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Versions: 00 01 02 03 04                                                
IPsecME Working Group                                             Y. Nir
Internet-Draft                                               Check Point
Intended status: Standards Track                         August 13, 2013
Expires: February 14, 2014

        Adopting Child SAs Following Re-Authentication in IKEv2


   This document describes an extension to the IKEv2 protocol whereby
   Child SAs are moved to the new IKE SA following re-authentication.
   This allows for a smoother transition with no loss of connectivity.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 14, 2014.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Conventions Used in This Document . . . . . . . . . . . . . 3
   2.  Adopting Child SAs  . . . . . . . . . . . . . . . . . . . . . . 4
     2.1.  The ADOPT_CHILD_SAS Notification  . . . . . . . . . . . . . 4
     2.2.  Calculating the Proof of Possession Value . . . . . . . . . 5
     2.3.  Verifying the Proof of Possesion Value  . . . . . . . . . . 5
   3.  Dealing With the Possible Race Condition  . . . . . . . . . . . 6
   4.  Interaction with Other Standards  . . . . . . . . . . . . . . . 6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Changes from Previous Versions  . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The Internet Key Exchange version 2 (IKEv2) protocol, as specified in
   [RFC5996bis] associates Child SAs with the IKE SAs under which the
   exchange that created them took place.  With the deletion of the IKE
   SA due to expiry, policy change, or an explicit message from the
   peer, the child SAs associated with it are implicitly closed as
   described in section 1.4.1 of the IKEv2 document.  This behavior is
   not desired when IKE SAs are replaced rather than deleted, because
   those child SAs could still be valid and there is no security reason
   to create new ones prematurely.

   There are two cases where an IKE SA is replaced.
   1.  Rekeying, where new keys are generated.  This is described in
       section 2.18 of RFC 5996.  This is done mainly for key freshness.
   2.  Re-Authentication, where both sides authenticate, and new keys
       are generated.  This is done as part of a risk management policy,
       to limit the time that compromised IKE SA keys can be used to
       provide the attacker access to the network.  No reauthentication
       exchange is specified in the RFC.  Instead, it's simply the
       Initial and Authentication exchanges done as if from scratch.
       This is described in section 2.8.3 of RFC 5996.

   For rekeying, RFC 5996 provides a way to avoid having to re-create
   all child SAs.  When an IKE SA is rekeyed, all the Child SAs under
   the old IKE SA are inherited by the new IKE SA, so that the
   subsequent deletion of the old IKE SA does not affect the Child SAs.
   This behavior is described in section 2.8 paragraph 4 of RFC 5996.

   For reauthentication, RFC 5996 does not provide a similar mechanism,
   and section 2.8.3 explicitly says that Child SAs need to be created
   from scratch.  This is often inconvenient, as IPsec systems usually
   create Child SAs only in response to traffic and multiple Child SAs
   may exist for a single IKE SA.  The protocol extension in this draft
   closes this gap.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The terms IKE SA, Child SA, Rekeying, and Reauthentication are as
   described in the RFC 5996.

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2.  Adopting Child SAs

   This document defines a new notification that is added to the
   IKE_AUTH exchange that is used to re-authenticate.  The notification
   proves that the current participant in the IKE_AUTH exchange is the
   same one that had participated in the old IKE SA.  If both peers send
   this notification, and it verifies correctly, all Child SAs belonging
   to the old IKE SA are immediately inherited by the new IKE SA.

   In addition to the Child SAs, any IP address assigned to either peer
   through the use of the CFG payload (as described in section 2.19 of
   RFC 5996, is also associated with the new IKE SA.

   Following a successful re-authentication exchange, the old IKE SA is
   deleted by the Initiator.

2.1.  The ADOPT_CHILD_SAS Notification

   The ADOPT_CHILD_SA notification is formatted as follows:

                            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!  RESERVED   !         Payload Length        !
       !  Protocol ID  !   SPI Size    ! ADOPT_CHILD_SAS Message Type  !
       |                                                               |
       ~                Security Parameter Index (SPI)                 ~
       |                                                               |
       |                                                               |
       ~                       Notification Data                       ~
       |                                                               |

   Figure 1

   o  Protocol ID (1 octet) MUST be 1, denoting an IKE SA.  Note that
      previous versions of RFC 5996 explicitly mentioned the
      possibility, but the current version omits this as prior to this
      specification there were no cases where the value 1 should have
      been used.
   o  SPI Size (1 octet) MUST be 16, as that is the size of the
      concatenation of the IKE SPIs.
   o  Security Parameter Index (16 octets) - contains the concatenated
      SPIs of the old IKE SA.  The Initiator SPI comes first, similar to
      the first 16 bytes of the IKE header.

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   o  ADOPT_CHILD_SAS Notify Message Type (2 octets) - MUST be xxxxx,
      the value assigned for ADOPT_CHILD_SAS.  TBA by IANA.
   o  Notification Data (variable) - contains the proof of ownership of
      the previous IKE SA.  Calculation of this field is described in
      Section 2.2.

