Internet Engineering Task Force (IETF)                        V. Smyslov
Request for Comments: 9242                                    ELVIS-PLUS
Category: Standards Track                                       May 2022
ISSN: 2070-1721


 Intermediate Exchange in the Internet Key Exchange Protocol Version 2
                                (IKEv2)

Abstract

   This document defines a new exchange, called "Intermediate Exchange",
   for the Internet Key Exchange Protocol Version 2 (IKEv2).  This
   exchange can be used for transferring large amounts of data in the
   process of IKEv2 Security Association (SA) establishment.  An example
   of the need to do this is using key exchange methods resistant to
   Quantum Computers (QCs) for IKE SA establishment.  The Intermediate
   Exchange makes it possible to use the existing IKE fragmentation
   mechanism (which cannot be used in the initial IKEv2 exchange),
   helping to avoid IP fragmentation of large IKE messages if they need
   to be sent before IKEv2 SA is established.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9242.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
   2.  Terminology and Notation
   3.  Intermediate Exchange Details
     3.1.  Support for Intermediate Exchange Negotiation
     3.2.  Using Intermediate Exchange
     3.3.  The IKE_INTERMEDIATE Exchange Protection and Authentication
       3.3.1.  Protection of IKE_INTERMEDIATE Messages
       3.3.2.  Authentication of IKE_INTERMEDIATE Exchanges
     3.4.  Error Handling in the IKE_INTERMEDIATE Exchange
   4.  Interaction with Other IKEv2 Extensions
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Example of IKE_INTERMEDIATE Exchange
   Acknowledgements
   Author's Address

1.  Introduction

   The Internet Key Exchange Protocol Version 2 (IKEv2) defined in
   [RFC7296] uses UDP as a transport for its messages.  If the size of a
   message is larger than the Path MTU (PMTU), IP fragmentation takes
   place, which has been shown to cause operational challenges in
   certain network configurations and devices.  The problem is described
   in more detail in [RFC7383], which also defines an extension to IKEv2
   called "IKE fragmentation".  This extension allows IKE messages to be
   fragmented at the IKE level, eliminating possible issues caused by IP
   fragmentation.  However, IKE fragmentation cannot be used in the
   initial IKEv2 exchange (IKE_SA_INIT).  In most cases, this limitation
   is not a problem, since the IKE_SA_INIT messages are usually small
   enough not to cause IP fragmentation.

   However, the situation has been changing recently.  One example of
   the need to transfer large amounts of data before an IKE SA is
   created is using the QC-resistant key exchange methods in IKEv2.
   Recent progress in quantum computing has led to concern that
   classical Diffie-Hellman key exchange methods will become insecure in
   the relatively near future and should be replaced with QC-resistant
   ones.  Currently, most QC-resistant key exchange methods have large
   public keys.  If these keys are exchanged in the IKE_SA_INIT
   exchange, then IP fragmentation will probably take place; therefore,
   all the problems caused by it will become inevitable.

   A possible solution to this problem would be to use TCP as a
   transport for IKEv2, as defined in [RFC8229].  However, this approach
   has significant drawbacks and is intended to be a last resort when
   UDP transport is completely blocked by intermediate network devices.

   This specification describes a way to transfer a large amount of data
   in IKEv2 using UDP transport.  For this purpose, the document defines
   a new exchange for IKEv2 called "Intermediate Exchange" or
   "IKE_INTERMEDIATE".  One or more of these exchanges may take place
   right after the IKE_SA_INIT exchange and prior to the IKE_AUTH
   exchange.  The IKE_INTERMEDIATE exchange messages can be fragmented
   using the IKE fragmentation mechanism, so these exchanges may be used
   to transfer large amounts of data that don't fit into the IKE_SA_INIT
   exchange without causing IP fragmentation.

   The Intermediate Exchange can be used to transfer large public keys
   of QC-resistant key exchange methods, but its application is not
   limited to this use case.  This exchange can also be used whenever
   some data needs to be transferred before the IKE_AUTH exchange and
   for some reason the IKE_SA_INIT exchange is not suited for this
   purpose.  This document defines the IKE_INTERMEDIATE exchange without
   tying it to any specific use case.  It is expected that separate
   specifications will define for which purposes and how the
   IKE_INTERMEDIATE exchange is used in IKEv2.  Some considerations must
   be taken into account when designing such specifications:

   *  The IKE_INTERMEDIATE exchange is not intended for bulk transfer.
      This document doesn't set a hard cap on the amount of data that
      can be safely transferred using this mechanism, as it depends on
      its application.  However, in most cases, it is anticipated that
      the amount of data will be limited to tens of kilobytes (a few
      hundred kilobytes in extreme cases), which is believed to cause no
      network problems (see [RFC6928] as an example of experiments with
      sending similar amounts of data in the first TCP flight).  See
      also Section 5 for the discussion of possible DoS attack vectors
      when the amount of data sent in the IKE_INTERMEDIATE exchange is
      too large.

