TLS Encrypted Client Hello
draft-ietf-tls-esni-15
| Document | Type | Active Internet-Draft (tls WG) | |
|---|---|---|---|
| Authors | Eric Rescorla , Kazuho Oku , Nick Sullivan , Christopher A. Wood | ||
| Last updated | 2022-10-03 | ||
| Replaces | draft-rescorla-tls-esni | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources |
GitHub Repository
Mailing list discussion |
||
| Stream | WG state | WG Document | |
| Associated WG milestone |
|
||
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-tls-esni-15
tls E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track K. Oku
Expires: 6 April 2023 Fastly
N. Sullivan
C. A. Wood
Cloudflare
3 October 2022
TLS Encrypted Client Hello
draft-ietf-tls-esni-15
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 6 April 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
Rescorla, et al. Expires 6 April 2023 [Page 1]
Internet-Draft TLS Encrypted Client Hello October 2022
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 . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Topologies . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Encrypted ClientHello (ECH) . . . . . . . . . . . . . . . 6
4. Encrypted ClientHello Configuration . . . . . . . . . . . . . 7
4.1. Configuration Identifiers . . . . . . . . . . . . . . . . 9
4.2. Configuration Extensions . . . . . . . . . . . . . . . . 9
5. The "encrypted_client_hello" Extension . . . . . . . . . . . 10
5.1. Encoding the ClientHelloInner . . . . . . . . . . . . . . 11
5.2. Authenticating the ClientHelloOuter . . . . . . . . . . . 13
6. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Offering ECH . . . . . . . . . . . . . . . . . . . . . . 14
6.1.1. Encrypting the ClientHello . . . . . . . . . . . . . 16
6.1.2. GREASE PSK . . . . . . . . . . . . . . . . . . . . . 17
6.1.3. Recommended Padding Scheme . . . . . . . . . . . . . 18
6.1.4. Determining ECH Acceptance . . . . . . . . . . . . . 19
6.1.5. Handshaking with ClientHelloInner . . . . . . . . . . 19
6.1.6. Handshaking with ClientHelloOuter . . . . . . . . . . 20
6.1.7. Authenticating for the Public Name . . . . . . . . . 21
6.2. GREASE ECH . . . . . . . . . . . . . . . . . . . . . . . 22
7. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Client-Facing Server . . . . . . . . . . . . . . . . . . 23
7.1.1. Sending HelloRetryRequest . . . . . . . . . . . . . . 25
7.2. Backend Server . . . . . . . . . . . . . . . . . . . . . 26
7.2.1. Sending HelloRetryRequest . . . . . . . . . . . . . . 27
8. Compatibility Issues . . . . . . . . . . . . . . . . . . . . 28
8.1. Misconfiguration and Deployment Concerns . . . . . . . . 28
8.2. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 28
9. Compliance Requirements . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
10.1. Security and Privacy Goals . . . . . . . . . . . . . . . 29
10.2. Unauthenticated and Plaintext DNS . . . . . . . . . . . 30
10.3. Client Tracking . . . . . . . . . . . . . . . . . . . . 31
10.4. Ignored Configuration Identifiers and Trial
Decryption . . . . . . . . . . . . . . . . . . . . . . 31
10.5. Outer ClientHello . . . . . . . . . . . . . . . . . . . 31
Rescorla, et al. Expires 6 April 2023 [Page 2]
Internet-Draft TLS Encrypted Client Hello October 2022
10.6. Related Privacy Leaks . . . . . . . . . . . . . . . . . 32
10.7. Cookies . . . . . . . . . . . . . . . . . . . . . . . . 33
10.8. Attacks Exploiting Acceptance Confirmation . . . . . . . 33
10.9. Comparison Against Criteria . . . . . . . . . . . . . . 34
10.9.1. Mitigate Cut-and-Paste Attacks . . . . . . . . . . . 34
10.9.2. Avoid Widely Shared Secrets . . . . . . . . . . . . 34
10.9.3. Prevent SNI-Based Denial-of-Service Attacks . . . . 34
10.9.4. Do Not Stick Out . . . . . . . . . . . . . . . . . . 35
10.9.5. Maintain Forward Secrecy . . . . . . . . . . . . . . 36
10.9.6. Enable Multi-party Security Contexts . . . . . . . . 36
10.9.7. Support Multiple Protocols . . . . . . . . . . . . . 36
