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IAB                                                       T. Hardie, Ed.
Internet-Draft                                          October 27, 2016
Intended status: Informational
Expires: April 30, 2017


         Confidentiality in the Face of Pervasive Surveillance
            draft-iab-privsec-confidentiality-mitigations-08

Abstract

   The IAB has published [RFC7624] in response to several revelations of
   pervasive attack on Internet communications.  This document surveys
   the most plausible mitigations to those threats currently available
   to the designers of Internet protocols.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 30, 2017.

Copyright Notice

   Copyright (c) 2016 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Available Mitigations . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Encryption  . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Forward Secrecy . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Covert Channel Reduction  . . . . . . . . . . . . . .   5
     3.2.  Auditing Authentication . . . . . . . . . . . . . . . . .   5
     3.3.  Metadata Minimization . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Length Hiding . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Anonymization . . . . . . . . . . . . . . . . . . . . . .   6
     3.5.  End-to-End Protection . . . . . . . . . . . . . . . . . .   7
   4.  Interplay among Mechanisms  . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Contributors {Contributors} . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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 describe
   more precisely the mitigations available for those threats and to lay
   out the interactions among them should they be deployed in
   combination.

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
   [RFC7624].  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.





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3.  Available Mitigations

   Given the threat model laid out in [RFC7624], how should the Internet
   technical community respond to pervasive attack?  The cost and risk
   considerations discussed in it provide a guide to responses.  Namely,
   responses to passive attack should close off avenues for those
   attacks that are safe, scalable, and cheap, forcing the attacker to
   mount attacks that expose it to higher cost and risk.  Protocols and
   security measures protecting against active attacks must also limit
   the impact of compromise and malfeasance by avoiding systems which
   grant universal credentials.

   In this section, we discuss a collection of high-level approaches to
   mitigating pervasive attacks.  These approaches are not meant to be
   exhaustive, but rather to provide general guidance to protocol
   designers in creating protocols that are resistant to pervasive
   attack.

   Many of these are basic tools which already exist.  As Edward Snowden
   put it, "properly implemented strong crypto systems are one of the
   few things you can rely on".  The task for the Internet community is
   to ensure that applications are able to use the strong crypto and
   other mitigations already available- and that these are properly
   implemented and commonly turned on.  Some of this work will require
   architectural changes to applications, e.g., in order to limit the
   information that is exposed to servers.  In many other cases,
   however, the need is simply to make the best use we can of the
   cryptographic tools we have.























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   +--------------------------+----------------------------------------+
   | Attack Class             | High-level mitigations                 |
   +--------------------------+----------------------------------------+
   | Passive observation      | Encryption for confidentiality         |
   |                          |                                        |
   | Passive inference        | Path differentiation                   |
   |                          |                                        |
   | Active                   | Authentication, monitoring             |
   |                          |                                        |
   | Metadata Analysis        | Data Minimization                      |
   |                          |                                        |
   | Static key exfiltration  | Encryption with per-session state      |
   |                          | (PFS)                                  |
   |                          |                                        |
   | Dynamic key exfiltration | Transparency, validation of end        |
   |                          | systems                                |
   |                          |                                        |
   | Content exfiltration     | Object encryption, distributed systems |
   +--------------------------+----------------------------------------+

                       Table 1: Table of Mitigations

3.1.  Encryption

   The traditional mitigation to passive attack is to render content
   unintelligible to the attacker by applying encryption, for example,
   by using TLS or IPsec [RFC5246][RFC4301].  Even without
   authentication, encryption will prevent a passive attacker from being
   able to read the encrypted content.  Exploiting unauthenticated
   encryption requires an active attack (man in the middle); with
   authentication, a key exfiltration attack is required.  For
   cryptographic systems providing forward secrecy, even exfiltration of
   long-term keys will not compromise data captured under session keys
   used before the exfiltration.

3.1.1.  Forward Secrecy

   An encrypted, authenticated session is safe from content-monitoring
   attacks in which neither end collaborates with the attacker, but can
   still be subverted by the endpoints.  The most common ciphersuites
   used for HTTPS today, for example, are based on using RSA encryption
   in such a way that if an attacker has the private key, the attacker
   can derive the session keys from passive observation of a session.
   These ciphersuites are thus vulnerable to a static key exfiltration
   attack - if the attacker obtains the server's private key once, then
   they can decrypt all past and future sessions for that server.





