



Network Working Group                                          D. Thaler
Internet-Draft                                                 Microsoft
Intended status: Informational                          October 31, 2016
Expires: May 4, 2017


      Privacy Considerations for IPv6 Adaptation Layer Mechanisms
                draft-ietf-6lo-privacy-considerations-04

Abstract

   This document discusses how a number of privacy threats apply to
   technologies designed for IPv6 over various link layer protocols, and
   provides advice to protocol designers on how to address such threats
   in adaptation layer specifications for IPv6 over such links.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on May 4, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Amount of Entropy Needed in Global Addresses  . . . . . . . .   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.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   RFC 6973 [RFC6973] discusses privacy considerations for Internet
   protocols, and Section 5.2 of that document covers a number of
   privacy-specific threats.  In the context of IPv6 addresses,
   Section 3 of [RFC7721] 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 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.  (On some Non-Broadcast Multi-Access (NBMA) links
      where all nodes aren't already privy to all on-link addresses,
      address scans might also be done by on-link attackers, but in most
      cases address scans are not an interesting threat from on-link
      attackers and thus address scans generally apply only to routable
      addresses.)

   For example, for links that may last for years, "enough" bits of
   entropy means at least 46 or so bits (see Section 2 for why) in a
   routable address; ideally all 64 bits of the IID should be used,
   although historically some bits have been excluded for reasons
   discussed in [RFC7421].  Link-local addresses can also be susceptible



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   to the same privacy threats from off-link attackers, since experience
   shows they are often leaked by upper-layer protocols such as SMTP,
   SIP, or DNS.

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

   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 fixed link-layer 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 Global Addresses

   In terms of privacy threats discussed in [RFC7721], the one with the
   need for the most entropy is address scans of routable addresses.  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 with that address) 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] or are behind a network firewall that will
   drop unsolicited inbound traffic (e.g., TCP SYNs, ICMP Echo Requests,
   etc.) and thus no TCP RST or ICMP Echo Reply will be sent.  In such
   cases, and when the device does not listen on a well-known TCP or UDP
   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.



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   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:

      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 link 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 the link 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 respond to
   unsolicited inbound packets, then the address scan is not limited by
   the ICMP unreachable rate limit in routers, since an adversary 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  | [RFC7668]                | 48                 |
     | DECT ULE      | [I-D.ietf-6lo-dect-ule]  | 40 or any EUI-48   |
     | MS/TP         | [I-D.ietf-6lo-6lobac]    | 7 or 64            |
     | ITU-T G.9959  | [RFC7428]                | 8                  |
     | NFC           | [I-D.ietf-6lo-nfc]       | 5                  |
     +---------------+--------------------------+--------------------+



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

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.

   Global addresses with interface identifiers formed from IEEE
   identifiers can have insufficient entropy to mitigate address scans
   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
   link-layer addresses with enough entropy compared to the link
   lifetime.  A number of such proposals can be found at
   <https://mentor.ieee.org/privecsg/documents>, and Section 10.8 of
   [BTCorev4.1] specifies one for Bluetooth.  Using routable IPv6
   addresses derived from such link-layer addresses would be roughly
   equivalent to those specified in [RFC7217].

   Correlation over time (for all addresses, not just routable
   addresses) can be mitigated if the link-layer address itself changes
   often enough, such as each time the link is established, if the link
   lifetime is short.  For further discussion, see
   [I-D.huitema-6man-random-addresses].

   Another potential concern is that of efficiency, such as avoiding
   Duplicate Address Detection (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 addresses.

3.2.  Short Addresses

   A routable IPv6 address with an interface identifier formed from the
   combination of 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 statistical methods to



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   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
   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
   compression/elision.

   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
   (Section 4.3 of [RFC6775]), 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 in routable addresses, 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 should define a way to include sufficient bits of
      entropy in the IPv6 interface identifier, based on the maximum
      link lifetime.  Specifying that randomized link-layer addresses
      can be used is one easy way to do so, for technologies that
      support such identifiers.

   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



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      extremely short or the short address is allocated by the network
      (rather than being constant in the node).  This also applies to
      link-local addresses if the same short address is used independent
      of network and is unique enough to allow location tracking.

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

   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.  Informative References

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <http://www.rfc-editor.org/info/rfc3972>.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
              <http://www.rfc-editor.org/info/rfc4429>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <http://www.rfc-editor.org/info/rfc4941>.

   [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,
              <http://www.rfc-editor.org/info/rfc4944>.






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   [RFC5535]  Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
              DOI 10.17487/RFC5535, June 2009,
              <http://www.rfc-editor.org/info/rfc5535>.

   [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,
              <http://www.rfc-editor.org/info/rfc6724>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <http://www.rfc-editor.org/info/rfc6775>.

   [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,
              <http://www.rfc-editor.org/info/rfc6973>.

   [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,
              <http://www.rfc-editor.org/info/rfc7217>.

   [RFC7288]  Thaler, D., "Reflections on Host Firewalls", RFC 7288,
              DOI 10.17487/RFC7288, June 2014,
              <http://www.rfc-editor.org/info/rfc7288>.

   [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,
              <http://www.rfc-editor.org/info/rfc7421>.

   [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
              over ITU-T G.9959 Networks", RFC 7428,
              DOI 10.17487/RFC7428, February 2015,
              <http://www.rfc-editor.org/info/rfc7428>.

   [RFC7668]  Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
              Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,
              <http://www.rfc-editor.org/info/rfc7668>.





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   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <http://www.rfc-editor.org/info/rfc7721>.

   [I-D.ietf-6man-default-iids]
              Gont, F., Cooper, A., Thaler, D., and S. LIU,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-16 (work in progress),
              September 2016.

   [I-D.ietf-6lo-6lobac]
              Lynn, K., Martocci, J., Neilson, C., and S. Donaldson,
              "Transmission of IPv6 over MS/TP Networks", draft-ietf-
              6lo-6lobac-05 (work in progress), June 2016.

   [I-D.ietf-6lo-dect-ule]
              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-07 (work in progress),
              October 2016.

   [I-D.ietf-6lo-nfc]
              Choi, Y., Youn, J., and Y. Hong, "Transmission of IPv6
              Packets over Near Field Communication", draft-ietf-6lo-
              nfc-05 (work in progress), October 2016.

   [I-D.huitema-6man-random-addresses]
              Huitema, C., "Implications of Randomized Link Layers
              Addresses for IPv6 Address Assignment", draft-huitema-
              6man-random-addresses-03 (work in progress), March 2016.

   [BTCorev4.1]
              Bluetooth Special Interest Group, "Bluetooth Core
              Specification Version 4.1", December 2013,
              <https://www.bluetooth.org/DocMan/handlers/
              DownloadDoc.ashx?doc_id=282159>.

Author's Address

   Dave Thaler
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: dthaler@microsoft.com




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