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Internet Engineering Task Force                              M. Campagna
Internet-Draft                                               E. Crockett
Intended status: Experimental Protocol                               AWS
Expires: January 12, 2021                                  July 06, 2020

  Hybrid Post-Quantum Key Encapsulation Methods (PQ KEM) for Transport
                        Layer Security 1.2 (TLS)
                 draft-campagna-tls-bike-sike-hybrid-04

Abstract

   Hybrid key exchange refers to executing two independent key exchanges
   and feeding the two resulting shared secrets into a Pseudo Random
   Function (PRF), with the goal of deriving a secret which is as secure
   as the stronger of the two key exchanges.  This document describes
   new hybrid key exchange schemes for the Transport Layer Security 1.2
   (TLS) protocol.  The key exchange schemes are based on combining
   Elliptic Curve Diffie-Hellman (ECDH) with a post-quantum key
   encapsulation method (PQ KEM) using the existing TLS PRF.

Context

   This draft is experimental.  It is intended to define hybrid key
   exchanges in sufficient detail to allow independent experimentations
   to interoperate.  While the NIST standardization process is still a
   few years away from being complete, we know that many TLS users have
   highly sensitive workloads that would benefit from the speculative
   additional protections provided by quantum-safe key exchanges.  These
   key exchanges are likely to change through the standardization
   process.  Early experiments serve to understand the real-world
   performance characteristics of these quantum-safe schemes as well as
   provide speculative additional confidentiality assurances against a
   future adversary with a large-scale quantum computer.

   Comments are solicited and can be sent to all authors at
   mcampagna@amazon.com.

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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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 January 12, 2021.

Copyright Notice


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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://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.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Key Exchange Algorithms  . . . . . . . . . . . . . . . . . . .  4
     2.1.  Key Encapsulation Method (KEM) . . . . . . . . . . . . . .  5
     2.2.  ECDHE_[KEM]  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Hybrid Premaster Secret  . . . . . . . . . . . . . . . . . . .  6
   4.  TLS Extension for Supported PQ KEM Parameters  . . . . . . . .  6
   5.  Data Structures and Computations . . . . . . . . . . . . . . .  7
     5.1.  Client Hello Extensions  . . . . . . . . . . . . . . . . .  7
       5.1.1.  When these extensions are sent . . . . . . . . . . . .  7
       5.1.2.  Meaning of these extensions  . . . . . . . . . . . . .  7
       5.1.3.  Structure of these extensions  . . . . . . . . . . . .  7
       5.1.4.  Actions of the sender  . . . . . . . . . . . . . . . .  7
       5.1.5.  Actions of the receiver  . . . . . . . . . . . . . . .  7
       5.1.6.  Supported PQ KEM Parameters Extension  . . . . . . . .  8
     5.2.  Server Key Exchange  . . . . . . . . . . . . . . . . . . .  9
       5.2.1.  When this message is sent  . . . . . . . . . . . . . .  9
       5.2.2.  Meaning of this message  . . . . . . . . . . . . . . .  9
       5.2.3.  Structure of this message  . . . . . . . . . . . . . .  9
       5.2.4.  Actions of the sender  . . . . . . . . . . . . . . . . 10
       5.2.5.  Actions of the receiver  . . . . . . . . . . . . . . . 11
     5.3.  Client Key Exchange  . . . . . . . . . . . . . . . . . . . 11
       5.3.1.  When this message is sent  . . . . . . . . . . . . . . 11
       5.3.2.  Meaning of the message . . . . . . . . . . . . . . . . 11
       5.3.3.  Structure of this message  . . . . . . . . . . . . . . 11
       5.3.4.  Actions of the sender  . . . . . . . . . . . . . . . . 12
       5.3.5.  Actions of the receiver  . . . . . . . . . . . . . . . 12
     5.4.  Derivation of the master secret for hybrid key agreement . 12
   6.  Cipher Suites  . . . . . . . . . . . . . . . . . . . . . . . . 13

