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Hybrid key exchange in TLS 1.3

Document Type Active Internet-Draft (tls WG)
Authors Douglas Stebila , Scott Fluhrer , Shay Gueron
Last updated 2023-02-27
Replaces draft-stebila-tls-hybrid-design
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Network Working Group                                         D. Stebila
Internet-Draft                                    University of Waterloo
Intended status: Informational                                S. Fluhrer
Expires: 31 August 2023                                    Cisco Systems
                                                               S. Gueron
                                           U. Haifa, Amazon Web Services
                                                        27 February 2023

                     Hybrid key exchange in TLS 1.3


   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 or on the GitHub repository which contains
   the draft:

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

   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 31 August 2023.

Copyright Notice

   Copyright (c) 2023 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 (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Revision history  . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Motivation for use of hybrid key exchange . . . . . . . .   5
     1.4.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.5.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Key encapsulation mechanisms  . . . . . . . . . . . . . . . .   8
   3.  Construction for hybrid key exchange  . . . . . . . . . . . .   9
     3.1.  Negotiation . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Transmitting public keys and ciphertexts  . . . . . . . .  10
     3.3.  Shared secret calculation . . . . . . . . . . . . . . . .  11
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Defined Hybrid Groups . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Kyber version . . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  Details of kyber components . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Related work . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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.

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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

   *  draft-ietf-tls-hybrid-design-06:

      -  Bump to version -06 to avoid expiry

   *  draft-ietf-tls-hybrid-design-05:

      -  Define four hybrid key exchange methods

      -  Updates to reflect NIST's selection of Kyber

      -  Clarifications and rewordings based on working group comments

   *  draft-ietf-tls-hybrid-design-04:

      -  Some wording changes

      -  Remove design considerations appendix

   *  draft-ietf-tls-hybrid-design-03:

      -  Remove specific code point examples and requested codepoint
         range for hybrid private use

      -  Change "Open questions" to "Discussion"

      -  Some wording changes

   *  draft-ietf-tls-hybrid-design-02:

      -  Bump to version -02 to avoid expiry

   *  draft-ietf-tls-hybrid-design-01:

      -  Forbid variable-length secret keys

      -  Use fixed-length KEM public keys/ciphertexts

   *  draft-ietf-tls-hybrid-design-00:

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      -  Allow key_exchange values from the same algorithm to be reused
         across multiple KeyShareEntry records in the same ClientHello.

   *  draft-stebila-tls-hybrid-design-03:

      -  Add requirement for KEMs to provide protection against key

      -  Clarify FIPS-compliance of shared secret concatenation method.

   *  draft-stebila-tls-hybrid-design-02:

      -  Design considerations from draft-stebila-tls-hybrid-design-00
         and draft-stebila-tls-hybrid-design-01 are moved to the

      -  A single construction is given in the main body.

   *  draft-stebila-tls-hybrid-design-01:

      -  Add (Comb-KDF-1) and (Comb-KDF-2) options.

      -  Add two candidate instantiations.

   *  draft-stebila-tls-hybrid-design-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:

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

   *  "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"

   "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

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   long as at least one of the component key exchange algorithms remains
   unbroken.  When one of the algorithms is traditional and one of them
   is postquantum, this is a Post-Quantum Traditional Hybrid Scheme
   [I-D.driscoll-pqt-hybrid-terminology]; while this is the initial use
   case for this draft, we do not limit this draft to that case.  We use
   the term "component" algorithms to refer to the algorithms combined
   in a hybrid key exchange.

   We note that some authors prefer the phrase "composite" to refer to
   the use of multiple algorithms, to distinguish from "hybrid public
   key encryption" in which a key encapsulation mechanism and data
   encapsulation mechanism are combined to create public key encryption.

   It is intended that the composite algorithms within a hybrid key
   exchange are to be performed, that is, negotiated and transmitted,
   within the TLS 1.3 handshake.  Any out-of-band method of exchanging
   keying material is considered out-of-scope.

   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 desired to
   mitigate risks 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
   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.

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   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 after next-generation algorithms are
   defined, and for a period of time thereafter, conservative users may
   not have full confidence in some algorithms.