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   Static key exfiltration attacks are prevented by including ephemeral,
   per-session secret information in the keys used for a session.  Most
   IETF security protocols include modes of operation that have this
   property.  These modes are known in the literature under the heading
   "perfect forward secrecy" (PFS) because even if an adversary has all
   of the secrets for one session, the next session will use new,
   different secrets and the attacker will not be able to decrypt it.
   The Internet Key Exchange (IKE) protocol used by IPsec supports PFS
   by default [RFC4306], and TLS supports PFS via the use of specific
   ciphersuites [RFC5246].

3.1.2.  Covert Channel Reduction

   Dynamic key exfiltration cannot be prevented by protocol means.  By
   definition, any secrets that are used in the protocol will be
   transmitted to the attacker and used to decrypt what the protocol
   encrypts.  Likewise, no technical means will stop a willing
   collaborator from sharing keys with an attacker.  However, this
   attack model also covers "unwitting collaborators", whose technical
   resources are collaborating with the attacker without their owners'
   knowledge.  This could happen, for example, if flaws are built into
   products or if malware is injected later on.

   Standards can also define protocols that provide greater or lesser
   opportunity for dynamic key exfiltration.  Collaborators engaging in
   key exfiltration through a standard protocol will need to use covert
   channels in the protocol to leak information that can be used by the
   attacker to recover the key.  Such use of covert channels has been
   demonstrated for SSL, TLS, and SSH.  Any protocol bits that can be
   freely set by the collaborator can be used as a covert channel,
   including, for example, TCP options or unencrypted traffic sent
   before a STARTTLS message in SMTP or XMPP.  Protocol designers should
   consider what covert channels their protocols expose, and how those
   channels can be exploited to exfiltrate key information.

3.2.  Auditing Authentication

   As with traditional, limited active attacks, a basic mitigation to
   pervasive active attack is to enable the endpoints of a communication
   to authenticate each other over the encrypted channel.  However,
   attackers that can mount pervasive active attacks can often subvert
   the authorities on which authentication systems rely.

   Thus, in order to make authentication systems more resilient to
   pervasive attack, it is beneficial to monitor these authorities to
   detect misbehavior that could enable active attack.  For example,
   DANE and Certificate Transparency both provide mechanisms for
   detecting when a CA has issued a certificate for a domain name



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   without the authorization of the holder of that domain name
   [RFC6962][RFC6698].  Other systems may use external notaries to
   detect certificate authority mismatch (e.g.  Convergence
   [Convergence]).

3.3.  Metadata Minimization

   The additional capabilities of a pervasive passive attacker, however,
   require some changes in how protocol designers evaluate what
   information is encrypted.  In addition to directly collecting
   unencrypted data, a pervasive passive attacker can also make
   inferences about the content of encrypted messages based on what is
   observable.  For example, if a user typically visits a particular set
   of web sites, then a pervasive passive attacker observing all of the
   user's behavior can track the user based on the hosts the user
   communicates with, even if the user changes IP addresses, and even if
   all of the connections are encrypted.

   Thus, in designing protocols to be resistant to pervasive passive
   attacks, protocol designers should consider what information is left
   unencrypted in the protocol, and how that information might be
   correlated with other traffic.  Some of the data left unencrypted may
   be considered "metadata" within the context of a single protocol, as
   it provides adjunct information used for delivery or display, rather
   than the data directly created or consumed by protocol users.  This
   does not mean it is not useful to attackers, however, and when this
   metadata is not protected by encryption it may leak substantial
   amounts of information.  Data minimization strategies should thus be
   applied to any data left unencrypted, whether it be payload or
   metadata.  Information that cannot be encrypted or omited should be
   be dissociated from other information.  For example, the TOR overlay
   routing network [TOR] anonymizes IP addresses by using multi-hop
   onion routing.

3.3.1.  Length Hiding

   One fundamental limitation of encryption is that it exposes the size
   of the plaintext that protects.  A passive attacker can use this to
   obtain information about the plaintext [CLINIC].  Protocols that use
   encryption can provide the ability to pad plaintext.  This enables
   control over the size of ciphertext by endpoints, which can be used
   to reduce the information available to passive attackers.