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   7.  Security Considerations [DRAFT]  . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . . 15
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  0
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   Quantum-safe (or post-quantum) key exchanges are being developed in
   order to provide secure key establishment against an adversary with
   access to a quantum computer.  Under such a threat model, the current
   key exchange mechanisms would be vulnerable.  BIKE [BIKEr2],  Kyber
   [KYBERr2] and SIKE [SIKEr2] are post-quantum candidates which were
   submitted to the NIST Call for Proposals for Post-Quantum
   Cryptographic Schemes.  While these schemes are still being analyzed
   as part of that process, there is already a need to protect the
   confidentiality of today's TLS connections against a future adversary
   with a quantum computer.  Hybrid key exchanges are designed to
   provide two parallel key exchanges: one which is classical (e.g.,
   ECDHE) and the other which is quantum-safe (e.g., SIKE).  The hybrid
   schemes we propose are at least as secure as ECDH against a classical
   adversary, and at least as secure as the PQ KEM against a quantum
   adversary.  This strategy is emerging as a method to speculatively
   provide additional security to existing protocols.

   This document describes additions to TLS to support PQ Hybrid Key
   Exchanges, applicable to TLS Version 1.2 [RFC5246].  These additions
   are designed to support most of the second-round candidates in the
   NIST Call for Proposals, but this document only defines ciphersuites
   for a small subset of possible hybrid key agreement methods.  In
   particular, it defines the use of the ECDHE together with BIKE, Kyber
   or SIKE, as a hybrid key agreement method.

   The remainder of this document is organized as follows.  [kexalgs]
   provides an overview of PQ KEM-based key exchange algorithms for TLS.
   [hybridpms] describes how a PQ KEM can be combined with ECDHE to form
   a premaster secret.  In [tlsexts], we present a TLS extension that
   allow a client to negotiate the use of specific PQ schemes and
   parameters.  [structs] specifies various data structures needed for a
   BIKE-, Kyber- or SIKE-based hybrid key exchange handshake, their
   encoding in TLS messages, and the processing of those messages.
   [ciphersuites] defines two new PQ KEM hybrid-based cipher suites and
   identifies a small subset of these as recommended for all
   implementations of this specification.  [security] discusses some
   security considerations.  [iana] describes IANA considerations for
   the name spaces created by this document.  [ack] gives
   acknowledgments.








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   Implementation of this specification requires familiarity with TLS
   [RFC5246], BIKE [BIKEr2], Kyber [KYBERr2], and SIKE [SIKEr2].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Key Exchange Algorithms

   This document introduces two new hybrid-based key exchange methods
   for TLS.  They use ECDHE with either BIKE, Kyber or SIKE in order to
   compute the TLS premaster secret.  The master secret derivation is
   augmented to include the ClientKeyExchange message.  The derivation
   of the encryption/MAC keys and initialization vectors is independent
   of the key exchange algorithm and not impacted by the introduction of
   these hybrid key exchanges.  While this specification only defines
   the use of a PQ KEM hybrid key exchange with BIKE, Kyber or SIKE, it
   is specifically designed so that it can be easily extended to include
   additional PQ KEM methods.

   The table below summarizes the new hybrid key exchange schemes.

         +---------------------------------+------------------+
         | Hybrid Key Exchange Scheme Name | Description      |
         +---------------------------------+------------------+
         | ECDHE_BIKE                      | ECDHE and BIKE.  |
         | ECDHE_KYBER                     | ECDHE and Kyber. |
         | ECDHE_SIKE                      | ECDHE and SIKE.  |
         +---------------------------------+------------------+

   These schemes are intended to provide quantum-safe forward secrecy.

    Client                                        Server
    ------                                        ------

    ClientHello          -------->
                                             ServerHello
                                             Certificate
                                       ServerKeyExchange
                                    CertificateRequest*+
                         <--------       ServerHelloDone
    Certificate*+
    ClientKeyExchange
    CertificateVerify*+
    [ChangeCipherSpec]
    Finished             -------->
                                      [ChangeCipherSpec]
                         <--------              Finished

    Application Data     <------->      Application Data

         * message is not sent under some conditions



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         + message is not sent unless client authentication
           is desired

   [hybrid_protocol] shows the messages involved in the TLS key
   establishment protocol (aka full handshake).  The addition of hybrid
   key exchanges has direct impact on the ClientHello, the ServerHello,
   the ServerKeyExchange, and the ClientKeyExchange messages.  Next, we
   describe each hybrid key exchange scheme in greater detail in terms
   of the content and processing of these messages.  For ease of
   exposition, we defer discussion of the optional extension for
   specifying the parameters supported by an implementation until
   [tlsexts].

