Mobopts Working Group Y. Qiu Internet-Draft Institute for Infocomm Research Expires: December 25, 2006 F. Zhao UC Davis R. Koodli Nokia Research Center June 23, 2006 Mobile IPv6 Location Privacy Solutions draft-irtf-mobopts-location-privacy-solutions-02 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/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 25, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract Mobile IPv6 [1] enables mobile nodes to remain reachable while roaming on the Internet. With its current specification, the location of a mobile node can be revealed and its movement can be tracked by simply monitoring its IP packets. In this document, we look into the MIP6 location privacy problem described in [2] and Qiu, et al. Expires December 25, 2006 [Page 1] Internet-Draft MIP6 location privacy solutions June 2006 propose efficient and secure techniques to protect the location privacy of a mobile node. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Brief Overview of Location Privacy in MIP6 . . . . . . . . . . 6 4. Location Privacy Using Return Routability Signaling . . . . . 7 4.1. Route-Optimized Binding Update to the Correspondent Node . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. Reverse-Tunneled Binding Update to the Correspondent Node . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. IP Address Location Privacy with Pseudo Home Address . . . . . 10 5.1. Pseudo Home Address Generation . . . . . . . . . . . . . . 10 5.1.1. Requirements . . . . . . . . . . . . . . . . . . . . . 10 5.1.2. The Shared Key, Kph . . . . . . . . . . . . . . . . . 10 5.1.3. Routable Pseudo Home Address Generation . . . . . . . 11 5.1.4. Dynamic Pseudo Home Address . . . . . . . . . . . . . 11 5.2. Home Binding Updates and Acknowledgements . . . . . . . . 12 5.2.1. Solution with IPsec Transport Mode . . . . . . . . . . 12 5.2.2. Solution with IPsec Tunneling Mode . . . . . . . . . . 15 5.3. Return Routability Signaling . . . . . . . . . . . . . . . 16 5.3.1. Mobile Node as the Initiator . . . . . . . . . . . . . 16 5.3.2. Correspondent Node as the Initiator . . . . . . . . . 19 5.4. Reverse-Tunneling Mode . . . . . . . . . . . . . . . . . . 22 5.5. Prefix Discovery . . . . . . . . . . . . . . . . . . . . . 23 6. Profiling Attack . . . . . . . . . . . . . . . . . . . . . . . 25 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 25 6.2.1. What Invariant should be Updated to Resist the Profiling Attack Effectively? . . . . . . . . . . . . 25 6.2.2. How Often these Invariants should be Updated? . . . . 26 6.2.3. What is the Scope of the Profiling Prevention? . . . 26 6.3. The Increment of Sequence Numbers in Correspondent Binding Updates . . . . . . . . . . . . . . . . . . . . . 26 6.4. The Increment of SPI . . . . . . . . . . . . . . . . . . . 27 7. Security Consideration . . . . . . . . . . . . . . . . . . . . 28 7.1. Home Binding Update Procedure . . . . . . . . . . . . . . 28 7.2. Reverse Tunneling Mode . . . . . . . . . . . . . . . . . . 28 7.3. Route Optimization Mode . . . . . . . . . . . . . . . . . 28 7.4. Return Routability Procedure . . . . . . . . . . . . . . . 29 8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 30 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 31 11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 31 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Qiu, et al. Expires December 25, 2006 [Page 2] Internet-Draft MIP6 location privacy solutions June 2006 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34 Intellectual Property and Copyright Statements . . . . . . . . . . 35 Qiu, et al. Expires December 25, 2006 [Page 3] Internet-Draft MIP6 location privacy solutions June 2006 1. Introduction IP address location privacy is about learning the location information of a mobile node from its IP addresses without authorization. In the presence of mobility, there are two related problems: disclosing a new IP address (care-of address) to a correspondent, and revealing a fixed IP address (home address) to an eavesdropper. To protect its location privacy, a mobile node must not disclose the binding between its care-of address and home address. Related to IP address location privacy is "profiling", where entities link and analyze the activities of a mobile node (just as they may do for any other node). The profiled activities can help attackers compromise the location privacy, especially when combined with additional out-of-band information. Furthermore, once the location privacy is compromised, it may lead to more targeted profiling. In addition to protecting IP address location privacy, solutions should consider how to thwart profiling of various fields, especially those specific to mobility protocol operations. The location privacy problem is described in detail in [2]. In this document, we focus on the location privacy related threats posed by passive attackers. To compromise the location privacy of mobile nodes, these attackers are required to be at certain locations, for example, an eavesdropper along the paths taken by the traffic flows of mobile nodes. The threats posed by active attackers are beyond the scope of this document. Furthermore, in order to simplify the problem, we assume that both correspondent nodes and home agents are fixed nodes. If either is mobile, the same analysis and solutions for mobile nodes may also apply. The basic idea is to use the "pseudo home address" to replace the real home address. One way to generate this pseudo home address is by making use of a pre-shared secret between the home agent and the mobile node. The real home address would be unrelated to the pseudo home address except for the potential commonality of network prefix. This requires a new mechanism between the home agent and the mobile node. The other approach is by masking the real home address using Return Routability parameters to generate the pseudo home address. This approach does not require a new mechanism between the mobile node and its home agent, but the pseudo home address is computed (securely) from the real home address (which would be invisible in data packets). The rest of this document is organized as follows. Section 3 presents a brief overview of MIP6 location privacy. Section 4 presents a mechanism where the pseudo home address is generated using the Return Routability test. Section 5 addresses the IP address location privacy issue. The profiling attack issue is addressed in Qiu, et al. Expires December 25, 2006 [Page 4] Internet-Draft MIP6 location privacy solutions June 2006 Section 6. Finally we present the security consideration and summarize related works in section 7 and 8. 2. Terminology Throughout this document we use the commonly adopted terminology defined in [1] and [2]. 