Design considerations for Metadata Insertion
draft-hardie-privsec-metadata-insertion-04

Network Working Group                                          T. Hardie
Internet-Draft                                          January 18, 2017
Intended status: Informational
Expires: July 22, 2017

              Design considerations for Metadata Insertion
               draft-hardie-privsec-metadata-insertion-04

Abstract

   The IAB has published [RFC7624] in response to several revelations of
   pervasive attack on Internet communications.  In this document we
   consider the implications of protocol designs which associate
   metadata with encrypted flows.
   In particular, we assert that designs which do so by explicit actions
   of the end system are preferable to designs in which middleboxes
   insert them.

Status of This Memo

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   This Internet-Draft will expire on July 22, 2017.

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   the Trust Legal Provisions and are provided without warranty as
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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 {Contributors} . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   To ensure that the Internet can be trusted by users, 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 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

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   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.

   As an example, RFC 7871 notes that it describes a deployed method and
   that it is unlikely a clean-slate design would have resulted in this
   mechanism.  If a clean-slate design were to follow the advice in this
   document, it 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 which a client subnet could be derived.  By
   negotiating an EDNS0 option which allowed them to self-populate this
   data, clients would be affirming their consent for its use and
   providing data at a granularity with which they were comfortable.

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   This variability would change the caching behavior for responses from
   participating servers, but the same considerations set out in section
   7.3.2 and 7.5 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 opt-in mechanism it
   provides.  The next section examines why, despite this, deployment
   considerations have sometimes trumped cleaner designs.

5.  Deployment considerations

   There are two 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 "Forward-for" method cited
   above provides an example of this.  When used with a proxy,
   Forwarded-for restores the original identity of the 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, using STUN [RFC5389]
   or a similar mechanism to first determine the identity 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
   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.

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   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 lowers the impact of mitigations to that attack.

8.  Contributors {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
   is gratefully acknowledged.

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>.

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9.2.  Informative References

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

   [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>.

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

   [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

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