3. ClientHello Malleability Mitigation +

+

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

+

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

+
+
+
+
+      Client              Attacker                       Server
+
+                                    handshake and ticket
+                                       for "example.com"
+                                       <-------->
+
+      ClientHello
+      + key_share
+      + ech
+      + ech_outer_extensions(pre_shared_key)
+      + pre_shared_key
+                  -------->
+                        (intercept)
+                        ClientHello
+                        + key_share
+                        + ech
+                           + ech_outer_extensions(pre_shared_key)
+                        + pre_shared_key'
+                                          -------->
+                                                         Alert
+                                                         -or-
+                                                   ServerHello
+                                                            ...
+                                                      Finished
+                                          <--------
+
+
+
Figure 5: +Message flow for malleable ClientHello +
+
+

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

+

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

+
+
+
+
+

+10.11.4. 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 is requested to create the following entries in the existing registry for +ExtensionType (defined in [RFC8446]):

+
    +
  1. +

    encrypted_client_hello(0xfe0d), with "TLS 1.3" column values set to +"CH, HRR, EE", and "Recommended" column set to "Yes".

    +
    2. +
  2. +

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

    +
    3. +
+
+
+
+
+

+11.2. Update of the TLS Alert Registry +

+

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

+
+
+
+
+

+11.3. ECH Configuration Extension Registry +

+

IANA is requested to create a new "ECHConfig Extension" registry in a new +"TLS Encrypted Client Hello (ECH) Configuration Extensions" page. New +registrations need to 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 extension is TLS WG recommends that the +extension be supported. This column is assigned a value of "N" unless +explicitly requested. Adding a value with 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 this registry are subject to the Specification Required +registration policy ([RFC8126], Section 4.6).

+

The registration policy for for the "ECHConfig Extension Type" registry +is Specification Required [RFC8126].

+
+
+
+
+
+
+

+12. ECHConfig Extension Guidance +

+

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

+
+
+
+

+13. References +

+
+
+

+13.1. Normative References +

+
+
[HPKE]
+
+Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180, , <https://www.rfc-editor.org/rfc/rfc9180>.
+
+
[HTTPS-RR]
+
+Schwartz, B. M., Bishop, M., and E. Nygren, "Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-dnsop-svcb-https-12, , <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-svcb-https-12>.
+
+
[RFC2119]
+
+Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
+
+
[RFC5890]
+
+Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, , <https://www.rfc-editor.org/rfc/rfc5890>.
+
+
[RFC7918]
+
+Langley, A., Modadugu, N., and B. Moeller, "Transport Layer Security (TLS) False Start", RFC 7918, DOI 10.17487/RFC7918, , <https://www.rfc-editor.org/rfc/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, , <https://www.rfc-editor.org/rfc/rfc8126>.
+
+
[RFC8174]
+
+Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
+
+
[RFC8446]
+
+Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <https://www.rfc-editor.org/rfc/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, , <https://www.rfc-editor.org/rfc/rfc9147>.
+
+
+
+
+
+
+

+13.2. Informative References +

+
+
[ECH-Analysis]
+
+"A Symbolic Analysis of Privacy for TLS 1.3 with Encrypted Client Hello", .
+
+
[EXPORTED-AUTHENTICATORS]
+
+Sullivan, N., "Exported Authenticators in TLS", RFC 9261, DOI 10.17487/RFC9261, , <https://www.rfc-editor.org/rfc/rfc9261>.
+
+
[I-D.kazuho-protected-sni]
+
+Oku, K., "TLS Extensions for Protecting SNI", Work in Progress, Internet-Draft, draft-kazuho-protected-sni-00, , <https://datatracker.ietf.org/doc/html/draft-kazuho-protected-sni-00>.
+
+
[RFC3986]
+
+Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <https://www.rfc-editor.org/rfc/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, , <https://www.rfc-editor.org/rfc/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, , <https://www.rfc-editor.org/rfc/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, , <https://www.rfc-editor.org/rfc/rfc7858>.
+
+
[RFC7924]
+
+Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", RFC 7924, DOI 10.17487/RFC7924, , <https://www.rfc-editor.org/rfc/rfc7924>.
+
+
[RFC8094]
+
+Reddy, T., Wing, D., and P. Patil, "DNS over Datagram Transport Layer Security (DTLS)", RFC 8094, DOI 10.17487/RFC8094, , <https://www.rfc-editor.org/rfc/rfc8094>.
+
+
[RFC8484]
+
+Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, , <https://www.rfc-editor.org/rfc/rfc8484>.
+
+
[RFC8701]
+
+Benjamin, D., "Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility", RFC 8701, DOI 10.17487/RFC8701, , <https://www.rfc-editor.org/rfc/rfc8701>.
+
+
[RFC8744]
+
+Huitema, C., "Issues and Requirements for Server Name Identification (SNI) Encryption in TLS", RFC 8744, DOI 10.17487/RFC8744, , <https://www.rfc-editor.org/rfc/rfc8744>.
+
+
[WHATWG-IPV4]
+
+"URL Living Standard - IPv4 Parser", , <https://url.spec.whatwg.org/#concept-ipv4-parser>.
+
+
+
+
+
+
+
+

+Appendix A. Alternative SNI Protection Designs +

+

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

+
+
+

+A.1. TLS-layer +

+
+
+

+A.1.1. TLS in Early Data +

+

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

+

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

+
+
+
+
+

+A.1.2. Combined Tickets +

+

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

+
+
+
+
+
+
+

+A.2. Application-layer +

+
+
+

+A.2.1. HTTP/2 CERTIFICATE Frames +

+

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

+
+
+
+
+
+
+
+
+

+Appendix B. Linear-time Outer Extension Processing +

+

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

+
    +
  1. +

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

    +
    2. +
  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.

      +
      • +
    +
    3. +
+
+
+
+
+

+Appendix C. Acknowledgements +

+

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

+
+
+
+
+

+Appendix D. Change Log +

+ +

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

+
+
+

+D.1. Since draft-ietf-tls-esni-16 +

+
    +
  • +

    Keep-alive

    +
    • +
+
+
+
+
+

+D.2. Since draft-ietf-tls-esni-15 +

+
    +
  • +

    Add CCS2022 reference and summary (#539)

    +
    • +
+
+
+
+
+

+D.3. Since draft-ietf-tls-esni-14 +

+
    +
  • +

    Keep-alive

    +
    • +
+
+
+
+
+

+D.4. Since draft-ietf-tls-esni-13 +

+
    +
  • +

    Editorial improvements

    +
    • +
+
+
+
+
+

+D.5. Since draft-ietf-tls-esni-12 +

+
    +
  • +

    Abort on duplicate OuterExtensions (#514)

    +
    • +
  • +

    Improve EncodedClientHelloInner definition (#503)

    +
    • +
  • +

    Clarify retry configuration usage (#498)

    +
    • +
  • +

    Expand on config_id generation implications (#491)

    +
    • +
  • +

    Server-side acceptance signal extension GREASE (#481)

    +
    • +
  • +

    Refactor overview, client implementation, and middlebox +sections (#480, #478, #475, #508)

    +
    • +
  • +

    Editorial iprovements (#485, #488, #490, #495, #496, #499, #500, +#501, #504, #505, #507, #510, #511)

    +
    • +
+
+
+
+
+

+D.6. Since draft-ietf-tls-esni-11 +

+
    +
  • +

    Move ClientHello padding to the encoding (#443)

    +
    • +
  • +

    Align codepoints (#464)

    +
    • +
  • +

    Relax OuterExtensions checks for alignment with RFC8446 (#467)

    +
    • +
  • +

    Clarify HRR acceptance and rejection logic (#470)

