IPv6 maintenance Working Group (6man) F. Gont Internet-Draft UK CPNI Updates: 4291, 4862 (if approved) March 31, 2012 Intended status: Standards Track Expires: October 2, 2012 A method for Generating Stable Privacy-Enhanced Addresses with IPv6 Stateless Address Autoconfiguration (SLAAC) draft-gont-6man-stable-privacy-addresses-01 Abstract This document specifies a method for generating IPv6 Interface Identifiers to be used with IPv6 Stateless Address Autoconfiguration (SLAAC), such that addresses configured using this method are stable within each subnet, but the Interface Identifier changes when hosts move from one network to another. The aforementioned method is meant to be an alternative to generating Interface Identifiers based on IEEE identifiers, such that the benefits of stable addresses can be achieved without sacrificing the privacy of users. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. This document may not be modified, and derivative works of it may not be created, and it may not be published except as an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on October 2, 2012. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents Gont Expires October 2, 2012 [Page 1] Internet-Draft Stable Privacy Addresses March 2012 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Design goals . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Algorithm specification . . . . . . . . . . . . . . . . . . . 6 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 7. Privacy issues still present with RFC 4941 . . . . . . . . . . 11 7.1. Host tracking . . . . . . . . . . . . . . . . . . . . . . 11 7.1.1. Tracking hosts across networks #1 . . . . . . . . . . 11 7.1.2. Tracking hosts across networks #2 . . . . . . . . . . 12 7.1.3. Revealing the identity of a devices performing server-like functions . . . . . . . . . . . . . . . . 12 7.2. Host scanning-attacks . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 8.1. Normative References . . . . . . . . . . . . . . . . . . . 14 8.2. Informative References . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 Gont Expires October 2, 2012 [Page 2] Internet-Draft Stable Privacy Addresses March 2012 1. Introduction [RFC4862] specifies the Stateless Address Autoconfiguration (SLAAC) for IPv6, which typically results in hosts configuring one or more "stable" addresses composed of a network prefix advertised by a local router, and an Interface Identifier (IID) that typically embeds a hardware address (e.g., using IEEE identifiers) [RFC4291]. Generally, static addresses are thought to simplify network management, since they simplify ACLs and logging. However, since IEEE identifiers are typically globally unique, the resulting IPv6 addresses can be leveraged to track and correlate the activity of a node over time and across multiple subnets and networks, thus negatively affecting the privacy of users. The "Privacy Extensions for Stateless Address Autoconfiguration in IPv6" [RFC4941] were introduced to complicate the task of eavesdroppers and other information collectors to correlate the activities of a node, and basically result in temporary (and random) Interface Identifiers that are typically more difficult to leverage than those based on IEEE identifiers. When privacy extensions are enabled, "privacy addresses" are employed for "outgoing communications", while the traditional IPv6 addresses based on IEEE identifiers are still used for "server" functions (i.e., receiving incoming connections). As noted in [RFC4941], "anytime a fixed identifier is used in multiple contexts, it becomes possible to correlate seemingly unrelated activity using this identifier". Therefore, since "privacy addresses" [RFC4941] do not eliminate the use of fixed identifiers for server-like functions, they only *partially* mitigate the correlation of host activities (see Section 7 for some example attacks that are still possible with privacy addresses). Therefore, it is vital that the privacy characteristics of "stable" addresses are improved such that the ability of an attacker correlating host activities across networks is reduced. Another important aspect not mitigated by "Privacy Addresses" [RFC4941] is that of host scanning. Since IPv6 addresses that embed IEEE identifiers have specific patterns, an attacker could leverage such patterns to greatly reduce the search space for "live" hosts. Since "privacy addresses" do not eliminate the use of IPv6 addresses that embed IEEE identifiers, host scanning attacks are still feasible even if "privacy extensions" are employed [Gont-DEEPSEC2011] [CPNI-IPv6]. This is yet another motivation to improve the privacy characteristics of "stable" addresses (currently embedding IEEE identifiers). Gont Expires October 2, 2012 [Page 3] Internet-Draft Stable Privacy Addresses March 2012 Privacy/temporary addresses can be challenging in a number of areas. For example, from a network-management point of view, they tend to increase the complexity of event logging, trouble-shooting, and enforcing access controls and quality of service, etc. As a result, some organizations disable the use of privacy addresses even at the expense of reduced privacy [Broersma]. Also, they result in increased complexity, which might not be possible or desirable in some