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jhoyla                                                        J. Hoyland
Internet-Draft                                           Cloudflare Ltd.
Intended status: Standards Track                               C.A. Wood
Expires: March 13, 2021                                       Cloudflare
                                                       September 9, 2020


                     TLS 1.3 Extended Key Schedule
               draft-jhoyla-tls-extended-key-schedule-02

Abstract

   TLS 1.3 is sometimes used in situations where it is necessary to
   inject extra key material into the handshake.  This draft aims to
   describe methods for doing so securely.  This key material must be
   injected in such a way that both parties agree on what is being
   injected and why, and further, in what order.

Note to Readers

   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/jhoyla/draft-jhoyla-tls-key-injection
   (https://github.com/jhoyla/draft-jhoyla-tls-key-injection).

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 13, 2021.






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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Key Schedule Extension  . . . . . . . . . . . . . . . . . . .   3
     3.1.  Handshake Secret Injection  . . . . . . . . . . . . . . .   3
     3.2.  Main Secret Injection . . . . . . . . . . . . . . . . . .   3
   4.  Key Schedule Injection Negotiation  . . . . . . . . . . . . .   4
   5.  Key Schedule Extension Structure  . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Introducing additional key material into the TLS handshake is a non-
   trivial process because both parties need to agree on the injection
   content and context.  If the two parties do not agree then an
   attacker may exploit the mismatch in so-called channel
   synchronization attacks, such as those described by [SLOTH].

   Injecting key material into the TLS handshake allows other protocols
   to be bound to the handshake.  For example, it may provide additional
   protections to the ClientHello message, which in the standard TLS
   handshake only receives protections after the server's Finished
   message has been received.  It may also permit the use of combined
   shared secrets, possibly from multiple key exchange algorithms, to be
   included in the key schedule.  This pattern is common for Post
   Quantum key exchange algorithms, as discussed in
   [I-D.ietf-tls-hybrid-design].



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   The goal of this document is to provide a standardised way for
   binding extra context into TLS 1.3 handshakes in a way that is easy
   to analyse from a security perspective, reducing the need for
   security analysis of extensions that affect the key schedule.  It
   separates the concerns of whether an extension achieves its goals
   from the concerns of whether an extension reduces the security of a
   TLS handshake, either directly or through some unforseen interaction
   with another extension.

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.  Key Schedule Extension

   This section describes two places in which additional secrets can be
   injected into the TLS 1.3 key schedule.

3.1.  Handshake Secret Injection

   To inject extra key material into the Handshake Secret it is
   recommended to prefix it, inside an appropriate frame, to the
   "(EC)DHE" input, where "||" represents concatenation.

                                    |
                                    v
                              Derive-Secret(., "derived", "")
                                    |
                                    v
     KeyScheduleInput || (EC)DHE -> HKDF-Extract = Handshake Secret
                                    |
                                    v

3.2.  Main Secret Injection

   To inject key material into the Main Secret it is recommended to
   prefix it, inside an appropriate frame, to the "0" input.










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                              |
                              v
                        Derive-Secret(., "derived", "")
                              |
                              v
     KeyScheduleInput || 0 -> HKDF-Extract = Main Secret
                              |
                              v

   This structure mirrors the Handshake Injection point.

4.  Key Schedule Injection Negotiation

   Applications which make use of additional key schedule inputs MUST
   define a mechanism for negotiating the content and type of that
   input.  This input MUST be framed in a KeyScheduleSecret struct, as
   defined in Section 5.  Applications must take care that any
   negotiation that takes place unambiguously agrees a secret.  It must
   be impossible, even under adversarial conditions, that a client and
   server agree on the transcript of the negotiation, but disagree on
   the secret that was negotiated.

5.  Key Schedule Extension Structure

   In some cases, protocols may require more than one secret to be
   injected at a particular stage in the key schedule.  Thus, we require
   a generic and extensible way of doing so.  To accomplish this, we use
   a structure - KeyScheduleInput - that encodes well-ordered sequences
   of secret material to inject into the key schedule.  KeyScheduleInput
   is defined as follows:

   struct {
       KeyScheduleSecretType type;
       opaque secret_data<0..2^16-1>;
   } KeyScheduleSecret;

   enum {
       (65535)
   } KeyScheduleSecretType;

   struct {
       KeyScheduleSecret secrets<0..2^16-1>;
   } KeyScheduleInput;

   Each secret included in a KeyScheduleInput structure has a type and
   corresponding secret data.  Each secret MUST have a unique
   KeyScheduleSecretType.  When encoding KeyScheduleInput as the key
   schedule Input value, the KeyScheduleSecret values MUST be in



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   ascending sorted order.  This ensures that endpoints always encode
   the same KeyScheduleInput value when using the same secret keying
   material.

6.  Security Considerations

   [BINDEL] provides a proof that the concatenation approach in
   Section 3 is secure as long as either the concatenated secret is
   secure or the existing KDF input is secure.

   [[OPEN ISSUE: Is this guarantee sufficient?  Do we also need to
   guarantee that a malicious prefix can't weaken the resulting PRF
   output?]]

7.  IANA Considerations

   This document requests the creation of a new IANA registry: TLS
   KeyScheduleInput Types.  This registry should be under the existing
   Transport Layer Security (TLS) Parameters heading.  It should be
   administered under a Specification Required policy [RFC8126].

   [[OPEN ISSUE: specify initial registry values]]

               +=======+=============+=========+===========+
               | Value | Description | DTLS-OK | Reference |
               +=======+=============+=========+===========+
               | TBD   | TBD         | TBD     | TBD       |
               +-------+-------------+---------+-----------+

                                  Table 1

8.  References

8.1.  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/info/rfc2119>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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/info/rfc8174>.



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8.2.  Informative References

   [BINDEL]   Bindel, N., Brendel, J., Fischlin, M., Goncalves, B., and
              D. Stebila, "Hybrid Key Encapsulation Mechanisms and
              Authenticated Key Exchange",
              DOI 10.1007/978-3-030-25510-7_12, Post-Quantum
              Cryptography pp. 206-226, 2019,
              <https://doi.org/10.1007/978-3-030-25510-7_12>.

   [I-D.ietf-tls-hybrid-design]
              Steblia, D., Fluhrer, S., and S. Gueron, "Hybrid key
              exchange in TLS 1.3", Work in Progress, Internet-Draft,
              draft-ietf-tls-hybrid-design-00, April 15, 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-tls-
              hybrid-design-00.txt>.

   [SLOTH]    Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH",
              DOI 10.14722/ndss.2016.23418, Proceedings 2016 Network and
              Distributed System Security Symposium, 2016,
              <https://doi.org/10.14722/ndss.2016.23418>.

Acknowledgments

   We thank Karthik Bhargavan for his comments.

Authors' Addresses

   Jonathan Hoyland
   Cloudflare Ltd.

   Email: jonathan.hoyland@gmail.com


   Christopher A. Wood
   Cloudflare

   Email: caw@heapingbits.net













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