James Kempf
Internet Draft DoCoMo Labs USA
Document: draft-kempf-mipshop-handover-key-00.txt Rajeev Koodli
Nokia Research
Center
Expires: November, 2006 June, 2006
Distributing a Symmetric FMIPv6 Handover Key using SEND
(draft-kempf-mipshop-handover-key-00.txt)
Status of this Memo
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Abstract
Fast Mobile IPv6 requires that a Fast Binding Update is secured
using a security association shared between an Access Router and a
Mobile Node in order to avoid certain attacks. In this document, a
method for distributing a shared key to secure this signaling is
defined. The method utilizes the RSA public key that the Mobile
Node used to generate its Cryptographically Generated Address in
SEND. The RSA public key is used to encrypt a shared key sent from
the Access Router to the Mobile Node prior to handover. The
ability of the Mobile Node to decrypt the shared key verifies its
possession of the private key corresponding to the CGA public key
used to generate the address. This allows the Mobile Node to use
the shared key to sign and authorize the routing changes triggered
by the Fast Binding Update.
Table of Contents
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1.0 Introduction.............................................2
2.0 Brief Review of SEND.....................................3
3.0 Handover Key Provisioning and Use........................3
4.0 Message Formats..........................................6
5.0 Security Considerations..................................8
6.0 IANA Considerations......................................9
7.0 Normative References.....................................9
8.0 Informative References...................................9
9.0 Author Information.......................................9
10.0 IPR Statements..........................................10
11.0 Disclaimer of Validity..................................10
12.0 Copyright Statement.....................................10
13.0 Acknowledgment..........................................10
1.0 Introduction
In Fast Mobile IPv6 (FMIPv6) [FMIP], a Fast Binding Update (FBU)
is sent from a Mobile Node (MN), undergoing IP handover, to the
previous Access Router (AR). The FBU causes a routing change so
traffic sent to the MN's previous care-of address on the previous
AR is tunneled to the new care-of address on the new AR. The
previous AR requires that only an authorized MN be able to change
the routing for the old care-of address. If such authorization is
not established, an attacker can redirect a victim MN's traffic at
will.
In this document, a lightweight mechanism is defined by which a
key for securing FMIP can be provisioned on the MN. The mechanism
utilizes the RSA public key with which the MN generates a care-of
Cryptographically Generated Address (CGA) in the SEND protocol
[SEND] to encrypt a shared handover key between the MN and the
AR". The shared handover key itself is established between the AR
and the MN at some arbitrary time prior to handover. In SEND, the
CGA public key is used to authorize possession of an address, and,
thereby, to perform operations associated with the address. The
connection between the address and the CGA public/private key pair
is called the key pair's CGA property. The shared handover key
derives its authorization potential from the ability of the MN to
decrypt the handover key using the CGA private key [CGA]. The
timing of the handover key provisioning is independent of the
handover timing, thus eliminating any potential additional latency
in handover.
Handover keys are an instantiation of the purpose built key
architectural principle [PBK].
1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in
RFC 2119 [RFC2119].
In addition, the following terminology is used:
CGA key Public key used to generate the CGA according to RFC 3972
[CGA].
2.0 Brief Review of SEND
SEND protects against a variety of threats to local link address
resolution (also known as Neighbor Discovery) and last hop router
(AR) discovery in IPv6 [RFC3756]. These threats are not exclusive
to wireless networks, but they generally are easier to mount on
certain wireless networks because the link between the access
point and MN can't be physically secured.
SEND utilizes CGAs in order to secure Neighbor Discovery signaling
[CGA]. Briefly, a CGA is formed by hashing together the IPv6
subnet prefix for a node's subnet, a random nonce, and an RSA
public key, and the CGA key. The CGA key is used to sign a
Neighbor Advertisement (NA) message sent to resolve the link layer
address to the IPv6 address. The combination of the CGA and the
signature on the NA proves to a receiving node the sender's
authorization to claim the address. The node may opportunistically
generate one or several keys specifically for SEND, or it may use
a certified key that it distributes more widely.