10.10. Padding Policy . . . . . . . . . . . . . . . . . . . . . 37
10.11. Active Attack Mitigations . . . . . . . . . . . . . . . 37
10.11.1. Client Reaction Attack Mitigation . . . . . . . . . 37
10.11.2. HelloRetryRequest Hijack Mitigation . . . . . . . . 38
10.11.3. ClientHello Malleability Mitigation . . . . . . . . 39
10.11.4. ClientHelloInner Packet Amplification Mitigation . 40
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
11.1. Update of the TLS ExtensionType Registry . . . . . . . . 41
11.2. Update of the TLS Alert Registry . . . . . . . . . . . . 41
12. ECHConfig Extension Guidance . . . . . . . . . . . . . . . . 41
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
13.1. Normative References . . . . . . . . . . . . . . . . . . 42
13.2. Informative References . . . . . . . . . . . . . . . . . 43
Appendix A. Alternative SNI Protection Designs . . . . . . . . . 44
A.1. TLS-layer . . . . . . . . . . . . . . . . . . . . . . . . 44
A.1.1. TLS in Early Data . . . . . . . . . . . . . . . . . . 44
A.1.2. Combined Tickets . . . . . . . . . . . . . . . . . . 45
A.2. Application-layer . . . . . . . . . . . . . . . . . . . . 45
A.2.1. HTTP/2 CERTIFICATE Frames . . . . . . . . . . . . . . 45
Appendix B. Linear-time Outer Extension Processing . . . . . . . 45
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 46
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 46
D.1. Since draft-ietf-tls-esni-15 . . . . . . . . . . . . . . 46
D.2. Since draft-ietf-tls-esni-14 . . . . . . . . . . . . . . 46
D.3. Since draft-ietf-tls-esni-13 . . . . . . . . . . . . . . 46
D.4. Since draft-ietf-tls-esni-12 . . . . . . . . . . . . . . 46
D.5. Since draft-ietf-tls-esni-11 . . . . . . . . . . . . . . 47
D.6. Since draft-ietf-tls-esni-10 . . . . . . . . . . . . . . 47
D.7. Since draft-ietf-tls-esni-09 . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
Rescorla, et al. Expires 6 April 2023 [Page 3]
Internet-Draft TLS Encrypted Client Hello October 2022
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 only supported with (D)TLS 1.3 [RFC8446] and newer versions of
the protocol.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. All TLS notation comes from [RFC8446],
Section 3.
Rescorla, et al. Expires 6 April 2023 [Page 4]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 5]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 6]
Internet-Draft TLS Encrypted Client Hello October 2022
4. Encrypted ClientHello Configuration
ECH uses HPKE for public key encryption [I-D.irtf-cfrg-hpke]. The
ECH configuration is defined by the following ECHConfig structure.
opaque HpkePublicKey<1..2^16-1>;
uint16 HpkeKemId; // Defined in I-D.irtf-cfrg-hpke
uint16 HpkeKdfId; // Defined in I-D.irtf-cfrg-hpke
uint16 HpkeAeadId; // Defined in I-D.irtf-cfrg-hpke
struct {
HpkeKdfId kdf_id;
HpkeAeadId aead_id;
} HpkeSymmetricCipherSuite;
struct {
uint8 config_id;
HpkeKemId kem_id;
HpkePublicKey public_key;
HpkeSymmetricCipherSuite cipher_suites<4..2^16-4>;
} HpkeKeyConfig;
struct {
HpkeKeyConfig key_config;
uint8 maximum_name_length;
opaque public_name<1..255>;
Extension extensions<0..2^16-1>;
} ECHConfigContents;
struct {
uint16 version;
uint16 length;
select (ECHConfig.version) {
case 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.
Rescorla, et al. Expires 6 April 2023 [Page 7]
Internet-Draft TLS Encrypted Client Hello October 2022
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 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.
Rescorla, et al. Expires 6 April 2023 [Page 8]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 9]
Internet-Draft TLS Encrypted Client Hello October 2022
The format is as defined in [RFC8446], Section 4.2. The same
interpretation rules apply: extensions MAY appear in any order, but
there MUST NOT be more than one extension of the same type in the
extensions block. An extension can be tagged as mandatory by using
an extension type codepoint with the high order bit set to 1.
