tls E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Experimental K. Oku
Expires: 3 December 2020 Fastly
N. Sullivan
C.A. Wood
Cloudflare
1 June 2020
TLS Encrypted Client Hello
draft-ietf-tls-esni-07
Abstract
This document describes a mechanism in Transport Layer Security (TLS)
for encrypting a ClientHello message under a server public key.
Status of This Memo
This 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 3 December 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Topologies . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Encrypted ClientHello (ECH) . . . . . . . . . . . . . . . 5
4. Encrypted ClientHello Configuration . . . . . . . . . . . . . 6
5. The "encrypted_client_hello" extension . . . . . . . . . . . 7
6. The "ech_nonce" extension . . . . . . . . . . . . . . . . . . 9
6.1. Incorporating Outer Extensions . . . . . . . . . . . . . 9
7. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Sending an encrypted ClientHello . . . . . . . . . . . . 10
7.2. Recommended Padding Scheme . . . . . . . . . . . . . . . 12
7.3. Handling the server response . . . . . . . . . . . . . . 12
7.3.1. Accepted ECH . . . . . . . . . . . . . . . . . . . . 13
7.3.2. Rejected ECH . . . . . . . . . . . . . . . . . . . . 13
7.3.3. HelloRetryRequest . . . . . . . . . . . . . . . . . . 14
7.4. GREASE extensions . . . . . . . . . . . . . . . . . . . . 15
8. Client-Facing Server Behavior . . . . . . . . . . . . . . . . 16
9. Compatibility Issues . . . . . . . . . . . . . . . . . . . . 18
9.1. Misconfiguration and Deployment Concerns . . . . . . . . 18
9.2. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10.1. Why is cleartext DNS OK? . . . . . . . . . . . . . . . . 19
10.2. Client Tracking . . . . . . . . . . . . . . . . . . . . 20
10.3. Optional Record Digests and Trial Decryption . . . . . . 20
10.4. Related Privacy Leaks . . . . . . . . . . . . . . . . . 20
10.5. Comparison Against Criteria . . . . . . . . . . . . . . 21
10.5.1. Mitigate against replay attacks . . . . . . . . . . 21
10.5.2. Avoid widely-deployed shared secrets . . . . . . . . 21
10.5.3. Prevent SNI-based DoS attacks . . . . . . . . . . . 21
10.5.4. Do not stick out . . . . . . . . . . . . . . . . . . 21
10.5.5. Forward secrecy . . . . . . . . . . . . . . . . . . 22
10.5.6. Proper security context . . . . . . . . . . . . . . 22
10.5.7. Split server spoofing . . . . . . . . . . . . . . . 22
10.5.8. Supporting multiple protocols . . . . . . . . . . . 22
10.6. Padding Policy . . . . . . . . . . . . . . . . . . . . . 22
10.7. Active Attack Mitigations . . . . . . . . . . . . . . . 22
10.7.1. Client Reaction Attack Mitigation . . . . . . . . . 23
10.7.2. HelloRetryRequest Hijack Mitigation . . . . . . . . 24
10.7.3. Resumption PSK Oracle Mitigation . . . . . . . . . . 25
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
11.1. Update of the TLS ExtensionType Registry . . . . . . . . 26
11.2. Update of the TLS Alert Registry . . . . . . . . . . . . 26
12. ECHConfig Extension Guidance . . . . . . . . . . . . . . . . 26
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
13.1. Normative References . . . . . . . . . . . . . . . . . . 26
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13.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Alternative SNI Protection Designs . . . . . . . . . 28
A.1. TLS-layer . . . . . . . . . . . . . . . . . . . . . . . . 28
A.1.1. TLS in Early Data . . . . . . . . . . . . . . . . . . 28
A.1.2. Combined Tickets . . . . . . . . . . . . . . . . . . 29
A.2. Application-layer . . . . . . . . . . . . . . . . . . . . 29
A.2.1. HTTP/2 CERTIFICATE Frames . . . . . . . . . . . . . . 29
Appendix B. Total Client Hello Encryption . . . . . . . . . . . 29
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
DISCLAIMER: This is very early a work-in-progress design and has not
yet seen significant (or really any) security analysis. It should
not be used as a basis for building production systems.
Although TLS 1.3 [RFC8446] encrypts most of the handshake, including
the server certificate, there are several ways in which an on-path
attacker can learn private information about the connection. The
cleartext Server Name Indication (SNI) extension in ClientHello
messages, which leaks the target domain for a given connection, is
perhaps the most sensitive information unencrypted in TLS 1.3.
The target domain may also be visible through other channels, such as
cleartext client DNS queries, visible server IP addresses (assuming
the server does not use domain-based virtual hosting), or other
indirect mechanisms such as traffic analysis. DoH
[I-D.ietf-doh-dns-over-https] and DPRIVE [RFC7858] [RFC8094] provide
mechanisms for clients to conceal DNS lookups from network
inspection, and many TLS servers host multiple domains on the same IP
address. In such environments, the SNI remains the primary explicit
signal used to determine the server's identity.
The TLS WG has extensively studied the problem of protecting the SNI,
but has been unable to develop a completely generic solution.
[I-D.ietf-tls-sni-encryption] provides a description of the problem
space and some of the proposed techniques. One of the more difficult
problems is "Do not stick out" ([I-D.ietf-tls-sni-encryption],
Section 3.4): if only sensitive or private services use SNI
encryption, then SNI encryption is a signal that a client is going to
such a service. For this reason, much recent work has focused on
concealing the fact that the SNI is being protected. Unfortunately,
the result often has undesirable performance consequences, incomplete
coverage, or both.