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 [3]. Qiu, et al. Expires December 25, 2006 [Page 5] Internet-Draft MIP6 location privacy solutions June 2006 3. Brief Overview of Location Privacy in MIP6 The current MIP6 specification lacks location privacy considerations. For example, both the home address and the care-of address are available in the following packets: o Home Binding Updates and Acknowledgements o Return Routability packets o Correspondent Binding Updates and Acknowledgements o Prefix Discovery messages o Data packets between mobile nodes and correspondent nodes in the Route Optimization mode Hence, correspondent nodes, eavesdroppers and of course the home agent(s) can learn the complete location information deterministically without authorization from a mobile node. In Route Optimization mode, the mobile node discloses its care-of address in order to receive the packets through the optimized route, thus it must conceal the real home address. If the mobile node is the initiator of the communication, it can conceal its home address from both correspondent nodes and eavesdroppers. When the correspondent node is the initiator, it may already know the real home address depending on how the mobile node publishes its real home address; in such a case, the mobile node can conceal its home address only from eavesdroppers. In Reverse Tunneling mode, a mobile node can hide its current location from its correspondent node and eavesdroppers along the HA-CN path. In the meanwhile, IPsec tunnel mode enables the mobile node to conceal its home address from any eavesdropper along the MN-HA path. Qiu, et al. Expires December 25, 2006 [Page 6] Internet-Draft MIP6 location privacy solutions June 2006 4. Location Privacy Using Return Routability Signaling In this section, we describe how to generate a pseudo home address by making use of information exchanged during the Return Routability procedure. This could provide an easier transition to MIP6 with location privacy. In this solution, there is no need to derive a pseudo home address with the home agent. The basic idea is that both the correspondent node and the mobile node derive a shared privacy management key, Kpm, from the keygen tokens exchanged in the home address and care-of address test procedures. Afterwards, the mobile node uses Kpm to hide its home address in the Binding Update to the correspondent node, and finally the correspondent node authenticates the received Binding Update and restores the mobile node'S home address therein. We describe this in the following sections. 4.1. Route-Optimized Binding Update to the Correspondent Node In the original Return Routability procedure, the home address is visible in the Binding Update to the correspondent node. The mobile node can make the home address invisible to eavesdroppers by replacing the real home address with a pseudo home address generated as follows. The mobile node sets a 'P' bit in the reserved field of the HoTI message to indicate it wishes to use a pseudo home address in place of the home address. The correspondent node, if it supports the 'P' bit, computes a privacy keygen token as follows: privacy keygen token = First (64, Kcn(home address set to all zeros | nonce | 2)) This computation is similar to computing the home keygen token except that the home address is set to all zeros. The privacy keygen token is returned in the HoT message as a Mobility Header Option along with the home keygen token. The care-of address test procedure is exactly the same as specified in MIP6 protocol [1]. The mobile node computes Kpm and the pseudo home address after the Return Routability procedure as follows: Kpm = SHA1 (privacy keygen token | care-of keygen token) pseudo home address = string XOR HoA Qiu, et al. Expires December 25, 2006 [Page 7] Internet-Draft MIP6 location privacy solutions June 2006 where String = First (128, HMAC_SHA1 (Kpm, (care-of address | Home nonce index | Care-of nonce index))) The mobile node then sends the following Binding Update to the correspondent node: o IPv6 header (source = care-of address, destination = correspondent node) o Destination Option * pseudo home address o Mobility header * Binding Update = (sequence number, home nonce index, care-of nonce index) * First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | Binding Update))) When a correspondent node receives a Binding Update with a new destination option carrying the pseudo home address, it must first compute Kpm as above. The computation is similar to how it would compute Kbm, except that the privacy keygen token is computed with the home address set to all zeros. With Kpm, the correspondent node computes the String and recovers the home address. It can then compute the home keygen token and Kbm, and verify the MAC for the Binding Update. If the Binding Update processing is successful, the pseudo home address is considered valid. The correspondent node then stores the nonce indices, and Kbm itself. It may also send a normal Binding Acknowledgment to the mobile node. The String is computed once by both the mobile node and the correspondent node, and hence the pseudo home address as computed above remains constant, until one of the address cookies expires or the mobile node undergoes a handover. 4.2. Reverse-Tunneled Binding Update to the Correspondent Node The mobile node may send the Binding Update not directly to the correspondent node, but via the home agent. No extension to the Return Routability signaling packets is required with reverse- tunneled Binding Updates. The privacy management key Kpm can be the same as the binding management key Kbm and the mobile node generates the pseudo home address as follows: Qiu, et al. Expires December 25, 2006 [Page 8] Internet-Draft MIP6 location privacy solutions June 2006 pseudo home address = Enc(Kpm, home address) Where Enc(.) is a symmetric key encryption algorithm, for example, AES. The format of the Correspondent Binding Update is as follows: o IPv6 header (source = care-of address, destination = home agent) o ESP header in tunnel mode o IPv6 header (source = home address, destination = correspondent node) o Destination Option * pseudo home address o Mobility header * Binding Update * Alternate Care-of Address option (care-of address) When the correspondent node receives a Binding Update with an Alternate Care-of Address option and a Pseudo Home Address option, it first computes Kbm, verifies the MAC for the Binding Update, and then recovers the home address from the pseudo home address, and verifies whether it is actually the same home address present as the source IP address. With this mechanism, the home address is visible as the source IP address along the HA-CN path. However the eavesdroppers on the HA-CN path can launch the attack to compromise the Return Routability procedure anyway. So, within the limitations of the existing Return Routability mechanism, this approach only requires a new destination option type and the associated processing to hide the home address from eavesdroppers. In the subsequent data packets that take the optimized route, only the care-of address and the pseudo home address are visible. Qiu, et al. Expires December 25, 2006 [Page 9] Internet-Draft MIP6 location privacy solutions June 2006 5. IP Address Location Privacy with Pseudo Home Address In this section, we present the mechanism to generate the pseudo home address between the home agent and the mobile node and illustrate the different packet formats when using this pseudo home address in the different scenarios. 