rfc9849.original.xml   rfc9849.xml 
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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft <rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft
-ietf-tls-esni-25" category="std" consensus="true" submissionType="IETF" tocIncl -ietf-tls-esni-25" category="std" consensus="true" submissionType="IETF" number=
ude="true" sortRefs="true" symRefs="true" version="3"> "9849" tocInclude="true" sortRefs="true" symRefs="true" version="3">
<!-- xml2rfc v2v3 conversion 3.28.1 --> <!-- xml2rfc v2v3 conversion 3.31.0 -->
<link href="https://datatracker.ietf.org/doc/draft-ietf-tls-esni-25" rel="prev
"/>
<front> <front>
<title abbrev="TLS Encrypted Client Hello">TLS Encrypted Client Hello</title > <title abbrev="TLS Encrypted Client Hello">TLS Encrypted Client Hello</title >
<seriesInfo name="Internet-Draft" value="draft-ietf-tls-esni-25"/> <seriesInfo name="RFC" value="9849"/>
<author initials="E." surname="Rescorla" fullname="Eric Rescorla"> <author initials="E." surname="Rescorla" fullname="Eric Rescorla">
<organization>Independent</organization> <organization>Knight-Georgetown Institute</organization>
<address> <address>
<email>ekr@rtfm.com</email> <email>ekr@rtfm.com</email>
</address> </address>
</author> </author>
<author initials="K." surname="Oku" fullname="Kazuho Oku"> <author initials="K." surname="Oku" fullname="Kazuho Oku">
<organization>Fastly</organization> <organization>Fastly</organization>
<address> <address>
<email>kazuhooku@gmail.com</email> <email>kazuhooku@gmail.com</email>
</address> </address>
</author> </author>
skipping to change at line 39 skipping to change at line 40
<address> <address>
<email>nicholas.sullivan+ietf@gmail.com</email> <email>nicholas.sullivan+ietf@gmail.com</email>
</address> </address>
</author> </author>
<author initials="C. A." surname="Wood" fullname="Christopher A. Wood"> <author initials="C. A." surname="Wood" fullname="Christopher A. Wood">
<organization>Cloudflare</organization> <organization>Cloudflare</organization>
<address> <address>
<email>caw@heapingbits.net</email> <email>caw@heapingbits.net</email>
</address> </address>
</author> </author>
<date year="2025" month="June" day="14"/> <date year="2025" month="November"/>
<area>SEC</area> <area>SEC</area>
<workgroup>tls</workgroup> <workgroup>tls</workgroup>
<keyword>Internet-Draft</keyword>
<abstract> <abstract>
<?line 67?> <?line 108?>
<!-- [rfced] References
a) Regarding [WHATWG-IPV4], this reference's date is May 2021.
The URL provided resolves to a page with "Last Updated 12 May 2025".
Note that WHATWG provides "commit snapshots" of their living standards and
there are several commit snapshots from May 2021 with the latest being from 20
May 2021. For example: 20 May 2021
(https://url.spec.whatwg.org/commit-snapshots/1b8b8c55eb4bed9f139c9a439fb1c1bf55
66b619/#concept-ipv4-parser)
We recommend updating this reference to the most current version of the WHATWG
Living Standard, replacing the URL with the more general URL to the standard
(https://url.spec.whatwg.org/), and adding a "commit snapshot" URL to the
reference.
Current:
[WHATWG-IPV4]
WHATWG, "URL - IPv4 Parser", WHATWG Living Standard, May
2021, <https://url.spec.whatwg.org/#concept-ipv4-parser>.
EKR: Per MT, WHATWG has asked us not to do that. We should leave
this as-is and change the date to December 2025.
rfced: For context, we reached out to WHATWG in September about a format for
references to their standards (see:
https://github.com/whatwg/meta/issues/363). The proposed update below for this
reference reflects the approved format. It would be helpful for the RPC to
know what WHATWG has asked authors to not do so that we can reach out for
clarification and update our recommended citation accordingly.
Perhaps:
[WHATWG-IPV4]
WHATWG, "URL - IPv4 Parser", WHATWG Living Standard,
<https://url.spec.whatwg.org/#concept-ipv4-parser>.
Commit snapshot:
https://url.spec.whatwg.org/commit-snapshots/1b8b8c55eb4bed9f139c9a43
9fb1c1bf5566b619/#concept-ipv4-parser
Regarding the date, we don't recommend using a future date for a reference as
it doesn't reflect the date for a currently published work (unless there is an
anticipated update to the WHATWG specification in December 2025).
d) FYI, RFCYYY1 (draft-ietf-tls-svcb-ech) will be updated during the XML stage.
OK.
-->
<t>This document describes a mechanism in Transport Layer Security (TLS) for <t>This document describes a mechanism in Transport Layer Security (TLS) for
encrypting a ClientHello message under a server public key.</t> encrypting a ClientHello message under a server public key.</t>
</abstract> </abstract>
<note removeInRFC="true">
<name>Discussion Venues</name>
<t>Source for this draft and an issue tracker can be found at
<eref target="https://github.com/tlswg/draft-ietf-tls-esni">https://github.com
/tlswg/draft-ietf-tls-esni</eref>.</t>
</note>
</front> </front>
<middle> <middle>
<?line 72?> <?line 161?>
<section anchor="intro"> <section anchor="intro">
<name>Introduction</name> <name>Introduction</name>
<t>Although TLS 1.3 <xref target="RFC8446"/> encrypts most of the handshak e, including the <t>Although TLS 1.3 <xref target="RFC8446"/> encrypts most of the handshak e, including the
server certificate, there are several ways in which an on-path attacker can server certificate, there are several ways in which an on-path attacker can
learn private information about the connection. The plaintext Server Name learn private information about the connection. The plaintext Server Name
Indication (SNI) extension in ClientHello messages, which leaks the target Indication (SNI) extension in ClientHello messages, which leaks the target
domain for a given connection, is perhaps the most sensitive information domain for a given connection, is perhaps the most sensitive information
left unencrypted in TLS 1.3.</t> left unencrypted in TLS 1.3.</t>
<t>This document specifies a new TLS extension, called Encrypted Client He <t>This document specifies a new TLS extension called Encrypted Client
llo Hello (ECH) that allows clients to encrypt their ClientHello to the
(ECH), that allows clients to encrypt their ClientHello to the TLS server. TLS server. This protects the SNI and other potentially sensitive
This protects the SNI and other potentially sensitive fields, such as the fields, such as the Application-Layer Protocol Negotiation (ALPN) list
Application Layer Protocol Negotiation (ALPN) <xref target="RFC7301"/>. Co-located servers with consistent externally visible
list <xref target="RFC7301"/>. Co-located servers with consistent externally vis TLS configurations and behavior, including supported versions and cipher suites
ible TLS and
configurations and behavior, including supported versions and cipher suites and
how they respond to incoming client connections, form an anonymity set. (Note how they respond to incoming client connections, form an anonymity set. (Note
that implementation-specific choices, such as extension ordering within TLS that implementation-specific choices, such as extension ordering within TLS
messages or division of data into record-layer boundaries, can result in messages or division of data into record-layer boundaries, can result in
different externally visible behavior, even for servers with consistent TLS different externally visible behavior, even for servers with consistent TLS
configurations.) Usage of this mechanism reveals that a client is connecting configurations.) Usage of this mechanism reveals that a client is connecting
to a particular service provider, but does not reveal which server from the to a particular service provider, but does not reveal which server from the
anonymity set terminates the connection. Deployment implications of this anonymity set terminates the connection. Deployment implications of this
feature are discussed in <xref target="deployment"/>.</t> feature are discussed in <xref target="deployment"/>.</t>
<t>ECH is not in itself sufficient to protect the identity of the server. <t>ECH is not in itself sufficient to protect the identity of the server.
The target domain may also be visible through other channels, such as The target domain may also be visible through other channels, such as
skipping to change at line 142 skipping to change at line 184
+--------------------+ +---------------------+ +--------------------+ +---------------------+
| | | | | | | |
| 2001:DB8::1111 | | 2001:DB8::EEEE | | 2001:DB8::1111 | | 2001:DB8::EEEE |
Client <----------------------------->| | Client <----------------------------->| |
| public.example.com | | private.example.org | | public.example.com | | private.example.org |
| | | | | | | |
+--------------------+ +---------------------+ +--------------------+ +---------------------+
Client-Facing Server Backend Server Client-Facing Server Backend Server
]]></artwork> ]]></artwork>
</figure> </figure>
<t>In Split Mode, the provider is not the origin server for private doma ins. <t>In split mode, the provider is not the origin server for private doma ins.
Rather, the DNS records for private domains point to the provider, and the 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 provider's server relays the connection back to the origin server, who
terminates the TLS connection with the client. Importantly, the service provider terminates the TLS connection with the client. Importantly, the service provider
does not have access to the plaintext of the connection beyond the unencrypted does not have access to the plaintext of the connection beyond the unencrypted
portions of the handshake.</t> portions of the handshake.</t>
<t>In the remainder of this document, we will refer to the ECH-service p rovider as <t>In the remainder of this document, we will refer to the ECH-service p rovider as
the "client-facing server" and to the TLS terminator as the "backend server". 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 These are the same entity in Shared Mode, but in split mode, the client-facing
and backend servers are physically separated.</t> and backend servers are physically separated.</t>
<t>See <xref target="security-considerations"/> for more discussion abou t the ECH threat model <t>See <xref target="security-considerations"/> for more discussion abou t the ECH threat model
and how it relates to the client, client-facing server, and backend server.</t> and how it relates to the client, client-facing server, and backend server.</t>
</section> </section>
<section anchor="encrypted-clienthello-ech"> <section anchor="encrypted-clienthello-ech">
<name>Encrypted ClientHello (ECH)</name> <name>Encrypted ClientHello (ECH)</name>
<t>A client-facing server enables ECH by publishing an ECH configuration , which <t>A client-facing server enables ECH by publishing an ECH configuration , which
is an encryption public key and associated metadata. Domains which wish to is an encryption public key and associated metadata. Domains which wish to
use ECH must publish this configuration, using the key associated use ECH must publish this configuration, using the key associated
with the client-facing server. This document with the client-facing server. This document
defines the ECH configuration's format, but delegates DNS publication details defines the ECH configuration's format, but delegates DNS publication details
to <xref target="RFC9460"/>. See to <xref target="RFC9460"/>. See
<xref target="ECH-IN-DNS"/> for specifics about how ECH configurations <xref target="RFCYYY1"/> for specifics about how ECH configurations
are advertised in SVCB and HTTPS records. Other delivery mechanisms are are advertised in SVCB and HTTPS records. Other delivery mechanisms are
also possible. For example, the client may have the ECH configuration also possible. For example, the client may have the ECH configuration
preconfigured.</t> preconfigured.</t>
<t>When a client wants to establish a TLS session with some backend serv er, it <t>When a client wants to establish a TLS session with some backend serv er, it
constructs a private ClientHello, referred to as the ClientHelloInner. constructs a private ClientHello, referred to as the ClientHelloInner.
The client then constructs a public ClientHello, referred to as the The client then constructs a public ClientHello, referred to as the
ClientHelloOuter. The ClientHelloOuter contains innocuous values for ClientHelloOuter. The ClientHelloOuter contains innocuous values for
sensitive extensions and an "encrypted_client_hello" extension sensitive extensions and an "encrypted_client_hello" extension
(<xref target="encrypted-client-hello"/>), which carries the encrypted ClientHel loInner. (<xref target="encrypted-client-hello"/>), which carries the encrypted ClientHel loInner.
Finally, the client sends ClientHelloOuter to the server.</t> Finally, the client sends ClientHelloOuter to the server.</t>
skipping to change at line 198 skipping to change at line 240
the client for application data. Instead, ECH rejection allows the client to the client for application data. Instead, ECH rejection allows the client to
retry with up-to-date configuration (<xref target="rejected-ech"/>).</t> retry with up-to-date configuration (<xref target="rejected-ech"/>).</t>
<t>The primary goal of ECH is to ensure that connections to servers in t he same <t>The primary goal of ECH is to ensure that connections to servers in t he same
anonymity set are indistinguishable from one another. Moreover, it should anonymity set are indistinguishable from one another. Moreover, it should
achieve this goal without affecting any existing security properties of TLS 1.3. achieve this goal without affecting any existing security properties of TLS 1.3.
See <xref target="goals"/> for more details about the ECH security and privacy g oals.</t> See <xref target="goals"/> for more details about the ECH security and privacy g oals.</t>
</section> </section>
</section> </section>
<section anchor="ech-configuration"> <section anchor="ech-configuration">
<name>Encrypted ClientHello Configuration</name> <name>Encrypted ClientHello Configuration</name>
<t>ECH uses HPKE for public key encryption <xref target="HPKE"/>. <t>ECH uses Hybrid Public Key Encryption (HPKE) for public key encryption <xref target="RFC9180"/>.
The ECH configuration is defined by the following <tt>ECHConfig</tt> structure.< /t> The ECH configuration is defined by the following <tt>ECHConfig</tt> structure.< /t>
<artwork><![CDATA[ <artwork><![CDATA[
opaque HpkePublicKey<1..2^16-1>; opaque HpkePublicKey<1..2^16-1>;
uint16 HpkeKemId; // Defined in RFC9180 uint16 HpkeKemId; // Defined in RFC 9180
uint16 HpkeKdfId; // Defined in RFC9180 uint16 HpkeKdfId; // Defined in RFC 9180
uint16 HpkeAeadId; // Defined in RFC9180 uint16 HpkeAeadId; // Defined in RFC 9180
uint16 ECHConfigExtensionType; // Defined in Section 11.3 uint16 ECHConfigExtensionType; // Defined in Section 11.3
struct { struct {
HpkeKdfId kdf_id; HpkeKdfId kdf_id;
HpkeAeadId aead_id; HpkeAeadId aead_id;
} HpkeSymmetricCipherSuite; } HpkeSymmetricCipherSuite;
struct { struct {
uint8 config_id; uint8 config_id;
HpkeKemId kem_id; HpkeKemId kem_id;
skipping to change at line 241 skipping to change at line 283
struct { struct {
uint16 version; uint16 version;
uint16 length; uint16 length;
select (ECHConfig.version) { select (ECHConfig.version) {
case 0xfe0d: ECHConfigContents contents; case 0xfe0d: ECHConfigContents contents;
} }
} ECHConfig; } ECHConfig;
]]></artwork> ]]></artwork>
<t>The structure contains the following fields:</t> <t>The structure contains the following fields:</t>
<dl> <dl>
<dt>version</dt> <dt>version:</dt>
<dd> <dd>
<t>The version of ECH for which this configuration is used. The versio n <t>The version of ECH for which this configuration is used. The versio n
is the same as the code point for the is the same as the code point for the
"encrypted_client_hello" extension. Clients MUST ignore any <tt>ECHConfig</tt> "encrypted_client_hello" extension. Clients MUST ignore any <tt>ECHConfig</tt>
structure with a version they do not support.</t> structure with a version they do not support.</t>
</dd> </dd>
<dt>length</dt> <dt>length:</dt>
<dd> <dd>
<t>The length, in bytes, of the next field. This length field allows <t>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 implementations to skip over the elements in such a list where they cannot
parse the specific version of ECHConfig.</t> parse the specific version of ECHConfig.</t>
</dd> </dd>
<dt>contents</dt> <dt>contents:</dt>
<dd> <dd>
<t>An opaque byte string whose contents depend on the version. For thi s <t>An opaque byte string whose contents depend on the version. For thi s
specification, the contents are an <tt>ECHConfigContents</tt> structure.</t> specification, the contents are an <tt>ECHConfigContents</tt> structure.</t>
</dd> </dd>
</dl> </dl>
<t>The <tt>ECHConfigContents</tt> structure contains the following fields: </t> <t>The <tt>ECHConfigContents</tt> structure contains the following fields: </t>
<dl> <dl>
<dt>key_config</dt> <dt>key_config:</dt>
<dd> <dd>
<t>A <tt>HpkeKeyConfig</tt> structure carrying the configuration infor mation <t>A <tt>HpkeKeyConfig</tt> structure carrying the configuration infor mation
associated with the HPKE public key (an "ECH key"). Note that this associated with the HPKE public key (an "ECH key"). Note that this
structure contains the <tt>config_id</tt> field, which applies to the entire structure contains the <tt>config_id</tt> field, which applies to the entire
ECHConfigContents.</t> ECHConfigContents.</t>
</dd> </dd>
<dt>maximum_name_length</dt> <dt>maximum_name_length:</dt>
<dd> <dd>
<t>The longest name of a backend server, if known. If not known, this value can <t>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 (<xref target="padding"/>) and doe s not be set to zero. It is used to compute padding (<xref target="padding"/>) and doe s not
constrain server name lengths. Names may exceed this length if, e.g., 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.</t> the server uses wildcard names or added new names to the anonymity set.</t>
</dd> </dd>
<dt>public_name</dt> <dt>public_name:</dt>
<dd> <dd>
<t>The DNS name of the client-facing server, i.e., the entity trusted <t>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, to update the ECH configuration. This is used to correct misconfigured clients,
as described in <xref target="rejected-ech"/>.</t> as described in <xref target="rejected-ech"/>.</t>
</dd> </dd>
<dt/> <dt/>
<dd> <dd>
<t>See <xref target="auth-public-name"/> for how the client interprets and validates the <t>See <xref target="auth-public-name"/> for how the client interprets and validates the
public_name.</t> public_name.</t>
</dd> </dd>
<dt>extensions</dt> <dt>extensions:</dt>
<dd> <dd>
<t>A list of ECHConfigExtension values that the client must take into <t>A list of ECHConfigExtension values that the client must take into
consideration when generating a ClientHello message. Each ECHConfigExtension consideration when generating a ClientHello message. Each ECHConfigExtension
