draft-tjhai-ipsecme-hybrid-qske-ikev2-03.xml

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<rfc submissionType="IETF" docName="draft-tjhai-ipsecme-hybrid-qske-ikev2-03" category="info"><?rfc compact="yes"?>
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	<front>
	<title abbrev="Hybrid PQKE for IKEv2">Framework to Integrate Post-quantum Key Exchanges into Internet Key Exchange Protocol Version 2 (IKEv2)</title>
	<author fullname="C. Tjhai" initials="C." surname="Tjhai">
	<organization>Post-Quantum</organization>
	<address><postal><street></street>
	</postal>
	<email>cjt@post-quantum.com</email>
	</address>
	</author>

	<author fullname="M. Tomlinson" initials="M." surname="Tomlinson">
	<organization>Post-Quantum</organization>
	<address><postal><street></street>
	</postal>
	<email>mt@post-quantum.com</email>
	</address>
	</author>

	<author fullname="G. Bartlett" initials="G." surname="Bartlett">
	<organization>Cisco Systems</organization>
	<address><postal><street></street>
	</postal>
	<email>grbartle@cisco.com</email>
	</address>
	</author>

	<author fullname="S. Fluhrer" initials="S." surname="Fluhrer">
	<organization>Cisco Systems</organization>
	<address><postal><street></street>
	</postal>
	<email>sfluhrer@cisco.com</email>
	</address>
	</author>

	<author fullname="D. Van Geest" initials="D." surname="Van Geest">
	<organization>ISARA Corporation</organization>
	<address><postal><street></street>
	</postal>
	<email>daniel.vangeest@isara.com</email>
	</address>
	</author>

	<author fullname="O. Garcia-Morchon" initials="O." surname="Garcia-Morchon">
	<organization>Philips</organization>
	<address><postal><street></street>
	</postal>
	<email>oscar.garcia-morchon@philips.com</email>
	</address>
	</author>

    <author fullname="Valery Smyslov" initials="V." surname="Smyslov">
    <organization>ELVIS-PLUS</organization>
    <address><postal><street></street>
    </postal>
    <email>svan@elvis.ru</email>
    </address>
    </author>

    <date/>
	<workgroup>Internet Engineering Task Force</workgroup>
	<abstract><t>
		This document describes how to extend Internet Key Exchange Protocol
		Version 2 (IKEv2) so that the shared secret exchanged between peers
		has resistance against quantum computer attacks.  The basic idea is
		to exchange one or more post-quantum key exchange payloads in
		conjunction with the existing (Elliptic Curve) Diffie-Hellman payload.
	</t></abstract>
	</front>

	<middle>
	<section title="Introduction" anchor="section-1">
		<section title="Problem Description" anchor="section-1.1"><t>
			Internet Key Exchange Protocol (IKEv2) as specified in RFC 7296
			<xref target="RFC7296"/> uses the Diffie-Hellman (DH) or Elliptic Curve
			Diffie-Hellman (ECDH) algorithm to establish a shared secret
			between an initiator and a responder.  The security of the DH and
			ECDH algorithms relies on the difficulty to solve a discrete logarithm
			problem in multiplicative and elliptic curve groups respectively when
			the order of the group parameter is large enough.  While solving such
			a problem remains difficult with current computing power, it is
			believed that general purpose quantum computers will be able to solve
			this problem, implying that the security of IKEv2 is compromised.
			There are, however, a number of cryptosystems that are conjectured to
			be resistant against quantum computer attack.  This family of
			cryptosystems are known as post-quantum cryptography (PQC).  It is
			sometimes also referred to as quantum-safe cryptography (QSC) or
			quantum-resistant cryptography (QRC).
		</t></section>

		<section title="Proposed Extension" anchor="section-1.2"><t>
			This document describes a framework to integrate QSC for IKEv2, while
			maintaining backwards compatibility, to derive a set of IKE keys that
			have resistance to quantum computer attacks.  Our framework allows the
			negotiation of one or more QSC algorithm to exchange data, in addition
			to the existing DH or ECDH key exchange data.  We believe that the
			feature of using more than one post-quantum algorithm is important as
			many of these algorithms are relatively new and there may be a need to
			hedge the security risk with multiple key exchange data from several
			distinct QSC algorithms.
	   </t>

		<t>
			The secrets established from each key exchange are combined in a way
			such that should the post-quantum secrets not be present, the derived
			shared secret is equivalent to that of the standard IKEv2; on the
			other hand, a post-quantum shared secret is obtained if both classical
			and post-quantum key exchange data are present.  This framework also
			applies to key exchanges in IKE Security Associations (SAs) for
			Encapsulating Security Payload (ESP) <xref target="RFC4303"/> or
			Authentication Header (AH) <xref target="RFC4302"/>, i.e. Child SAs,
			in order to provide a stronger guarantee of forward security.</t>