    +
    • +
  • +

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

    +
    • +
+
+
+
+
+

+D.7. Since draft-ietf-tls-esni-10 +

+
    +
  • +

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

    +
    • +
  • +

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

    +
    • +
  • +

    Simplify ClientHelloOuterAAD generation (#438, #442)

    +
    • +
  • +

    Allow empty enc in ECHClientHello (#444)

    +
    • +
  • +

    Authenticate ECHClientHello extensions position in ClientHelloOuterAAD (#410)

    +
    • +
  • +

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

    +
    • +
  • +

    Compress ECHConfigContents (#409)

    +
    • +
  • +

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

    +
    • +
  • +

    Validate ClientHelloInner contents (#411)

    +
    • +
  • +

    Note split-mode challenges for HRR (#418)

    +
    • +
  • +

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

    +
    • +
+
+
+
+
+

+D.8. Since draft-ietf-tls-esni-09 +

+
    +
  • +

    Finalize HPKE dependency (#390)

    +
    • +
  • +

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

    +
    • +
  • +

    Rename ECHConfigs to ECHConfigList (#391)

    +
    • +
  • +

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

    +
    • +
+
+
+
+
+
+
+

+Authors' Addresses +

+
+
Eric Rescorla
+
RTFM, Inc.
+ +
+
+
Kazuho Oku
+
Fastly
+ +
+
+
Nick Sullivan
+
Cloudflare
+ +
+
+
Christopher A. Wood
+
Cloudflare
+ +
+
+
+ + + diff --git a/caw/ech-config-extensions/draft-ietf-tls-esni.txt b/caw/ech-config-extensions/draft-ietf-tls-esni.txt new file mode 100644 index 00000000..bf4c6107 --- /dev/null +++ b/caw/ech-config-extensions/draft-ietf-tls-esni.txt @@ -0,0 +1,2238 @@ + + + + +tls E. Rescorla +Internet-Draft RTFM, Inc. +Intended status: Standards Track K. Oku +Expires: 11 April 2024 Fastly + N. Sullivan + C. A. Wood + Cloudflare + 9 October 2023 + + + TLS Encrypted Client Hello + draft-ietf-tls-esni-latest + +Abstract + + This document describes a mechanism in Transport Layer Security (TLS) + for encrypting a ClientHello message under a server public key. + +Discussion Venues + + This note is to be removed before publishing as an RFC. + + Source for this draft and an issue tracker can be found at + https://github.com/tlswg/draft-ietf-tls-esni + (https://github.com/tlswg/draft-ietf-tls-esni). + +Status of This Memo + + This Internet-Draft is submitted in full conformance with the + provisions of BCP 78 and BCP 79. + + Internet-Drafts are working documents of the Internet Engineering + Task Force (IETF). Note that other groups may also distribute + working documents as Internet-Drafts. The list of current Internet- + Drafts is at https://datatracker.ietf.org/drafts/current/. + + Internet-Drafts are draft documents valid for a maximum of six months + and may be updated, replaced, or obsoleted by other documents at any + time. It is inappropriate to use Internet-Drafts as reference + material or to cite them other than as "work in progress." + + This Internet-Draft will expire on 11 April 2024. + +Copyright Notice + + Copyright (c) 2023 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 + 6.2. GREASE ECH + 7. Server Behavior + 7.1. Client-Facing Server + 7.1.1. Sending HelloRetryRequest + 7.2. Backend Server + 7.2.1. Sending HelloRetryRequest + 8. Compatibility Issues + 8.1. Misconfiguration and Deployment Concerns + 8.2. Middleboxes + 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. Related Privacy Leaks + 10.7. Cookies + 10.8. Attacks Exploiting Acceptance Confirmation + 10.9. Comparison Against Criteria + 10.9.1. Mitigate Cut-and-Paste Attacks + 10.9.2. Avoid Widely Shared Secrets + 10.9.3. Prevent SNI-Based Denial-of-Service Attacks + 10.9.4. Do Not Stick Out + 10.9.5. Maintain Forward Secrecy + 10.9.6. Enable Multi-party Security Contexts + 10.9.7. Support Multiple Protocols + 10.10. Padding Policy + 10.11. Active Attack Mitigations + 10.11.1. Client Reaction Attack Mitigation + 10.11.2. HelloRetryRequest Hijack Mitigation + 10.11.3. ClientHello Malleability Mitigation + 10.11.4. 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. ECHConfig Extension Guidance + 13. References + 13.1. Normative References + 13.2. Informative References + Appendix A. Alternative SNI Protection Designs + A.1. TLS-layer + A.1.1. TLS in Early Data + A.1.2. Combined Tickets + A.2. Application-layer + A.2.1. HTTP/2 CERTIFICATE Frames + Appendix B. Linear-time Outer Extension Processing + Appendix C. Acknowledgements + Appendix D. Change Log + D.1. Since draft-ietf-tls-esni-16 + D.2. Since draft-ietf-tls-esni-15 + D.3. Since draft-ietf-tls-esni-14 + D.4. Since draft-ietf-tls-esni-13 + D.5. Since draft-ietf-tls-esni-12 + D.6. Since draft-ietf-tls-esni-11 + D.7. Since draft-ietf-tls-esni-10 + D.8. Since draft-ietf-tls-esni-09 + Authors' Addresses + +1. Introduction + + DISCLAIMER: This draft is work-in-progress and has not yet seen + significant (or really any) security analysis. It should not be used + as a basis for building production systems. This published version + of the draft has been designated an "implementation draft" for + testing and interop purposes. + + Although TLS 1.3 [RFC8446] encrypts most of the handshake, including + the server certificate, there are several ways in which an on-path + attacker can learn private information about the connection. The + plaintext Server Name Indication (SNI) extension in ClientHello + messages, which leaks the target domain for a given connection, is + perhaps the most sensitive, unencrypted information in TLS 1.3. + + The target domain may also be visible through other channels, such as + plaintext client DNS queries or visible server IP addresses. + However, DoH [RFC8484] and DPRIVE [RFC7858] [RFC8094] provide + mechanisms for clients to conceal DNS lookups from network + inspection, and many TLS servers host multiple domains on the same IP + address. 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 used to determine the server's + identity. + + This document specifies a new TLS extension, called Encrypted Client + Hello (ECH), that allows clients to encrypt their ClientHello to such + a deployment. This protects the SNI and other potentially sensitive + fields, such as the ALPN list [RFC7301]. Co-located servers with + consistent externally visible TLS configurations, including supported + versions and cipher suites, form an anonymity set. 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. + + 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". + +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.com | + | | | | + +--------------------+ +---------------------+ + Client-Facing Server Backend Server + + Figure 2: Split Mode Topology + + In Split Mode, the provider is not the origin server for private + domains. Rather, the DNS records for private domains point to the + provider, and the provider's server relays the connection back to the + origin server, who terminates the TLS connection with the client. + Importantly, 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. + +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. The server must publish this for all the domains it serves + via Shared or Split Mode. This document defines the ECH + configuration's format, but delegates DNS publication details to + [HTTPS-RR]. 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 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 RFC9180 + uint16 HpkeKdfId; // Defined in RFC9180 + uint16 HpkeAeadId; // Defined in RFC9180 + + 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 ech_config_extension_type; + opaque extension_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. + Beginning with draft-08, the version is the same as the code point + for the "encrypted_client_hello" extension. Clients MUST ignore + any ECHConfig structure with a version they do not support. + + length The length, in bytes, of the next field. 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. 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. + + Clients MUST ignore any ECHConfig structure whose public_name is + not parsable as a dot-separated sequence of LDH labels, as defined + in [RFC5890], Section 2.3.1 or which begins or end with an ASCII + dot. + + Clients SHOULD ignore the ECHConfig if it contains an encoded IPv4 + address. To determine if a public_name value is an IPv4 address, + clients can invoke the IPv4 parser algorithm in [WHATWG-IPV4]. It + returns a value when the input is an IPv4 address. + + See Section 6.1.7 for how the client interprets and validates the + public_name. + + extensions A list of ECH configuration extensions that the client + must take into consideration when generating a ClientHello + message. These are described below (Section 4.2). + + [[OPEN ISSUE: determine if clients should enforce a 63-octet label + limit for public_name]] [[OPEN ISSUE: fix reference to WHATWG-IPV4]] + + The HpkeKeyConfig structure contains the following fields: + + config_id A one-byte identifier for the given HPKE key + configuration. This is used by clients to indicate the key used + for ClientHello encryption. Section 4.1 describes how client- + facing servers allocate this value. + + kem_id The HPKE KEM identifier corresponding to public_key. Clients + MUST ignore any ECHConfig structure with a key using a KEM they do + not support. + + public_key The HPKE public key used by the client to encrypt + ClientHelloInner. + + cipher_suites The list of HPKE KDF and AEAD identifier pairs clients + can use for encrypting ClientHelloInner. 