2.1.  Key Encapsulation Method (KEM)

   A key encapsulation mechanism (KEM) is a set of three algorithms

   o  key generation (KeyGen)

   o  encapsulation (Encaps)

   o  decapsulation (Decaps)

   and a defined key space, where

   o  "KeyGen()": returns a public and a secret key (pk, sk).

   o  "Encaps(pk)": takes pk as input and outputs ciphertext c and a key
      K from the key space.

   o  "Decaps(sk, c)": takes sk and c as input, and returns a key K or
      ERROR.  K is called the session key.

   The security of a KEM is discussed in [security].  BIKE, Kyber and
   SIKE are KEMs.

2.2.  ECDHE_[KEM]

   This section describes the nearly identical hybrid key exchanges
   ECDHE_BIKE, ECDHE_KYBER and ECDHE_SIKE.  For the remainder of this
   section [KEM] refers to either BIKE, Kyber or SIKE.  The server sends
   its ephemeral ECDH public key and an ephemeral [KEM] public key
   generated using the corresponding curve and [KEM] parameters in the
   ServerKeyExchange message.  This specification requires that these
   parameters MUST be signed using a signature algorithm corresponding
   to the public key in the server's certificate.

   The client generates an ECDHE key pair on the same curve as the
   server's ephemeral ECDH key, and computes a ciphertext value based on
   the [KEM] public key provided by the server, and sends them in the
   ClientKeyExchange message.  The client computes and holds the PQ KEM-
   encapsulated key (K) as a contribution to the premaster secret.





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   Both client and server perform an ECDH operation and use the
   resultant shared secret (Z) as part of the premaster secret.  The
   server computes the PQ KEM decapsulation routine to compute the
   encapsulated key (K), or to produce an error message in case the
   decapsulation fails.

3.  Hybrid Premaster Secret

   This section defines the mechanism for combining the ECDHE and [KEM]
   secrets into a TLS 1.2 [RFC5246] pre-master secret.  In the hybrid
   key exchange, both the server and the client compute two shared
   secrets: the previously defined ECDHE shared secret Z from RFC 8422
   [RFC8422], and another shared secret K from the underlying PQ key
   encapsulation method.

   Form the premaster secret for ECDHE_[KEM] hybrid key exchanges as the
   concatenation of the ECDHE shared secret Z with the KEM key K to form
   the opaque data value "premaster_secret = Z || K".

4.  TLS Extension for Supported PQ KEM Parameters

   A new TLS extension for post-quantum key encapsulation methods is
   defined in this specification.

   This allows negotiating the use of specific PQ KEM parameter sets
   during a handshake starting a new session.  The extension is
   especially relevant for constrained clients that may only support a
   limited number of PQ KEM parameter sets.  They follow the general
   approach outlined in RFC 5246 [RFC5246]; message details are
   specified in [structs].  The client enumerates the BIKE, Kyber and
   SIKE parameters it supports by including the PQ KEM extension in its
   ClientHello message.

   A TLS client that proposes PQ KEM cipher suites in its ClientHello
   message SHOULD include this extension.  Servers implementing a PQ KEM
   cipher suite MUST support this extension, and when a client uses this
   extension, servers MUST NOT negotiate the use of a PQ KEM parameter
   set unless they can complete the handshake while respecting the
   choice of parameters specified by the client.  This eliminates the
   possibility that a negotiated hybrid handshake will be subsequently
   aborted due to a client's inability to deal with the server's PQ KEM
   key.

   The client MUST NOT include the PQ KEM extension in the ClientHello
   message if it does not propose any PQ KEM cipher suites.
   Additionally, the client MUST NOT include parameters in the PQ KEM
   extension for PQ KEM cipher suites it does not propose.  That is, if
   a client does not support BIKE, it must not include the BIKE
   parameters in the extension, similarly for Kyber and SIKE.  A client
   that proposes a PQ KEM scheme may choose not to include this
   extension.  In this case, the server is free to choose any one of the
   parameter sets listed in [structs].  That section also describes the
   structure and processing of this extension in greater detail.



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   In the case of session resumption, the server simply ignores the
   Supported PQ KEM Parameters extension appearing in the current
   ClientHello message.  These extensions only play a role during
   handshakes negotiating a new session.