		<t>
			Some post-quantum key exchange payloads may have size larger than
			the standard MTU size, and therefore there could be issues with
			fragmentation at IP layer.  IKE does allow transmission over TCP
			where fragmentation is not an issue <xref target="RFC8229"/>;
			however, we believe that a UDP-based solution will be required
			too.  IKE does have a mechanism to handle fragmentation within
			UDP <xref target="RFC7383"/>, however that is only applicable to
			messages exchanged after the IKE_SA_INIT.  To use this mechanism,
			we use the INTERMEDIATE exchange as outlined in
			<xref target="I-D.smyslov-ipsecme-ikev2-aux"/>.  With this
			mechanism, we do an initial key exchange, using a smaller, possibly
			non-quantum resistant primitive, such as ECDH.  Then, before we do
			the IKE_AUTH exchange, we perform one or more INTERMEDIATE exchanges,
			each of which includes a secondary key exchange.  As the INTERMEDIATE
			exchange is encrypted, the IKE fragmentation protocol RFC7383
			can be used.  The IKE SK values will be updated after each exchange,
			and so the final IKE SK values will depend on all the key exchanges,
			hence they are secure if any of the key exchanges are secure.</t>

		<t>
			Note that readers should consider the approach in this document as
			providing a long term solution in upgrading the IKEv2 protocol to
			support post-quantum algorithms.  A short term solution to make IKEv2
			key exchange quantum secure is to use post-quantum pre-shared keys as
			discussed in <xref target="I-D.ietf-ipsecme-qr-ikev2"/>.</t>
		</section>

	<section title="Changes" anchor="section-1.3"><t>
   Changes in this draft in each version iterations.</t>

	 <t>draft-tjhai-ipsecme-hybrid-qske-ikev2-02</t>
	 <t><list style="symbols">
	 <t>Use new transform types to negotiate additional key exchanges,
	 rather than using the KE payloads of IKE SA.</t>
 	 </list></t>

	 <t>draft-tjhai-ipsecme-hybrid-qske-ikev2-01</t>

	 <t><list style="symbols">
 		<t>Use INTERMEDIATE to perform multiple key exchanges in succession.</t>

 		<t>Handle fragmentation by keeping the first key exchange (a standard
 			IKE_SA_INIT with a few extra notifies) small, and encrypting the rest
 			of the key exchanges.</t>

 		<t>Simplify the negotiation of the ‘extra’ key exchanges.</t>
 	</list></t>

	<t>draft-tjhai-ipsecme-hybrid-qske-ikev2-00</t>

	<t><list style="symbols">
		<t>We added a feature to allow more than one post-quantum key
      exchange algorithms to be negotiated and used to exchange a post-
      quantum shared secret.</t>
		<t>Instead of relying on TCP encapsulation to deal with IP level
		  fragmentation, we introduced a new key exchange payload that can
		  be sent as multiple fragments within IKE_SA_INIT message.</t>
	</list>
</t>

</section>

	<section title="Document Organization" anchor="section-1.4"><t>
   The remainder of this document is organized as follows.  <xref target="section-2"/>
   summarizes design criteria.  <xref target="section-3"/> describes how
	 post-quantum key exchange is performed between two IKE peers and how
	 keying materials are derived for both SAs and child SAs.  A summary of alternative
	 approaches that have been considered, but later discarded, are described
	 in <xref target="section-4"/>.  <xref target="section-5"/>
   discusses IANA considerations for the namespaces introduced in this
	 document, and lastly <xref target="section-6"/> discusses security considerations.</t>

    <t> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", 
    "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted 
    as described in BCP 14 <xref target="RFC2119" /> <xref target="RFC8174" /> when, and only when, 
    they appear in all capitals, as shown here.
    </t>
 	</section>
	</section>

	<section title="Design Criteria" anchor="section-2"><t>
   The design of the proposed post-quantum IKEv2 is driven by the
   following criteria:</t>

	<t><list style="hanging" hangIndent="5"><t hangText="1)">
	Need for post-quantum cryptography in IPsec.  Quantum computers
	<vspace blankLines="0"/>
	might become feasible in the next 5-10 years.  If current
        Internet communications are monitored and recorded today (D),
        the communications could be decrypted as soon as a quantum-
        computer is available (e.g., year Q) if key negotiation only
        relies on non post-quantum primitives.  This is a high threat
        for any information that must remain confidential for a long
        period of time T &gt; Q-D.  The need is obvious if we assume that Q
        is 2040, D is 2020, and T is 30 years.  Such a value of T is
        typical in classified or healthcare data.
	</t>

	<t hangText="2)">
	Hybrid.  Currently, there does not exist a post-quantum key
	<vspace blankLines="0"/>
	exchange that is trusted at the level that ECDH is trusted
        against conventional (non-quantum) adversaries.  A hybrid
        approach allows introducing promising post-quantum candidates
        next to well-established primitives, since the overall security
        is at least as strong as each individual primitive.
	</t>