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<1..2^16-1>; + + The ECHConfigList structure contains one or more ECHConfig structures + in decreasing order of preference. This allows a server to support + multiple versions of ECH and multiple sets of ECH parameters. + +4.1. Configuration 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. See Section 12 for guidance on which types + of extensions are appropriate for this structure. + + 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. 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. + +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. + The outer extension has the following fields: + + config_id The ECHConfigContents.key_config.config_id for the chosen + ECHConfig. + + cipher_suite The cipher suite used to encrypt ClientHelloInner. + This MUST match a value provided in the corresponding + ECHConfigContents.cipher_suites list. + + enc The HPKE encapsulated key, used by servers to decrypt the + corresponding payload field. This field is empty in a + ClientHelloOuter sent in response to HelloRetryRequest. + + payload The serialized and encrypted ClientHelloInner structure, + encrypted using HPKE as described in Section 6.1. + + 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 a 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. Note this field uses the ClientHello structure, defined in + Section 4.1.2 of [RFC8446] which does not include the Handshake + structure's four byte header. The zeros field MUST be all zeroes. + + 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 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, C, while + ClientHelloOuter contains extensions A, D, B, C, E, 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 10.11.4. + + Implementations SHOULD bound the time to compute a ClientHelloInner + proportionally to the ClientHelloOuter size. If the cost is + disproportionately large, a malicious client could exploit this in a + denial of service attack. Appendix B describes a linear-time + procedure that may be used for this purpose. + +5.2. Authenticating the ClientHelloOuter + + To prevent a network attacker from modifying the reconstructed + ClientHelloInner (see Section 10.11.3), ECH authenticates + ClientHelloOuter by passing ClientHelloOuterAAD as the associated + data for HPKE sealing and opening operations. The + ClientHelloOuterAAD is a serialized ClientHello structure, defined in + Section 4.1.2 of [RFC8446], which matches the ClientHelloOuter except + the payload field of the "encrypted_client_hello" is replaced with a + byte string of the same length but whose contents are zeros. This + value does not include the four-byte header from the Handshake + structure. + + The client follows the procedure in Section 6.1.1 to first construct + ClientHelloOuterAAD with a placeholder payload field, then replace + the field with the encrypted value to compute ClientHelloOuter. + + The server then receives ClientHelloOuter and computes + ClientHelloOuterAAD by making a copy and replacing the portion + corresponding to the payload field with zeros. + + The payload and the placeholder strings have the same length, so it + is not necessary for either side to recompute length prefixes when + applying the above transformations. + + The decompression process in Section 5.1 forbids + "encrypted_client_hello" in OuterExtensions. This ensures the + unauthenticated portion of ClientHelloOuter is not incorporated into + ClientHelloInner. + +6. Client Behavior + + Clients that implement the ECH extension behave in one of two ways: + either they offer a real ECH extension, as described in Section 6.1; + or they send a GREASE ECH extension, as described in Section 6.2. + Clients of the latter type do not negotiate ECH. Instead, they + generate a dummy ECH extension that is ignored by the server. (See + Section 10.9.4 for an explanation.) The client offers ECH if it is + in possession of a compatible ECH configuration and sends GREASE ECH + otherwise. + +6.1. Offering ECH + + To offer ECH, the client first chooses a suitable ECHConfig from the + server's ECHConfigList. To determine if a given ECHConfig is + suitable, it checks that it supports the KEM algorithm identified by + ECHConfig.contents.kem_id, at least one KDF/AEAD algorithm identified + by ECHConfig.contents.cipher_suites, and the version of ECH indicated + by ECHConfig.contents.version. Once a suitable configuration is + found, the client selects the cipher suite it will use for + encryption. It MUST NOT choose a cipher suite or version not + advertised by the configuration. If no compatible configuration is + found, then the client SHOULD proceed as described in Section 6.2. + + Next, the client constructs the ClientHelloInner message just as it + does a standard ClientHello, with the exception of the following + rules: + + 1. It MUST NOT offer to negotiate TLS 1.2 or below. This is + necessary to ensure the backend server does not negotiate a TLS + version that is incompatible with ECH. + + 2. It MUST NOT offer to resume any session for TLS 1.2 and below. + + 3. If it intends to compress any extensions (see Section 5.1), it + MUST order those extensions consecutively. + + 4. It MUST include the "encrypted_client_hello" extension of type + inner as described in Section 5. (This requirement is not + applicable when the "encrypted_client_hello" extension is + generated as described in Section 6.2.) + + 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.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. + + 4. It MAY copy any other field from the ClientHelloInner except + ClientHelloInner.random. Instead, It MUST generate a fresh + ClientHelloOuter.random using a secure random number generator. + (See Section 10.11.1.) + + 5. The value of ECHConfig.contents.public_name MUST be placed in the + "server_name" extension. + + 6. When the client offers the "pre_shared_key" extension in + ClientHelloInner, it SHOULD also include a GREASE + "pre_shared_key" extension in ClientHelloOuter, generated in the + manner described in Section 6.1.2. The client MUST NOT use this + extension to advertise a PSK to the client-facing server. (See + Section 10.11.3.) When the client includes a GREASE + "pre_shared_key" extension, it MUST also copy the + "psk_key_exchange_modes" from the ClientHelloInner into the + ClientHelloOuter. + + 7. When the client offers the "early_data" extension in + ClientHelloInner, it MUST also include the "early_data" extension + in ClientHelloOuter. This allows servers that reject ECH and use + ClientHelloOuter to safely ignore any early data sent by the + client per [RFC8446], Section 4.2.10. + + Note that these rules may change in the presence of an application + profile specifying otherwise. + + 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 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) + + Finally, the client replaces payload with final_payload to obtain + ClientHelloOuter. The two values have the same length, so it is not + necessary to recompute length prefixes in the serialized structure. + + Note this construction requires the "encrypted_client_hello" be + computed after all other extensions. This is possible because the + ClientHelloOuter's "pre_shared_key" extension is either omitted, or + uses a random binder (Section 6.1.2). + +6.1.2. GREASE PSK + + When offering ECH, the client is not permitted to advertise PSK + identities in the ClientHelloOuter. However, the client can send a + "pre_shared_key" extension in the ClientHelloInner. In this case, + when resuming a session with the client, the backend server sends a + "pre_shared_key" extension in its ServerHello. This would appear to + a network observer as if the the server were sending this extension + without solicitation, which would violate the extension rules + described in [RFC8446]. Sending a GREASE "pre_shared_key" extension + in the ClientHelloOuter makes it appear to the network as if the + extension were negotiated properly. + + The client generates the extension payload by constructing an + OfferedPsks structure (see [RFC8446], Section 4.2.11) as follows. + For each PSK identity advertised in the ClientHelloInner, the client + generates a random PSK identity with the same length. It also + generates a random, 32-bit, unsigned integer to use as the + obfuscated_ticket_age. Likewise, for each inner PSK binder, the + client generates a random string of the same length. + + Per the rules of