Network Working Group                                     T. Hardie, Ed.
Intended status: Informational                          October 12, 2015
Expires: April 14, 2016

              Design considerations for Metadata Insertion


   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 April 14, 2016.

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   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 . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Deployment considerations . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Contributors {Contributors} . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 a few terms that are specific to the attacks discussed in this
   document.  Note especially that "passive" and "active" below do not
   refer to the effort used to mount the attack; a "passive attack" is
   any attack that accesses a flow but does not modify it, while an
   "active attack" is any attack that modifies a flow.  Some passive
   attacks involve active interception and modifications of devices,
   rather than simple access to the medium.  The introduced terms are:

   Pervasive Attack:  An attack on Internet communications that makes
      use of access at a large number of points in the network, or
      otherwise provides the attacker with access to a large amount of
      Internet traffic; see [RFC7258].

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   Passive Pervasive Attack:    An eavesdropping attack undertaken by a
      pervasive attacker, in which the packets in a traffic stream
      between two endpoints are intercepted, but in which the attacker
      does not modify the packets in the traffic stream between two
      endpoints, modify the treatment of packets in the traffic stream
      (e.g. delay, routing), or add or remove packets in the traffic
      stream.  Passive pervasive attacks are undetectable from the
      endpoints.  Equivalent to passive wiretapping as defined in
      [RFC4949]; we use an alternate term here since the methods
      employed are wider than those implied by the word "wiretapping",
      including the active compromise of intermediate systems.

   Active Pervasive Attack:  An attack undertaken by a pervasive
      attacker, which in addition to the elements of a passive pervasive
      attack, also includes modification, addition, or removal of
      packets in a traffic stream, or modification of treatment of
      packets in the traffic stream.  Active pervasive attacks provide
      more capabilities to the attacker at the risk of possible
      detection at the endpoints.  Equivalent to active wiretapping as
      defined in [RFC4949].

   Observation:  Information collected directly from communications by
      an eavesdropper or observer.  For example, the knowledge that
      <> sent a message to <> via SMTP
      taken from the headers of an observed SMTP message would be an

   Inference:  Information derived from analysis of information
      collected directly from communications by an eavesdropper or
      observer.  For example, the knowledge that a given web page was
      accessed by a given IP address, by comparing the size in octets of
      measured network flow records to fingerprints derived from known
      sizes of linked resources on the web servers involved, would be an

   Collaborator:  An entity that is a legitimate participant in a
      communication, and provides information about that communication
      to an attacker.  Collaborators may either deliberately or
      unwittingly cooperate with the attacker, in the latter case
      because the attacker has subverted the collaborator through
      technical, social, or other means.

   Key Exfiltration:  The transmission of cryptographic keying material
      for an encrypted communication from a collaborator, deliberately
      or unwittingly, to an attacker.

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   Content Exfiltration:  The transmission of the content of a
      communication from a collaborator, deliberately or unwittingly, to
      an attacker.

   Data Minimization: With respect to protocol design, refers to the
   practice of only exposing the minimum amount of data or metadata
   necessary for the task supported by that protocol to the other
   endpoint(s) and/or devices along the path.

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
   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

   While it may be tempting to restore previous information flows, this
   design pattern should be avoided, as it contributes to the overall
   loss of confidentiality for the Internet.  This document recommends
   against restoration in these cases unless a positive affirmation of
   approval for restoration has been received from the actor whose data
   will be added.  In general, we recommend that the actor add such
   metadata themselves so that it flows end-to-end, rather than
   requiring the action of other parties.  Where this is not possible,
   opt-in methods for consent are strongly recommended; opt-out systems,

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   especially for previously deployed systems, may provide sufficient
   targeting that the most vulnerable users would be reluctant to employ

4.  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

   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

   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.

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5.  IANA Considerations

   This memo makes no request of IANA.

6.  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.

7.  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.

8.  References

8.1.  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,

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", FYI
              36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [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,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [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,

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8.2.  Informative References

              Contavalli, C., Gaast, W., Lawrence, D., and W. Kumari,
              "Client Subnet in DNS Queries", draft-ietf-dnsop-edns-
              client-subnet-04 (work in progress), September 2015.

   [RFC2015]  Elkins, M., "MIME Security with Pretty Good Privacy
              (PGP)", RFC 2015, DOI 10.17487/RFC2015, October 1996,

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <>.

   [RFC4306]  Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
              Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,

   [RFC5750]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Certificate
              Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,

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   [STRINT]   S Farrell, ., "Strint Workshop Report", April 2014,

Author's Address

   Ted Hardie (editor)


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