   *  It is expected that the IKE_INTERMEDIATE exchange will only be
      used for transferring data that is needed to establish IKE SA and
      not for data that can be sent later when this SA is established.

2.  Terminology and Notation

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   It is expected that readers are familiar with the terms used in the
   IKEv2 specification [RFC7296].  Notation for the payloads contained
   in IKEv2 messages is defined in Section 1.2 of [RFC7296].

3.  Intermediate Exchange Details

3.1.  Support for Intermediate Exchange Negotiation

   The initiator indicates its support for Intermediate Exchange by
   including a notification of type INTERMEDIATE_EXCHANGE_SUPPORTED in
   the IKE_SA_INIT request message.  If the responder also supports this
   exchange, it includes this notification in the response message.

   Initiator                                 Responder
   -----------                               -----------
   HDR, SAi1, KEi, Ni,
   [N(INTERMEDIATE_EXCHANGE_SUPPORTED)] -->
                                      <-- HDR, SAr1, KEr, Nr, [CERTREQ],
                                    [N(INTERMEDIATE_EXCHANGE_SUPPORTED)]

   The INTERMEDIATE_EXCHANGE_SUPPORTED is a Status Type IKEv2
   notification with Notify Message Type 16438.  When it is sent, the
   Protocol ID and SPI Size fields in the Notify payload are both set to
   0.  This specification doesn't define any data that this notification
   may contain, so the Notification Data is left empty.  However, future
   enhancements to this specification may override this.
   Implementations MUST ignore non-empty Notification Data if they don't
   understand its purpose.

3.2.  Using Intermediate Exchange

   If both peers indicated their support for the Intermediate Exchange,
   the initiator may use one or more these exchanges to transfer
   additional data.  Using the Intermediate Exchange is optional; the
   initiator may find it unnecessary even when support for this exchange
   has been negotiated.

   The Intermediate Exchange is denoted as IKE_INTERMEDIATE; its
   Exchange Type is 43.

   Initiator                                 Responder
   -----------                               -----------
   HDR, ..., SK {...}  -->
                                        <--  HDR, ..., SK {...}

   The initiator may use several IKE_INTERMEDIATE exchanges if
   necessary.  Since window size is initially set to 1 for both peers
   (Section 2.3 of [RFC7296]), these exchanges MUST be sequential and
   MUST all be completed before the IKE_AUTH exchange is initiated.  The
   IKE SA MUST NOT be considered as established until the IKE_AUTH
   exchange is successfully completed.

   The Message IDs for IKE_INTERMEDIATE exchanges MUST be chosen
   according to the standard IKEv2 rule, described in Section 2.2 of
   [RFC7296], i.e., it is set to 1 for the first IKE_INTERMEDIATE
   exchange, 2 for the next (if any), and so on.  Implementations MUST
   verify that Message IDs in the IKE_INTERMEDIATE messages they receive
   actually follow this rule.  The Message ID for the first pair of
   IKE_AUTH messages is one more than the value used in the last
   IKE_INTERMEDIATE exchange.

   If the presence of NAT is detected in the IKE_SA_INIT exchange via
   NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP
   notifications, then the peers switch to port 4500 in the first
   IKE_INTERMEDIATE exchange and use this port for all subsequent
   exchanges, as described in Section 2.23 of [RFC7296].

   The content of the IKE_INTERMEDIATE exchange messages depends on the
   data being transferred and will be defined by specifications
   utilizing this exchange.  However, since the main motivation for the
   IKE_INTERMEDIATE exchange is to avoid IP fragmentation when large
   amounts of data need to be transferred prior to the IKE_AUTH
   exchange, the Encrypted payload MUST be present in the
   IKE_INTERMEDIATE exchange messages, and payloads containing large
   amounts of data MUST be placed inside it.  This will allow IKE
   fragmentation [RFC7383] to take place, provided it is supported by
   the peers and negotiated in the initial exchange.