Section 6.1, the server is not permitted to resume a + connection in the outer handshake. If ECH is rejected and the + client-facing server replies with a "pre_shared_key" extension in its + ServerHello, then the client MUST abort the handshake with an + "illegal_parameter" alert. + +6.1.3. Recommended Padding Scheme + + This section describes a deterministic padding mechanism based on the + following observation: individual extensions can reveal sensitive + information through their length. Thus, each extension in the inner + ClientHello may require different amounts of padding. This padding + may be fully determined by the client's configuration or may require + server input. + + By way of example, clients typically support a small number of + application profiles. For instance, a browser might support HTTP + with ALPN values ["http/1.1", "h2"] and WebRTC media with ALPNs + ["webrtc", "c-webrtc"]. Clients SHOULD pad this extension by + rounding up to the total size of the longest ALPN extension across + all application profiles. The target padding length of most + ClientHello extensions can be computed in this way. + + In contrast, clients do not know the longest SNI value in the client- + facing server's anonymity set without server input. Clients SHOULD + use the ECHConfig's maximum_name_length field as follows, where L is + the maximum_name_length value. + + 1. If the ClientHelloInner contained a "server_name" extension with + a name of length D, add max(0, L - D) bytes of padding. + + 2. If the ClientHelloInner did not contain a "server_name" extension + (e.g., if the client is connecting to an IP address), add L + 9 + bytes of padding. This is the length of a "server_name" + extension with an L-byte name. + + Finally, the client SHOULD pad the entire message as follows: + + 1. Let L be the length of the EncodedClientHelloInner with all the + padding computed so far. + + 2. Let N = 31 - ((L - 1) % 32) and add N bytes of padding. + + This rounds the length of EncodedClientHelloInner up to a multiple of + 32 bytes, reducing the set of possible lengths across all clients. + + In addition to padding ClientHelloInner, clients and servers will + also need to pad all other handshake messages that have sensitive- + length fields. For example, if a client proposes ALPN values in + ClientHelloInner, the server-selected value will be returned in an + EncryptedExtension, so that handshake message also needs to be padded + using TLS record layer padding. + +6.1.4. 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, it is either a ServerHello or + 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 it has a length other than 8, the client + aborts 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. + + [[OPEN ISSUE: Depending on what we do for issue#450, it may be + appropriate to change the client behavior if the HRR extension is + present but with the wrong value.]] + + 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 10.11.2. + + 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 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 keys as securely replaced by the server. It SHOULD + retry the handshake with a new transport connection, using the retry + configurations supplied by the server. The retry configurations may + only be applied to the retry connection. The client MUST NOT use + retry configurations for connections beyond the retry. This avoids + introducing pinning concerns or a tracking vector, should a malicious + server present client-specific retry configurations in order to + identify the client in a subsequent ECH handshake. + + If none of the values provided in "retry_configs" contains a + supported version, 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 implement a limit on retries caused by receipt of + "retry_configs" or servers which do not acknowledge the + "encrypted_client_hello" extension. If the client does not retry in + either scenario, it MUST report an error to the calling application. + +6.1.7. 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. Clients + that incorporate DNS names and IP addresses into the same syntax + (e.g. [RFC3986], Section 7.4 and [WHATWG-IPV4]) MUST reject names + that would be interpreted as IPv4 addresses. Clients that enforce + this by checking and rejecting encoded IPv4 addresses in + ECHConfig.contents.public_name do not need to repeat the check at + this layer. + + Note that authenticating a connection for the public name does not + authenticate it for the origin. The TLS implementation MUST NOT + report such connections as successful to the application. It + additionally MUST ignore all session tickets and session IDs + presented by the server. These connections are only used to trigger + retries, as described in Section 6.1.6. This may be implemented, for + instance, by reporting a failed connection with a dedicated error + code. + +6.2. GREASE ECH + + If the client attempts to connect to a server and does not have an + ECHConfig structure available for the server, it SHOULD send a GREASE + [RFC8701] "encrypted_client_hello" extension in the first ClientHello + as follows: + + * Set the config_id field to a random byte. + + * Set the cipher_suite field to a supported + HpkeSymmetricCipherSuite. The selection SHOULD vary to exercise + all supported configurations, but MAY be held constant for + successive connections to the same server in the same session. + + * Set the enc field to a randomly-generated valid encapsulated + public key output by the HPKE KEM. + + * Set the payload field to a randomly-generated string of L+C bytes, + where C is the ciphertext expansion of the selected AEAD scheme + and L is the size of the EncodedClientHelloInner the client would + compute when offering ECH, padded according to Section 6.1.3. + + 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. + +7. Server Behavior + + Servers that support ECH play one of two roles, depending on the + payload of the "encrypted_client_hello" extension in the initial + ClientHello: + + * If ECHClientHello.type is outer, then the server acts as a client- + facing server and proceeds as described in Section 7.1 to extract + a ClientHelloInner, if available. + + * If ECHClientHello.type is inner, then the server acts as a backend + server and proceeds as described in Section 7.2. + + * Otherwise, if ECHClientHello.type is not a valid + ECHClientHelloType, then the server MUST abort with an + "illegal_parameter" alert. + + If the "encrypted_client_hello" is not present, then the server + completes the handshake normally, as described in [RFC8446]. + +7.1. Client-Facing Server + + Upon receiving an "encrypted_client_hello" extension in an initial + ClientHello, the client-facing server determines if it will accept + ECH, prior to negotiating any other TLS parameters. Note that + successfully decrypting the extension will result in a new + ClientHello to process, so even the client's TLS version preferences + may have changed. + + 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: + + The server verifies that the ECHConfig supports the cipher suite + indicated by the ECHClientHello.cipher_suite and that the version of + ECH indicated by the client matches the ECHConfig.version. If not, + the server continues to the next candidate ECHConfig. + + Next, the server decrypts ECHClientHello.payload, using the private + key skR corresponding to ECHConfig, as follows: + + context = SetupBaseR(ECHClientHello.enc, skR, + "tls ech" || 0x00 || ECHConfig) + EncodedClientHelloInner = context.Open(ClientHelloOuterAAD, + ECHClientHello.payload) + + ClientHelloOuterAAD is computed from ClientHelloOuter as described in + Section 5.2. The info parameter to SetupBaseR is the concatenation + "tls ech", a zero byte, and the serialized ECHConfig. If decryption + fails, the server continues to the next candidate ECHConfig. + Otherwise, the server reconstructs ClientHelloInner from + EncodedClientHelloInner, as described in Section 5.1. It then stops + iterating over the candidate ECHConfig values. + + Upon determining the ClientHelloInner, the client-facing server + checks that the message includes a well-formed + "encrypted_client_hello" extension of type inner and that it does not + offer TLS 1.2 or below. If either of these checks fails, the client- + facing server MUST abort with an "illegal_parameter" alert. + + 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 10.9.4 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). Instead, servers can + measure occurrences of the "ech_required" alert to detect this case. + +7.1.1. Sending HelloRetryRequest + + After sending or forwarding a HelloRetryRequest, the client-facing + server does not repeat the steps in Section 7.1 with the second + ClientHelloOuter. Instead, it continues with the ECHConfig selection + from the first ClientHelloOuter as follows: + + If the client-facing server accepted ECH, it checks the second + ClientHelloOuter also contains the "encrypted_client_hello" + extension. If not, it MUST abort