	<t hangText="3)">
	Focus on quantum-resistant confidentiality. A passive attacker
	<vspace blankLines="0"/>
	can eavesdrop on IPsec communication today and decrypt it once a
        quantum computer is available in the future. This is a very
        serious attack for which we do not have a solution. An attacker
        can only perform active attacks such as impersonation of the
        communicating peers once a quantum computer is available,
        sometime in the future. Thus, our design focuses on quantum-
        resistant confidentiality due to the urgency of this problem.
        This document does not address quantum-resistant authentication
        since it is less urgent at this stage.
	</t>

	<t hangText="4)">
	Limit amount of exchanged data.  The protocol design should be
	<vspace blankLines="0"/>
	such that the amount of exchanged data, such as public-keys, is
        kept as small as possible even if initiator and responder need
        to agree on a hybrid group or multiple public-keys need to be
        exchanged.
	</t>

	<t hangText="5)">
	Future proof.  Any cryptographic algorithm could be potentially
	<vspace blankLines="0"/>
	broken in the future by currently unknown or impractical
        attacks: quantum computers are merely the most concrete example
        of this.  The design does not categorize algorithms as "post-quantum"
				or "non post-quantum" and does not create assumptions
        about the properties of the algorithms, meaning that if
        algorithms with different properties become necessary in the future,
        this framework can be used unchanged to facilitate migration to
        those algorithms.
	</t>

	<t hangText="6)">
	Limited amount of changes.  A key goal is to limit the number of
	<vspace blankLines="0"/>
	changes required when enabling a post-quantum handshake.  This
        ensures easier and quicker adoption in existing implementations.
	</t>

	<t hangText="7)">
	Localized changes.  Another key requirement is that changes to
	<vspace blankLines="0"/>
	the protocol are limited in scope, in particular, limiting
        changes in the exchanged messages and in the state machine, so
        that they can be easily implemented.
	</t>

	<t hangText="8)">
	Deterministic operation.  This requirement means that the hybrid
	<vspace blankLines="0"/>
	post-quantum exchange, and thus, the computed key, will be based
        on algorithms that both client and server wish to support.
	</t>

	<t hangText="9)">
	Fragmentation support.  Some PQC algorithms could be relatively
	<vspace blankLines="0"/>
	bulky and they might require fragmentation.  Thus, a design goal
        is the adaptation and adoption of an existing fragmentation
        method or the design of a new method that allows for the
        fragmentation of the key shares.
	</t>

	<t hangText="10)">
	Backwards compatibility and interoperability.  This is a
	<vspace blankLines="0"/>
	fundamental requirement to ensure that hybrid post-quantum IKEv2
        and a non-post-quantum IKEv2 implementations are interoperable.
	</t>

	<t hangText="11)">
	FIPS compliance.  IPsec is widely used in Federal Information
	<vspace blankLines="0"/>
	Systems and FIPS certification is an important requirement.
        However, algorithms that are believed to be post-quantum are not
        FIPS compliant yet.  Still, the goal is that the overall hybrid
        post-quantum IKEv2 design can be FIPS compliant.
	</t>

	</list>
	</t>

	</section>

	<section title="The Framework of Hybrid Post-Quantum Key Exchange" anchor="section-3">
		<section title="Overall design" anchor="section-3.1"><t>
			This design assigns new group identifiers (Transform Type 4) to the
			various post-quantum key exchanges (which will be defined later).  We
			specifically do not make a distinction between classical (DH and ECDH)
			and post-quantum key exchanges, nor post-quantum algorithms which are
			true key exchanges versus post-quantum algorithms that act as key
			transport mechanisms; all are treated equivalently by the
			protocol.  In order to support both hybrid key exchanges (that is,
			relying on distinct key exchanges) and fragmentation, the proposed
			hybrid post-quantum IKEv2 protocol extends IKE <xref target="RFC7296"/>
			by adding additional key exchange messages (INTERMEDIATE) between the
			IKE_SA_INIT and the IKE_AUTH exchanges.  In order to minimize
			communication overhead, only the key shares that are agreed to be used
			are actually exchanged.  In order to achieve this, the IKE_SA_INIT
			exchange now includes notify payloads that negotiate the extra key
			exchanges to be used.  The initiator IKE_SA_INIT message includes a
			notify that lists the extra key exchange policy required by the
			initiator; the responder selects one of the listed policies, and
			includes that as a notify in the response IKE_SA_INIT message.  Then,
			the initiator and the responder perform one (or possibly more) INTERMEDIATE
			exchange; each such exchange includes a KE payload for the key exchange
			that was negotiated.</t>

		<t>Here is an overview of the initial exchanges:</t>

  <figure><artwork><![CDATA[
     Initiator                                Responder
  --------------------------------------------------------
  <-- IKE_SA_INIT  (and extra key exchange negotiation) -->

