NTP Client Data Minimization
draft-dfranke-ntp-data-minimization-00

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Author Daniel Franke 
Last updated 2016-10-17
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Network Working Group                                          D. Franke
Internet-Draft                                                    Akamai
Updates: 5905 (if approved)                             October 17, 2016
Intended status: Standards Track
Expires: April 20, 2017

                      NTP Client Data Minimization
                 draft-dfranke-ntp-data-minimization-00

Abstract

   This memo proposes backward-compatible updates to the Network Time
   Protocol to strip unnecessary identifying information from client
   requests and to improve resilience against blind spoofing of
   unauthenticated server responses.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 20, 2017.

Copyright Notice

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Client Packet Format  . . . . . . . . . . . . . . . . . . . .   2
   4.  Security and Privacy Considerations . . . . . . . . . . . . .   3
     4.1.  Data Minimization . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Transmit Timestamp Randomization  . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Network Time Protocol packets, as specified by RFC 5905 [RFC5905],
   carry a great deal of information about the state of the NTP daemon
   which transmitted them.  In the case of mode 4 packets (responses
   sent from server to client), as well as in broadcast and symmetric
   modes, most of this information is essential for accurate and
   reliable time synchronizaton.  However, in mode 3 packets (requests
   sent from client to server), these fields serve no purpose.  Server
   implementations never need to inspect them, and they can achieve
   nothing by doing so.  Populating these fields with accurate
   information is harmful to privacy because it allows a passive
   observer to fingerprint clients and track them as they move across
   networks.

   This memo updates RFC 5905 to redact unnecessary data from mode 3
   packets.  It calls for no changes on the server side, and clients
   which implement these updates will remain fully interoperable with
   existing servers.

2.  Requirements Language

   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].

3.  Client Packet Format

   In every client-mode packet sent by a Network Time Protocol [RFC5905]
   implementation:

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      The first octet, which contains the leap indicator, version
      number, and mode fields, SHALL be set to 0x23 (LI = 0, VN = 4,
      Mode = 3).

      The Transmit Timestamp field SHALL be set uniformly at random,
      generated by a mechanism suitable for cryptographic purposes.
      [RFC4086] provides guidance on the generation of random values.

      All other header fields, specifically the Stratum, Poll,
      Precision, Root Delay, Root Dispersion, Reference ID, Reference
      Timestamp, Origin Timestamp, and Receive Timestamp, SHALL be set
      to zero.

4.  Security and Privacy Considerations

4.1.  Data Minimization

   Zeroing out unused fields in client requests prevents disclosure of
   information that can be used for fingerprinting [RFC6973].

   While populating any of these fields with authentic data reveals at
   least some identifying information about the client, the Origin
   Timestamp and Receive Timestamp fields constitute a particularly
   severe information leak.  RFC 5905 calls for clients to copy the
   transmit timestamp and destination timestamp of the server's most
   recent response into the origin timestamp and receive timestamp
   (respectively) of their next request to that server.  Therefore, when
   a client moves between networks, a passive observer of both network
   paths can determine with high confidence that the old and new IP
   addresses belong to the same system by noticing that the transmit
   timestamp of a response sent to the old IP matches the origin
   timestamp of a request sent from the new one.

4.2.  Transmit Timestamp Randomization

   While this memo calls for most fields in client packets to be set to
   zero, the transmit timestamp is randomized.  This decision is
   motivated by security as well as privacy.

   NTP servers copy the transmit timestamp from the client's request
   into the origin timestamp of the response; this memo calls for no
   change in this behavior.  Clients discard any response whose origin
   timestamp does not match the transmit timestamp of any request
   currently in flight.

   In the absence of cryptographic authentication, verification of
   origin timestamps is clients' primary defense against blind spoofing
   of NTP responses.  It is therefore important that clients' transmit

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   timestamps be unpredictable.  Their role in this regard is closely
   analagous to that of TCP Initial Sequence Numbers [RFC6528].

   The traditional behavior of the NTP reference implementation is to
   randomize only a few (typically 10-15 depending on the precision of
   the system clock) low-order bits of transmit timestamp, with all
   higher bits representing the system time, as measured just before the
   packet was sent.  This is suboptimal, because with so few random
   bits, an adversary sending spoofed packets at high volume will have a
   good chance of correctly guessing a valid origin timestamp.

5.  IANA Considerations

   [RFC EDITOR: DELETE PRIOR TO PUBLICATION]

   This memo introduces no new IANA considerations.

6.  References

6.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

6.2.  Informative References

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
              2012, <http://www.rfc-editor.org/info/rfc6528>.

   [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>.

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Appendix A.  Acknowledgements

   The author thanks Prof. Sharon Goldberg, Miroslav Lichvar, and
   Aanchal Malhotra for calling attention to the issues addressed in
   this memo.

Author's Address

   Daniel Fox Franke
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA  02142
   United States

   Email: dafranke@akamai.com
   URI:   https://www.dfranke.us

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