implementations (e.g., some embedded devices). In scenarios in which "Privacy Extensions" are deliberately not used (possibly for any of the aforementioned reasons), all a host is left with is the addresses that have been generated using e.g. IEEE identifiers, and this is yet another case in which it is also vital that the privacy characteristics of these stable addresses are improved. We note that in most (if not all) of those scenarios in which "Privacy Extensions" are disabled, there is usually no actual desire to negatively affect user privacy, but rather a desire to simplify operation of the network (simplify the use of ACLs, logging, etc.). This document specifies a method to generate interface identifiers that are stable/constant within each subnet, but that change as hosts move from one network to another, thus keeping the "stability" properties of the interface identifiers specified in [RFC4291], while still mitigating host-scanning attacks and preventing correlation of the activities of a node as it moves from one network to another. For nodes that currently disable "Privacy extensions" [RFC4941] for some of the reasons stated above, this mechanism provides stable privacy-enhanced addresses which may already address most of the privacy concerns related to addresses that embed IEEE identifiers [RFC4291]. On the other hand, in scenarios in which "Privacy Extensions" are employed, implementation of the mechanism described in this document would mitigate host-scanning attacks and also mitigate correlation of host activities. 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]. Gont Expires October 2, 2012 [Page 4] Internet-Draft Stable Privacy Addresses March 2012 2. Design goals This document specifies a method for selecting interface identifiers to be used with IPv6 SLAAC, with the following goals: o The resulting interface identifier remains constant/stable for each prefix used with SLAAC within each subnet. That is, the algorithm generates the same interface identifier when configuring an address belonging to the same prefix within the same subnet. o The resulting interface identifier does change when addresses are configured for different prefixes. That is, if different autoconfiguration prefixes are used to configure addresses for the same network interface card, the resulting interface identifiers must be (statistically) different. o It must be difficult for an outsider to predict the interface identifiers that will be generated by the algorithm, even with knowledge of the interface identifiers generated for configuring other addresses. o The aforementioned interface identifiers are meant to be an alternative to those based on IEEE identifiers, as specified in [RFC4291]. We note that of use of the algorithm specified in this document is (to a large extent) orthogonal to the use of "Privacy Extensions" [RFC4941]. Hosts that do not implement/use "Privacy Extensions" would have the benefit that they would not be subject to the host- tracking and host scanning issues discussed in the previous section. On the other hand, in the case of hosts employing "Privacy Extensions", the method specified in this document would prevent host scanning attacks and correlation of node activities across networks (see Section 7). Gont Expires October 2, 2012 [Page 5] Internet-Draft Stable Privacy Addresses March 2012 3. Algorithm specification IPv6 implementations conforming to this specification MUST generate interface identifiers using the algorithm specified in this section. The aforementioned algorithm MUST be employed for generating the interface identifiers for all the IPv6 addresses configured with SLAAC for a given interface, including IPv6 link-local addresses. Unless otherwise noted, all of the parameters included in the expression below MUST be included when generating an Interface ID. 1. Compute a random (but stable) identifier with the expression: RID = F(Prefix, Modified_EUI64, Network_ID, secret_key) Where: RID: Random (but stable) identifier F(): A pseudorandom function (PRF) that is not computable from the outside (without knowledge of the secret key). The PRF could be implemented as a cryptographic hash of the concatenation of each of the function parameters . Prefix: The prefix to be used for SLAAC, as learned from an ICMPv6 Router Advertisement message. Modified_EUI64: The Modified EUI-64 format identifier corresponding to this network interface. Network_ID: Some network specific data that identifies the subnet to which this interface is attached. For example the IEEE 802.11 SSID corresponding to the network to which this interface is associated. This parameter is OPTIONAL. secret_key: A secret key that is not known by the attacker. The secret key MUST be initialized at system installation time to the concatenation of a pseudo-random number (see [RFC4086] for randomness requirements for security) and the machine's serial number. An implementation MAY provide the means for the user to change the secret key. Gont Expires October 2, 2012 [Page 6] Internet-Draft Stable Privacy Addresses March 2012 2. The Interface Identifier is finally obtained by taking the leftmost 64 bits of the RID value computed in the previous step, and and setting bit 6 (the leftmost bit is numbered 0) to zero. This