Secure Password Framework for Internet Key Exchange Version 2 (IKEv2)
RFC 6467

Internet Engineering Task Force (IETF)                        T. Kivinen
Request for Comments: 6467                                     AuthenTec
Category: Informational                                    December 2011
ISSN: 2070-1721

 Secure Password Framework for Internet Key Exchange Version 2 (IKEv2)

Abstract

   This document defines a generic way for Internet Key Exchange version
   2 (IKEv2) to use any of the symmetric secure password authentication
   methods.  Multiple methods are already specified in other documents,
   and this document does not add any new one.  This document specifies
   a way to agree on which method is to be used in the current
   connection.  This document also provides a common way to transmit,
   between peers, payloads that are specific to secure password
   authentication methods.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6467.

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Copyright Notice

   Copyright (c) 2011 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 ....................................................2
   2. Method Negotiation ..............................................4
   3. Generic Secure Password Method Payload ..........................6
   4. IKE_AUTH Exchange ...............................................7
   5. Security Considerations .........................................9
   6. IANA Considerations .............................................9
   7. References .....................................................10
      7.1. Normative References ......................................10
      7.2. Informative References ....................................10

1.  Introduction

   The IPsecME working group was chartered to provide for IKEv2
   ([RFC5996]) a symmetric secure password authentication protocol that
   supports the use of low-entropy shared secrets, and to protect
   against off-line dictionary attacks without requiring the use of
   certificates or the Extensible Authentication Protocol (EAP).  There
   are multiple such methods, and the working group was to pick one.
   Unfortunately, the working group failed to pick one protocol, and
   there are multiple candidates going forward as separate documents.
   As each of those older versions of those documents used a different
   technique to negotiate the use of the method and also used different
   payload formats, it is very hard to try to make an implementation
   where multiple such systems could co-exist.

   Current document versions ([SPSK-AUTH], [PACE], and [PAKE]) use the
   method described in this document.

   This document describes IKEv2 payload formats that can be used for
   multiple secure password methods to negotiate and transmit data so
   each different method can easily co-exist in the same implementation.

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   This document consists of two major parts:

   o  How to negotiate which secure password method negotiation is used.

   o  How to transmit data, between peers, that is specific to secure
      password methods.

   The secure password methods are not usually meant to be used in the
   normal end user (remote access VPN) cases.  In such cases, EAP-based
   authentication works fine, and the asymmetric nature of EAP does not
   matter.  In such scenarios, the authentication is usually backed up
   with the back-end Authentication, Authorization, and Accounting (AAA)
   servers and other infrastructure.  That is, in such scenarios,
   neither of the IKEv2 peers really knows the secret, as on one end it
   is typed in by the user when it is needed, and on the other end it is
   authenticated by the back-end AAA server.

   The new secure password methods are meant to be used, for example, in
   the authentication between two servers or routers.  These scenarios
   are usually symmetric: both peers know the shared secret, no back-end
   authentication servers are involved, and either end can initiate an
   IKEv2 connection.  Note that such a model could also be supported by
   EAP when an EAP method that can run in symmetric fashion is in use,
   and the EAP method is directly implemented on both peers and no AAA
   is in use.

   In many cases, each implementation will use only one of the proposed
   secure password authentication methods but can include support for
   multiple methods even when only one of them will be used.  For
   example, a general-purpose operating system running IPsec and IKEv2
   and supporting secure password authentication methods to protect
   services provided by the system might need to implement support for
   several methods.  It is then up to the administrator which one is to
   be used.  As the server might need to connect to multiple other
   servers, each implementing a different set of methods, it may not be
   possible to pick one method that would serve all cases.

   The secure password methods mostly keep the existing IKEv2
   IKE_SA_INIT exchange and modify the IKE_AUTH authentication step.  As
   those methods do not want to add new round trips, negotiating which
   of the secure password methods to use needs to happen during the
   IKE_SA_INIT.  As the identity of the other end is only provided
   inside IKE_AUTH, the responder needs to select the list of supported
   methods based only on the IP address of the initiator.  This could
   lead to problems if only certain methods would be acceptable for
   certain identified peers.  Fortunately, as the authentication is done
   based on the secret shared between both peers, the shared secret
   should be usable in all of the methods; thus, a remote peer usually

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   does not need to restrict selection of the method based on the
   initiator's identity but only based on the supported methods and the
   administrative policy.