the handshake with a + "missing_extension" alert. Otherwise, it checks that + ECHClientHello.cipher_suite and ECHClientHello.config_id are + unchanged, and that ECHClientHello.enc is empty. If not, it MUST + abort the handshake with an "illegal_parameter" alert. + + Finally, it decrypts the new ECHClientHello.payload as a second + message with the previous HPKE context: + + EncodedClientHelloInner = context.Open(ClientHelloOuterAAD, + ECHClientHello.payload) + + 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. + + The backend server MUST NOT perform this operation if it negotiated + TLS 1.2 or below. Note that doing so would overwrite the downgrade + signal for TLS 1.3 (see [RFC8446], Section 4.1.3). + +7.2.1. Sending HelloRetryRequest + + When the backend server sends HelloRetryRequest in response to the + ClientHello, it similarly confirms ECH acceptance by adding a + confirmation signal to its HelloRetryRequest. But instead of + embedding the signal in the HelloRetryRequest.random (the value of + which is specified by [RFC8446]), it sends the signal in an + extension. + + The backend server begins by computing HelloRetryRequest as usual, + except that it also contains an "encrypted_client_hello" extension + with a payload of 8 zero bytes. It then computes the transcript hash + for the first ClientHelloInner, denoted ClientHelloInner1, up to and + including the modified HelloRetryRequest. Let + transcript_hrr_ech_conf denote the output. Finally, the backend + server overwrites the payload of the "encrypted_client_hello" + extension with the following string: + + hrr_accept_confirmation = HKDF-Expand-Label( + HKDF-Extract(0, ClientHelloInner1.random), + "hrr ech accept confirmation", + transcript_hrr_ech_conf, + 8) + + In the subsequent ServerHello message, the backend server sends the + accept_confirmation value as described in Section 7.2. + +8. Compatibility Issues + + Unlike most TLS extensions, placing the SNI value in an ECH extension + is not interoperable with existing servers, which expect the value in + the existing plaintext extension. Thus server operators SHOULD + ensure servers understand a given set of ECH keys before advertising + them. Additionally, servers SHOULD retain support for any + previously-advertised keys for the duration of their validity. + + However, in more complex deployment scenarios, this may be difficult + to fully guarantee. Thus this protocol was designed to be robust in + case of inconsistencies between systems that advertise ECH keys and + servers, at the cost of extra round-trips due to a retry. Two + specific scenarios are detailed below. + +8.1. Misconfiguration and Deployment Concerns + + It is possible for ECH advertisements and servers to become + inconsistent. This may occur, for instance, from DNS + misconfiguration, caching issues, or an incomplete rollout in a + multi-server deployment. This may also occur if a server loses its + ECH keys, or if a deployment of ECH must be rolled back on the + server. + + The retry mechanism repairs inconsistencies, provided the server is + authoritative for the public name. If server and advertised keys + mismatch, the server will 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. + +8.2. Middleboxes + + When connecting through a TLS-terminating proxy that does not support + this extension, [RFC8446], Section 9.3 requires the proxy still act + as a conforming TLS client and server. 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 public name, 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. + +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 + +10.1. Security and Privacy Goals + + ECH considers two types of attackers: passive and active. Passive + attackers can read packets from the network, but they cannot perform + any sort of active behavior such as probing servers or querying DNS. + A middlebox that filters based on plaintext packet contents is one + example of a passive attacker. In contrast, active attackers can + also write packets into the network for malicious purposes, such as + interfering with existing connections, probing servers, and querying + DNS. In short, an active attacker corresponds to the conventional + threat model for TLS 1.3 [RFC8446]. + + Given these types of attackers, the primary goals of ECH are as + follows. + + 1. Security preservation. Use of ECH does not weaken the security + properties of TLS without ECH. + + 2. Handshake privacy. TLS connection establishment to a host with a + specific ECHConfig and TLS configuration is indistinguishable + from a connection to any other host with the same ECHConfig and + TLS configuration. (The set of hosts which share the same + ECHConfig and TLS configuration is referred to as the anonymity + set.) + + 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 host, 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 hosts as possible. An attacker can distinguish + two hosts that have different ECHConfig values based on the + ECHClientHello.config_id value. This also means public information + in a TLS handshake should be consistent across hosts. For example, + if a client-facing server services many backend origin hosts, only + one of which supports some cipher suite, it may be possible to + identify that host based on the contents of unencrypted handshake + messages. + + Beyond these primary security and privacy goals, ECH also aims to + hide, to some extent, the fact that it is being used at all. + Specifically, the GREASE ECH extension described in Section 6.2 does + not change the security properties of the TLS handshake at all. Its + goal is to provide "cover" for the real ECH protocol (Section 6.1), + as a means of addressing the "do not stick out" requirements of + [RFC8744]. See Section 10.9.4 for details. + +10.2. Unauthenticated and Plaintext DNS + + In comparison to [I-D.kazuho-protected-sni], wherein DNS Resource + Records are signed via a server private key, ECH records have no + authenticity or provenance information. This means that any attacker + which can inject DNS responses or poison DNS caches, which is a + common scenario in client access networks, can supply clients with + fake ECH records (so that the client encrypts data to them) or strip + the ECH record from the response. However, in the face of an + attacker that controls DNS, no encryption scheme can work because the + attacker can replace the IP address, thus blocking client + connections, or substitute a unique IP address which is 1:1 with the + DNS name that was looked up (modulo DNS wildcards). Thus, allowing + the ECH records in the clear does not make the situation + significantly worse. + + Clearly, DNSSEC (if the client validates and hard fails) is a defense + against this form of attack, but DoH/DPRIVE are also defenses against + DNS attacks by attackers on the local network, which is a common case + where ClientHello and SNI encryption are desired. Moreover, as noted + in the introduction, SNI encryption is less useful without encryption + of DNS queries in transit via DoH or DPRIVE mechanisms. + +10.3. Client Tracking + + A malicious client-facing server could distribute unique, per-client + ECHConfig structures as a way of tracking clients across subsequent + connections. On-path adversaries which know about these unique keys + could also track clients in this way by observing TLS connection + attempts. + + The cost of this type of attack scales linearly with the desired + number of target clients. Moreover, DNS caching behavior makes + targeting individual users for extended periods of time, e.g., using + per-client ECHConfig structures delivered via HTTPS RRs with high + TTLs, challenging. Clients can help mitigate this problem by + flushing any DNS or ECHConfig state upon changing networks. + +10.4. 