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Versions: 00 01 02 03 draft-ietf-tls-hybrid-design

Network Working Group                                         D. Stebila
Internet-Draft                                    University of Waterloo
Intended status: Informational                                S. Fluhrer
Expires: August 15, 2020                                   Cisco Systems
                                                               S. Gueron
                                           U. Haifa, Amazon Web Services
                                                       February 12, 2020


                     Hybrid key exchange in TLS 1.3
                   draft-stebila-tls-hybrid-design-03

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.
   It is motivated by transition to post-quantum cryptography.  This
   document provides a construction for hybrid key exchange in the
   Transport Layer Security (TLS) protocol version 1.3.

   Discussion of this work is encouraged to happen on the TLS IETF
   mailing list tls@ietf.org or on the GitHub repository which contains
   the draft: https://github.com/dstebila/draft-stebila-tls-hybrid-
   design.

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
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   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 August 15, 2020.

Copyright Notice

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




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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Revision history  . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Motivation for use of hybrid key exchange . . . . . . . .   5
     1.4.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.5.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Key encapsulation mechanisms  . . . . . . . . . . . . . . . .   7
   3.  Construction for hybrid key exchange  . . . . . . . . . . . .   8
     3.1.  Negotiation . . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Transmitting public keys and ciphertexts  . . . . . . . .   9
     3.3.  Shared secret calculation . . . . . . . . . . . . . . . .  10
   4.  Open questions  . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Related work . . . . . . . . . . . . . . . . . . . .  18
   Appendix B.  Design Considerations  . . . . . . . . . . . . . . .  19
     B.1.  (Neg) How to negotiate hybridization and component
           algorithms? . . . . . . . . . . . . . . . . . . . . . . .  21
       B.1.1.  Key exchange negotiation in TLS 1.3 . . . . . . . . .  21
       B.1.2.  (Neg-Ind) Negotiating component algorithms
               individually  . . . . . . . . . . . . . . . . . . . .  21
       B.1.3.  (Neg-Comb) Negotiating component algorithms as a
               combination . . . . . . . . . . . . . . . . . . . . .  22
       B.1.4.  Benefits and drawbacks  . . . . . . . . . . . . . . .  23
     B.2.  (Num) How many component algorithms to combine? . . . . .  24
       B.2.1.  (Num-2) Two . . . . . . . . . . . . . . . . . . . . .  24
       B.2.2.  (Num-2+) Two or more  . . . . . . . . . . . . . . . .  24
       B.2.3.  Benefits and Drawbacks  . . . . . . . . . . . . . . .  24
     B.3.  (Shares) How to convey key shares?  . . . . . . . . . . .  24
       B.3.1.  (Shares-Concat) Concatenate key shares  . . . . . . .  25
       B.3.2.  (Shares-Multiple) Send multiple key shares  . . . . .  25
       B.3.3.  (Shares-Ext-Additional) Extension carrying additional



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               key shares  . . . . . . . . . . . . . . . . . . . . .  25
       B.3.4.  Benefits and Drawbacks  . . . . . . . . . . . . . . .  25
     B.4.  (Comb) How to use keys? . . . . . . . . . . . . . . . . .  26
       B.4.1.  (Comb-Concat) Concatenate keys  . . . . . . . . . . .  26
       B.4.2.  (Comb-KDF-1) KDF keys . . . . . . . . . . . . . . . .  27
       B.4.3.  (Comb-KDF-2) KDF keys . . . . . . . . . . . . . . . .  28
       B.4.4.  (Comb-XOR) XOR keys . . . . . . . . . . . . . . . . .  29
       B.4.5.  (Comb-Chain) Chain of KDF applications for each key .  30
       B.4.6.  (Comb-AltInput) Second shared secret in an alternate
               KDF input . . . . . . . . . . . . . . . . . . . . . .  31
       B.4.7.  Benefits and Drawbacks  . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   This document gives a construction for hybrid key exchange in TLS
   1.3.  The overall design approach is a simple, "concatenation"-based
   approach: each hybrid key exchange combination should be viewed as a
   single new key exchange method, negotiated and transmitted using the
   existing TLS 1.3 mechanisms.

   This document does not propose specific post-quantum mechanisms; see
   Section 1.4 for more on the scope of this document.

1.1.  Revision history

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

   Earlier versions of this document categorized various design
   decisions one could make when implementing hybrid key exchange in TLS
   1.3.  These have been moved to the appendix of the current draft, and
   will be eventually be removed.

   o  draft-03:

      *  Add requirement for KEMs to provide protection against key
         reuse.

      *  Clarify FIPS-compliance of shared secret concatenation method.

   o  draft-02:

      *  Design considerations from draft-00 and draft-01 are moved to
         the appendix.

      *  A single construction is given in the main body.




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   o  draft-01:

      *  Add (Comb-KDF-1) (Appendix B.4.2) and (Comb-KDF-2)
         (Appendix B.4.3) options.

      *  Add two candidate instantiations.

   o  draft-00: Initial version.

1.2.  Terminology

   For the purposes of this document, it is helpful to be able to divide
   cryptographic algorithms into two classes:

   o  "Traditional" algorithms: Algorithms which are widely deployed
      today, but which may be deprecated in the future.  In the context
      of TLS 1.3 in 2019, examples of traditional key exchange
      algorithms include elliptic curve Diffie-Hellman using secp256r1
      or x25519, or finite-field Diffie-Hellman.

   o  "Next-generation" (or "next-gen") algorithms: Algorithms which are
      not yet widely deployed, but which may eventually be widely
      deployed.  An additional facet of these algorithms may be that we
      have less confidence in their security due to them being
      relatively new or less studied.  This includes "post-quantum"
      algorithms.

   "Hybrid" key exchange, in this context, means the use of two (or
   more) key exchange algorithms based on different cryptographic
   assumptions, e.g., one traditional algorithm and one next-gen
   algorithm, with the purpose of the final session key being secure as
   long as at least one of the component key exchange algorithms remains
   unbroken.  We use the term "component" algorithms to refer to the
   algorithms combined in a hybrid key exchange.

   The primary motivation of this document is preparing for post-quantum
   algorithms.  However, it is possible that public key cryptography
   based on alternative mathematical constructions will be required
   independent of the advent of a quantum computer, for example because
   of a cryptanalytic breakthrough.  As such we opt for the more generic
   term "next-generation" algorithms rather than exclusively "post-
   quantum" algorithms.

   Note that TLS 1.3 uses the phrase "groups" to refer to key exchange
   algorithms - for example, the "supported_groups" extension - since
   all key exchange algorithms in TLS 1.3 are Diffie-Hellman-based.  As
   a result, some parts of this document will refer to data structures




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   or messages with the term "group" in them despite using a key
   exchange algorithm that is not Diffie-Hellman-based nor a group.

1.3.  Motivation for use of hybrid key exchange

   A hybrid key exchange algorithm allows early adopters eager for post-
   quantum security to have the potential of post-quantum security
   (possibly from a less-well-studied algorithm) while still retaining
   at least the security currently offered by traditional algorithms.
   They may even need to retain traditional algorithms due to regulatory
   constraints, for example FIPS compliance.

   Ideally, one would not use hybrid key exchange: one would have
   confidence in a single algorithm and parameterization that will stand
   the test of time.  However, this may not be the case in the face of
   quantum computers and cryptanalytic advances more generally.

