Network Working Group J. Kempf

Network Working Group J. Kempf
Request for Comments: 5269 DoCoMo Labs USA
Category: Standards Track R. Koodli
Starent Networks
June 2008

Distributing a Symmetric Fast Mobile IPv6 (FMIPv6) Handover Key Using
SEcure Neighbor Discovery (SEND)

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.

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 provisioning a shared key from the Access Router to the Mobile
Node is defined to protect this signaling. The Mobile Node generates
a public/private key pair using the same public key algorithm as for
SEND (RFC 3971). The Mobile Node sends the public key to the Access
Router. The Access Router encrypts a shared handover key using the
public key and sends it back to the Mobile Node. The Mobile Node
decrypts the shared handover key using the matching private key, and
the handover key is then available for generating an authenticator on
a Fast Binding Update. The Mobile Node and Access Router use the
Router Solicitation for Proxy Advertisement and Proxy Router
Advertisement from Fast Mobile IPv6 for the key exchange. The key
exchange messages are required to have SEND security; that is, the
source address is a Cryptographically Generated Address (CGA) and the
messages are signed using the CGA private key of the sending node.
This allows the Access Router, prior to providing the shared handover
key, to verify the authorization of the Mobile Node to claim the
address so that the previous care-of CGA in the Fast Binding Update
can act as the name of the key.

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RFC 5269 FMIP Security June 2008

Table of Contents

1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Overview of the Protocol ........................................3
2.1. Brief Review of SEND .......................................3
2.2. Protocol Overview ..........................................4
3. Handover Key Provisioning and Use ...............................4
3.1. Sending Router Solicitations for Proxy Advertisement .......4
3.2. Receiving Router Solicitations for Proxy
Advertisement and Sending Proxy Router Advertisements ......5
3.3. Receiving Proxy Router Advertisements ......................6
3.4. Sending FBUs ...............................................7
3.5. Receiving FBUs .............................................7
3.6. Key Generation and Lifetime ................................7
3.7. Protocol Constants .........................................8
4. Message Formats .................................................8
4.1. Handover Key Request Option ................................8
4.2. Handover Key Reply Option .................................10
5. Security Considerations ........................................11
6. IANA Considerations ............................................11
7. References .....................................................12
7.1. Normative References ......................................12
7.2. Informative References ....................................12

1. 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's link is
tunneled to the new Care-of Address on the new AR's link. Only an MN
authorized to claim the address should be able to change the routing
for the previous 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
shared handover key for securing FMIP can be provisioned on the MN by
the AR. The mechanism utilizes SEND [SEND] and an additional
public/private key pair, generated on the MN using the same public
key algorithm as SEND, to encrypt/decrypt a shared handover key sent
from the AR to the MN. The key provisioning occurs at some arbitrary
time prior to handover, thereby relieving any performance overhead on
the handover process. The message exchange between the MN and AR to
provision the handover key is required to be protected by SEND; that
is, the source address for the key provisioning messages must be a
CGA and the messages must be signed with the CGA private key. This
allows the AR to establish the MN's authorization to operate on the

Kempf & Koodli Standards Track [Page 2]

RFC 5269 FMIP Security June 2008

CGA. The AR uses the CGA to name the handover key. The SEND key
pair is, however, independent from the handover encryption/decryption
key pair and from the actual handover key. Once the shared handover
key has been established, when the MN undergoes IP handover, the MN
generates an authorization Message Authentication Code (MAC) on the
FBU. The previous care-of CGA included in the FBU is used by the AR
to find the right handover key for checking the authorization.

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 "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

In addition, the following terminology is used:

CGA public key

Public key used to generate the CGA according to RFC 3972
[CGA].

CGA private key

Private key corresponding to the CGA public key.

Handover key encryption public key

Public key generated by the MN and sent to the current AR to
encrypt the shared handover key.

Handover key encryption private key

Private key corresponding to handover key encryption public
key, held by the MN.

2. Overview of the Protocol

2.1. 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.

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RFC 5269 FMIP Security June 2008

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,
called the CGA public key. The CGA private 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.

