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Versions: 01 02 03 04 RFC 4449

IETF Mobile IP Working Group                          Charles E. Perkins
INTERNET-DRAFT                                     Nokia Research Center
                                                         21 October 2004


        Precomputable Binding Management Key Kbm for Mobile IPv6
                   <draft-ietf-mip6-precfgkbm-01.txt>


Status of This Memo

   This document is a submission by the IETF MIPv6 Working Group Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the mip6@ietf.org mailing list.

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
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   RFC 3668.

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Abstract

   A mobile node and a correspondent node may preconfigure data useful
   for precomputing a Binding Management Key that can subsequently be
   used for authorizing Binding Updates.













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1. Precomputing a Binding Management Key (Kbm)

   A mobile node and a correspondent node may preconfigure data useful
   for creating a Binding Management Key (Kbm), which can then be used
   for authorizing binding management messages, especially Binding
   Update and Binding Acknowledgement messages.  This data is as
   follows:

    -  A shared key (Kcn) used to generate keygen tokens, at least 20
       octets long

    -  A nonce for use when generating the care-of keygen token

    -  A nonce for use when generating the home keygen token

   The keygen tokens MUST be generated from Kcn and the nonces as
   specified in Mobile IPv6 [1] return routability.  Likewise, the
   binding management key Kbm must subsequently be generated from the
   keygen tokens in the same way as specified in Mobile IPv6 [1].  The
   preconfigured data is associated to the mobile node's home address.

   Replay protection for Binding Update messages using Kbm computed from
   the preconfigured data depends upon the value of the sequence number
   field in the Binding Update.  If the correspondent node does not
   maintain information about the recently used values of that field,
   then there may be an opportunity for a malicious node to replay old
   Binding Update messages and fool the correspondent node into routing
   towards an old care-of address.  For this reason, a correspondent
   node that uses a precomputable Kbm also MUST keep track of the most
   recent value of the Sequence Number field of Binding Update messages
   using the precomputable Kbm value.

   When a Binding Update is to be authenticated using such a
   precomputable binding key (Kbm), the Binding Authorization Data
   suboption MUST be present.  The Nonce Indices option SHOULD NOT
   be present.  If it is present, the nonce indices supplied MAY be
   ignored and are not included as part of the calculation for the
   authentication data, which is to be carried exactly as specified
   in [1].


2. Applicability Statement

   Preconfigured shared keys (such as Kcn) between a mobile node and a
   correspondent node are useful in several specific scenarios:

    -  mobile node and correspondent node are administered within the
       same domain, and the correspondent node has good reason to trust
       the actions of the mobile node





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    -  the correspondent node has some guarantee that the mobile node
       will behave properly (perhaps by contractual agreement)

    -  the method of assignment for keys between the correspondent node
       and mobile node results in a stronger security association than
       what can be provided by the Return Routability procedure.

    -  diagnostic procedures

    -  software development and testing

   Generally speaking, the required level of trust that the
   correspondent node needs for enabling a precomputable Kbm with a
   mobile node is more often found within relatively small, closed
   groups of users who are personally familiar with each other, or who
   have some external basis for establishing trustworthy interactions.


3. Security Considerations

   A correspondent node and a mobile node MAY use a precomputable
   binding management key (Kbm) to manage the authentication
   requirements for binding cache management messages.  Such keys must
   be handled carefully to avoid inadvertent exposure to the threats
   outlined in [2].

   A mobile node MUST use a different value for Kcn for each node in
   its Binding Update List, and a correspondent node MUST ensure that
   every mobile node uses a different value of Kcn.  This ensures that
   the sender of a Binding Update can always be uniquely determined.
   This is necessary, as this authorization method does not provide any
   guarantee that the given care-of address is legitimate.  For the
   same reason, this method SHOULD only be applied between nodes that
   are under the same administration.  The return routability procedure
   is RECOMMENDED for all general use and MUST be the default, unless
   the user explicitly overrides this by entering the abovementioned
   preconfigured data for a particular peer.

   Replay protection for the Binding Authorization Data option
   authentication mechanism is provided by the Sequence Number field
   of the Binding Update.  This method of providing replay protection
   fails when the Binding Update sequence numbers cycle through the 16
   bit counter (i.e., not more than 65,536 distinct uses of Kbm for
   any particular care-of address), or if the sequence numbers are not
   protected against reboots.  If the mobile node were to send a fresh
   Binding Update to its correspondent node every hour, 24 hours a day,
   every day of the year, and utilize the same care-of address every
   time, this would require changing keys every 7 years.  Even if the
   mobile node were to do so every minute, this would provide protection
   for over a month.  Given typical mobility patterns, there is little
   danger of replay problems; nodes for which problems might arise are



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   expected to use methods other than manual configuration for Kcn and
   the associated nonces.  When the sequence number field rolls over,
   the parties SHOULD configure a new value for Kcn, so that new Kbm
   values will be computed.

   If a correspondent node does NOT keep track of the Sequence Number
   for Binding Update messages from a particular mobile node, then the
   correspondent node could be fooled into accepting an old value for
   the mobile node's care-of address.  In the unlikely event that this
   address was reallocated to another IPv6 node in the meantime, that
   IPv6 node would then be vulnerable to unwanted traffic emanating from
   the correspondent node.  In order to circumvent this possibility,
   correspondent nodes are mandated to keep track of the most recent
   Sequence Number value in a Binding Update message from the mobile
   node.

   There is no upper bound on the lifetime defined for the precomputable
   Kbm.  As noted, the key is very, very likely to be quite secure
   over the lifetime of the security association and usefulness of
   applications between a mobile node and correspondent node that fit
   the terms specified in section 2.


4. IANA Considerations

   No new protocol numbers are required.


5. Acknowledgement

   Thanks are due to everyone who reviewed the discussion of issue #146.


References

   [1] D. Johnson, C. Perkins, and J. Arkko.  Mobility support in IPv6
       (work in progress).  Internet Draft, Internet Engineering Task
       Force, May 2003.

   [2] P. Nikander, T. Aura, J. Arkko, G. Montenegro, and E. Nordmark.
       Mobile IP version 6 Route Optimization Security Design
       Background.  Internet Draft, Internet Engineering Task Force,
       June 2003.

   The first citation is normative, and the second citation is
   informative only.








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Author's Address

   Questions about this document can also be directed to the author:


     Charles E. Perkins
     Nokia Research Center
     313 Fairchild Drive
     Mountain View, CA 94043
     USA

     Phone:  +1 650 625-2986
     Fax:  +1 650 625-2502
     E-mail:  charles.perkins@nokia.com








































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