   Many (though not all) post-quantum algorithms currently under
   consideration are relatively new; they have not been subject to the
   same depth of study as RSA and finite-field or elliptic curve Diffie-
   Hellman, and thus the security community does not necessarily have as
   much confidence in their fundamental security, or the concrete
   security level of specific parameterizations.

   Moreover, it is possible that even by the end of the NIST Post-
   Quantum Cryptography Standardization Project, and for a period of
   time thereafter, conservative users may not have full confidence in
   some algorithms.

   As such, there may be users for whom hybrid key exchange is an
   appropriate step prior to an eventual transition to next-generation
   algorithms.

1.4.  Scope

   This document focuses on hybrid ephemeral key exchange in TLS 1.3
   [TLS13].  It intentionally does not address:

   o  Selecting which next-generation algorithms to use in TLS 1.3, nor
      algorithm identifiers nor encoding mechanisms for next-generation
      algorithms.  This selection will be based on the recommendations
      by the Crypto Forum Research Group (CFRG), which is currently
      waiting for the results of the NIST Post-Quantum Cryptography
      Standardization Project [NIST].

   o  Authentication using next-generation algorithms.  If a
      cryptographic assumption is broken due to the advent of a quantum
      computer or some other cryptanalytic breakthrough, confidentiality



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      of information can be broken retroactively by any adversary who
      has passively recorded handshakes and encrypted communications.
      In contrast, session authentication cannot be retroactively
      broken.

1.5.  Goals

   The primary goal of a hybrid key exchange mechanism is to facilitate
   the establishment of a shared secret which remains secure as long as
   as one of the component key exchange mechanisms remains unbroken.

   In addition to the primary cryptographic goal, there may be several
   additional goals in the context of TLS 1.3:

   o  *Backwards compatibility:* Clients and servers who are "hybrid-
      aware", i.e., compliant with whatever hybrid key exchange standard
      is developed for TLS, should remain compatible with endpoints and
      middle-boxes that are not hybrid-aware.  The three scenarios to
      consider are:

      1.  Hybrid-aware client, hybrid-aware server: These parties should
          establish a hybrid shared secret.

      2.  Hybrid-aware client, non-hybrid-aware server: These parties
          should establish a traditional shared secret (assuming the
          hybrid-aware client is willing to downgrade to traditional-
          only).

      3.  Non-hybrid-aware client, hybrid-aware server: These parties
          should establish a traditional shared secret (assuming the
          hybrid-aware server is willing to downgrade to traditional-
          only).

      Ideally backwards compatibility should be achieved without extra
      round trips and without sending duplicate information; see below.

   o  *High performance:* Use of hybrid key exchange should not be
      prohibitively expensive in terms of computational performance.  In
      general this will depend on the performance characteristics of the
      specific cryptographic algorithms used, and as such is outside the
      scope of this document.  See [BCNS15], [CECPQ1], [FRODO] for
      preliminary results about performance characteristics.

   o  *Low latency:* Use of hybrid key exchange should not substantially
      increase the latency experienced to establish a connection.
      Factors affecting this may include the following.





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      *  The computational performance characteristics of the specific
         algorithms used.  See above.

      *  The size of messages to be transmitted.  Public key and
         ciphertext sizes for post-quantum algorithms range from
         hundreds of bytes to over one hundred kilobytes, so this impact
         can be substantially.  See [BCNS15], [FRODO] for preliminary
         results in a laboratory setting, and [LANGLEY] for preliminary
         results on more realistic networks.

      *  Additional round trips added to the protocol.  See below.

   o  *No extra round trips:* Attempting to negotiate hybrid key
      exchange should not lead to extra round trips in any of the three
      hybrid-aware/non-hybrid-aware scenarios listed above.

   o  *Minimal duplicate information:* Attempting to negotiate hybrid
      key exchange should not mean having to send multiple public keys
      of the same type.

2.  Key encapsulation mechanisms

   In the context of the NIST Post-Quantum Cryptography Standardization
   Project, key exchange algorithms are formulated as key encapsulation
   mechanisms (KEMs), which consist of three algorithms:

   o  "KeyGen() -> (pk, sk)": A probabilistic key generation algorithm,
      which generates a public key "pk" and a secret key "sk".

   o  "Encaps(pk) -> (ct, ss)": A probabilistic encapsulation algorithm,
      which takes as input a public key "pk" and outputs a ciphertext
      "ct" and shared secret "ss".

   o  "Decaps(sk, ct) -> ss": A decapsulation algorithm, which takes as
      input a secret key "sk" and ciphertext "ct" and outputs a shared
      secret "ss", or in some cases a distinguished error value.

   The main security property for KEMs is indistinguishability under
   adaptive chosen ciphertext attack (IND-CCA2), which means that shared
   secret values should be indistinguishable from random strings even
   given the ability to have arbitrary ciphertexts decapsulated.  IND-
   CCA2 corresponds to security against an active attacker, and the
   public key / secret key pair can be treated as a long-term key or
   reused.  A common design pattern for obtaining security under key
   reuse is to apply the Fujisaki-Okamoto (FO) transform [FO] or a
   variant thereof [HHK].





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   A weaker security notion is indistinguishability under chosen
   plaintext attack (IND-CPA), which means that the shared secret values
   should be indistinguishable from random strings given a copy of the
   public key.  IND-CPA roughly corresponds to security against a
   passive attacker, and sometimes corresponds to one-time key exchange.

   Key exchange in TLS 1.3 is phrased in terms of Diffie-Hellman key
   exchange in a group.  DH key exchange can be modeled as a KEM, with
   "KeyGen" corresponding to selecting an exponent "x" as the secret key
   and computing the public key "g^x"; encapsulation corresponding to
   selecting an exponent "y", computing the ciphertext "g^y" and the
   shared secret "g^(xy)", and decapsulation as computing the shared
   secret "g^(xy)".  See [I-D.irtf-cfrg-hpke] for more details of such
   Diffie-Hellman-based key encapsulation mechanisms.

   TLS 1.3 does not require that ephemeral public keys be used only in a
   single key exchange session; some implementations may reuse them, at
   the cost of limited forward secrecy.  As a result, any KEM used in
   this document MUST explicitly be designed to be secure in the event
   that the public key is re-used, such as achieving IND-CCA2 security
   or having a transform like the Fujisaki-Okamoto transform [FO] [HHK]
   applied.  While it is recommended that implementations avoid reuse of
   KEM public keys, implementations that do reuse KEM public keys MUST
   ensure that the number of reuses of a KEM public key abides by any
   bounds in the specification of the KEM or subsequent security
   analyses.  Implementations MUST NOT reuse randomness in the
   generation of KEM ciphertexts.

3.  Construction for hybrid key exchange

3.1.  Negotiation

   Each particular combination of algorithms in a hybrid key exchange
   will be represented as a "NamedGroup" and sent in the
   "supported_groups" extension.  No internal structure or grammar is
   implied or required in the value of the identifier; they are simply
   opaque identifiers.

   Each value representing a hybrid key exchange will correspond to an
   ordered pair of two algorithms.  For example, a future document could
   specify that hybrid value 0x2000 corresponds to
   secp256r1+ntruhrss701, and 0x2001 corresponds to x25519+ntruhrss701.
   (We note that this is independent from future documents standardizing
   solely post-quantum key exchange methods, which would have to be
   assigned their own identifier.)

