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Versions: 00 01 draft-ietf-6man-stable-privacy-addresses

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   and derivative works of it may not be created, and it may not be
   published except as an Internet-Draft.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
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   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



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   (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
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   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























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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).



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   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].











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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).

















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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.





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   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.












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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.














































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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.















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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.





































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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).







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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



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   identifiers in the Interface-ID.  The method specified in this
   document would mitigate the aforementioned host-scanning attacks.

















































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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, <http://
              www.si6networks.com/presentations/deepsec2011/
              fgont-deepsec2011-ipv6-security.pdf>.

   [Broersma]
              Broersma, R., "IPv6 Everywhere: Living with a Fully IPv6-
              enabled environment",  Australian IPv6 Summit 2010,
              Melbourne, VIC Australia, October 2010,
              <http://www.ipv6.org.au/summit/talks/Ron_Broersma.pdf>.

   [CPNI-IPv6]
              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).






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Author's Address

   Fernando Gont
   UK CPNI

   Email: fgont@si6networks.com
   URI:   http://www.cpni.gov.uk












































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