has a 2-octet type and opaque data value, where the data value is encoded has a 2-octet type and opaque data value, where the data value is encoded
with a 2-octet integer representing the length of the data, in network byte with a 2-octet integer representing the length of the data, in network byte
order. ECHConfigExtension values are described below (<xref target="config-exten sions"/>).</t> order. ECHConfigExtension values are described below (<xref target="config-exten sions"/>).</t>
</dd> </dd>
</dl> </dl>
<t>The <tt>HpkeKeyConfig</tt> structure contains the following fields:</t> <t>The <tt>HpkeKeyConfig</tt> structure contains the following fields:</t>
<dl> <dl>
<dt>config_id</dt> <dt>config_id:</dt>
<dd> <dd>
<t>A one-byte identifier for the given HPKE key configuration. This is used by <t>A one-byte identifier for the given HPKE key configuration. This is used by
clients to indicate the key used for ClientHello encryption. <xref target="confi g-ids"/> clients to indicate the key used for ClientHello encryption. <xref target="confi g-ids"/>
describes how client-facing servers allocate this value.</t> describes how client-facing servers allocate this value.</t>
</dd> </dd>
<dt>kem_id</dt> <dt>kem_id:</dt>
<dd> <dd>
<t>The HPKE Key Encapsulation Mechanism (KEM) identifier corresponding <t>The HPKE Key Encapsulation Mechanism (KEM) identifier corresponding
to <tt>public_key</tt>. Clients MUST ignore any <tt>ECHConfig</tt> structure wit h a to <tt>public_key</tt>. Clients MUST ignore any <tt>ECHConfig</tt> structure wit h a
key using a KEM they do not support.</t> key using a KEM they do not support.</t>
</dd> </dd>
<dt>public_key</dt> <dt>public_key:</dt>
<dd> <dd>
<t>The HPKE public key used by the client to encrypt ClientHelloInner. </t> <t>The HPKE public key used by the client to encrypt ClientHelloInner. </t>
</dd> </dd>
<dt>cipher_suites</dt> <dt>cipher_suites:</dt>
<dd> <dd>
<t>The list of HPKE KDF and AEAD identifier pairs clients can use for encrypting <t>The list of HPKE Key Derivation Function (KDF) and Authenticated En cryption with Associated Data (AEAD) identifier pairs clients can use for encryp ting
ClientHelloInner. See <xref target="real-ech"/> for how clients choose from this list.</t> ClientHelloInner. See <xref target="real-ech"/> for how clients choose from this list.</t>
</dd> </dd>
</dl> </dl>
<t>The client-facing server advertises a sequence of ECH configurations to clients, <t>The client-facing server advertises a sequence of ECH configurations to clients,
serialized as follows.</t> serialized as follows.</t>
<artwork><![CDATA[ <artwork><![CDATA[
ECHConfig ECHConfigList<4..2^16-1>; ECHConfig ECHConfigList<4..2^16-1>;
]]></artwork> ]]></artwork>
<t>The <tt>ECHConfigList</tt> structure contains one or more <tt>ECHConfig </tt> structures in <t>The <tt>ECHConfigList</tt> structure contains one or more <tt>ECHConfig </tt> structures in
decreasing order of preference. This allows a server to support multiple decreasing order of preference. This allows a server to support multiple
versions of ECH and multiple sets of ECH parameters.</t> versions of ECH and multiple sets of ECH parameters.</t>
<section anchor="config-ids"> <section anchor="config-ids">
<name>Configuration Identifiers</name> <name>Configuration Identifiers</name>
<t>A client-facing server has a set of known ECHConfig values, with corr esponding <t>A client-facing server has a set of known ECHConfig values with corre sponding
private keys. This set SHOULD contain the currently published values, as well as 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 previous values that may still be in use, since clients may cache DNS records
up to a TTL or longer.</t> up to a TTL or longer.</t>
<t><xref target="client-facing-server"/> describes a trial decryption pr ocess for decrypting the <t><xref target="client-facing-server"/> describes a trial decryption pr ocess for decrypting the
ClientHello. This can impact performance when the client-facing server maintains ClientHello. This can impact performance when the client-facing server maintains
many known ECHConfig values. To avoid this, the client-facing server SHOULD many known ECHConfig values. To avoid this, the client-facing server SHOULD
allocate distinct <tt>config_id</tt> values for each ECHConfig in its known set. The allocate distinct <tt>config_id</tt> values for each ECHConfig in its known set. The
RECOMMENDED strategy is via rejection sampling, i.e., to randomly select RECOMMENDED strategy is via rejection sampling, i.e., to randomly select
<tt>config_id</tt> repeatedly until it does not match any known ECHConfig.</t> <tt>config_id</tt> repeatedly until it does not match any known ECHConfig.</t>
<t>It is not necessary for <tt>config_id</tt> values across different cl ient-facing <t>It is not necessary for <tt>config_id</tt> values across different cl ient-facing
skipping to change at line 364 skipping to change at line 406
in any order, but there MUST NOT be more than one extension of the in any order, but there MUST NOT be more than one extension of the
same type in the extensions block. Unlike TLS extensions, an extension same type in the extensions block. Unlike TLS extensions, an extension
can be tagged as mandatory by using an extension type codepoint with can be tagged as mandatory by using an extension type codepoint with
the high order bit set to 1.</t> the high order bit set to 1.</t>
<t>Clients MUST parse the extension list and check for unsupported manda tory <t>Clients MUST parse the extension list and check for unsupported manda tory
extensions. If an unsupported mandatory extension is present, clients MUST extensions. If an unsupported mandatory extension is present, clients MUST
ignore the <tt>ECHConfig</tt>.</t> ignore the <tt>ECHConfig</tt>.</t>
<t>Any future information or hints that influence ClientHelloOuter SHOUL D be <t>Any future information or hints that influence ClientHelloOuter SHOUL D be
specified as ECHConfig extensions. This is primarily because the outer specified as ECHConfig extensions. This is primarily because the outer
ClientHello exists only in support of ECH. Namely, it is both an envelope for ClientHello exists only in support of ECH. Namely, it is both an envelope for
the encrypted inner ClientHello and enabler for authenticated key mismatch the encrypted inner ClientHello and an enabler for authenticated key mismatch
signals (see <xref target="server-behavior"/>). In contrast, the inner ClientHel lo is the signals (see <xref target="server-behavior"/>). In contrast, the inner ClientHel lo is the
true ClientHello used upon ECH negotiation.</t> true ClientHello used upon ECH negotiation.</t>
</section> </section>
</section> </section>
<section anchor="encrypted-client-hello"> <section anchor="encrypted-client-hello">
<name>The "encrypted_client_hello" Extension</name> <name>The "encrypted_client_hello" Extension</name>
<t>To offer ECH, the client sends an "encrypted_client_hello" extension in the <t>To offer ECH, the client sends an "encrypted_client_hello" extension in the
ClientHelloOuter. When it does, it MUST also send the extension in ClientHelloOuter. When it does, it MUST also send the extension in
ClientHelloInner.</t> ClientHelloInner.</t>
<artwork><![CDATA[ <artwork><![CDATA[
skipping to change at line 402 skipping to change at line 444
Empty; Empty;
}; };
} ECHClientHello; } ECHClientHello;
]]></artwork> ]]></artwork>
<t>The outer extension uses the <tt>outer</tt> variant and the inner exten sion uses the <t>The outer extension uses the <tt>outer</tt> variant and the inner exten sion uses the
<tt>inner</tt> variant. The inner extension has an empty payload, which is inclu ded <tt>inner</tt> variant. The inner extension has an empty payload, which is inclu ded
because TLS servers are not allowed to provide extensions in ServerHello because TLS servers are not allowed to provide extensions in ServerHello
which were not included in ClientHello. The outer extension has the following which were not included in ClientHello. The outer extension has the following
fields:</t> fields:</t>
<dl> <dl>
<dt>config_id</dt> <dt>config_id:</dt>
<dd> <dd>
<t>The ECHConfigContents.key_config.config_id for the chosen ECHConfig .</t> <t>The ECHConfigContents.key_config.config_id for the chosen ECHConfig .</t>
</dd> </dd>
<dt>cipher_suite</dt> <dt>cipher_suite:</dt>
<dd> <dd>
<t>The cipher suite used to encrypt ClientHelloInner. This MUST match a value <t>The cipher suite used to encrypt ClientHelloInner. This MUST match a value
provided in the corresponding <tt>ECHConfigContents.cipher_suites</tt> list.</t> provided in the corresponding <tt>ECHConfigContents.cipher_suites</tt> list.</t>
</dd> </dd>
<dt>enc</dt> <dt>enc:</dt>
<dd> <dd>
<t>The HPKE encapsulated key, used by servers to decrypt the correspon ding <t>The HPKE encapsulated key used by servers to decrypt the correspond ing
<tt>payload</tt> field. This field is empty in a ClientHelloOuter sent in respon se to <tt>payload</tt> field. This field is empty in a ClientHelloOuter sent in respon se to
HelloRetryRequest.</t> HelloRetryRequest.</t>
</dd> </dd>
<dt>payload</dt> <dt>payload:</dt>
<dd> <dd>
<t>The serialized and encrypted EncodedClientHelloInner structure, enc rypted <t>The serialized and encrypted EncodedClientHelloInner structure, enc rypted
using HPKE as described in <xref target="real-ech"/>.</t> using HPKE as described in <xref target="real-ech"/>.</t>
</dd> </dd>
</dl> </dl>
<t>When a client offers the <tt>outer</tt> version of an "encrypted_client _hello" <t>When a client offers the <tt>outer</tt> version of an "encrypted_client _hello"
extension, the server MAY include an "encrypted_client_hello" extension in its extension, the server MAY include an "encrypted_client_hello" extension in its
EncryptedExtensions message, as described in <xref target="client-facing-server" />, with the EncryptedExtensions message, as described in <xref target="client-facing-server" />, with the
following payload:</t> following payload:</t>
<artwork><![CDATA[ <artwork><![CDATA[
struct { struct {
ECHConfigList retry_configs; ECHConfigList retry_configs;
} ECHEncryptedExtensions; } ECHEncryptedExtensions;
]]></artwork> ]]></artwork>
<t>The response is valid only when the server used the ClientHelloOuter. I f the <t>The response is valid only when the server used the ClientHelloOuter. I f the
server sent this extension in response to the <tt>inner</tt> variant, then the c lient server sent this extension in response to the <tt>inner</tt> variant, then the c lient
MUST abort with an "unsupported_extension" alert.</t> MUST abort with an "unsupported_extension" alert.</t>
<dl> <dl>
<dt>retry_configs</dt> <dt>retry_configs:</dt>
<dd> <dd>
<t>An ECHConfigList structure containing one or more ECHConfig structu res, in <t>An ECHConfigList structure containing one or more ECHConfig structu res, in
decreasing order of preference, to be used by the client as described in decreasing order of preference, to be used by the client as described in
<xref target="rejected-ech"/>. These are known as the server's "retry configurat ions".</t> <xref target="rejected-ech"/>. These are known as the server's "retry configurat ions".</t>
</dd> </dd>
</dl> </dl>
<t>Finally, when the client offers the "encrypted_client_hello", if the pa yload is <t>Finally, when the client offers the "encrypted_client_hello", if the pa yload is
the <tt>inner</tt> variant and the server responds with HelloRetryRequest, it MU ST the <tt>inner</tt> variant and the server responds with HelloRetryRequest, it MU ST
include an "encrypted_client_hello" extension with the following payload:</t> include an "encrypted_client_hello" extension with the following payload:</t>
<artwork><![CDATA[ <artwork><![CDATA[
skipping to change at line 459 skipping to change at line 501
} ECHHelloRetryRequest; } ECHHelloRetryRequest;
]]></artwork> ]]></artwork>
<t>The value of ECHHelloRetryRequest.confirmation is set to <t>The value of ECHHelloRetryRequest.confirmation is set to
<tt>hrr_accept_confirmation</tt> as described in <xref target="backend-server-hr r"/>.</t> <tt>hrr_accept_confirmation</tt> as described in <xref target="backend-server-hr r"/>.</t>
<t>This document also defines the "ech_required" alert, which the client M UST send <t>This document also defines the "ech_required" alert, which the client M UST send
when it offered an "encrypted_client_hello" extension that was not accepted by when it offered an "encrypted_client_hello" extension that was not accepted by
the server. (See <xref target="alerts"/>.)</t> the server. (See <xref target="alerts"/>.)</t>
<section anchor="encoding-inner"> <section anchor="encoding-inner">
<name>Encoding the ClientHelloInner</name> <name>Encoding the ClientHelloInner</name>
<t>Before encrypting, the client pads and optionally compresses ClientHe lloInner <t>Before encrypting, the client pads and optionally compresses ClientHe lloInner
into a EncodedClientHelloInner structure, defined below:</t> into an EncodedClientHelloInner structure, defined below:</t>
<artwork><![CDATA[ <artwork><![CDATA[
struct { struct {
ClientHello client_hello; ClientHello client_hello;
uint8 zeros[length_of_padding]; uint8 zeros[length_of_padding];
} EncodedClientHelloInner; } EncodedClientHelloInner;
]]></artwork> ]]></artwork>
<t>The <tt>client_hello</tt> field is computed by first making a copy of ClientHelloInner <t>The <tt>client_hello</tt> field is computed by first making a copy of ClientHelloInner
and setting the <tt>legacy_session_id</tt> field to the empty string. In TLS, th is and setting the <tt>legacy_session_id</tt> field to the empty string. In TLS, th is
field uses the ClientHello structure defined in <xref section="4.1.2" sectionFor mat="of" target="RFC8446"/>. field uses the ClientHello structure defined in <xref section="4.1.2" sectionFor mat="of" target="RFC8446"/>.
In DTLS, it uses the ClientHello structured defined in In DTLS, it uses the ClientHello structured defined in
skipping to change at line 492 skipping to change at line 534
} ExtensionType; } ExtensionType;
ExtensionType OuterExtensions<2..254>; ExtensionType OuterExtensions<2..254>;
]]></artwork> ]]></artwork>
<t>OuterExtensions contains the removed ExtensionType values. Each value references <t>OuterExtensions contains the removed ExtensionType values. Each value references
the matching extension in ClientHelloOuter. The values MUST be ordered the matching extension in ClientHelloOuter. The values MUST be ordered
contiguously in ClientHelloInner, and the "ech_outer_extensions" extension MUST contiguously in ClientHelloInner, and the "ech_outer_extensions" extension MUST
be inserted in the corresponding position in EncodedClientHelloInner. be inserted in the corresponding position in EncodedClientHelloInner.
Additionally, the extensions MUST appear in ClientHelloOuter in the same Additionally, the extensions MUST appear in ClientHelloOuter in the same
relative order. However, there is no requirement that they be contiguous. For relative order. However, there is no requirement that they be contiguous. For
example, OuterExtensions may contain extensions A, B, C, while ClientHelloOuter example, OuterExtensions may contain extensions A, B, and C, while ClientHelloOu
contains extensions A, D, B, C, E, F.</t> ter
contains extensions A, D, B, C, E, and F.</t>
<t>The "ech_outer_extensions" extension can only be included in <t>The "ech_outer_extensions" extension can only be included in
EncodedClientHelloInner, and MUST NOT appear in either ClientHelloOuter or EncodedClientHelloInner and MUST NOT appear in either ClientHelloOuter or
ClientHelloInner.</t> ClientHelloInner.</t>
<t>Finally, the client pads the message by setting the <tt>zeros</tt> fi eld to a byte <t>Finally, the client pads the message by setting the <tt>zeros</tt> fi eld to a byte
string whose contents are all zeros and whose length is the amount of padding string whose contents are all zeros and whose length is the amount of padding
to add. <xref target="padding"/> describes a recommended padding scheme.</t> to add. <xref target="padding"/> describes a recommended padding scheme.</t>
<t>The client-facing server computes ClientHelloInner by reversing this process. <t>The client-facing server computes ClientHelloInner by reversing this process.
First it parses EncodedClientHelloInner, interpreting all bytes after First, it parses EncodedClientHelloInner, interpreting all bytes after
<tt>client_hello</tt> as padding. If any padding byte is non-zero, the server MU ST <tt>client_hello</tt> as padding. If any padding byte is non-zero, the server MU ST
abort the connection with an "illegal_parameter" alert.</t> abort the connection with an "illegal_parameter" alert.</t>
<t>Next it makes a copy of the <tt>client_hello</tt> field and copies th e <t>Next, it makes a copy of the <tt>client_hello</tt> field and copies t he
<tt>legacy_session_id</tt> field from ClientHelloOuter. It then looks for an <tt>legacy_session_id</tt> field from ClientHelloOuter. It then looks for an
"ech_outer_extensions" extension. If found, it replaces the extension with the "ech_outer_extensions" extension. If found, it replaces the extension with the
corresponding sequence of extensions in the ClientHelloOuter. The server MUST corresponding sequence of extensions in the ClientHelloOuter. The server MUST
abort the connection with an "illegal_parameter" alert if any of the following abort the connection with an "illegal_parameter" alert if any of the following
are true:</t> are true:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>Any referenced extension is missing in ClientHelloOuter.</t> <t>Any referenced extension is missing in ClientHelloOuter.</t>
</li> </li>
<li> <li>
skipping to change at line 526 skipping to change at line 568
</li> </li>
<li> <li>
<t>"encrypted_client_hello" is referenced in OuterExtensions.</t> <t>"encrypted_client_hello" is referenced in OuterExtensions.</t>
</li> </li>
<li> <li>
<t>The extensions in ClientHelloOuter corresponding to those in Oute rExtensions <t>The extensions in ClientHelloOuter corresponding to those in Oute rExtensions
do not occur in the same order.</t> do not occur in the same order.</t>
</li> </li>
</ul> </ul>
<t>These requirements prevent an attacker from performing a packet ampli fication <t>These requirements prevent an attacker from performing a packet ampli fication
attack, by crafting a ClientHelloOuter which decompresses to a much larger attack by crafting a ClientHelloOuter which decompresses to a much larger
ClientHelloInner. This is discussed further in <xref target="decompression-amp"/ >.</t> ClientHelloInner. This is discussed further in <xref target="decompression-amp"/ >.</t>
<t>Implementations SHOULD construct the ClientHelloInner in linear <t>Implementations SHOULD construct the ClientHelloInner in linear
time. Quadratic time implementations (such as may happen via naive time. Quadratic time implementations (such as may happen via naive
copying) create a denial of service risk. copying) create a denial-of-service risk.