creates an interface identifier with the universal/local bit indicating local significance only. Note that the result of F() in the algorithm above is no more secure than the secret key. If an attacker is aware of the PRF that is being used by the victim (which we should expect), and the attacker can obtain enough material (i.e. addresses configured by the victim), the attacker may simply search the entire secret-key space to find matches. To protect against this, the secret key should be of a reasonable length. Key lengths of at least 128 bits should be adequate. The secret key is initialized at installation time to the concatenation of a pseudo-random number and the machine's serial number. This allows this mechanism to be enabled/used automatically, without user intervention. The machine's serial number is concatenated to the pseudo-random number, such that the entropy of the key is increased (since at installation time the entropy of the Pseudo-Random Number Generator might be reduced). Including the SLAAC prefix in the PRF computation causes the Interface ID to vary across networks that employ different prefixes, thus mitigating host-tracking attacks and any other attacks that benefit from predictable Interface IDs (such as host scanning). Including the optional Network_ID parameter when computing the RID value above would cause the algorithm to produce a different Interface Identifier when connecting to different networks, even when configuring addresses belonging to the same prefix. This means that a host would employ a different Interface ID as it moves from one network to another even for IPv6 link-local addresses or Unique Local Addresses (ULAs). Note that there are a number of ways in which these addresses might leak out. For example, an attacker could use ICMPv6 Node Information queries [RFC4620] to obtain such addresses. Gont Expires October 2, 2012 [Page 7] Internet-Draft Stable Privacy Addresses March 2012 4. IANA Considerations There are no IANA registries within this document. The RFC-Editor can remove this section before publication of this document as an RFC. Gont Expires October 2, 2012 [Page 8] Internet-Draft Stable Privacy Addresses March 2012 5. Security Considerations This document specifies an algorithm for generating interface identifiers to be used with IPv6 Stateless Address Autoconfiguration (SLAAC), replacing e.g. the Modified EUI-64 format identifiers. When compared to modified EUI-64 format identifiers, the identifiers specified in this document have a number of advantages: o They prevent trivial host-tracking, since when a host moves from one network to another the network prefix used for autoconfiguration and/or the Network ID (e.g., IEEE 802.11 SSID) will typically change, and hence the resulting interface identifier will also change (see Section 7. o They mitigate host-scanning techniques which leverage predictable interface identifiers (e.g., known Organizational Unique Identifiers). We note that this algorithm is meant to replace Modified EUI-64 format identifiers, but not the temporary-addresses such as those specified in [RFC4941]. Clearly, temporary addresses may help to mitigate the correlation of activities of a node within the same network, and may also reduce the attack exposure window (since the lifetime of privacy/temporary IPv6 address is reduced when compared to that of addresses generated with the method specified in this document). We note that implementation of this algorithm would still benefit those hosts employing "Privacy Addresses", since it would mitigate host-tracking vectors still present when privacy addresses are used (Section 7, and would also mitigate host-scanning techniques that leverage patterns in IPv6 addresses that embed IEEE identifiers. Finally, we note that the method described in this document may mitigate most of the privacy concerns arising from the use of IPv6 addresses that embed IEEE identifiers, without the use of temporary addresses, thus possibly offering an interesting trade-off for those scenarios in which the use of temporary addresses is not feasible. Gont Expires October 2, 2012 [Page 9] Internet-Draft Stable Privacy Addresses March 2012 6. Acknowledgements The author would like to thank (in alphabetical order) Steven Bellovin, Dominik Elsbroek, Ray Hunter, Jong-Hyouk Lee, and Michael Richardson, for providing valuable comments on earlier versions of this document. This document is based on the technical report "Security Assessment of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by Fernando Gont on behalf of the UK Centre for the Protection of National Infrastructure (CPNI). Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for their continued support. Gont Expires October 2, 2012 [Page 10] Internet-Draft Stable Privacy Addresses March 2012 7. Privacy issues still present with RFC 4941 This aims to clarify the motivation of using the method specified in this document even when privacy/temporary addresses are employed. It has been incorporated in the document to clarify a number of questions that arose during the presentation of this document at IETF 83 (Paris). This entire section might be removed prior to publication of this document as an RFC. 7.1. Host tracking Some 6man participants questioned the inclusion of the SLAAC prefix in PRF function, and noted that the ID of "stable" addresses need not change across networks, since privacy/temporary addresses already mitigate host tracking. This section describes one possible attack scenario that illustrates that host-tracking may still be possible when privacy/temporary addresses are employed. 