   Specific values shall be standardized by IANA in the TLS Supported
   Groups registry.  We suggest that values 0x2000 through 0x2EFF are



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   suitable for hybrid key exchange methods (the leading "2" suggesting
   that there are 2 algorithms), noting that 0x2A2A is reserved as a
   GREASE value [GREASE].  This document requests that values 0x2F00
   through 0x2FFF be reserved for Private Use for hybrid key exchange.

       enum {

             /* Elliptic Curve Groups (ECDHE) */
             secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
             x25519(0x001D), x448(0x001E),

             /* Finite Field Groups (DHE) */
             ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096(0x0102),
             ffdhe6144(0x0103), ffdhe8192(0x0104),

             /* Hybrid Key Exchange Methods */
             TBD(0xTBD), ...,

             /* Reserved Code Points */
             ffdhe_private_use(0x01FC..0x01FF),
             hybrid_private_use(0x2F00..0x2FFF),
             ecdhe_private_use(0xFE00..0xFEFF),
             (0xFFFF)
       } NamedGroup;

3.2.  Transmitting public keys and ciphertexts

   We take the relatively simple "concatenation approach": the messages
   from the two algorithms being hybridized will be concatenated
   together and transmitted as a single value, to avoid having to change
   existing data structures.  However we do add structure in the
   concatenation procedure, specifically including length fields, so
   that the concatenation operation is unambiguous.  Note that among the
   Round 2 candidates in the NIST Post-Quantum Cryptography
   Standardization Project, not all algorithms have fixed public key
   sizes; for example, the SIKE key encapsulation mechanism permits
   compressed or uncompressed public keys at each security level, and
   the compressed and uncompressed formats are interoperable.

   Recall that in TLS 1.3 a KEM public key or KEM ciphertext is
   represented as a "KeyShareEntry":

       struct {
           NamedGroup group;
           opaque key_exchange<1..2^16-1>;
       } KeyShareEntry;





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   These are transmitted in the "extension_data" fields of
   "KeyShareClientHello" and "KeyShareServerHello" extensions:

       struct {
           KeyShareEntry client_shares<0..2^16-1>;
       } KeyShareClientHello;

       struct {
           KeyShareEntry server_share;
       } KeyShareServerHello;

   The client's shares are listed in descending order of client
   preference; the server selects one algorithm and sends its
   corresponding share.

   For a hybrid key exchange, the "key_exchange" field of a
   "KeyShareEntry" is the following data structure:

       struct {
           opaque key_exchange_1<1..2^16-1>;
           opaque key_exchange_2<1..2^16-1>;
       } HybridKeyExchange

   The order of shares in the "HybridKeyExchange" struct is the same as
   the order of algorithms indicated in the definition of the
   "NamedGroup".

   For the client's share, the "key_exchange_1" and "key_exchange_2"
   values are the "pk" outputs of the corresponding KEMs' "KeyGen"
   algorithms, if that algorithm corresponds to a KEM; or the (EC)DH
   ephemeral key share, if that algorithm corresponds to an (EC)DH
   group.  For the server's share, the "key_exchange_1" and
   "key_exchange_2" values are the "ct" outputs of the corresponding
   KEMs' "Encaps" algorithms, if that algorithm corresponds to a KEM; or
   the (EC)DH ephemeral key share, if that algorithm corresponds to an
   (EC)DH group.

3.3.  Shared secret calculation

   Here we also take a simple "concatenation approach": the two shared
   secrets are concatenated together and used as the shared secret in
   the existing TLS 1.3 key schedule.  In this case, we do not add any
   additional structure (length fields) in the concatenation procedure:
   among all Round 2 candidates, once the algorithm and variant are
   specified, the shared secret output length is fixed.

   In other words, the shared secret is calculated as




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       concatenated_shared_secret = shared_secret_1 || shared_secret_2

   and inserted into the TLS 1.3 key schedule in place of the (EC)DHE
   shared secret:

                                       0
                                       |
                                       v
                         PSK ->  HKDF-Extract = Early Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
   concatenated_shared_secret -> HKDF-Extract = Handshake Secret
   ^^^^^^^^^^^^^^^^^^^^^^^^^^          |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
                            0 -> HKDF-Extract = Master Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)

   *FIPS-compliance of shared secret concatenation.* [NIST-SP-800-56C]
   or [NIST-SP-800-135] give NIST recommendations for key derivation
   methods in key exchange protocols.  Some hybrid combinations may
   combine the shared secret from a NIST-approved algorithm (e.g., ECDH
   using the nistp256/secp256r1 curve) with a shared secret from a non-
   approved algorithm (e.g., post-quantum).  Although the simple
   concatenation approach above is not currently an approved method in
   [NIST-SP-800-56C] or [NIST-SP-800-135], NIST indicated in January
   2020 that a forthcoming revision of [NIST-SP-800-56C] will list
   simple concatenation as an approved method [NIST-FAQ].







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4.  Open questions

   *Larger public keys and/or ciphertexts.* The "HybridKeyExchange"
   struct in Section 3.2 limits public keys and ciphertexts to 2^16-1
   bytes; this is bounded by the same (2^16-1)-byte limit on the
   "key_exchange" field in the "KeyShareEntry" struct.  Some post-
   quantum KEMs have larger public keys and/or ciphertexts; for example,
   Classic McEliece's smallest parameter set has public key size 261,120
   bytes.  Hence this draft can not accommodate all current NIST Round 2
   candidates.

   If it is desired to accommodate algorithms with public keys or
   ciphertexts larger than 2^16-1 bytes, options include a) revising the
   TLS 1.3 standard to allow longer "key_exchange" fields; b) creating
   an alternative extension which is sufficiently large; or c) providing
   a reference to an external public key, e.g. a URL at which to look up
   the public key (along with a hash to verify).

   *Duplication of key shares.* Concatenation of public keys in the
   "HybridKeyExchange" struct as described in Section 3.2 can result in
   sending duplicate key shares.  For example, if a client wanted to
   offer support for two combinations, say "secp256r1+sikep503" and
   "x25519+sikep503", it would end up sending two sikep503 public keys,
   since the "KeyShareEntry" for each combination contains its own copy
   of a sikep503 key.  This duplication may be more problematic for
   post-quantum algorithms which have larger public keys.

   If it is desired to avoid duplication of key shares, options include
   a) disconnect the use of a combination for the algorithm identifier
   from the use of concatenation of public keys by introducing new logic
   and/or data structures (see Appendix B.3.2 or Appendix B.3.3); or b)
   provide some back reference from a later key share entry to an
   earlier one.

   *Variable-length shared secrets.* The shared secret calculation in
   Section 3.3 directly concatenates the shared secret values of each
   scheme, rather than encoding them with length fields.  This
   implicitly assumes that the length of each shared secret is fixed
   once the algorithm is fixed.  This is the case for all Round 2
   candidates.

   However, if it is envisioned that this specification be used with
   algorithms which do not have fixed-length shared secrets (after the
   variant has been fixed by the algorithm identifier in the
   "NamedGroup" negotiation in Section 3.1), then Section 3.3 should be
   revised to use an unambiguous concatenation method such as the
   following:




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       struct {
           opaque shared_secret_1<1..2^16-1>;
           opaque shared_secret_2<1..2^16-1>;
       } HybridSharedSecret

   Guidance from the working group is particularly requested on this
   point.