5.  Data Structures and Computations

   This section specifies the data structures and computations used by
   PQ KEM hybrid-key agreement mechanisms specified in Sections 2
   [kexalgs], 3 [hybridpms], and 4 [tlsexts].  The presentation language
   used here is the same as that used in TLS 1.2 [RFC5246].

5.1.  Client Hello Extensions

   This section specifies the Supported PQ KEM Parameters extension that
   can be included with the ClientHello message as described in RFC 5246
   [RFC5246].

5.1.1.  When these extensions are sent

   The extensions SHOULD be sent along with any ClientHello message that
   proposes the associated PQ KEM cipher suites.

5.1.2.  Meaning of these extensions

   These extensions allow a client to enumerate the PQ KEM parameters
   sets it supports for any supported PQ KEM.

5.1.3.  Structure of these extensions

   The general structure of TLS extensions is described in RFC 5246
   [RFC5246], and this specification adds a new type to ExtensionType.

   enum {
       pq_kem_parameters(0xFE01)
     } ExtensionType;

   where

   o  "pq_kem_parameters" (Supported PQ KEM Parameters extension):
      Indicates the set of post-quantum KEM parameters supported by the
      client.  For this extension, the opaque extension_data field
      contains PQKEMParametersExtension.  See [pqkemxts] for details.

5.1.4.  Actions of the sender

   A client that proposes PQ KEM hybrid key exchange cipher suites in
   its ClientHello message appends these extensions (along with any
   others), enumerating the parameters it supports.  Clients SHOULD send
   the PQ KEM parameter sets it supports if it supports PQ KEM hybrid
   key exchange cipher suites.

5.1.5.  Actions of the receiver



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   A server that receives a ClientHello containing this extension MUST
   use the client's enumerated capabilities to guide its selection of an
   appropriate cipher suite.  One of the proposed PQ KEM cipher suites
   must be negotiated only if the server can successfully complete the
   handshake while using the PQ KEM parameters supported by the client
   (cf.  [pqkemxts].)

   If a server does not understand the Supported PQ KEM Parameters
   extension, or is unable to complete the PQ KEM handshake while
   restricting itself to the enumerated parameters, it MUST NOT
   negotiate the use of the corresponding PQ KEM cipher suite.
   Depending on what other cipher suites are proposed by the client and
   supported by the server, this may result in a fatal handshake failure
   alert due to the lack of common cipher suites.

5.1.6.  Supported PQ KEM Parameters Extension

   This section defines the contents of the Supported PQ KEM Parameters
   extension.  In the language of RFC 5246 [RFC5246], the
   "extension_data" is the "PQKEMParametersExtension" type defined
   below.

   enum {
       BIKE1-L1-R1 (1),
       BIKE1-L3-R1 (2),
       BIKE1-L5-R1 (3),
       BIKE2-L1-R1 (4),
       BIKE2-L3-R1 (5),
       BIKE2-L5-R1 (6),
       BIKE3-L1-R1 (7),
       BIKE3-L3-R1 (8),
       BIKE3-L5-R1 (9),
       SIKE-P503-R1 (10),
       SIKE-P751-R1 (11),
       SIKE-p964-R1 (12),
       BIKE1-CCA-L1-R2 (13),
       BIKE1-CCA-L3-R2 (14),
       BIKE1-CCA-L5-R2 (15),
       BIKE2-CCA-L1-R2 (16),
       BIKE2-CCA-L3-R2 (17),
       BIKE2-CCA-L5-R2 (18),
       SIKE-P434-R2 (19),
       SIKE-P503-R2 (20),
       SIKE-P610-R2 (21),
       SIKE-P751-R2 (22),
       KYBER-512-R2 (23),
       KYBER-512-90s-R2 (24)
     } NamedPQKEM (2^16-1);

   "BIKE1-L1-R1", etc: Indicates support of the corresponding BIKE
   parameters defined in BIKE Round 1 [BIKEr1], the round 1 candidate to
   the NIST Post-Quantum Cryptography Standardization Process.  (NIST
   PQC)



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   "BIKE1-CCA-L1-R2", etc: Indicates support of the corresponding BIKE
   CCA parameters defined in BIKE Round 2 [BIKEr2], the latest revision
   of the round 2 CCA candidate submitted to NIST PQC available at the
   time of this draft specification.

