James Kempf Internet Draft DoCoMo Labs USA Document: draft-ietf-mipshop-handover-key-00.txt Rajeev Koodli Nokia Research Center Expires: August, 2007 February, 2007 Distributing a Symmetric FMIPv6 Handover Key using SEND (draft-ietf-mipshop-handover-key-00.txt) Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 Kempf & Koodli Expires August, 2007 [Page 1]
Internet Draft FMIP Security February, 2007 1.0 Introduction......................................................2 2.0 Overview of the Protocol..........................................3 3.0 Handover Key Provisioning and Use.................................3 4.0 Message Formats...................................................6 5.0 Security Considerations...........................................8 6.0 IANA Considerations...............................................8 7.0 Normative References..............................................8 8.0 Informative References............................................9 9.0 Author Information................................................9 10.0 IPR Statements....................................................9 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 Kempf & Koodli Expires August, 2007 [Page 2]
Internet Draft FMIP Security February, 2007 "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. 2.0 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. 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 RS/RA exchange to transport an encrypted, shared handover key from the AR to the MN. When the AR receives an RS from an MN having a CGA source address and including a CGA Option and SEND Signature Option, it includes a Handover Key Option in the unicast RA sent as a reply. The MN ignores the option if it does not support SEND handover keys; otherwise, the option is processed and utilized by the MN to provide address authorization on an FBU when the MN moves to another AR. 3.0 Handover Key Provisioning and Use 3.1 Sending Router Solicitations Kempf & Koodli Expires August, 2007 [Page 3]
Internet Draft FMIP Security February, 2007 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. 3.2 Receiving Router Solicitations and Sending Router Advertisements When an FMIPv6 capable AR with SEND receives an RS from a MN including a SEND Signature Option and a CGA Option with the AT field set, and the source address is a CGA, the AR MUST first validate the RS using SEND as described in RFC 3971. If the RS can not be validated, 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. The AR SHOULD proceed to process such an RS as described in [SEND]. If RS can be validated, the AR MUST then determine whether the CGA already has an associated shared handover key. If the CGA has 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 MN's CGA public 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 algorithm type indicated in the AT field of the CGA Option, 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 at the CGA destination address. The handover key MUST be stored by the AR for future use, indexed by the CGA, and the authentication algorithm type MUST be recorded with the key. 3.3 Receiving Router Advertisements Upon receipt of one or more RAs secured with SEND and having the Handover Key Option, the MN MUST first validate the RAs as described in RFC 3971. From the RAs that validate, the MN SHOULD choose an RA with an AT flag in the Handover Key Option indicating an authentication algorithm that the MN supports, decrypt the handover key using its CGA private key, and store the 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 indicated in the RA. The MN MUST index the handover keys with the AR's IPv6 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. If Kempf & Koodli Expires August, 2007 [Page 4]
Internet Draft FMIP Security February, 2007 none of the RAs contains an algorithm type indicator corresponding to an algorithm the MN supports, the MN MAY resend the RS 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. 3.4 Sending FBUs 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 authenticator for the message. The MN MUST select the appropriate key for the AR using the AR's address and the care-of CGA. The MN MUST generate the MAC using the handover key and the appropriate algorithm, then include the MAC in the FBU message as defined by the FMIPv6 document. As specified by FMIPv6 [FMIP], the MN MUST include the care-of CGA in a Home Address Option. The FMIPv6 document provides more detail about authenticator algorithm selection and the construction of the authenticator. 3.5 Receiving FBUs When the AR receives an FBU message containing an authenticator, 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 authenticator. 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. 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. The default time for keeping the key valid corresponds to the default time during which forwarding from the previous AR to the new AR is performed for FMIP. The FMIPv6 document [FMIP] provides more detail about the default FMIP forwarding time default. If the MN returns to a previous AR prior to the expiration of the handover key, the AR MAY send and the MN MAY receive the same Kempf & Koodli Expires August, 2007 [Page 5]
Internet Draft FMIP Security February, 2007 handover key as 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 router in an RA. The MN SHOULD discard the handover key after MIPv6 binding update is complete on the new AR. The previous AR MUST discard the key after FMIPv6 forwarding for the previous care-of address times out. 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. The value 0 is invalid. The receiver MUST discard a message that contains this value. Key Length: Length of the encrypted handover key, in units of octets. Encrypted Handover Key: The encrypted handover key. The option is padded to an 8 octet boundary, as required for IPv6 Neighbor Discovery Protocol options. As an example, suppose the authenticator consists of an AES encrypted SHA-1 message digest. Since AES has 128 bit keys, the value of the length field is 24 and the value of the Key Length field is 16. The 16 octets of the key are followed by 4 octets of zeros as padding, to round out the length to a multiple of 8. 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. Kempf & Koodli Expires August, 2007 [Page 6]
Internet Draft FMIP Security February, 2007 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. 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 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 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 Kempf & Koodli Expires August, 2007 [Page 7]
Internet Draft FMIP Security February, 2007 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. As an example of the calculation for the CGA Parameters field, suppose a 128 byte RSA key is used for the CGA public key. Then the Length field is 160 and the Pad Length field is 3. The length of the CGA Parameters field is then 160 - 3 - 4 = 153. 5.0 Security Considerations This document describes a key distribution protocol for the FMIPv6 handover optimization protocol. The key distribution protocol utilizes the CGA public 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. This protocol is subject to the same risks from replay attacks and DoS attacks using the RS as the SEND protocol [SEND]. 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. To avoid state depletion attacks, the handover key MUST NOT be generated prior to SEND processing verifying the RS. This includes processing of the time stamp option to ensure that the RS has not been replayed. State depletion attacks are possible if this ordering is not respected. For other FMIPv6 security considerations, please see the FMIPv6 document [FMIP]. 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 3971, March 2005. [CGA] Aura, T., "Cryptographically Generated Addresses", RFC 3972, March 2005. Kempf & Koodli Expires August, 2007 [Page 8]
Internet Draft FMIP Security February, 2007 [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. [RFC3775] Johnson, D., Perkins, C., and Arkko, J., "Mobility Support in IPv6", RFC 3775, June, 2004. 8.0 Informative References [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 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 The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. Kempf & Koodli Expires August, 2007 [Page 9]
Internet Draft FMIP Security February, 2007 The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. 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, THE IETF TRUST 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 ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 12.0 Copyright Statement Copyright (C) The IETF Trust (2007). 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 Funding for the RFC Editor function is currently provided by the Internet Society. Kempf & Koodli Expires August, 2007 [Page 10]