   *Resumption.* TLS 1.3 allows for session resumption via a PSK.  When
   a PSK is used during session establishment, an ephemeral key exchange
   can also be used to enhance forward secrecy.  If the original key
   exchange was hybrid, should an ephemeral key exchange in a resumption
   of that original key exchange be required to use the same hybrid
   algorithms?

   *Failures.* Some post-quantum key exchange algorithms have non-
   trivial failure rates: two honest parties may fail to agree on the
   same shared secret with non-negligible probability.  Does a non-
   negligible failure rate affect the security of TLS?  How should such
   a failure be treated operationally?  What is an acceptable failure
   rate?

5.  IANA Considerations

   Identifiers for specific key exchange algorithm combinations will be
   defined in later documents.  This document requests IANA reserve
   values 0x2F00..0x2FFF in the TLS Supported Groups registry for
   private use for hybrid key exchange methods.

6.  Security Considerations

   The shared secrets computed in the hybrid key exchange should be
   computed in a way that achieves the "hybrid" property: the resulting
   secret is secure as long as at least one of the component key
   exchange algorithms is unbroken.  See [GIACON] and [BINDEL] for an
   investigation of these issues.  Under the assumption that shared
   secrets are fixed length once the combination is fixed, the
   construction from Section 3.3 corresponds to the dual-PRF combiner of
   [BINDEL] which is shown to preserve security under the assumption
   that the hash function is a dual-PRF.

   As noted in Section 2, KEMs used in this document MUST explicitly be
   designed to be secure in the event that the public key is re-used,
   such as achieving IND-CCA2 security or having a transform like the
   Fujisaki-Okamoto transform applied.  Some IND-CPA-secure post-quantum
   KEMs (i.e., without countermeasures such as the FO transform) are
   completely insecure under public key reuse; for example, some




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   lattice-based IND-CPA-secure KEMs are vulnerable to attacks that
   recover the private key after just a few thousand samples [FLUHRER].

7.  Acknowledgements

   These ideas have grown from discussions with many colleagues,
   including Christopher Wood, Matt Campagna, Eric Crockett, authors of
   the various hybrid Internet-Drafts and implementations cited in this
   document, and members of the TLS working group.  The immediate
   impetus for this document came from discussions with attendees at the
   Workshop on Post-Quantum Software in Mountain View, California, in
   January 2019.  Martin Thomson suggested the (Comb-KDF-1)
   (Appendix B.4.2) approach.  Daniel J.  Bernstein and Tanja Lange
   commented on the risks of reuse of ephemeral public keys.

8.  References

8.1.  Normative References

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

8.2.  Informative References

   [BCNS15]   Bos, J., Costello, C., Naehrig, M., and D. Stebila, "Post-
              Quantum Key Exchange for the TLS Protocol from the Ring
              Learning with Errors Problem", 2015 IEEE Symposium on
              Security and Privacy, DOI 10.1109/sp.2015.40, May 2015.

   [BERNSTEIN]
              "Post-Quantum Cryptography", Springer Berlin
              Heidelberg book, DOI 10.1007/978-3-540-88702-7, 2009.

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

   [CECPQ1]   Braithwaite, M., "Experimenting with Post-Quantum
              Cryptography", July 2016,
              <https://security.googleblog.com/2016/07/experimenting-
              with-post-quantum.html>.

   [CECPQ2]   Langley, A., "CECPQ2", December 2018,
              <https://www.imperialviolet.org/2018/12/12/cecpq2.html>.





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   [DFGS15]   Dowling, B., Fischlin, M., Guenther, F., and D. Stebila,
              "A Cryptographic Analysis of the TLS 1.3 Handshake
              Protocol Candidates", Proceedings of the 22nd ACM SIGSAC
              Conference on Computer and Communications Security -
              CCS '15, DOI 10.1145/2810103.2813653, 2015.

   [DODIS]    Dodis, Y. and J. Katz, "Chosen-Ciphertext Security of
              Multiple Encryption", Theory of Cryptography pp. 188-209,
              DOI 10.1007/978-3-540-30576-7_11, 2005.

   [DOWLING]  Dowling, B., "Provable Security of Internet Protocols",
              Queensland University of Technology dissertation,
              DOI 10.5204/thesis.eprints.108960, n.d..

   [ETSI]     Campagna, M., Ed. and . others, "Quantum safe cryptography
              and security: An introduction, benefits, enablers and
              challengers", ETSI White Paper No. 8 , June 2015,
              <https://www.etsi.org/images/files/ETSIWhitePapers/
              QuantumSafeWhitepaper.pdf>.

   [EVEN]     Even, S. and O. Goldreich, "On the Power of Cascade
              Ciphers", Advances in Cryptology pp. 43-50,
              DOI 10.1007/978-1-4684-4730-9_4, 1984.

   [EXTERN-PSK]
              Housley, R., "TLS 1.3 Extension for Certificate-based
              Authentication with an External Pre-Shared Key", draft-
              ietf-tls-tls13-cert-with-extern-psk-07 (work in progress),
              December 2019.

   [FLUHRER]  Fluhrer, S., "Cryptanalysis of ring-LWE based key exchange
              with key share reuse", Cryptology ePrint Archive, Report
              2016/085 , January 2016,
              <https://eprint.iacr.org/2016/085>.

   [FO]       Fujisaki, E. and T. Okamoto, "Secure Integration of
              Asymmetric and Symmetric Encryption Schemes", Journal of
              Cryptology Vol. 26, pp. 80-101,
              DOI 10.1007/s00145-011-9114-1, December 2011.

   [FRODO]    Bos, J., Costello, C., Ducas, L., Mironov, I., Naehrig,
              M., Nikolaenko, V., Raghunathan, A., and D. Stebila,
              "Frodo", Proceedings of the 2016 ACM SIGSAC Conference on
              Computer and Communications Security - CCS'16,
              DOI 10.1145/2976749.2978425, 2016.






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   [GIACON]   Giacon, F., Heuer, F., and B. Poettering, "KEM Combiners",
              Public-Key Cryptography - PKC 2018 pp. 190-218,
              DOI 10.1007/978-3-319-76578-5_7, 2018.

   [GREASE]   Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-04 (work in progress), August 2019.

   [HARNIK]   Harnik, D., Kilian, J., Naor, M., Reingold, O., and A.
              Rosen, "On Robust Combiners for Oblivious Transfer and
              Other Primitives", Lecture Notes in Computer Science pp.
              96-113, DOI 10.1007/11426639_6, 2005.

   [HHK]      Hofheinz, D., Hoevelmanns, K., and E. Kiltz, "A Modular
              Analysis of the Fujisaki-Okamoto Transformation", Theory
              of Cryptography pp. 341-371,
              DOI 10.1007/978-3-319-70500-2_12, 2017.

   [HOFFMAN]  Hoffman, P., "The Transition from Classical to Post-
              Quantum Cryptography", draft-hoffman-c2pq-06 (work in
              progress), November 2019.

   [I-D.irtf-cfrg-hpke]
              Barnes, R. and K. Bhargavan, "Hybrid Public Key
              Encryption", draft-irtf-cfrg-hpke-02 (work in progress),
              November 2019.