   "SIKE1-P503-R1", etc: Indicates support of the corresponding SIKE
   parameters defined in SIKE Round 1 [SIKEr1], the round 1 candidate to
   NIST PQC.

   "SIKE1-P434-R2", etc: Indicates support of the corresponding SIKE
   parameters defined in SIKE Round 2 [SIKEr2], the round 2 candidate to
   NIST PQC.

   "KYBER-512-R2", etc: Indicates support of the corresponding KYBER
   parameters defined in Kyber [KYBERr2], the round 2 candidate to NIST
   PQC.

   struct {
       NamedPQKEM pq_kem_parameters_list <1..2^16-1>
     } PQKEMParametersExtension;

   Items in "pq_kem_parameters_list" are ordered according to the
   client's preferences (favorite choice first).

   As an example, a client that only supports BIKE1-L1-R1 ( value 1 =
   0x0001), BIKE2-L1-R1 ( value 4 = 0x0004) and SIKE-P434-R2 ( value 19
   = 0x0013) and prefers to use SIKE-P434-R2 would include a TLS
   extension consisting of the following octets:

   FE 01 00 08 00 06 00 13 00 01 00 04

   Note that the first two octets (FE 01) indicate the extension type
   (Supported PQ KEM Parameters extension), the next two octets
   indicates the length of the extension in bytes (00 08), and the next
   two octets indicate the length of enumerated values in bytes (00 06).

5.2.  Server Key Exchange

5.2.1.  When this message is sent

   This message is sent when using an ECDHE_[KEM] hybrid key exchange
   algorithms.

5.2.2.  Meaning of this message

   This message is used to convey the server's ephemeral ECDH and [KEM]
   public keys to the client.

5.2.3.  Structure of this message

   struct {
       opaque public_key <1,...,2^24 - 1>;
     } PQKEMPublicKey;



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   public_key:  This is a byte string representation of the [KEM] public
   key following the conversion defined by the [KEM] implementation.

   struct {
       NamedPQKEM      named_params;
       PQKEMPublicKey  public;
     } ServerPQKEMParams;

   The ServerKeyExchange message is extended as follows:

   struct {
       ServerECDHParams      ecdh_params;
       ServerPQKEMParams     pq_kem_params;
       Signature             signed_params;
     } ServerKeyExchange;

   where

   o  "ecdh_params":   Specifies the ECDHE public key and associated
      domain parameters.

   o  "pq_kem_params":   Specifies the [KEM] public key and associated
      parameters.

   o  "signed_params":   a signature over the server's key exchange
      parameters.  Note that only ciphersuites which include a signature
      algorithm are supported; see [ciphersuites].  The private key
      corresponding to the certified public key in the server's
      Certificate message is used for signing.

   digitally-signed struct {
       opaque              client_random[32];
       opaque              server_random[32];
       ServerDHParams      ecdh_params;
       ServerPQKEMParams   pq_kem_params;
     } Signature;

   The parameters are hashed as part of the signing algorithm as
   follows, where H is the hash function used for generating the
   signature:

   For ECDHE_[KEM]:

      "H( client_random[32] + server_random[32] + ecdh_params +
      pq_kem_params)."

   NOTE: This specification only defines hybrid ciphersuites with RSA
   and ECDSA signatures.  See [RFC5246] and RFC 8422 [RFC8422],
   respectively, for details on their use in TLS 1.2.

5.2.4.  Actions of the sender





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   The server selects elliptic curve domain parameters and an ephemeral
   ECDH public key corresponding to these parameters according to RFC
   8422 [RFC8422].  The server SHOULD generate a fresh ephemeral ECDH
   key for each key exchange so that the hybrid key exchange scheme
   provides forward secrecy.  The server selects a PQ KEM parameter set,
   and uses "KeyGen()" for the corresponding parameters of BIKE Round 1
   [BIKEr1], BIKE Round 2 [BIKEr2], Kyber [KYBERr2], SIKE Round 1
   [SIKEr1], or SIKE Round 2 [SIKEr2] to generate an ephemeral public
   key pair.  The server MUST generate a fresh PQ KEM key for each key
   exchange.  A server that receives a Supported PQ KEM Parameters
   extension MUST use the client's enumerated capabilities to guide its
   selection of an appropriate cipher suite.  The server MUST NOT
   negotiate the use of a PQ KEM parameter set unless they can complete
   the handshake while respecting the choice of parameters specified by
   the client (cf.  [pqkemxts]).  If the client does not include the PQ
   KEM Parameters extension, the server is free to choose any one of the
   parameters listed in [pqkemxts].