   Also, as the initiator already knows which peer it is connecting
   with, it can limit which methods it proposes to the other peer.  And
   as secure password methods are meant to be used in symmetric cases,
   both ends should have similar configuration; i.e., they have the same
   shared secret and, most likely, also a list of acceptable
   authentication methods to be used.  This could also be interpreted so
   that there is no need to support method negotiation, as both ends can
   already see this from the configuration.  On the other hand, in most
   cases, either end does not really care which method is used but is
   willing to use any secure method that the other end supports.  In
   such cases, the automatic negotiation provides a way to make the
   configuration easy, i.e., no need to pick one method to be used
   between the peers.

   The reason for using the common IKEv2 payload to transmit, between
   peers, data that is specific to the secure password method is that
   the payload type field in the IKEv2 is only an 8-bit field, and 62.5%
   of the range is already reserved (50% to the private use numbers, and
   12.5% to the IKEv1 payload numbers).  This leaves 95 usable numbers,
   out of which 16 are already in use.  Initially, it was proposed that
   five payload type numbers be consumed.  Those five new payload types
   would already represent a 31% increase in the number of currently
   allocated payload types.

2.  Method Negotiation

   Because all of the methods modify the IKE_AUTH exchange, the
   negotiation of the secure password method to be used needs to happen
   during the IKE_SA_INIT exchange.  The secure password negotiation
   exchange would be:

   Initiator                         Responder
   -------------------------------------------------------------------
   HDR(SPIi=xxx, SPIr=0, IKE_SA_INIT,
       Flags: Initiator, Message ID=0),
       SAi1, KEi, Ni, [N(SECURE_PASSWORD_METHODS)]  -->

                      <--  HDR(SPIi=xxx, SPIr=yyy, IKE_SA_INIT,
                               Flags: Response, Message ID=0),
                               SAr1, KEr, Nr, [CERTREQ],
                               [N(SECURE_PASSWORD_METHODS)]

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   If the N(SECURE_PASSWORD_METHODS) Notify payload is missing, then
   normal IKEv2 authentication methods are used.  If the Notify payloads
   are included, then the negotiation of the secure password methods
   happens inside those payloads.

   As it might be possible that future secure password methods will
   modify the IKE_AUTH payload in a more substantial way, it is better
   that as an end result of the negotiation we have exactly one secure
   password method that will be used.  The initiator will know which
   methods are usable when talking to that responder, so the initiator
   will send a list of acceptable methods in its IKE_SA_INIT request.
   The responder will pick exactly one method and put that in its
   response.

   The secure password methods are identified by the 16-bit IANA-
   allocated numbers stored in the Notify payload notification data
   field.  If a method supports multiple different password
   preprocessing methods, each of those may be allocated a separate
   number from this space, or the method might do its own negotiation of
   the preprocessing method later.  As the initiator has already
   selected the shared secret it will be using, it will also know which
   preprocessing it might need, so it should propose only those
   preprocessing methods suitable for the selected shared secret.  This
   means that it is recommended that separate IANA numbers be allocated
   for different preprocessing methods.

   The actual Notify payload will look like this:

                        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    |      Notify Message Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                Security Parameter Index (SPI)                 ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Notification Data                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Protocol ID will be zero, and the SPI Size will also be zero,
   meaning that the SPI field will be empty.  The Notify Message Type
   will be 16424.

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   The Notification Data contains the list of the 16-bit secure password
   method numbers:

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Secure Password Method #1     | Secure Password Method #2     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Secure Password Method #3     | ...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The response Notify payload contains exactly one 16-bit secure
   password method number inside the Notification Data field.

3.  Generic Secure Password Method Payload

   This payload will contain the data that is specific to the secure
   password payload.  The IKE_AUTH exchanges might have a number of
   these inside, depending on what is required and specified by the
   secure password method.  As the secure password method is already
   selected during IKE_SA_INIT, there is no need to repeat the
   information of the selected secure password method; thus, this
   payload only contains the method-specific data.  As some secure
   password methods require multiple different payloads, they are
   assumed to include their method-specific payload type inside the
   payload -- for example, inside the first octet of the data.  However,
   this is method-specific, and a method is free to format the payload
   data as it wants.

   The Generic Secure Password Method (GSPM) payload will look
   like this:

                        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        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~         Data Specific to the Secure Password Method           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Payload Type for this payload is 49, and the name used in this
   document is "GSPM payload."

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   If the method uses payload subtypes (which are specific to the secure
   password method) inside the GSPM payload, the format will be
   like this:

                        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        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Subtype*    |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                                                               |
   ~         Data Specific to the Secure Password Method           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * method-specific subtype field

   This representation is here only for illustrative purposes; the
   secure password method will define the exact format of the payload
   contents.

