Network Working Group                                          D. Thaler
Internet-Draft                                                 Microsoft
Intended status: Informational                          October 18, 2015
Expires: April 20, 2016

 Privacy Considerations for IPv6 over Networks of Resource-Constrained


   This document discusses how a number of privacy threats apply to
   technologies designed for IPv6 over networks of resource-constrained
   nodes, and provides advice to protocol designers on how to address
   such threats in IPv6-over-foo adaptation layer specifcations.

Status of This Memo

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   This Internet-Draft will expire on April 20, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Amount of Entropy Needed  . . . . . . . . . . . . . . . . . .   3
   3.  Potential Approaches  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  IEEE-Identifier-Based Addresses . . . . . . . . . . . . .   5
     3.2.  Short Addresses . . . . . . . . . . . . . . . . . . . . .   5
   4.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   RFC 6973 [RFC6973] discusses privacy considerations for Internet
   protocols, and Section 5.2 in particular covers a number of privacy-
   specific threats.  In the context of IPv6 addresses, Section 3 of
   [I-D.ietf-6man-ipv6-address-generation-privacy] provides further
   elaboration on the applicability of the privacy threats.

   When interface identifiers (IIDs) are generated without sufficient
   entropy compared to the link lifetime, devices and users can become
   vulnerable to the various threats discussed there, including:

   o  Correlation of activities over time, if the same identifier is
      used for Internet traffic over period of time

   o  Location tracking, if the same interface identifier is used with
      different prefixes as a device moves between different networks

   o  Device-specific vulnerability exploitation, if the identifier
      helps identify a vendor or version or protocol and hence suggests
      what types of attacks to try

   o  Address scanning, which enables all of the above attacks by off-
      link attackers.

   Typically "enough" bits of entropy means at least 46 bits (see
   Section 2 for why); ideally all 64 bits of the IID should be used,
   although historically some bits have been excluded for reasons
   discussed in [RFC7421].

   For these reasons, [I-D.ietf-6man-default-iids] recommends using an
   address generation scheme in [RFC7217], rather than addresses
   generated from a fixed IEEE identifier.

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   Furthermore, to mitigate the threat of correlation of activities over
   time on long-lived links, [RFC4941] specifies the notion of a
   "temporary" address to be used for transport sessions (typically
   locally-initiated outbound traffic to the Internet) that should not
   be linkable to a more permanent identifier such as a DNS name, user
   name, or stable hardware address.  Indeed, the default address
   selection rules [RFC6724] now prefer temporary addresses by default
   for outgoing connections.  If a device needs to simultaneously
   support unlinkable traffic as well as traffic that is linkable to
   such a stable identifier, this necessitates supporting simultaneous
   use of multiple addresses per device.

2.  Amount of Entropy Needed

   In terms of privacy threats discussed in
   [I-D.ietf-6man-ipv6-address-generation-privacy], the one with the
   need for the most entropy is address scans.  To mitigate address
   scans, one needs enough entropy to make the probability of a
   successful address probe be negligible.  Typically this is measured
   in the length of time it would take to have a 50% probability of
   getting at least one hit.  Address scans often rely on sending a
   packet such as a TCP SYN or ICMP Echo Request, and determining
   whether the reply is an ICMP unreachable error (if no host exists) or
   a TCP response or ICMP Echo Reply (if a host exists), or neither in
   which case nothing is known for certain.

   Many privacy-sensitive devices support a "stealth mode" as discussed
   in Section 5 of [RFC7288] whereby they will not send a TCP RST or
   ICMP Echo Reply.  In such cases, and when the device does not listen
   on a well-known TCP port known to the scanner, the effectiveness of
   an address scan is limited by the ability to get ICMP unreachable
   errors, since the attacker can only infer the presence of a host
   based on the absense of an ICMP unreachable error.

   Generation of ICMP unreachable errors is typically rate limited to 2
   per second (the default in routers such as Cisco routers running IOS
   12.0 or later).  Such a rate results in taking about a year to
   completely scan 26 bits of space.

