Internet DRAFT - draft-ietf-tls-cross-sni-resumption
draft-ietf-tls-cross-sni-resumption
TLS Working Group V. Vasiliev
Internet-Draft Google
Intended status: Standards Track 6 December 2021
Expires: 9 June 2022
Transport Layer Security (TLS) Resumption across Server Names
draft-ietf-tls-cross-sni-resumption-02
Abstract
This document specifies a way for the parties in the Transport Layer
Security (TLS) protocol to indicate that an individual session ticket
can be used to perform resumption even if the Server Name of the new
connection does not match the Server Name of the original.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the TLS Working Group
mailing list (tls@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/tls/
(https://mailarchive.ietf.org/arch/browse/tls/).
Source for this draft and an issue tracker can be found at
https://github.com/vasilvv/tls-cross-sni-resumption
(https://github.com/vasilvv/tls-cross-sni-resumption).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 9 June 2022.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. The Flag . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Transport Layer Security protocol [RFC8446] allows the clients to use
an abbreviated handshake in cases where the client has previously
established a secure session with the same server. This mechanism is
known as "session resumption", and its positive impact on performance
makes it desirable to be able to use it as frequently as possible.
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Modern application-level protocols, HTTP in particular, often require
accessing multiple servers within a single workflow. Since the
identity of the server is established through its certificate, in the
ideal case, the resumption would be possible to all of the domains
for which the certificate is valid (see [PERF] for a survey of
potential practical impact of such approach). TLS, starting with
version 1.3, defines the SNI value to be a property of an individual
connection that is not retained across sessions ([RFC8446],
Section 4.2.11). However, in the absence of additional signals, it
discourages using a session ticket when the SNI value does not match
([RFC8446], Section 4.6.1), as there is normally no reason to assume
that all servers sharing the same certificate would also share the
same session keys. The extension defined in this document allows the
server to provide such a signal in-band.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. The Flag
Resumption across server names is negotiated using the TLS flags
extension [I-D.draft-ietf-tls-tlsflags]. The server MAY send a
resumption_across_names(8) flag in a NewSessionTicket message; the
flag is an assertion by the server that any server for any identity
presented in its certificate would be capable of accepting that
ticket. A client receiving a ticket with this flag MAY attempt
resumption for any server name corresponding to an identity in the
server certificate even if the new server name value does not match
the one used in the original session; note that this requires the
client to retain the list of the names specified in the original
server certificate. The flag cannot be used in TLS versions before
1.3, as the NewSessionTicket message does not exist in those
versions.
4. Security Considerations
This document does not alter any of the security requirements of
[RFC8446], but merely lifts a performance-motivated "SHOULD NOT"
recommendation from Section 4.6.1. Notably, it still relies on the
client ensuring that the server certificate is valid for the new SNI
at the time of session resumption.
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If the original server's assertion regarding supporting cross-name
resumption turns out to be incorrect, a different server that
receives a misdirected ticket will not be able to decrypt it and will
therefore be unable to resume. The protocol will gracefully recover
from such situations, as session resumption may be safely rejected
for any reason. However, such misconfiguration will waste tickets
stored in the client's cache, as TLS tickets may be single-use,
leading to a potential performance regression.
When providing the SNI value to the application, TLS 1.3 requires the
value from the most recent ClientHello to be used ([RFC8446],
Section 4.6.1). If the server TLS implementation violates that
requirement and instead reports the SNI value of the original
session, this can lead to a confusion attack where the client and the
server disagree on the server name being used (similar to the attacks
described in [DB15]). The implementers MUST ensure that this aspect
of SNI processing is handled correctly before enabling cross-name
resumption.
Cross-domain resumption implies that any certificate the client
provides for one host would become available to the other hosts using
the same server certificate. Because of that, when performing cross-
domain resumption, the client MUST use the same policy on whether to
present said certificate to the server as if it were a new TLS
session. For instance, if the client would show a certificate choice
prompt for every individual domain it connects to, it MUST show that
prompt for the new host when performing cross-domain resumption.
Cross-domain resumption, like other similar mechanisms (e.g. cross-
domain HTTP connection reuse), can incentivize the server deployments
to create server certificates valid for a wider range of domains than
they would otherwise. However, any increase in the scope of a
certificate comes at a cost: the wider is the scope of the
certificate, the wider is the impact of the key compromise for that
certificate. In addition, creating a certificate that is valid for
multiple hostnames can lead to complications if some of those
hostnames change ownership, or otherwise require a different
operational domain.
Session tickets can contain arbitrary information, and thus could be
potentially used to re-identify a user from a previous connection.
Cross-domain resumption expands the potential list of servers to
which an individual ticket could be presented. Client applications
should partition the session cache between connections that are meant
to be uncorrelated. For example, the Web use case uses network
partition keys to separate cache lookups [FETCH].
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5. IANA Considerations
IANA (will add/has added) the following entry to the "TLS Flags"
table of the "Transport Layer Security (TLS) Extensions" registry:
Value 0x8
Flag Name resumption_across_names
Message NST
Recommended N
Reference This document
6. References
6.1. Normative References
[I-D.draft-ietf-tls-tlsflags]
Nir, Y., "A Flags Extension for TLS 1.3", Work in
Progress, Internet-Draft, draft-ietf-tls-tlsflags-07, 25
October 2021, <https://datatracker.ietf.org/doc/html/
draft-ietf-tls-tlsflags-07>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
6.2. Informative References
[DB15] Delignat-Lavaud, A. and K. Bhargavan, "Network-based
Origin Confusion Attacks against HTTPS Virtual Hosting",
15 March 2015.
[FETCH] WHATWG, "Fetch Standard", December 2021,
<https://fetch.spec.whatwg.org/>.
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[PERF] Sy, E., Moennich, M., Mueller, T., Federrath, H., and M.
Fischer, "Enhanced Performance for the encrypted Web
through TLS Resumption across Hostnames", 7 February 2019,
<https://arxiv.org/pdf/1902.02531.pdf>.
Acknowledgments
Cross-name resumption has been previously implemented in the QUIC
Crypto protocol as a preloaded list of hostnames.
Erik Sy has previously proposed a similar mechanism for TLS, draft-
sy-tls-resumption-group (https://datatracker.ietf.org/doc/draft-sy-
tls-resumption-group/). This document incorporates ideas from that
draft.
This document has benefited from contributions and suggestions from
Carrick Bartle, David Benjamin, Nick Harper, Eric Rescorla, David
Schinazi, Ryan Sleevi, Ian Swett, Martin Thomson, Christopher Wood,
and many others.
Author's Address
Victor Vasiliev
Google
Email: vasilvv@google.com
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