IETF Mobile IP Working Group Charles E. Perkins
INTERNET-DRAFT Nokia Research Center
30 June 2005
Securing Mobile IPv6 Route Optimization Using a Static Shared Key
<draft-ietf-mip6-precfgkbm-03.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
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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. 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 a use a specific home address, as well as the
validity of the claimed care-of address. This mechanism requires no
configuration and no trusted entities beyond the mobile node's home
agent.
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 address is
assured in a stronger manner than in [1]. On the other hand, the
applicability of this mechanisms is limited due to the need for
pre-configuration. This mechanism is also limited to use only in
scenarios where mobile nodes can be trusted to not misbehave, as the
validity of the claimed care-of addresses is not verified.
2. 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
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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].
3. 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 [2].
- The configuration effort related to this mechanism is acceptable.
- 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 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 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 use is typically not possible
due to the configuration effort. Application at a public network
operator is typically not possible due to requirements placed on the
trustworthiness of mobile nodes.
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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 [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 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 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. In order to circumvent this possibility,
correspondent nodes SHOULD keep track of the most recent Sequence
Number value in a Binding Update message from the mobile 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
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or claims to not 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 3.
5. IANA Considerations
No new protocol numbers are required.
6. Acknowledgement
Thanks are due to everyone who reviewed the discussion of issue #146.
Thanks to Jari Arkko for supplying text for the Introduction.
References
[1] D. Johnson, C. Perkins, and J. Arkko. Mobility Support in IPv6.
Request for Comments (Proposed Standard), Internet Engineering
Task Force, July 2004.
[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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