5.1. Pseudo Home Address Generation The mobile node can generate a pseudo home address based on a shared secret with its home agent and use this pseudo home address to replace its real home address. When receiving the incoming packets from the mobile node, the home agent derives the real home address thereafter and uses the real home address as one of selectors to check with the local IPsec policy, just like described in RFC 3776 [4]. Afterwards, the home agent updates its Binding Cache to store the recent pseudo home address in addition to the real home address. 5.1.1. Requirements The mechanism to generate the pseudo home address needs to fulfill the following requirements: o Secure: The attacker could not learn the real home address from the eavesdropped pseudo home address. o Routable: When used in the Return Routability procedure, the pseudo home address must be routable, i.e. this IPv6 address should use one of home network prefixes. o Dynamic: To prevent the profiling attack based on the pseudo home address, it is desired that this pseudo home address can be updated periodically. Note that the update must not break the continuity of the current upper layer session(s). 5.1.2. The Shared Key, Kph The pseudo home address is generated based on a shared secret, denoted by Kph, between the mobile node and the home agent. As specified in RFC 3776 [4], IPsec is required to protect the signaling messages between the mobile node and the home agent; thus the trust relationship is in the form of an IPsec security association established either manually or through IKE [5] [6]. If this security association is manually established, Kph can be generated from the shared manual key, denoted by Ks, as follows: Kph = HMAC_SHA1(Ks, 0) Qiu, et al. Expires December 25, 2006 [Page 10] Internet-Draft MIP6 location privacy solutions June 2006 If this security association is established through IKE, Kph is negotiated and renewed by IKE as well, for example, by running the quick mode protected by a previously established IKE security association in phase 2. Either way, Kph is associated with the relevant security association entry in SAD. The location privacy protection option can be negotiated between the home agent and the mobile node. The home agent can distinuguish the regular MIP6 signaling packets from those providing the location proivacy based on the security association and process them appropriately. 5.1.3. Routable Pseudo Home Address Generation The mobile node could formulate its home address in either stateful or stateless manner. The computation of a routable pseudo home address is as follows: pseudo home address = one of home network prefixes || Enc(Kph, interface ID) where Enc(.) can be either a block cipher or a stream cipher AES is a popular block cipher that takes a 128-bit block as input and generates a 128-bit block as output. When AES is applied, the mobile node and the home agent need to append some padding, such as a sequence of zeros, to the Interface ID since it is typically shorter than 128 bits. Also only the first n bits from the output of AES are used so that the pseudo home address is still 128 bit long. If a stream cipher, such as RC4, is used, the interface ID is masked by a sequence of random bits, thus no additional padding or trimming is required. More details regarding how to process inbound and outbound packets are presented in the following sections. Note that the home agent should know the length of home network prefix, for example by looking up a home network prefix table; thus it can correctly identify the encrypted portion in the pseudo home address. Also, the mobile node may choose any from all the available home network prefixes when generating a specific pseudo home address. Preferably, the mobile node should choose the prefix which is not used in its real home address. 5.1.4. Dynamic Pseudo Home Address To update the pseudo home address, one possible way is to generate a sequence of secret keys, {K0, K1, ..., Kn}, from Kph and use these derived keys to generate new pseudo home addresses as follows: Ki = HMAC_SHA1(Kph, i) Qiu, et al. Expires December 25, 2006 [Page 11] Internet-Draft MIP6 location privacy solutions June 2006 pseudo home address = home network prefix || Enc(Ki, interface ID) To avoid maintaining a counter between the mobile node and the home agent, Ki can leverage on the sequence number in the IPsec header. Ki = HMAC_SHA1(Kph, IPsec sequence number) Whenever the mobile node sends a new Home Binding Update, it generates a new key with Kph and the current IPsec sequence number as inputs. As the sequence number in the IPsec header is incremented by at least one every time, the pseudo home address will look different to eavesdroppers on the MN-HA path. Also the mobile node and the home agent do not need to maintain some state when generating the pseudo home address; IPsec anti-replay service, if supported, can detect the reused pseudo home address. If the home agent does not support the anti-replay service, for example when a manual key is used, the mobile node should still use a new sequence number every time; although an eavesdropper could replay the eavesdropped pseudo home address, it is not a new vulnerability. If IKE is used, Kph is updated whenever an IPsec security association expires. If the lifetime of the IPsec security association is based on the number of packets sent, given that the extended sequence number is 64 bits, it is expected that there is no duplicated pseudo home address within a long time period. On the other hand, if Kph is derived from a manual secret key, the same output of Enc(Ki, interface ID) may appear after the sequence number wraps around. However, it may not be a new problem, because the output of Enc(.) (the same length as interface ID) may be not longer than IPsec extened sequece number. In summary, the real home address cannot be revealed from the pseudo home address without the knowledge of Kph. And the pseudo home address fulfills the requirements of being routable and dynamic. 