   Some users may want to accelerate adoption of post-quantum
   cryptography due to the threat of retroactive decryption: if a
   cryptographic assumption is broken due to the advent of a quantum
   computer or some other cryptanalytic breakthrough, confidentiality of
   information can be broken retroactively by any adversary who has
   passively recorded handshakes and encrypted communications.  Hybrid
   key exchange enables potential security against retroactive
   decryption while not fully abandoning traditional cryptosystems.

   As such, there may be users for whom hybrid key exchange is an
   appropriate step prior to an eventual transition to next-generation
   algorithms.  Users should consider the confidence they have in each
   hybrid component to assess that the hybrid system meets the desired

1.4.  Scope

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

   *  Selecting which next-generation algorithms to use in TLS 1.3, or
      algorithm identifiers or 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].

   *  Authentication using next-generation algorithms.  While quantum
      computers could retroactively decrypt previous sessions, 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.

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

   *  *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-

      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-

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

   *  *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 [PST] for preliminary results about
      performance characteristics.

   *  *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.

      -  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 substantial.  See [PST] 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.

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   *  *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.

   *  *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

   This document models key agreement as key encapsulation mechanisms
   (KEMs), which consist of three algorithms:

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

   *  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.

   *  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 other 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].

   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 [HPKE] for more details of such Diffie--Hellman-based
   key encapsulation mechanisms.  Diffie--Hellman key exchange, when

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   viewed as a KEM, does not formally satisfy IND-CCA2 security, but is
   still safe to use for ephemeral key exchange in TLS 1.3, see e.g.

   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
   the manner described in this document MUST explicitly be designed to
   be secure in the event that the public key is reused.  Finite-field
   and elliptic-curve Diffie--Hellman key exchange methods used in TLS
   1.3 satisfy this criteria.  For generic KEMs, this means satisfying
   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 or more algorithms.  For example, a future
   document could specify that one identifier corresponds to
   secp256r1+Kyber512, and another corresponds to x25519+Kyber512.  (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.

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       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 */
             x25519_kyber768(TBD), secp384r1_kyber768(TBD),
             x25519_kyber512(TBD), secp256r1_kyber512(TBD), ...,

             /* Reserved Code Points */
       } NamedGroup;

3.2.  Transmitting public keys and ciphertexts

   We take the relatively simple "concatenation approach": the messages
   from the two or more algorithms being hybridized will be concatenated
   together and transmitted as a single value, to avoid having to change
   existing data structures.  The values are directly concatenated,
   without any additional encoding or length fields; this assumes that
   the representation and length of elements is fixed once the algorithm
   is fixed.  If concatenation were to be used with values that are not
   fixed-length, a length prefix or other unambiguous encoding must be
   used to ensure that the composition of the two values is injective
   and requires a mechanism different from that specified in this

   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;

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

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       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 concatenation of the key_exchange field for each of the
   constituent algorithms.  The order of shares in the concatenation is
   the same as the order of algorithms indicated in the definition of
   the NamedGroup.

   For the client's share, the key_exchange value contains the
   concatenation of 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 value contains
   concatenation of 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

   [TLS13] requires that ``The key_exchange values for each
   KeyShareEntry MUST be generated independently.'' In the context of
   this document, since the same algorithm may appear in multiple named
   groups, we relax the above requirement to allow the same key_exchange
   value for the same algorithm to be reused in multiple KeyShareEntry
   records sent in within the same ClientHello.  However, key_exchange
   values for different algorithms MUST be generated independently.

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.  Again, we do not add any
   additional structure (length fields) in the concatenation procedure:
   for both the traditional groups and Kyber, the shared secret output
   length is fixed for a specific elliptic curve or parameter set.

   In other words, the shared secret is calculated as

       concatenated_shared_secret = shared_secret_1 || shared_secret_2

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

                         PSK ->  HKDF-Extract = Early Secret
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                 Derive-Secret(., "derived", "")
   concatenated_shared_secret -> HKDF-Extract = Handshake Secret
   ^^^^^^^^^^^^^^^^^^^^^^^^^^          |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                 Derive-Secret(., "derived", "")
                            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).  [NIST-SP-800-56C] lists
   simple concatenation as an approved method for generation of a hybrid
   shared secret in which one of the constituent shared secret is from
   an approved method.