   The actual math is as follows.  Let 2^N be the number of devices on
   the subnet.  Let 2^M be the size of the space to scan (i.e., M bits
   of entropy).  Let S be the number of scan attempts.  The formula for
   a 50% chance of getting at least one hit in S attempts is: P(at least
   one success) = 1 - (1 - 2^N/2^M)^S = 1/2.  Assuming 2^M >> S, this
   simplifies to: S * 2^N/2^M = 1/2, giving S = 2^(M-N-1), or M = N + 1
   + log_2(S).  Using a scan rate of 2 per second, this results in the
   following rule of thumb:

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      Bits of entropy needed = log_2(# devices per link) + log_2(seconds
      of link lifetime) + 2

   For example, for a network with at most 2^16 devices on the same
   long-lived link, and the average lifetime of a device being 8 years
   (2^28 seconds) or less, this results in a need for at least 46 bits
   of entropy (16+28+2) so that an address scan would need to be
   sustained for longer than the lifetime of devices to have a 50%
   chance of getting a hit.

   Although 46 bits of entropy may be enough to provide privacy in such
   cases, 59 or more bits of entropy would be needed if addresses are
   used to provide security against attacks such as spoofing, as CGAs
   [RFC3972] and HBAs [RFC5535] do, since attacks are not limited by
   ICMP rate limiting but by the processing power of the attacker.  See
   those RFCs for more discussion.

   If, on the other hand, the devices being scanned for do not implement
   a "stealth mode", but respond with TCP RST or ICMP Echo Reply
   packets, then the address scan is not limited by the ICMP unreachable
   rate limit in routers, since the attacker can determine the presence
   of a host without them.  In such cases, more bits of entropy would be
   needed to provide the same level of protection.

3.  Potential Approaches

   The table below shows the number of bits of entropy currently
   available in various technologies:

     | Technology    | Reference                | Bits of Entropy    |
     | 802.15.4      | [RFC4944]                | 16+ or any EUI-64  |
     | Bluetooth LE  | [I-D.ietf-6lo-btle]      | 48                 |
     | DECT ULE      | [I-D.ietf-6lo-dect-ule]  | 40 or any EUI-48   |
     | MS/TP         | [I-D.ietf-6lo-6lobac]    | 8 or 64            |
     | ITU-T G.9959  | [RFC7428]                | 8                  |
     | NFC           | [I-D.ietf-6lo-nfc]       | 6 or ???           |

   Such technologies generally support either IEEE identifiers or so
   called "Short Addresses", or both, as link layer addresses.  We
   discuss each in turn.

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3.1.  IEEE-Identifier-Based Addresses

   Some technologies allow the use of IEEE EUI-48 or EUI-64 identifiers,
   or allow using an arbitrary 64-bit identifier.  Using such an
   identifier to construct IPv6 addresses makes it easy to use the
   normal LOWPAN_IPHC encoding with stateless compression, allowing such
   IPv6 addresses to be fully elided in common cases.

   Interfaces identifiers formed from IEEE identifiers can have
   insufficient entropy unless the IEEE identifier itself has sufficient
   entropy, and enough bits of entropy are carried over into the IPv6
   address to sufficiently mitigate the threats.  Privacy threats other
   than "Correlation over time" can be mitigated using per-network
   randomized IEEE identifiers with 46 or more bits of entropy.  A
   number of such proposals can be found at
   <>, and Section 10.8 of
   [BTCorev4.1] specifies one for Bluetooth.  Using IPv6 addresses
   derived from such IEEE identifiers would be roughly equivalent to
   those specified in [RFC7217].

   Correlation over time can be mitigated if the IEEE identifier itself
   changes often enough, such as each time the link is established, if
   the link lifetime is short.  For further discussion, see

   Another potential concern is that of efficiency, such as avoiding DAD
   all together when IPv6 addresses are IEEE-identifier-based.
   Appendix A of [RFC4429] provides an analysis of address collision
   probability based on the number of bits of entropy.  A simple web
   search on "duplicate MAC addresses" will show that collisions do
   happen with MAC addresses, and thus based on the analysis in
   [RFC4429], using sufficient bits of entropy in random addresses can
   provide greater protection against collision than using MAC

3.2.  Short Addresses

   An IPv6 interface identifier formed from a "Short Address" and a set
   of well-known constant bits (such as padding with 0's) lacks
   sufficient entropy to mitigate address scanning unless the link
   lifetime is extremely short.  Furthermore, an adversary could also
   use statisical methods to determine the size of the L2 address space
   and thereby make some inference regarding the underlying technology
   on a given link, and target further attacks accordingly.