   Appendix A contains an example of using an IKE_INTERMEDIATE exchange
   in creating an IKE SA.

3.3.  The IKE_INTERMEDIATE Exchange Protection and Authentication

3.3.1.  Protection of IKE_INTERMEDIATE Messages

   The keys SK_e[i/r] and SK_a[i/r] for the protection of
   IKE_INTERMEDIATE exchanges are computed in the standard fashion, as
   defined in Section 2.14 of [RFC7296].

   Every subsequent IKE_INTERMEDIATE exchange uses the most recently
   calculated IKE SA keys before this exchange is started.  So, the
   first IKE_INTERMEDIATE exchange always uses SK_e[i/r] and SK_a[i/r]
   keys that were computed as a result of the IKE_SA_INIT exchange.  If
   additional key exchange is performed in the first IKE_INTERMEDIATE
   exchange, resulting in the update of SK_e[i/r] and SK_a[i/r], then
   these updated keys are used for protection of the second
   IKE_INTERMEDIATE exchange.  Otherwise, the original SK_e[i/r] and
   SK_a[i/r] keys are used again, and so on.

   Once all the IKE_INTERMEDIATE exchanges are completed, the most
   recently calculated SK_e[i/r] and SK_a[i/r] keys are used for
   protection of the IKE_AUTH exchange and all subsequent exchanges.

3.3.2.  Authentication of IKE_INTERMEDIATE Exchanges

   The IKE_INTERMEDIATE messages must be authenticated in the IKE_AUTH
   exchange, which is performed by adding their content into the AUTH
   payload calculation.  It is anticipated that in many use cases,
   IKE_INTERMEDIATE messages will be fragmented using the IKE
   fragmentation [RFC7383] mechanism.  According to [RFC7383], when IKE
   fragmentation is negotiated, the initiator may first send a request
   message in unfragmented form, but later turn on IKE fragmentation and
   resend it fragmented if no response is received after a few
   retransmissions.  In addition, peers may resend a fragmented message
   using different fragment sizes to perform simple PMTU discovery.

   The requirement to support this behavior makes authentication
   challenging: it is not appropriate to add on-the-wire content of the
   IKE_INTERMEDIATE messages into the AUTH payload calculation, because
   implementations are generally unaware of which form these messages
   are received by peers.  Instead, a more complex scheme is used;
   authentication is performed by adding the content of these messages
   before their encryption and possible fragmentation, so that the data
   to be authenticated doesn't depend on the form the messages are
   delivered in.

   If one or more IKE_INTERMEDIATE exchanges took place, the definition
   of the blob to be signed (or MACed) from Section 2.15 of [RFC7296] is
   modified as follows:

   InitiatorSignedOctets = RealMsg1 | NonceRData | MACedIDForI | IntAuth
   ResponderSignedOctets = RealMsg2 | NonceIData | MACedIDForR | IntAuth

   IntAuth =  IntAuth_iN | IntAuth_rN | IKE_AUTH_MID

   IntAuth_i1 = prf(SK_pi1,              IntAuth_i1A [| IntAuth_i1P])
   IntAuth_i2 = prf(SK_pi2, IntAuth_i1 | IntAuth_i2A [| IntAuth_i2P])
   IntAuth_i3 = prf(SK_pi3, IntAuth_i2 | IntAuth_i3A [| IntAuth_i3P])
   ...
   IntAuth_iN = prf(SK_piN, IntAuth_iN-1 | IntAuth_iNA [| IntAuth_iNP])

   IntAuth_r1 = prf(SK_pr1,              IntAuth_r1A [| IntAuth_r1P])
   IntAuth_r2 = prf(SK_pr2, IntAuth_r1 | IntAuth_r2A [| IntAuth_r2P])
   IntAuth_r3 = prf(SK_pr3, IntAuth_r2 | IntAuth_r3A [| IntAuth_r3P])
   ...
   IntAuth_rN = prf(SK_prN, IntAuth_rN-1 | IntAuth_rNA [| IntAuth_rNP])

   The essence of this modification is that a new chunk called "IntAuth"
   is appended to the string of octets that is signed (or MACed) by the
   peers.  IntAuth consists of three parts: IntAuth_iN, IntAuth_rN, and
   IKE_AUTH_MID.