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4.  Discussion

   *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.
   However, all defined parameter sets for Kyber have public keys and
   ciphertexts that fall within the TLS constraints.

   *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+kyber512" and
   "x25519+kyber512", it would end up sending two kyber512 public keys,
   since the KeyShareEntry for each combination contains its own copy of
   a kyber512 key.  This duplication may be more problematic for post-
   quantum algorithms which have larger public keys.  On the other hand,
   if the client wants to offer, for example "secp256r1+kyber512" and
   "secp256r1" (for backwards compatibility), there is relatively little
   duplicated data (as the secp256r1 keys are comparatively small).

   *Failures.* Some post-quantum key exchange algorithms, including
   Kyber, have non-zero probability of failure, meaning two honest
   parties may derive different shared secrets.  This would cause a
   handshake failure.  Kyber has a cryptographically small failure rate;
   if other algorithms are used, implementers should be aware of the
   potential of handshake failure.  Clients can retry if a failure is

5.  Defined Hybrid Groups

   This document defines four initial hybrids for use within TLS 1.3

   | Hybrid name        | Hybrid components   | Named Group |
   | x25519_kyber768    | x25519, kyber768    | TBD         |
   | secp384r1_kyber768 | secp384r1, kyber768 | TBD         |
   | x25519_kyber512    | x25519, kyber512    | TBD         |
   | secp256r1_kyber512 | secp256r1, kyber512 | TBD         |

   where the components x25519, secp384r1, secp256r1 are the existing
   named groups.

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   The intention is that the first two combinations (using kyber768) are
   for normal TLS sessions, while the latter two (using kyber512) are
   for sessions that have limits in record size or it is important to
   limit the total amount of communication.

5.1.  Kyber version

   For kyber512 and kyber768, this document refers to the same named
   parameter sets defined in the Round 3 submission of Kyber to NIST.
   That submission defines two variants for each parameter set based on
   the symmetric primitives used.  This document uses the FIPS 202
   varient (and not the "90s" varient); the FIPS 202 varient uses SHA-3
   and SHAKE as its internal symmetric primitives.

   The Kyber team has updated their documentation twice since submitting
   to Round 3 (these updates are labeled as version 3.0.1 and 3.0.2),
   however neither modifies the FIPS 202 variant of Kyber.

5.2.  Details of kyber components

   The listed kyber512, kyber768 components are the named parameter sets
   of the key exchange method kyber [Kyber].  When it is used, the
   client selects an ephemeral private key, generates the corresponding
   public key, and transmits that (as a component) within its keyshare.
   When the server receives this keyshare, it extracts the kyber public
   key, generates a ciphertext and shared secret.  It then transmits the
   ciphertext (as a component) within its keyshare.  When the client
   receives this keyshare, it extracts the kyber ciphertext, and uses
   its private key to generate the shared secret.  Both sides uses their
   copy of the shared secret as a component within the hybrid shared
   secret. where the client's key share is the Kyber public key, and the
   server's key share is the

6.  IANA Considerations

   IANA will assign identifiers from the TLS TLS Supported Groups
   section for the hybrid combinations defined in this document.  These
   assignments should be made in a range that is distinct from the
   Elliptic Curve Groups and the Finite Field Groups ranges.

7.  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

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   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 the manner described in this
   document MUST explicitly be designed to be secure in the event that
   the public key is reused, such as achieving IND-CCA2 security or
   having a transform like the Fujisaki--Okamoto transform applied.
   Kyber has such security properties.  However, some other post-quantum
   KEMs are designed to be IND-CPA-secure (i.e., without countermeasures
   such as the FO transform) are completely insecure under public key
   reuse; for example, some lattice-based IND-CPA-secure KEMs are
   vulnerable to attacks that recover the private key after just a few
   thousand samples [FLUHRER].

