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


   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 (, which is archived at

   Source for this draft and an issue tracker can be found at

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   This Internet-Draft will expire on 9 June 2022.

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Copyright Notice

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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",
   "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

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

   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

              Nir, Y., "A Flags Extension for TLS 1.3", Work in
              Progress, Internet-Draft, draft-ietf-tls-tlsflags-07, 25
              October 2021, <

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

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,

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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,


   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 (
   tls-resumption-group/).  This document incorporates ideas from that

   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


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