  <-- {INTERMEDIATE (hybrid post-quantum key exchange)} -->
                             ...
  <-- {INTERMEDIATE (hybrid post-quantum key exchange)} -->

                     <-- {IKE_AUTH} -->
]]></artwork></figure>

	<t>The extra post-quantum key exchanges can use algorithms that are
		currently considered to be resistant to quantum computer attacks.  These
		algorithms are collectively referred to as post-quantum algorithms in
		this document.</t>

    <t> Most post-quantum key agreement algorithms are relatively new, and
        thus are not fully trusted.  There are also many proposed algorithms,
        with different trade-offs and relying on different hard problems.  The
        concern is that some of these hard problems may turn out to be easier
        to solve than anticipated (and thus the key agreement algorithm not be
        as secure as expected).  A hybrid solution allows us to deal with this
        uncertainty by combining a classical key exchanges with a post-quantum
        one, as well as leaving open the possibility of multiple post-quantum
        key exchanges.</t>

    <t>The method that we use to perform hybrid key exchange also addresses
        the fragmentation issue.  The initial IKE_INIT messages do not have any
        inherent fragmentation support within IKE; however that can include a
        relatively short KE payload (e.g. one for group 14, 19 or 31).  The
        rest of the KE payloads are encrypted within INTERMEDIATE messages; because
        they are encrypted, the standard IKE fragmentation solution
        <xref target="RFC7383"/> is available.</t>

	</section>

	<section title="Overall Protocol" anchor="section-3.2"><t>
		In the simplest case, the initiator is happy with a single key exchange
		(and has no interest in supporting multiple), and he is not concerned
		with possible fragmentation of the IKE_SA_INIT messages (either because
		the key exchange he selects is small enough not to fragment, or he is
		confident that fragmentation will be handled either by IP fragmentation,
		or transport via TCP).  In the following we overview the two protocol
		rounds involved in the hybrid post-quantum protocol.</t>

	<t>In this case, the initiator performs the IKE_SA_INIT as standard,
		inserting this preferred key exchange (which is possibly a post-quantum
		algorithm) as the listed Transform Type 4, and including the initiator
		KE payload.  If the responder accepts the policy, he responds with an
		IKE_SA_INIT response, and IKE continues as usual.</t>

	<t>If the initiator desires to negotiate multiple key exchanges, or he
		needs IKE to handle any possible fragmentation, then he uses the protocol
		listed below.</t>

    <section title="IKE_SA_INIT Round: Negotiation" >
        <t> Multiple key exchanges are negotiated using the standard IKEv2 mechanism, via SA payload.
        For this purpose several new transform types, namely Additional Key Exchange 1, Additional Key Exchange 2,
        Additional Key Exchange 3, etc., are defined. They are collectively called Additional Key Exchanges 
        and have slightly different semantics than existing IKEv2 transform types.
        They are interpreted as additional key exchanges that peers agreed to perform
        in a series of INTERMEDIATE exchanges. The possible transform IDs for these transform types 
        are the same as IDs for the transform type 4 (Diffie-Hellman Group), so they all share
        a single IANA registry for transform IDs.
        </t>
        <t> Key exchange method negotiated via transform type 4 MUST always take place
        in the IKE_SA_INIT exchange. Additional Key Exchanges negotiated via newly
        defined transforms MUST take place in series of INTERMEDIATE exchanges, in an order of the values of their transform types, 
        so that key exchange negotiated using transform type N always precedes that of
        transform type N + 1. Each INTERMEDIATE exchange MUST bear exactly one key exchange method. 
        Note that with this semantics, Additional Key Exchanges transforms are not associated
        with any particular type of key exchange and don't have any specific per transform type transform ID IANA registry. 
        Instead they all share a single registry for transform IDs - "Diffie-Hellman Group Transform IDs", as well as Transform Type 4. 
        All new key exchange algorithms (both classical or quantum safe) should be added to this registry.
        This approach gives peers flexibility in defining the ways they want 
        to combine different key exchange methods. 
        </t>
        <t> When forming a proposal the initiator adds transforms for the IKE_SA_INIT exchange
        using transform type 4. In most cases they will contain classical key exchange methods, however
        it is not a requirement. Additional key exchange methods are proposed using Additional Key Exchanges 
        transform types. All these transform types are optional, the initiator is free 
        to select any of them for proposing additional key exchange methods. Consequently, 
        if none of Additional Key Exchanges are included in the proposal, then this proposal
        indicates performing standard IKEv2, as defined in <xref target="RFC7296"/>.
        If the initiator includes any transform of type N (where N is among Additional Key Exchanges) in the proposal, 
        the responder MUST select one of the algorithms proposed using this type. A transform ID NONE 
        may be added to those transform types which contain key exchange methods that the initiator believes are optional.
        </t>
        <t> The responder performs negotiation using standard IKEv2 procedure described in Section 3.3 of <xref target="RFC7296"/>.
        However, for the Additional Key Exchange types the responder's choice MUST NOT contain equal transform IDs (apart from NONE), 
        and the ID selected for Transform Type 4 MUST NOT appear in any of Additional Key Exchange transforms.
        In other words, all selected key exchange methods must be different.
        </t>
    </section>