   [IKE-HYBRID]
              Tjhai, C., Tomlinson, M., grbartle@cisco.com, g., Fluhrer,
              S., Geest, D., Garcia-Morchon, O., and V. Smyslov,
              "Framework to Integrate Post-quantum Key Exchanges into
              Internet Key Exchange Protocol Version 2 (IKEv2)", draft-
              tjhai-ipsecme-hybrid-qske-ikev2-04 (work in progress),
              July 2019.

   [IKE-PSK]  Fluhrer, S., Kampanakis, P., McGrew, D., and V. Smyslov,
              "Mixing Preshared Keys in IKEv2 for Post-quantum
              Security", draft-ietf-ipsecme-qr-ikev2-11 (work in
              progress), January 2020.

   [KIEFER]   Kiefer, F. and K. Kwiatkowski, "Hybrid ECDHE-SIDH Key
              Exchange for TLS", draft-kiefer-tls-ecdhe-sidh-00 (work in
              progress), November 2018.

   [KPW13]    Krawczyk, H., Paterson, K., and H. Wee, "On the Security
              of the TLS Protocol: A Systematic Analysis", Advances in
              Cryptology - CRYPTO 2013 pp. 429-448,
              DOI 10.1007/978-3-642-40041-4_24, 2013.




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   [LANGLEY]  Langley, A., "Post-quantum confidentiality for TLS", April
              2018, <https://www.imperialviolet.org/2018/04/11/
              pqconftls.html>.

   [NIELSEN]  Nielsen, M. and I. Chuang, "Quantum Computation and
              Quantum Information", Cambridge University Press , 2000.

   [NIST]     National Institute of Standards and Technology (NIST),
              "Post-Quantum Cryptography", n.d.,
              <https://www.nist.gov/pqcrypto>.

   [NIST-FAQ]
              National Institute of Standards and Technology (NIST),
              "Post-Quantum Cryptography - FAQs", January 2020,
              <https://csrc.nist.gov/Projects/post-quantum-cryptography/
              faqs>.

   [NIST-SP-800-135]
              National Institute of Standards and Technology (NIST),
              "Recommendation for Existing Application-Specific Key
              Derivation Functions", December 2011,
              <https://doi.org/10.6028/NIST.SP.800-135r1>.

   [NIST-SP-800-56C]
              National Institute of Standards and Technology (NIST),
              "Recommendation for Key-Derivation Methods in Key-
              Establishment Schemes", April 2018,
              <https://doi.org/10.6028/NIST.SP.800-56Cr1>.

   [OQS-102]  Open Quantum Safe Project, "OQS-OpenSSL-1-0-2_stable",
              November 2018, <https://github.com/open-quantum-
              safe/openssl/tree/OQS-OpenSSL_1_0_2-stable>.

   [OQS-111]  Open Quantum Safe Project, "OQS-OpenSSL-1-1-1_stable",
              November 2018, <https://github.com/open-quantum-
              safe/openssl/tree/OQS-OpenSSL_1_1_1-stable>.

   [SCHANCK]  Schanck, J. and D. Stebila, "A Transport Layer Security
              (TLS) Extension For Establishing An Additional Shared
              Secret", draft-schanck-tls-additional-keyshare-00 (work in
              progress), April 2017.

   [WHYTE12]  Schanck, J., Whyte, W., and Z. Zhang, "Quantum-Safe Hybrid
              (QSH) Ciphersuite for Transport Layer Security (TLS)
              version 1.2", draft-whyte-qsh-tls12-02 (work in progress),
              July 2016.





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   [WHYTE13]  Whyte, W., Zhang, Z., Fluhrer, S., and O. Garcia-Morchon,
              "Quantum-Safe Hybrid (QSH) Key Exchange for Transport
              Layer Security (TLS) version 1.3", draft-whyte-qsh-
              tls13-06 (work in progress), October 2017.

   [XMSS]     Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
              Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
              RFC 8391, DOI 10.17487/RFC8391, May 2018,
              <https://www.rfc-editor.org/info/rfc8391>.

   [ZHANG]    Zhang, R., Hanaoka, G., Shikata, J., and H. Imai, "On the
              Security of Multiple Encryption or CCA-security+CCA-
              security=CCA-security?", Public Key Cryptography - PKC
              2004 pp. 360-374, DOI 10.1007/978-3-540-24632-9_26, 2004.

Appendix A.  Related work

   Quantum computing and post-quantum cryptography in general are
   outside the scope of this document.  For a general introduction to
   quantum computing, see a standard textbook such as [NIELSEN].  For an
   overview of post-quantum cryptography as of 2009, see [BERNSTEIN].
   For the current status of the NIST Post-Quantum Cryptography
   Standardization Project, see [NIST].  For additional perspectives on
   the general transition from classical to post-quantum cryptography,
   see for example [ETSI] and [HOFFMAN], among others.

   There have been several Internet-Drafts describing mechanisms for
   embedding post-quantum and/or hybrid key exchange in TLS:

   o  Internet-Drafts for TLS 1.2: [WHYTE12]

   o  Internet-Drafts for TLS 1.3: [KIEFER], [SCHANCK], [WHYTE13]

   There have been several prototype implementations for post-quantum
   and/or hybrid key exchange in TLS:

   o  Experimental implementations in TLS 1.2: [BCNS15], [CECPQ1],
      [FRODO], [OQS-102]

   o  Experimental implementations in TLS 1.3: [CECPQ2], [OQS-111]

   These experimental implementations have taken an ad hoc approach and
   not attempted to implement one of the drafts listed above.

   Unrelated to post-quantum but still related to the issue of combining
   multiple types of keying material in TLS is the use of pre-shared
   keys, especially the recent TLS working group document on including
   an external pre-shared key [EXTERN-PSK].



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   Considering other IETF standards, there is work on post-quantum
   preshared keys in IKEv2 [IKE-PSK] and a framework for hybrid key
   exchange in IKEv2 [IKE-HYBRID].  The XMSS hash-based signature scheme
   has been published as an informational RFC by the IRTF [XMSS].

   In the academic literature, [EVEN] initiated the study of combining
   multiple symmetric encryption schemes; [ZHANG], [DODIS], and [HARNIK]
   examined combining multiple public key encryption schemes, and
   [HARNIK] coined the term "robust combiner" to refer to a compiler
   that constructs a hybrid scheme from individual schemes while
   preserving security properties.  [GIACON] and [BINDEL] examined
   combining multiple key encapsulation mechanisms.

Appendix B.  Design Considerations

   This appendix discusses choices one could make along four distinct
   axes when integrating hybrid key exchange into TLS 1.3:

   1.  How to negotiate the use of hybridization in general and
       component algorithms specifically?

   2.  How many component algorithms can be combined?

   3.  How should multiple key shares (public keys / ciphertexts) be
       conveyed?

   4.  How should multiple shared secrets be combined?

   The construction in the main body illustrates one selection along
   each of these axes.  The remainder of this appendix outlines various
   options we have identified for each of these choices.  Immediately
   below we provide a summary list.  Options are labelled with a short
   code in parentheses to provide easy cross-referencing.

   1.  (Neg) (Appendix B.1) How to negotiate the use of hybridization in
       general and component algorithms specifically?

