[Search] [txt|pdfized|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: 00 01 02 03 04 05 06 07 08 rfc8165               Informational
Network Working Group                                          T. Hardie
Internet-Draft                                            March 03, 2017
Intended status: Informational
Expires: September 4, 2017


              Design considerations for Metadata Insertion
               draft-hardie-privsec-metadata-insertion-07

Abstract

   The IAB has published RFC7624 in response to several revelations of
   pervasive attack on Internet communications.  This document considers
   the implications of protocol designs which associate metadata with
   encrypted flows.  In particular, it asserts that designs which do so
   by explicit actions at the host are preferable to designs in which
   middleboxes insert them.

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
   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 September 4, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (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




Hardie                  Expires September 4, 2017               [Page 1]


Internet-Draft         privsec-metadata-insertion             March 2017


   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Design patterns . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  Advice  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Deployment considerations . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   To minimize the risks associated with pervasive surveillance, it is
   necessary for the Internet technical community to address the
   vulnerabilities exploited in the attacks document in [RFC7258] and
   the threats described in [RFC7624].  The goal of this document is to
   address a common design pattern which emerges from the increase in
   encryption: explicit association of metadata which would previously
   have been inferred from the plaintext protocol.

2.  Terminology

   This document makes extensive use of standard security and privacy
   terminology; see [RFC4949] and [RFC6973].  Terms used from [RFC6973]
   include Eavesdropper, Observer, Initiator, Intermediary, Recipient,
   Attack (in a privacy context), Correlation, Fingerprint, Traffic
   Analysis, and Identifiability (and related terms).  In addition, we
   use terms that are specific to the attacks discussed in [RFC7624].
   Terms introduced terms from there include: Pervasive Attack, Passive
   Pervasive Attack, Active Pervasive Attack, Observation, Inference,
   and Collaborator.

3.  Design patterns

   One of the core mitigations for the loss of confidentiality in the
   presence of pervasive surveillance is data minimization, which limits
   the amount of data disclosed to those elements absolutely required to
   complete the relevant protocol exchange.  When data minimization is
   in effect, some information which was previously available may be
   removed from specific protocol exchanges.  The information may be



Hardie                  Expires September 4, 2017               [Page 2]


Internet-Draft         privsec-metadata-insertion             March 2017


   removed explicitly (by a browser suppressing cookies during private
   modes, as an example) or by other means.  As noted in [RFC7624], some
   topologies which aggregate or alter the network path also acted to
   reduce the ease with which metadata is available to eavesdroppers.

   In some cases, other actors within a protocol context will continue
   to have access to the information which has been thus withdrawn from
   specific protocol exchanges.  If those actors attach the information
   as metadata to those protocol exchange, the confidentiality effect of
   data minimization is lost.

   The restoration of information is particularly tempting for systems
   whose primary function is not to provide confidentiality.  A proxy
   providing compression, for example, may wish to restore the identity
   of the requesting party; similarly a VPN system used to provide
   channel security may believe that origin IP should be restored.
   Actors considering restoring metadata may believe that they
   understand the relevant privacy considerations or believe that,
   because the primary purpose of the service was not privacy-related,
   none exist.  Examples of this design pattern include [RFC7239] and
   [RFC7871].

4.  Advice

   Avoid this design pattern.  It contributes to the overall loss of
   confidentiality for the Internet and trust in the Internet as a
   medium.  Do not add metadata to flows at intermediary devices unless
   a positive affirmation of approval for restoration has been received
   from the actor whose data will be added.

   Instead, design the protocol so that the actor can add such metadata
   themselves so that it flows end-to-end, rather than requiring the
   action of other parties.  In addition to improving privacy, this
   approach ensures consistent availability between the communicating
   parties, no matter what path is taken.  (Note that this document does
   not attempt to describe how an actor sets policies on providing this
   metadata, as the range of systems which might be implied is very
   broad).

   As an example, RFC 7871 describes a method that had already been
   deployed and notes that it is unlikely that a clean-slate design
   would result in this mechanism.  If a clean-slate design were to
   follow the advice in this document, that design would likely reverse
   a core element of RFC 7871: rather than adding metadata at a proxy,
   it would provide facilities for end systems to add it to their
   initial queries.  In the case of RFC 7871, the relevant metadata is
   relatively easy for an end system to derive, as STUN [RFC5389]
   provides a method for learning the reflexive transport address from



Hardie                  Expires September 4, 2017               [Page 3]


Internet-Draft         privsec-metadata-insertion             March 2017


   which a client subnet could be derived.  This would allow clients to
   populate this data themselves, thus affirming their consent and
   providing data at a granularity with which they were comfortable.  As
   in RFC 7871, the addition of this data would require confirmation
   that the upstream DNS resolver understood what to do with it, but the
   same negotiation mechanism, an EDNS0 option [RFC6891], could be used.
   Because of this negotiation, there would be a new variability in
   responses that would change the caching behavior for data supplied by
   participating servers.  This not a major change from the current
   design, however, as the same considerations set out in section 7.3.2
   and 7.5 of RFC 7871 would apply to client-supplied subnets as well as
   they do for proxy supplied subnets.