	<section title="INTERMEDIATE Round: Additional Key Exchanges" anchor="section-3.2.2"><t>
		For each extra key exchange agreed to in the IKE_SA_INIT exchange,
		the initiator and the responder perform an INTERMEDIATE exchange, as
		described in <xref target="I-D.smyslov-ipsecme-ikev2-aux"/>.</t>

   <t>This exchange is as follows:</t>
   <figure><artwork><![CDATA[
      Initiator                     Responder
      -------------------------------------------------
      HDR, SK {Ni2, KEi2}    -->
                             <--    HDR, SK {Nr2, KEr2}
   ]]></artwork></figure>

   <t>The initiator sends a nonce in the Ni2 payload, and the key exchange
		 payload in the KEi2; the group id of the KEi2 payload MUST match the
		 negotiated extra key exchange.  This packet is encrypted with the
		 current IKE SK keys.</t>

   <t>On receiving this, the responder sends a nonce in the Nr2 payload,
		 and the key exchange payload KEr2; again, this packet is encrypted
		 with the current IKE SA keys.</t>

   <t>Once this exchange is done, then both sides compute an updated
		 keying material:</t>
   <figure><artwork><![CDATA[
        SKEYSEED = prf(SK_d(old), KE2result | Ni2 | Nr2)
   ]]></artwork></figure>
   <t>where KE2result is the shared secret of the key exchange.  Then,
		 SK_d, SK_ai, SK_ar, SK_ei, SK_er, SK_pi, SK_pr are updated as:</t>
   <figure><artwork><![CDATA[
        {SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr}
                = prf+ (SKEYSEED, Ni2 | Nr2 | SPIi | SPIr)
   ]]></artwork></figure>

   <t>Note that the negotiated transform types (the encryption type,
		 hash type, prf type) are not modified.</t>

   <t>Both the initiator and the responder will use this updated key
		 values for the next message.</t>

	</section>

	<section title="IKE_AUTH Exchange" anchor="section-3.2.3"><t>
		After the INTERMEDIATE exchanges have completed, then the initiator and
		the responder will perform an IKE_AUTH exchange.  This exchange is
		the standard IKE exchange, except that the initiator and responder
		signed octets are modified as described in
		<xref target="I-D.smyslov-ipsecme-ikev2-aux"/>.</t>
	</section>

    <section title="CREATE_CHILD_SA Exchange">
        <t> The CREATE_CHILD_SA exchange is used in IKEv2 for the purpose 
        of creating additional Child SAs, rekeying them and rekeying IKE SA itself.
        When creating or rekeying Child SAs, the peers may optionally 
        perform a Diffie-Hellmann key exchange to add a fresh entropy into the session keys, 
        in case of IKE SA rekeying, the key exchange is mandatory.
        </t>
        <t> If the IKE SA was created using multiple key exchange methods, the peers 
        may want continue using multiple key exchanges in the CREATE_CHILD_SA exchange too.
        If the initiator includes any Additional Key Exchanges transform in the SA payload
        (along with Transform Type 4) and the responder agrees to perform additional
        key exchanges, then the additional key exchanges are performed in a series 
        of the INFORMATIONAL exchanges that follows the CREATE_CHILD_SA exchange
        in an order of the values of their transform types, so that 
        key exchange negotiated using transform type N always precedes key exchange negotiated using
        transform type N + 1. Each INFORMATIONAL exchange MUST bear exactly one key exchange method. 
        Key exchange negotiated via Transform Type 4 always takes place
        in the CREATE_CHILD_SA exchange, as per IKEv2 specification. 
        </t>
        <t> Since after IKE SA is created the window size may be greater than one, and multiple
        concurrent exchanges may be active, it is essential to link the INFORMATIONAL exchanges together and 
        with the CREATE_CHILD_SA exchange. A new status type notification ADDITIONAL_KEY_EXCHANGE
        is used for this purpose. Its Notify Message Type is &lt;TBA by IANA&gt;, Protocol ID and SPI Size 
        are both set to 0. The data associated with this notification is a blob meaningful 
        only to the responder, so that the responder can correctly link successive
        exchanges. For the initiator the content of this notification is an opaque blob.
        </t>
        <t> The responder MUST include this notification in a CREATE_CHILD_SA or 
        INFORMATIONAL response message in case next exchange is expected, filling it with
        some data that would allow linking this exchange to the next one. The initiator 
        MUST copy the received notification with its content intact into the request 
        message of the next exchange. 
        </t>

