Internet DRAFT - draft-ietf-tls-svcb-ech
draft-ietf-tls-svcb-ech
TLS Working Group B. Schwartz
Internet-Draft Google
Intended status: Standards Track M. Bishop
Expires: 29 March 2024 E. Nygren
Akamai Technologies
26 September 2023
Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings
draft-ietf-tls-svcb-ech-00
Abstract
To use TLS Encrypted ClientHello (ECH) the client needs to learn 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.
Status of This Memo
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This Internet-Draft will expire on 29 March 2024.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SvcParam for ECH configuration . . . . . . . . . . . . . . . 2
3. Server behavior . . . . . . . . . . . . . . . . . . . . . . . 3
4. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Disabling fallback . . . . . . . . . . . . . . . . . . . 3
4.2. ClientHello construction . . . . . . . . . . . . . . . . 3
4.3. Performance optimizations . . . . . . . . . . . . . . . . 3
5. Interaction with HTTP Alt-Svc . . . . . . . . . . . . . . . . 4
5.1. Security Considerations . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Overview
The Service Bindings framework [SVCB] allows server operators to
publish a detailed description of their service in the Domain Name
System [RFC1034] using SVCB or HTTPS records. Each SVCB record
describes a single "alternative endpoint", and contains a collection
of "SvcParams" that can be extended with new kinds of information
that may be of interest to a client. Clients can use the SvcParams
to improve the privacy, security, and performance of their connection
to this endpoint.
This specification defines a new SvcParam to enable the use of TLS
Encrypted ClientHello [ECH] in TLS-based protocols. This SvcParam
can be used in SVCB, HTTPS or any future SVCB-compatible DNS records,
and is intended to serve as the primary bootstrap mechanism for ECH.
2. SvcParam for ECH configuration
The "ech" SvcParamKey is defined for conveying the ECH configuration
of an alternative endpoint. It is applicable to all TLS-based
protocols (including DTLS [RFC9147] and QUIC version 1 [RFC9001])
unless otherwise specified.
In wire format, the value of the parameter is an ECHConfigList
(Section 4 of [ECH]), including the redundant length prefix. In
presentation format, the value is the ECHConfigList in Base 64
Encoding (Section 4 of [RFC4648]). Base 64 is used here to simplify
integration with TLS server software. To enable simpler parsing,
this SvcParam MUST NOT contain escape sequences.
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3. Server behavior
When publishing a record containing an "ech" parameter, the publisher
MUST ensure that all IP addresses of TargetName correspond to servers
that have access to the corresponding private key or are
authoritative for the public name. (See Section 7.2.2 of [ECH] for
more details about the public name.) Otherwise, connections will
fail entirely.
4. Client behavior
This section describes client behavior in using ECH configurations
provided in SVCB or HTTPS records.
4.1. Disabling fallback
The SVCB-optional client behavior specified in (Section 3 of [SVCB])
permits clients to fall back to a direct connection if all SVCB
options fail. This behavior is not suitable for ECH, because
fallback would negate the privacy benefits of ECH. Accordingly, ECH-
capable SVCB-optional clients MUST switch to SVCB-reliant connection
establishment if SVCB resolution succeeded (as defined in Section 3
of [SVCB]) and all alternative endpoints have an "ech" SvcParam.
4.2. ClientHello construction
When ECH is in use, the TLS ClientHello is divided into an
unencrypted "outer" and an encrypted "inner" ClientHello. The outer
ClientHello is an implementation detail of ECH, and its contents are
controlled by the ECHConfig in accordance with [ECH]. The inner
ClientHello is used for establishing a connection to the service, so
its contents may be influenced by other SVCB parameters. For
example, the requirements related to ALPN protocol identifiers in
Section 7.1.2 of [SVCB] apply only to the inner ClientHello.
Similarly, it is the inner ClientHello whose Server Name Indication
identifies the desired service.
4.3. Performance optimizations
Prior to retrieving the SVCB records, the client does not know
whether they contain an "ech" parameter. As a latency optimization,
clients MAY prefetch DNS records that will only be used if this
parameter is not present (i.e. only in SVCB-optional mode).
The "ech" SvcParam alters the contents of the TLS ClientHello if it
is present. Therefore, clients that support ECH MUST NOT issue any
TLS ClientHello until after SVCB resolution has completed. (See
Section 5.1 of [SVCB]).
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5. Interaction with HTTP Alt-Svc
HTTP clients that implement both HTTP Alt-Svc [RFC7838] and the HTTPS
record type [SVCB] can use them together, provided that they only
perform connection attempts that are "consistent" with both sets of
parameters (Section 9.3 of [SVCB]). At the time of writing, there is
no defined parameter related to ECH for Alt-Svc. Accordingly, a
connection attempt that uses ECH is considered "consistent" with an
Alt-Svc Field Value that does not mention ECH.
Origins that publish an "ech" SvcParam in their HTTPS record SHOULD
also publish an HTTPS record with the "ech" SvcParam for every alt-
authority offered in its Alt-Svc Field Values. Otherwise, clients
might reveal the name of the server in an unencrypted ClientHello to
an alt-authority.
If all HTTPS records for an alt-authority contain "ech" SvcParams,
the client MUST adopt SVCB-reliant behavior (as in Section 4.1) for
that RRSet. This precludes the use of certain connections that Alt-
Svc would otherwise allow, as discussed in Section 9.3 of [SVCB].
5.1. Security Considerations
A SVCB RRSet containing some RRs with "ech" and some without is
vulnerable to a downgrade attack: a network intermediary can block
connections to the endpoints that support ECH, causing the client to
fall back to a non-ECH endpoint. This configuration is NOT
RECOMMENDED. Zone owners who do use such a mixed configuration
SHOULD mark the RRs with "ech" as more preferred (i.e. lower
SvcPriority value) than those without, in order to maximize the
likelihood that ECH will be used in the absence of an active
adversary.
Use of ECH yields an anonymity set of cardinality equal to the number
of ECH-enabled server domains supported by a given client-facing
server. Thus, even with an encrypted ClientHello, an attacker who
can enumerate the set of ECH-enabled domains supported by a client-
facing server can guess the correct SNI with probability at least 1/
K, where K is the size of this ECH-enabled server anonymity set.
This probability may be increased via traffic analysis or other
mechanisms.
6. IANA Considerations
IANA is instructed to modify the Service Binding (SVCB) Parameter
Keys Registry entry for "ech" as follows:
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+========+======+====================+===========+============+
| Number | Name | Meaning | Format | Change |
| | | | Reference | Controller |
+========+======+====================+===========+============+
| 5 | ech | TLS Encrypted | (This | IETF |
| | | ClientHello Config | document) | |
+--------+------+--------------------+-----------+------------+
Table 1
7. References
7.1. Normative References
[ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-16, 6 April 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-16>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/rfc/rfc1034>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[SVCB] Schwartz, B. M., Bishop, M., and E. Nygren, "Service
binding and parameter specification via the DNS (DNS SVCB
and HTTPS RRs)", Work in Progress, Internet-Draft, draft-
ietf-dnsop-svcb-https-12, 11 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-12>.
7.2. Informative References
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/rfc/rfc7838>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>.
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[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
Authors' Addresses
Ben Schwartz
Google
Email: ietf@bemasc.net
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Erik Nygren
Akamai Technologies
Email: erik+ietf@nygren.org
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