   The IKE_AUTH_MID chunk is a value of the Message ID field from the
   IKE Header of the first round of the IKE_AUTH exchange.  It is
   represented as a four-octet integer in network byte order (in other
   words, exactly as it appears on the wire).

   The IntAuth_iN and IntAuth_rN chunks represent the cumulative result
   of applying the negotiated Pseudorandom Function (PRF) to all
   IKE_INTERMEDIATE exchange messages sent during IKE SA establishment
   by the initiator and the responder, respectively.  After the first
   IKE_INTERMEDIATE exchange is complete, peers calculate the IntAuth_i1
   value by applying the negotiated PRF to the content of the request
   message from this exchange and calculate the IntAuth_r1 value by
   applying the negotiated PRF to the content of the response message.
   For every subsequent IKE_INTERMEDIATE exchange (if any), peers
   recalculate these values as follows: after the nth exchange is
   complete, they compute IntAuth_[i/r]n by applying the negotiated PRF
   to the concatenation of IntAuth_[i/r](n-1) (computed for the previous
   IKE_INTERMEDIATE exchange) and the content of the request (for
   IntAuth_in) or response (for IntAuth_rn) messages from this exchange.
   After all IKE_INTERMEDIATE exchanges are over, the resulted
   IntAuth_[i/r]N values (assuming N exchanges took place) are used in
   computing the AUTH payload.

   For the purpose of calculating the IntAuth_[i/r]* values, the content
   of the IKE_INTERMEDIATE messages is represented as two chunks of
   data: mandatory IntAuth_[i/r]*A, optionally followed by IntAuth_[i/
   r]*P.

   The IntAuth_[i/r]*A chunk consists of the sequence of octets from the
   first octet of the IKE Header (not including the prepended four
   octets of zeros, if UDP encapsulation or TCP encapsulation of ESP
   packets is used) to the last octet of the generic header of the
   Encrypted payload.  The scope of IntAuth_[i/r]*A is identical to the
   scope of Associated Data defined for the use of AEAD algorithms in
   IKEv2 (see Section 5.1 of [RFC5282]), which is stressed by using the
   "A" suffix in its name.  Note that calculation of IntAuth_[i/r]*A
   doesn't depend on whether an AEAD algorithm or a plain cipher is used
   in IKE SA.

   The IntAuth_[i/r]*P chunk is present if the Encrypted payload is not
   empty.  It consists of the content of the Encrypted payload that is
   fully formed but not yet encrypted.  The Initialization Vector,
   Padding, Pad Length, and Integrity Checksum Data fields (see
   Section 3.14 of [RFC7296]) are not included into the calculation.  In
   other words, the IntAuth_[i/r]*P chunk is the inner payloads of the
   Encrypted payload in plaintext form, which is stressed by using the
   "P" suffix in its name.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ ^
   |                       IKE SA Initiator's SPI                  | | |
   |                                                               | | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I |
   |                       IKE SA Responder's SPI                  | K |
   |                                                               | E |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |
   |  Next Payload | MjVer | MnVer | Exchange Type |     Flags     | H |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ d |
   |                          Message ID                           | r A
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
   |                       Adjusted Length                         | | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v |
   |                                                               |   |
   ~                 Unencrypted payloads (if any)                 ~   |
   |                                                               |   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ |
   | Next Payload  |C|  RESERVED   |    Adjusted Payload Length    | | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | v
   |                                                               | |
   ~                     Initialization Vector                     ~ E
   |                                                               | E
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ c ^
   |                                                               | r |
   ~             Inner payloads (not yet encrypted)                ~   P
   |                                                               | P |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ l v
   |              Padding (0-255 octets)           |  Pad Length   | d
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
   |                                                               | |
   ~                    Integrity Checksum Data                    ~ |
   |                                                               | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v

      Figure 1: Data to Authenticate in the IKE_INTERMEDIATE Exchange
                                  Messages

   Figure 1 illustrates the layout of the IntAuth_[i/r]*A (denoted as A)
   and the IntAuth_[i/r]*P (denoted as P) chunks in case the Encrypted
   payload is not empty.