3.0 Handover Key Provisioning and Use
3.1 Mobile Node Considerations
At some time prior to handover, the MN MUST send an IPv6 Router
Solicitation (RS) [RFC2461] exactly as specified for IPv6 Router
Discovery. A CGA for the MN MUST be the source address on the
packet, and the MN MUST include the SEND CGA Option and SEND
Signature Option with the packet, as specified in [SEND]. The MN
indicates that it wants to receive a shared handover key by
setting the handover authentication Algorithm Type (AT) extension
field in the CGA Option (described in Section 4.2) to the MN's
preferred authentication algorithm.
Receiving routers that are enabled to perform FMIPv6 with SEND
handover key distribution reply directly to the CGA with a Router
Advertisement (RA) including a Handover Key Option as described in
the next section, containing the encrypted, shared handover key
and the authentication algorithm type. The MN SHOULD choose an AR
from the returned RAs, decrypt the handover key using the private
key corresponding to the CGA key, and store the associated
handover key for later use along with the algorithm type. If more
than one router responds to the RS, the MN MAY keep track of all
such keys. The MN MUST use the returned algorithm type provided by
the ARs. The MN MUST index the handover keys with the AR's IPv6
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address, to which the MN later sends the FBU, and the CGA. This
allows the MN to select the proper key when communicating with a
previous AR.
When the MN needs to signal the previous AR using an FMIPv6 FBU,
the MN MUST utilize the handover key and the corresponding
authentication algorithm to generate an appropriate authenticator
for the message. The MN MUST select the appropriate key for the AR
using the AR's destination address and the care-of CGA. The MN
MUST generate the MAC using the handover key and include it in the
FBU message as defined by the FMIPv6 spec using the appropriate
algorithm. As specified by FMIPv6 [FMIP], the MN MUST include the
care-of CGA in a Home Address Option.
3.2 Access Router Considerations
When an FMIPv6 capable AR with SEND receives an RS from a MN
including a SEND CGA Option with the AT field set and a Signature
Option, and the source address is a CGA, the AR MUST verify the
signature and CGA as described in [SEND]. If the signature and CGA
verify, the AR MUST then determine whether the CGA key already has
an associated shared handover key. If the CGA key has an existing
handover key, the AR MUST return the existing handover key to the
MN. If the CGA key does not have a shared handover key, the AR
MUST construct a shared handover key as described in Section 3.3.
The AR MUST encrypt the handover key with the MN's CGA key. The AR
MUST insert the encrypted handover key into a Handover Key Option
(described in Section 4.1) and MUST attach the Handover Key Option
to the RA. The AR SHOULD set the AT field of the Handover Key
Option to the MN's preferred field if it is supported; otherwise,
the AR MUST select an authentication algorithm which is of
equivalent strength and set the field to that. The RA is then
unicast back to the MN with the CGA as the destination address.
The handover key MUST be stored by the AR for future use, indexed
by the CGA key and the CGA, and the authentication algorithm type
recorded with the key.
If either the CGA or the signature do not verify, the AR MUST NOT
include a Handover Key Option in the reply. The AR also MUST NOT
change any existing key record for the address, since the message
may be an attempt by an attacker to disrupt communications for a
legitimate MN.
When the AR receives an FBU message containing appropriate
authorization, the AR MUST find the corresponding handover key
using the care-of CGA in the Home Address Option as the index. If
a handover key is found, the AR MUST utilize the handover key and
the appropriate algorithm to verify the MAC in the Binding
Authorization Option according to the procedure described in the
FMIPv6 specification.
3.3 Key Generation and Lifetime
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The AR MUST randomly generate a key having sufficient strength to
match the authentication algorithm. The actual size of the key
depends on the authentication algorithm, but should be
sufficiently large to mitigate birthday attacks. Some
authentication algorithms may specify a required key size. The AR
MUST generate a unique key for each CGA key, and SHOULD take care
that the key generation is uncorrelated between keys.
The handover key lifetime depends on the lifetime of the CGA key
[CGA], which, in turn, is determined by the lifetime of the
addresses generated using the CGA key. The CGA key and handover
key SHOULD be renewed by the MN when the preferred lifetime of the
last address generated with the CGA key expires and MUST be
discarded if the valid lifetime of the last address generated with
the key expires [RFC2462]. The handover key is renewed by sending
a SEND-secured RS as described in Section 3.1 for one of the CGAs
associated with the handover key.
