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Network Working Group                                     T. Hardie, Ed.
Internet-Draft                                            March 20, 2016
Intended status: Informational
Expires: September 19, 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

   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 September 19, 2016.

Copyright Notice

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

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2

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   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Design patterns  . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Forward-for in Forwarded HTTP Extension  . . . . . . . . .  4
     3.2.  IP Address Propagation in DNS Requests . . . . . . . . . .  4
   4.  Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Deployment considerations  . . . . . . . . . . . . . . . . . .  5
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   8.  Contributors {Contributors}  . . . . . . . . . . . . . . . . .  6
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  6
     9.2.  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].

   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",

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      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
      <alice@example.com> sent a message to <bob@example.com> 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.

   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

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   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 act 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 "Forward-for"
   described in [RFC7239] and anointing DNS queries with originating
   network information as described in [I-D.ietf-dnsop-edns-client-

3.1.  Forward-for in Forwarded HTTP Extension

   [RFC7239] defines an HTTP header extension that seeks to add back
   certain network metadata that can be lost in the process of proxying
   a connection.  The Forwarded-for extension allows a proxy to include
   the originating IP address as part of the HTTP request.  While there
   are many types of HTTP proxies, some proxies seek to specifically
   disassociate the origin IP address from the request, and adding back
   this metadata without some explicit action of the user may
   unwittingly expose metadata that users are specifically seeking to
   protect through the use of such a proxy.

3.2.  IP Address Propagation in DNS Requests

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   [I-D.ietf-dnsop-edns-client-subnet] describes and EDNS0 extension
   that can propagate the IP address of the originating DNS query
   through the DNS hierarchy of Recursive Resolvers to Authoritative
   Nameservers.  Many Authoritative Nameservers will tailor their
   responses based on the IP address of the query, to provide a response
   that is network-topologically more "close" to the query IP address.
   Increasingly, Recursive Resolvers that clients use may not be close
   to the originating IP address, so by carrying the originating query
   IP address through to the Authoritative Nameserver, that server can
   provide a more topologically-relevant response to the user.  DNS
   privacy is a significant challenge [RFC7626] which would only be
   exacerbated by recursive resolvers no longer serving as aggregation
   points for DNS queries and instead propagating those addresses up
   through to the Authoritative Nameservers which would then be in a
   position to profile the DNS traffic they receive based on originating
   IP address.

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.

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

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   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.

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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   [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/

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

9.2.  Informative References

              Contavalli, C., Gaast, W., tale, t.  and W. Kumari,
              "Client Subnet in DNS Queries", Internet-Draft draft-ietf-
              dnsop-edns-client-subnet-06, December 2015.

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

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

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, 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, <http://www.rfc-

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

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

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

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

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015, <http://www.rfc-

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

Author's Address

   Ted Hardie, editor

   Email: ted.ietf@gmail.com

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