        <t> Below is an example of three additional key exchanges.
        </t>

        <figure><artwork><![CDATA[
Initiator                             Responder
-----------------------------------------------------------------------
HDR(CREATE_CHILD_SA), SK {SA, Ni, KEi} -->
                             <--  HDR(CREATE_CHILD_SA), SK {SA, Nr, KEr,
                                      N(ADDITIONAL_KEY_EXCHANGE)(link1)}

HDR(INFORMATIONAL), SK {Ni2, KEi2,
 N(ADDITIONAL_KEY_EXCHANGE)(link1)} -->
                                 <--  HDR(INFORMATIONAL), SK {Nr2, KEr2,
                                      N(ADDITIONAL_KEY_EXCHANGE)(link2)}

HDR(INFORMATIONAL), SK {Ni3, KEi3,
 N(ADDITIONAL_KEY_EXCHANGE)(link2)} -->
                                 <--  HDR(INFORMATIONAL), SK {Nr3, KEr3,
                                      N(ADDITIONAL_KEY_EXCHANGE)(link3)}

HDR(INFORMATIONAL), SK {Ni4, KEi4,
 N(ADDITIONAL_KEY_EXCHANGE)(link3)} -->
                                 <--  HDR(INFORMATIONAL), SK {Nr4, KEr4}
        ]]></artwork></figure>


    </section>
  </section>
  </section>

	<section title="Alternative Design" anchor="section-4"><t>
   This section gives an overview on a number of alternative approaches
   that we have considered, but later discarded.  These approaches are:</t>

	<t><list style="symbols">
		<t>Sending the classical and post-quantum key
		exchanges as a single transform<vspace blankLines="1"/>
		We considered combining the various key exchanges into a single large
		KE payload; this effort is documented in a previous version of this
		draft (draft-tjhai-ipsecme-hybrid-qske-ikev2-01).  This does allow us
		to cleanly apply hybrid key exchanges during the child SA; however it
		does add considerable complexity, and requires an independent
		fragmentation solution.
	</t>

  <t>Sending post-quantum proposals and policies in KE payload
		only<vspace blankLines="1"/>
		With the objective of not introducing unnecessary notify
		payloads, we considered communicating the hybrid post-quantum proposal
		in the KE payload during the first pass of the protocol
		exchange.   Unfortunately, this design is susceptible to the following
		downgrade attack.  Consider the scenario where there is an MitM attacker
		sitting between an initiator and a responder.  The initiator proposes,
		through SAi payload, to use a hybrid post-quantum group and as a backup
		a Diffie-Hellman group, and through KEi payload, the initiator proposes
		a list of hybrid post-quantum proposals and policies.  The MitM attacker
		intercepts this traffic and replies with N(INVALID_KE_PAYLOAD) suggesting
		to downgrade to the backup Diffie-Hellman group instead.  The initiator
		then resends the same SAi payload and the KEi payload containing the
		public value of the backup Diffie-Hellman group.  Note that the attacker
		may forward the second IKE_SA_INIT message only to the responder, and
		therefore at this point in time, the responder will not have the
		information that the initiator prefers the hybrid group.  Of course,
		it is possible for the responder to have a policy to reject an
		IKE_SA_INIT message that (a) offers a hybrid group but not offering
		the corresponding public value in the KEi payload; and (b) the
		responder has not specifically acknowledged that it does not
		supported the requested hybrid group.  However, the checking of this
		policy introduces unnecessary protocol complexity.  Therefore, in
		order to fully prevent any downgrade attacks, using KE payload alone
		is not sufficient and that the initiator MUST always indicate its
		preferred post-quantum proposals and policies in a notify payload
		in the subsequent IKE_SA_INIT messages following a
		N(INVALID_KE_PAYLOAD) response.</t>

	<t>New payload types to negotiate hybrid proposal and to carry post-
      quantum public values<vspace blankLines="1"/>
	Semantically, it makes sense to use a new payload type, which
	mimics the SA payload, to carry a hybrid proposal.  Likewise, another new
	payload type that mimics the KE payload, could be used to transport hybrid
	public value.  Although, in theory a new payload type could be made
	backwards compatible by not setting its critical flag as per Section 2.5
	of RFC7296, we believe that it may not be that simple in practice.  Since
	the original release of IKEv2 in RFC4306, no new payload type has ever
	been proposed and therefore, this creates a potential risk of having a
	backward compatibility issue from non-conforming RFC IKEv2
	implementations.  Since we could not see any other compelling advantages
	apart from a semantic one, we use the existing transform type and
	notify payloads instead.  In fact, as described above, we use the KE
	payload in the first IKE_SA_INIT request round and the notify payload
	to carry the post-quantum proposals and policies. We use one or more
	of the existing KE payloads to carry the hybrid public values.
	</t>