   *Public keys, ciphertexts, and secrets should be constant length.*
   This document assumes that the length of each public key, ciphertext,
   and shared secret is fixed once the algorithm is fixed.  This is the
   case for Kyber.

   Note that variable-length secrets are, generally speaking, dangerous.
   In particular, when using key material of variable length and
   processing it using hash functions, a timing side channel may arise.
   In broad terms, when the secret is longer, the hash function may need
   to process more blocks internally.  In some unfortunate
   circumstances, this has led to timing attacks, e.g. the Lucky
   Thirteen [LUCKY13] and Raccoon [RACCOON] attacks.

   Furthermore, [AVIRAM] identified a risk of using variable-length
   secrets when the hash function used in the key derivation function is
   no longer collision-resistant.

   Therefore, this specification MUST only be used with algorithms which
   have fixed-length shared secrets (after the variant has been fixed by
   the algorithm identifier in the NamedGroup negotiation in
   Section 3.1).

8.  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.  Daniel J.  Bernstein and Tanja Lange commented on the
   risks of reuse of ephemeral public keys.  Matt Campagna and the team
   at Amazon Web Services provided additional suggestions.  Nimrod

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   Aviram proposed restricting to fixed-length secrets.

9.  References

9.1.  Normative References

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

9.2.  Informative References

   [AVIRAM]   Nimrod Aviram, Benjamin Dowling, Ilan Komargodski, Kenny
              Paterson, Eyal Ronen, and Eylon Yogev, "[TLS] Combining
              Secrets in Hybrid Key Exchange in TLS 1.3", 1 September
              2021, <

   [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,

              "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,

   [CAMPAGNA] Campagna, M. and E. Crockett, "Hybrid Post-Quantum Key
              Encapsulation Methods (PQ KEM) for Transport Layer
              Security 1.2 (TLS)", Work in Progress, Internet-Draft,
              draft-campagna-tls-bike-sike-hybrid-07, 2 September 2021,

   [CECPQ1]   Braithwaite, M., "Experimenting with Post-Quantum
              Cryptography", 7 July 2016,

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   [CECPQ2]   Langley, A., "CECPQ2", 12 December 2018,

   [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., Fischlin, M., Günther, F., and D. Stebila, "A
              Cryptographic Analysis of the TLS 1.3 Handshake Protocol",
              Journal of Cryptology vol. 34, no. 4,
              DOI 10.1007/s00145-021-09384-1, July 2021,

   [ETSI]     Campagna, M., Ed. and others, "Quantum safe cryptography
              and security: An introduction, benefits, enablers and
              challengers", ETSI White Paper No. 8 , June 2015,

   [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,

              Housley, R., "TLS 1.3 Extension for Certificate-Based
              Authentication with an External Pre-Shared Key", RFC 8773,
              DOI 10.17487/RFC8773, March 2020,

   [FLUHRER]  Fluhrer, S., "Cryptanalysis of ring-LWE based key exchange
              with key share reuse", Cryptology ePrint Archive, Report
              2016/085 , January 2016,

   [FO]       Fujisaki, E. and T. Okamoto, "Secure Integration of
              Asymmetric and Symmetric Encryption Schemes", Journal of
              Cryptology vol. 26, no. 1, pp. 80-101,
              DOI 10.1007/s00145-011-9114-1, December 2011,

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   [FRODO]    Bos, J., Costello, C., Ducas, L., Mironov, I., Naehrig,
              M., Nikolaenko, V., Raghunathan, A., and D. Stebila,
              "Frodo: Take off the Ring! Practical, Quantum-Secure Key
              Exchange from LWE", Proceedings of the 2016 ACM SIGSAC
              Conference on Computer and Communications Security,
              DOI 10.1145/2976749.2978425, October 2016,

   [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,

   [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., Hövelmanns, 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. E., "The Transition from Classical to Post-
              Quantum Cryptography", Work in Progress, Internet-Draft,
              draft-hoffman-c2pq-07, 26 May 2020,

   [HPKE]     Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <>.