2.2. Protocol Overview

The protocol utilizes the SEND secured Router Solicitation for Proxy
Advertisement (RtSolPr)/Proxy Router Advertisement (PrRtAdv) [FMIP]
exchange between the MN and the AR to transport an encrypted, shared
handover key from the AR to the MN. First, the MN generates the
necessary key pair and associated CGA addresses so that the MN can
employ SEND. Then, the MN generates a public/private key pair for
encrypting/decrypting the shared handover key, using the same public
key algorithm as was used for SEND. The MN then sends an RtSolPr
message with a Handover Key Request Option containing the handover
key encryption public key. The source address of the RtSolPr message
is the MN's care-of CGA on the AR's link, the RtSolPr message is
signed with the MN's CGA key, and contains the CGA Parameters option,
in accordance with RFC 3971 [SEND]. The AR verifies the message
using SEND, then utilizes the handover key encryption public key to
encrypt a shared handover key, which is included with the PrRtAdv in
the Handover Key Reply Option. The MN decrypts the shared handover
key and uses it to establish an authorization MAC when it sends an
FBU to the previous AR.

3. Handover Key Provisioning and Use

3.1. Sending Router Solicitations for Proxy Advertisement

At some time prior to handover, the MN MUST generate a handover key
encryption public/private key pair, using exactly the same public key
algorithm with exactly the same parameters (key size, etc.) as for
SEND [SEND]. The MN can reuse the key pair on different access
routers but MUST NOT use the key pair for any other encryption or for
signature operation. In order to prevent cryptanalysis, the key pair
SHOULD be discarded after either a duration of HKEPK-LIFETIME or
HKEPK-HANDOVERS number of handovers, whichever occurs first. See
Section 3.7 for definitions of protocol constants.

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RFC 5269 FMIP Security June 2008

The MN MUST send a Router Solicitation for Proxy Advertisement
(RtSolPr) containing a Handover Key Request Option with the handover
encryption public key. 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 SEND
signature covers all fields in the RtSolPr, including the 128-bit
source and destination addresses and ICMP checksum as described in
RFC 3971, except for the Signature Option itself. The MN also sets
the handover authentication Algorithm Type (AT) extension field in
the Handover Key Request Option to the MN's preferred FBU
authentication algorithm. The SEND Nonce MUST also be included for
anti-replay protection.

3.2. Receiving Router Solicitations for Proxy Advertisement and Sending
Proxy Router Advertisements

When an FMIPv6 capable AR with SEND receives an RtSolPr from an MN
protected with SEND and including a Handover Key Request Option, the
AR MUST first validate the RtSolPr using SEND as described in RFC
3971. If the RtSolPr can not be validated, the AR MUST NOT include a
Handover Key Reply 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.
The AR SHOULD respond to the RtSolPr but MUST NOT perform handover
key provisioning.

If the RtSolPr can be validated, the AR MUST then determine whether
the CGA is already associated with a shared handover key. If the CGA
is associated with an existing handover key, the AR MUST return the
existing handover key to the MN. If the CGA does not have a shared
handover key, the AR MUST construct a shared handover key as
described in Section 3.6. The AR MUST encrypt the handover key with
the handover key encryption public key included in the Handover Key
Request Option. The AR MUST insert the encrypted handover key into a
Handover Key Reply Option and MUST attach the Handover Key Reply
Option to the PrRtAdv. The lifetime of the key, HK-LIFETIME, MUST
also be included in the Handover Key Reply Option. The AR SHOULD set
the AT field of the Handover Key Option to the MN's preferred
algorithm type indicated in the AT field of the Handover Key Request
Option, if it is supported; otherwise, the AR MUST select an
authentication algorithm that is of equivalent strength or stronger,
and set the field to that. The AR MUST also include the SEND nonce
from the RtSolPr for anti-replay protection. The AR MUST have a
certificate suitable for a SEND-capable router, support SEND
certificate discovery, and include a SEND CGA Option and a SEND
Signature Option in the PrRtAdv messages it sends. Similarly, the
mobile nodes MUST be configured with one or more SEND trust anchors
so that they can verify these messages. The SEND signature covers

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RFC 5269 FMIP Security June 2008

all fields in the PrRtAdv, including the 128-bit source and
destination addresses and ICMP checksum as described in RFC 3971,
except for the Signature Option itself. The PrRtAdv is then unicast
back to the MN at the MN's care-of CGA that was the source address on
the RtSolPr. The handover key MUST be stored by the AR for future
use, indexed by the CGA, and the authentication algorithm type (i.e.,
the resolution of the AT field processing) and HK-LIFETIME MUST be
recorded with the key.

