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jhoyla                                                        J. Hoyland
Internet-Draft                                           Cloudflare Ltd.
Intended status: Standards Track                                 C. Wood
Expires: May 7, 2020                                         Apple, Inc.
                                                       November 04, 2019


                     TLS 1.3 Extended Key Schedule
               draft-jhoyla-tls-extended-key-schedule-00

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/ [1].

   Source for this draft and an issue tracker can be found at
   https://github.com/jhoyla/draft-jhoyla-tls-key-injection [2].

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 May 7, 2020.








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Copyright Notice

   Copyright (c) 2019 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.  Early Secret Injection  . . . . . . . . . . . . . . . . .   3
     3.2.  Handshake Secret Injection  . . . . . . . . . . . . . . .   4
   4.  Key Schedule Extension Structure  . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   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.

   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




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   Quantum key exchange algorithms, as discussed in
   [I-D.stebila-tls-hybrid-design].

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 ways in which additional secrets can be
   injected into the TLS 1.3 key schedule.

3.1.  Early Secret Injection

   TLS provides exporter keys that allow for other protocols to provide
   data authenticated by the TLS channel.  This can be used to bind a
   protocol to a specific TLS handshake, giving joint authentication
   guarantees.  In a similar way, one may wish to introduce externally
   authenticated and pre-shared data to the early secret derivation.
   This can be used to bind TLS to an external protocol.

   To achieve this, pre-shared keys modify the binder key computation.
   This is needed since it ensures that both parties agree on both the
   authenticated data and the context in which it was used.

   The binder key computation change is as follows:

                0
                |
                v
      PSK ->  HKDF-Extract = Early Secret
                |
                +-----> Derive-Secret(., "ext binder"
                |                      | "res binder"
                |                      | "imp ext binder"
                |                      | "imp res binder", "")
                |                     = binder_key
                v

   Use of the "imp ext binder" label implies that both parties agree
   that there is some context that has been agreed, and that they are
   using an external PSK.  Use of the "imp res binder" label implies
   that both parties agree that there is some context that has been
   agreed, and that they are using an resumption PSK.  This assumes the



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   PSK has some mechanism by which additional context is included.
   [I-D.ietf-tls-external-psk-importer] describes one way by which such
   context may be included.

     struct {
       opaque external_identity<1...2^16-1>;
       opaque context<0...2^16>;
     } PSKIDWithAdditionalData;

   external_identity  is the "PSK_ID" that would have been used if the
      additional data were not agreed upon.

   context  is an opaque value that is bound to the agreed upon
      additional data.

   Those using the "imp ext binder" or "imp res binder" label MUST
   include a "context" field, to allow the additional data.

   Note that this structure is recursive.  If this mechanism is used
   multiple times then the "external_identity" field will contain
   previous contexts in sequential order.  If the client does not know
   in advance which pieces of additional data the server will be willing
   to agree on, it can provide multiple binders with different subsets
   of the additional data.  The server can then select a binder with
   which it is willing to proceed.  The binders MUST be verified in an
   all-or-nothing manner, and only one binder SHOULD be checked.  A
   server MUST NOT accept a binder for which it only agrees upon some of
   the data.

3.2.  Handshake Secret Injection

   To inject key material into the Handshake Secret it is recommended to
   use an extra derive secret.

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



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   As shown in the figure above, the key schedule has an extra derive
   secret and HKDF-Extract step.  This extra step isolates the Input
   material from the rest of the handshake secret, such that even
   maliciously chosen values cannot weaken the security of the key
   schedule overall.

   The additional Derive-Secret with the "derived early" label enforces
   the separation of the key schedule from vanilla TLS handshakes,
   because HKDFs can be assumed to ensure that keys derived with
   different labels are independent.

4.  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
   ascending sorted order.  This ensures that endpoints always encode
   the same KeyScheduleInput value when using the same secret keying
   material.

5.  Security Considerations

   [[OPEN ISSUE: This draft has not seen any security analysis.]]







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6.  IANA Considerations

   [[TODO: define secret registry structure]]

7.  References

7.1.  Normative References

   [I-D.ietf-tls-external-psk-importer]
              Benjamin, D. and C. Wood, "Importing External PSKs for
              TLS", draft-ietf-tls-external-psk-importer-01 (work in
              progress), October 2019.

   [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>.

   [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>.

7.2.  Informative References

   [I-D.stebila-tls-hybrid-design]
              Steblia, D., Fluhrer, S., and S. Gueron, "Design issues
              for hybrid key exchange in TLS 1.3", draft-stebila-tls-
              hybrid-design-01 (work in progress), July 2019.

7.3.  URIs

   [1] https://mailarchive.ietf.org/arch/browse/tls/

   [2] https://github.com/jhoyla/draft-jhoyla-tls-key-injection

Acknowledgments

   We thank Karthik Bhargavan for his comments.

Authors' Addresses

   Jonathan Hoyland
   Cloudflare Ltd.

   Email: jonathan.hoyland@gmail.com






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   Christopher A. Wood
   Apple, Inc.

   Email: cawood@apple.com















































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