<xref target="linear-outer-extensions"/> describes a linear-time procedure that may be used <xref target="linear-outer-extensions"/> describes a linear-time procedure that may be used
for this purpose.</t> for this purpose.</t>
</section> </section>
<section anchor="authenticating-outer"> <section anchor="authenticating-outer">
<name>Authenticating the ClientHelloOuter</name> <name>Authenticating the ClientHelloOuter</name>
<t>To prevent a network attacker from modifying the <tt>ClientHelloOuter </tt> <t>To prevent a network attacker from modifying the <tt>ClientHelloOuter </tt>
while keeping the same encrypted <tt>ClientHelloInner</tt> while keeping the same encrypted <tt>ClientHelloInner</tt>
(see <xref target="flow-clienthello-malleability"/>), ECH authenticates ClientHe lloOuter (see <xref target="flow-clienthello-malleability"/>), ECH authenticates ClientHe lloOuter
by passing ClientHelloOuterAAD as the associated data for HPKE sealing by passing ClientHelloOuterAAD as the associated data for HPKE sealing
and opening operations. The ClientHelloOuterAAD is a serialized and opening operations. The ClientHelloOuterAAD is a serialized
skipping to change at line 552 skipping to change at line 594
<xref section="5.3" sectionFormat="of" target="RFC9147"/> for DTLS, which matche s the ClientHelloOuter except <xref section="5.3" sectionFormat="of" target="RFC9147"/> for DTLS, which matche s the ClientHelloOuter except
that the <tt>payload</tt> field of the "encrypted_client_hello" is replaced with a byte that the <tt>payload</tt> 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 string of the same length but whose contents are zeros. This value does not
include Handshake structure's four-byte header in TLS nor twelve-byte header in include Handshake structure's four-byte header in TLS nor twelve-byte header in
DTLS.</t> DTLS.</t>
</section> </section>
</section> </section>
<section anchor="client-behavior"> <section anchor="client-behavior">
<name>Client Behavior</name> <name>Client Behavior</name>
<t>Clients that implement the ECH extension behave in one of two ways: eit her they <t>Clients that implement the ECH extension behave in one of two ways: eit her they
offer a real ECH extension, as described in <xref target="real-ech"/>; or they s end a offer a real ECH extension, as described in <xref target="real-ech"/>, or they s end a
Generate Random Extensions And Sustain Extensibility (GREASE) <xref target="RFC8 701"/> Generate Random Extensions And Sustain Extensibility (GREASE) <xref target="RFC8 701"/>
ECH extension, as described in <xref target="grease-ech"/>. Clients of the latte r type do not ECH extension, as described in <xref target="grease-ech"/>. Clients of the latte r type do not
negotiate ECH. Instead, they generate a dummy ECH extension that is ignored by negotiate ECH. Instead, they generate a dummy ECH extension that is ignored by
the server. (See <xref target="dont-stick-out"/> for an explanation.) The client offers ECH the server. (See <xref target="dont-stick-out"/> for an explanation.) The client offers ECH
if it is in possession of a compatible ECH configuration and sends GREASE ECH if it is in possession of a compatible ECH configuration and sends GREASE ECH
(see <xref target="grease-ech"/>) otherwise.</t> (see <xref target="grease-ech"/>) otherwise.</t>
<section anchor="real-ech"> <section anchor="real-ech">
<name>Offering ECH</name> <name>Offering ECH</name>
<t>To offer ECH, the client first chooses a suitable ECHConfig from the server's <t>To offer ECH, the client first chooses a suitable ECHConfig from the server's
ECHConfigList. To determine if a given <tt>ECHConfig</tt> is suitable, it checks that ECHConfigList. To determine if a given <tt>ECHConfig</tt> is suitable, it checks that
skipping to change at line 651 skipping to change at line 693
allows servers that reject ECH and use ClientHelloOuter to safely ignore any allows servers that reject ECH and use ClientHelloOuter to safely ignore any
early data sent by the client per <xref section="4.2.10" sectionFormat="comma" t arget="RFC8446"/>.</t> early data sent by the client per <xref section="4.2.10" sectionFormat="comma" t arget="RFC8446"/>.</t>
</li> </li>
</ol> </ol>
<t>The client might duplicate non-sensitive extensions in both messages. However, <t>The client might duplicate non-sensitive extensions in both messages. However,
implementations need to take care to ensure that sensitive extensions are not implementations need to take care to ensure that sensitive extensions are not
offered in the ClientHelloOuter. See <xref target="outer-clienthello"/> for addi tional offered in the ClientHelloOuter. See <xref target="outer-clienthello"/> for addi tional
guidance.</t> guidance.</t>
<t>Finally, the client encrypts the EncodedClientHelloInner with the abo ve values, <t>Finally, the client encrypts the EncodedClientHelloInner with the abo ve values,
as described in <xref target="encrypting-clienthello"/>, to construct a ClientHe lloOuter. It as described in <xref target="encrypting-clienthello"/>, to construct a ClientHe lloOuter. It
sends this to the server, and processes the response as described in sends this to the server and processes the response as described in
<xref target="determining-ech-acceptance"/>.</t> <xref target="determining-ech-acceptance"/>.</t>
<section anchor="encrypting-clienthello"> <section anchor="encrypting-clienthello">
<name>Encrypting the ClientHello</name> <name>Encrypting the ClientHello</name>
<t>Given an EncodedClientHelloInner, an HPKE encryption context and <t t>enc</tt> value, <t>Given an EncodedClientHelloInner, an HPKE encryption context and <t t>enc</tt> value,
and a partial ClientHelloOuterAAD, the client constructs a ClientHelloOuter as and a partial ClientHelloOuterAAD, the client constructs a ClientHelloOuter as
follows.</t> follows.</t>
<t>First, the client determines the length L of encrypting EncodedClie ntHelloInner <t>First, the client determines the length L of encrypting EncodedClie ntHelloInner
with the selected HPKE AEAD. This is typically the sum of the plaintext length 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 and the AEAD tag length. The client then completes the ClientHelloOuterAAD with
an "encrypted_client_hello" extension. This extension value contains the outer an "encrypted_client_hello" extension. This extension value contains the outer
skipping to change at line 696 skipping to change at line 738
]]></artwork> ]]></artwork>
<t>Including <tt>ClientHelloOuterAAD</tt> as the HPKE AAD binds the <t t>ClientHelloOuter</tt> <t>Including <tt>ClientHelloOuterAAD</tt> as the HPKE AAD binds the <t t>ClientHelloOuter</tt>
to the <tt>ClientHelloInner</tt>, thus preventing attackers from modifying to the <tt>ClientHelloInner</tt>, thus preventing attackers from modifying
<tt>ClientHelloOuter</tt> while keeping the same <tt>ClientHelloInner</tt>, as d escribed in <tt>ClientHelloOuter</tt> while keeping the same <tt>ClientHelloInner</tt>, as d escribed in
<xref target="flow-clienthello-malleability"/>.</t> <xref target="flow-clienthello-malleability"/>.</t>
<t>Finally, the client replaces <tt>payload</tt> with <tt>final_payloa d</tt> to obtain <t>Finally, the client replaces <tt>payload</tt> with <tt>final_payloa d</tt> to obtain
ClientHelloOuter. The two values have the same length, so it is not necessary ClientHelloOuter. The two values have the same length, so it is not necessary
to recompute length prefixes in the serialized structure.</t> to recompute length prefixes in the serialized structure.</t>
<t>Note this construction requires the "encrypted_client_hello" be com puted after <t>Note this construction requires the "encrypted_client_hello" be com puted after
all other extensions. This is possible because the ClientHelloOuter's all other extensions. This is possible because the ClientHelloOuter's
"pre_shared_key" extension is either omitted, or uses a random binder "pre_shared_key" extension is either omitted or uses a random binder
(<xref target="grease-psk"/>).</t> (<xref target="grease-psk"/>).</t>
</section> </section>
<section anchor="grease-psk"> <section anchor="grease-psk">
<name>GREASE PSK</name> <name>GREASE PSK</name>
<t>When offering ECH, the client is not permitted to advertise PSK ide ntities in <t>When offering ECH, the client is not permitted to advertise PSK ide ntities in
the ClientHelloOuter. However, the client can send a "pre_shared_key" extension 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, 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 the backend server sends a "pre_shared_key" extension in its ServerHello. This
would appear to a network observer as if the server were sending this would appear to a network observer as if the server were sending this
extension without solicitation, which would violate the extension rules extension without solicitation, which would violate the extension rules
skipping to change at line 726 skipping to change at line 768
client generates a random string of the same length.</t> client generates a random string of the same length.</t>
<t>Per the rules of <xref target="real-ech"/>, the server is not permi tted to resume a <t>Per the rules of <xref target="real-ech"/>, the server is not permi tted to resume a
connection in the outer handshake. If ECH is rejected and the client-facing 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 server replies with a "pre_shared_key" extension in its ServerHello, then the
client MUST abort the handshake with an "illegal_parameter" alert.</t> client MUST abort the handshake with an "illegal_parameter" alert.</t>
</section> </section>
<section anchor="padding"> <section anchor="padding">
<name>Recommended Padding Scheme</name> <name>Recommended Padding Scheme</name>
<t>If the ClientHelloInner is encrypted without padding, then the leng th of <t>If the ClientHelloInner is encrypted without padding, then the leng th of
the <tt>ClientHelloOuter.payload</tt> can leak information about <tt>ClientHello Inner</tt>. the <tt>ClientHelloOuter.payload</tt> can leak information about <tt>ClientHello Inner</tt>.
In order to prevent this the <tt>EncodedClientHelloInner</tt> structure In order to prevent this, the <tt>EncodedClientHelloInner</tt> structure
has a padding field. This section describes a deterministic mechanism for has a padding field. This section describes a deterministic mechanism for
computing the required amount of padding based on the following computing the required amount of padding based on the following
observation: individual extensions can reveal sensitive information through observation: individual extensions can reveal sensitive information through
their length. Thus, each extension in the inner ClientHello may require their length. Thus, each extension in the inner ClientHello may require
different amounts of padding. This padding may be fully determined by the different amounts of padding. This padding may be fully determined by the
client's configuration or may require server input.</t> client's configuration or may require server input.</t>
<t>By way of example, clients typically support a small number of appl ication <t>By way of example, clients typically support a small number of appl ication
profiles. For instance, a browser might support HTTP with ALPN values profiles. For instance, a browser might support HTTP with ALPN values
["http/1.1", "h2"] and WebRTC media with ALPNs ["webrtc", "c-webrtc"]. Clients ["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 SHOULD pad this extension by rounding up to the total size of the longest ALPN
skipping to change at line 813 skipping to change at line 855
<t>It computes a second ClientHelloInner based on the first Client HelloInner, as <t>It computes a second ClientHelloInner based on the first Client HelloInner, as
in <xref section="4.1.4" sectionFormat="of" target="RFC8446"/>. The ClientHelloI nner's in <xref section="4.1.4" sectionFormat="of" target="RFC8446"/>. The ClientHelloI nner's
"encrypted_client_hello" extension is left unmodified.</t> "encrypted_client_hello" extension is left unmodified.</t>
</li> </li>
<li> <li>
<t>It constructs EncodedClientHelloInner as described in <xref tar get="encoding-inner"/>.</t> <t>It constructs EncodedClientHelloInner as described in <xref tar get="encoding-inner"/>.</t>
</li> </li>
<li> <li>
<t>It constructs a second partial ClientHelloOuterAAD message. Thi s message MUST <t>It constructs a second partial ClientHelloOuterAAD message. Thi s message MUST
be syntactically valid. The extensions MAY be copied from the original be syntactically valid. The extensions MAY be copied from the original
ClientHelloOuter unmodified, or omitted. If not sensitive, the client MAY ClientHelloOuter unmodified or omitted. If not sensitive, the client MAY
copy updated extensions from the second ClientHelloInner for compression.</t> copy updated extensions from the second ClientHelloInner for compression.</t>
</li> </li>
<li> <li>
<t>It encrypts EncodedClientHelloInner as described in <t>It encrypts EncodedClientHelloInner as described in
<xref target="encrypting-clienthello"/>, using the second partial ClientHelloOut erAAD, to <xref target="encrypting-clienthello"/>, using the second partial ClientHelloOut erAAD, to
obtain a second ClientHelloOuter. It reuses the original HPKE encryption obtain a second ClientHelloOuter. It reuses the original HPKE encryption
context computed in <xref target="real-ech"/> and uses the empty string for <tt> enc</tt>. </t> context computed in <xref target="real-ech"/> and uses the empty string for <tt> enc</tt>. </t>
<t> <t>
The HPKE context maintains a sequence number, so this operation internally The HPKE context maintains a sequence number, so this operation internally
uses a fresh nonce for each AEAD operation. Reusing the HPKE context avoids uses a fresh nonce for each AEAD operation. Reusing the HPKE context avoids
skipping to change at line 848 skipping to change at line 890
<xref target="auth-public-name"/>. If authentication or the handshake fails, the client MUST <xref target="auth-public-name"/>. If authentication or the handshake fails, the client MUST
return a failure to the calling application. It MUST NOT use the retry return a failure to the calling application. It MUST NOT use the retry
configurations. It MUST NOT treat this as a secure signal to configurations. It MUST NOT treat this as a secure signal to
disable ECH.</t> disable ECH.</t>
<t>If the server supplied an "encrypted_client_hello" extension in its <t>If the server supplied an "encrypted_client_hello" extension in its
EncryptedExtensions message, the client MUST check that it is syntactically EncryptedExtensions message, the client MUST check that it is syntactically
valid and the client MUST abort the connection with a "decode_error" alert 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 otherwise. If an earlier TLS version was negotiated, the client MUST NOT enable
the False Start optimization <xref target="RFC7918"/> for this handshake. If bot h the False Start optimization <xref target="RFC7918"/> for this handshake. If bot h
authentication and the handshake complete successfully, the client MUST perform authentication and the handshake complete successfully, the client MUST perform
the processing described below then abort the connection with an "ech_required" the processing described below and then abort the connection with an "ech_requir ed"
alert before sending any application data to the server.</t> alert before sending any application data to the server.</t>
<t>If the server provided "retry_configs" and if at least one of the <t>If the server provided "retry_configs" and if at least one of the
values contains a version supported by the client, the client can values contains a version supported by the client, the client can
regard the ECH configuration as securely replaced by the server. It regard the ECH configuration as securely replaced by the server. It
SHOULD retry the handshake with a new transport connection, using the SHOULD retry the handshake with a new transport connection using the
retry configurations supplied by the server.</t> retry configurations supplied by the server.</t>
<t>Clients can implement a new transport connection in a way that best <t>Clients can implement a new transport connection in a way that best
suits their deployment. For example, clients can reuse the same server suits their deployment. For example, clients can reuse the same server
IP address when establishing the new transport connection or they can IP address when establishing the new transport connection or they can
choose to use a different IP address if provided with options from choose to use a different IP address if provided with options from
DNS. ECH does not mandate any specific implementation choices when DNS. ECH does not mandate any specific implementation choices when
establishing this new connection.</t> establishing this new connection.</t>
<t>The retry configurations are meant to be used for retried connectio ns. Further <t>The retry configurations are meant to be used for retried connectio ns. Further
use of retry configurations could yield a tracking vector. In settings where use of retry configurations could yield a tracking vector. In settings where
the client will otherwise already let the server track the client, e.g., the client will otherwise already let the server track the client, e.g.,
skipping to change at line 888 skipping to change at line 930
a node with configuration B in the second. Note that this guidance a node with configuration B in the second. Note that this guidance
does not apply to the cases in the previous paragraph where the server does not apply to the cases in the previous paragraph where the server
has securely disabled ECH.</t> has securely disabled ECH.</t>
<t>If a client does not retry, it MUST report an error to the calling <t>If a client does not retry, it MUST report an error to the calling
application.</t> application.</t>
</section> </section>
<section anchor="auth-public-name"> <section anchor="auth-public-name">
<name>Authenticating for the Public Name</name> <name>Authenticating for the Public Name</name>
<t>When the server rejects ECH, it continues with the handshake using the plaintext <t>When the server rejects ECH, it continues with the handshake using the plaintext
"server_name" extension instead (see <xref target="server-behavior"/>). Clients that offer "server_name" extension instead (see <xref target="server-behavior"/>). Clients that offer
ECH then authenticate the connection with the public name, as follows:</t> ECH then authenticate the connection with the public name as follows:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>The client MUST verify that the certificate is valid for <t>The client MUST verify that the certificate is valid for
ECHConfig.contents.public_name. If invalid, it MUST abort the connection with ECHConfig.contents.public_name. If invalid, it MUST abort the connection with
the appropriate alert.</t> the appropriate alert.</t>
</li> </li>
<li> <li>
<t>If the server requests a client certificate, the client MUST re spond with an <t>If the server requests a client certificate, the client MUST re spond with an
empty Certificate message, denoting no client certificate.</t> empty Certificate message, denoting no client certificate.</t>
</li> </li>
</ul> </ul>
<t>In verifying the client-facing server certificate, the client MUST <t>In verifying the client-facing server certificate, the client MUST
interpret the public name as a DNS-based reference identity interpret the public name as a DNS-based reference identity
<xref target="RFC6125"/>. Clients that incorporate DNS names and IP addresses in to <xref target="RFC9525"/>. Clients that incorporate DNS names and IP addresses in to
the same syntax (e.g. <xref section="7.4" sectionFormat="of" target="RFC3986"/> and <xref target="WHATWG-IPV4"/>) the same syntax (e.g. <xref section="7.4" sectionFormat="of" target="RFC3986"/> and <xref target="WHATWG-IPV4"/>)
MUST reject names that would be interpreted as IPv4 addresses. MUST reject names that would be interpreted as IPv4 addresses.
Clients that enforce this by checking ECHConfig.contents.public_name Clients that enforce this by checking ECHConfig.contents.public_name
do not need to repeat the check when processing ECH rejection.</t> do not need to repeat the check when processing ECH rejection.</t>
<t>Note that authenticating a connection for the public name does not authenticate <t>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 it for the origin. The TLS implementation MUST NOT report such connections as
successful to the application. It additionally MUST ignore all session tickets successful to the application. It additionally MUST ignore all session tickets
and session IDs presented by the server. These connections are only used to and session IDs presented by the server. These connections are only used to
trigger retries, as described in <xref target="rejected-ech"/>. This may be impl emented, for trigger retries, as described in <xref target="rejected-ech"/>. This may be impl emented, for
instance, by reporting a failed connection with a dedicated error code.</t> instance, by reporting a failed connection with a dedicated error code.</t>
<t>Prior to attempting a connection, a client SHOULD validate the <tt> ECHConfig</tt>. <t>Prior to attempting a connection, a client SHOULD validate the <tt> ECHConfig</tt>.