   If a server is unable to complete the PQ KEM handshake while
   restricting itself to the enumerated parameters, it MUST NOT
   negotiate the use of the corresponding PQ KEM cipher suite.
   Depending on what other cipher suites are proposed by the client and
   supported by the server, this may result in a fatal handshake failure
   alert due to the lack of common cipher suites.

   After selecting a ciphersuite and appropriate parameters, the server
   conveys this information to the client in the ServerKeyExchange
   message using the format defined above.

5.2.5.  Actions of the receiver

   The client verifies the signature and retrieves the server's elliptic
   curve domain parameters and ephemeral ECDH public key and the [KEM]
   parameter set and public key from the ServerKeyExchange message.

   A possible reason for a fatal handshake failure is that the client's
   capabilities for handling elliptic curves and point formats are
   exceeded (see RFC 8422 [RFC8422]), the PQ KEM parameters are not
   supported (see [chello]), or the signature does not verify.

5.3.  Client Key Exchange

5.3.1.  When this message is sent

   This message is sent in all key exchange algorithms.  In the key
   exchanges defined in this document, it contains the client's
   ephemeral ECDH public key and the [KEM] ciphertext value.

5.3.2.  Meaning of the message

   This message is used to convey ephemeral data relating to the key
   exchange belonging to the client (such as its ephemeral ECDH public
   key and the [KEM] ciphertext value).

5.3.3.  Structure of this message

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   The TLS ClientKeyExchange message is extended as follows.

   struct {
       opaque ciphertext <1,..., 2^24 - 1>;
     } PQKEMCiphertext;

   where

   o  "ciphertext": This is a byte string representation of the PQ
      ciphertext of the KEM construction.  Since the underlying calling
      convention of the KEM API handles the ciphertext byte string
      directly it is sufficient to pass this as single byte string array
      in the protocol.

   struct {
       ClientECDiffieHellmanPublic    ecdh_public;
       PQKEMCiphertext                ciphertext;
     } ClientKeyExchange;

5.3.4.  Actions of the sender

   The client selects an ephemeral ECDH public key corresponding to the
   parameters it received from the server according to RFC 8422
   [RFC8422].  The client SHOULD generate a fresh ephemeral ECDH key for
   each key exchange so that the hybrid key exchange scheme provides
   forward secrecy.  Using the "Encaps(pk)" function corresponding to
   the PQ KEM and named parameters in ServerKeyExchange message, the
   client computes a [KEM] ciphertext.  It conveys this information to
   the server in the ClientKeyExchange message using the format defined
   above.

5.3.5.  Actions of the receiver

   The server retrieves the client's ephemeral ECDH public key and the
   [KEM] ciphertext from the ClientKeyExchange message and checks that
   it is on the same elliptic curve as the server's ECDHE key, and that
   the [KEM] ciphertexts conform to the domain parameters selected by
   the server.  The server uses the "Decaps(pk)" function corresponding
   to the PQ KEM and named parameters in ServerKeyExchange message to
   compute the KEM shared secret.

   In the case of BIKE and Kyber there is a decapsulation failure rate
   no greater than 10^(-7).  In the case of a decapsulation failure, an
   implementation MUST abort the handshake.

5.4.  Derivation of the master secret for hybrid key agreement

   This section defines a new hybrid master secret derivation.  It is
   defined under the assumption that we use the concatenated premaster
   secret defined in Section 3.1 [hybridpms].  Recall in this case the
   premaster_secret = Z || K, where Z it the ECDHE shared secret, and K
   is the KEM shared secret.

   We define the master secret as follows:


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   master_secret[48] = TLS-PRF(secret, label, seed)

   where

   o  "secret": the premaster_secret,

   o  "label": the string "hybrid master secret", and

   o  "seed": the concatenation of "ClientHello.random ||
      ServerHello.random || ClientKeyExchange"

6.  Cipher Suites

   The table below defines new hybrid key exchange cipher suites that
   use the key exchange algorithms specified in Section 2 [kexalgs].