The design in this document takes a different approach: it assumes
that private origins will co-locate with or hide behind a provider
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(CDN, application server, etc.) which can protect SNIs for all of the
domains it hosts. As a result, SNI protection does not indicate that
the client is attempting to reach a private origin, but only that it
is going to a particular service provider, which the observer could
already tell from the visible IP address.
The design in this document introduces a new extension, called
Encrypted Client Hello (ECH), which allows clients to encrypt the
entirety of their ClientHello to a supporting server. This protects
the SNI and other potentially sensitive fields, such as the ALPN
list. This extension is only supported with (D)TLS 1.3 [RFC8446] and
newer versions of the protocol.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. All TLS notation comes from [RFC8446],
Section 3.
3. Overview
This document is designed to operate in one of two primary topologies
shown below, which we call "Shared Mode" and "Split Mode"
3.1. Topologies
+---------------------+
| |
| 2001:DB8::1111 |
| |
Client <-----> | private.example.org |
| |
| public.example.com |
| |
+---------------------+
Server
Figure 1: Shared Mode Topology
In Shared Mode, the provider is the origin server for all the domains
whose DNS records point to it and clients form a TLS connection
directly to that provider, which has access to the plaintext of the
connection.
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+--------------------+ +---------------------+
| | | |
| 2001:DB8::1111 | | 2001:DB8::EEEE |
Client <------------------------------------>| |
| public.example.com | | private.example.com |
| | | |
+--------------------+ +---------------------+
Client-Facing Server Backend Server
Figure 2: Split Mode Topology
In Split Mode, the provider is _not_ the origin server for private
domains. Rather the DNS records for private domains point to the
provider, and the provider's server relays the connection back to the
backend server, which is the true origin server. The provider does
not have access to the plaintext of the connection.
3.2. Encrypted ClientHello (ECH)
ECH works by encrypting the entire ClientHello, including the SNI and
any additional extensions such as ALPN. This requires that each
provider publish a public key and metadata which is used for
ClientHello encryption for all the domains for which it serves
directly or indirectly (via Split Mode). This document defines the
format of the SNI encryption public key and metadata, referred to as
an ECH configuration, and delegates DNS publication details to
[HTTPSSVC], though other delivery mechanisms are possible. In
particular, if some of the clients of a private server are
applications rather than Web browsers, those applications might have
the public key and metadata preconfigured.
When a client wants to form a TLS connection to any of the domains
served by an ECH-supporting provider, it constructs a ClientHello in
the regular fashion containing the true SNI value (ClientHelloInner)
and then encrypts it using the public key for the provider. It then
constructs a new ClientHello (ClientHelloOuter) with an innocuous SNI
(and potentially innocuous versions of other extensions such as ALPN
[RFC7301]) and containing the encrypted ClientHelloInner as an
extension. It sends ClientHelloOuter to the server.
Upon receiving ClientHelloOuter, the server can then decrypt
ClientHelloInner and either terminate the connection (in Shared Mode)
or forward it to the backend server (in Split Mode).
Note that both ClientHelloInner and ClientHelloOuter are both valid,
complete ClientHello messages. ClientHelloOuter carries an encrypted
representation of ClientHelloInner in a "encrypted_client_hello"
extension, defined in Section 5.
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4. Encrypted ClientHello Configuration
ClientHello encryption configuration information is conveyed with the
following ECHConfigs structure.
opaque HpkePublicKey<1..2^16-1>;
uint16 HkpeKemId; // Defined in I-D.irtf-cfrg-hpke
uint16 HkpeKdfId; // Defined in I-D.irtf-cfrg-hpke
uint16 HkpeAeadId; // Defined in I-D.irtf-cfrg-hpke
struct {
HkpeKdfId kdf_id;
HkpeAeadId aead_id;
} HpkeCipherSuite;
struct {
opaque public_name<1..2^16-1>;
HpkePublicKey public_key;
HkpeKemId kem_id;
HpkeCipherSuite cipher_suites<4..2^16-2>;
uint16 maximum_name_length;
Extension extensions<0..2^16-1>;
} ECHConfigContents;
struct {
uint16 version;
uint16 length;
select (ECHConfig.version) {
case 0xff07: ECHConfigContents;
}
} ECHConfig;
ECHConfig ECHConfigs<1..2^16-1>;
The ECHConfigs 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.
The ECHConfig structure contains the following fields:
version The version of the structure. For this specification, that
value SHALL be 0xff07. Clients MUST ignore any ECHConfig
structure with a version they do not understand.
contents An opaque byte string whose contents depend on the version
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of the structure. For this specification, the contents are an
ECHConfigContents structure.
The ECHConfigContents structure contains the following fields:
public_name The non-empty name of the entity trusted to update these
encryption keys. This is used to repair misconfigurations, as
described in Section 7.3.
public_key The HPKE [I-D.irtf-cfrg-hpke] public key which can be
used by the client to encrypt the ClientHello.
kem_id The HPKE [I-D.irtf-cfrg-hpke] KEM identifier corresponding to
public_key. Clients MUST ignore any ECHConfig structure with a
key using a KEM they do not support.
cipher_suites The list of HPKE [I-D.irtf-cfrg-hpke] AEAD and KDF
identifier pairs clients can use for encrypting the ClientHello.
maximum_name_length The largest name the server expects to support,
if known. If this value is not known it can be set to zero, in
which case clients SHOULD use the inner ClientHello padding scheme
described below. That could happen if wildcard names are in use,
or if names can be added or removed from the anonymity set during
the lifetime of a particular resource record value.
extensions A list of extensions that the client can take into
consideration when generating a ClientHello message. The purpose
of the field is to provide room for additional functionality in
the future. See Section 12 for guidance on what type of
extensions are appropriate for this structure.
The format is defined in [RFC8446], Section 4.2. The same
interpretation rules apply: extensions MAY appear in any order, but
there MUST NOT be more than one extension of the same type in the
extensions block. An extension can be tagged as mandatory by using
an extension type codepoint with the high order bit set to 1. A
client which receives a mandatory extension they do not understand
MUST reject the ECHConfig content.
