TLS Key Share Prediction
draft-ietf-tls-key-share-prediction-01

Transport Layer Security                                     D. Benjamin
Internet-Draft                                                Google LLC
Updates: 8446 (if approved)                            10 September 2024
Intended status: Standards Track                                        
Expires: 14 March 2025

                        TLS Key Share Prediction
                 draft-ietf-tls-key-share-prediction-01

Abstract

   This document defines a mechanism for servers to communicate key
   share preferences in DNS.  Clients may use this information to reduce
   TLS handshake round-trips.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://tlswg.github.io/tls-key-share-prediction/draft-ietf-tls-key-
   share-prediction.html.  Status information for this document may be
   found at https://datatracker.ietf.org/doc/draft-ietf-tls-key-share-
   prediction/.

   Discussion of this document takes place on the Transport Layer
   Security Working Group mailing list (mailto:tls@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/tls/.  Subscribe
   at https://www.ietf.org/mailman/listinfo/tls/.

   Source for this draft and an issue tracker can be found at
   https://github.com/tlswg/tls-key-share-prediction.

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
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   This Internet-Draft will expire on 14 March 2025.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  DNS Service Parameter . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Format  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Configuring Services  . . . . . . . . . . . . . . . . . .   4
     3.3.  Client Behavior . . . . . . . . . . . . . . . . . . . . .   4
     3.4.  Misprediction . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Named groups in TLS 1.3 [RFC8446] are negotiated with two lists in
   the ClientHello: The client sends its full preferences in the
   supported_groups extension, but also generates key shares for a
   subset in the key_share extension.  Named groups in this subset may
   be used in one, while named groups outside the subset requires a
   HelloRetryRequest and two round trips.  The additional round trip is
   undesirable for performance, but unused key shares consume network
   and computational resources, so clients often do not generate key
   shares for all groups.

   Post-quantum key encapsulation methods (KEMs) have large keys and
   ciphertexts, so network costs are particularly pronounced.  As a TLS
   ecosystem transitions from one post-quantum KEM to another, it is
   challenging to pick key shares without prior knowledge of the
   server's policies:

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   1.  Predicting both post-quantum KEMs consumes excessive bandwidth on
       the unused option.

   2.  Predicting the old post-quantum KEM adds a round-trip cost to
       newer servers.  Servers will be unlikely to transition as a
       result.

   3.  Predicting the new post-quantum KEM adds a round-trip cost to
       older servers.  Particularly early in the transition, when most
       servers do not implement the new KEM, this may significantly
       regress performance.

   This document defines a method for servers to declare their named
   group preferences in DNS, using SVCB or HTTPS resource records
   [RFC9460].  This allows the client to predict key shares more
   accurately.

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.  DNS Service Parameter

   This document defines the tls-supported-groups SvcParamKey [RFC9460],
   which specifies the endpoint's TLS supported group preferences, as a
   non-empty sequence of TLS NamedGroup codepoints in order of
   decreasing preference, with no duplicates.  This allows clients
   connecting to the endpoint to reduce the likelihood of needing a
   HelloRetryRequest.

3.1.  Format

   The presentation value of the SvcParamValue is a non-empty comma-
   separated list (Appendix A.1 of [RFC9460]) of decimal integers
   between 0 and 65535 (inclusive) in ASCII, with no duplicate integers.
   Any other value is a syntax error.  To enable simpler parsing, this
   SvcParam MUST NOT contain escape sequences.

   The wire format of the SvcParamValue is a sequence of 2-octet numeric
   values in network byte order.  An empty list of values is invalid, as
   is a list containing duplicates.

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   For example, a TLS server which prefers x25519 (29) and also supports
   secp256r1 (23) would a tls-supported-groups SvcParamValue containing
   29 and 23.  The presentation value would be "29,23".  The wire format
   of the SvcParamValue would be four octets, represented in hexadecimal
   as 001d0017.

   The following is an example of the value appearing in a complete DNS
   record in the presentation syntax:

   example.net.  7200  IN SVCB 3 server.example.net. (
       port="8004" tls-supported-groups=29,23 )

3.2.  Configuring Services

   Services SHOULD include supported TLS named groups, in order of
   decreasing preference in the tls-supported-groups parameter of their
   HTTPS or SVCB endpoints.  As TLS preferences are updated, services
   SHOULD update the DNS record to match.  Services MAY include GREASE
   values [RFC8701] in this list.