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, 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 a different ALPN lists to different servers + or in different application contexts. A client that treats this + context as sensitive SHOULD NOT send context-specific values in + ClientHelloOuter. + + Values which are independent of the true server name, or other + information the client wishes to protect, MAY be included in + ClientHelloOuter. If they match the corresponding ClientHelloInner, + they MAY be compressed as described in Section 5.1. However, note + the payload length reveals information about which extensions are + compressed, so inner extensions which only sometimes match the + corresponding outer extension SHOULD NOT be compressed. + + Clients MAY include additional extensions in ClientHelloOuter to + avoid signaling unusual behavior to passive observers, provided the + choice of value and value itself are not sensitive. See + Section 10.9.4. + +10.6. Related Privacy Leaks + + ECH requires encrypted DNS to be an effective privacy protection + mechanism. However, verifying the server's identity from the + Certificate message, particularly when using the X509 + CertificateType, may result in additional network traffic that may + reveal the server identity. Examples of this traffic may include + requests for revocation information, such as OCSP or CRL traffic, or + requests for repository information, such as + authorityInformationAccess. It may also include implementation- + specific traffic for additional information sources as part of + verification. + + Implementations SHOULD avoid leaking information that may identify + the server. Even when sent over an encrypted transport, such + requests may result in indirect exposure of the server's identity, + such as indicating a specific CA or service being used. To mitigate + this risk, servers SHOULD deliver such information in-band when + possible, such as through the use of OCSP stapling, and clients + SHOULD take steps to minimize or protect such requests during + certificate validation. + + Attacks that rely on non-ECH traffic to infer server identity in an + ECH connection are out of scope for this document. For example, a + client that connects to a particular host prior to ECH deployment may + later resume a connection to that same host after ECH deployment. An + adversary that observes this can deduce that the ECH-enabled + connection was made to a host that the client previously connected to + and which is within the same anonymity set. + +10.7. Cookies + + Section 4.2.2 of [RFC8446] defines a cookie value that servers may + send in HelloRetryRequest for clients to echo in the second + ClientHello. While ECH encrypts the cookie in the second + ClientHelloInner, the backend server's HelloRetryRequest is + unencrypted.This means differences in cookies between backend + servers, such as lengths or cleartext components, may leak + information about the server identity. + + Backend servers in an anonymity set SHOULD NOT reveal information in + the cookie which identifies the server. This may be done by handling + HelloRetryRequest statefully, thus not sending cookies, or by using + the same cookie construction for all backend servers. + + Note that, if the cookie includes a key name, analogous to Section 4 + of [RFC5077], this may leak information if different backend servers + issue cookies with different key names at the time of the connection. + In particular, if the deployment operates in Split Mode, the backend + servers may not share cookie encryption keys. Backend servers may + mitigate this by either handling key rotation with trial decryption, + or coordinating to match key names. + +10.8. Attacks Exploiting Acceptance Confirmation + + To signal acceptance, the backend server overwrites 8 bytes of its + ServerHello.random with a value derived from the + ClientHelloInner.random. (See Section 7.2 for details.) This + behavior increases the likelihood of the ServerHello.random colliding + with the ServerHello.random of a previous session, potentially + reducing the overall security of the protocol. However, the + remaining 24 bytes provide enough entropy to ensure this is not a + practical avenue of attack. + + On the other hand, the probability that two 8-byte strings are the + same is non-negligible. This poses a modest operational risk. + Suppose the client-facing server terminates the connection (i.e., ECH + is rejected or bypassed): if the last 8 bytes of its + ServerHello.random coincide with the confirmation signal, then the + client will incorrectly presume acceptance and proceed as if the + backend server terminated the connection. However, the probability + of a false positive occurring for a given connection is only 1 in + 2^64. This value is smaller than the probability of network + connection failures in practice. + + Note that the same bytes of the ServerHello.random are used to + implement downgrade protection for TLS 1.3 (see [RFC8446], + Section 4.1.3). These mechanisms do not interfere because the + backend server only signals ECH acceptance in TLS 1.3 or higher. + +10.9. Comparison Against Criteria + + [RFC8744] lists several requirements for SNI encryption. In this + section, we re-iterate these requirements and assess the ECH design + against them. + +10.9.1. Mitigate Cut-and-Paste Attacks + + Since servers process either ClientHelloInner or ClientHelloOuter, + and because ClientHelloInner.random is encrypted, it is not possible + for an attacker to "cut and paste" the ECH value in a different + Client Hello and learn information from ClientHelloInner. + +10.9.2. Avoid Widely Shared Secrets + + This design depends upon DNS as a vehicle for semi-static public key + distribution. Server operators may partition their private keys + however they see fit provided each server behind an IP address has + the corresponding private key to decrypt a key. Thus, when one ECH + key is provided, sharing is optimally bound by the number of hosts + that share an IP address. Server operators may further limit sharing + by publishing different DNS records containing ECHConfig values with + different keys using a short TTL. + +10.9.3. Prevent SNI-Based Denial-of-Service Attacks + + This design requires servers to decrypt ClientHello messages with + ECHClientHello extensions carrying valid digests. Thus, it is + possible for an attacker to force decryption operations on the + server. This attack is bound by the number of valid TCP connections + an attacker can open. + +10.9.4. Do Not Stick Out + + As a means of reducing the impact of network ossification, [RFC8744] + recommends SNI-protection mechanisms be designed in such a way that + network operators do not differentiate connections using the + mechanism from connections not using the mechanism. To that end, ECH + is designed to resemble a standard TLS handshake as much as possible. + The most obvious difference is the extension itself: as long as + middleboxes ignore it, as required by [RFC8446], the rest of the + handshake is designed to look very much as usual. + + The GREASE ECH protocol described in Section 6.2 provides a low-risk + way to evaluate the deployability of ECH. It is designed to mimic + the real ECH protocol (Section 6.1) without changing the security + properties of the handshake. The underlying theory is that if GREASE + ECH is deployable without triggering middlebox misbehavior, and real + ECH looks enough like GREASE ECH, then ECH should be deployable as + well. Thus, our strategy for mitigating network ossification is to + deploy GREASE ECH widely enough to disincentivize differential + treatment of the real ECH protocol by the network. + + Ensuring that networks do not differentiate between real ECH and + GREASE ECH may not be feasible for all implementations. While most + middleboxes will not treat them differently, some operators may wish + to block real ECH usage but allow GREASE ECH. This specification + aims to provide a baseline security level that most deployments can + achieve easily, while providing implementations enough flexibility to + achieve stronger security where possible. Minimally, real ECH is + designed to be indifferentiable from GREASE ECH for passive + adversaries with following capabilities: + + 1. The attacker does not know the ECHConfigList used by the server. + + 2. The attacker keeps per-connection state only. In particular, it + does not track endpoints across connections. + + 3. ECH and GREASE ECH are designed so that the following features do + not vary: the code points of extensions negotiated in the clear; + the length of messages; and the values of plaintext alert + messages. + + This leaves a variety of practical differentiators out-of-scope. + including, though not limited to, the following: + + 1. the value of the configuration identifier; + + 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. + These mitigations are out-of-scope for this specification. + +10.9.5. Maintain Forward Secrecy + + This design is not forward secret because the server's ECH key is + static. However, the window of exposure is bound by the key + lifetime. It is RECOMMENDED that servers rotate keys frequently. + +10.9.6. Enable Multi-party Security Contexts + + This design permits servers operating in Split Mode to forward + connections directly to backend origin servers. The client + authenticates the identity of the backend origin server, thereby + avoiding unnecessary MiTM attacks. + + Conversely, assuming ECH records retrieved from DNS are + authenticated, e.g., via DNSSEC or fetched from a trusted Recursive + Resolver, spoofing a client-facing server operating in Split Mode is + not possible. See Section 10.2 for more details regarding plaintext + DNS. + + Authenticating the ECHConfig structure naturally authenticates the + included public name. This also authenticates any retry signals from + the client-facing server because the client validates the server + certificate against the public name before retrying. + +10.9.7. Support Multiple Protocols + + This design has no impact on application layer protocol negotiation. + It may affect connection routing, server certificate selection, and + client certificate verification. Thus, it is