Clients SHOULD ignore any Clients SHOULD ignore any
<tt>ECHConfig</tt> structure with a public_name that is not a valid host name in <tt>ECHConfig</tt> structure with a public_name that is not a valid host name in
preferred name syntax (see <xref section="2" sectionFormat="of" target="DNS-TERM S"/>). That is, to be preferred name syntax (see <xref section="2" sectionFormat="of" target="RFC9499" />). That is, to be
valid, the public_name needs to be a dot-separated sequence of LDH labels, as valid, the public_name needs to be a dot-separated sequence of LDH labels, as
defined in <xref section="2.3.1" sectionFormat="of" target="RFC5890"/>, where:</ t> defined in <xref section="2.3.1" sectionFormat="of" target="RFC5890"/>, where:</ t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>the sequence does not begin or end with an ASCII dot, and</t> <t>the sequence does not begin or end with an ASCII dot, and</t>
</li> </li>
<li> <li>
<t>all labels are at most 63 octets.</t> <t>all labels are at most 63 octets.</t>
</li> </li>
</ul> </ul>
<t>Clients additionally SHOULD ignore the structure if the final LDH <t>Clients additionally SHOULD ignore the structure if the final LDH
label either consists of all ASCII digits (i.e. '0' through '9') or is label either consists of all ASCII digits (i.e., '0' through '9') or is
"0x" or "0X" followed by some, possibly empty, sequence of ASCII "0x" or "0X" followed by some, possibly empty, sequence of ASCII
hexadecimal digits (i.e. '0' through '9', 'a' through 'f', and 'A' hexadecimal digits (i.e., '0' through '9', 'a' through 'f', and 'A'
through 'F'). This avoids public_name values that may be interpreted through 'F'). This avoids public_name values that may be interpreted
as IPv4 literals.</t> as IPv4 literals.</t>
</section> </section>
<section anchor="impact-of-retry-on-future-connections"> <section anchor="impact-of-retry-on-future-connections">
<name>Impact of Retry on Future Connections</name> <name>Impact of Retry on Future Connections</name>
<t>Clients MAY use information learned from a rejected ECH for future <t>Clients MAY use information learned from a rejected ECH for future
connections to avoid repeatedly connecting to the same server and connections to avoid repeatedly connecting to the same server and
being forced to retry. However, they MUST handle ECH rejection for being forced to retry. However, they MUST handle ECH rejection for
those connections as if it were a fresh connection, rather than those connections as if it were a fresh connection, rather than
enforcing the single retry limit from <xref target="rejected-ech"/>. The reason enforcing the single retry limit from <xref target="rejected-ech"/>. The reason
for this requirement is that if the server sends a "retry_config" for this requirement is that if the server sends a "retry_config"
and then immediately rejects the resulting connection, it is and then immediately rejects the resulting connection, it is
most likely misconfigured. However, if the server sends a "retry_config" most likely misconfigured. However, if the server sends a "retry_config"
and then the client tries to use that to connect some time and then the client tries to use that to connect some time
later, it is possible that the server has changed later, it is possible that the server has changed
its configuration again and is now trying to recover.</t> its configuration again and is now trying to recover.</t>
<t>Any persisted information MUST be associated with the ECHConfig sou rce <t>Any persisted information MUST be associated with the ECHConfig sou rce
used to bootstrap the connection, such as a DNS SVCB ServiceMode record used to bootstrap the connection, such as a DNS SVCB ServiceMode record
<xref target="ECH-IN-DNS"/>. Clients MUST limit any sharing of persisted ECH-rel ated <xref target="RFCYYY1"/>. Clients MUST limit any sharing of persisted ECH-relate d
state to connections that use the same ECHConfig source. Otherwise, it state to connections that use the same ECHConfig source. Otherwise, it
might become possible for the client to have the wrong public name for might become possible for the client to have the wrong public name for
the server, making recovery impossible.</t> the server, making recovery impossible.</t>
<t>ECHConfigs learned from ECH rejection can be used as a tracking <t>ECHConfigs learned from ECH rejection can be used as a tracking
vector. Clients SHOULD impose the same lifetime and scope restrictions vector. Clients SHOULD impose the same lifetime and scope restrictions
that they apply to other server-based that they apply to other server-based
tracking vectors such as PSKs.</t> tracking vectors such as PSKs.</t>
<t>In general, the safest way for clients to minimize ECH retries is t o <t>In general, the safest way for clients to minimize ECH retries is t o
comply with any freshness rules (e.g., DNS TTLs) imposed by the ECH comply with any freshness rules (e.g., DNS TTLs) imposed by the ECH
configuration.</t> configuration.</t>
</section> </section>
</section> </section>
<section anchor="grease-ech"> <section anchor="grease-ech">
<name>GREASE ECH</name> <name>GREASE ECH</name>
<t>The GREASE ECH mechanism allows a connection between and ECH-capable client <t>The GREASE ECH mechanism allows a connection between an ECH-capable c lient
and a non-ECH server to appear to use ECH, thus reducing the extent to and a non-ECH server to appear to use ECH, thus reducing the extent to
which ECH connections stick out (see <xref target="dont-stick-out"/>).</t> which ECH connections stick out (see <xref target="dont-stick-out"/>).</t>
<section anchor="client-greasing"> <section anchor="client-greasing">
<name>Client Greasing</name> <name>Client Greasing</name>
<t>If the client attempts to connect to a server and does not have an ECHConfig <t>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 <xref target="RFC870 1"/> structure available for the server, it SHOULD send a GREASE <xref target="RFC870 1"/>
"encrypted_client_hello" extension in the first ClientHello as follows:</t> "encrypted_client_hello" extension in the first ClientHello as follows:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>Set the <tt>config_id</tt> field to a random byte.</t> <t>Set the <tt>config_id</tt> field to a random byte.</t>
</li> </li>
<li> <li>
<t>Set the <tt>cipher_suite</tt> field to a supported HpkeSymmetri cCipherSuite. The <t>Set the <tt>cipher_suite</tt> field to a supported HpkeSymmetri cCipherSuite. The
selection SHOULD vary to exercise all supported configurations, but MAY be selection SHOULD vary to exercise all supported configurations, but MAY be
held constant for successive connections to the same server in the same held constant for successive connections to the same server in the same
session.</t> session.</t>
</li> </li>
<li> <li>
<t>Set the <tt>enc</tt> field to a randomly-generated valid encaps ulated public key <t>Set the <tt>enc</tt> field to a randomly generated valid encaps ulated public key
output by the HPKE KEM.</t> output by the HPKE KEM.</t>
</li> </li>
<li> <li>
<t>Set the <tt>payload</tt> field to a randomly-generated string o f L+C bytes, where C <t>Set the <tt>payload</tt> field to a randomly generated string o f L+C bytes, where C
is the ciphertext expansion of the selected AEAD scheme and L is the size of 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 the EncodedClientHelloInner the client would compute when offering ECH, padded
according to <xref target="padding"/>.</t> according to <xref target="padding"/>.</t>
</li> </li>
</ul> </ul>
<t>If sending a second ClientHello in response to a HelloRetryRequest, the <t>If sending a second ClientHello in response to a HelloRetryRequest, the
client copies the entire "encrypted_client_hello" extension from the first client copies the entire "encrypted_client_hello" extension from the first
ClientHello. The identical value will reveal to an observer that the value of 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 "encrypted_client_hello" was fake, but this only occurs if there is a
HelloRetryRequest.</t> HelloRetryRequest.</t>
skipping to change at line 1015 skipping to change at line 1057
MAY offer to resume sessions established without ECH.</t> MAY offer to resume sessions established without ECH.</t>
</section> </section>
<section anchor="server-greasing"> <section anchor="server-greasing">
<name>Server Greasing</name> <name>Server Greasing</name>
<t><xref target="config-extensions-iana"/> describes a set of Reserved extensions <t><xref target="config-extensions-iana"/> describes a set of Reserved extensions
which will never be registered. These can be used by servers to which will never be registered. These can be used by servers to
"grease" the contents of the ECH configuration, as inspired by "grease" the contents of the ECH configuration, as inspired by
<xref target="RFC8701"/>. This helps ensure clients process ECH extensions <xref target="RFC8701"/>. This helps ensure clients process ECH extensions
correctly. When constructing ECH configurations, servers SHOULD correctly. When constructing ECH configurations, servers SHOULD
randomly select from reserved values with the high-order bit randomly select from reserved values with the high-order bit
clear. Correctly-implemented client will ignore those extensions.</t> clear. Correctly implemented clients will ignore those extensions.</t>
<t>The reserved values with the high-order bit set are mandatory, as <t>The reserved values with the high-order bit set are mandatory, as
defined in <xref target="config-extensions"/>. Servers SHOULD randomly select fr om defined in <xref target="config-extensions"/>. Servers SHOULD randomly select fr om
these values and include them in extraneous ECH configurations. these values and include them in extraneous ECH configurations.
Correctly-implemented clients will ignore these configurations because Correctly implemented clients will ignore these configurations because
they do not recognize the mandatory extension. Servers SHOULD ensure they do not recognize the mandatory extension. Servers SHOULD ensure
that any client using these configurations encounters a warning or error that any client using these configurations encounters a warning or error
message. This can be accomplished in several ways, including:</t> message. This can be accomplished in several ways, including:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>By giving the extraneous configurations distinctive config IDs or <t>By giving the extraneous configurations distinctive config IDs or
public names, and rejecting the TLS connection or inserting an public names, and rejecting the TLS connection or inserting an
application-level warning message when these are observed.</t> application-level warning message when these are observed.</t>
</li> </li>
<li> <li>
<t>By giving the extraneous configurations an invalid public <t>By giving the extraneous configurations an invalid public
key and a public name not associated with the server, so that key and a public name not associated with the server so that
the initial ClientHelloOuter will not be decryptable and the initial ClientHelloOuter will not be decryptable and
the server cannot perform the recovery flow described the server cannot perform the recovery flow described
in <xref target="rejected-ech"/>.</t> in <xref target="rejected-ech"/>.</t>
</li> </li>
</ul> </ul>
</section> </section>
</section> </section>
</section> </section>
<section anchor="server-behavior"> <section anchor="server-behavior">
<name>Server Behavior</name> <name>Server Behavior</name>
<t>As described in <xref target="topologies"/>, servers can play two roles , either as <t>As described in <xref target="topologies"/>, servers can play two roles , either as
the client-facing server or as the back-end server. the client-facing server or as the back-end server.
Depending on the server role, the <tt>ECHClientHello</tt> will be different:</t> Depending on the server role, the <tt>ECHClientHello</tt> will be different:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>A client-facing server expects a <tt>ECHClientHello.type</tt> of <t t>outer</tt>, and <t>A client-facing server expects an <tt>ECHClientHello.type</tt> of < tt>outer</tt>, and
proceeds as described in <xref target="client-facing-server"/> to extract a proceeds as described in <xref target="client-facing-server"/> to extract a
ClientHelloInner, if available.</t> ClientHelloInner, if available.</t>
</li> </li>
<li> <li>
<t>A backend server expects a <tt>ECHClientHello.type</tt> of <tt>inne r</tt>, and <t>A backend server expects an <tt>ECHClientHello.type</tt> of <tt>inn er</tt>, and
proceeds as described in <xref target="backend-server"/>.</t> proceeds as described in <xref target="backend-server"/>.</t>
</li> </li>
</ul> </ul>
<t>In split mode, a client-facing server which receives a <tt>ClientHello< /tt> <t>In split mode, a client-facing server which receives a <tt>ClientHello< /tt>
with <tt>ECHClientHello.type</tt> of <tt>inner</tt> MUST abort with an with <tt>ECHClientHello.type</tt> of <tt>inner</tt> MUST abort with an
"illegal_parameter" alert. Similarly, in split mode, a backend server "illegal_parameter" alert. Similarly, in split mode, a backend server
which receives a <tt>ClientHello</tt> with <tt>ECHClientHello.type</tt> of <tt>o uter</tt> which receives a <tt>ClientHello</tt> with <tt>ECHClientHello.type</tt> of <tt>o uter</tt>
MUST abort with an "illegal_parameter" alert.</t> MUST abort with an "illegal_parameter" alert.</t>
<t>In shared mode, a server plays both roles, first decrypting the <t>In shared mode, a server plays both roles, first decrypting the
<tt>ClientHelloOuter</tt> and then using the contents of the <tt>ClientHelloOuter</tt> and then using the contents of the
skipping to change at line 1074 skipping to change at line 1116
<tt>ClientHello</tt> with <tt>ECHClientHello.type</tt> of <tt>inner</tt> MUST ab ort with an <tt>ClientHello</tt> with <tt>ECHClientHello.type</tt> of <tt>inner</tt> MUST ab ort with an
"illegal_parameter" alert, because such a <tt>ClientHello</tt> should never "illegal_parameter" alert, because such a <tt>ClientHello</tt> should never
be received directly from the network.</t> be received directly from the network.</t>
<t>If <tt>ECHClientHello.type</tt> is not a valid <tt>ECHClientHelloType</ tt>, then <t>If <tt>ECHClientHello.type</tt> is not a valid <tt>ECHClientHelloType</ tt>, then
the server MUST abort with an "illegal_parameter" alert.</t> the server MUST abort with an "illegal_parameter" alert.</t>
<t>If the "encrypted_client_hello" is not present, then the server complet es the <t>If the "encrypted_client_hello" is not present, then the server complet es the
handshake normally, as described in <xref target="RFC8446"/>.</t> handshake normally, as described in <xref target="RFC8446"/>.</t>
<section anchor="client-facing-server"> <section anchor="client-facing-server">
<name>Client-Facing Server</name> <name>Client-Facing Server</name>
<t>Upon receiving an "encrypted_client_hello" extension in an initial <t>Upon receiving an "encrypted_client_hello" extension in an initial
ClientHello, the client-facing server determines if it will accept ECH, prior ClientHello, the client-facing server determines if it will accept ECH prior
to negotiating any other TLS parameters. Note that successfully decrypting the 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 extension will result in a new ClientHello to process, so even the client's TLS
version preferences may have changed.</t> version preferences may have changed.</t>
<t>First, the server collects a set of candidate ECHConfig values. This list is <t>First, the server collects a set of candidate ECHConfig values. This list is
determined by one of the two following methods:</t> determined by one of the two following methods:</t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
<t>Compare ECHClientHello.config_id against identifiers of each know n ECHConfig <t>Compare ECHClientHello.config_id against identifiers of each know n ECHConfig
and select the ones that match, if any, as candidates.</t> and select the ones that match, if any, as candidates.</t>
</li> </li>
<li> <li>
skipping to change at line 1118 skipping to change at line 1160
]]></artwork> ]]></artwork>
<t>ClientHelloOuterAAD is computed from ClientHelloOuter as described in <t>ClientHelloOuterAAD is computed from ClientHelloOuter as described in
<xref target="authenticating-outer"/>. The <tt>info</tt> parameter to SetupBaseR is the <xref target="authenticating-outer"/>. The <tt>info</tt> parameter to SetupBaseR is the
concatenation "tls ech", a zero byte, and the serialized ECHConfig. If concatenation "tls ech", a zero byte, and the serialized ECHConfig. If
decryption fails, the server continues to the next candidate ECHConfig. decryption fails, the server continues to the next candidate ECHConfig.
Otherwise, the server reconstructs ClientHelloInner from Otherwise, the server reconstructs ClientHelloInner from
EncodedClientHelloInner, as described in <xref target="encoding-inner"/>. It the n stops EncodedClientHelloInner, as described in <xref target="encoding-inner"/>. It the n stops
iterating over the candidate ECHConfig values.</t> iterating over the candidate ECHConfig values.</t>
<t>Once the server has chosen the correct ECHConfig, it MAY verify that the value <t>Once the server has chosen the correct ECHConfig, it MAY verify that the value
in the ClientHelloOuter "server_name" extension matches the value of in the ClientHelloOuter "server_name" extension matches the value of
ECHConfig.contents.public_name, and abort with an "illegal_parameter" alert if ECHConfig.contents.public_name and abort with an "illegal_parameter" alert if
these do not match. This optional check allows the server to limit ECH these do not match. This optional check allows the server to limit ECH
connections to only use the public SNI values advertised in its ECHConfigs. connections to only use the public SNI values advertised in its ECHConfigs.