3.3. Receiving Proxy Router Advertisements

Upon receipt of one or more PrRtAdvs secured with SEND and having the
Handover Key Reply Option, the MN MUST first validate the PrRtAdvs as
described in RFC 3971. Normally, the MN will have obtained the
router's certification path to validate an RA prior to sending the
PrRtSol and the MN MUST check to ensure that the key used to sign the
PrRtAdv is the router's certified public key. If the MN does not
have the router's certification path cached, it MUST use the SEND
CPS/CPA messages to obtain the certification path to validate the
key. If a certified key from the router was not used to sign the
message, the message MUST be dropped.

From the messages that validate, the MN SHOULD choose one with an AT
flag in the Handover Key Reply Option indicating an authentication
algorithm that the MN supports. From that message, the MN MUST
determine which handover key encryption public key to use in the
event the MN has more than one. The MN finds the right public key to
use by matching the SEND nonce from the RtSolPr. If no such match
occurs, the MN MUST drop the PrRtAdv. The MN MUST use the matching
private key to decrypt the handover key using its handover key
encryption private key, and store the handover key for later use,
named with the AR's CGA, along with the algorithm type and
HK-LIFETIME. The MN MUST use the returned algorithm type indicated
in the PrRtAdv. The MN MUST index the handover keys with the AR's
IPv6 address, to which the MN later sends the FBU, and the MN's CGA
to which the handover key applies. This allows the MN to select the
proper key when communicating with a previous AR. Prior to
HK-LIFETIME expiring, the MN MUST request a new key from the AR if
FMIPv6 service is still required from the router.

If more than one router responds to the RtSolPr, the MN MAY keep
track of all such keys. If none of the PrRtAdvs contains an
algorithm type indicator corresponding to an algorithm the MN
supports, the MN MAY re-send the RtSolPr requesting a different
algorithm, but to prevent bidding down attacks from compromised
routers, the MN SHOULD NOT request an algorithm that is weaker than
its original request.

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RFC 5269 FMIP Security June 2008

3.4. Sending FBUs

When the MN needs to signal the Previous AR (PAR) using an FMIPv6
FBU, the MN MUST utilize the handover key and the corresponding
authentication algorithm to generate an authenticator for the
message. The MN MUST select the appropriate key for the PAR using
the PAR's CGA and the MN's previous care-of CGA on the PAR's link.
As defined by the FMIPv6 [FMIP], the MN MUST generate the
authentication MAC using the handover key and the appropriate
algorithm and MUST include the MAC in the FBU message. As specified
by FMIPv6, the MN MUST include the old care-of CGA in a Home Address
Option. The FMIPv6 document provides more detail about the
construction of the authenticator on the FBU.

3.5. Receiving FBUs

When the PAR receives an FBU message containing an authenticator, the
PAR MUST find the corresponding handover key using the MN's care-of
CGA in the Home Address Option as the index. If a handover key is
found, the PAR MUST utilize the handover key and the appropriate
algorithm to verify the authenticator. If the handover key is not
found, the PAR MUST NOT change forwarding for the care-of CGA. The
FMIPv6 document [FMIP] provides more detail on how the AR processes
an FBU containing an authenticator.

3.6. Key Generation and Lifetime

The AR MUST randomly generate a key having sufficient strength to
match the authentication algorithm. Some authentication algorithms
specify a required key size. The AR MUST generate a unique key for
each CGA public key, and SHOULD take care that the key generation is
uncorrelated between handover keys, and between handover keys and CGA
keys. The actual algorithm used to generate the key is not important
for interoperability since only the AR generates the key; the MN
simply uses it.

A PAR 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 PAR, and the MN MAY as a consequence initialize
another, subsequent handover optimization to move traffic from the
PAR to another new AR. The default time for keeping the key valid
corresponds to the default time during which forwarding from the PAR
to the new AR is performed for FMIP. The FMIPv6 document [FMIP]
provides more detail about the FMIP forwarding time default.

If the MN returns to a PAR prior to the expiration of the handover
key, the PAR MAY send and the MN MAY receive the same handover key as

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RFC 5269 FMIP Security June 2008

was previously returned, if the MN generates the same CGA for its
Care-of Address. However, the MN MUST NOT assume that it can
continue to use the old key without actually receiving the handover
key again from the PAR. The MN SHOULD discard the handover key after
MIPv6 binding update is complete on the new AR. The PAR MUST discard
the key after FMIPv6 forwarding for the previous Care-of Address
times out or when HK-LIFETIME expires.

3.7. Protocol Constants

The following are protocol constants with suggested defaults:

HKEPK-LIFETIME: The maximum lifetime for the handover key
encryption public key. Default is 12 hours.

HKEPK-HANDOVERS: The maximum number of handovers for which the
handover key encryption public key should be
reused. Default is 10.