   For the purpose of prf calculation, the Length field in the IKE
   Header and the Payload Length field in the Encrypted payload header
   are adjusted so that they don't count the lengths of Initialization
   Vector, Integrity Checksum Data, Padding, and Pad Length fields.  In
   other words, the Length field in the IKE Header (denoted as Adjusted
   Length in Figure 1) is set to the sum of the lengths of IntAuth_[i/
   r]*A and IntAuth_[i/r]*P, and the Payload Length field in the
   Encrypted payload header (denoted as Adjusted Payload Length in
   Figure 1) is set to the length of IntAuth_[i/r]*P plus the size of
   the Encrypted payload header (four octets).

   The prf calculations MUST be applied to whole messages only, before
   possible IKE fragmentation.  This ensures that the IntAuth will be
   the same regardless of whether or not IKE fragmentation takes place.
   If the message was received in fragmented form, it MUST be
   reconstructed before calculating the prf as if it were received
   unfragmented.  While reconstructing, the RESERVED field in the
   reconstructed Encrypted payload header MUST be set to the value of
   the RESERVED field in the Encrypted Fragment payload header from the
   first fragment (with the Fragment Number field set to 1).

   Note that it is possible to avoid actual reconstruction of the
   message by incrementally calculating prf on decrypted (or ready to be
   encrypted) fragments.  However, care must be taken to properly
   replace the content of the Next Header and the Length fields so that
   the result of computing the prf is the same as if it were computed on
   the reconstructed message.

   Each calculation of IntAuth_[i/r]* uses its own keys SK_p[i/r]*,
   which are the most recently updated SK_p[i/r] keys available before
   the corresponded IKE_INTERMEDIATE exchange is started.  The first
   IKE_INTERMEDIATE exchange always uses the SK_p[i/r] keys that were
   computed in the IKE_SA_INIT exchange as SK_p[i/r]1.  If the first
   IKE_INTERMEDIATE exchange performs additional key exchange resulting
   in an SK_p[i/r] update, then these updated SK_p[i/r] keys are used as
   SK_p[i/r]2; otherwise, the original SK_p[i/r] keys are used, and so
   on.  Note that if keys are updated, then for any given
   IKE_INTERMEDIATE exchange, the keys SK_e[i/r] and SK_a[i/r] used for
   protection of its messages (see Section 3.3.1) and the key SK_p[i/r]
   for its authentication are always from the same generation.

3.4.  Error Handling in the IKE_INTERMEDIATE Exchange

   Since messages of the IKE_INTERMEDIATE exchange are not authenticated
   until the IKE_AUTH exchange successfully completes, possible errors
   need to be handled with care.  There is a trade-off between providing
   better diagnostics of the problem and risk of becoming part of a DoS
   attack.  Sections 2.21.1 and 2.21.2 of [RFC7296] describe how errors
   are handled in initial IKEv2 exchanges; these considerations are also
   applied to the IKE_INTERMEDIATE exchange with the qualification that
   not all error notifications may appear in the IKE_INTERMEDIATE
   exchange (for example, errors concerning authentication are generally
   only applicable to the IKE_AUTH exchange).

4.  Interaction with Other IKEv2 Extensions

   The IKE_INTERMEDIATE exchanges MAY be used during the IKEv2 Session
   Resumption [RFC5723] between the IKE_SESSION_RESUME and the IKE_AUTH
   exchanges.  To be able to use it, peers MUST negotiate support for
   Intermediate Exchange by including INTERMEDIATE_EXCHANGE_SUPPORTED
   notifications in the IKE_SESSION_RESUME messages.  Note that a flag
   denoting whether peers supported the IKE_INTERMEDIATE exchange is not
   stored in the resumption ticket and is determined each time from the
   IKE_SESSION_RESUME exchange.

5.  Security Considerations

   The data that is transferred by means of the IKE_INTERMEDIATE
   exchanges is not authenticated until the subsequent IKE_AUTH exchange
   is complete.  However, if the data is placed inside the Encrypted
   payload, then it is protected from passive eavesdroppers.  In
   addition, the peers can be certain that they receive messages from
   the party they performed the IKE_SA_INIT exchange with if they can
   successfully verify the Integrity Checksum Data of the Encrypted
   payload.

   The main application for the Intermediate Exchange is to transfer
   large amounts of data before an IKE SA is set up, without causing IP
   fragmentation.  For that reason, it is expected that IKE
   fragmentation will be employed in IKE_INTERMEDIATE exchanges in most
   cases.  Section 5 of [RFC7383] contains security considerations for
   IKE fragmentation.