Clients MUST parse the extension list and check for unsupported
mandatory extensions. If an unsupported mandatory extension is
present, clients MUST reject the ECHConfig value.
5. The "encrypted_client_hello" extension
The encrypted ClientHelloInner is carried in an
"encrypted_client_hello" extension, defined as follows:
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enum {
encrypted_client_hello(0xff02), (65535)
} ExtensionType;
For clients (in ClientHello), this extension contains the following
ClientEncryptedCH structure:
struct {
HpkeCipherSuite suite;
opaque record_digest<0..2^16-1>;
opaque enc<1..2^16-1>;
opaque encrypted_ch<1..2^16-1>;
} ClientEncryptedCH;
suite The HpkeCipherSuite cipher suite used to encrypt
ClientHelloInner. This MUST match a value provided in the
corresponding ECHConfig.cipher_suites list.
record_digest A cryptographic hash of the ECHConfig structure from
which the ECH key was obtained, i.e., from the first byte of
"version" to the end of the structure. This hash is computed
using the hash function associated with "suite", i.e., the
corresponding HPKE KDF algorithm hash.
enc The HPKE encapsulated key, used by servers to decrypt the
corresponding encrypted_ch field.
encrypted_ch The serialized and encrypted ClientHelloInner
structure, AEAD-encrypted using HPKE with the selected KEM, KDF,
and AEAD algorithm and key generated as described below.
If the server accepts ECH, it does not send this extension. If it
rejects ECH, then it sends the following structure in
EncryptedExtensions:
struct {
ECHConfigs retry_configs;
} ServerEncryptedCH;
retry_configs An ECHConfigs structure containing one or more
ECHConfig structures in decreasing order of preference that the
client should use on subsequent connections to encrypt the
ClientHelloInner structure.
This protocol also defines the "ech_required" alert, which is sent by
the client when it offered an "encrypted_client_hello" extension
which was not accepted by the server.
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6. The "ech_nonce" extension
When using ECH, the client MUST also add an extension of type
"ech_nonce" to the ClientHelloInner (but not to the outer
ClientHello). This nonce ensures that the server's encrypted
Certificate can only be read by the entity which sent this
ClientHello. This extension is defined as follows:
enum {
ech_nonce(0xff03), (65535)
} ExtensionType;
struct {
uint8 nonce[16];
} ECHNonce;
nonce A 16-byte nonce exported from the HPKE encryption context.
See Section 7.1 for details about its computation.
Finally, requirements in Section 7 and Section 8 require
implementations to track, alongside each PSK established by a
previous connection, whether the connection negotiated this extension
with the "ech_accept" response type. If so, this is referred to as
an "ECH PSK". Otherwise, it is a "non-ECH PSK". This may be
implemented by adding a new field to client and server session
states.
6.1. Incorporating Outer Extensions
Some TLS 1.3 extensions can be quite large and having them both in
the inner and outer ClientHello will lead to a very large overall
size. One particularly pathological example is "key_share" with
post-quantum algorithms. In order to reduce the impact of duplicated
extensions, the client may use the "outer_extensions" extension.
enum {
outer_extension(0xff04), (65535)
} ExtensionType;
struct {
ExtensionType outer_extensions<2..254>;
uint8 hash<32..255>;
} OuterExtensions;
OuterExtensions MUST only be used in ClientHelloInner. It consists
of one or more ExtensionType values, each of which reference an
extension in ClientHelloOuter, and a digest of the complete
ClientHelloInner.
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When sending ClientHello, the client first computes ClientHelloInner,
including any PSK binders, and then MAY substitute extensions which
it knows will be duplicated in ClientHelloOuter. To do so, the
client computes a hash H of the entire ClientHelloInner message with
the same hash as for the KDF used to encrypt ClienHelloInner. Then,
the client removes and and replaces extensions from ClientHelloInner
with a single "outer_extensions" extension. The list of
outer_extensions include those which were removed from
ClientHelloInner, in the order in which they were removed. The hash
contains the full ClientHelloInner hash H computed above.
This process is reversed by client-facing servers upon receipt.
Specifically, the server replaces the "outer_extensions" with
extensions contained in ClientHelloOuter. The server then computes a
hash H' of the reconstructed ClientHelloInner. If H' does not equal
OuterExtensions.hash, the server aborts the connection with an
"illegal_parameter" alert.
Clients SHOULD only use this mechanism for extensions which are
large. All other extensions SHOULD appear in both ClientHelloInner
and ClientHelloOuter even if they have identical values.
7. Client Behavior
7.1. Sending an encrypted ClientHello
In order to send an encrypted ClientHello, the client first
determines if it supports the server's chosen KEM, as identified by
ECHConfig.kem_id. If one is supported, the client MUST select an
appropriate HpkeCipherSuite from the list of suites offered by the
server. If the client does not support the corresponding KEM or is
unable to select an appropriate group or HpkeCipherSuite, it SHOULD
ignore that ECHConfig value and MAY attempt to use another value
provided by the server. The client MUST NOT send ECH using HPKE
algorithms not advertised by the server.
Given a compatible ECHConfig with fields public_key and kem_id,
carrying the HpkePublicKey and KEM identifier corresponding to the
server, clients compute an HPKE encryption context as follows:
pkR = HPKE.KEM.Unmarshal(ECHConfig.public_key)
enc, context = SetupBaseS(pkR, "tls13-ech")
ech_nonce_value = context.Export("tls13-ech-nonce", 16)
ech_hrr_key = context.Export("tls13-ech-hrr-key", 16)
Note that the HPKE algorithm identifiers are those which match the
client's chosen preference from ECHConfig.cipher_suites. The client
MAY replace any large, duplicated extensions in ClientHelloInner with
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the corresponding "outer_extensions" extension, as described in
Section 6.1.