3.3.  Client Behavior

   When connecting to a service endpoint whose HTTPS or SVCB record
   contains the tls-supported-groups parameter, the client evaluates the
   server preferences against its own to predict which named group will
   be chosen.  When evaluating the server preferences, the client MUST
   ignore any codepoints that it does not support or recognize.  If
   there is a named group in common, the client MAY send a key_share
   extension containing just that named group in the initial
   ClientHello.  To avoid downgrade attacks, the client MUST continue to
   send its full preferences in the supported_groups extension.  See
   Section 4 for additional discussion on downgrades.

3.4.  Misprediction

   Although this service parameter is intended to reduce key share
   mispredictions, mispredictions may still occur in some scenarios.
   For example:

   *  The client has fetched a stale HTTPS or SVCB record that no longer
      reflects the server preferences

   *  The server is in the process of deploying a change to named group
      preferences, and different server instances temporary evaluate
      different preferences

   *  The client was unable to fetch the HTTPS or SVCB record

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   *  The client and server implement incompatible selection algorithms,
      such that client's evaluation of the service parameter did not
      match the server's final selection

   Clients and servers MUST correctly handle mispredictions by
   responding to and sending HelloRetryRequest, respectively.

4.  Security Considerations

   This document introduces a mechanism for clients to vary the
   key_share extension based on DNS.  DNS responses are unauthenticated
   in many deployments, so this can permit attacker influence over the
   client's predicted named groups.  That, in turn, can influence the
   named group selected by the TLS server, as TLS's downgrade
   protections only extend to the ClientHello itself.  However, the
   client continues to send its full preferences in supported_groups, so
   this influence is limited by the server's named group selection
   policy:

   Servers which select purely based on preference orders will first
   select a named group on supported_groups, and then consider key_share
   only to send HelloRetryRequest or ServerHello.  When connecting to
   such servers, attackers cannot influence the selection with this
   mechanism.

   However, some servers prioritize round-trip times over preference
   orders.  That is, when choosing between a named group in key_share
   and a more preferable (e.g. more secure) named group not in
   key_share, these servers will select the less preferable one in
   key_share.  In this case, an attacker may be able to influence the
   selection by forging an HTTPS or SVCB record.  Per Section 4.2.8 of
   [RFC8446], the client's key_share extension does not reflect its full
   preference list in supported_groups.  Thus, this server behavior is
   only appropriate when the two options are of comparable preference,
   such that round trip concerns dominate.  In particular, it is NOT
   RECOMMENDED when choosing between post-quantum and classical named
   groups.

   As these semantics were already prescribed in [RFC8446], it is safe
   for clients to admit attacker control over the set of named groups
   preferred in key_share, provided supported_groups always reflects the
   true client preference.  Servers are expected to evaluate the
   combination of key_share and supported_groups according to the
   defined semantics and their selection goals.

   To reduce the risk of downgrade attacks with incorrectly deployed
   servers, clients MAY choose to ignore tls-supported-groups when the
   result would be to a predict a less preferred group.  For example, a

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   client that implements a combination of post-quantum groups and ECDH
   groups MAY limit its influence to predicting post-quantum groups.
   This optimizes transitions between post-quantum groups, where the
   bandwidth concerns are more pronounced, but means ECDH-only servers
   cannot take advantage of the mechanism.

5.  IANA Considerations

   This document updates the Service Parameter Keys registry [RFC9460]
   with the following entry:

   +======+======================+===========+===========+============+
   |Number| Name                 | Meaning   | Format    | Change     |
   |      |                      |           | Reference | Controller |
   +======+======================+===========+===========+============+
   |TBD   | tls-supported-groups | Supported | (this     | IETF       |
   |      |                      | groups in | document) |            |
   |      |                      | TLS       | Section   |            |
   |      |                      |           | 3.1       |            |
   +------+----------------------+-----------+-----------+------------+

                                 Table 1

6.  Normative References

   [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>.

   [RFC8701]  Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,
              <https://www.rfc-editor.org/rfc/rfc8701>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.

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Acknowledgments

   The author would like to thank David Adrian, Bob Beck, Sophie
   Schmieg, Martin Thomson, and Bas Westerbaan for discussions and
   review of this document.

Author's Address

   David Benjamin
   Google LLC
   Email: davidben@google.com

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