	<t>Hybrid public value payload<vspace blankLines="1"/>
	One way to transport the negotiated hybrid public payload, which contains
	one classical Diffie-Hellman public value and one or more post-quantum
	public values, is to bundle these into a single KE payload.  Alternatively,
	these could also be transported in a single new hybrid public value
	payload, but following the same reasoning as above, this may not be
	a good idea from a backward compatibility perspective.  Using a single
	KE payload would require an encoding or formatting to be defined so
	that both peers are able to compose and extract the individual public
	values.  However, we believe that it is cleaner to send the hybrid
	public values in multiple KE payloads--one for each group or
	algorithm.  Furthermore, at this point in the protocol exchange, both
	peers should have indicated support of handling multiple KE payloads.
	</t>

	<t>Fragmentation<vspace blankLines="1"/>
	Handling of large IKE_SA_INIT messages has been one of the most
      challenging tasks.  A number of approaches have been considered
      and the two prominent ones that we have discarded are outlined as
      follows.
	<vspace blankLines="1"/>
	The first approach was to treat the entire IKE_SA_INIT message as
      a stream of bytes, which we then split it into a number of
      fragments, each of which is wrapped onto a payload that would fit
      into the size of the network MTU.  The payload that wraps each
      fragment is a new payload type and it was envisaged that this new
      payload type will not cause a backward compatibility issue because
      at this stage of the protocol, both peers should have indicated
      support of fragmentation in the first pass of the IKE_SA_INIT
      exchange.  The negotiation of fragmentation is performed using  a
      notify payload, which also defines supporting parameters such as
      the size of fragment in octets and the fragment identifier.  The
      new payload that wraps each fragment of the messages in this
      exchange is assigned the same fragment identifier. Furthermore, it
      also has other parameters such as a fragment index and total
      number of fragments.  We decided to discard this approach due to
      its blanket approach to fragmentation.  In cases where only a few
      payloads need to be fragmented, we felt that this approach is
      overly complicated.
	<vspace blankLines="1"/>
	Another idea that was discarded was fragmenting an individual
      payload without introducing a new payload type.  The idea was to
      use the 9-th bit (the bit after the critical flag in the RESERVED
      field) in the generic payload header as a flag to mark that this
      payload is fragmented.  As an example, if a KE payload is to be
      fragmented, it may look as follows.
	</t>

	</list>
	</t>

	<figure><artwork><![CDATA[
                     1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|F| RESERVED  |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Diffie-Hellman Group Number  |     Fragment Identifier       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Fragment Index        |        Total Fragments        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Total KE Payload Data Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Fragmented KE Payload                   ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
	</figure>
	<t><list hangIndent="3" style="hanging"><t>
      When the flag F is set, this means the current KE payload is a
      fragment of a larger KE payload.  The Payload Length field denotes
      the size of this payload fragment in octets--including the size of
      the generic payload header.  The two-octet RESERVED field
      following Diffie-Hellman Group Number was to be used as a fragment
      identifier to help assembly and disassembly of fragments.  The
      Fragment Index and Total Fragments fields are self-explanatory.
      The Total KE Payload Data Length indicates the size of the
      assembled KE payload data in octets.  Finally, the actual fragment
      is carried in Fragment KE Payload field.</t>

	</list>
	</t>

	<t><list hangIndent="3" style="hanging"><t>
      We discarded this approach because we believe that the working
      group may not be happy using the RESERVED field to change the
      format of a packet and that implementers may not like the
      complexity added from checking the fragmentation flag in each
      received payload.  More importantly, fragmenting the messages
			in this way may leave the system to be more prone to denial of
			service (DoS) attacks.  By using INTERMEDIATE to transport the large
			post-quantum key exchange payloads, there is no longer any issue
			with fragmentation.</t>
	</list>
	</t>

	<t><list style="symbols"><t>Group sub-identifier<vspace blankLines="1"/>
	As discussed before, each group identifier
	is used to
      distinguish a post-quantum algorithm.  Further classification
      could be made on a particular post-quantum algorithm by assigning
      additional value alongside the group identifier.  This sub-
      identifier value may be used to assign different security
      parameter sets to a given post-quantum algorithm.  However, this
      level of details does not fit the principles of the document where
      it should deal with generic hybrid key exchange protocol, not a
      specific ciphersuite.  Furthermore, there are enough Diffie-
      Hellman group identifiers should this be required in the future.
	</t>

	</list>
	</t>

	</section>

    <section title="IANA Considerations" anchor="section-5">
        <t>This document also adds the following Transform Types to the "Transform Type Values" registry:</t>
        <figure align="left">
            <artwork align="left"><![CDATA[
Type Description                Used In                        Reference
------------------------------------------------------------------------
6    Additional Key Exchange 1  (optional in IKE, AH and ESP)  [RFCXXXX]
7    Additional Key Exchange 2  (optional in IKE, AH and ESP)  [RFCXXXX]
8    Additional Key Exchange 3  (optional in IKE, AH and ESP)  [RFCXXXX]
9    Additional Key Exchange 4  (optional in IKE, AH and ESP)  [RFCXXXX]
10   Additional Key Exchange 5  (optional in IKE, AH and ESP)  [RFCXXXX]
11   Additional Key Exchange 6  (optional in IKE, AH and ESP)  [RFCXXXX]
12   Additional Key Exchange 7  (optional in IKE, AH and ESP)  [RFCXXXX]
            ]]></artwork>
        </figure>
        <t>This document also defines a new Notify Message Types in the "Notify Message Types - Status Types" registry:</t>
        <figure align="center">
            <artwork align="left"><![CDATA[
<TBA>       ADDITIONAL_KEY_EXCHANGE
            ]]></artwork>
        </figure>
    </section>


