IAB                                                       T. Hardie, Ed.
Internet-Draft
Intended status: Informational                             June 02, 2015
Expires: December 4, 2015

         Confidentiality in the Face of Pervasive Surveillance
          draft-iab-privsec-confidentiality-mitigations-01
          draft-iab-privsec-confidentiality-mitigations-02

Abstract

   The IAB has published [I-D.iab-privsec-confidentiality-threat] in
   response to several revelations of pervasive attack on Internet
   communications.  In this document we survey the mitigations to those
   threats which are currently available or which might plausibly be
   deployed.  We discuss these primarily in the context of Internet
   protocol design, focusing on robustness to pervasive monitoring and
   avoidance of unwanted cross-mitigation impacts.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Available Mitigations . . . . . . . . . . . . . . . . . . . .   4
   4.  Interplay among Mitigations . . . . . . . . . . . . . . . . .   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 [I-D.iab-privsec-confidentiality-threat].  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 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",
      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
      observation.

   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
      inference.

   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.  Available Mitigations

   Given the threat model laid out in
   [I-D.iab-privsec-confidentiality-threat]., 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.

   +--------------------------+----------------------------------------+
   | 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 |
   +--------------------------+----------------------------------------+

                      Figure 1: Table of Mitigations

   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.

   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 anonymizes IP addresses of by using multi-hop onion
   routing.

   As with traditional, limited active attacks, the basic mitigation to
   pervasive active attack is to enable the endpoints of a communication
   to authenticate each other.  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 without the authorization of the holder of that
   domain name [RFC6962][RFC6698].

   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
   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 corelate 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.

   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-corelation.

   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.

   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].

   Dynamic key exfiltration cannot be prevent 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.

   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
   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 mechansims protect against attackers within
   intermediate nodes.  Thus, neither of these mechanisms provides
   complete protection by itself.  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.

   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.

   In summary, many of the basic tools for mitigating pervasive attack
   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 systems we have defined - for
   example, TLS with PFS ciphersuites - and that these properly
   implemented.  (And, one might add, 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.

   Some tools that we currently have can also be used for mitigating
   pervasive attacks, but since they have not generally been designed
   with this in mind, they may need elaboration or adjustment to be
   completely suitable.  The next section examines one common reason for
   such adjustment: managing the integration of one mitigation with the
   environment in which it is deployed.

4.  Interplay among Mitigations

   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 a different mitigation which is working at a different time scale
   or with a different aim.

   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 a mitigation in which the MAC address varies 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, proposes 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 observors, 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.

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, 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

   [I-D.iab-privsec-confidentiality-threat]
              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", draft-iab-privsec-
              confidentiality-threat-07 (work in progress), May 2015.

   [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", RFC
              4949, 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, July
              2013.

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

8.2.  Informative References

   [I-D.ietf-dnsop-edns-client-subnet]
              Contavalli, C., Gaast, W., Lawrence, D., and W. Kumari,
              "Client Subnet in DNS Querys", draft-ietf-dnsop-edns-
              client-subnet-01 (work in progress), May 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.

   [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.

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

Author's Address

   Ted Hardie (editor)

   Email: ted.ietf@gmail.com