The server MUST be careful not to unnecessarily reject connections if the same The server MUST be careful not to unnecessarily reject connections if the same
ECHConfig id or keypair is used in multiple ECHConfigs with distinct public ECHConfig id or keypair is used in multiple ECHConfigs with distinct public
names.</t> names.</t>
<t>Upon determining the ClientHelloInner, the client-facing server check s that the <t>Upon determining the ClientHelloInner, the client-facing server check s that the
message includes a well-formed "encrypted_client_hello" extension of type message includes a well-formed "encrypted_client_hello" extension of type
<tt>inner</tt> and that it does not offer TLS 1.2 or below. If either of these c hecks <tt>inner</tt> and that it does not offer TLS 1.2 or below. If either of these c hecks
fails, the client-facing server MUST abort with an "illegal_parameter" alert.</t > fails, the client-facing server MUST abort with an "illegal_parameter" alert.</t >
<t>If these checks succeed, the client-facing server then forwards the <t>If these checks succeed, the client-facing server then forwards the
ClientHelloInner to the appropriate backend server, which proceeds as in ClientHelloInner to the appropriate backend server, which proceeds as in
<xref target="backend-server"/>. If the backend server responds with a HelloRetr yRequest, the <xref target="backend-server"/>. If the backend server responds with a HelloRetr yRequest, the
client-facing server forwards it, decrypts the client's second ClientHelloOuter client-facing server forwards it, decrypts the client's second ClientHelloOuter
using the procedure in <xref target="client-facing-server-hrr"/>, and forwards t he resulting using the procedure in <xref target="client-facing-server-hrr"/>, and forwards t he resulting
second ClientHelloInner. The client-facing server forwards all other TLS second ClientHelloInner. The client-facing server forwards all other TLS
messages between the client and backend server unmodified.</t> messages between the client and backend server unmodified.</t>
<t>Otherwise, if all candidate ECHConfig values fail to decrypt the exte nsion, the <t>Otherwise, if all candidate ECHConfig values fail to decrypt the exte nsion, the
client-facing server MUST ignore the extension and proceed with the connection client-facing server MUST ignore the extension and proceed with the connection
using ClientHelloOuter, with the following modifications:</t> using ClientHelloOuter with the following modifications:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>If sending a HelloRetryRequest, the server MAY include an <t>If sending a HelloRetryRequest, the server MAY include an
"encrypted_client_hello" extension with a payload of 8 random bytes; see "encrypted_client_hello" extension with a payload of 8 random bytes; see
<xref target="dont-stick-out"/> for details.</t> <xref target="dont-stick-out"/> for details.</t>
</li> </li>
<li> <li>
<t>If the server is configured with any ECHConfigs, it MUST include the <t>If the server is configured with any ECHConfigs, it MUST include the
"encrypted_client_hello" extension in its EncryptedExtensions with the "encrypted_client_hello" extension in its EncryptedExtensions with the
"retry_configs" field set to one or more ECHConfig structures with up-to-date "retry_configs" field set to one or more ECHConfig structures with up-to-date
skipping to change at line 1164 skipping to change at line 1206
and rely on the client to abort if ECH was required. In particular, the and rely on the client to abort if ECH was required. In particular, the
unrecognized value alone does not indicate a misconfigured ECH advertisement unrecognized value alone does not indicate a misconfigured ECH advertisement
(<xref target="misconfiguration"/>). Instead, servers can measure occurrences of the (<xref target="misconfiguration"/>). Instead, servers can measure occurrences of the
"ech_required" alert to detect this case.</t> "ech_required" alert to detect this case.</t>
<section anchor="client-facing-server-hrr"> <section anchor="client-facing-server-hrr">
<name>Sending HelloRetryRequest</name> <name>Sending HelloRetryRequest</name>
<t>After sending or forwarding a HelloRetryRequest, the client-facing server does <t>After sending or forwarding a HelloRetryRequest, the client-facing server does
not repeat the steps in <xref target="client-facing-server"/> with the second not repeat the steps in <xref target="client-facing-server"/> with the second
ClientHelloOuter. Instead, it continues with the ECHConfig selection from the ClientHelloOuter. Instead, it continues with the ECHConfig selection from the
first ClientHelloOuter as follows:</t> first ClientHelloOuter as follows:</t>
<t>If the client-facing server accepted ECH, it checks the second Clie ntHelloOuter <t>If the client-facing server accepted ECH, it checks that the second ClientHelloOuter
also contains the "encrypted_client_hello" extension. If not, it MUST abort the also contains the "encrypted_client_hello" extension. If not, it MUST abort the
handshake with a "missing_extension" alert. Otherwise, it checks that handshake with a "missing_extension" alert. Otherwise, it checks that
ECHClientHello.cipher_suite and ECHClientHello.config_id are unchanged, and 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 ECHClientHello.enc is empty. If not, it MUST abort the handshake with an
"illegal_parameter" alert.</t> "illegal_parameter" alert.</t>
<t>Finally, it decrypts the new ECHClientHello.payload as a second mes sage with the <t>Finally, it decrypts the new ECHClientHello.payload as a second mes sage with the
previous HPKE context:</t> previous HPKE context:</t>
<artwork><![CDATA[ <artwork><![CDATA[
EncodedClientHelloInner = context.Open(ClientHelloOuterAAD, EncodedClientHelloInner = context.Open(ClientHelloOuterAAD,
ECHClientHello.payload) ECHClientHello.payload)
skipping to change at line 1267 skipping to change at line 1309
client-facing and backend server requires care, as deployment mistakes client-facing and backend server requires care, as deployment mistakes
can lead to compatibility issues. These are discussed in <xref target="compat-is sues"/>.</t> can lead to compatibility issues. These are discussed in <xref target="compat-is sues"/>.</t>
<t>Beyond coordination difficulties, ECH deployments may also induce chall enges <t>Beyond coordination difficulties, ECH deployments may also induce chall enges
for use cases of information that ECH protects. In particular, for use cases of information that ECH protects. In particular,
use cases which depend on this unencrypted information may no longer work use cases which depend on this unencrypted information may no longer work
as desired. This is elaborated upon in <xref target="no-sni"/>.</t> as desired. This is elaborated upon in <xref target="no-sni"/>.</t>
<section anchor="compat-issues"> <section anchor="compat-issues">
<name>Compatibility Issues</name> <name>Compatibility Issues</name>
<t>Unlike most TLS extensions, placing the SNI value in an ECH extension is not <t>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 interoperable with existing servers, which expect the value in the existing
plaintext extension. Thus server operators SHOULD ensure servers understand a plaintext extension. Thus, server operators SHOULD ensure servers understand a
given set of ECH keys before advertising them. Additionally, servers SHOULD given set of ECH keys before advertising them. Additionally, servers SHOULD
retain support for any previously-advertised keys for the duration of their retain support for any previously advertised keys for the duration of their
validity.</t> validity.</t>
<t>However, in more complex deployment scenarios, this may be difficult to fully <t>However, in more complex deployment scenarios, this may be difficult to fully
guarantee. Thus this protocol was designed to be robust in case of guarantee. Thus, this protocol was designed to be robust in case of
inconsistencies between systems that advertise ECH keys and servers, at the cost 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.< /t> of extra round-trips due to a retry. Two specific scenarios are detailed below.< /t>
<section anchor="misconfiguration"> <section anchor="misconfiguration">
<name>Misconfiguration and Deployment Concerns</name> <name>Misconfiguration and Deployment Concerns</name>
<t>It is possible for ECH advertisements and servers to become inconsi stent. This <t>It is possible for ECH advertisements and servers to become inconsi stent. This
may occur, for instance, from DNS misconfiguration, caching issues, or an 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 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. </t> loses its ECH keys, or if a deployment of ECH must be rolled back on the server. </t>
<t>The retry mechanism repairs inconsistencies, provided the TLS serve r <t>The retry mechanism repairs inconsistencies, provided the TLS serve r
has a certificate for the public name. If server and advertised keys has a certificate for the public name. If server and advertised keys
mismatch, the server will reject ECH and respond with mismatch, the server will reject ECH and respond with
"retry_configs". If the server does "retry_configs". If the server does
not understand not understand the "encrypted_client_hello" extension at all, it will ignore it
the "encrypted_client_hello" extension at all, it will ignore it as required by as required by <xref section="4.1.2" sectionFormat="of" target="RFC8446"/>. Prov
<xref section="4.1.2" sectionFormat="of" target="RFC8446"/>. Provided the server ided the server can present a certificate
can present a certificate
valid for the public name, the client can safely retry with updated settings, valid for the public name, the client can safely retry with updated settings,
as described in <xref target="rejected-ech"/>.</t> as described in <xref target="rejected-ech"/>.</t>
<t>Unless ECH is disabled as a result of successfully establishing a c onnection to <t>Unless ECH is disabled as a result of successfully establishing a c onnection to
the public name, the client MUST NOT fall back to using unencrypted 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 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 ClientHello, including the SNI. It MAY attempt to use another server from the
DNS results, if one is provided.</t> DNS results, if one is provided.</t>
<t>In order to ensure that the retry mechanism works successfully serv ers <t>In order to ensure that the retry mechanism works successfully, ser vers
SHOULD ensure that every endpoint which might receive a TLS connection SHOULD ensure that every endpoint which might receive a TLS connection
is provisioned with an appropriate certificate for the public name. is provisioned with an appropriate certificate for the public name.
This is especially important during periods of server reconfiguration This is especially important during periods of server reconfiguration
when different endpoints might have different configurations.</t> when different endpoints might have different configurations.</t>
</section> </section>
<section anchor="middleboxes"> <section anchor="middleboxes">
<name>Middleboxes</name> <name>Middleboxes</name>
<t>The requirements in <xref section="9.3" sectionFormat="comma" targe t="RFC8446"/> which require proxies to <t>The requirements in <xref section="9.3" sectionFormat="comma" targe t="RFC8446"/> which require proxies to
act as conforming TLS client and server provide interoperability act as conforming TLS client and server provide interoperability
with TLS-terminating proxies even in cases where the server supports with TLS-terminating proxies even in cases where the server supports
ECH but the proxy does not, as detailed below.</t> ECH but the proxy does not, as detailed below.</t>
<t>The proxy must ignore unknown parameters, and <t>The proxy must ignore unknown parameters and
generate its own ClientHello containing only parameters it understands. Thus, generate its own ClientHello containing only parameters it understands. Thus,
when presenting a certificate to the client or sending a ClientHello to the when presenting a certificate to the client or sending a ClientHello to the
server, the proxy will act as if connecting to the ClientHelloOuter server, the proxy will act as if connecting to the ClientHelloOuter
server_name, which SHOULD match the public name (see <xref target="real-ech"/>), without server_name, which SHOULD match the public name (see <xref target="real-ech"/>) without
echoing the "encrypted_client_hello" extension.</t> echoing the "encrypted_client_hello" extension.</t>
<t>Depending on whether the client is configured to accept the proxy's certificate <t>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 as authoritative for the public name, this may trigger the retry logic described
in <xref target="rejected-ech"/> or result in a connection failure. A proxy whic h is not in <xref target="rejected-ech"/> or result in a connection failure. A proxy whic h is not
authoritative for the public name cannot forge a signal to disable ECH.</t> authoritative for the public name cannot forge a signal to disable ECH.</t>
</section> </section>
</section> </section>
<section anchor="no-sni"> <section anchor="no-sni">
<name>Deployment Impact</name> <name>Deployment Impact</name>
<t>Some use cases which depend on information ECH encrypts may break wit h the <t>Some use cases which depend on information ECH encrypts may break wit h the
skipping to change at line 1345 skipping to change at line 1386
intercept and decrypt client TLS connections. The feasibility of alternative intercept and decrypt client TLS connections. The feasibility of alternative
solutions is specific to individual deployments.</t> solutions is specific to individual deployments.</t>
</section> </section>
</section> </section>
<section anchor="compliance"> <section anchor="compliance">
<name>Compliance Requirements</name> <name>Compliance Requirements</name>
<t>In the absence of an application profile standard specifying otherwise, <t>In the absence of an application profile standard specifying otherwise,
a compliant ECH application MUST implement the following HPKE cipher suite:</t> a compliant ECH application MUST implement the following HPKE cipher suite:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>KEM: DHKEM(X25519, HKDF-SHA256) (see <xref section="7.1" sectionFor mat="of" target="HPKE"/>)</t> <t>KEM: DHKEM(X25519, HKDF-SHA256) (see <xref section="7.1" sectionFor mat="of" target="RFC9180"/>)</t>
</li> </li>
<li> <li>
<t>KDF: HKDF-SHA256 (see <xref section="7.2" sectionFormat="of" target ="HPKE"/>)</t> <t>KDF: HKDF-SHA256 (see <xref section="7.2" sectionFormat="of" target ="RFC9180"/>)</t>
</li> </li>
<li> <li>
<t>AEAD: AES-128-GCM (see <xref section="7.3" sectionFormat="of" targe t="HPKE"/>)</t> <t>AEAD: AES-128-GCM (see <xref section="7.3" sectionFormat="of" targe t="RFC9180"/>)</t>
</li> </li>
</ul> </ul>
</section> </section>
<section anchor="security-considerations"> <section anchor="security-considerations">
<name>Security Considerations</name> <name>Security Considerations</name>
<t>This section contains security considerations for ECH.</t> <t>This section contains security considerations for ECH.</t>
<section anchor="goals"> <section anchor="goals">
<name>Security and Privacy Goals</name> <name>Security and Privacy Goals</name>
<t>ECH considers two types of attackers: passive and active. Passive att ackers can <t>ECH considers two types of attackers: passive and active. Passive att ackers can
read packets from the network, but they cannot perform any sort of active 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 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, 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, active attackers can also write packets into the network for malicious purposes,
such as interfering with existing connections, probing servers, and querying such as interfering with existing connections, probing servers, and querying
DNS. In short, an active attacker corresponds to the conventional threat model DNS. In short, an active attacker corresponds to the conventional threat model
<xref target="RFC3552"/> for TLS 1.3 <xref target="RFC8446"/>.</t> <xref target="RFC3552"/> for TLS 1.3 <xref target="RFC8446"/>.</t>
<t>Passive and active attackers can exist anywhere in the network, inclu ding <t>Passive and active attackers can exist anywhere in the network, inclu ding
between the client and client-facing server, as well as between the between the client and client-facing server, as well as between the
client-facing and backend servers when running ECH in Split Mode. However, client-facing and backend servers when running ECH in split mode. However,
for Split Mode in particular, ECH makes two additional assumptions:</t> for split mode in particular, ECH makes two additional assumptions:</t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
<t>The channel between each client-facing and each backend server is <t>The channel between each client-facing and each backend server is
authenticated such that the backend server only accepts messages from trusted authenticated such that the backend server only accepts messages from trusted
client-facing servers. The exact mechanism for establishing this authenticated client-facing servers. The exact mechanism for establishing this authenticated
channel is out of scope for this document.</t> channel is out of scope for this document.</t>
</li> </li>
<li> <li>
<t>The attacker cannot correlate messages between client and client- facing <t>The attacker cannot correlate messages between a client and clien t-facing
server with messages between client-facing and backend server. Such correlation server with messages between client-facing and backend server. Such correlation
could allow an attacker to link information unique to a backend server, such as could allow an attacker to link information unique to a backend server, such as
their server name or IP address, with a client's encrypted ClientHelloInner. their server name or IP address, with a client's encrypted ClientHelloInner.
Correlation could occur through timing analysis of messages across the Correlation could occur through timing analysis of messages across the
client-facing server, or via examining the contents of messages sent between client-facing server, or via examining the contents of messages sent between
client-facing and backend servers. The exact mechanism for preventing this sort client-facing and backend servers. The exact mechanism for preventing this sort
of correlation is out of scope for this document.</t> of correlation is out of scope for this document.</t>
</li> </li>
</ol> </ol>
<t>Given this threat model, the primary goals of ECH are as follows.</t> <t>Given this threat model, the primary goals of ECH are as follows.</t>
skipping to change at line 1419 skipping to change at line 1460
name, then each anonymity set has size k = 1. Client-facing servers name, then each anonymity set has size k = 1. Client-facing servers
SHOULD deploy ECH in such a way so as to maximize the size of the 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 anonymity set where possible. This means client-facing servers should
use the same ECHConfig for as many server names as possible. An use the same ECHConfig for as many server names as possible. An
attacker can distinguish two server names that have different attacker can distinguish two server names that have different
ECHConfig values based on the ECHClientHello.config_id value.</t> ECHConfig values based on the ECHClientHello.config_id value.</t>
<t>This also means public information in a TLS handshake should be <t>This also means public information in a TLS handshake should be
consistent across server names. For example, if a client-facing server consistent across server names. For example, if a client-facing server
services many backend origin server names, only one of which supports some services many backend origin server names, only one of which supports some
cipher suite, it may be possible to identify that server name based on the cipher suite, it may be possible to identify that server name based on the
contents of unencrypted handshake message. Similarly, if a backend contents of the unencrypted handshake message. Similarly, if a backend
origin reuses KeyShare values, then that provides a unique identifier origin reuses KeyShare values, then that provides a unique identifier
for that server.</t> for that server.</t>
<t>Beyond these primary security and privacy goals, ECH also aims to hid e, to some <t>Beyond these primary security and privacy goals, ECH also aims to hid e, to some
extent, the fact that it is being used at all. Specifically, the GREASE ECH extent, the fact that it is being used at all. Specifically, the GREASE ECH
extension described in <xref target="grease-ech"/> does not change the security properties of extension described in <xref target="grease-ech"/> does not change the security properties of
the TLS handshake at all. Its goal is to provide "cover" for the real ECH the TLS handshake at all. Its goal is to provide "cover" for the real ECH
protocol (<xref target="real-ech"/>), as a means of addressing the "do not stick out" protocol (<xref target="real-ech"/>), as a means of addressing the "do not stick out"
requirements of <xref target="RFC8744"/>. See <xref target="dont-stick-out"/> fo r details.</t> requirements of <xref target="RFC8744"/>. See <xref target="dont-stick-out"/> fo r details.</t>
</section> </section>
<section anchor="plaintext-dns"> <section anchor="plaintext-dns">
<name>Unauthenticated and Plaintext DNS</name> <name>Unauthenticated and Plaintext DNS</name>
<t>ECH supports delivery of configurations through the DNS using SVCB or HTTPS <t>ECH supports delivery of configurations through the DNS using SVCB or HTTPS
records, without requiring any verifiable authenticity or provenance records without requiring any verifiable authenticity or provenance
information <xref target="ECH-IN-DNS"/>. This means that any attacker which can information <xref target="RFCYYY1"/>. This means that any attacker which can inj
inject ect
DNS responses or poison DNS caches, which is a common scenario in DNS responses or poison DNS caches, which is a common scenario in
client access networks, can supply clients with fake ECH configurations (so client access networks, can supply clients with fake ECH configurations (so
that the client encrypts data to them) or strip the ECH configurations from that the client encrypts data to them) or strip the ECH configurations from
the response. However, in the face of an attacker that controls DNS, the response. However, in the face of an attacker that controls DNS,
no encryption scheme can work because the attacker can replace the IP no encryption scheme can work because the attacker can replace the IP
address, thus blocking client connections, or substitute a unique IP address, thus blocking client connections, or substitute a unique IP
address for each DNS name that was looked up. Thus, using DNS records address for each DNS name that was looked up. Thus, using DNS records
without additional authentication does not make the situation significantly without additional authentication does not make the situation significantly
worse.</t> worse.</t>
<t>Clearly, DNSSEC (if the client validates and hard fails) is a defense <t>Clearly, DNSSEC (if the client validates and hard fails) is a defense
skipping to change at line 1528 skipping to change at line 1569
In particular, timing side channels can reveal information about the contents In particular, timing side channels can reveal information about the contents
of ClientHelloInner. Implementations should take such side channels into of ClientHelloInner. Implementations should take such side channels into
consideration when reasoning about the privacy properties that ECH provides.</t> consideration when reasoning about the privacy properties that ECH provides.</t>
</section> </section>
<section anchor="related-privacy-leaks"> <section anchor="related-privacy-leaks">
<name>Related Privacy Leaks</name> <name>Related Privacy Leaks</name>
<t>ECH requires encrypted DNS to be an effective privacy protection mech anism. <t>ECH requires encrypted DNS to be an effective privacy protection mech anism.
However, verifying the server's identity from the Certificate message, However, verifying the server's identity from the Certificate message,
particularly when using the X509 CertificateType, may result in additional particularly when using the X509 CertificateType, may result in additional
network traffic that may reveal the server identity. Examples of this traffic 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 may include requests for revocation information, such as Online Certificate Stat
requests for repository information, such as authorityInformationAccess. It may us Protocol (OCSP) or Certificate Revocation List (CRL) traffic, or requests for
also include implementation-specific traffic for additional information sources repository information, such as authorityInformationAccess. It may also include
as part of verification.</t> implementation-specific traffic for additional information sources as part of v
erification.</t>
<t>Implementations SHOULD avoid leaking information that may identify th e server. <t>Implementations SHOULD avoid leaking information that may identify th e server.