   From a protocol perspective, in other words, this approach would be a
   minor change from RFC 7871, would be as fully featured and would
   provide better privacy properties than the on-path update mechanism
   RFC 7871 provides.  The next section examines why, despite this,
   deployment considerations have sometimes trumped cleaner designs.

5.  Deployment considerations

   There are a few common tensions associated with the deployment of
   systems which restore metadata.  The first is the trade-off in speed
   of deployment for different actors.  The Forwarded HTTP Extension in
   [RFC7239] provides an example of this.  When used with a proxy, it
   restores information related to the original requesting party, thus
   allowing a responding server to tailor responses according to the
   original party's region, network, or other characteristics associated
   with the identity.  It would, of course, be possible for the
   originating client to add this data itself, after using STUN
   [RFC5389] or a similar mechanism to first determine the information
   to declare.  This would require, however, full specification and
   adoption of this mechanism by the end systems.  It would not be
   available at all during this period, and would thereafter be limited
   to those systems which have been upgraded to include it.  The long
   tail of browser deployments indicates that many systems might go
   without upgrades for a significant period of time.  The proxy
   infrastructure, in contrast, is commonly under more active management
   and represents a much smaller number of elements; this impacts both
   the general deployment difficulty and the number of systems which the
   origin server must trust.

   The second common tension is between the metadata minimization and
   the desire to tailor content responses.  For origin servers whose
   content is common across users, the loss of metadata may have limited
   impact on the system's functioning.  For other systems, which
   commonly tailor content by region or network, the loss of metadata
   may imply a loss of functionality.  Where the user desires this



Hardie                  Expires September 4, 2017               [Page 4]


Internet-Draft         privsec-metadata-insertion             March 2017


   functionality, restoration can commonly be achieved by the use of
   other identifiers or login procedures.  Where the user does not
   desire this functionality, but it is a preference of the server or a
   third party, adjustment is more difficult.  At the extreme, content
   blocking by network origin may be a regulatory requirement.  Trusting
   a network intermediary to provide accurate data is, of course,
   fragile in this case, but it may be a part of the regulatory
   framework.

   There also tensions with latency of operation.  For example, where
   the end system does not initially know the information which would be
   added by on-path devices, it must engage the protocol mechanisms to
   determine it.  Determining a public IP address to include in a
   locally supplied header might require a STUN exchange, and the
   additional latency of this exchange discourages deployment of host-
   based solutions.  To minimize this latency, engaging those mechanisms
   may need to be done in parallel with or in advance of the core
   protocol exchanges with which this metadata would be supplied.

   These tensions do not change the basic recommendation, but they
   suggest that the parties who are introducing encryption and data
   minimization for existing protocols consider carefully whether the
   work also implies introducing mechanisms for the end-to-end
   provisioning of metadata when a user has actively consented to
   provide it.

6.  IANA Considerations

   This memo makes no request of IANA.

7.  Security Considerations

   This memorandum describes a design pattern related emerging from
   responses to the attacks described in [RFC7258].  Continued use of
   this design pattern, which uses mid-flow devices to restore metadat,
   lowers the impact of mitigations to that attack.

   Note that some emergency service recipients, notably PSAPs (Public
   Safety Answering Points) may prefer data provided by a network to
   data provided by end system, because an end system could use false
   data to attack others or consume resources.  While this has the
   consequence that the data available to the PSAP is often more coarse
   than that available to the end system, the risk of false data being
   provided involved a risk to the lives of those targeted.







Hardie                  Expires September 4, 2017               [Page 5]


Internet-Draft         privsec-metadata-insertion             March 2017


8.  Contributors

   This document is derived in part from the work initially done on the
   Perpass mailing list and at the [STRINT] workshop.  It has been
   discussed with the IAB's Privacy and Security program, whose review
   and input is gratefully acknowledged.  The document also benefited
   from an extensive review by Mohamed Boucadair.

9.  References

9.1.  Normative References

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <http://www.rfc-editor.org/info/rfc4949>.

   [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>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <http://www.rfc-editor.org/info/rfc7624>.

9.2.  Informative References

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <http://www.rfc-editor.org/info/rfc5389>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <http://www.rfc-editor.org/info/rfc6891>.

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,
              <http://www.rfc-editor.org/info/rfc7239>.



Hardie                  Expires September 4, 2017               [Page 6]


Internet-Draft         privsec-metadata-insertion             March 2017


   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,
              <http://www.rfc-editor.org/info/rfc7871>.

   [STRINT]   S Farrell, ., "Strint Workshop Report", April 2014,
              <https://www.w3.org/2014/strint/draft-iab-strint-
              report.html>.

Author's Address

   Ted Hardie

   Email: ted.ietf@gmail.com





































Hardie                  Expires September 4, 2017               [Page 7]