Design issues for hybrid key exchange in TLS 1.3
draft-stebila-tls-hybrid-design-00

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Network Working Group                                         D. Stebila
Internet-Draft                                    University of Waterloo
Intended status: Informational                                 S. Gueron
Expires: September 12, 2019                U. Haifa, Amazon Web Services
                                                          March 11, 2019

            Design issues for hybrid key exchange in TLS 1.3
                   draft-stebila-tls-hybrid-design-00

Abstract

   Hybrid key exchange refers to using multiple key exchange algorithms
   simultaneously and combining the result with the goal of providing
   security even if all but one of the component algorithms is broken,
   and is motivated by transition to post-quantum cryptography.  This
   document categorizes various design considerations for using hybrid
   key exchange in the Transport Layer Security (TLS) protocol version
   1.3.

Status of This Memo

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   This Internet-Draft will expire on September 12, 2019.

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Stebila & Gueron       Expires September 12, 2019               [Page 1]
Internet-Draft          stebila-tls-hybrid-design             March 2019

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Motivation for use of hybrid key exchange . . . . . . . .   4
     1.3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.4.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.5.  Related work  . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Design options  . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  (Neg) How to negotiate hybridization and component
           algorithms? . . . . . . . . . . . . . . . . . . . . . . .   9
       3.1.1.  Key exchange negotiation in TLS 1.3 . . . . . . . . .   9
       3.1.2.  (Neg-Ind) Negotiating component algorithms
               individually  . . . . . . . . . . . . . . . . . . . .   9
       3.1.3.  (Neg-Comb) Negotiating component algorithms as a
               combination . . . . . . . . . . . . . . . . . . . . .  10
       3.1.4.  Benefits and drawbacks  . . . . . . . . . . . . . . .  11
     3.2.  (Num) How many component algorithms to combine? . . . . .  12
       3.2.1.  (Num-2) Two . . . . . . . . . . . . . . . . . . . . .  12
       3.2.2.  (Num-2+) Two or more  . . . . . . . . . . . . . . . .  12
       3.2.3.  Benefits and Drawbacks  . . . . . . . . . . . . . . .  12
     3.3.  (Shares) How to convey key shares?  . . . . . . . . . . .  12
       3.3.1.  (Shares-Concat) Concatenate key shares  . . . . . . .  13
       3.3.2.  (Shares-Multiple) Send multiple key shares  . . . . .  13
       3.3.3.  (Shares-Ext-Additional) Extension carrying additional
               key shares  . . . . . . . . . . . . . . . . . . . . .  13
       3.3.4.  Benefits and Drawbacks  . . . . . . . . . . . . . . .  13
     3.4.  (Comb) How to use keys? . . . . . . . . . . . . . . . . .  14
       3.4.1.  (Comb-Concat) Concatenate keys then KDF . . . . . . .  14
       3.4.2.  (Comb-XOR) XOR keys then KDF  . . . . . . . . . . . .  15
       3.4.3.  (Comb-Chain) Chain of KDF applications for each key .  15
       3.4.4.  (Comb-AltInput) Second shared secret in an alternate
               KDF input . . . . . . . . . . . . . . . . . . . . . .  16
       3.4.5.  Benefits and Drawbacks  . . . . . . . . . . . . . . .  17
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18