5.2. Home Binding Updates and Acknowledgements 5.2.1. Solution with IPsec Transport Mode When the mobile node moves to a new foreign subnet, it sends the following modified Home Binding Update to its home agent, which usually happens before any other signaling message. o IPv6 header (source = care-of address, destination = home agent) o Destination option header Qiu, et al. Expires December 25, 2006 [Page 12] Internet-Draft MIP6 location privacy solutions June 2006 * Home Address option (pseudo home address) o ESP header in transport mode o Mobility header * Home Binding Update * Alternative Care-of Address option (care-of address) When the home agent receives the Binding Update from the mobile node, it first looks up its SAD using SPI, optionally together with IPsec protocol type and destination IP address. This should return the established security association between the home agent and the mobile node. RFC 3776 [4] represents the corresponding inbound SAD and SPD entries as follows: o home agent SAD: SA1(IN, spi_a, home_agent_1, ESP, TRANSPORT): source = home_address_1 & destination = home_agent_1 & proto = MH o home agent SPD IN: IF source = home_address_1 & destination = home_agent_1 & proto = MH THEN USE SA SA1 The home agent checks whether this is a replayed packet; if not, it uses this security association to process the received IPsec packet. The home agent also checks with its IPsec SPD by using the home address as one of selectors. If a block cipher is used to generate this pseudo home address, the home agent regenerates the pseudo home address from the real home address retrieved. This procedure is the same as described before. The home agent compares the output with the pseudo home address received in the Destination option. If they match, the home agent accepts this Binding Update message. On the other hand, if the stream cipher is used, the home agent recovers the real home address by decrypting the received pseudo home address and the rest is similar with the procedure documented in RFC 3776 [4]. The encryption/decryption operation over a small payload is efficient, thus there is no vulnerability to Denial-of-Service attacks. Note that the home agent should restore the network prefix associated with the mobile node's real home address if a different home network prefix is used to generate the pseudo home address. Qiu, et al. Expires December 25, 2006 [Page 13] Internet-Draft MIP6 location privacy solutions June 2006 If it succeeds, the home agent stores the pseudo home address in the home Binding Cache. The organization of the Binding Cache is extended by adding a new field of pseudo home address as follows: +-------------------+------------+---------------+--------+---+ |pseudo home address|home address|care-of address|lifetime|...| +-------------------+------------+---------------+--------+---+ If the pseduo home address is unique in any snapshot of the Binding Cache, the home agent can look up its Binding Cache by using either the pseduo home address or the home address. The home agent replies to the mobile node with the following modified Home Binding Acknowledgement: o IPv6 header (source = home agent, destination = care-of address) o Routing header (type 2) * pseudo home address o ESP header in transport mode o Mobility header * Home Binding Acknowledgement RFC 3776 [4] specifies the corresponding outbound SAD and SPD entries as follows: o home agent SAD: SA2(OUT, spi_b, home_address_1, ESP, TRANSPORT): source = home_agent_1 & destination = home_address_1 & proto = MH o home agent SPD OUT: IF source = home_agent_1 & destination = home_address_1 & proto = MH THEN USE SA SA2 The detailed procedure is as follows: the home agent generates the Home Binding Acknowledgement with the mobile node's home address as the destination IP address, and then this packet is processed based on the IPsec security association, finally the home agent replaces Qiu, et al. Expires December 25, 2006 [Page 14] Internet-Draft MIP6 location privacy solutions June 2006 the real home address with the appropriate pseudo home address. How the home agent derives the pseudo home address to be used in the Home Binidng Acknowledgement, especially when the mobile node uses different pseudo home addresses with different correspondent nodes, is implementation specific and the details are beyond the scope of this document. For example, the home agent may record the IPsec sequence number received in the Home Binding Update and genenrate the pseudo home address, or the home agent marks the recent unacknowledged Binding Cache entry and uses the pseudo home address therein. The home agent can acknowledge the Home Binding Update in the ascending order of the IPsec sequence number or the time when the Binding Cache entry is created. Compared with the packet formats defined in RFC 3776 [4], the pseudo home address replaces the real home address. In case that the mobile node fails to receive the Binding Acknowledgement, it will retransmit the Binding Update but with a new IPsec sequence number and thus a new pseudo home address, which prevents the replay attack and the profiling attack targeting at the pseudo home address. 5.2.2. Solution with IPsec Tunneling Mode The packet formats above follow the fashion in RFC 3776 [4], in the following we show an alternative that uses the similar packet formats as in [7]. This is applicable when using IKEv2 [6] and the revised IPsec Architecture [8]. Binding Update: o IPv6 header (source = care-of address, destination = home agent) o ESP header in tunnel mode o IPv6 header (source = home address, destination = home agent) o Mobility header * Home Binding Update * Alternative Care-of Address option (care-of address) The home agent processes this Binding Update in the same way as specified in [7]. Additionally, the home agent uses the retrieved Kph to generate the pseudo home address and replaces the previous pseudo home address in respective existing home Binding Cache entry, if any. The Binding Acknowledgement format looks as follows: Qiu, et al. Expires December 25, 2006 [Page 15] Internet-Draft MIP6 location privacy solutions June 2006 o IPv6 header (source = home agent, destination = care-of address) o ESP header in tunnel mode o IPv6 header (source = home agent, destination = home address) o Mobility header * Home Binding Acknowledgement When the mobile node returns home, it can use the pseudo home address or the real home address as the source IP address in the communication with its home agent, for example, for the de- registration Binding Update. The packet formats are similar to those defined in RFC 3776 [4]. 5.3. Return Routability Signaling 5.3.1. Mobile Node as the Initiator When initiating the communication with its correspondent node, the mobile node sends HoTI to its home agent in the following format: o IPv6 header (source = care-of address, destination = home agent) o ESP header in tunneling mode o IPv6 header (source = pseudo home address, destination = correspondent node) o Mobility header * HoTI The home agent would process the received HoTI message in a similar way as described in RFC 3776 [4]. Furthermore, it may derive the real home address by using pseudo home address as a key to look up its binding cache and verify the SPD using the real home address as one of selectors. After that, the home agent generates the following HoTI and forwards it to the correspondent node: o IPv6 header (source = pseudo home address, destination = correspondent node) o Mobility header * HoTI Qiu, et al. Expires December 25, 2006 [Page 16] Internet-Draft MIP6 location privacy solutions June 2006 The correspondent node processes this received HoTI message in the same way as in RFC 3775 [1] and sends the following HoT message to the home agent. o IPv6 header (source = correspondent node, destination = pseudo home address) o Mobility header * HoT = (home init cookie, home keygen token, home nonce index) where home keygen token = First (64, HMAC_SHA1(Kcn, (pseudo home address | nonce | 0))) and Kcn is the correspondent node's local secret [1]. Since the pseudo home address is routable, the HoT message is forwarded to the home network and intercepted by the home agent there. Upon the reception, the home agent uses the pseudo home address as a key to look up its Binding Cache. The search should return the real home address of the mobile node. Then the home agent uses the corresponding security association to process and forward the HoT message to the mobile node. The packet format is as follows: o IPv6 header (source = home agent, destination = care-of address) o ESP header in tunneling mode o IPv6 header (source = correspondent node, destination = pseudo home address) o Mobility header * HoT = (home init cookie, home keygen token, home nonce index) The care-of address test is exactly the same as specified in RFC 3775 [1]. After receiving both HoT and CoT messages, the mobile node sends the Binding Update to the correspondent node in the following format: o IPv6 header (source = care-of address, destination = correspondent node) o Destination Option * pseudo home address Qiu, et al. Expires December 25, 2006 [Page 17] Internet-Draft MIP6 location privacy solutions June 2006 o Mobility header * Binding Update = (sequence number, home nonce index, care-of nonce index) * First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | Binding Update))) where Kbm is the binding management key given by Kbm = SHA1 (home keygen token | care-of keygen token) home keygen token = First (64, HMAC_SHA1(Kcn, (pseudo home address | nonce | 0))) care-of keygen token = First (64, HMAC_SHA1(Kcn, (CoA | nonce | 1))) After receiving the Binding Update, the correspondent node first computes the home keygen token and the care-of keygen token, then computes Kbm and verifies the MAC. If the MAC is valid, it keeps the pseudo home address in the Binding Cache. The correspondent node then generates the following binding acknowledgement and sends back to the mobile node: o IPv6 header (source = correspondent node, destination = care-of address) o Mobility header * sequence number (within the Binding Update message header) * First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | BA))) The subsequent data traffic between the mobile node and the correspondent node will follow the same procedure and the packet formats as specified in [1] except that the pseudo home address is used in place of the home address. Data packets from the mobile node to the correspondent node: o IPv6 header (source = care-of address, destination = correspondent node) o Destination option Qiu, et al. Expires December 25, 2006 [Page 18] Internet-Draft MIP6 location privacy solutions June 2006 * pseudo home address o Payload Data packets from the correspondent node to the mobile node: o IPv6 header (source = correspondent node, destination = care-of address) o Routing Header * pseudo home address o Payload Note that it may be desirable for the mobile node use different pseudo home addresses when communicating with different correspondent nodes. To do so, the mobile node needs to inform the home agent of a new pseudo home address by sending the Home Binding Update before communicating with a new correspondent node. And during the communication with a specific correspondent node, the mobile node uses the same pseudo home address. The mobile node usually can check its Correspondent Binding list to see whether a new pseudo home address is needed. If the correspondent node appears in the Correspondent Binding list, the mobile node uses the existing pseudo home address. Otherwise, the mobile node sends a Home Binding Update to the home agent. With a new IPsec sequence number, both the home agent and the mobile node will generate a new pseudo home address for this correspondent node. Note that the mobile node may extend its Correspondent Binding list to store the pseudo home address associated with a correspondent node. When the communication with a correspondent node is ended, the mobile node may send an explicit de- registration to the home agent to withdraw the corresponding pseudo home address. The home agnet may also implicitly withdraw the pseudo home address, for example, when the Return Routability procedure is not renewed within a certain time period. The strategy to update the home agent's Binding Cache is beyond the scope of this document. The mobile node decides whether a new pseudo home address is needed or an old pseudo home should be withdrawn based on the communication activities with the correspondent node. Besides the solution described above, another way is to leverage on the availability of upper layer connection information; however it may require an interface between the IP layer and the upper transport layer. 5.3.2. Correspondent Node as the Initiator In this case, the correspondent node may already know the real home Qiu, et al. Expires December 25, 2006 [Page 19] Internet-Draft MIP6 location privacy solutions June 2006 address of the mobile node. When this is a concern, the mobile node should not publish its home address, e.g. via DNS. It may be able to make use of runtime binding of user identity to a dynamic home address, for instance using SIP Proxies. When the correspondent node contacts the mobile node at its home address, the mobile node may wish to communicate with the correspondent node via an optimized route. In the following we present the detailed packet formats to protect the location privacy against eavesdroppers in this procedure. The idea is to reverse-tunnel the Correspondent Binding Update via the home agent. In this way, the home address and care-of address test packets are exactly the same as specified in MIP6 protocol [1]. After receiving the HoT message and the CoT message, the mobile node generates the pseudo home address as follows: pseudo home address = Enc(Kbm, home address) Where Enc(.) is a symmetric key encryption algorithm, for example, AES and Kbm is the binding management key given by Kbm = SHA1 (home keygen token | care-of keygen token) home keygen token = First (64, HMAC_SHA1(Kcn, (home address | nonce | 0))) care-of keygen token = First (64, HMAC_SHA1(Kcn, (care-of address | nonce | 1))) The mobile node sends the Binding Update to the correspondent node in the following packet format: o IPv6 header (source = care-of address, destination = home agent) o ESP header in tunnel mode o IPv6 header (source = home address, destination = correspondent node) o Destination Option * pseudo home address o Mobility header * Binding Update Qiu, et al. Expires December 25, 2006 [Page 20] Internet-Draft MIP6 location privacy solutions June 2006 * Alternate Care-of Address option (care-of address) This packet goes through the MN-HA path, and finally arrives at the home agent. After processing this received packet, the home agent forwards the following packet to the correspondent node via the HA-CN path: o IPv6 header (source = home address, destination = correspondent node) o Destination Option * pseudo home address o Mobility header * Binding Update * Alternate Care-of Address option (care-of address) Note that both the home address and the care-of address are made visible in order for the correspondent node to recover the home address and verify the MAC. Although eavesdroppers on the HA-CN path can learn the location of the mobile node, the attackers on this path can compromise the security of Return Routability procedure already. Thus this solution protects the location privacy without introduing too many changes to the Return Routability packets or making the security weaker. After receiving this Binding Update, the correspondent node first computes the home keygen token and the care-of keygen token. It then computes Kbm and decrypts the pseudo home address to recover the home address. The correspondent node keeps both the home address and the pseudo home address in its Binding Cache. It may also send a regular Binding Acknowledgement to the mobile node's home address. In the subsequent data packets between the mobile node and the correspondent node, the home address is replaced by the pseudo home address. The packet from the mobile node to the correspondent node is as follows: o IPv6 header (source = care-of address, destination = correspondent node) o Destination option * pseudo home address Qiu, et al. Expires December 25, 2006 [Page 21] Internet-Draft MIP6 location privacy solutions June 2006 o Payload Note that the correspondent node does not have to decrypt the pseudo home address for every data packet received because it can keep pseudo home address in its Binding Cache. Since the correspondent node needs to search its Binding Cache to verify the incoming data packets with a destination option anyway, it does not cause additional overhead. The packet from the correspondent node to the mobile node is as follows: o IPv6 header (source = correspondent node, destination = care-of address) o Routing Header * pseudo home address o Payload Similarly, the mobile node does not have to recover the home address for each data packet received either, simply by keeping the pseudo home address in its Correspondent Binding list. 5.4. Reverse-Tunneling Mode To hide its care-of address from the correspondent node and its home address from eavesdroppers on the MN-HA path, the mobile node sends IP data packets via the IPsec-protected reverse tunneling in the following format. o IPv6 header (source = care-of address, destination = home agent) o ESP header in tunnel mode o IPv6 header (source = home address, destination = correspondent node) o data payload The home agent forwards the data packet to the correspondent node in the following format. o IPv6 header (source = home address, destination = correspondent node) Qiu, et al. Expires December 25, 2006 [Page 22] Internet-Draft MIP6 location privacy solutions June 2006 o data payload The correspondent node replies with the following data packet that would be intercepted by the home agent. o IPv6 header (source = correspondent node, destination = home address) o data payload The data packet forwarded by the home agent to the mobile node is as follows. o IPv6 header (source = home agent, destination = care-of address) o ESP header in tunnel mode o IPv6 header (source = correspondent node, destination = home address) o data payload Note that if the mobile node is the initiator of the communication with the correspondent node, it may also use the pseudo home address rather than the real home address in the Reverse Tunneling mode, which may require the home agent to look up its Binding Cache and to map the home address to the pseudo home address or the other way around. 5.5. Prefix Discovery Similar with that described in RFC 3776 [4], the following packet format is used for requests for prefixes from the mobile node to the home agent: o IPv6 header (source = care-of address, destination = home agent) o Destination Options header * Home Address option (pseudo home address) o ESP header in transport mode o ICMPv6 * Mobile Prefix Solicitation Similarly, solicited and unsolicited prefix information Qiu, et al. Expires December 25, 2006 [Page 23] Internet-Draft MIP6 location privacy solutions June 2006 advertisements from the home agent to the mobile node use the following format: o IPv6 header (source = home agent, destination = care-of address) o Routing header (type 2) * pseudo home address o ESP header in transport mode o ICMPv6 * Mobile Prefix Advertisement The packet formats similar with those described in [7] can also be used. o IPv6 header (source = care-of address, destination = home agent) o ESP header in tunnel mode o IPv6 header (source = home address, destination = home agent) o ICMPv6 * Mobile Prefix Solicitation and o IPv6 header (source = home agent, destination = care-of address) o ESP header in tunnel mode o IPv6 header (source = home agent, destination = home address) o ICMPv6 * Mobile Prefix Advertisement Qiu, et al. Expires December 25, 2006 [Page 24] Internet-Draft MIP6 location privacy solutions June 2006 6. Profiling Attack 6.1. Overview Pseudo home address provides the IP address location privacy; however, eavesdroppers could still collect, link, and (either online or offline) analyze the activities of mobile nodes based on certain observed fields. The more information collected, the higher probability to compromise the location privacy of mobile nodes, which in return results in more targeted profiling. In the presence of mobility, there exist many invariants, such as fields in the packets and communication patterns, which allows eavesdroppers to easily correlate the observed activities. For example, eavesdroppers can use the following, but not limited to, information to profile the activities of mobile nodes. o On the MN-HA path: the care-of address, the home address, the pseudo home address, the IPsec sequence number, SPI, Initialization Vector (IV), the timing of HoTI messages o On the HA-CN path: eavesdroppers on this path could intercept the traffic to or from mobile nodes, thus we do not consider the threats arising from this path. o On the MN-CN path: the care-of address, the home address or the pseudo home address, the sequence number in the Correspondent Binding Update, the interval of Return Routability packets, etc. 6.2. Discussion To resist the profiling attack, these invariants need to be updated periodically. RFC 3041 [9] takes a similar approach to provide the privacy protection: the IPv6 address is updated over time. In the context of mobility support, there are the following three specific issues to be addressed. 