Even when sent over an encrypted transport, such requests may result in indirect 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 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 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 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 SHOULD take steps to minimize or protect such requests during certificate
validation.</t> validation.</t>
<t>Attacks that rely on non-ECH traffic to infer server identity in an E CH <t>Attacks that rely on non-ECH traffic to infer server identity in an E CH
connection are out of scope for this document. For example, a client that 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 connects to a particular host prior to ECH deployment may later resume a
skipping to change at line 1560 skipping to change at line 1598
encrypts the cookie in the second ClientHelloInner, the backend server's encrypts the cookie in the second ClientHelloInner, the backend server's
HelloRetryRequest is unencrypted.This means differences in cookies between HelloRetryRequest is unencrypted.This means differences in cookies between
backend servers, such as lengths or cleartext components, may leak information backend servers, such as lengths or cleartext components, may leak information
about the server identity.</t> about the server identity.</t>
<t>Backend servers in an anonymity set SHOULD NOT reveal information in the cookie <t>Backend servers in an anonymity set SHOULD NOT reveal information in the cookie
which identifies the server. This may be done by handling HelloRetryRequest which identifies the server. This may be done by handling HelloRetryRequest
statefully, thus not sending cookies, or by using the same cookie construction statefully, thus not sending cookies, or by using the same cookie construction
for all backend servers.</t> for all backend servers.</t>
<t>Note that, if the cookie includes a key name, analogous to <xref sect ion="4" sectionFormat="of" target="RFC5077"/>, this may leak information if diff erent backend servers issue <t>Note that, if the cookie includes a key name, analogous to <xref sect ion="4" sectionFormat="of" target="RFC5077"/>, this may leak information if diff erent backend servers issue
cookies with different key names at the time of the connection. In particular, 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 if the deployment operates in split mode, the backend servers may not share
cookie encryption keys. Backend servers may mitigate this by either handling cookie encryption keys. Backend servers may mitigate this either by handling
key rotation with trial decryption, or coordinating to match key names.</t> key rotation with trial decryption or by coordinating to match key names.</t>
</section> </section>
<section anchor="attacks-exploiting-acceptance-confirmation"> <section anchor="attacks-exploiting-acceptance-confirmation">
<name>Attacks Exploiting Acceptance Confirmation</name> <name>Attacks Exploiting Acceptance Confirmation</name>
<t>To signal acceptance, the backend server overwrites 8 bytes of its <t>To signal acceptance, the backend server overwrites 8 bytes of its
ServerHello.random with a value derived from the ClientHelloInner.random. (See ServerHello.random with a value derived from the ClientHelloInner.random. (See
<xref target="backend-server"/> for details.) This behavior increases the likeli hood of the <xref target="backend-server"/> for details.) This behavior increases the likeli hood of the
ServerHello.random colliding with the ServerHello.random of a previous session, ServerHello.random colliding with the ServerHello.random of a previous session,
potentially reducing the overall security of the protocol. However, the potentially reducing the overall security of the protocol. However, the
remaining 24 bytes provide enough entropy to ensure this is not a practical remaining 24 bytes provide enough entropy to ensure this is not a practical
avenue of attack.</t> avenue of attack.</t>
skipping to change at line 1589 skipping to change at line 1627
positive occurring for a given connection is only 1 in 2^64. This value is 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.</t> smaller than the probability of network connection failures in practice.</t>
<t>Note that the same bytes of the ServerHello.random are used to implem ent <t>Note that the same bytes of the ServerHello.random are used to implem ent
downgrade protection for TLS 1.3 (see <xref section="4.1.3" sectionFormat="comma " target="RFC8446"/>). These downgrade protection for TLS 1.3 (see <xref section="4.1.3" sectionFormat="comma " target="RFC8446"/>). These
mechanisms do not interfere because the backend server only signals ECH mechanisms do not interfere because the backend server only signals ECH
acceptance in TLS 1.3 or higher.</t> acceptance in TLS 1.3 or higher.</t>
</section> </section>
<section anchor="comparison-against-criteria"> <section anchor="comparison-against-criteria">
<name>Comparison Against Criteria</name> <name>Comparison Against Criteria</name>
<t><xref target="RFC8744"/> lists several requirements for SNI encryptio n. <t><xref target="RFC8744"/> lists several requirements for SNI encryptio n.
In this section, we re-iterate these requirements and assess the ECH design In this section, we reiterate these requirements and assess the ECH design
against them.</t> against them.</t>
<section anchor="mitigate-cut-and-paste-attacks"> <section anchor="mitigate-cut-and-paste-attacks">
<name>Mitigate Cut-and-Paste Attacks</name> <name>Mitigate Cut-and-Paste Attacks</name>
<t>Since servers process either ClientHelloInner or ClientHelloOuter, and because <t>Since servers process either ClientHelloInner or ClientHelloOuter, and because
ClientHelloInner.random is encrypted, it is not possible for an attacker to "cut 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 and paste" the ECH value in a different Client Hello and learn information from
ClientHelloInner.</t> ClientHelloInner.</t>
</section> </section>
<section anchor="avoid-widely-shared-secrets"> <section anchor="avoid-widely-shared-secrets">
<name>Avoid Widely Shared Secrets</name> <name>Avoid Widely Shared Secrets</name>
skipping to change at line 1637 skipping to change at line 1675
(<xref target="real-ech"/>) without changing the security properties of the hand shake. The (<xref target="real-ech"/>) without changing the security properties of the hand shake. The
underlying theory is that if GREASE ECH is deployable without triggering underlying theory is that if GREASE ECH is deployable without triggering
middlebox misbehavior, and real ECH looks enough like GREASE ECH, then ECH middlebox misbehavior, and real ECH looks enough like GREASE ECH, then ECH
should be deployable as well. Thus, the strategy for mitigating network should be deployable as well. Thus, the strategy for mitigating network
ossification is to deploy GREASE ECH widely enough to disincentivize ossification is to deploy GREASE ECH widely enough to disincentivize
differential treatment of the real ECH protocol by the network.</t> differential treatment of the real ECH protocol by the network.</t>
<t>Ensuring that networks do not differentiate between real ECH and GR EASE ECH may <t>Ensuring that networks do not differentiate between real ECH and GR EASE ECH may
not be feasible for all implementations. While most middleboxes will not treat 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 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 GREASE ECH. This specification aims to provide a baseline security level that
most deployments can achieve easily, while providing implementations enough most deployments can achieve easily while providing implementations enough
flexibility to achieve stronger security where possible. Minimally, real ECH is flexibility to achieve stronger security where possible. Minimally, real ECH is
designed to be indifferentiable from GREASE ECH for passive adversaries with designed to be indifferentiable from GREASE ECH for passive adversaries with
following capabilities:</t> following capabilities:</t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
<t>The attacker does not know the ECHConfigList used by the server .</t> <t>The attacker does not know the ECHConfigList used by the server .</t>
</li> </li>
<li> <li>
<t>The attacker keeps per-connection state only. In particular, it does not <t>The attacker keeps per-connection state only. In particular, it does not
track endpoints across connections.</t> track endpoints across connections.</t>
</li> </li>
skipping to change at line 1693 skipping to change at line 1731
</section> </section>
<section anchor="maintain-forward-secrecy"> <section anchor="maintain-forward-secrecy">
<name>Maintain Forward Secrecy</name> <name>Maintain Forward Secrecy</name>
<t>This design does not provide forward secrecy for the inner ClientHe llo <t>This design does not provide forward secrecy for the inner ClientHe llo
because the server's ECH key is static. However, the window of because the server's ECH key is static. However, the window of
exposure is bound by the key lifetime. It is RECOMMENDED that servers exposure is bound by the key lifetime. It is RECOMMENDED that servers
rotate keys regularly.</t> rotate keys regularly.</t>
</section> </section>
<section anchor="enable-multi-party-security-contexts"> <section anchor="enable-multi-party-security-contexts">
<name>Enable Multi-party Security Contexts</name> <name>Enable Multi-party Security Contexts</name>
<t>This design permits servers operating in Split Mode to forward conn ections <t>This design permits servers operating in split mode to forward conn ections
directly to backend origin servers. The client authenticates the identity of directly to backend origin servers. The client authenticates the identity of
the backend origin server, thereby allowing the backend origin server the backend origin server, thereby allowing the backend origin server
to hide behind the client-facing server without the client-facing to hide behind the client-facing server without the client-facing
server decrypting and reencrypting the connection.</t> server decrypting and reencrypting the connection.</t>
<t>Conversely, if the DNS records used for configuration are <t>Conversely, if the DNS records used for configuration are
authenticated, e.g., via DNSSEC, authenticated, e.g., via DNSSEC,
spoofing a client-facing server operating in Split Mode is not spoofing a client-facing server operating in split mode is not
possible. See <xref target="plaintext-dns"/> for more details regarding plaintex t possible. See <xref target="plaintext-dns"/> for more details regarding plaintex t
DNS.</t> DNS.</t>
<t>Authenticating the ECHConfig structure naturally authenticates the included <t>Authenticating the ECHConfig structure naturally authenticates the included
public name. This also authenticates any retry signals from the client-facing public name. This also authenticates any retry signals from the client-facing
server because the client validates the server certificate against the public server because the client validates the server certificate against the public
name before retrying.</t> name before retrying.</t>
</section> </section>
<section anchor="support-multiple-protocols"> <section anchor="support-multiple-protocols">
<name>Support Multiple Protocols</name> <name>Support Multiple Protocols</name>
<t>This design has no impact on application layer protocol negotiation . It may <t>This design has no impact on application layer protocol negotiation . It may
skipping to change at line 1748 skipping to change at line 1786
information about its verification process by a timing side channel), the information about its verification process by a timing side channel), the
attacker learns that its test certificate name was incorrect. As an example, 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 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 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 verification failure alert because of the mismatch faster than it would due to
the Certificate signature validation, information about the name leaks. Note the Certificate signature validation, information about the name leaks. Note
that the attacker can also withhold the CertificateVerify message. In that that the attacker can also withhold the CertificateVerify message. In that
scenario, a client which first verifies the Certificate would then respond scenario, a client which first verifies the Certificate would then respond
similarly and leak the same information.</t> similarly and leak the same information.</t>
<figure anchor="flow-diagram-client-reaction"> <figure anchor="flow-diagram-client-reaction">
<name>Client reaction attack</name> <name>Client Reaction Attack</name>
<artwork><![CDATA[ <artwork><![CDATA[
Client Attacker Server Client Attacker Server
ClientHello ClientHello
+ key_share + key_share
+ ech ------> (intercept) -----> X (drop) + ech ------> (intercept) -----> X (drop)
ServerHello ServerHello
+ key_share + key_share
{EncryptedExtensions} {EncryptedExtensions}
{CertificateRequest*} {CertificateRequest*}
skipping to change at line 1785 skipping to change at line 1823
To begin, the attacker intercepts and forwards a legitimate ClientHello with an To begin, the attacker intercepts and forwards a legitimate ClientHello with an
"encrypted_client_hello" (ech) extension to the server, which triggers a "encrypted_client_hello" (ech) extension to the server, which triggers a
legitimate HelloRetryRequest in return. Rather than forward the retry to the legitimate HelloRetryRequest in return. Rather than forward the retry to the
client, the attacker attempts to generate its own ClientHello in response based client, the attacker attempts to generate its own ClientHello in response based
on the contents of the first ClientHello and HelloRetryRequest exchange with the 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 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 used the SNI from the first ClientHello and the key share from the second
(attacker-controlled) ClientHello, the Certificate produced would leak the (attacker-controlled) ClientHello, the Certificate produced would leak the
client's chosen SNI to the attacker.</t> client's chosen SNI to the attacker.</t>
<figure anchor="flow-diagram-hrr-hijack"> <figure anchor="flow-diagram-hrr-hijack">
<name>HelloRetryRequest hijack attack</name> <name>HelloRetryRequest Hijack Attack</name>
<artwork><![CDATA[ <artwork><![CDATA[
Client Attacker Server Client Attacker Server
ClientHello ClientHello
+ key_share + key_share
+ ech ------> (forward) -------> + ech ------> (forward) ------->
HelloRetryRequest HelloRetryRequest
+ key_share + key_share
(intercept) <------- (intercept) <-------
ClientHello ClientHello
skipping to change at line 1837 skipping to change at line 1875
incorrectly apply parameters from ClientHelloOuter to the handshake.</t> incorrectly apply parameters from ClientHelloOuter to the handshake.</t>
<t>To begin, the attacker first interacts with a server to obtain a re sumption <t>To begin, the attacker first interacts with a server to obtain a re sumption
ticket for a given test domain, such as "example.com". Later, upon receipt of a 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 ClientHelloOuter, it modifies it such that the server will process the
resumption ticket with ClientHelloInner. If the server only accepts resumption 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 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 alert when ClientHelloInner is for "example.com" but silently ignore the PSK
and continue when ClientHelloInner is for any other name. This introduces an and continue when ClientHelloInner is for any other name. This introduces an
oracle for testing encrypted SNI values.</t> oracle for testing encrypted SNI values.</t>
<figure anchor="tls-clienthello-malleability"> <figure anchor="tls-clienthello-malleability">
<name>Message flow for malleable ClientHello</name> <name>Message Flow for Malleable ClientHello</name>
<artwork><![CDATA[ <artwork><![CDATA[
Client Attacker Server Client Attacker Server
handshake and ticket handshake and ticket
for "example.com" for "example.com"
<--------> <-------->
ClientHello ClientHello
+ key_share + key_share
+ ech + ech
skipping to change at line 1874 skipping to change at line 1912
]]></artwork> ]]></artwork>
</figure> </figure>
<t>This attack may be generalized to any parameter which the server va ries by <t>This attack may be generalized to any parameter which the server va ries by
server name, such as ALPN preferences.</t> server name, such as ALPN preferences.</t>
<t>ECH mitigates this attack by only negotiating TLS parameters from <t>ECH mitigates this attack by only negotiating TLS parameters from
ClientHelloInner and authenticating all inputs to the ClientHelloInner ClientHelloInner and authenticating all inputs to the ClientHelloInner
(EncodedClientHelloInner and ClientHelloOuter) with the HPKE AEAD. See (EncodedClientHelloInner and ClientHelloOuter) with the HPKE AEAD. See
<xref target="authenticating-outer"/>. The decompression process in <xref target ="encoding-inner"/> <xref target="authenticating-outer"/>. The decompression process in <xref target ="encoding-inner"/>
forbids "encrypted_client_hello" in OuterExtensions. This ensures the forbids "encrypted_client_hello" in OuterExtensions. This ensures the
unauthenticated portion of ClientHelloOuter is not incorporated into unauthenticated portion of ClientHelloOuter is not incorporated into
ClientHelloInner. ClientHelloInner. An earlier iteration of this specification only
An earlier iteration of this specification only
encrypted and authenticated the "server_name" extension, which left the overall encrypted and authenticated the "server_name" extension, which left the overall
ClientHello vulnerable to an analogue of this attack.</t> ClientHello vulnerable to an analogue of this attack.</t>
</section> </section>
<section anchor="decompression-amp"> <section anchor="decompression-amp">
<name>ClientHelloInner Packet Amplification Mitigation</name> <name>ClientHelloInner Packet Amplification Mitigation</name>
<t>Client-facing servers must decompress EncodedClientHelloInners. A m alicious <t>Client-facing servers must decompress EncodedClientHelloInners. A m alicious
attacker may craft a packet which takes excessive resources to decompress attacker may craft a packet which takes excessive resources to decompress
or may be much larger than the incoming packet:</t> or may be much larger than the incoming packet:</t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
<t>If looking up a ClientHelloOuter extension takes time linear in the number of <t>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 extensions, the overall decoding process would take O(M*N) time, where
M is the number of extensions in ClientHelloOuter and N is the M is the number of extensions in ClientHelloOuter and N is the
size of OuterExtensions.</t> size of OuterExtensions.</t>
</li> </li>
<li> <li>
<t>If the same ClientHelloOuter extension can be copied multiple t imes, <t>If the same ClientHelloOuter extension can be copied multiple t imes,
an attacker could cause the client-facing server to construct a large an attacker could cause the client-facing server to construct a large
ClientHelloInner by including a large extension in ClientHelloOuter, ClientHelloInner by including a large extension in ClientHelloOuter
of length L, and an OuterExtensions list referencing N copies of that 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 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 response to O(N+L) bandwidth from the client. In split mode, an
O(N*L) sized packet would then be transmitted to the O(N*L)-sized packet would then be transmitted to the
backend server.</t> backend server.</t>
</li> </li>
</ul> </ul>
<t>ECH mitigates this attack by requiring that OuterExtensions be refe renced in <t>ECH mitigates this attack by requiring that OuterExtensions be refe renced in
order, that duplicate references be rejected, and by recommending that order, that duplicate references be rejected, and by recommending that
client-facing servers use a linear scan to perform decompression. These client-facing servers use a linear scan to perform decompression. These
requirements are detailed in <xref target="encoding-inner"/>.</t> requirements are detailed in <xref target="encoding-inner"/>.</t>
</section> </section>
</section> </section>
</section> </section>
<section anchor="iana-considerations"> <section anchor="iana-considerations">
<name>IANA Considerations</name> <name>IANA Considerations</name>
<section anchor="update-of-the-tls-extensiontype-registry"> <section anchor="update-of-the-tls-extensiontype-registry">
<name>Update of the TLS ExtensionType Registry</name> <name>Update of the TLS ExtensionType Registry</name>
<t>IANA is requested to create the following entries in the existing reg <t>IANA has created the following entries in the existing
istry for "TLS ExtensionType Values" registry (defined in <xref target="RFC8446"/>):</t>
ExtensionType (defined in <xref target="RFC8446"/>):</t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
<t>encrypted_client_hello(0xfe0d), with "TLS 1.3" column values set to <t>encrypted_client_hello (0xfe0d), with "TLS 1.3" column values set to
"CH, HRR, EE", "DTLS-Only" column set to "N", and "Recommended" column set "CH, HRR, EE", "DTLS-Only" column set to "N", and "Recommended" column set
to "Yes".</t> to "Y".</t>
</li> </li>
<li> <li>
<t>ech_outer_extensions(0xfd00), with the "TLS 1.3" column values se <t>ech_outer_extensions (0xfd00), with the "TLS 1.3" column values s
t to "CH", et to "CH",
"DTLS-Only" column set to "N", "Recommended" column set to "Yes", and the "DTLS-Only" column set to "N", "Recommended" column set to "Y", and the
"Comment" column set to "Only appears in inner CH."</t> "Comment" column set to "Only appears in inner CH."</t>
</li> </li>
</ol> </ol>
</section> </section>
<section anchor="alerts"> <section anchor="alerts">
<name>Update of the TLS Alert Registry</name> <name>Update of the TLS Alert Registry</name>
<t>IANA is requested to create an entry, ech_required(121) in the existi <t>IANA has created an entry, ech_required (121) in the existing "TLS
ng registry Alerts" registry (defined in <xref target="RFC8446"/>), with the "DTLS-OK" colum
for Alerts (defined in <xref target="RFC8446"/>), with the "DTLS-OK" column set n
to set to "Y".</t>
"Y".</t>
</section> </section>
<section anchor="config-extensions-iana"> <section anchor="config-extensions-iana">
<name>ECH Configuration Extension Registry</name> <name>ECH Configuration Extension Registry</name>
<t>IANA is requested to create a new "ECHConfig Extension" registry in a <t>IANA has created a new "TLS ECHConfig Extension" registry in a new
new "TLS Encrypted Client Hello (ECH) Configuration Extensions" registry group. New
"TLS Encrypted Client Hello (ECH) Configuration Extensions" page. New registrations will list the following attributes:</t>
registrations need to list the following attributes:</t>
<dl spacing="compact"> <dl spacing="compact">
<dt>Value:</dt> <dt>Value:</dt>
<dd> <dd>
<t>The two-byte identifier for the ECHConfigExtension, i.e., the <t>The two-byte identifier for the ECHConfigExtension, i.e., the
ECHConfigExtensionType</t> ECHConfigExtensionType</t>
</dd> </dd>
<dt>Extension Name:</dt> <dt>Extension Name:</dt>
<dd> <dd>
<t>Name of the ECHConfigExtension</t> <t>Name of the ECHConfigExtension</t>
</dd> </dd>
skipping to change at line 1964 skipping to change at line 2001
</dd> </dd>
<dt>Reference:</dt> <dt>Reference:</dt>
<dd> <dd>
<t>The specification where the ECHConfigExtension is defined</t> <t>The specification where the ECHConfigExtension is defined</t>
</dd> </dd>
<dt>Notes:</dt> <dt>Notes:</dt>
<dd> <dd>
<t>Any notes associated with the entry</t> <t>Any notes associated with the entry</t>
</dd> </dd>
</dl> </dl>
<t>New entries in the "ECHConfig Extension" registry are subject to the <t>New entries in the "TLS ECHConfig Extension" registry are subject to the
Specification Required registration policy (<xref section="4.6" sectionFormat="c omma" target="RFC8126"/>), with the policies described in <xref section="17" sec tionFormat="comma" target="RFC8447"/>. IANA Specification Required registration policy (<xref section="4.6" sectionFormat="c omma" target="RFC8126"/>), with the policies described in <xref section="17" sec tionFormat="comma" target="RFC8447"/>. IANA
[shall add/has added] the following note to the TLS ECHConfig Extension has added the following note to the "TLS ECHConfig Extension"
registry:</t> registry:</t>
<t>Note: The role of the designated expert is described in RFC 8447. <t>Note: The role of the designated expert is described in RFC 8447.