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.
Rescorla, et al. Expires 6 April 2023 [Page 10]
Internet-Draft TLS Encrypted Client Hello October 2022
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:
Rescorla, et al. Expires 6 April 2023 [Page 11]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 12]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 13]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 14]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 15]
Internet-Draft TLS Encrypted Client Hello October 2022
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
Rescorla, et al. Expires 6 April 2023 [Page 16]
Internet-Draft TLS Encrypted Client Hello October 2022
* 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.
Rescorla, et al. Expires 6 April 2023 [Page 17]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 18]
Internet-Draft TLS Encrypted Client Hello October 2022
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:
Rescorla, et al. Expires 6 April 2023 [Page 19]
Internet-Draft TLS Encrypted Client Hello October 2022
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
Rescorla, et al. Expires 6 April 2023 [Page 20]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 21]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 22]
Internet-Draft TLS Encrypted Client Hello October 2022
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_config" 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.
Rescorla, et al. Expires 6 April 2023 [Page 23]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 24]
Internet-Draft TLS Encrypted Client Hello October 2022
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)
Rescorla, et al. Expires 6 April 2023 [Page 25]
Internet-Draft TLS Encrypted Client Hello October 2022
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:
Rescorla, et al. Expires 6 April 2023 [Page 26]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 27]
Internet-Draft TLS Encrypted Client Hello October 2022
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
Rescorla, et al. Expires 6 April 2023 [Page 28]
Internet-Draft TLS Encrypted Client Hello October 2022
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 [I-D.irtf-cfrg-hpke],
Section 7.1)
* KDF: HKDF-SHA256 (see [I-D.irtf-cfrg-hpke], Section 7.2)
* AEAD: AES-128-GCM (see [I-D.irtf-cfrg-hpke], Section 7.3)
10. Security Considerations
10.1. Security and Privacy Goals
ECH considers two types of attackers: passive and active. Passive
attackers can read packets from the network, but they cannot perform
any sort of active behavior such as probing servers or querying DNS.
A middlebox that filters based on plaintext packet contents is one
example of a passive attacker. In contrast, active attackers can
also write packets into the network for malicious purposes, such as
interfering with existing connections, probing servers, and querying
DNS. In short, an active attacker corresponds to the conventional
threat model for TLS 1.3 [RFC8446].
Given these types of attackers, the primary goals of ECH are as
follows.
1. 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.)
Rescorla, et al. Expires 6 April 2023 [Page 29]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 30]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 31]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 32]
Internet-Draft TLS Encrypted Client Hello October 2022
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
Rescorla, et al. Expires 6 April 2023 [Page 33]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 34]
Internet-Draft TLS Encrypted Client Hello October 2022
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;
Rescorla, et al. Expires 6 April 2023 [Page 35]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 36]
Internet-Draft TLS Encrypted Client Hello October 2022
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
------>
Rescorla, et al. Expires 6 April 2023 [Page 37]
Internet-Draft TLS Encrypted Client Hello October 2022
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
Rescorla, et al. Expires 6 April 2023 [Page 38]
Internet-Draft TLS Encrypted Client Hello October 2022
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.
Rescorla, et al. Expires 6 April 2023 [Page 39]
Internet-Draft TLS Encrypted Client Hello October 2022
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:
Rescorla, et al. Expires 6 April 2023 [Page 40]
Internet-Draft TLS Encrypted Client Hello October 2022
* 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".
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.
Rescorla, et al. Expires 6 April 2023 [Page 41]
Internet-Draft TLS Encrypted Client Hello October 2022
13. References
13.1. Normative References
[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-10, 24 May 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-10>.
[I-D.ietf-tls-exported-authenticator]
Sullivan, N., "Exported Authenticators in TLS", Work in
Progress, Internet-Draft, draft-ietf-tls-exported-
authenticator-15, 4 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
exported-authenticator-15>.
[I-D.irtf-cfrg-hpke]
Barnes, R., Bhargavan, K., Lipp, B., and C. A. Wood,
"Hybrid Public Key Encryption", Work in Progress,
Internet-Draft, draft-irtf-cfrg-hpke-12, 2 September 2021,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
hpke-12>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/rfc/rfc5890>.
[RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", RFC 7918,
DOI 10.17487/RFC7918, August 2016,
<https://www.rfc-editor.org/rfc/rfc7918>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
Rescorla, et al. Expires 6 April 2023 [Page 42]
Internet-Draft TLS Encrypted Client Hello October 2022
13.2. Informative References
[ECH-Analysis]
"A Symbolic Analysis of Privacy for TLS 1.3 with Encrypted
Client Hello", November 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, <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, January 2005,
<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,
January 2008, <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,
July 2014, <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, May
2016, <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, July 2016,
<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, February 2017,
<https://www.rfc-editor.org/rfc/rfc8094>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/rfc/rfc8484>.
Rescorla, et al. Expires 6 April 2023 [Page 43]
Internet-Draft TLS Encrypted Client Hello October 2022
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/rfc/rfc8701>.
[RFC8744] Huitema, C., "Issues and Requirements for Server Name
Identification (SNI) Encryption in TLS", RFC 8744,
DOI 10.17487/RFC8744, July 2020,
<https://www.rfc-editor.org/rfc/rfc8744>.
[WHATWG-IPV4]
"URL Living Standard - IPv4 Parser", May 2021,
<https://url.spec.whatwg.org/#concept-ipv4-parser>.
Appendix A. Alternative SNI Protection Designs
Alternative approaches to encrypted SNI may be implemented at the TLS
or application layer. In this section we describe several
alternatives and discuss drawbacks in comparison to the design in
this document.
A.1. TLS-layer
A.1.1. TLS in Early Data
In this variant, TLS Client Hellos are tunneled within early data
payloads belonging to outer TLS connections established with the
client-facing server. This requires clients to have established a
previous session --- and obtained PSKs --- with the server. The
client-facing server decrypts early data payloads to uncover Client
Hellos destined for the backend server, and forwards them onwards as
necessary. Afterwards, all records to and from backend servers are
forwarded by the client-facing server -- unmodified. This avoids
double encryption of TLS records.
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.
Rescorla, et al. Expires 6 April 2023 [Page 44]
Internet-Draft TLS Encrypted Client Hello October 2022
A.1.2. Combined Tickets
In this variant, client-facing and backend servers coordinate to
produce "combined tickets" that are consumable by both. Clients
offer combined tickets to client-facing servers. The latter parse
them to determine the correct backend server to which the Client
Hello should be forwarded. This approach is problematic due to non-
trivial coordination between client-facing and backend servers for
ticket construction and consumption. Moreover, it requires a
bootstrapping step similar to that of the previous variant. In
contrast, encrypted SNI requires no such coordination.
A.2. Application-layer
A.2.1. HTTP/2 CERTIFICATE Frames
In this variant, clients request secondary certificates with
CERTIFICATE_REQUEST HTTP/2 frames after TLS connection completion.
In response, servers supply certificates via TLS exported
authenticators [I-D.ietf-tls-exported-authenticator] in CERTIFICATE
frames. Clients use a generic SNI for the underlying client-facing
server TLS connection. Problems with this approach include: (1) one
additional round trip before peer authentication, (2) non-trivial
application-layer dependencies and interaction, and (3) obtaining the
generic SNI to bootstrap the connection. In contrast, encrypted SNI
induces no additional round trip and operates below the application
layer.
Appendix B. Linear-time Outer Extension Processing
The following procedure processes the "ech_outer_extensions"
extension (see Section 5.1) in linear time, 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.
Rescorla, et al. Expires 6 April 2023 [Page 45]
Internet-Draft TLS Encrypted Client Hello October 2022
* 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-15
* Add CCS2022 reference and summary (#539)
D.2. Since draft-ietf-tls-esni-14
* Keep-alive
D.3. Since draft-ietf-tls-esni-13
* Editorial improvements
D.4. 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)
Rescorla, et al. Expires 6 April 2023 [Page 46]
Internet-Draft TLS Encrypted Client Hello October 2022
D.5. 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.6. 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.7. 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)
Rescorla, et al. Expires 6 April 2023 [Page 47]
Internet-Draft TLS Encrypted Client Hello October 2022
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
Rescorla, et al. Expires 6 April 2023 [Page 48]