compatible with + multiple application and transport protocols. By encrypting the + entire ClientHello, this design additionally supports encrypting the + ALPN extension. + +10.10. Padding Policy + + Variations in the length of the ClientHelloInner ciphertext could + leak information about the corresponding plaintext. Section 6.1.3 + describes a RECOMMENDED padding mechanism for clients aimed at + reducing potential information leakage. + +10.11. Active Attack Mitigations + + This section describes the rationale for ECH properties and mechanics + as defenses against active attacks. In all the attacks below, the + attacker is on-path between the target client and server. The goal + of the attacker is to learn private information about the inner + ClientHello, such as the true SNI value. + +10.11.1. Client Reaction Attack Mitigation + + This attack uses the client's reaction to an incorrect certificate as + an oracle. The attacker intercepts a legitimate ClientHello and + replies with a ServerHello, Certificate, CertificateVerify, and + Finished messages, wherein the Certificate message contains a "test" + certificate for the domain name it wishes to query. If the client + decrypted the Certificate and failed verification (or leaked + information about its verification process by a timing side channel), + the attacker learns that its test certificate name was incorrect. As + an example, suppose the client's SNI value in its inner ClientHello + is "example.com," and the attacker replied with a Certificate for + "test.com". If the client produces a verification failure alert + because of the mismatch faster than it would due to the Certificate + signature validation, information about the name leaks. Note that + the attacker can also withhold the CertificateVerify message. In + that scenario, a client which first verifies the Certificate would + then respond similarly and leak the same information. + + Client Attacker Server + ClientHello + + key_share + + ech ------> (intercept) -----> X (drop) + + ServerHello + + key_share + {EncryptedExtensions} + {CertificateRequest*} + {Certificate*} + {CertificateVerify*} + <------ + Alert + ------> + + Figure 3: Client reaction attack + + ClientHelloInner.random prevents this attack. In particular, since + the attacker does not have access to this value, it cannot produce + the right transcript and handshake keys needed for encrypting the + Certificate message. Thus, the client will fail to decrypt the + Certificate and abort the connection. + +10.11.2. HelloRetryRequest Hijack Mitigation + + This attack aims to exploit server HRR state management to recover + information about a legitimate ClientHello using its own attacker- + controlled ClientHello. To begin, the attacker intercepts and + forwards a legitimate ClientHello with an "encrypted_client_hello" + (ech) extension to the server, which triggers a legitimate + HelloRetryRequest in return. Rather than forward the retry to the + client, the attacker attempts to generate its own ClientHello in + response based on the contents of the first ClientHello and + HelloRetryRequest exchange with the result that the server encrypts + the Certificate to the attacker. If the server used the SNI from the + first ClientHello and the key share from the second (attacker- + controlled) ClientHello, the Certificate produced would leak the + client's chosen SNI to the attacker. + + Client Attacker Server + ClientHello + + key_share + + ech ------> (forward) -------> + HelloRetryRequest + + key_share + (intercept) <------- + + ClientHello + + key_share' + + ech' -------> + ServerHello + + key_share + {EncryptedExtensions} + {CertificateRequest*} + {Certificate*} + {CertificateVerify*} + {Finished} + <------- + (process server flight) + + Figure 4: HelloRetryRequest hijack attack + + This attack is mitigated by using the same HPKE context for both + ClientHello messages. The attacker does not possess the context's + keys, so it cannot generate a valid encryption of the second inner + ClientHello. + + If the attacker could manipulate the second ClientHello, it might be + possible for the server to act as an oracle if it required parameters + from the first ClientHello to match that of the second ClientHello. + For example, imagine the client's original SNI value in the inner + ClientHello is "example.com", and the attacker's hijacked SNI value + in its inner ClientHello is "test.com". A server which checks these + for equality and changes behavior based on the result can be used as + an oracle to learn the client's SNI. + +10.11.3. ClientHello Malleability Mitigation + + This attack aims to leak information about secret parts of the + encrypted ClientHello by adding attacker-controlled parameters and + observing the server's response. In particular, the compression + mechanism described in Section 5.1 references parts of a potentially + attacker-controlled ClientHelloOuter to construct ClientHelloInner, + or a buggy server may incorrectly apply parameters from + ClientHelloOuter to the handshake. + + To begin, the attacker first interacts with a server to obtain a + resumption ticket for a given test domain, such as "example.com". + Later, upon receipt of a ClientHelloOuter, it modifies it such that + the server will process the resumption ticket with ClientHelloInner. + If the server only accepts resumption PSKs that match the server + name, it will fail the PSK binder check with an alert when + ClientHelloInner is for "example.com" but silently ignore the PSK and + continue when ClientHelloInner is for any other name. This + introduces an oracle for testing encrypted SNI values. + + Client Attacker Server + + handshake and ticket + for "example.com" + <--------> + + ClientHello + + key_share + + ech + + ech_outer_extensions(pre_shared_key) + + pre_shared_key + --------> + (intercept) + ClientHello + + key_share + + ech + + ech_outer_extensions(pre_shared_key) + + pre_shared_key' + --------> + Alert + -or- + ServerHello + ... + Finished + <-------- + + Figure 5: Message flow for malleable ClientHello + + This attack may be generalized to any parameter which the server + varies by server name, such as ALPN preferences. + + ECH mitigates this attack by only negotiating TLS parameters from + ClientHelloInner and authenticating all inputs to the + ClientHelloInner (EncodedClientHelloInner and ClientHelloOuter) with + the HPKE AEAD. See Section 5.2. An earlier iteration of this + specification only encrypted and authenticated the "server_name" + extension, which left the overall ClientHello vulnerable to an + analogue of this attack. + +10.11.4. 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 is requested to create the following entries in the existing + registry for ExtensionType (defined in [RFC8446]): + + 1. encrypted_client_hello(0xfe0d), with "TLS 1.3" column values set + to "CH, HRR, EE", and "Recommended" column set to "Yes". + + 2. ech_outer_extensions(0xfd00), with the "TLS 1.3" column values + set to "", and "Recommended" column set to "Yes". + +11.2. Update of the TLS Alert Registry + + IANA is requested to create an entry, ech_required(121) in the + existing registry for Alerts (defined in [RFC8446]), with the "DTLS- + OK" column set to "Y". + +11.3. ECH Configuration Extension Registry + + IANA is requested to create a new "ECHConfig Extension" registry in a + new "TLS Encrypted Client Hello (ECH) Configuration Extensions" page. + New registrations need to 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 extension is TLS + WG recommends that the extension be supported. This column is + assigned a value of "N" unless explicitly requested. Adding a + value with 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 this registry are subject to the Specification + Required registration policy ([RFC8126], Section 4.6). + + The registration policy for for the "ECHConfig Extension Type" + registry is Specification Required [RFC8126]. + +12. ECHConfig Extension Guidance + + Any future information or hints that influence ClientHelloOuter + SHOULD be specified as ECHConfig extensions. This is primarily + because the outer ClientHello exists only in support of ECH. Namely, + it is both an envelope for the encrypted inner ClientHello and + enabler for authenticated key mismatch signals (see Section 7). In + contrast, the inner ClientHello is the true ClientHello used upon ECH + negotiation. + +13. References + +13.1. Normative References + + [HPKE] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid + Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180, + February 2022, . + + [HTTPS-RR] Schwartz, B. M., Bishop, M., and E. Nygren, "Service + binding and parameter specification via the DNS (DNS SVCB + and HTTPS RRs)", Work in Progress, Internet-Draft, draft- + ietf-dnsop-svcb-https-12, 11 March 2023, + . + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, + DOI 10.17487/RFC2119, March 1997, + . + + [RFC5890] Klensin, J., "Internationalized Domain Names for + Applications (IDNA): Definitions and