   +----------------------------------------------------------------+
   | Ciphersuite                                                    |
   +----------------------------------------------------------------+
   | TLS_ECDHE_BIKE_ECDSA_WITH_AES_128_GCM_SHA256 = { 0xFF, 0x01 }  |
   | TLS_ECDHE_BIKE_ECDSA_WITH_AES_256_GCM_SHA384 = { 0xFF, 0x02 }  |
   | TLS_ECDHE_BIKE_RSA_WITH_AES_128_GCM_SHA256   = { 0xFF, 0x03 }  |
   | TLS_ECDHE_BIKE_RSA_WITH_AES_256_GCM_SHA384   = { 0xFF, 0x04 }  |
   | TLS_ECDHE_SIKE_ECDSA_WITH_AES_128_GCM_SHA256 = { 0xFF, 0x05 }  |
   | TLS_ECDHE_SIKE_ECDSA_WITH_AES_256_GCM_SHA384 = { 0xFF, 0x06 }  |
   | TLS_ECDHE_SIKE_RSA_WITH_AES_128_GCM_SHA256   = { 0xFF, 0x07 }  |
   | TLS_ECDHE_SIKE_RSA_WITH_AES_256_GCM_SHA384   = { 0xFF, 0x08 }  |
   | TLS_ECDHE_KYBER_ECDSA_WITH_AES_128_GCM_SHA256 = { 0xFF, 0x09 } |
   | TLS_ECDHE_KYBER_ECDSA_WITH_AES_256_GCM_SHA384 = { 0xFF, 0x0A } |
   | TLS_ECDHE_KYBER_RSA_WITH_AES_128_GCM_SHA256   = { 0xFF, 0x0B } |
   | TLS_ECDHE_KYBER_RSA_WITH_AES_256_GCM_SHA384   = { 0xFF, 0x0C } |
   +----------------------------------------------------------------+

   The key exchange method, signature algorithm, cipher, and hash
   algorithm for each of these cipher suites are easily determined by
   examining the name.  Ciphers and hash algorithms are defined in RFC
   5288 [RFC5288].

   It is recommended that any implementation of this specification
   include both of the following ciphersuites:

   o  TLS_ECDHE_BIKE_RSA_WITH_AES_256_GCM_SHA384   = { 0xFF, 0x04 }

   o  TLS_ECDHE_SIKE_RSA_WITH_AES_256_GCM_SHA384   = { 0xFF, 0x08 }

   using the parameters BIKE1-CCA-L1-R2 and SIKE-P434-R2.

7.  Security Considerations [DRAFT]

   The security considerations in TLS 1.2 [RFC5246] and RFC 8422
   [RFC8422] apply to this document as well.  In addition, as described
   in RFC 5288 [RFC5288] and RFC 5289 [RFC5289], these cipher suites may
   only be used with TLS 1.2 or greater.



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   The description of a KEM is provided in [kems].  The security of the
   KEM is defined through the indistinguishability against a chosen-
   plaintext (IND-CPA) and against a chosen-ciphertext (IND-CCA)
   adversary.  We are focused here on the IND-CPA security of the KEM.
   As a result, implementations MUST NOT use a KEM key more than once,
   as reusing keys with IND-CPA KEMs can result in chosen ciphertext
   attacks like the GJS attack against BIKE [GJS].

   In the IND-CPA experiment of KEMs, an oracle generates keys (sk, pk)
   with "KeyGen()", computes (c, K) with "Encaps(pk)", and draws
   uniformly at random a value R from the key space, and a random bit b.
   The adversary is an algorithm A that is given (pk, c, K) if b=1, and
   (pk, c, R) if b=0.  Algorithm A outputs a bit b' as a guess for b,
   and wins if b' = b.

   All of the ciphersuites described in this document are intended to
   provide forward secrecy.  The hybrid key exchange mechanism described
   in this specification achieves forward secrecy when all ephemeral
   keys are single-use.  This specification requires single-use PQ KEM
   keys, so ephemeral ECDH keys SHOULD also be single-use so that
   forward secrecy is achieved.