Unless the MN renews the handover key with another RS, the AR MUST
discard the handover key when the valid lifetime of the last CGA
to be generated with the key expires. Note that CGAs generated
with the CGA key for which there is an associated handover key may
expire prior to the expiration of the key, if the MN does not
renew the CGAs prior to the expiration of the CGAs' valid
lifetime.
The AR SHOULD NOT discard the handover key immediately after use
if it is still valid. It is possible that the MN may undergo rapid
movement to another AR prior to the completion of Mobile IPv6
binding update on the new AR, and the MN MAY as a consequence
initialize another, subsequent handover optimization to move
traffic from the previous AR to another new AR. In that case,
keeping the key active until the expiration of the address ensures
that the MN can continue to use the handover key for FMIP
signaling purposes if necessary.
If the MN returns to a previous AR prior to the expiration of the
handover key, the MN MAY receive the same handover key as was
previously returned, if the MN uses the same CGA key for address
generation and the previous care-of CGA has not yet expired.
However, the MN MUST NOT assume that it can continue to use the
old key without actually receiving the handover key again from the
router in an RA, regardless of how much time is left on the valid
lifetime of the care-of CGAs.
3.4 Signaling Optimization
As described here, the signaling for handover key provisioning may
require an additional RS-RA exchange beyond that used for basic IP
level movement detection [DNA]. This is because a host performing
router discovery typically includes a link local IPv6 address as
the source address for the RS sent to perform movement detection,
and not a global IPv6 address. The care-of address, however, is a
global address. Since a MN may not have the collection of prefixes
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on the subnet when it sends the RS, it may not be able to generate
a global IPv6 address until the RA returns with the prefixes
supported on the link. While it is possible that the MN may have
another source of information about prefixes supported on the link
(for example, from a Proxy Router Advertisement [FMIP]), the usual
case is that the MN learns these prefixes as part of the initial
RS-RA exchange used to perform movement detection. If that is the
case, the MN must later perform another RS-RA exchange with the
MN's global care-of address as the source address of the RS, and
destination address of the returned RA, in order to obtain a
handover key tied to the CGA.
One possible way to eliminate the need for an additional RS-RA
exchange is to tie the handover key on the MN to both the link
local IPv6 address and the global IPv6 care-of addresses. However,
if this is done, the same CGA key MUST be used for both the link
local IPv6 address and the global IPv6 care-of addresses. If the
MN requires multiple global IPv6 addresses, it MUST either utilize
different subnet prefixes for the different global addresses or
use a different 16 octet modifier for the CGA calculation. Note
that this optimization does not affect the ability of the MN to
generate privacy care-of addresses [RFC3041], since the MN can
utilize a different 16 octet modifier for each address.
4.0 Message Formats
4.1 Handover Key Option
The Handover Key Option is a standard IPv6 Neighbor Discovery
option in TLV format.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Key Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Handover Key . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: To be assigned by IANA.
Length: The length of the option in units of 8 octets,
including the Type and Length fields.
Key Length: Length of the encrypted handover key, in units of
octets.
Encrypted Handover Key:
The encrypted handover key.
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The option is padded to an 8 octet boundary, as required for IPv6
Neighbor Discovery Protocol options.
4.2 Handover Authentication Algorithm Type Field
Handover keys extend the SEND CGA Option to include an Algorithm
Type (AT) field. This allows the MN to ask for and the AR to
acknowledge a particular algorithm for FBU authentication.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Pad Length | AT | Resrvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. CGA Parameters .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 11
Length: The length of the option, including the Type and
Length fields, in units of 8 octets.
Pad Length: The number of padding octets beyond the end of the
CGA Parameters field but within the length
specified by the Length field. Padding octets MUST
be set to zero by senders and ignored by
receivers.
AT: A 4-bit algorithm type field describing the
algorithm used by FMIPv6 to calculate the
authenticator. See [FMIP] for details.
Reserved: A 4-bit field reserved for future use. The value
MUST be initialized to zero by the sender and MUST
be ignored by the receiver.