   When Short Addresses are desired on links that are not guaranteed to
   have a short enough lifetime, the mechanism for constructing an IPv6
   interface identifier from a Short Address could be designed to

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   sufficiently mitigate the problem.  For example, if all nodes on a
   given L2 network have a shared secret (such as the key needed to get
   on the layer-2 network), the 64-bit IID might be generated using a
   one-way hash that includes (at least) the shared secret together with
   the Short Address.  The use of such a hash would result in the IIDs
   being spread out among the full range of IID address space, thus
   mitigating address scans, while still allowing full stateless

   For long-lived links, "temporary" addresses might even be generated
   in the same way by (for example) also including in the hash the
   Version Number from the Authoritative Border Router Option (ABDO) if
   any.  This would allow changing temporary addresses whenever the
   Version Number is changed, even if the set of prefix or context
   information is unchanged.

   In summary, any specification using Short Addresses should carefully
   construct an IID generation mechanism so as to provide sufficient
   entropy compared to the link lifetime.

4.  Recommendations

   The following are recommended for adaptation layer specifications:

   o  Security (privacy) sections should say how address scans are
      mitigated.  An address scan might be mitigated by having a link
      always be short-lived, or might be mitigated by having a large
      number of bits of entropy, or some combination.  Thus, a
      specification should explain what the maximum lifetime of a link
      is in practice, and show how the number of bits of entropy is
      sufficient given that lifetime.

   o  Technologies must define a way to include sufficient bits of
      entropy in the IPv6 interface identifier, based on the maximum
      link lifetime.  Specifying that a random EUI-48 or EUI-64 can be
      used is one easy way to do so, for technologies that support such

   o  Specifications should not simply construct an IPv6 interface
      identifier by padding a short address with a set of other well-
      known constant bits, unless the link lifetime is guaranteed to be
      extremely short.

   o  Specifications should make sure that an IPv6 address can change
      over long periods of time.  For example, the interface identifier
      might change each time a device connects to the network (if
      connections are short), or might change each day (if connections

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      can be long).  This is necessary to mitigate correlation over

   o  If a device can roam between networks, and more than a few bits of
      entropy exist in the IPv6 interface identifier, then make sure
      that the interface identifier can vary per network as the device
      roams.  This is necessary to mitigate location tracking.

5.  IANA Considerations

   This document has no actions for IANA.

6.  Security Considerations

   This entire document is about security considerations and how to
   specify possible mitigations.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

7.2.  Informative References

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

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   [RFC5535]  Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
              DOI 10.17487/RFC5535, June 2009,

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,

   [RFC7288]  Thaler, D., "Reflections on Host Firewalls", RFC 7288,
              DOI 10.17487/RFC7288, June 2014,

   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,

   [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
              over ITU-T G.9959 Networks", RFC 7428,
              DOI 10.17487/RFC7428, February 2015,

              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              draft-ietf-6man-ipv6-address-generation-privacy-08 (work
              in progress), September 2015.

              Gont, F., Cooper, A., Thaler, D., and S. LIU,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-08 (work in progress),
              October 2015.

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              Lynn, K., Martocci, J., Neilson, C., and S. Donaldson,
              "Transmission of IPv6 over MS/TP Networks", draft-ietf-
              6lo-6lobac-02 (work in progress), July 2015.

              Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
              Energy", draft-ietf-6lo-btle-17 (work in progress), August

              Mariager, P., Petersen, J., Shelby, Z., Logt, M., and D.
              Barthel, "Transmission of IPv6 Packets over DECT Ultra Low
              Energy", draft-ietf-6lo-dect-ule-03 (work in progress),
              September 2015.

              Hong, Y. and J. Youn, "Transmission of IPv6 Packets over
              Near Field Communication", draft-ietf-6lo-nfc-02 (work in
              progress), October 2015.

              Huitema, C., "Implications of Randomized Link Layers
              Addresses for IPv6 Address Assignment", draft-huitema-
              6man-random-addresses-02 (work in progress), August 2015.

              Bluetooth Special Interest Group, "Bluetooth Core
              Specification Version 4.1", December 2013,

Author's Address

   Dave Thaler
   One Microsoft Way
   Redmond, WA  98052


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