8.  IANA Considerations

   This document describes three new name spaces for use with the TLS
   protocol:

9.  Acknowledgements

   This specification is based on ideas discussed with Ian Goldberg,
   Michele Mosca, Douglas Stebila and William Whyte during preparations
   for the first ETSI-IQC Quantum Safe Cryptography Workshop in 2013.
   The specification was developed through collaboration on the open
   source s2n project with Nicholas Allen, Nir Drucker, Shay Gueron,
   Andrew Hopkins, Colm MacCarthaigh and Alex Weibel.

10.  References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997, <https://www.rfc-editor.org/info/
              rfc2119>.

   [RFC8422]  Nir, Y., Josefsson, S. and M. Pegourie-Gonnard, "Elliptic
              Curve Cryptography (ECC) Cipher Suites for Transport Layer
              Security (TLS) Versions 1.2 and Earlier", RFC 8422, DOI
              10.17487/RFC8422, August 2018, <https://www.rfc-editor.org
              /info/rfc8422>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008, <https://www.rfc-editor.org/info/
              rfc5246>.



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   [RFC5288]  Salowey, J., Choudhury, A. and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, DOI
              10.17487/RFC5288, August 2008, <https://www.rfc-editor.org
              /info/rfc5288>.

   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with
              SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              DOI 10.17487/RFC5289, August 2008, <https://www.rfc-
              editor.org/info/rfc5289>.

   [GJS]      Guo, Q, Johansson, T and P Stankovski, "A Key Recovery
              Attack on MDPC with CCA Security Using Decoding Failures",
              2016, <https://eprint.iacr.org/2016/858.pdf>.

   [BIKEr1]   Misoczki, R, Aragon, N, Barreto, P, Bettaieb, S, Bidoux,
              L, Blazy, O, Deneuville, J, Gaborit, P, Gueron, S,
              Guneysu, T, Melchor, C, Persichetti, E, Sendrier, N,
              Tillich, J and G Zemor, "BIKE: Bit Flipping Key
              Encapsulation", November 2017, <http://http://
              bikesuite.org/files/BIKE.pdf>.

   [BIKEr2]   Misoczki, R, Aragon, N, Barreto, P, Bettaieb, S, Bidoux,
              L, Blazy, O, Deneuville, J, Gaborit, P, Gueron, S,
              Guneysu, T, Melchor, C, Persichetti, E, Sendrier, N,
              Tillich, J, Vasseur, V and G Zemor, "BIKE: Bit Flipping
              Key Encapsulation, version 3.2 ", February 2020, <https://
              bikesuite.org/files/round2/spec/BIKE-
              Spec-2020.02.07.1.pdf>.

   [KYBERr2]  Avanzi, R, Bos, J, Ducas, L, Kiltz, E, Lepoint, T,
              Lyubashevsky, V, Schanck, J, Schwabe, P, Seiler, G and D
              Stehle, "CRYSTALS-Kyber", March 2019, <https://
              csrc.nist.gov/CSRC/media/Projects/Post-Quantum-
              Cryptography/documents/round-2/submissions/CRYSTALS-Kyber-
              Round2.zip>.

   [SIKEr1]   Jao, D, Azarderakhsh, R, Campagna, M, Costello, C, De Feo,
              L, Hess, B, Jalali, A, Koziel, B, LaMacchia, B, Longa, P,
              Naehrig, M, Renes, J, Soukharev, V and D Urbanik,
              "Supersingular Isogeny Key Encapsulation", November 2017,
              <https://csrc.nist.gov/CSRC/media/Projects/Post-Quantum-
              Cryptography/documents/round-1/submissions/SIKE.zip>.

   [SIKEr2]   Jao, D, Azarderakhsh, R, Campagna, M, Costello, C, De Feo,
              L, Hess, B, Jalali, A, Koziel, B, LaMacchia, B, Longa, P,
              Naehrig, M, Pereira, G, Renes, J, Soukharev, V and D
              Urbanik, "Supersingular Isogeny Key Encapsulation", April
              2019, <https://csrc.nist.gov/CSRC/media/Projects/Post-
              Quantum-Cryptography/documents/round-2/submissions/SIKE-
              Round2.zip>.

Appendix A.  Additional Stuff

   This becomes an Appendix.


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Index

Authors' Addresses

   Matt Campagna
   AWS

   Email: campagna@amazon.com


   Eric Crockett
   AWS

   Email: ericcro@amazon.com











































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