6.2.1. What Invariant should be Updated to Resist the Profiling Attack Effectively? Different invariants allow eavesdroppers to correlate the observed activities with the different levels of assurance. Obviously a constant identity allows eavesdroppers to link the activities of a mobile node in a deterministic way; and other invariants may be less reliable because they are affected by other factors. For example, a malicious entity may profile the traffic based on the care-of address, however the mobile node may renew its care-of address via DHCP or IPv6 address privacy extension; the sequence numbers Qiu, et al. Expires December 25, 2006 [Page 25] Internet-Draft MIP6 location privacy solutions June 2006 appearing in the IPsec headers as well as the Correspondent Binding Updates in one flow may mix with those in another flow; the timing of MIP6 Return Routability packets is easily affetced by the background traffic and routing dynamics. Nevertheless, these fields and phenomena provide additional hints to malicious entities. We must update the identity of mobile nodes and should update other invariants as much as possible. 6.2.2. How Often these Invariants should be Updated? Generally, the more frequent the update is, the more likely the profiling attack is prevented and also the higher costs in terms of communication and processing overheads. As the malicious entity has many choices to profile the activities, one might consider updating all the possible invariants with same frequency because the granularity of profiling depends on the longest interval of updation. In other words, from the cost-effectiveness perspective, it is not necessary to update some invariants too frequently if other invariants cannot be updated so frequently. 6.2.3. What is the Scope of the Profiling Prevention? From the perspective of a mobile node, the activities when communicating with different correspondent nodes should not be correlated, nor should the different sessions with the same correspondent node. The former case requires that the mobile node use different pseudo home addresses when communicating with different correspondent nodes and the latter case requires that the mobile node use different pseudo home addresses in the different sessions with the same correspondent node. If the mobile node performs handover during the communication with its correspondent node, it is desired that eavesdroppers near the correspondent node cannot track the movements of the mobile node. Different scope of the profiling prevention results in different levels of complexity. In the previous sections, the packet formats when the mobile node uses different pseudo home addresses when communicating with different correspondent nodes are described. It also is worth bearing in mind that attackers must be attached to certain paths. It seems reasonable to assume that if the mobile node roams to another foreign subnet, eavesdroppers attaching to the previous MN-HA and MN-CN paths cannot access the new MN-HA and MN-CN paths. If this is the case, we only need to consider updating the invariants when the mobile node stays in the same location. 6.3. The Increment of Sequence Numbers in Correspondent Binding Updates RFC 3775 [1] only requires that the sequence number in the Binding Qiu, et al. Expires December 25, 2006 [Page 26] Internet-Draft MIP6 location privacy solutions June 2006 Update is greater than that received in the previous valid Binding Update for this home address. However, if the increment of sequence number is fixed, an eavesdropper is able to identify the activities of mobile node. We propose the increment of sequence number as follows: o seq#_increment = First(8, HMAC_SHA1(Kbm, home nonce index | care nonce index)) o If seq#_increment = 0, then set seq#_increment = 1 o Seq# = (previous Seq# + seq#_increment) modulo (2^16) To avoid causing the sequence number wrapping around quickly and generate enough randomness, the first 8 bits of the keyed hash function output is used. 6.4. The Increment of SPI To prevent eavesdroppers on the MN-HA path correlating the packets based on the constant SPI, both the mobile node and the home agent can update the SPI based on the following method: o SPI_increment = First(8, HMAC_SHA1(Kph, the current SPI)) o If SPI_increment = 0, then set SPI_increment = 1 o the new SPI = (the current SPI + SPI_increment) modulo (2^32) The mobile node and the home agent could update the SPI when a Home Binding Update is sent or received. The new SPI is applied to the next Home Binding Update procedure. Qiu, et al. Expires December 25, 2006 [Page 27] Internet-Draft MIP6 location privacy solutions June 2006 7. Security Consideration This document addresses a security issue in the mobile environment, location privacy. The proposed solutions do not introduce any new vulnerability. 7.1. Home Binding Update Procedure When the mobile node roams to a new foreign subnet, it sends the modified Home Binding Update to its home agent and receives the modified Home Binding Acknowledgement from its home agent. In both messages, the pseudo home address is used in place of the home address. Eavesdroppers is unable to derive the real home address from the pseudo home address and thus to correlate the care-of address with the home address. Moreover, the pseudo home address can be updated to prevent eavesdroppers linking the mobile node's ongoing activities together. The home agent can derive the real home address from the received pseudo home address efficiently because the encryption/decryption operation is done over a small amount of data (in this case, less than 128 bits), thus the home agent could resist the Denial-of- Service attack when attackers flood with the forged Home Binding Updates. 7.2. Reverse Tunneling Mode In this mode, the correspondent node sends data packets to the mobile node's home address, thus it is not aware of the movement of the mobile node. The home agent intercepts the data packets from the correspondent node and tunnels them to the mobile node's care-of address by IPsec ESP tunneling mode. Thus the home address is not visible to the eavesdroppers on the MN-HA path either since the inner IPv6 header is encrypted. 7.3. Route Optimization Mode In this mode, since the mobile node communicates with the correspondent node using its care-of address, the mobile node has to hide its home address from eavesdroppers and even correspondent nodes. This is accomplished as follows. If the mobile node is the initiator of the communication with the correspondent node, it performs the modified Correspondent Binding Update procedure as described in section 3. By replacing the home address with the pseudo home address in the messages involved, the binding between the home address and the care-of address is not disclosed to eavesdroppers and the correspondent node. And the Qiu, et al. Expires December 25, 2006 [Page 28] Internet-Draft MIP6 location privacy solutions June 2006 continuity of the current session is kept. If the correspondent node is the initiator of the communication with the mobile node, the mobile node also performs the modified Correspondent Binding Update procedure with the correspondent node after the first contact. The mobile node can conceal its home address to eavesdroppers only since the correspondent node already knows its real home address. Note the same analysis also applies to the data packets. 