7.1.1. Tracking hosts across networks #1 A host configures the stable addresses without including the Prefix in the F() (the PRF). The aforementioned host now runs any application that needs to perform a server-like function (e.g. a peer-to-peer application). As a result of that, an attacker/user participating in the same application (e.g., P2P) would learn the Interface-ID used for the stable address. Some time later, the same host moves to a completely different network, and uses the same P2P application, probably even with a different user. The attacker now interacts with the same host again, and hence can learn the "new" stable address. Since the interface ID is the same as the one used before, the attacker can infer that it is communicating with the same device as before. Had the host included the Prefix when computing the Interface-ID (with the hash function F()) as RECOMMENDED in this document, the Interface-ID would have been different, and this privacy attack would not have been possible. This is just *one* possible attack scenario, which should remind us that one should not disclose more than it is really needed for achieving a specific goal (and an Interface-ID that is constant across different networks does exactly that: it discloses more information than is needed for providing a stable address). Gont Expires October 2, 2012 [Page 11] Internet-Draft Stable Privacy Addresses March 2012 7.1.2. Tracking hosts across networks #2 Once an attacker learns the fixed Interface-ID employed by the victim host for its stable address, the attacker is able to "probe" a network for the presence of such host at any given network. See Section 7.1.1 for just one example of how an attacker could learn such prefix. Other examples include being able to share the same network segment at some point in time (think about sharing a conference network with 1000+ peers), etc. For example, if an attacker learns that in one network the victim used the prefix 1111:2222:3333:4444 for its stable addresses, then we could subsequently probe for the presence of such device in the network 2011:db8::/64 by sending a probe packet (ICMPv6 Echo Request, or your favourite probe packet) to the address 2001:db8::1111:2222: 3333:4444. 7.1.3. Revealing the identity of a devices performing server-like functions Some devices may typically perform server-like functions and may be usually moved from one network to another (e.g., from storage devices to printers). Such devices are likely to simply disable (or not even implement) privacy/temporary addresses [RFC4941]. If the aforementioned devices employ Interface-IDs that are constant across networks, it would be trivial for an attacker to tell whether the same device is being used across networks by simply looking at the Interface ID. Clearly, performing server-like should not imply that a device discloses its identity (i.e., that the attacker can tell whether it is the same device providing some function in two different networks, at two different points in time. The scheme proposed in this document prevents such information leakage by causing nodes to generate different Interface-IDs when moving to one network to another, thus mitigating this kind of privacy attack. 7.2. Host scanning-attacks While it is usually assumed that host-scanning attacks are unfeasible, an attack can leverage patterns in IPv6 address generation to greatly reduce the search space. As noted earlier in this document, privacy/temporary addresses do not eliminate the use of IPv6 addresses that embed IEEE identifiers, and hence such hosts would still be vulnerable to host-scanning attacks unless they eliminate the patterns introduced by embedding IEEE Gont Expires October 2, 2012 [Page 12] Internet-Draft Stable Privacy Addresses March 2012 identifiers in the Interface-ID. The method specified in this document would mitigate the aforementioned host-scanning attacks. Gont Expires October 2, 2012 [Page 13] Internet-Draft Stable Privacy Addresses March 2012 8. References 8.1. Normative References [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, September 2007. 8.2. Informative References [RFC4620] Crawford, M. and B. Haberman, "IPv6 Node Information Queries", RFC 4620, August 2006. [Gont-DEEPSEC2011] Gont, "Results of a Security Assessment of the Internet Protocol version 6 (IPv6)", DEEPSEC 2011 Conference, Vienna, Austria, November 2011, . [Broersma] Broersma, R., "IPv6 Everywhere: Living with a Fully IPv6- enabled environment", Australian IPv6 Summit 2010, Melbourne, VIC Australia, October 2010, . [CPNI-IPv6] Gont, F., "Security Assessment of the Internet Protocol version 6 (IPv6)", UK Centre for the Protection of National Infrastructure, (available on request). Gont Expires October 2, 2012 [Page 14] Internet-Draft Stable Privacy Addresses March 2012 Author's Address Fernando Gont UK CPNI Email: fgont@si6networks.com URI: http://www.cpni.gov.uk Gont Expires October 2, 2012 [Page 15]