       *  (Neg-Ind) (Appendix B.1.2) Negotiating component algorithms
          individually

          +  (Neg-Ind-1) (Appendix B.1.2.1) Traditional algorithms in
             "ClientHello" "supported_groups" extension, next-gen
             algorithms in another extension

          +  (Neg-Ind-2) (Appendix B.1.2.2) Both types of algorithms in
             "supported_groups" with external mapping to tradition/next-
             gen.




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          +  (Neg-Ind-3) (Appendix B.1.2.3) Both types of algorithms in
             "supported_groups" separated by a delimiter.

       *  (Neg-Comb) (Appendix B.1.3) Negotiating component algorithms
          as a combination

          +  (Neg-Comb-1) (Appendix B.1.3.1) Standardize "NamedGroup"
             identifiers for each desired combination.

          +  (Neg-Comb-2) (Appendix B.1.3.2) Use placeholder identifiers
             in "supported_groups" with an extension defining the
             combination corresponding to each placeholder.

          +  (Neg-Comb-3) (Appendix B.1.3.3) List combinations by
             inserting grouping delimiters into "supported_groups" list.

   2.  (Num) (Appendix B.2) How many component algorithms can be
       combined?

       *  (Num-2) (Appendix B.2.1) Two.

       *  (Num-2+) (Appendix B.2.2) Two or more.

   3.  (Shares) (Appendix B.3) How should multiple key shares (public
       keys / ciphertexts) be conveyed?

       *  (Shares-Concat) (Appendix B.3.1) Concatenate each combination
          of key shares.

       *  (Shares-Multiple) (Appendix B.3.2) Send individual key shares
          for each algorithm.

       *  (Shares-Ext-Additional) (Appendix B.3.3) Use an extension to
          convey key shares for component algorithms.

   4.  (Comb) (Appendix B.4) How should multiple shared secrets be
       combined?

       *  (Comb-Concat) (Appendix B.4.1) Concatenate the shared secrets
          then use directly in the TLS 1.3 key schedule.

       *  (Comb-KDF-1) (Appendix B.4.2) and (Comb-KDF-2)
          (Appendix B.4.3) KDF the shared secrets together, then use the
          output in the TLS 1.3 key schedule.

       *  (Comb-XOR) (Appendix B.4.4) XOR the shared secrets then use
          directly in the TLS 1.3 key schedule.




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       *  (Comb-Chain) (Appendix B.4.5) Extend the TLS 1.3 key schedule
          so that there is a stage of the key schedule for each shared
          secret.

       *  (Comb-AltInput) (Appendix B.4.6) Use the second shared secret
          in an alternate (otherwise unused) input in the TLS 1.3 key
          schedule.

B.1.  (Neg) How to negotiate hybridization and component algorithms?

B.1.1.  Key exchange negotiation in TLS 1.3

   Recall that in TLS 1.3, the key exchange mechanism is negotiated via
   the "supported_groups" extension.  The "NamedGroup" enum is a list of
   standardized groups for Diffie-Hellman key exchange, such as
   "secp256r1", "x25519", and "ffdhe2048".

   The client, in its "ClientHello" message, lists its supported
   mechanisms in the "supported_groups" extension.  The client also
   optionally includes the public key of one or more of these groups in
   the "key_share" extension as a guess of which mechanisms the server
   might accept in hopes of reducing the number of round trips.

   If the server is willing to use one of the client's requested
   mechanisms, it responds with a "key_share" extension containing its
   public key for the desired mechanism.

   If the server is not willing to use any of the client's requested
   mechanisms, the server responds with a "HelloRetryRequest" message
   that includes an extension indicating its preferred mechanism.

B.1.2.  (Neg-Ind) Negotiating component algorithms individually

   In these three approaches, the parties negotiate which traditional
   algorithm and which next-gen algorithm to use independently.  The
   "NamedGroup" enum is extended to include algorithm identifiers for
   each next-gen algorithm.

B.1.2.1.  (Neg-Ind-1)

   The client advertises two lists to the server: one list containing
   its supported traditional mechanisms (e.g. via the existing
   "ClientHello" "supported_groups" extension), and a second list
   containing its supported next-generation mechanisms (e.g., via an
   additional "ClientHello" extension).  A server could then select one
   algorithm from the traditional list, and one algorithm from the next-
   generation list.  (This is the approach in [SCHANCK].)




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B.1.2.2.  (Neg-Ind-2)

   The client advertises a single list to the server which contains both
   its traditional and next-generation mechanisms (e.g., all in the
   existing "ClientHello" "supported_groups" extension), but with some
   external table provides a standardized mapping of those mechanisms as
   either "traditional" or "next-generation".  A server could then
   select two algorithms from this list, one from each category.

B.1.2.3.  (Neg-Ind-3)

   The client advertises a single list to the server delimited into
   sublists: one for its traditional mechanisms and one for its next-
   generation mechanisms, all in the existing "ClientHello"
   "supported_groups" extension, with a special code point serving as a
   delimiter between the two lists.  For example, "supported_groups =
   secp256r1, x25519, delimiter, nextgen1, nextgen4".

B.1.3.  (Neg-Comb) Negotiating component algorithms as a combination

   In these three approaches, combinations of key exchange mechanisms
   appear as a single monolithic block; the parties negotiate which of
   several combinations they wish to use.

B.1.3.1.  (Neg-Comb-1)

   The "NamedGroup" enum is extended to include algorithm identifiers
   for each *combination* of algorithms desired by the working group.
   There is no "internal structure" to the algorithm identifiers for
   each combination, they are simply new code points assigned
   arbitrarily.  The client includes any desired combinations in its
   "ClientHello" "supported_groups" list, and the server picks one of
   these.  This is the approach in [KIEFER] and [OQS-111].

B.1.3.2.  (Neg-Comb-2)

   The "NamedGroup" enum is extended to include algorithm identifiers
   for each next-gen algorithm.  Some additional field/extension is used
   to convey which combinations the parties wish to use.  For example,
   in [WHYTE13], there are distinguished "NamedGroup" called
   "hybrid_marker 0", "hybrid_marker 1", "hybrid_marker 2", etc.  This
   is complemented by a "HybridExtension" which contains mappings for
   each numbered "hybrid_marker" to the set of component key exchange
   algorithms (2 or more) for that proposed combination.







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B.1.3.3.  (Neg-Comb-3)

   The client lists combinations in "supported_groups" list, using a
   special delimiter to indicate combinations.  For example,
   "supported_groups = combo_delimiter, secp256r1, nextgen1,
   combo_delimiter, secp256r1, nextgen4, standalone_delimiter,
   secp256r1, x25519" would indicate that the client's highest
   preference is the combination secp256r1+nextgen1, the next highest
   preference is the combination secp2561+nextgen4, then the single
   algorithm secp256r1, then the single algorithm x25519.  A hybrid-
   aware server would be able to parse these; a hybrid-unaware server
   would see "unknown, secp256r1, unknown, unknown, secp256r1, unknown,
   unknown, secp256r1, x25519", which it would be able to process,
   although there is the potential that every "projection" of a hybrid
   list that is tolerable to a client does not result in list that is
   tolerable to the client.

B.1.4.  Benefits and drawbacks

   *Combinatorial explosion.* (Neg-Comb-1) (Appendix B.1.3.1) requires
   new identifiers to be defined for each desired combination.  The
   other 4 options in this section do not.