CGA Parameters:
A variable-length field containing the CGA
Parameters data structure described in Section 4
of [CGA]. This specification requires that if both
the CGA option and the RSA Signature option are
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present, then the public key found from the CGA
Parameters field in the CGA option MUST be that
referred by the Key Hash field in the RSA
Signature option. Packets received with two
different keys MUST be silently discarded. Note
that a future extension may provide a mechanism
allowing the owner of an address and the signer to
be different parties.
Padding: A variable-length field making the option length a
multiple of 8, containing as many octets as
specified in the Pad Length field.
5.0 Security Considerations
This document describes a key distribution protocol for the FMIPv6
handover optimization protocol. The key distribution protocol
utilizes the CGA key of SEND to bootstrap a shared key for
authorizing changes due to handover associated with the MN's
former address on the wireless interface of the AR. General
security considerations involving CGAs apply to the protocol
described in this document, see [CGA] for a discussion of security
considerations around CGAs.
The shared handover key is indexed by the CGA key on the AR.
Multiple addresses can be generated using the same CGA key, and
handover for these addresses is authorized by the same handover
key. If the handover key corresponding to the CGA key used to
generate the addresses is compromised, handover authorization for
all addresses generated using the CGA key is also compromised.
This is similar to the case when the private key corresponding to
the public key used to generate the CGAs is compromised, resulting
in SEND security for the CGAs being compromised. These risks can
be mitigated by using different CGA keys to generate different
addresses, at the expense of additional signaling to establish the
handover keys.
The protocol described in this document coupled with the FBU
authorization protocol described in [FMIP] provides protection
against redirection of traffic on the previous AR by nodes that
are not authorized to claim the previous care-of CGA. This
includes nodes having authorized care-of CGAs on the previous AR's
wireless link that attempt to redirect traffic for addresses for
which they are not authorized. However, this protocol does not
protect against redirection attacks against nodes on the new AR's
link. In such an attack, the MN sends an FBU to the previous AR
with its previous care-of CGA in the Home Address Option, but the
address for another node as the new care-of address. The victim on
the new link is them bombarded with the MN's traffic. The FMIPv6
specification [FMIP] includes a few recommendations about how to
mitigate redirection attacks of this sort.
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6.0 IANA Considerations
A new IPv6 Neighbor Discovery option, the Handover Key Option, is
defined, and requires a IPv6 Neighbor Discovery option type code
from IANA.
7.0 Normative References
[FMIP] Koodli, R., editor, "Fast Handovers for Mobile IPv6", RFC
4068, July 2005.
[SEND] Arkko, J., editor, Kempf, J., Zill, B., and Nikander, P.,
"SEcure Neighbor Discovery (SEND)", RFC 2971, March 2005.
[CGA] Aura, T., "Cryptographically Generated Addresses", RFC 3972,
March 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC3756] Nikander, P., editor, Kempf, J., and Nordmark, E., "
IPv6 Neighbor Discovery (ND) Trust Models and Threats",
RFC 3756, May 2004.
[RFC2461] Narten, T., and Nordmark, E., "Neighbor Discovery for IP
version 6 (IPv6)", RFC 2461, December 1998.
[RFC2462] Thomas, S., and Narten, T., "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC3041] Narten, T., and Draves, R., "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
8.0 Informative References
[DNA] Kempf, J., Narayanan, S., Nordmark, E., Pentland, B., and
Choi, JH., "Detecting Network Attachment in IPv6 Networks
(DNAv6)", Internet Draft, work in progress.
[PBK] Bradner, S., Mankin, A., and Schiller, J., "A Framework for
Purpose-Built Keys (PBK)", Internet Draft, work in
progress.
9.0 Author Information
James Kempf Phone: +1 408 451 4711
DoCoMo Labs USA Email: kempf@docomolabs-usa.com
181 Metro Drive
Suite 300
San Jose, CA
95110
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USA
Rajeev Koodli Phone: +1 650 625 2359
Nokia Research Center Fax: +1 650 625 2502
313 Fairchild Drive Email: Rajeev.Koodli@nokia.com
Mountain View, CA
94043
USA
10.0 IPR Statements
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11.0 Disclaimer of Validity
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
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PARTICULAR PURPOSE.
12.0 Copyright Statement
Copyright (C) The Internet Society (2006). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights.
13.0 Acknowledgment
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Funding for the RFC Editor function is currently provided by the
Internet Society.
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