The designated expert <xref target="RFC8126"/> ensures that the specificat ion is The designated expert <xref target="RFC8126"/> ensures that the specificat ion is
publicly available. It is sufficient to have an Internet-Draft publicly available. It is sufficient to have an Internet-Draft
(that is posted and never published as an RFC) or a document from (that is posted and never published as an RFC) or a document from
another standards body, industry consortium, university site, etc. another standards body, industry consortium, university site, etc.
The expert may provide more in depth reviews, but their approval The expert may provide more in-depth reviews, but their approval
should not be taken as an endorsement of the extension.</t> should not be taken as an endorsement of the extension.</t>
<t>This document defines several Reserved values for ECH configuration e xtensions <t>This document defines several Reserved values for ECH configuration e xtensions
to be used for "greasing" as described in <xref target="server-greasing"/>.</t> to be used for "greasing" as described in <xref target="server-greasing"/>.</t>
<t>The initial contents for this registry consists of multiple reserved values, <t>The initial contents for this registry consists of multiple reserved values
with the following attributes, which are repeated for each registration:</t> with the following attributes, which are repeated for each registration:</t>
<dl spacing="compact"> <dl spacing="compact">
<dt>Value:</dt> <dt>Value:</dt>
<dd> <dd>
<t>0x0000, 0x1A1A, 0x2A2A, 0x3A3A, 0x4A4A, 0x5A5A, 0x6A6A, 0x7A7A, 0 x8A8A, <t>0x0000, 0x1A1A, 0x2A2A, 0x3A3A, 0x4A4A, 0x5A5A, 0x6A6A, 0x7A7A, 0 x8A8A,
0x9A9A, 0xAAAA, 0xBABA, 0xCACA, 0xDADA, 0xEAEA, 0xFAFA</t> 0x9A9A, 0xAAAA, 0xBABA, 0xCACA, 0xDADA, 0xEAEA, 0xFAFA</t>
</dd> </dd>
<dt>Extension Name:</dt> <dt>Extension Name:</dt>
<dd> <dd>
<t>RESERVED</t> <t>RESERVED</t>
</dd> </dd>
<dt>Recommended:</dt> <dt>Recommended:</dt>
<dd> <dd>
<t>Y</t> <t>Y</t>
</dd> </dd>
<dt>Reference:</dt> <dt>Reference:</dt>
<dd> <dd>
<t>This document</t> <t>RFC 9849</t>
</dd> </dd>
<dt>Notes:</dt> <dt>Notes:</dt>
<dd> <dd>
<t>Grease entries.</t> <t>GREASE entries</t>
</dd> </dd>
</dl> </dl>
</section> <!-- [rfced] We note that the following terms use fixed-width font
inconsistently. Please review these terms and let us know how we should update
or if there are any specific patterns that should be followed (e.g.,
fixed-width font used for field names, variants, etc.).
accept_confirmation
cipher_suite
ClientHello
ClientHelloInner
ClientHelloOuter
ClientHelloOuterAAD
config_id
ECHClientHello
ECHConfig
ECHConfig.contents.public_name
ECHConfigContents
ECHConfigList
EncodedClientHelloInner
inner
maximum_name_length
outer
payload
public_key
ServerHello.random
zeros
EKR: Thanks. Fixed width should be used for field names and other PDUs.
I notice that some of these are regular words (zeros) so you have to determine f
rom context whether it's referring to some protocol element or just to the conce
pt "carries an encrypted payload" versus "the payload field". Do you want to tak
e a cut at changing as many of these as make sense and then I can review, or wou
ld you prefer I make the changes?
One question is what to do in definition lists. My sense is that the list heds s
hould be non-fixed-width but maybe you have a convention.
rfced: Thank you for offering to make changes. Please feel free to attach an upd
ated markdown file containing the
updates for terms using fixed-width font.
For definition lists, we typically leave this up to the authors to determine how
they would like the terms to appear for consistency. For an example of terms in
a definition list using a fixed-width font, see: https://www.rfc-editor.org/rfc
/rfc9623.html#section-5.1.1.
-->
</section>
</section> </section>
</middle> </middle>
<back> <back>
<displayreference target="RFC9180" to="HPKE"/>
<displayreference target="RFC9499" to="DNS-TERMS"/>
<displayreference target="I-D.kazuho-protected-sni" to="PROTECTED-SNI"/>
<references anchor="sec-combined-references"> <references anchor="sec-combined-references">
<name>References</name> <name>References</name>
<references anchor="sec-normative-references"> <references anchor="sec-normative-references">
<name>Normative References</name> <name>Normative References</name>
<reference anchor="RFC2119"> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2
<front> 119.xml"/>
<title>Key words for use in RFCs to Indicate Requirement Levels</tit <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
le> 918.xml"/>
<author fullname="S. Bradner" initials="S." surname="Bradner"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
<date month="March" year="1997"/> 180.xml"/>
<abstract> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
<t>In many standards track documents several words are used to sig 446.xml"/>
nify the requirements in the specification. These words are often capitalized. T <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
his document defines these words as they should be interpreted in IETF documents 147.xml"/>
. This document specifies an Internet Best Current Practices for the Internet Co <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
mmunity, and requests discussion and suggestions for improvements.</t> 174.xml"/>
</abstract> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
</front> 460.xml"/>
<seriesInfo name="BCP" value="14"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
<seriesInfo name="RFC" value="2119"/> 525.xml"/>
<seriesInfo name="DOI" value="10.17487/RFC2119"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
</reference> 890.xml"/>
<reference anchor="RFC7918"> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
<front> 126.xml"/>
<title>Transport Layer Security (TLS) False Start</title> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
<author fullname="A. Langley" initials="A." surname="Langley"/> 447.xml"/>
<author fullname="N. Modadugu" initials="N." surname="Modadugu"/> </references>
<author fullname="B. Moeller" initials="B." surname="Moeller"/> <references anchor="sec-informative-references">
<date month="August" year="2016"/> <name>Informative References</name>
<abstract> <reference anchor="RFCYYY1" target="https://www.rfc-editor.org/info/rfcY
<t>This document specifies an optional behavior of Transport Layer YY1">
Security (TLS) client implementations, dubbed "False Start". It affects only pr
otocol timing, not on-the-wire protocol data, and can be implemented unilaterall
y. A TLS False Start reduces handshake latency to one round trip.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="7918"/>
<seriesInfo name="DOI" value="10.17487/RFC7918"/>
</reference>
<reference anchor="RFC8446">
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.3</titl
e>
<author fullname="E. Rescorla" initials="E." surname="Rescorla"/>
<date month="August" year="2018"/>
<abstract>
<t>This document specifies version 1.3 of the Transport Layer Secu
rity (TLS) protocol. TLS allows client/server applications to communicate over t
he Internet in a way that is designed to prevent eavesdropping, tampering, and m
essage forgery.</t>
<t>This document updates RFCs 5705 and 6066, and obsoletes RFCs 50
77, 5246, and 6961. This document also specifies new requirements for TLS 1.2 im
plementations.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="8446"/>
<seriesInfo name="DOI" value="10.17487/RFC8446"/>
</reference>
<reference anchor="RFC9147">
<front>
<title>The Datagram Transport Layer Security (DTLS) Protocol Version
1.3</title>
<author fullname="E. Rescorla" initials="E." surname="Rescorla"/>
<author fullname="H. Tschofenig" initials="H." surname="Tschofenig"/
>
<author fullname="N. Modadugu" initials="N." surname="Modadugu"/>
<date month="April" year="2022"/>
<abstract>
<t>This document specifies version 1.3 of the Datagram Transport L
ayer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to com
municate over the Internet in a way that is designed to prevent eavesdropping, t
ampering, and message forgery.</t>
<t>The DTLS 1.3 protocol is based on the Transport Layer Security
(TLS) 1.3 protocol and provides equivalent security guarantees with the exceptio
n of order protection / non-replayability. Datagram semantics of the underlying
transport are preserved by the DTLS protocol.</t>
<t>This document obsoletes RFC 6347.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="9147"/>
<seriesInfo name="DOI" value="10.17487/RFC9147"/>
</reference>
<reference anchor="RFC8174">
<front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</ti
tle>
<author fullname="B. Leiba" initials="B." surname="Leiba"/>
<date month="May" year="2017"/>
<abstract>
<t>RFC 2119 specifies common key words that may be used in protoco
l specifications. This document aims to reduce the ambiguity by clarifying that
only UPPERCASE usage of the key words have the defined special meanings.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="8174"/>
<seriesInfo name="DOI" value="10.17487/RFC8174"/>
</reference>
<reference anchor="RFC9460">
<front>
<title>Service Binding and Parameter Specification via the DNS (SVCB
and HTTPS Resource Records)</title>
<author fullname="B. Schwartz" initials="B." surname="Schwartz"/>
<author fullname="M. Bishop" initials="M." surname="Bishop"/>
<author fullname="E. Nygren" initials="E." surname="Nygren"/>
<date month="November" year="2023"/>
<abstract>
<t>This document specifies the "SVCB" ("Service Binding") and "HTT
PS" DNS resource record (RR) types to facilitate the lookup of information neede
d to make connections to network services, such as for HTTP origins. SVCB record
s allow a service to be provided from multiple alternative endpoints, each with
associated parameters (such as transport protocol configuration), and are extens
ible to support future uses (such as keys for encrypting the TLS ClientHello). T
hey also enable aliasing of apex domains, which is not possible with CNAME. The
HTTPS RR is a variation of SVCB for use with HTTP (see RFC 9110, "HTTP Semantics
"). By providing more information to the client before it attempts to establish
a connection, these records offer potential benefits to both performance and pri
vacy.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="9460"/>
<seriesInfo name="DOI" value="10.17487/RFC9460"/>
</reference>
<reference anchor="ECH-IN-DNS">
<front> <front>
<title>Bootstrapping TLS Encrypted ClientHello with DNS Service Bind ings</title> <title>Bootstrapping TLS Encrypted ClientHello with DNS Service Bind ings</title>
<author fullname="Benjamin M. Schwartz" initials="B. M." surname="Sc <author initials="B." surname="Schwartz" fullname="Benjamin M. Schwa
hwartz"> rtz">
<organization>Meta Platforms, Inc.</organization> <organization/>
</author> </author>
<author fullname="Mike Bishop" initials="M." surname="Bishop"> <author initials="M." surname="Bishop" fullname="Mike Bishop">
<organization>Akamai Technologies</organization> <organization/>
</author> </author>
<author fullname="Erik Nygren" initials="E." surname="Nygren"> <author initials="E." surname="Nygren" fullname="Erik Nygren">
<organization>Akamai Technologies</organization> <organization/>
</author> </author>
<date day="12" month="February" year="2025"/> <date year="2025" month="November"/>
<abstract>
<t> To use TLS Encrypted ClientHello (ECH) the client needs to l
earn the
ECH configuration for a server before it attempts a connection to the
server. This specification provides a mechanism for conveying the
ECH configuration information via DNS, using a SVCB or HTTPS record.
</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-tls-svcb-ech-07"/>
</reference>
<reference anchor="HPKE">
<front>
<title>Hybrid Public Key Encryption</title>
<author fullname="R. Barnes" initials="R." surname="Barnes"/>
<author fullname="K. Bhargavan" initials="K." surname="Bhargavan"/>
<author fullname="B. Lipp" initials="B." surname="Lipp"/>
<author fullname="C. Wood" initials="C." surname="Wood"/>
<date month="February" year="2022"/>
<abstract>
<t>This document describes a scheme for hybrid public key encrypti
on (HPKE). This scheme provides a variant of public key encryption of arbitrary-
sized plaintexts for a recipient public key. It also includes three authenticate
d variants, including one that authenticates possession of a pre-shared key and
two optional ones that authenticate possession of a key encapsulation mechanism
(KEM) private key. HPKE works for any combination of an asymmetric KEM, key deri
vation function (KDF), and authenticated encryption with additional data (AEAD)
encryption function. Some authenticated variants may not be supported by all KEM
s. We provide instantiations of the scheme using widely used and efficient primi
tives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based ke
y derivation function (HKDF), and SHA2.</t>
<t>This document is a product of the Crypto Forum Research Group (
CFRG) in the IRTF.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="9180"/>
<seriesInfo name="DOI" value="10.17487/RFC9180"/>
</reference>
<reference anchor="RFC6125">
<front>
<title>Representation and Verification of Domain-Based Application S
ervice Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Cer
tificates in the Context of Transport Layer Security (TLS)</title>
<author fullname="P. Saint-Andre" initials="P." surname="Saint-Andre
"/>
<author fullname="J. Hodges" initials="J." surname="Hodges"/>
<date month="March" year="2011"/>
<abstract>
<t>Many application technologies enable secure communication betwe
en two entities by means of Internet Public Key Infrastructure Using X.509 (PKIX
) certificates in the context of Transport Layer Security (TLS). This document s
pecifies procedures for representing and verifying the identity of application s
ervices in such interactions. [STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="6125"/>
<seriesInfo name="DOI" value="10.17487/RFC6125"/>
</reference>
<reference anchor="RFC5890">
<front>
<title>Internationalized Domain Names for Applications (IDNA): Defin
itions and Document Framework</title>
<author fullname="J. Klensin" initials="J." surname="Klensin"/>
<date month="August" year="2010"/>
<abstract>
<t>This document is one of a collection that, together, describe t
he protocol and usage context for a revision of Internationalized Domain Names f
or Applications (IDNA), superseding the earlier version. It describes the docume
nt collection and provides definitions and other material that are common to the
set. [STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="5890"/>
<seriesInfo name="DOI" value="10.17487/RFC5890"/>
</reference>
<reference anchor="RFC8126">
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs
</title>
<author fullname="M. Cotton" initials="M." surname="Cotton"/>
<author fullname="B. Leiba" initials="B." surname="Leiba"/>
<author fullname="T. Narten" initials="T." surname="Narten"/>
<date month="June" year="2017"/>
<abstract>
<t>Many protocols make use of points of extensibility that use con
stants to identify various protocol parameters. To ensure that the values in the
se fields do not have conflicting uses and to promote interoperability, their al
locations are often coordinated by a central record keeper. For IETF protocols,
that role is filled by the Internet Assigned Numbers Authority (IANA).</t>
<t>To make assignments in a given registry prudently, guidance des
cribing the conditions under which new values should be assigned, as well as whe
n and how modifications to existing values can be made, is needed. This document
defines a framework for the documentation of these guidelines by specification
authors, in order to assure that the provided guidance for the IANA Consideratio
ns is clear and addresses the various issues that are likely in the operation of
a registry.</t>
<t>This is the third edition of this document; it obsoletes RFC 52
26.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="26"/>
<seriesInfo name="RFC" value="8126"/>
<seriesInfo name="DOI" value="10.17487/RFC8126"/>
</reference>
<reference anchor="RFC8447">
<front>
<title>IANA Registry Updates for TLS and DTLS</title>
<author fullname="J. Salowey" initials="J." surname="Salowey"/>
<author fullname="S. Turner" initials="S." surname="Turner"/>
<date month="August" year="2018"/>
<abstract>
<t>This document describes a number of changes to TLS and DTLS IAN
A registries that range from adding notes to the registry all the way to changin
g the registration policy. These changes were mostly motivated by WG review of t
he TLS- and DTLS-related registries undertaken as part of the TLS 1.3 developmen
t process.</t>
<t>This document updates the following RFCs: 3749, 5077, 4680, 524
6, 5705, 5878, 6520, and 7301.</t>
</abstract>
</front> </front>
<seriesInfo name="RFC" value="8447"/> <seriesInfo name="RFC" value="YYY1"/>
<seriesInfo name="DOI" value="10.17487/RFC8447"/> <seriesInfo name="DOI" value="10.17487/RFCYYY1"/>
</reference> </reference>
</references>
<references anchor="sec-informative-references">
<name>Informative References</name>
<reference anchor="WHATWG-IPV4" target="https://url.spec.whatwg.org/#con cept-ipv4-parser"> <reference anchor="WHATWG-IPV4" target="https://url.spec.whatwg.org/#con cept-ipv4-parser">
<front> <front>
<title>URL Living Standard - IPv4 Parser</title> <title>URL - IPv4 Parser</title>
<author> <author>
<organization/> <organization>WHATWG</organization>
</author> </author>
<date year="2021" month="May"/> <date year="2021" month="May"/>
</front> </front>
<refcontent>WHATWG Living Standard</refcontent>
</reference> </reference>
<reference anchor="ECH-Analysis" target="https://www.cs.ox.ac.uk/people/ vincent.cheval/publis/BCW-ccs22.pdf"> <reference anchor="ECH-Analysis" target="https://www.cs.ox.ac.uk/people/ vincent.cheval/publis/BCW-ccs22.pdf">
<front> <front>
<title>A Symbolic Analysis of Privacy for TLS 1.3 with Encrypted Cli ent Hello</title> <title>A Symbolic Analysis of Privacy for TLS 1.3 with Encrypted Cli ent Hello</title>
<author> <author initials="K." surname="Bhargavan">
<organization/> <organization>Inria</organization>
</author>
<author initials="V." surname="Cheval">
<organization>Inria</organization>
</author>
<author initials="C." surname="Wood">
<organization>Cloudflare</organization>
</author> </author>
<date year="2022" month="November"/> <date year="2022" month="November"/>
</front> </front>
<seriesInfo name="DOI" value="10.1145/3548606.3559360"/>
<refcontent>CCS '22: Proceedings of the 2022 ACM SIGSAC Conference on
Computer and Communications Security, pp. 365-379</refcontent>
</reference> </reference>
<reference anchor="RFC7301"> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
<front> 499.xml"/>
<title>Transport Layer Security (TLS) Application-Layer Protocol Neg <xi:include href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.