   *Extensions.* (Neg-Ind-1) (Appendix B.1.2.1) and (Neg-Comb-2)
   (Appendix B.1.3.2) require new extensions to be defined.  The other
   options in this section do not.

   *New logic.* All options in this section except (Neg-Comb-1)
   (Appendix B.1.3.1) require new logic to process negotiation.

   *Matching security levels.* (Neg-Ind-1) (Appendix B.1.2.1), (Neg-Ind-
   2) (Appendix B.1.2.2), (Neg-Ind-3) (Appendix B.1.2.3), and (Neg-Comb-
   2) (Appendix B.1.3.2) allow algorithms of different claimed security
   level from their corresponding lists to be combined.  For example,
   this could result in combining ECDH secp256r1 (classical security
   level 128) with NewHope-1024 (classical security level 256).
   Implementations dissatisfied with a mismatched security levels must
   either accept this mismatch or attempt to renegotiate.  (Neg-Ind-1)
   (Appendix B.1.2.1), (Neg-Ind-2) (Appendix B.1.2.2), and (Neg-Ind-3)
   (Appendix B.1.2.3) give control over the combination to the server;
   (Neg-Comb-2) (Appendix B.1.3.2) gives control over the combination to
   the client.  (Neg-Comb-1) (Appendix B.1.3.1) only allows standardized
   combinations, which could be set by TLS working group to have
   matching security (provided security estimates do not evolve
   separately).

   *Backwards-compability.* TLS 1.3-compliant hybrid-unaware servers
   should ignore unreocgnized elements in "supported_groups" (Neg-Ind-2)



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   (Appendix B.1.2.2), (Neg-Ind-3) (Appendix B.1.2.3), (Neg-Comb-1)
   (Appendix B.1.3.1), (Neg-Comb-2) (Appendix B.1.3.2) and unrecognized
   "ClientHello" extensions (Neg-Ind-1) (Appendix B.1.2.1), (Neg-Comb-2)
   (Appendix B.1.3.2).  In (Neg-Ind-3) (Appendix B.1.2.3) and (Neg-Comb-
   3) (Appendix B.1.3.3), a server that is hybrid-unaware will ignore
   the delimiters in "supported_groups", and thus might try to negotiate
   an algorithm individually that is only meant to be used in
   combination; depending on how such an implementation is coded, it may
   also encounter bugs when the same element appears multiple times in
   the list.

B.2.  (Num) How many component algorithms to combine?

B.2.1.  (Num-2) Two

   Exactly two algorithms can be combined together in hybrid key
   exchange.  This is the approach taken in [KIEFER] and [SCHANCK].

B.2.2.  (Num-2+) Two or more

   Two or more algorithms can be combined together in hybrid key
   exchange.  This is the approach taken in [WHYTE13].

B.2.3.  Benefits and Drawbacks

   Restricting the number of component algorithms that can be hybridized
   to two substantially reduces the generality required.  On the other
   hand, some adopters may want to further reduce risk by employing
   multiple next-gen algorithms built on different cryptographic
   assumptions.

B.3.  (Shares) How to convey key shares?

   In ECDH ephmeral key exchange, the client sends its ephmeral public
   key in the "key_share" extension of the "ClientHello" message, and
   the server sends its ephmeral public key in the "key_share" extension
   of the "ServerHello" message.

   For a general key encapsulation mechanism used for ephemeral key
   exchange, we imagine that that client generates a fresh KEM public
   key / secret pair for each connection, sends it to the client, and
   the server responds with a KEM ciphertext.  For simplicity and
   consistency with TLS 1.3 terminology, we will refer to both of these
   types of objects as "key shares".

   In hybrid key exchange, we have to decide how to convey the client's
   two (or more) key shares, and the server's two (or more) key shares.




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B.3.1.  (Shares-Concat) Concatenate key shares

   The client concatenates the bytes representing its two key shares and
   uses this directly as the "key_exchange" value in a "KeyShareEntry"
   in its "key_share" extension.  The server does the same thing.  Note
   that the "key_exchange" value can be an octet string of length at
   most 2^16-1.  This is the approach taken in [KIEFER], [OQS-111], and
   [WHYTE13].

B.3.2.  (Shares-Multiple) Send multiple key shares

   The client sends multiple key shares directly in the "client_shares"
   vectors of the "ClientHello" "key_share" extension.  The server does
   the same.  (Note that while the existing "KeyShareClientHello" struct
   allows for multiple key share entries, the existing
   "KeyShareServerHello" only permits a single key share entry, so some
   modification would be required to use this approach for the server to
   send multiple key shares.)

B.3.3.  (Shares-Ext-Additional) Extension carrying additional key shares

   The client sends the key share for its traditional algorithm in the
   original "key_share" extension of the "ClientHello" message, and the
   key share for its next-gen algorithm in some additional extension in
   the "ClientHello" message.  The server does the same thing.  This is
   the approach taken in [SCHANCK].

B.3.4.  Benefits and Drawbacks

   *Backwards compatibility.* (Shares-Multiple) (Appendix B.3.2) is
   fully backwards compatible with non-hybrid-aware servers.  (Shares-
   Ext-Additional) (Appendix B.3.3) is backwards compatible with non-
   hybrid-aware servers provided they ignore unrecognized extensions.
   (Shares-Concat) (Appendix B.3.1) is backwards-compatible with non-
   hybrid aware servers, but may result in duplication / additional
   round trips (see below).

   *Duplication versus additional round trips.* If a client wants to
   offer multiple key shares for multiple combinations in order to avoid
   retry requests, then the client may ended up sending a key share for
   one algorithm multiple times when using (Shares-Ext-Additional)
   (Appendix B.3.3) and (Shares-Concat) (Appendix B.3.1).  (For example,
   if the client wants to send an ECDH-secp256r1 + McEliece123 key
   share, and an ECDH-secp256r1 + NewHope1024 key share, then the same
   ECDH public key may be sent twice.  If the client also wants to offer
   a traditional ECDH-only key share for non-hybrid-aware
   implementations and avoid retry requests, then that same ECDH public




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   key may be sent another time.)  (Shares-Multiple) (Appendix B.3.2)
   does not result in duplicate key shares.

B.4.  (Comb) How to use keys?

   Each component key exchange algorithm establishes a shared secret.
   These shared secrets must be combined in some way that achieves the
   "hybrid" property: the resulting secret is secure as long as at least
   one of the component key exchange algorithms is unbroken.

B.4.1.  (Comb-Concat) Concatenate keys

   Each party concatenates the shared secrets established by each
   component algorithm in an agreed-upon order, then feeds that through
   the TLS key schedule.  In the context of TLS 1.3, this would mean
   using the concatenated shared secret in place of the (EC)DHE input to
   the second call to "HKDF-Extract" in the TLS 1.3 key schedule:

                                       0
                                       |
                                       v
                         PSK ->  HKDF-Extract = Early Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
   concatenated_shared_secret -> HKDF-Extract = Handshake Secret
   ^^^^^^^^^^^^^^^^^^^^^^^^^^          |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
                            0 -> HKDF-Extract = Master Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)

   This is the approach used in [KIEFER], [OQS-111], and [WHYTE13].



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   [GIACON] analyzes the security of applying a KDF to concatenated KEM
   shared secrets, but their analysis does not exactly apply here since
   the transcript of ciphertexts is included in the KDF application
   (though it should follow relatively straightforwardly).