Document Framework", + RFC 5890, DOI 10.17487/RFC5890, August 2010, + . + + [RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport + Layer Security (TLS) False Start", RFC 7918, + DOI 10.17487/RFC7918, August 2016, + . + + [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, + . + + [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC + 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, + May 2017, . + + [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol + Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, + . + + [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, + . + +13.2. Informative References + + [ECH-Analysis] + "A Symbolic Analysis of Privacy for TLS 1.3 with Encrypted + Client Hello", November 2022. + + [EXPORTED-AUTHENTICATORS] + Sullivan, N., "Exported Authenticators in TLS", RFC 9261, + DOI 10.17487/RFC9261, July 2022, + . + + [I-D.kazuho-protected-sni] + Oku, K., "TLS Extensions for Protecting SNI", Work in + Progress, Internet-Draft, draft-kazuho-protected-sni-00, + 18 July 2017, . + + [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, + . + + [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, . + + [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, . + + [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, . + + [RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security + (TLS) Cached Information Extension", RFC 7924, + DOI 10.17487/RFC7924, July 2016, + . + + [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram + Transport Layer Security (DTLS)", RFC 8094, + DOI 10.17487/RFC8094, February 2017, + . + + [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS + (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, + . + + [RFC8701] Benjamin, D., "Applying Generate Random Extensions And + Sustain Extensibility (GREASE) to TLS Extensibility", + RFC 8701, DOI 10.17487/RFC8701, January 2020, + . + + [RFC8744] Huitema, C., "Issues and Requirements for Server Name + Identification (SNI) Encryption in TLS", RFC 8744, + DOI 10.17487/RFC8744, July 2020, + . + + [WHATWG-IPV4] + "URL Living Standard - IPv4 Parser", May 2021, + . + +Appendix A. Alternative SNI Protection Designs + + Alternative approaches to encrypted SNI may be implemented at the TLS + or application layer. In this section we describe several + alternatives and discuss drawbacks in comparison to the design in + this document. + +A.1. TLS-layer + +A.1.1. TLS in Early Data + + In this variant, TLS Client Hellos are tunneled within early data + payloads belonging to outer TLS connections established with the + client-facing server. This requires clients to have established a + previous session -— and obtained PSKs —- with the server. The + client-facing server decrypts early data payloads to uncover Client + Hellos destined for the backend server, and forwards them onwards as + necessary. Afterwards, all records to and from backend servers are + forwarded by the client-facing server -- unmodified. This avoids + double encryption of TLS records. + + Problems with this approach are: (1) servers may not always be able + to distinguish inner Client Hellos from legitimate application data, + (2) nested 0-RTT data may not function correctly, (3) 0-RTT data may + not be supported -- especially under DoS -- leading to availability + concerns, and (4) clients must bootstrap tunnels (sessions), costing + an additional round trip and potentially revealing the SNI during the + initial connection. In contrast, encrypted SNI protects the SNI in a + distinct Client Hello extension and neither abuses early data nor + requires a bootstrapping connection. + +A.1.2. Combined Tickets + + In this variant, client-facing and backend servers coordinate to + produce "combined tickets" that are consumable by both. Clients + offer combined tickets to client-facing servers. The latter parse + them to determine the correct backend server to which the Client + Hello should be forwarded. This approach is problematic due to non- + trivial coordination between client-facing and backend servers for + ticket construction and consumption. Moreover, it requires a + bootstrapping step similar to that of the previous variant. In + contrast, encrypted SNI requires no such coordination. + +A.2. Application-layer + +A.2.1. HTTP/2 CERTIFICATE Frames + + In this variant, clients request secondary certificates with + CERTIFICATE_REQUEST HTTP/2 frames after TLS connection completion. + In response, servers supply certificates via TLS exported + authenticators [EXPORTED-AUTHENTICATORS] in CERTIFICATE frames. + Clients use a generic SNI for the underlying client-facing server TLS + connection. Problems with this approach include: (1) one additional + round trip before peer authentication, (2) non-trivial application- + layer dependencies and interaction, and (3) obtaining the generic SNI + to bootstrap the connection. In contrast, encrypted SNI induces no + additional round trip and operates below the application layer. + +Appendix B. Linear-time Outer Extension Processing + + The following procedure processes the "ech_outer_extensions" + extension (see Section 5.1) in linear time, ensuring that each + referenced extension in the ClientHelloOuter is included at most + once: + + 1. Let I be zero and N be 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. + +Appendix C. Acknowledgements + + This document draws extensively from ideas in + [I-D.kazuho-protected-sni], but is a much more limited mechanism + because it depends on the DNS for the protection of the ECH key. + Richard Barnes, Christian Huitema, Patrick McManus, Matthew Prince, + Nick Sullivan, Martin Thomson, and David Benjamin also provided + important ideas and contributions. + +Appendix D. Change Log + + *RFC Editor's Note:* Please remove this section prior to + publication of a final version of this document. + + Issue and pull request numbers are listed with a leading octothorp. + +D.1. Since draft-ietf-tls-esni-16 + + * Keep-alive + +D.2. Since draft-ietf-tls-esni-15 + + * Add CCS2022 reference and summary (#539) + +D.3. Since draft-ietf-tls-esni-14 + + * Keep-alive + +D.4. Since draft-ietf-tls-esni-13 + + * Editorial improvements + +D.5. Since draft-ietf-tls-esni-12 + + * Abort on duplicate OuterExtensions (#514) + + * Improve EncodedClientHelloInner definition (#503) + + * Clarify retry configuration usage (#498) + + * Expand on config_id generation implications (#491) + + * Server-side acceptance signal extension GREASE (#481) + + * Refactor overview, client implementation, and middlebox sections + (#480, #478, #475, #508) + + * Editorial iprovements (#485, #488, #490, #495, #496, #499, #500, + #501, #504, #505, #507, #510, #511) + +D.6. Since draft-ietf-tls-esni-11 + + * Move ClientHello padding to the encoding (#443) + + * Align codepoints (#464) + + * Relax OuterExtensions checks for alignment with RFC8446 (#467) + + * Clarify HRR acceptance and rejection logic (#470) + + * Editorial improvements (#468, #465, #462, #461) + +D.7. Since draft-ietf-tls-esni-10 + + * Make HRR confirmation and ECH acceptance explicit (#422, #423) + + * Relax computation of the acceptance signal (#420, #449) + + * Simplify ClientHelloOuterAAD generation (#438, #442) + + * Allow empty enc in ECHClientHello (#444) + + * Authenticate ECHClientHello extensions position in + ClientHelloOuterAAD (#410) + + * Allow clients to send a dummy PSK and early_data in + ClientHelloOuter when applicable (#414, #415) + + * Compress ECHConfigContents (#409) + + * Validate ECHConfig.contents.public_name (#413, #456) + + * Validate ClientHelloInner contents (#411) + + * Note split-mode challenges for HRR (#418) + + * Editorial improvements (#428, #432, #439, #445, #458, #455) + +D.8. Since draft-ietf-tls-esni-09 + + * Finalize HPKE dependency (#390) + + * Move from client-computed to server-chosen, one-byte config + identifier (#376, #381) + + * Rename ECHConfigs to ECHConfigList (#391) + + * Clarify some security and privacy properties (#385, #383) + +Authors' Addresses + + Eric Rescorla + RTFM, Inc. + Email: ekr@rtfm.com + + + Kazuho Oku + Fastly + Email: kazuhooku@gmail.com + + + Nick Sullivan + Cloudflare + Email: nick@cloudflare.com + + + Christopher A. Wood + Cloudflare + Email: caw@heapingbits.net diff --git a/caw/ech-config-extensions/index.html b/caw/ech-config-extensions/index.html new file mode 100644 index 00000000..a3feb3e8 --- /dev/null +++ b/caw/ech-config-extensions/index.html @@ -0,0 +1,45 @@ + + + + tlswg/draft-ietf-tls-esni caw/ech-config-extensions preview + + + + +

Editor's drafts for caw/ech-config-extensions branch of tlswg/draft-ietf-tls-esni

+ + + + + + +
TLS Encrypted Client Helloplain textsame as master
+ + + diff --git a/index.html b/index.html index 7ada9183..8cbf64e8 100644 --- a/index.html +++ b/index.html @@ -33,6 +33,14 @@

Preview for branch draft-ietf-tls-esni-17

Preview for branch caw

+

Preview for branch caw/ech-config-extensions

+ + + + + + +
TLS Encrypted Client Helloplain textdiff with master

Preview for branch caw/cite-ech-bootstrapping