The client then generates a ClientHelloInner value. In addition to
the normal values, ClientHelloInner MUST also contain:
* an "ech_nonce" extension, containing "ech_nonce_value" derived
above
* TLS padding [RFC7685] (see Section 7.2)
When offering an encrypted ClientHello, the client MUST NOT offer to
resume any non-ECH PSKs. It additionally MUST NOT offer to resume
any sessions for TLS 1.2 or below.
The encrypted ClientHello value is then computed as:
encrypted_ch = context.Seal("", ClientHelloInner)
Finally, the client MUST generate a ClientHelloOuter message
containing the "encrypted_client_hello" extension with the values as
indicated above. In particular,
* suite contains the client's chosen HpkeCipherSuite;
* record_digest contains the digest of the corresponding ECHConfig
structure;
* enc contains the encapsulated key as output by SetupBaseS; and
* encrypted_ch contains the HPKE encapsulated key (enc) and the
ClientHelloInner ciphertext (encrypted_ch_inner).
The client MUST place the value of ECHConfig.public_name in the
ClientHelloOuter "server_name" extension. The ClientHelloOuter MUST
NOT contain a "cached_info" extension [RFC7924] with a CachedObject
entry whose CachedInformationType is "cert", since this indication
would divulge the true server name. The remaining contents of the
ClientHelloOuter MAY be identical to those in ClientHelloInner but
MAY also differ.
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7.2. 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. For the
"server_name" extension with length D, clients SHOULD use the
server's length hint L (ECHCOnfig.maximum_name_length) when computing
the padding as follows:
1. If L > D, add L - D bytes of padding. This rounds to the
server's advertised hint, i.e., ECHConfig.maximum_name_length.
2. Otherwise, add 32 - (D % 32) bytes of padding. This rounds D up
to the nearest multiple of 32 bytes.
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.
7.3. Handling the server response
As described in Section 8, the server MAY either accept ECH and use
ClientHelloInner or reject it and use ClientHelloOuter. However,
there is no indication in ServerHello of which one the server has
done and the client must therefore use trial decryption in order to
determine this.
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7.3.1. Accepted ECH
If the server used ClientHelloInner, the client proceeds with the
connection as usual, authenticating the connection for the origin
server.
7.3.2. Rejected ECH
If the server used ClientHelloOuter, the client proceeds with the
handshake, authenticating for ECHConfig.public_name as described in
Section 7.3.2.1. If authentication or the handshake fails, the
client MUST return a failure to the calling application. It MUST NOT
use the retry keys.
Otherwise, when the handshake completes successfully with the public
name authenticated, the client MUST abort the connection with an
"ech_required" alert. It then processes the "retry_keys" field from
the server's "encrypted_client_hello" extension.
If one of the values contains a version supported by the client, it
can regard the ECH keys as securely replaced by the server. It
SHOULD retry the handshake with a new transport connection, using
that value to encrypt the ClientHello. The value may only be applied
to the retry connection. The client MUST continue to use the
previously-advertised keys for subsequent connections. This avoids
introducing pinning concerns or a tracking vector, should a malicious
server present client-specific retry keys to identify clients.
If none of the values provided in "retry_keys" contains a supported
version, the client can regard ECH as securely disabled by the
server. As below, it SHOULD then retry the handshake with a new
transport connection and ECH disabled.
If the field contains any other value, the client MUST abort the
connection with an "illegal_parameter" alert.
If the server negotiates an earlier version of TLS, or if it does not
provide an "encrypted_client_hello" extension in EncryptedExtensions,
the client proceeds with the handshake, authenticating for
ECHConfigContents.public_name as described in Section 7.3.2.1. If an
earlier version was negotiated, the client MUST NOT enable the False
Start optimization [RFC7918] for this handshake. If authentication
or the handshake fails, the client MUST return a failure to the
calling application. It MUST NOT treat this as a secure signal to
disable ECH.
Otherwise, when the handshake completes successfully with the public
name authenticated, the client MUST abort the connection with an
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"ech_required" alert. The client can then regard ECH as securely
disabled by the server. It SHOULD retry the handshake with a new
transport connection and ECH disabled.
Clients SHOULD implement a limit on retries caused by
"ech_retry_request" or servers which do not acknowledge the
"encrypted_client_hello" extension. If the client does not retry in
either scenario, it MUST report an error to the calling application.
7.3.2.1. Authenticating for the public name
When the server cannot decrypt or does not process the
"encrypted_client_hello" extension, it continues with the handshake
using the cleartext "server_name" extension instead (see Section 8).
Clients that offer ECH then authenticate the connection with the
public name, as follows:
* If the server resumed a session or negotiated a session that did
not use a certificate for authentication, the client MUST abort
the connection with an "illegal_parameter" alert. This case is
invalid because Section 7.1 requires the client to only offer ECH-
established sessions, and Section 8 requires the server to decline
ECH-established sessions if it did not accept ECH.
* The client MUST verify that the certificate is valid for
ECHConfigContents.public_name. If invalid, it MUST abort the
connection with the appropriate alert.
* If the server requests a client certificate, the client MUST
respond with an empty Certificate message, denoting no client
certificate.
Note that authenticating a connection for the public name does not
authenticate it for the origin. The TLS implementation MUST NOT
report such connections as successful to the application. It
additionally MUST ignore all session tickets and session IDs
presented by the server. These connections are only used to trigger
retries, as described in Section 7.3. This may be implemented, for
instance, by reporting a failed connection with a dedicated error
code.