HK-LIFETIME: The maximum lifetime for the handover key. Default
is 12 hours (43200 seconds).

4. Message Formats

4.1. Handover Key Request Option

The Handover Key Request Option is a standard IPv6 Neighbor Discovery
[RFC4861] option in TLV format. The Handover Key Request Option is
included in the RtSolPr message along with the SEND CGA Option, RSA
Signature Option, and Nonce Option.

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.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Handover Key Encryption Public Key .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 5269 FMIP Security June 2008

Fields:

Type: 27

Length: The length of the option in units of 8 octets,
including the Type and Length fields. The value 0
is invalid. The receiver MUST discard a message
that contains this value.

Pad Length: The number of padding octets beyond the end of the
Handover Key Encryption Public Key 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.

Resrvd.: 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.

Handover Key Encryption Public Key:
The handover key encryption public key. The key
MUST be formatted according to the same
specification as the CGA key in the CGA Parameters
Option [CGA] of the message, and MUST have the same
parameters as the CGA key.

Padding: A variable-length field making the option length a
multiple of 8, containing as many octets as
specified in the Pad Length field.

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RFC 5269 FMIP Security June 2008

4.2. Handover Key Reply Option

The Handover Key Reply Option is a standard IPv6 Neighbor Discovery
[RFC4861] option in TLV format. The Handover Key Reply Option is
included in the PrRtAdv message along with the SEND CGA Option, RSA
Signature Option, and Nonce Option.

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.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Lifetime | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
. .
. Encrypted Handover Key .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

Type: 28

Length: The length of the option in units of 8 octets,
including the Type and Length fields. The value 0
is invalid. The receiver MUST discard a message
that contains this value.

Pad Length: The number of padding octets beyond the end of the
Encrypted Handover Key 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.

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RFC 5269 FMIP Security June 2008

Resrvd.: 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.

Key Lifetime: Lifetime of the handover key, HK-LIFETIME, in
seconds.

Encrypted Handover Key:
The shared handover key, encrypted with the MN's
handover key encryption public key, using the
RSAES-PKCS1-v1_5 format [RFC3447].

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. Security Considerations

This document describes a shared key provisioning protocol for the
FMIPv6 handover optimization protocol. The key provisioning protocol
utilizes a public key generated with the same public key algorithm as
SEND to bootstrap a shared key for authorizing changes due to
handover associated with the MN's former address on the PAR. General
security considerations involving CGAs apply to the protocol
described in this document, see [CGA] for a discussion of security
considerations around CGAs. This protocol is subject to the same
risks from replay attacks and denial-of-service (DoS) attacks using
the RtSolPr as the SEND protocol [SEND] for RS. The measures
recommended in RFC 3971 for mitigating replay attacks and DoS attacks
apply here as well. An additional consideration involves when to
generate the handover key on the AR. To avoid state depletion
attacks, the handover key SHOULD NOT be generated prior to SEND
processing that verifies the originator of RtSolPr. State depletion
attacks can be addressed by techniques, such as rate limiting
RtSolPr, restricting the amount of state reserved for unresolved
solicitations, and clever cache management. These techniques are the
same as used in implementing Neighbor Discovery.

For other FMIPv6 security considerations, please see the FMIPv6
document [FMIP].

6. IANA Considerations

IANA has assigned IPv6 Neighbor Discovery option type codes for the
two new IPv6 Neighbor Discovery options, the Handover Key Request
Option (27) and Handover Key Reply Option (28), defined in this
document.

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RFC 5269 FMIP Security June 2008

7. References

7.1. Normative References

[FMIP] Koodli, R., Ed., "Mobile IPv6 Fast Handovers", RFC 5268,
June 2008.

[SEND] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.

[CGA] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.

[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.

7.2. Informative References

[RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
Neighbor Discovery (ND) Trust Models and Threats", RFC
3756, May 2004.

[PBK] Bradner, S., Mankin, A., and Schiller, J., "A Framework for
Purpose-Built Keys (PBK)", Work in Progress, June 2003.
Progress.

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RFC 5269 FMIP Security June 2008

Authors' Addresses

James Kempf
DoCoMo Labs USA
3240 Hillview Avenue
Palo Alto, CA 94303
USA

Phone: +1 650 496 4711
EMail: [email protected]

Rajeev Koodli
Starent Networks
30 International Place
Tewksbury, MA 01876
USA

Phone: +1 408 735 7679
EMail: [email protected]

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RFC 5269 FMIP Security June 2008

Full Copyright Statement

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