              D, F., "Terminology for Post-Quantum Traditional Hybrid
              Schemes", Work in Progress, Internet-Draft, draft-
              driscoll-pqt-hybrid-terminology-01, 20 October 2022,

              Tjhai, C., Tomlinson, M.,, Fluhrer, S.,
              Van Geest, D., Garcia-Morchon, O., and V. Smyslov,
              "Framework to Integrate Post-quantum Key Exchanges into
              Internet Key Exchange Protocol Version 2 (IKEv2)", Work in
              Progress, Internet-Draft, draft-tjhai-ipsecme-hybrid-qske-

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              ikev2-04, 9 July 2019,

   [IKE-PSK]  Fluhrer, S., Kampanakis, P., McGrew, D., and V. Smyslov,
              "Mixing Preshared Keys in the Internet Key Exchange
              Protocol Version 2 (IKEv2) for Post-quantum Security",
              RFC 8784, DOI 10.17487/RFC8784, June 2020,

   [KIEFER]   Kiefer, F. and K. Kwiatkowski, "Hybrid ECDHE-SIDH Key
              Exchange for TLS", Work in Progress, Internet-Draft,
              draft-kiefer-tls-ecdhe-sidh-00, 5 November 2018,

   [Kyber]    Roberto Avanzi, Joppe Bos, Léo Ducas, Eike Kiltz, Tancrède
              Lepoint, Vadim Lyubashevsky, John M Schanck, Peter
              Schwabe, Gregor Seiler, Damien Stehlé, "Crystals-Kyber
              NIST Round 3 submission", 1 October 2020,

   [LANGLEY]  Langley, A., "Post-quantum confidentiality for TLS", 11
              April 2018, <

   [LUCKY13]  Al Fardan, N. J. and K. G. Paterson, "Lucky Thirteen:
              Breaking the TLS and DTLS record protocols", n.d.,

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

   [NIST]     National Institute of Standards and Technology (NIST),
              "Post-Quantum Cryptography", n.d.,

              National Institute of Standards and Technology (NIST),
              "Recommendation for Existing Application-Specific Key
              Derivation Functions", December 2011,

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              National Institute of Standards and Technology (NIST),
              "Recommendation for Key-Derivation Methods in Key-
              Establishment Schemes", August 2020,

   [OQS-102]  Open Quantum Safe Project, "OQS-OpenSSL-1-0-2_stable",
              November 2018, <

   [OQS-111]  Open Quantum Safe Project, "OQS-OpenSSL-1-1-1_stable",
              January 2022, <

   [PST]      Paquin, C., Stebila, D., and G. Tamvada, "Benchmarking
              Post-quantum Cryptography in TLS", Post-Quantum
              Cryptography pp. 72-91, DOI 10.1007/978-3-030-44223-1_5,
              2020, <>.

   [RACCOON]  Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J.,
              Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and
              Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)",
              September 2020, <>.

   [S2N]      Amazon Web Services, "Post-quantum TLS now supported in
              AWS KMS", 4 November 2019,

   [SCHANCK]  Schanck, J. M. and D. Stebila, "A Transport Layer Security
              (TLS) Extension For Establishing An Additional Shared
              Secret", Work in Progress, Internet-Draft, draft-schanck-
              tls-additional-keyshare-00, 17 April 2017,

   [WHYTE12]  Schanck, J. M., Whyte, W., and Z. Zhang, "Quantum-Safe
              Hybrid (QSH) Ciphersuite for Transport Layer Security
              (TLS) version 1.2", Work in Progress, Internet-Draft,
              draft-whyte-qsh-tls12-02, 22 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", Work in Progress,
              Internet-Draft, draft-whyte-qsh-tls13-06, 3 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,

   [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 traditional 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:

   *  Internet-Drafts for TLS 1.2: [WHYTE12], [CAMPAGNA]

   *  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:

   *  Experimental implementations in TLS 1.2: [BCNS15], [CECPQ1],
      [FRODO], [OQS-102], [S2N]

   *  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.

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   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].

   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.

Authors' Addresses

   Douglas Stebila
   University of Waterloo

   Scott Fluhrer
   Cisco Systems

   Shay Gueron
   University of Haifa and Amazon Web Services

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