7.3.3. HelloRetryRequest
If the server sends a HelloRetryRequest in response to the
ClientHello, the client sends a second updated ClientHello per the
rules in [RFC8446]. However, at this point, the client does not know
whether the server processed ClientHelloOuter or ClientHelloInner,
and MUST regenerate both values to be acceptable. Note: if the inner
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and outer ClientHellos use different groups for their key shares or
differ in some other way, then the HelloRetryRequest may actually be
invalid for one or the other ClientHello, in which case a fresh
ClientHello MUST be generated, ignoring the instructions in
HelloRetryRequest. Otherwise, the usual rules for HelloRetryRequest
processing apply.
Clients bind encryption of the second ClientHelloInner to encryption
of the first ClientHelloInner via the derived ech_hrr_key by
modifying HPKE setup as follows:
pkR = HPKE.KEM.Unmarshal(ECHConfig.public_key)
enc, context = SetupPSKS(pkR, "tls13-ech-hrr", ech_hrr_key, "")
ech_nonce_value = context.Export("tls13-ech-hrr-nonce", 16)
Clients then encrypt the second ClientHelloInner using this new HPKE
context. In doing so, the encrypted value is also authenticated by
ech_hrr_key. The rationale for this is described in Section 10.7.2.
Client-facing servers perform the corresponding process when
decrypting second ClientHelloInner messages. In particular, upon
receipt of a second ClientHello message with a ClientEncryptedCH
value, servers setup their HPKE context and decrypt ClientEncryptedCH
as follows:
context = SetupPSKR(ClientEncryptedCH.enc, skR, "tls13-ech-hrr", ech_hrr_key, "")
ClientHelloInner = context.Open("", ClientEncryptedCH.encrypted_ch)
ech_nonce_value = context.Export("tls13-ech-hrr-nonce", 16)
[[OPEN ISSUE: Should we be using the PSK input or the info input? On
the one hand, the requirements on info seem weaker, but maybe
actually this needs to be secret? Analysis needed.]]
7.4. GREASE extensions
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 as follows:
* Set the "suite" field to a supported HpkeCipherSuite. 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 "record_digest" field to a randomly-generated string of
hash_length bytes, where hash_length is the length of the hash
function associated with the chosen HpkeCipherSuite.
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* Set the "enc" field to a randomly-generated valid encapsulated
public key output by the HPKE KEM.
* Set the "encrypted_ch" field to a randomly-generated string of L
bytes, where L is the size of the ClientHelloInner message the
client would use given an ECHConfig structure, padded according to
Section 7.2.
If the server sends an "encrypted_client_hello" extension, the client
MUST check the extension syntactically and abort the connection with
a "decode_error" alert if it is invalid.
Offering a GREASE extension is not considered offering an encrypted
ClientHello for purposes of requirements in Section 7. In
particular, the client MAY offer to resume sessions established
without ECH.
8. Client-Facing Server Behavior
Upon receiving an "encrypted_client_hello" extension, the client-
facing server MUST check that it is able to negotiate TLS 1.3 or
greater. If not, it MUST abort the connection with a
"handshake_failure" alert.
The ClientEncryptedCH value is said to match a known ECHConfig if
there exists an ECHConfig that can be used to successfully decrypt
ClientEncryptedCH.encrypted_ch. This matching procedure should be
done using one of the following two checks:
1. Compare ClientEncryptedCH.record_digest against cryptographic
hashes of known ECHConfig and choose the one that matches.
2. Use trial decryption of ClientEncryptedCH.encrypted_ch with known
ECHConfig and choose the one that succeeds.
Some uses of ECH, such as local discovery mode, may omit the
ClientEncryptedCH.record_digest since it can be used as a tracking
vector. In such cases, trial decryption should be used for matching
ClientEncryptedCH to known ECHConfig. Unless specified by the
application using (D)TLS or externally configured on both sides,
implementations MUST use the first method.
If the ClientEncryptedCH value does not match any known ECHConfig
structure, it MUST ignore the extension and proceed with the
connection, with the following added behavior:
* It MUST include the "encrypted_client_hello" extension with the
"retry_keys" field set to one or more ECHConfig structures with
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up-to-date keys. Servers MAY supply multiple ECHConfig values of
different versions. This allows a server to support multiple
versions at once.
* The server MUST ignore all PSK identities in the ClientHello which
correspond to ECH PSKs. ECH PSKs offered by the client are
associated with the ECH name. The server was unable to decrypt
then ECH name, so it should not resume them when using the
cleartext SNI name. This restriction allows a client to reject
resumptions in Section 7.3.2.1.
Note that an unrecognized ClientEncryptedCH.record_digest value may
be a GREASE ECH extension (see Section 7.4), 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 9.1). Instead, servers can measure occurrences of the
"ech_required" alert to detect this case.
If the ClientEncryptedCH value matches a known ECHConfig, the server
then decrypts ClientEncryptedCH.encrypted_ch, using the private key
skR corresponding to ECHConfig, as follows:
context = SetupBaseR(ClientEncryptedCH.enc, skR, "tls13-ech")
ClientHelloInner = context.Open("", ClientEncryptedCH.encrypted_ch)
ech_nonce_value = context.Export("tls13-ech-nonce", 16)
ech_hrr_key = context.Export("tls13-ech-hrr-key", 16)
If decryption fails, the server MUST abort the connection with a
"decrypt_error" alert. Moreover, if there is no "ech_nonce"
extension, or if its value does not match the derived ech_nonce, the
server MUST abort the connection with a "decrypt_error" alert. Next,
the server MUST scan ClientHelloInner for any "outer_extension"
extensions and substitute their values with the values in
ClientHelloOuter. It MUST first verify that the hash found in the
extension matches the hash of the extension to be interpolated in and
if it does not, abort the connections with a "decrypt_error" alert.
