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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RFC 4449 Shared Data for Precomputable Kbm June 2006
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