   Since authentication of peers occurs only in the IKE_AUTH exchange, a
   malicious initiator may use the Intermediate Exchange to mount a DoS
   attack on the responder.  In this case, it starts creating an IKE SA,
   negotiates using the Intermediate Exchanges, and transfers a lot of
   data to the responder that may also require computationally expensive
   processing.  Then, it aborts the SA establishment before the IKE_AUTH
   exchange.  Specifications utilizing the Intermediate Exchange MUST
   NOT allow an unlimited number of these exchanges to take place at the
   initiator's discretion.  It is recommended that these specifications
   be defined in such a way that the responder would know (possibly via
   negotiation with the initiator) the exact number of these exchanges
   that need to take place.  In other words, after the IKE_SA_INIT
   exchange is complete, it is preferred that both the initiator and the
   responder know the exact number of IKE_INTERMEDIATE exchanges they
   have to perform; it is possible that some IKE_INTERMEDIATE exchanges
   are optional and are performed at the initiator's discretion, but if
   a specification defines optional use of IKE_INTERMEDIATE, then the
   maximum number of these exchanges must be hard capped by the
   corresponding specification.  In addition, [RFC8019] provides
   guidelines for the responder of how to deal with DoS attacks during
   IKE SA establishment.

   Note that if an attacker was able to break the key exchange in real
   time (e.g., by means of a quantum computer), then the security of the
   IKE_INTERMEDIATE exchange would degrade.  In particular, such an
   attacker would be able to both read data contained in the Encrypted
   payload and forge it.  The forgery would become evident in the
   IKE_AUTH exchange (provided the attacker cannot break the employed
   authentication mechanism), but the ability to inject forged
   IKE_INTERMEDIATE exchange messages with a valid Integrity Check Value
   (ICV) would allow the attacker to mount a DoS attack.  Moreover, in
   this situation, if the negotiated PRF was not secure against a second
   preimage attack with known key, then the attacker could forge the
   IKE_INTERMEDIATE exchange messages without later being detected in
   the IKE_AUTH exchange.  To do this, the attacker would find the same
   IntAuth_[i/r]* value for the forged message as for the original.

6.  IANA Considerations

   This document defines a new Exchange Type in the "IKEv2 Exchange
   Types" registry:

   +=======+==================+===========+
   | Value | Exchange Type    | Reference |
   +=======+==================+===========+
   | 43    | IKE_INTERMEDIATE | RFC 9242  |
   +-------+------------------+-----------+

        Table 1: IKEv2 Exchange Types

   This document also defines a new Notify Message Type in the "IKEv2
   Notify Message Types - Status Types" registry:

   +=======+=================================+===========+
   | Value | NOTIFY MESSAGES - STATUS TYPES  | Reference |
   +=======+=================================+===========+
   | 16438 | INTERMEDIATE_EXCHANGE_SUPPORTED | RFC 9242  |
   +-------+---------------------------------+-----------+

      Table 2: IKEv2 Notify Message Types - Status Types

7.  References

7.1.  Normative 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>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,
              <https://www.rfc-editor.org/info/rfc7383>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [RFC5282]  Black, D. and D. McGrew, "Using Authenticated Encryption
              Algorithms with the Encrypted Payload of the Internet Key
              Exchange version 2 (IKEv2) Protocol", RFC 5282,
              DOI 10.17487/RFC5282, August 2008,
              <https://www.rfc-editor.org/info/rfc5282>.

   [RFC5723]  Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
              Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
              DOI 10.17487/RFC5723, January 2010,
              <https://www.rfc-editor.org/info/rfc5723>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.

   [RFC8019]  Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
              Protocol Version 2 (IKEv2) Implementations from
              Distributed Denial-of-Service Attacks", RFC 8019,
              DOI 10.17487/RFC8019, November 2016,
              <https://www.rfc-editor.org/info/rfc8019>.

   [RFC8229]  Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
              August 2017, <https://www.rfc-editor.org/info/rfc8229>.

Appendix A.  Example of IKE_INTERMEDIATE Exchange

   This appendix contains an example of the messages using
   IKE_INTERMEDIATE exchanges.  This appendix is purely informative; if
   it disagrees with the body of this document, the other text is
   considered correct.

   In this example, there is one IKE_SA_INIT exchange and two
   IKE_INTERMEDIATE exchanges, followed by the IKE_AUTH exchange to
   authenticate all initial exchanges.  The xxx in the HDR(xxx,MID=yyy)
   indicates the Exchange Type, and yyy indicates the Message ID used
   for that exchange.  The keys used for each SK {} payload are
   indicated in the parenthesis after the SK.  Otherwise, the payload
   notation is the same as is used in [RFC7296].