4.  IKE_AUTH Exchange

   As the negotiation takes place during IKE_SA_INIT, the secure
   password methods may modify the IKE_AUTH exchange if needed.  To
   easily enable implementing multiple methods, it is recommended that
   IKE_AUTH exchange not be modified unnecessarily.  Adding zero, one,
   or multiple GSPM payloads to each exchange is needed, as is the
   modification to how the AUTH payload is calculated, but all other
   changes should be kept minimal.

   The IKE_AUTH exchange should look similar to when EAP is used,
   meaning that the first request includes IDi, SAi2, TSi, TSr, and some
   number of GSPM payloads.  The response should include IDr and, again,
   a number of GSPM payloads.  There may be multiple exchanges, each
   consisting of some number of GSPM payloads; finally, when
   authentication is done, there should be one final exchange where the
   request includes the AUTH payload (along with some number of GSPM
   payloads) and the response contains AUTH, SAr2, TSi, TSr, and some
   number of GSPM payloads.  The number of GSPM payloads is up to the
   secure password method but usually will be less than 3.  However,
   depending on the method, it might be more.

   The AUTH payload calculation should include all the data that would
   normally be included, in addition to the extra data needed by the
   secure password method.  The secure password method needs to define
   how the AUTH payload is calculated.

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   As the AUTH payload calculation is changed, the secure payload method
   should not use any of the existing authentication method numbers in
   the AUTH Payload Auth Method field but instead should use the number
   allocated in this document.  This number is meant to be used by all
   secure password authentication methods.

   Initiator                         Responder
   -------------------------------------------------------------------
   HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
       Flags: Initiator, Message ID=1),
       SK {IDi, [CERTREQ,]
           GSPM, [GSPM, ...,]
           [IDr,] SAi2,
           TSi, TSr}  -->

                     <--  HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
                                 Response, Message ID=1),
                                 SK {IDr, [CERT,]
                                     GSPM, [GSPM, ...]}

   HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
       Flags: Initiator, Message ID=2),
       SK {GSPM, [GSPM, ...,]}  -->

                     <--  HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
                                 Response, Message ID=2),
                                 SK {GSPM, [GSPM, ...]}
   ...

   HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
       Flags: Initiator, Message ID=x),
       SK {[GSPM, ...,], AUTH}  -->

                     <--  HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
                                 Response, Message ID=x),
                                 SK {[GSPM, ...,] AUTH, SAr2,
                                     TSi, TSr}

   Note that the number of the GSPM payloads and other payloads in each
   packet will be defined only by the secure password method
   documentation, and representations in this document are only for
   illustrative purposes.

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5.  Security Considerations

   As this document does not describe an exact protocol, the security
   considerations are not relevant.  Any secure password method
   documentation using payload types described here needs to also
   describe the security properties of the protocol it defines or
   discusses.

6.  IANA Considerations

   This document allocates one new IKEv2 message type in the "Notify
   Messages Types - Status Types" registry:

      16424   SECURE_PASSWORD_METHODS

   This document also allocates one new number in the "IKEv2
   Authentication Method" registry:

      12   Generic Secure Password Authentication Method

   This document also adds one new payload type to the "IKEv2 Payload
   Types" registry:

      49   Generic Secure Password Method      GSPM

   This document creates a new IANA registry -- "IKEv2 Secure Password
   Methods":

      0            Reserved

   Values 1-1023 are unassigned.  Values 1024-65535 are for private use
   among mutually consenting parties.  Changes and additions to this
   registry are done by Expert Review.

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7.  References

7.1.  Normative References

   [RFC5996]    Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
                "Internet Key Exchange Protocol Version 2 (IKEv2)",
                RFC 5996, September 2010.

7.2.  Informative References

   [PACE]       Kuegler, D. and Y. Sheffer, "Password Authenticated
                Connection Establishment with IKEv2", Work in Progress,
                September 2011.

   [PAKE]       Shin, S. and K. Kobara, "Most Efficient Augmented
                Password-Only Authentication and Key Exchange for
                IKEv2", Work in Progress, July 2011.

   [SPSK-AUTH]  Harkins, D., "Secure PSK Authentication for IKE", Work
                in Progress, July 2011.

Author's Address

   Tero Kivinen
   AuthenTec
   Eerikinkatu 28
   HELSINKI  FI-00180
   Finland

   EMail: kivinen@iki.fi

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