3.4.  Anonymization

   While encryption and authentication protect the security of
   individual sessions, these sessions may still leak information, such
   as IP addresses or server names, that a pervasive attacker can use to



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   correlate sessions and derive additional information about the
   target.  Thus, pervasive attack highlights the need for anonymization
   technologies, which make correlation more difficult.  Typical
   approaches to anonymization against traffic analysis include:

   o  Aggregation: Routing sessions for many endpoints through a common
      mid-point (e.g, an HTTP proxy).  The midpoint appears as the
      origin of the communication when traffic analysis is conducted
      from points after it, so individual sources cannot be
      distinguished.  If traffic analysis is being conducted prior to
      the mid-point, all flows appear to be destined to the same point,
      which leaks very little information.  Even when traffic analysis
      is being performed both before and after the mid-point,
      simultaneous connections may make it difficult to correlate the
      traffic going into and out of the mid-point.  For this to be
      effective as a mitigation, traffic to the mid-point must be
      encrypted and traffic from the mid-point should be.

   o  Onion routing: Routing a session through several mid-points,
      rather than directly end-to-end, with encryption that guarantees
      that each node can only see the previous and next hops.  This
      ensures that the source and destination of a communication are
      never revealed simultaneously.  Note, however, that onion routing
      anonymity guarantees depend on an attacker being unable to control
      many of the routing nodes [TorPaper].

   o  Multi-path: Routing different sessions via different paths (even
      if they originate from the same endpoint).  This reduces the
      probability that the same attacker will be able to collect many
      sessions or associate them with the same individual.  If, for
      example, a device has both a cellular and 802.11 interface,
      routing some traffic across the cellular network and other traffic
      over the 802.11 interface means that traffic analysis conducted
      only with one network will be incomplete.  Even if conducted in
      both, it may be more difficult for the attacker to associate the
      traffic in each network with the other.  For this to be effective
      as a mitigation, signalling protocols which gather and transmit
      data about multiple interfaces (such as SIP) must be encrypted to
      avoid the information being used in cross-correlation.

3.5.  End-to-End Protection

   Content exfiltration has some similarity to the dynamic exfiltration
   case, in that nothing can prevent a collaborator from revealing what
   they know, and the mitigations against becoming an unwitting
   collaborator apply.  In this case, however, applications can limit
   what the collaborator is able to reveal.  For example, the S/MIME and
   PGP systems for secure email both deny intermediate servers access to



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   certain parts of the message [RFC5750][RFC2015].  Even if a server
   were to provide an attacker with full access, the attacker would
   still not be able to read the protected parts of the message.

   Mechanisms like S/MIME and PGP are often referred to as "end-to-end"
   security mechanisms, as opposed to "hop-by-hop" or "end-to-middle"
   mechanisms like the use of SMTP over TLS.  These two different
   mechanisms address different types of attackers: Hop-by-hop
   mechanisms protect from attackers on the wire (passive or active),
   while end-to-end mechanisms protect against attackers within
   intermediate nodes as well as those on the wire.  Even end-to-end
   mechanisms are not complete protection in themselves, as intermediate
   nodes can still access some metadata.  For example:

   o  Two users messaging via Facebook over HTTPS are protected against
      passive and active attackers in the network between the users and
      Facebook.  However, if Facebook is a collaborator in an
      exfiltration attack, their communications can still be monitored.
      They would need to encrypt their messages end-to-end in order to
      protect themselves against this risk.

   o  Two users exchanging PGP-protected email have protected the
      content of their exchange from network attackers and intermediate
      servers, but the header information (e.g., To and From addresses)
      is unnecessarily exposed to passive and active attackers that can
      see communications among the mail agents handling the email
      messages.  These mail agents need to use hop-by-hop encryption and
      traffic analysis mitigation to address this risk.

   Mechanisms such as S/MIME and PGP are also known as "object-based"
   security mechanisms (as opposed to "communications security"
   mechanisms), since they operate at the level of objects, rather than
   communications sessions.  Such secure object can be safely handled by
   intermediaries in order to realize, for example, store and forward
   messaging.  In the examples above, the encrypted instant messages or
   email messages would be the secure objects.  Hop-to-hop security
   mechanisms may be susceptible to downgrade attacks (e.g., STARTTLS-
   secured SMTP has been downgraded by intermediate network nodes
   [WaPo-STARTTLS]) in which case end-to-end mechanisms are advised.

   The mitigations to the content exfiltration case regard participants
   in the protocol as potential passive attackers themselves, and apply
   the mitigations discussed above with regard to passive attack.
   Information that is not necessary for these participants to fulfill
   their role in the protocol can be encrypted, and other information
   can be anonymized.





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4.  Interplay among Mechanisms

   One of the key considerations in selecting mitigations is how to
   manage the interplay among different mechanisms.  Care must be taken
   to avoid situations where a mitigation is rendered fruitless because
   of mechanisms which working at a different time scale or with a
   different aim.