Upon determining the true SNI, the client-facing server then either
serves the connection directly (if in Shared Mode), in which case it
executes the steps in the following section, or forwards the TLS
connection to the backend server (if in Split Mode). In the latter
case, it does not make any changes to the TLS messages, but just
blindly forwards them.
If the server sends a NewSessionTicket message, the corresponding ECH
PSK MUST be ignored by all other servers in the deployment when not
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negotiating ECH, including servers which do not implement this
specification.
9. 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 cleartext 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.
9.1. Misconfiguration and Deployment Concerns
It is possible for ECH advertisements and servers to become
inconsistent. This may occur, for instance, from DNS
misconfiguration, caching issues, or an incomplete rollout in a
multi-server deployment. This may also occur if a server loses its
ECH keys, or if a deployment of ECH must be rolled back on the
server.
The retry mechanism repairs inconsistencies, provided the server is
authoritative for the public name. If server and advertised keys
mismatch, the server will respond with ech_retry_requested. If the
server does not understand the "encrypted_client_hello" extension at
all, it will ignore it as required by [RFC8446]; Section 4.1.2.
Provided the server can present a certificate valid for the public
name, the client can safely retry with updated settings, as described
in Section 7.3.
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.
Client-facing servers with non-uniform cryptographic configurations
across backend origin servers segment the ECH anonymity set based on
these configurations. For example, if a client-facing server hosts k
backend origin servers, and exactly one of those backend origin
servers supports a different set of cryptographic algorithms than the
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other (k - 1) servers, it may be possible to identify this single
server based on the contents of the ServerHello as this message is
not encrypted.
9.2. Middleboxes
A more serious problem is MITM proxies which do not support this
extension. [RFC8446], Section 9.3 requires that such proxies remove
any extensions they do not understand. The handshake will then
present a certificate based on the public name, without eching the
"encrypted_client_hello" extension to the client.
Depending on whether the client is configured to accept the proxy's
certificate as authoritative for the public name, this may trigger
the retry logic described in Section 7.3 or result in a connection
failure. A proxy which is not authoritative for the public name
cannot forge a signal to disable ECH.
A non-conformant MITM proxy which instead forwards the ECH extension,
substituting its own KeyShare value, will result in the client-facing
server recognizing the key, but failing to decrypt the SNI. This
causes a hard failure. Clients SHOULD NOT attempt to repair the
connection in this case.
10. Security Considerations
10.1. Why is cleartext DNS OK?
In comparison to [I-D.kazuho-protected-sni], wherein DNS Resource
Records are signed via a server private key, ECH records have no
authenticity or provenance information. This means that any attacker
which can inject DNS responses or poison DNS caches, which is a
common scenario in client access networks, can supply clients with
fake ECH records (so that the client encrypts data to them) or strip
the ECH record from the response. However, in the face of an
attacker that controls DNS, no encryption scheme can work because the
attacker can replace the IP address, thus blocking client
connections, or substituting a unique IP address which is 1:1 with
the DNS name that was looked up (modulo DNS wildcards). Thus,
allowing the ECH records in the clear does not make the situation
significantly worse.
Clearly, DNSSEC (if the client validates and hard fails) is a defense
against this form of attack, but DoH/DPRIVE are also defenses against
DNS attacks by attackers on the local network, which is a common case
where ClientHello and SNI encryption are desired. Moreover, as noted
in the introduction, SNI encryption is less useful without encryption
of DNS queries in transit via DoH or DPRIVE mechanisms.
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10.2. 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 HTTPSSVC RRs with high
TTLs, challenging. Clients can help mitigate this problem by
flushing any DNS or ECHConfig state upon changing networks.
10.3. Optional Record Digests and Trial Decryption
Optional record digests 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, servers must perform trial decrypt upon receipt of an
empty record digest, which may exacerbate DoS attacks. Specifically,
an adversary may send malicious ClientHello messages, i.e., those
which will not decrypt with any known ECH key, in order to force
wasteful decryption. Servers that support this feature should, for
example, implement some form of rate limiting mechanism to limit the
damage caused by such attacks.
10.4. 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
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SHOULD take steps to minimize or protect such requests during
certificate validation.
10.5. Comparison Against Criteria
[I-D.ietf-tls-sni-encryption] lists several requirements for SNI
encryption. In this section, we re-iterate these requirements and
assess the ECH design against them.
10.5.1. Mitigate against replay attacks
Since servers process either ClientHelloInner or ClientHelloOuter,
and ClientHelloInner contains an HPKE-derived nonce, it is not
possible for an attacker to "cut and paste" the ECH value in a
different Client Hello and learn information from ClientHelloInner.
This is because the attacker lacks access to the HPKE-derived nonce
used to derive the handshake secrets.
10.5.2. Avoid widely-deployed shared secrets
This design depends upon DNS as a vehicle for semi-static public key
distribution. Server operators may partition their private keys
however they see fit provided each server behind an IP address has
the corresponding private key to decrypt a key. Thus, when one 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.
10.5.3. Prevent SNI-based DoS attacks
This design requires servers to decrypt ClientHello messages with
ClientEncryptedCH extensions carrying valid digests. Thus, it is
possible for an attacker to force decryption operations on the
server. This attack is bound by the number of valid TCP connections
an attacker can open.
10.5.4. Do not stick out
The only explicit signal indicating possible use of ECH is the
ClientHello "encrypted_client_hello" extension. Server handshake
messages do not contain any signal indicating use or negotiation of
ECH. Clients MAY GREASE the "encrypted_client_hello" extension, as
described in Section 7.4, which helps ensure the ecosystem handles
ECH correctly. Moreover, as more clients enable ECH support, e.g.,
as normal part of Web browser functionality, with keys supplied by
shared hosting providers, the presence of ECH extensions becomes less
unusual and part of typical client behavior. In other words, if all
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Web browsers start using ECH, the presence of this value will not
signal unusual behavior to passive eavesdroppers.
