





Network Working Group                                         C. Perkins
Request for Comments: 4449                         Nokia Research Center
Category: Standards Track                                      June 2006


   Securing Mobile IPv6 Route Optimization Using a Static Shared Key

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

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.

Table of Contents

   1. Introduction ....................................................1
   2. Applicability Statement .........................................2
   3. Precomputing a Binding Management Key (Kbm) .....................3
   4. Security Considerations .........................................4
   5. Acknowledgement .................................................5
   6. References ......................................................5
      6.1. Normative References .......................................5
      6.2. Informative References .....................................6

1.  Introduction

   This specification introduces an alternative, low-latency security
   mechanism for protecting signaling related to the route optimization
   in Mobile IPv6.  The default mechanism specified in [1] uses a
   periodic return routability test to verify both the "right" of the
   mobile node to use a specific home address, as well as the validity
   of the claimed care-of address.  That mechanism requires no
   configuration and no trusted entities beyond the mobile node's home
   agent.





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   The mechanism specified in this document, however, requires the
   configuration of a shared secret between mobile node and its
   correspondent node.  As a result, messages relating to the
   routability tests can be omitted, leading to significantly smaller
   latency.  In addition, the right to use a specific home address is
   ensured in a stronger manner than in [1].  On the other hand, the
   applicability of this mechanisms is limited due to the need for
   preconfiguration.  This mechanism is also limited to use only in
   scenarios where mobile nodes can be trusted not to misbehave, as the
   validity of the claimed care-of addresses is not verified.

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].  Other
   terminology is used as already defined in [1].

2.  Applicability Statement

   This mechanism is useful in scenarios where the following conditions
   are all met:

    -  Mobile node and correspondent node are administered within the
       same domain.

    -  The correspondent node has good reason to trust the actions of
       the mobile node.  In particular, the correspondent node needs to
       be certain that the mobile node will not launch flooding attacks
       against a third party as described in [5].

    -  The configuration effort related to this mechanism is acceptable.
       Users MUST be able to generate/select a sufficiently good value
       for Kcn (see [3])

    -  There is a desire to take advantage of higher efficiency or
       greater assurance with regards to the correctness of the home
       address offered via this mechanism.

    -  This mechanism is used only for authenticating Binding Update
       messages (and not, e.g., data), so the total volume of traffic is
       low (see RFC 4107 [4], and the discussion in section 4).

   This mechanism can also be useful in software development, testing,
   and diagnostics related to mobility signaling.

   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



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   have some external basis for establishing trustworthy interactions.
   A typical example scenario where this mechanism is applicable is
   within a corporation, or between specific users.  Application in the
   general Internet is typically not possible due to the effort that is
   required to manually configure the correspondent nodes.  Application
   at a public network operator is typically not possible due to
   requirements placed on the trustworthiness of mobile nodes.

3.  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.
   Kcn MUST be generated with sufficient randomness (see RFC 4086 [3]).

   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
   toward 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 (for example, by committing it to stable
   storage).










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   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 SHOULD be
   ignored and are not included as part of the calculation for the
   authentication data, which is to be performed exactly as specified in
   [1].

4.  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 [5].  Many requirements listed in this document are
   intended to ensure the safety of the manual configuration.  In
   particular, Kcn MUST be generated with sufficient randomness (see RFC
   4086 [3]), as noted in Section 3.

   Manually configured keys MUST be used in conformance with RFC 4107
   [4].  Used according to the applicability statement, and with the
   other security measures mandated in this specification, the keys will
   satisfy the properties in that document.  In order to ensure
   protection against dictionary attacks, the configured security
   information is intended to be used ONLY for authenticating Binding
   Update messages.

   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 aforementioned
   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), 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, this would require
   changing keys every 7 years.  Even if the mobile node were to do so



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   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 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.

   Note that where a node has been configured to use the mechanism
   specified in this document with a particular peer, it SHOULD NOT
   attempt to use another mechanism, even if the peer requests this or
   claims not to support the mechanism in this document.  This is
   necessary in order to prevent bidding down attacks.

   There is no upper bound on the lifetime defined for the precomputable
   Kbm.  As noted, the key is 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.

5. Acknowledgement

   Thanks are due to everyone who reviewed the discussion of issue #146.
   Thanks to Jari Arkko for supplying text for the Introduction.

6. References

6.1.  Normative References

   [1] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
       IPv6", RFC 3775, June 2004.

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

   [3] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness
       Requirements for Security", BCP 106, RFC 4086, June 2005.

   [4] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key
       Management", BCP 107, RFC 4107, June 2005.



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6.2. Informative References

   [5] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
       Nordmark, "Mobile IP Version 6 Route Optimization Security Design
       Background", RFC 4226, December 2005.

Author's Address

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

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


































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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

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