   [BINDEL] analyzes the security of the (Comb-Concat) approach as
   abstracted in their "dualPRF" combiner.  They show that, if the
   component KEMs are IND-CPA-secure (or IND-CCA-secure), then the
   values output by "Derive-Secret" are IND-CPA-secure (respectively,
   IND-CCA-secure).  An important aspect of their analysis is that each
   ciphertext is input to the final PRF calls; this holds for TLS 1.3
   since the "Derive-Secret" calls that derive output keys (application
   traffic secrets, and exporter and resumption master secrets) include
   the transcript hash as input.

B.4.2.  (Comb-KDF-1) KDF keys

   Each party feeds the shared secrets established by each component
   algorithm in an agreed-upon order into a KDF, then feeds that through
   the TLS key schedule.  In the context of TLS 1.3, this would mean
   first applying "HKDF-Extract" to the shared secrets, then using the
   output in place of the (EC)DHE input to the second call to "HKDF-
   Extract" in the TLS 1.3 key schedule:




























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                                       0
                                       |
                                       v
                         PSK ->  HKDF-Extract = Early Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                  Next-Gen             |
                      |                v
     (EC)DHE -> HKDF-Extract     Derive-Secret(., "derived", "")
                      |                |
                      v                v
                   output -----> HKDF-Extract = Handshake Secret
                   ^^^^^^              |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
                            0 -> HKDF-Extract = Master Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)

B.4.3.  (Comb-KDF-2) KDF keys

   Each party concatenates the shared secrets established by each
   component algorithm in an agreed-upon order then feeds that into a
   KDF, then feeds the result through the TLS key schedule.

   Compared with (Comb-KDF-1) (Appendix B.4.2), this method concatenates
   the (2 or more) shared secrets prior to input to the KDF, whereas
   (Comb-KDF-1) puts the (exactly 2) shared secrets in the two different
   input slots to the KDF.

   Compared with (Comb-Concat) (Appendix B.4.1), this method has an
   extract KDF application.  While this adds computational overhead,
   this may provide a cleaner abstraction of the hybridization mechanism
   for the purposes of formal security analysis.







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                                       0
                                       |
                                       v
                         PSK ->  HKDF-Extract = Early Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                      v
     concatenated     0
     shared           |
     secret  -> HKDF-Extract     Derive-Secret(., "derived", "")
     ^^^^^^           |                |
                      v                v
                   output -----> HKDF-Extract = Handshake Secret
                   ^^^^^^              |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
                            0 -> HKDF-Extract = Master Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)

B.4.4.  (Comb-XOR) XOR keys

   Each party XORs the shared secrets established by each component
   algorithm (possibly after padding secrets of different lengths), then
   feeds that through the TLS key schedule.  In the context of TLS 1.3,
   this would mean using the XORed shared secret in place of the (EC)DHE
   input to the second call to "HKDF-Extract" in the TLS 1.3 key
   schedule.

   [GIACON] analyzes the security of applying a KDF to the XORed KEM
   shared secrets, but their analysis does not quite apply here since
   the transcript of ciphertexts is included in the KDF application
   (though it should follow relatively straightforwardly).







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B.4.5.  (Comb-Chain) Chain of KDF applications for each key

   Each party applies a chain of key derivation functions to the shared
   secrets established by each component algorithm in an agreed-upon
   order; roughly speaking: "F(k1 || F(k2))".  In the context of TLS
   1.3, this would mean extending the key schedule to have one round of
   the key schedule applied for each component algorithm's shared
   secret:

                                       0
                                       |
                                       v
                         PSK ->  HKDF-Extract = Early Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
    traditional_shared_secret -> HKDF-Extract
    ^^^^^^^^^^^^^^^^^^^^^^^^^          |
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
       next_gen_shared_secret -> HKDF-Extract = Handshake Secret
       ^^^^^^^^^^^^^^^^^^^^^^          |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
                            0 -> HKDF-Extract = Master Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)

   This is the approach used in [SCHANCK].

   [BINDEL] analyzes the security of this approach as abstracted in
   their nested dual-PRF "N" combiner, showing a similar result as for
   the dualPRF combiner that it preserves IND-CPA (or IND-CCA) security.



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   Again their analysis depends on each ciphertext being input to the
   final PRF ("Derive-Secret") calls, which holds for TLS 1.3.

B.4.6.  (Comb-AltInput) Second shared secret in an alternate KDF input

   In the context of TLS 1.3, the next-generation shared secret is used
   in place of a currently unused input in the TLS 1.3 key schedule,
   namely replacing the "0" "IKM" input to the final "HKDF-Extract":

                                       0
                                       |
                                       v
                         PSK ->  HKDF-Extract = Early Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
    traditional_shared_secret -> HKDF-Extract = Handshake Secret
    ^^^^^^^^^^^^^^^^^^^^^^^^^          |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
       next_gen_shared_secret -> HKDF-Extract = Master Secret
       ^^^^^^^^^^^^^^^^^^^^^^          |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)

   This approach is not taken in any of the known post-quantum/hybrid
   TLS drafts.  However, it bears some similarities to the approach for
   using external PSKs in [EXTERN-PSK].

B.4.7.  Benefits and Drawbacks

   *New logic.* While (Comb-Concat) (Appendix B.4.1), (Comb-KDF-1)
   (Appendix B.4.2), and (Comb-KDF-2) (Appendix B.4.3) require new logic
   to compute the concatenated shared secret, this value can then be
   used by the TLS 1.3 key schedule without changes to the key schedule



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   logic.  In contrast, (Comb-Chain) (Appendix B.4.5) requires the TLS
   1.3 key schedule to be extended for each extra component algorithm.

   *Philosophical.* The TLS 1.3 key schedule already applies a new stage
   for different types of keying material (PSK versus (EC)DHE), so
   (Comb-Chain) (Appendix B.4.5) continues that approach.

   *Efficiency.* (Comb-KDF-1) (Appendix B.4.2), (Comb-KDF-2)
   (Appendix B.4.3), and (Comb-Chain) (Appendix B.4.5) increase the
   number of KDF applications for each component algorithm, whereas
   (Comb-Concat) (Appendix B.4.1) and (Comb-AltInput) (Appendix B.4.6)
   keep the number of KDF applications the same (though with potentially
   longer inputs).

   *Extensibility.* (Comb-AltInput) (Appendix B.4.6) changes the use of
   an existing input, which might conflict with other future changes to
   the use of the input.

   *More than 2 component algorithms.* The techniques in (Comb-Concat)
   (Appendix B.4.1) and (Comb-Chain) (Appendix B.4.5) can naturally
   accommodate more than 2 component shared secrets since there is no
   distinction to how each shared secret is treated.  (Comb-AltInput)
   (Appendix B.4.6) would have to make some distinct, since the 2
   component shared secrets are used in different ways; for example, the
   first shared secret is used as the "IKM" input in the 2nd "HKDF-
   Extract" call, and all subsequent shared secrets are concatenated to
   be used as the "IKM" input in the 3rd "HKDF-Extract" call.

Authors' Addresses

   Douglas Stebila
   University of Waterloo

   Email: dstebila@uwaterloo.ca


   Scott Fluhrer
   Cisco Systems

   Email: sfluhrer@cisco.com


   Shay Gueron
   University of Haifa and Amazon Web Services

   Email: shay.gueron@gmail.com





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