   The tools that we currently have have not generally been designed
   with all of these mitigations in mind, so they may need elaboration
   or adjustment to be completely suitable.  Thus, managing the
   integration of one mitigation with the environment in which it is
   deployed is critical.

   As an example, there is work in progress in IEEE 802 to standardize a
   method for the randomization of MAC Addresses.  This work aims to
   enable the MAC address to vary as the device connects to different
   networks, or connects at different times.  In theory, the
   randomization will mitigate tracking by MAC address.  However, the
   randomization will be defeated if the adversary can link the
   randomized MAC address to other identifiers such as the interface
   identifier used in IPv6 addresses, the unique identifiers used in
   DHCP or DHCPv6, or unique identifiers used in various link-local
   discovery protocols.

   For mitigations which rely on aggregation to separate the origin of
   traffic from its destination, care must be taken that the protocol
   mechanics do not expose origin IP through secondary means.
   [I-D.ietf-dnsop-edns-client-subnet] for example, documents a method
   to carry the IP address or subnet of a querying party through a
   recursive resolver to an authoritative resolver.  Even with a
   truncated IP address, this mechanism increases the likelihood that a
   pervasive monitor would be able to associate query traffic and
   responses.

   If a client wished to ensure that its traffic did not expose this
   data, it would need to require that its stub resolver emit any
   privacy-sensitive queries with a source NETMASK set to 0, as detailed
   in Section 5.1 of [I-D.ietf-dnsop-edns-client-subnet].  Given that
   setting this only occasionally might also be used a signal to
   observers, any client wishing to have any privacy sensitive traffic
   would, in essence have to emit this for every query.  While this
   would succeed at providing the required privacy, given the mechanism
   proposed, it would also mean no split-DNS adjustments in response
   would be possible for the privacy sensitive client.






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5.  IANA Considerations

   This memo makes no request of IANA.

6.  Security Considerations

   This memorandum describes a series of mitigations to the attacks
   described in [RFC7258].  No such list could possibly be
   comprehensive, nor is the attack therein described the only possible
   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.  Work from Brian
   Trammell, Bruce Schneier, Christian Huitema, Cullen Jennings, Daniel
   Borkmann, Martin Thomson, and Richard Barnes is incorporated here, as
   are ideas and commentary from Jeff Hodges, Phillip Hallam-Baker, and
   Stephen Farrell.

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

   [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>.









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   [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>.

8.2.  Informative References

   [CLINIC]   Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
              Why You Went to the Clinic: Risks and Realization of HTTPS
              Traffic Analysis", March 2014.

   [Convergence]
              M Marlinspike, ., "Convergence Project", August 2011,
              <http://convergenc.io>.

   [I-D.ietf-dnsop-edns-client-subnet]
              Contavalli, C., Gaast, W., tale, t., and W. Kumari,
              "Client Subnet in DNS Queries", draft-ietf-dnsop-edns-
              client-subnet-08 (work in progress), April 2016.

   [RFC2015]  Elkins, M., "MIME Security with Pretty Good Privacy
              (PGP)", RFC 2015, DOI 10.17487/RFC2015, October 1996,
              <http://www.rfc-editor.org/info/rfc2015>.

   [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>.

   [RFC4306]  Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
              Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005,
              <http://www.rfc-editor.org/info/rfc4306>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [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,
              <http://www.rfc-editor.org/info/rfc5750>.

   [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, <http://www.rfc-editor.org/info/rfc6698>.



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   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <http://www.rfc-editor.org/info/rfc6962>.

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

   [TOR]      The Tor Project, "Tor", 2013,
              <https://www.torproject.org/>.

   [TorPaper]
              Dingledine, R., Mathewson, N., and P. Syverson, "Tor: The
              Second-Generation Onion Router", 2004,
              <http://static.usenix.org/event/sec04/tech/full_papers/
              dingledine/dingledine.pdf>.

   [WaPo-STARTTLS]
              Scola, N. and A. Soltani, "Mobile ISP Cricket was
              thwarting encrypted emails, researchers find", 2014,
              <https://www.washingtonpost.com/news/the-
              switch/wp/2014/10/28/mobile-isp-thwarted-customers-
              attempts-to-send-encrypted-e-mails-research-finds/>.

Author's Address

   Ted Hardie (editor)

   Email: ted.ietf@gmail.com






















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