   Initiator                         Responder
   -----------                       -----------
   HDR(IKE_SA_INIT,MID=0),
   SAi1, KEi, Ni,
   N(INTERMEDIATE_EXCHANGE_SUPPORTED)  -->

                                <--  HDR(IKE_SA_INIT,MID=0),
                                     SAr1, KEr, Nr, [CERTREQ],
                                     N(INTERMEDIATE_EXCHANGE_SUPPORTED)

   At this point, peers calculate SK_* and store them as SK_*1.  SK_e[i/
   r]1 and SK_a[i/r]1 will be used to protect the first IKE_INTERMEDIATE
   exchange, and SK_p[i/r]1 will be used for its authentication.

   Initiator                         Responder
   -----------                       -----------
   HDR(IKE_INTERMEDIATE,MID=1),
   SK(SK_ei1,SK_ai1) {...}  -->

            <Calculate IntAuth_i1 = prf(SK_pi1, ...)>

                                <--  HDR(IKE_INTERMEDIATE,MID=1),
                                     SK(SK_er1,SK_ar1) {...}

            <Calculate IntAuth_r1 = prf(SK_pr1, ...)>

   If the SK_*1 keys are updated (e.g., as a result of a new key
   exchange) after completing this IKE_INTERMEDIATE exchange, then the
   peers store the updated keys as SK_*2; otherwise, they use SK_*1 as
   SK_*2.  SK_e[i/r]2 and SK_a[i/r]2 will be used to protect the second
   IKE_INTERMEDIATE exchange, and SK_p[i/r]2 will be used for its
   authentication.

   Initiator                         Responder
   -----------                       -----------
   HDR(IKE_INTERMEDIATE,MID=2),
   SK(SK_ei2,SK_ai2) {...}  -->

            <Calculate IntAuth_i2 = prf(SK_pi2, ...)>

                                <--  HDR(IKE_INTERMEDIATE,MID=2),
                                     SK(SK_er2,SK_ar2) {...}

            <Calculate IntAuth_r2 = prf(SK_pr2, ...)>

   If the SK_*2 keys are updated (e.g., as a result of a new key
   exchange) after completing the second IKE_INTERMEDIATE exchange, then
   the peers store the updated keys as SK_*3; otherwise, they use SK_*2
   as SK_*3.  SK_e[i/r]3 and SK_a[i/r]3 will be used to protect the
   IKE_AUTH exchange, SK_p[i/r]3 will be used for authentication, and
   SK_d3 will be used for derivation of other keys (e.g., for Child
   SAs).

   Initiator                         Responder
   -----------                       -----------
   HDR(IKE_AUTH,MID=3),
   SK(SK_ei3,SK_ai3)
   {IDi, [CERT,] [CERTREQ,]
   [IDr,] AUTH, SAi2, TSi, TSr}  -->
                                <--  HDR(IKE_AUTH,MID=3),
                                     SK(SK_er3,SK_ar3)
                                     {IDr, [CERT,] AUTH, SAr2, TSi, TSr}

   In this example, two IKE_INTERMEDIATE exchanges took place;
   therefore, SK_*3 keys would be used as SK_* keys for further
   cryptographic operations in the context of the created IKE SA, as
   defined in [RFC7296].

Acknowledgements

   The idea to use an Intermediate Exchange between the IKE_SA_INIT and
   IKE_AUTH exchanges was first suggested by Tero Kivinen.  He also
   helped to write the example IKE_INTERMEDIATE exchange shown in
   Appendix A.  Scott Fluhrer and Daniel Van Geest identified a possible
   problem with authentication of the IKE_INTERMEDIATE exchange and
   helped to resolve it.  The author is grateful to Tobias Brunner, who
   raised good questions concerning authentication of the
   IKE_INTERMEDIATE exchange and proposed how to make the size of
   authentication chunks constant regardless of the number of exchanges.
   The author is also grateful to Paul Wouters and Benjamin Kaduk, who
   suggested a lot of text improvements for the document.

Author's Address

   Valery Smyslov
   ELVIS-PLUS
   PO Box 81
   Moscow (Zelenograd)
   124460
   Russian Federation
   Phone: +7 495 276 0211
   Email: svan@elvis.ru