otiation Extension</title> kazuho-protected-sni.xml"/>
<author fullname="S. Friedl" initials="S." surname="Friedl"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
<author fullname="A. Popov" initials="A." surname="Popov"/> 301.xml"/>
<author fullname="A. Langley" initials="A." surname="Langley"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
<author fullname="E. Stephan" initials="E." surname="Stephan"/> 484.xml"/>
<date month="July" year="2014"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
<abstract> 858.xml"/>
<t>This document describes a Transport Layer Security (TLS) extens <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
ion for application-layer protocol negotiation within the TLS handshake. For ins 094.xml"/>
tances in which multiple application protocols are supported on the same TCP or <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9
UDP port, this extension allows the application layer to negotiate which protoco 250.xml"/>
l will be used within the TLS connection.</t> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
</abstract> 701.xml"/>
</front> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3
<seriesInfo name="RFC" value="7301"/> 986.xml"/>
<seriesInfo name="DOI" value="10.17487/RFC7301"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3
</reference> 552.xml"/>
<reference anchor="RFC8484"> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
<front> 744.xml"/>
<title>DNS Queries over HTTPS (DoH)</title> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
<author fullname="P. Hoffman" initials="P." surname="Hoffman"/> 924.xml"/>
<author fullname="P. McManus" initials="P." surname="McManus"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
<date month="October" year="2018"/> 077.xml"/>
<abstract>
<t>This document defines a protocol for sending DNS queries and ge
tting DNS responses over HTTPS. Each DNS query-response pair is mapped into an H
TTP exchange.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="8484"/>
<seriesInfo name="DOI" value="10.17487/RFC8484"/>
</reference>
<reference anchor="RFC7858">
<front>
<title>Specification for DNS over Transport Layer Security (TLS)</ti
tle>
<author fullname="Z. Hu" initials="Z." surname="Hu"/>
<author fullname="L. Zhu" initials="L." surname="Zhu"/>
<author fullname="J. Heidemann" initials="J." surname="Heidemann"/>
<author fullname="A. Mankin" initials="A." surname="Mankin"/>
<author fullname="D. Wessels" initials="D." surname="Wessels"/>
<author fullname="P. Hoffman" initials="P." surname="Hoffman"/>
<date month="May" year="2016"/>
<abstract>
<t>This document describes the use of Transport Layer Security (TL
S) to provide privacy for DNS. Encryption provided by TLS eliminates opportuniti
es for eavesdropping and on-path tampering with DNS queries in the network, such
as discussed in RFC 7626. In addition, this document specifies two usage profil
es for DNS over TLS and provides advice on performance considerations to minimiz
e overhead from using TCP and TLS with DNS.</t>
<t>This document focuses on securing stub-to-recursive traffic, as
per the charter of the DPRIVE Working Group. It does not prevent future applica
tions of the protocol to recursive-to-authoritative traffic.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="7858"/>
<seriesInfo name="DOI" value="10.17487/RFC7858"/>
</reference>
<reference anchor="RFC8094">
<front>
<title>DNS over Datagram Transport Layer Security (DTLS)</title>
<author fullname="T. Reddy" initials="T." surname="Reddy"/>
<author fullname="D. Wing" initials="D." surname="Wing"/>
<author fullname="P. Patil" initials="P." surname="Patil"/>
<date month="February" year="2017"/>
<abstract>
<t>DNS queries and responses are visible to network elements on th
e path between the DNS client and its server. These queries and responses can co
ntain privacy-sensitive information, which is valuable to protect.</t>
<t>This document proposes the use of Datagram Transport Layer Secu
rity (DTLS) for DNS, to protect against passive listeners and certain active att
acks. As latency is critical for DNS, this proposal also discusses mechanisms to
reduce DTLS round trips and reduce the DTLS handshake size. The proposed mechan
ism runs over port 853.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="8094"/>
<seriesInfo name="DOI" value="10.17487/RFC8094"/>
</reference>
<reference anchor="RFC9250">
<front>
<title>DNS over Dedicated QUIC Connections</title>
<author fullname="C. Huitema" initials="C." surname="Huitema"/>
<author fullname="S. Dickinson" initials="S." surname="Dickinson"/>
<author fullname="A. Mankin" initials="A." surname="Mankin"/>
<date month="May" year="2022"/>
<abstract>
<t>This document describes the use of QUIC to provide transport co
nfidentiality for DNS. The encryption provided by QUIC has similar properties to
those provided by TLS, while QUIC transport eliminates the head-of-line blockin
g issues inherent with TCP and provides more efficient packet-loss recovery than
UDP. DNS over QUIC (DoQ) has privacy properties similar to DNS over TLS (DoT) s
pecified in RFC 7858, and latency characteristics similar to classic DNS over UD
P. This specification describes the use of DoQ as a general-purpose transport fo
r DNS and includes the use of DoQ for stub to recursive, recursive to authoritat
ive, and zone transfer scenarios.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="9250"/>
<seriesInfo name="DOI" value="10.17487/RFC9250"/>
</reference>
<reference anchor="RFC8701">
<front>
<title>Applying Generate Random Extensions And Sustain Extensibility
(GREASE) to TLS Extensibility</title>
<author fullname="D. Benjamin" initials="D." surname="Benjamin"/>
<date month="January" year="2020"/>
<abstract>
<t>This document describes GREASE (Generate Random Extensions And
Sustain Extensibility), a mechanism to prevent extensibility failures in the TLS
ecosystem. It reserves a set of TLS protocol values that may be advertised to e
nsure peers correctly handle unknown values.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="8701"/>
<seriesInfo name="DOI" value="10.17487/RFC8701"/>
</reference>
<reference anchor="RFC3986">
<front>
<title>Uniform Resource Identifier (URI): Generic Syntax</title>
<author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee
"/>
<author fullname="R. Fielding" initials="R." surname="Fielding"/>
<author fullname="L. Masinter" initials="L." surname="Masinter"/>
<date month="January" year="2005"/>
<abstract>
<t>A Uniform Resource Identifier (URI) is a compact sequence of ch
aracters that identifies an abstract or physical resource. This specification de
fines the generic URI syntax and a process for resolving URI references that mig
ht be in relative form, along with guidelines and security considerations for th
e use of URIs on the Internet. The URI syntax defines a grammar that is a supers
et of all valid URIs, allowing an implementation to parse the common components
of a URI reference without knowing the scheme-specific requirements of every pos
sible identifier. This specification does not define a generative grammar for UR
Is; that task is performed by the individual specifications of each URI scheme.
[STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="STD" value="66"/>
<seriesInfo name="RFC" value="3986"/>
<seriesInfo name="DOI" value="10.17487/RFC3986"/>
</reference>
<reference anchor="DNS-TERMS">
<front>
<title>DNS Terminology</title>
<author fullname="P. Hoffman" initials="P." surname="Hoffman"/>
<author fullname="K. Fujiwara" initials="K." surname="Fujiwara"/>
<date month="March" year="2024"/>
<abstract>
<t>The Domain Name System (DNS) is defined in literally dozens of
different RFCs. The terminology used by implementers and developers of DNS proto
cols, and by operators of DNS systems, has changed in the decades since the DNS
was first defined. This document gives current definitions for many of the terms
used in the DNS in a single document.</t>
<t>This document updates RFC 2308 by clarifying the definitions of
"forwarder" and "QNAME". It obsoletes RFC 8499 by adding multiple terms and cla
rifications. Comprehensive lists of changed and new definitions can be found in
Appendices A and B.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="219"/>
<seriesInfo name="RFC" value="9499"/>
<seriesInfo name="DOI" value="10.17487/RFC9499"/>
</reference>
<reference anchor="RFC3552">
<front>
<title>Guidelines for Writing RFC Text on Security Considerations</t
itle>
<author fullname="E. Rescorla" initials="E." surname="Rescorla"/>
<author fullname="B. Korver" initials="B." surname="Korver"/>
<date month="July" year="2003"/>
<abstract>
<t>All RFCs are required to have a Security Considerations section
. Historically, such sections have been relatively weak. This document provides
guidelines to RFC authors on how to write a good Security Considerations section
. This document specifies an Internet Best Current Practices for the Internet Co
mmunity, and requests discussion and suggestions for improvements.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="72"/>
<seriesInfo name="RFC" value="3552"/>
<seriesInfo name="DOI" value="10.17487/RFC3552"/>
</reference>
<reference anchor="RFC8744">
<front>
<title>Issues and Requirements for Server Name Identification (SNI)
Encryption in TLS</title>
<author fullname="C. Huitema" initials="C." surname="Huitema"/>
<date month="July" year="2020"/>
<abstract>
<t>This document describes the general problem of encrypting the S
erver Name Identification (SNI) TLS parameter. The proposed solutions hide a hid
den service behind a fronting service, only disclosing the SNI of the fronting s
ervice to external observers. This document lists known attacks against SNI encr
yption, discusses the current "HTTP co-tenancy" solution, and presents requireme
nts for future TLS-layer solutions.</t>
<t>In practice, it may well be that no solution can meet every req
uirement and that practical solutions will have to make some compromises.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="8744"/>
<seriesInfo name="DOI" value="10.17487/RFC8744"/>
</reference>
<reference anchor="RFC7924">
<front>
<title>Transport Layer Security (TLS) Cached Information Extension</
title>
<author fullname="S. Santesson" initials="S." surname="Santesson"/>
<author fullname="H. Tschofenig" initials="H." surname="Tschofenig"/
>
<date month="July" year="2016"/>
<abstract>
<t>Transport Layer Security (TLS) handshakes often include fairly
static information, such as the server certificate and a list of trusted certifi
cation authorities (CAs). This information can be of considerable size, particul
arly if the server certificate is bundled with a complete certificate chain (i.e
., the certificates of intermediate CAs up to the root CA).</t>
<t>This document defines an extension that allows a TLS client to
inform a server of cached information, thereby enabling the server to omit alrea
dy available information.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="7924"/>
<seriesInfo name="DOI" value="10.17487/RFC7924"/>
</reference>
<reference anchor="RFC5077">
<front>
<title>Transport Layer Security (TLS) Session Resumption without Ser
ver-Side State</title>
<author fullname="J. Salowey" initials="J." surname="Salowey"/>
<author fullname="H. Zhou" initials="H." surname="Zhou"/>
<author fullname="P. Eronen" initials="P." surname="Eronen"/>
<author fullname="H. Tschofenig" initials="H." surname="Tschofenig"/
>
<date month="January" year="2008"/>
<abstract>
<t>This document describes a mechanism that enables the Transport
Layer Security (TLS) server to resume sessions and avoid keeping per-client sess
ion state. The TLS server encapsulates the session state into a ticket and forwa
rds it to the client. The client can subsequently resume a session using the obt
ained ticket. This document obsoletes RFC 4507. [STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="5077"/>
<seriesInfo name="DOI" value="10.17487/RFC5077"/>
</reference>
<reference anchor="I-D.kazuho-protected-sni">
<front>
<title>TLS Extensions for Protecting SNI</title>
<author fullname="Kazuho Oku" initials="K." surname="Oku">
</author>
<date day="18" month="July" year="2017"/>
<abstract>
<t> This memo introduces TLS extensions and a DNS Resource Recor
d Type
that can be used to protect attackers from obtaining the value of the
Server Name Indication extension being transmitted over a Transport
Layer Security (TLS) version 1.3 handshake.
</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft" value="draft-kazuho-protected-sni-00
"/>
</reference>
</references> </references>
</references> </references>
<?line 2017?> <?line 2140?>
<section anchor="linear-outer-extensions"> <section anchor="linear-outer-extensions">
<name>Linear-time Outer Extension Processing</name> <name>Linear-Time Outer Extension Processing</name>
<t>The following procedure processes the "ech_outer_extensions" extension (see <t>The following procedure processes the "ech_outer_extensions" extension (see
<xref target="encoding-inner"/>) in linear time, ensuring that each referenced e xtension <xref target="encoding-inner"/>) in linear time, ensuring that each referenced e xtension
in the ClientHelloOuter is included at most once:</t> in the ClientHelloOuter is included at most once:</t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
<t>Let I be initialized to zero and N be set to the number of extensio ns <t>Let I be initialized to zero and N be set to the number of extensio ns
in ClientHelloOuter.</t> in ClientHelloOuter.</t>
</li> </li>
<li> <li>
<t>For each extension type, E, in OuterExtensions: </t> <t>For each extension type, E, in OuterExtensions: </t>
<ul spacing="normal"> <ul spacing="normal">
skipping to change at line 2436 skipping to change at line 2195
alert and terminate this procedure.</t> alert and terminate this procedure.</t>
</li> </li>
<li> <li>
<t>Otherwise, the I-th extension of ClientHelloOuter has type E. C opy <t>Otherwise, the I-th extension of ClientHelloOuter has type E. C opy
it to the EncodedClientHelloInner and increment I.</t> it to the EncodedClientHelloInner and increment I.</t>
</li> </li>
</ul> </ul>
</li> </li>
</ol> </ol>
</section> </section>
<section anchor="acknowledgements"> <section numbered="false" anchor="acknowledgements">
<name>Acknowledgements</name> <name>Acknowledgements</name>
<t>This document draws extensively from ideas in <xref target="I-D.kazuho- protected-sni"/>, but <t>This document draws extensively from ideas in <xref target="I-D.kazuho- protected-sni"/>, but
is a much more limited mechanism because it depends on the DNS for the 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, protection of the ECH key. <contact fullname="Richard Barnes"/>, <contact fullna
Matthew Prince, Nick Sullivan, Martin Thomson, and David Benjamin also provided me="Christian Huitema"/>, <contact fullname="Patrick McManus"/>,
<contact fullname="Matthew Prince"/>, <contact fullname="Nick Sullivan"/>, <cont
act fullname="Martin Thomson"/>, and <contact fullname="David Benjamin"/> also p
rovided
important ideas and contributions.</t> important ideas and contributions.</t>
</section> </section>
<section anchor="change-log">
<name>Change Log</name>
<ul empty="true">
<li>
<t><strong>RFC Editor's Note:</strong> Please remove this section prio
r to publication of a
final version of this document.</t>
</li>
</ul>
<t>Issue and pull request numbers are listed with a leading octothorp.</t>
<section anchor="since-draft-ietf-tls-esni-16">
<name>Since draft-ietf-tls-esni-16</name>
<ul spacing="normal">
<li>
<t>Keep-alive</t>
</li>
</ul>
</section>
<section anchor="since-draft-ietf-tls-esni-15">
<name>Since draft-ietf-tls-esni-15</name>
<ul spacing="normal">
<li>
<t>Add CCS2022 reference and summary (#539)</t>
</li>
</ul>
</section>
<section anchor="since-draft-ietf-tls-esni-14">
<name>Since draft-ietf-tls-esni-14</name>
<ul spacing="normal">
<li>
<t>Keep-alive</t>
</li>
</ul>
</section>
<section anchor="since-draft-ietf-tls-esni-13">
<name>Since draft-ietf-tls-esni-13</name>
<ul spacing="normal">
<li>
<t>Editorial improvements</t>
</li>
</ul>
</section>
<section anchor="since-draft-ietf-tls-esni-12">
<name>Since draft-ietf-tls-esni-12</name>
<ul spacing="normal">
<li>
<t>Abort on duplicate OuterExtensions (#514)</t>
</li>
<li>
<t>Improve EncodedClientHelloInner definition (#503)</t>
</li>
<li>
<t>Clarify retry configuration usage (#498)</t>
</li>
<li>
<t>Expand on config_id generation implications (#491)</t>
</li>
<li>
<t>Server-side acceptance signal extension GREASE (#481)</t>
</li>
<li>
<t>Refactor overview, client implementation, and middlebox
sections (#480, #478, #475, #508)</t>
</li>
<li>
<t>Editorial iprovements (#485, #488, #490, #495, #496, #499, #500,
#501, #504, #505, #507, #510, #511)</t>
</li>
</ul>
</section>
<section anchor="since-draft-ietf-tls-esni-11">
<name>Since draft-ietf-tls-esni-11</name>
<ul spacing="normal">
<li>
<t>Move ClientHello padding to the encoding (#443)</t>
</li>
<li>
<t>Align codepoints (#464)</t>
</li>
<li>
<t>Relax OuterExtensions checks for alignment with RFC8446 (#467)</t
>
</li>
<li>
<t>Clarify HRR acceptance and rejection logic (#470)</t>
</li>
<li>
<t>Editorial improvements (#468, #465, #462, #461)</t>
</li>
</ul>
</section>
<section anchor="since-draft-ietf-tls-esni-10">
<name>Since draft-ietf-tls-esni-10</name>
<ul spacing="normal">
<li>
<t>Make HRR confirmation and ECH acceptance explicit (#422, #423)</t
>
</li>
<li>
<t>Relax computation of the acceptance signal (#420, #449)</t>
</li>
<li>
<t>Simplify ClientHelloOuterAAD generation (#438, #442)</t>
</li>
<li>
<t>Allow empty enc in ECHClientHello (#444)</t>
</li>
<li>
<t>Authenticate ECHClientHello extensions position in ClientHelloOut
erAAD (#410)</t>
</li>
<li>
<t>Allow clients to send a dummy PSK and early_data in ClientHelloOu
ter when
applicable (#414, #415)</t>
</li>
<li>
<t>Compress ECHConfigContents (#409)</t>
</li>
<li>
<t>Validate ECHConfig.contents.public_name (#413, #456)</t>
</li>
<li>
<t>Validate ClientHelloInner contents (#411)</t>
</li>
<li>
<t>Note split-mode challenges for HRR (#418)</t>
</li>
<li>
<t>Editorial improvements (#428, #432, #439, #445, #458, #455)</t>
</li>
</ul>
</section>
<section anchor="since-draft-ietf-tls-esni-09">
<name>Since draft-ietf-tls-esni-09</name>
<ul spacing="normal">
<li>
<t>Finalize HPKE dependency (#390)</t>
</li>
<li>
<t>Move from client-computed to server-chosen, one-byte config
identifier (#376, #381)</t>
</li>
<li>
<t>Rename ECHConfigs to ECHConfigList (#391)</t>
</li>
<li>
<t>Clarify some security and privacy properties (#385, #383)</t>
</li>
</ul>
</section>
</section>
</back> </back>
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