	<section title="Security Considerations" anchor="section-6"><t>
	The key length of the Encryption Algorithm (Transform Type 1), the
	Pseudorandom Function (Transform Type 2) and the Integrity Algorithm
	(Transform Type 3), all have to be of sufficient length to prevent
	attacks using Grover's algorithm <xref target="GROVER"/>.  In order to use the
	extension proposed in this document, the key lengths of these
	transforms SHALL be at least 256 bits long in order to provide
	sufficient resistance to  quantum attacks.  Accordingly the
	post-quantum security level achieved is at least 128 bits.</t>

	<t>SKEYSEED is calculated from shared, KEx, using an algorithm defined
	in Transform Type 2.  While a quantum attacker may learn the value
	of KEx', if this value is obtained by means of a classical key exchange,
	other KEx values generated by means of a quantum-resistant algorithm
	ensure that the final SKEYSEED is not compromised.  This assumes that
	the algorithm defined in the Transform Type 2 is post-quantum.
	</t>

  <t>
	The main focus of this document is to prevent a passive attacker
	performing a "harvest and decrypt" attack.  In other words, an attacker
	that records messages exchanges today and proceeds to decrypt them once
	he owns a quantum computer.  This attack is prevented due to the hybrid
	nature of the key exchange.  Other attacks involving an active attacker
	using a quantum-computer are not completely solved by this
	document.  This is for two reasons.</t>

  <t>
		The first reason is because the authentication step remains
		classical.  In particular, the authenticity of the SAs established
		under IKEv2 is protected using a pre-shared key, RSA, DSA, or ECDSA
		algorithms.  Whilst the pre-shared key option, provided the key is
		long enough, is post-quantum, the other algorithms are not.   Moreover,
		in implementations where scalability is a requirement, the pre-shared
		key method may not be suitable.  Quantum-safe authenticity may be
		provided by using a quantum-safe digital signature and several
		quantum-safe digital signature methods are being explored by
		IETF.  For example, if the implementation is able to reliably
		track state, the hash based method, XMSS has the status of an RFC,
		see <xref target="RFC8391"/>.  Currently, quantum-safe authentication
		methods are not specified in this document, but are planned to be
		incorporated in due course.
	</t>

	<t>It should be noted that the purpose of post-quantum algorithms is
		to provide resistance to attacks mounted in the future.  The current
		threat is that encrypted sessions are subject to eavesdropping and
		archived with decryption by quantum computers taking place at some
		point in the future.  Until quantum computers become available there
		is no point in attacking the authenticity of a connection because
		there are no possibilities for exploitation.  These only occur at
		the time of the connection, for example by mounting a MitM
		attack.  Consequently there is not such a pressing need for
		quantum-safe authenticity.</t>

	<t>This draft does not attempt to address key exchanges with KE payloads
		longer than 64k; the current IKE payload format does not allow that as
		a possibility.  If such huge KE payloads are required, a work around
		(such as making the KE payload a URL and a hash of the real payload)
		would be needed.  At the current time, it appears likely that there
		will be plenty of key exchanges available that would not require such
		a workaround.</t>
	</section>

	</middle>

	<back>
        <references title='Normative References'>

    	&RFC2119;
    
    	&RFC8174;
    
    	&RFC7296;
    
	    &I-D.smyslov-ipsecme-ikev2-aux;

	</references>
    <references title='Informative References'>

        &RFC4302;

        &RFC4303;

        &RFC7383;
    
    	&RFC8229;
    
        <reference anchor="GROVER"><front>
        <title>A Fast Quantum Mechanical Algorithm for Database Search</title>
        <author fullname="L. Grover" initials="L." surname="Grover">
        </author>
        <date year="1996"/>
        </front>
        <seriesInfo name="Proc." value="of the Twenty-Eighth Annual ACM Symposium on the Theory of Computing (STOC 1996)"/>
        </reference>

	    &I-D.ietf-ipsecme-qr-ikev2;

    	&RFC8391;
    
    </references>

	<section title="Acknowledgements" numbered="no" anchor="acknowledgements"><t>
   The authors would like to thanks Frederic Detienne and Olivier
   Pelerin for their comments and suggestions, including the idea to
   negotiate the post-quantum algorithms using the existing KE payload.</t>

	</section>

	</back>

	</rfc>