2.2.  Calculating the Proof of Possession Value

   The notification data field in the ADOPT_CHILD_SAS notification is
   calculated as follows:

       InitiatorPOP = prf(SK_pi, "Adopting Child SAs for Initiator")

       ResponderPOP = ptr(SK_pr, "Adopting Child SAs for Responder")

   InitiatorPOP and ResponderPOP are respectively sent the initiator and
   responder in the IKE_AUTH exchange that creates the reauthenticated
   IKE SA.  The roles may be reversed from those of the original IKE SA,
   but it is still the new Initiator that uses the old SK_pi value.  The
   algorithms used, the PRF keys and the length of the output are all
   those from the old IKE SA, not the new one.

2.3.  Verifying the Proof of Possesion Value

   Both sides of the IKE_AUTH exchange should be in possession of the
   SK_pi and SK_pr values from the previous IKE SA.  This allows both
   sides to make the calculation and verify that it is correct.  This
   verification MUST be done only after the other side has been
   authenticated.  If the value does not verify, the IKE_AUTH exchange
   MUST be terminated, and an INVALID_SYNTAX notification MUST be sent.

   To go through with the new IKE SA inheriting the SAs of the old IKE
   SA, all of the following MUST apply:
   o  Both sides have to be successfully authenticated.
   o  The authenticated identities of both sides are the same as those
      in the old SA.  If the authenticated identity of one peer differs
      from the authenticated identity that it had in the previous IKE
      SA, the other side MUST respond with an INVALID_SYNTAX
      notification.  See Section 3 for a discussion of a possible race
   o  The proof of possession values in the ADOPT_CHILD_SAS notification
      both validated.  The responder MUST NOT continue in sending the
      last IKE_AUTH packet if this condition is not satisfied.  See
      Section 3 for a discussion of what happens if the responder's
      notification does not validate.

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3.  Dealing With the Possible Race Condition

   The sections above describe two kinds of failures in the IKE_AUTH
   1.  An authentication failure.  This could be something as sinister
       as an attack, or as innocent as a temporary failure to contact an
       OCSP server.
   2.  A validation failure of the ADOPT_CHILD_SAS notification.

   If either of those failures occurs for the Initiator, there is no
   problem.  The IKE_AUTH exchange is aborted, the old IKE SA is still
   valid, and all the Child SAs belong to that old IKE SA.

   If, however, the failure occurs for the Responder, we may have a
   problem.  Having sent the last IKE_AUTH response, the responder is
   confident that the exchange has completed successfully, and can
   transfer the Child SAs to the new IKE SA.  However, when the
   Initiator sees that last response, one of the two errors happens, and
   this leads it to delete the new IKE SA.  The Responder erases the new
   IKE SA, deleting with it all the Child SAs.  The result is a mismatch
   in databases, where the Initiator still has the valid SAs, while the
   Responder does not.

   If the Child SAs have been transferred, and the new IKE SA has been
   deleted, but the old IKE SA has not yet been deleted, then the
   Responder MUST delete the old IKE SA (using a DELETE payload)
   immediately after receiving the deletion of the new IKE SA.  If the
   Child SAs have not yet been transferred, then the Responder MAY keep
   the old IKE SA along with the Child SAs until they are deleted by the
   peer or expire according to policy.

   The Initiator MUST NOT delete the old IKE SA because of a failure of
   IKE to create a new IKE SA.  The old IKE SA may only be deleted if
   policy dictates it, such as when a reauthentication timer expires.

   Following a successful verification and transfer of the Child SAs,
   the Initiator SHOULD delete the old IKE SA.

4.  Interaction with Other Standards

   This document changes things so that there is often no need to create
   new Child SAs along with the new IKE SA when reauthenticating.  This
   makes the full IKE_AUTH exchange with the piggy-backed Child SA
   exchange (as described in RFC 5996) superfluous.  Implementations
   should consider implementing the childless extension of IKEv2
   ([RFC6023] in addition to this specification.

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5.  IANA Considerations

   IANA is requested to assign a notify message type from the status
   types range (16418-40959) of the "IKEv2 Notify Message Types"
   registry with name "ADOPT_CHILD_SAS"

6.  Security Considerations

   Comparing the authenticated identities of the new IKE SA with those
   of the old IKE SA is critical.  Without it, attackers would be able
   to authenticate as themselves, steal the Child SAs, and then close
   them.  The proof of possession seems to be superfluous, and in most
   cases it really is.  However, there are some uses of IKE by multiple
   entities with a shared identity and a shared credential.  Calculating
   and verifying the proof of possession blocks such entities from
   stealing each others SAs.

   An on-path attacker may get the Initiator to send the ADOPT_CHILD_SAS
   notification before failing authentication.  This notification is a
   PRF calculated with a secret key over a known message.  The security
   properties of PRFs are such that this does not reveal any secret data
   such as IKE SA keys.

7.  Changes from Previous Versions

   First version

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

              Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", draft-kivinen-ipsecme-ikev2-rfc5996bis-00 (work
              in progress), August 2013.

8.2.  Informative References

   [RFC6023]  Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A
              Childless Initiation of the Internet Key Exchange Version
              2 (IKEv2) Security Association (SA)", RFC 6023,

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              October 2010.

Author's Address

   Yoav Nir
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  6789735

   Email: ynir@checkpoint.com

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