10.5.5. 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.
10.5.6. Proper security context
This design permits servers operating in Split Mode to forward
connections directly to backend origin servers, thereby avoiding
unnecessary MiTM attacks.
10.5.7. Split server spoofing
Assuming ECH records retrieved from DNS are authenticated, e.g., via
DNSSEC or fetched from a trusted Recursive Resolver, spoofing a
server operating in Split Mode is not possible. See Section 10.1 for
more details regarding cleartext DNS.
Authenticating the ECHConfigs structure naturally authenticates the
included public name. This also authenticates any retry signals from
the server because the client validates the server certificate
against the public name before retrying.
10.5.8. Supporting multiple protocols
This design has no impact on application layer protocol negotiation.
It may affect connection routing, server certificate selection, and
client certificate verification. Thus, it is compatible with
multiple protocols.
10.6. Padding Policy
Variations in the length of the ClientHelloInner ciphertext could
leak information about the corresponding plaintext. Section 7.2
describes a RECOMMENDED padding mechanism for clients aimed at
reducing potential information leakage.
10.7. 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.
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10.7.1. Client Reaction Attack Mitigation
This attack uses the client's reaction to an incorrect certificate as
an oracle. The attacker intercepts a legitimate ClientHello and
replies with a ServerHello, Certificate, CertificateVerify, and
Finished messages, wherein the Certificate message contains a "test"
certificate for the domain name it wishes to query. If the client
decrypted the Certificate and failed verification (or leaked
information about its verification process by a timing side channel),
the attacker learns that its test certificate name was incorrect. As
an example, suppose the client's SNI value in its inner ClientHello
is "example.com," and the attacker replied with a Certificate for
"test.com". If the client produces a verification failure alert
because of the mismatch faster than it would due to the Certificate
signature validation, information about the name leaks. Note that
the attacker can also withhold the CertificateVerify message. In
that scenario, a client which first verifies the Certificate would
then respond similarly and leak the same information.
Client Attacker Server
ClientHello
+ key_share
+ ech ------> (intercept) -----> X (drop)
ServerHello
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
<------
Alert
------>
Figure 3: Client reaction attack
The "ech_nonce" extension in the inner ClientHello 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.
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10.7.2. HelloRetryRequest Hijack Mitigation
This attack aims to exploit server HRR state management to recover
information about a legitimate ClientHello using its own attacker-
controlled ClientHello. To begin, the attacker intercepts and
forwards a legitimate ClientHello with an "encrypted_client_hello"
(ech) extension to the server, which triggers a legitimate
HelloRetryRequest in return. Rather than forward the retry to the
client, the attacker, attempts to generate its own ClientHello in
response based on the contents of the first ClientHello and
HelloRetryRequest exchange with the result that the server encrypts
the Certificate to the attacker. If the server used the SNI from the
first ClientHello and the key share from the second (attacker-
controlled) ClientHello, the Certificate produced would leak the
client's chosen SNI to the attacker.
Client Attacker Server
ClientHello
+ key_share
+ ech ------> (forward) ------->
HelloRetryRequest
+ key_share
(intercept) <-------
ClientHello
+ key_share'
+ ech' ------->
ServerHello
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
{Finished}
<-------
(process server flight)
Figure 4: HelloRetryRequest hijack attack
This attack is mitigated by binding the first and second ClientHello
messages together. In particular, since the attacker does not
possess the ech_hrr_key, it cannot generate a valid encryption of the
second inner ClientHello. The server will attempt decryption using
ech_hrr_key, detect failure, and fail the connection.
If the second ClientHello were not bound to the first, 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.
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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.
10.7.3. Resumption PSK Oracle Mitigation
This attack uses resumption PSKs as an oracle for dictionary attacks
against a given ClientHello's true SNI. To begin, the attacker first
interacts with a server to obtain a resumption ticket for a given
test domain, such as "test.com". Later, upon receipt of a legitimate
ClientHello without a PSK binder, it computes a PSK binder using its
own ticket and forwards the resulting ClientHello. Assume the server
then validates the PSK binder on the outer ClientHello and chooses
connection parameters based on the inner ClientHello. A server which
then validates information in the outer ClientHello ticket against
information in the inner ClientHello, such as the SNI, introduces an
oracle that can be used to test the encrypted SNI value of specific
ClientHello messages.
Client Attacker Server
handshake and ticket
for "test.com"
<-------->
ClientHello
+ key_share
+ ech --------> (intercept)
ClientHello
+ key_share
+ ech
+ pre_shared_key
-------->
Alert
<--------
Figure 5: Message flow for resumption and PSK
ECH mitigates against this attack by requiring servers not mix-and-
match information from the inner and outer ClientHello. For example,
if the server accepts the inner ClientHello, it does not validate
binders in the outer ClientHello. This means that ECH PSKs are used
within the HPKE encryption envelope.
11. IANA Considerations
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11.1. Update of the TLS ExtensionType Registry
IANA is requested to create the following two entries in the existing
registry for ExtensionType (defined in [RFC8446]):
1. encrypted_client_hello(0xff02), with "TLS 1.3" column values
being set to "CH, EE", and "Recommended" column being set to
"Yes".
2. ech_nonce(0xff03), with the "TLS 1.3" column values being set to
"CH", and "Recommended" column being set to "Yes".
3. outer_extension(0xff04), with the "TLS 1.3" column values being
set to "CH", and "Recommended" column being 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 being set to "Y".
12. ECHConfig Extension Guidance
Any future information or hints that influence the outer ClientHello
SHOULD be specified as ECHConfig extensions, or in an entirely new
version of ECHConfig. 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 8). In contrast, the
inner ClientHello is the true ClientHello used upon ECH negotiation.