7.4. Return Routability Procedure As the pseudo home address is required to be routable, the modified Return Routability procedure provides the same security strength as in RFC 3775 [1]. Qiu, et al. Expires December 25, 2006 [Page 29] Internet-Draft MIP6 location privacy solutions June 2006 8. Related Work Our work benefits from previous works and discussions. Similar with this document, many drafts proposed using a temporary identity to replace the mobile node's home address in IPsec SA, MIP6 signaling messages and data packets. However, the details of how to generate and update this additional identity are absent. RFC 3041 [9] specifies the mechanism to update a stateless IPv6 address periodically. Although it is possible to update the care-of address and the home address based on RFC 3041, we further consider the shortest interval to do so in order to resist the profiling attack effectively and efficiently. The draft [10] proposes using a temporary identity, TMI, to replace the home address in the scenarios of mobile client and mobile server, and also discussed the feasibility of utilizing CBID/CGA/MAP to further protect the location privacy. However, as a 128 bit random number, TMI is not suitable to be the source IP address in the HoTI message forwarded by the home agent to the correspondent node because TMI is not routable and the home agent cannot receive the HoT message from the correspondent node. Furthermore the draft does not specify how to update TMI or address profiling attacks. The draft [11] proposes to update the identity based on a key and a previous identity. The packet formats are presented. The draft [12] proposes to update the mobile node's home address periodically to hide the movement. The new identity is generated from the current local network prefix, the binding update session key and the previous home address. The new home address is random, routable, recognizable and recoverable. Also it seems that the home address is updated as frequently as the Return Routeability procedure. The draft [13] intends to achieve both route optimization and location privacy at the same time. The proposed solution is to reverse-tunnel the traffic to an additional entity. This kind of architectural solution achieves only the recoverable location privacy instead. 9. IANA Considerations This document may specify IANA Type assignment(s) in subsequent versions. Qiu, et al. Expires December 25, 2006 [Page 30] Internet-Draft MIP6 location privacy solutions June 2006 10. Conclusion In this document, we introduced efficient and secure solutions to protect location privacy of a mobile node. The central idea is to use a pseudo home address instead of the mobile node's real home address in IP packets of this mobile node. It is possible to update this pseudo home address whenever the mobile node moves to a new location or starts a communication with a new correspondent node. This results in the binding between the care-of address and the home address is hidden to eavesdroppers or even correspondent nodes in some scenarios. Moreover, this pseudo home address is routable, thus the security of this proposed return routeability test is not weakened. We intend to make the best tradeoffs among many related factors during the design. Also we present the thorough analyses of MIP6 location privacy issues and also some best practices to enhance the location privacy. This would help design alternative solutions when a different tradeoff is desired. Furthermore, the mobile node may also desire to hide its movement to the home agent in some cases; the details are beyond the scope of this document. 11. Acknowledgement The authors wish to thank the co-authors of previous drafts from which this draft is derived: Vijay Devarapalli, Hannu Flinck, Charlie Perkins, Feng Bao, Robert Deng, James Kempf, and Jianying Zhou. In addition, sincere appreciation is also extended to Wassim Haddad, Claude Castelluccia, Francis Dupont, Gabriel Montenegro, Greg Daley, Kilian Weniger and Takashi Aramaki for their valuable contributions and discussions. 12. References [1] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004. [2] Koodli, R., "IP Address Location Privacy and Mobile IPv6: Problem Statement", draft-ietf-mip6-location-privacy-ps-01 (work in progress), March 2006. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [4] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to Protect Mobile IPv6 Signaling Between Mobile Nodes and Home Agents", RFC 3776, June 2004. Qiu, et al. Expires December 25, 2006 [Page 31] Internet-Draft MIP6 location privacy solutions June 2006 [5] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [6] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [7] Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with IKEv2 and the revised IPsec Architecture", draft-ietf-mip6-ikev2-ipsec-06 (work in progress), April 2006. [8] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [9] Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001. [10] Castelluccia, C., Dupont, F., and G. Montenegro, "Protocol for Protecting Movement of Mobile Nodes in Mobile IPv6", draft-dupont-mip6-privacyext-02 (work in progress), July 2005. [11] Bao, F., Deng, R., Kempf, J., Qiu, Y., and J. Zhou, "Protocol for Protecting Movement of Mobile Nodes in Mobile IPv6", draft-qiu-mip6-mnprivacy-00 (work in progress), March 2005. [12] Bao, F., Deng, R., Kempf, J., Qiu, Y., and J. Zhou, "Protocol for Protecting Movement of Mobile Nodes in Mobile IPv6", draft-qiu-mip6-hiding-movement-00 (work in progress), March 2005. [13] Weniger, K. and T. Aramaki, "Route Optimization and Location Privacy using Tunneling Agents (ROTA)", draft-weniger-rota-01 (work in progress), October 2005. [14] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [15] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [16] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [17] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [18] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998. Qiu, et al. Expires December 25, 2006 [Page 32] Internet-Draft MIP6 location privacy solutions June 2006 [19] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [20] Koodli, R., Devarapalli, V., Flinck, H., and C. Perkins, "Solutions for IP Address Location Privacy in the presence of IP Mobility", draft-koodli-mip6-location-privacy-solutions-00 (work in progress), February 2005. [21] Daley, G., "Location Privacy and Mobile IPv6", draft-daley-mip6-locpriv-00 (work in progress), January 2004. Qiu, et al. Expires December 25, 2006 [Page 33] Internet-Draft MIP6 location privacy solutions June 2006 Authors' Addresses Ying Qiu Institute for Infocomm Research 21 Heng Mui Keng Terrace Singapore 119613 Phone: +65-6874-6742 Email: qiuying@i2r.a-star.edu.sg Fan Zhao University of California Davis One Shields Avenue Davis, CA 95616 US Phone: +1 530 752 3128 Email: fanzhao@ucdavis.edu Rajeev Koodli Nokia Research Center 313 Fairchild Drive Mountain View, CA 94043 US Email: rajeev.koodl@nokia.com Qiu, et al. Expires December 25, 2006 [Page 34] Internet-Draft MIP6 location privacy solutions June 2006 Intellectual Property Statement 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. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Qiu, et al. Expires December 25, 2006 [Page 35]