13. References
13.1. Normative References
[HTTPSSVC] Schwartz, B., Bishop, M., and E. Nygren, "Service binding
and parameter specification via the DNS (DNS SVCB and
HTTPSSVC)", Work in Progress, Internet-Draft, draft-
nygren-dnsop-svcb-httpssvc-00, 23 September 2019,
<http://www.ietf.org/internet-drafts/draft-nygren-dnsop-
svcb-httpssvc-00.txt>.
[I-D.ietf-tls-exported-authenticator]
Sullivan, N., "Exported Authenticators in TLS", Work in
Progress, Internet-Draft, draft-ietf-tls-exported-
authenticator-12, 15 May 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-tls-exported-authenticator-
12.txt>.
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[I-D.irtf-cfrg-hpke]
Barnes, R., Bhargavan, K., and C. Wood, "Hybrid Public Key
Encryption", Work in Progress, Internet-Draft, draft-irtf-
cfrg-hpke-04, 8 May 2020, <http://www.ietf.org/internet-
drafts/draft-irtf-cfrg-hpke-04.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7685] Langley, A., "A Transport Layer Security (TLS) ClientHello
Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
October 2015, <https://www.rfc-editor.org/info/rfc7685>.
[RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", RFC 7918,
DOI 10.17487/RFC7918, August 2016,
<https://www.rfc-editor.org/info/rfc7918>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<https://www.rfc-editor.org/info/rfc7924>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
13.2. Informative References
[I-D.ietf-doh-dns-over-https]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", Work in Progress, Internet-Draft, draft-ietf-doh-
dns-over-https-14, 16 August 2018, <http://www.ietf.org/
internet-drafts/draft-ietf-doh-dns-over-https-14.txt>.
[I-D.ietf-tls-sni-encryption]
Huitema, C. and E. Rescorla, "Issues and Requirements for
SNI Encryption in TLS", Work in Progress, Internet-Draft,
draft-ietf-tls-sni-encryption-09, 28 October 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-tls-sni-
encryption-09.txt>.
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[I-D.kazuho-protected-sni]
Oku, K., "TLS Extensions for Protecting SNI", Work in
Progress, Internet-Draft, draft-kazuho-protected-sni-00,
18 July 2017, <http://www.ietf.org/internet-drafts/draft-
kazuho-protected-sni-00.txt>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/info/rfc8701>.
Appendix A. Alternative SNI Protection Designs
Alternative approaches to encrypted SNI may be implemented at the TLS
or application layer. In this section we describe several
alternatives and discuss drawbacks in comparison to the design in
this document.
A.1. TLS-layer
A.1.1. TLS in Early Data
In this variant, TLS Client Hellos are tunneled within early data
payloads belonging to outer TLS connections established with the
client-facing server. This requires clients to have established a
previous session --- and obtained PSKs --- with the server. The
client-facing server decrypts early data payloads to uncover Client
Hellos destined for the backend server, and forwards them onwards as
necessary. Afterwards, all records to and from backend servers are
forwarded by the client-facing server - unmodified. This avoids
double encryption of TLS records.
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Problems with this approach are: (1) servers may not always be able
to distinguish inner Client Hellos from legitimate application data,
(2) nested 0-RTT data may not function correctly, (3) 0-RTT data may
not be supported - especially under DoS - leading to availability
concerns, and (4) clients must bootstrap tunnels (sessions), costing
an additional round trip and potentially revealing the SNI during the
initial connection. In contrast, encrypted SNI protects the SNI in a
distinct Client Hello extension and neither abuses early data nor
requires a bootstrapping connection.
A.1.2. Combined Tickets
In this variant, client-facing and backend servers coordinate to
produce "combined tickets" that are consumable by both. Clients
offer combined tickets to client-facing servers. The latter parse
them to determine the correct backend server to which the Client
Hello should be forwarded. This approach is problematic due to non-
trivial coordination between client-facing and backend servers for
ticket construction and consumption. Moreover, it requires a
bootstrapping step similar to that of the previous variant. In
contrast, encrypted SNI requires no such coordination.
A.2. Application-layer
A.2.1. HTTP/2 CERTIFICATE Frames
In this variant, clients request secondary certificates with
CERTIFICATE_REQUEST HTTP/2 frames after TLS connection completion.
In response, servers supply certificates via TLS exported
authenticators [I-D.ietf-tls-exported-authenticator] in CERTIFICATE
frames. Clients use a generic SNI for the underlying client-facing
server TLS connection. Problems with this approach include: (1) one
additional round trip before peer authentication, (2) non-trivial
application-layer dependencies and interaction, and (3) obtaining the
generic SNI to bootstrap the connection. In contrast, encrypted SNI
induces no additional round trip and operates below the application
layer.
Appendix B. Total Client Hello Encryption
The design described here only provides encryption for the SNI, but
not for other extensions, such as ALPN. Another potential design
would be to encrypt all of the extensions using the same basic
structure as we use here for ECH. That design has the following
advantages:
* It protects all the extensions from ordinary eavesdroppers
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* If the encrypted block has its own KeyShare, it does not
necessarily require the client to use a single KeyShare, because
the client's share is bound to the SNI by the AEAD (analysis
needed).
It also has the following disadvantages:
* The client-facing server can still see the other extensions. By
contrast we could introduce another EncryptedExtensions block that
was encrypted to the backend server and not the client-facing
server.
* It requires a mechanism for the client-facing server to provide
the extension-encryption key to the backend server and thus cannot
be used with an unmodified backend server.
* A conforming middlebox will strip every extension, which might
result in a ClientHello which is just unacceptable to the server
(more analysis needed).
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.
Authors' Addresses
Eric Rescorla
RTFM, Inc.
Email: ekr@rtfm.com
Kazuho Oku
Fastly
Email: kazuhooku@gmail.com
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
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Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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