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Versions: 00 01                                                         
Network Working Group                                          R. Barnes
Internet-Draft                                               B. Schneier
Intended status: Informational                               C. Jennings
Expires: July 10, 2014                                         T. Hardie
                                                        January 06, 2014

         Pervasive Attack: A Threat Model and Problem Statement


   Documents published in 2013 have revealed several classes of
   "pervasive" attack on Internet communications.  In this document, we
   review the main attacks that have been published, and develop a
   threat model that describes these pervasive attacks.  Based on this
   threat model, we discuss the techniques that can be employed in
   Internet protocol design to increase the protocols robustness to
   pervasive attacks.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 10, 2014.

Copyright Notice

   Copyright (c) 2014 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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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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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1.  Introduction

   Starting in the June 2013, documents released to the press by Edward
   Snowden have revealed several operations undertaken by intelligence
   agencies to exploit Internet communications for intelligence
   purposes.  These attacks were largely based on protocol
   vulnerabilities that were already known to exist.  The attacks were
   nonetheless striking in their pervasive nature, both in terms of the
   amount of Internet communications targeted, and in terms of the
   diversity of attack techniques employed.

   To ensure that the Internet can be trusted by users, it is necessary
   for the Internet technical community to address the vulnerabilities
   exploited in these attacks [I-D.farrell-perpass-attack].  The goal of
   this document is to describe more precisely the threats posed by
   these pervasive attacks, and based on those threats, lay out the
   problems that need to be solved in order to secure the Internet in
   the face of those threats.

   The remainder of this document is structured as follows.  In
   Section 3, we provide a brief summary of the attacks that have been
   disclosed.  Section 4 describes a threat model based on these
   attacks, focusing on classes of attack that have not been a focus of
   Internet engineering to date.  Section 5 provides some high-level
   guidance on how Internet protocols can defend against the threats
   described here.

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2.  Terminology

   This document makes extensive use of standard security terminology;
   see, for example, [RFC4949].  In addition, we use a few terms that
   are specific to the attacks discussed here:

   Pervasive Attack:  An attack on Internet protocols 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

   Collaborator:  An entity that is a legitimate participant in a
      protocol, but who provides information about that interaction
      (keys or data) to an attacker.

   Key Exfiltration:  The transmission of keying material for an
      encrypted communication from a collaborator to an attacker

   Content Exfiltration:  The transmission of the content of a
      communication from a collaborator to an attacker

   Unwitting Collaborator:  A collaborator that provides information to
      the attacker not deliberately, but because the attacker has
      exploited some technology used by the collaborator.

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3.  Reported Instances of Large-Scale Attacks

   Through recent revelations of sensitive documents in several media
   outlets, the Internet community has been made aware of several
   intelligence activities conducted by US and UK national intelligence
   agencies, particularly the US National Security Agency (NSA) and the
   UK Government Communications Headquarters (GCHQ).  These documents
   have revealed the methods that these agencies use to attack Internet
   applications and obtain sensitive user information.  Theses documents
   suggest the following types of attacks have occurred:

   o  Large scale passive collection of Internet traffic
      [pass1][pass2][pass3][pass4].  For example:

      *  The NSA XKEYSCORE system accesses data from multiple access
         points and searches for "selectors" such as email addresses, at
         the scale of tens of terabytes of data per day.

      *  The GCHQ Tempora system appears to have access to around 1,500
         major cables passing through the UK.

      *  The NSA MUSCULAR program tapped cables between data centers
         belonging to major service providers.

      *  Several programs appear perform wide-scale collection of
         cookies in web traffic and location data from location-aware
         portable devices such as smartphones.

   o  Decryption of TLS-protected Internet sessions [dec1][dec2][dec3].
      For example, the NSA BULLRUN project appears to have had a budget
      of around $250M per year to undermine encryption through multiple

   o  Insertion of NSA devices as a man in the middle of Internet
      transactions [TOR1][TOR2].  For example, the NSA QUANTUM system
      appears to use several different techniques to hijack HTTP
      connections, ranging from DNS response injection to HTTP 302

   o  Direct acquisition of bulk data and metadata from service
      providers [dir1][dir2][dir3].  For example, the NSA PRISM program
      provides the agency with access to many types of user data (e.g.,
      email, chat, VoIP).

   o  Use of implants (covert modifications or malware) to undermine
      security and anonymity features [dec2][TOR1][TOR2].  For example:

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      *  NSA appears to use the QUANTUM man-in-the-middle system to
         direct users to a FOXACID server, which delivers an implant
         that makes the TOR anonymity service less effective.

      *  The BULLRUN program mentioned above includes the addition of
         covert modifications to software as one means to undermine

      *  There is also some suspicion that NSA modifications to the
         DUAL_EC_DRBG random number generator were made to ensure that
         keys generated using that generator could be predicted by NSA.
         These suspicions have been reinforced by reports that RSA
         Security was paid roughly $10M to make DUAL_EC_DRBG the default
         in their products.

   We use the term "pervasive attack" to collectively describe these
   operations.  The term "pervasive" is used because the attacks are
   designed to gather as much data as possible and to apply selective
   analysis on targets after the fact.  This means that all, or nearly
   all, Internet communications are targets for these attacks.  To
   achieve this scale, the attacks are physically pervasive; they affect
   a large number of Internet communications.  They are pervasive in
   content, consuming and exploiting any information revealed by the
   protocol.  And they are pervasive in technology, exploiting many
   different vulnerabilities in many different protocols.

   It's important to note that although the attacks mentioned above were
   executed by NSA and GCHQ, there are many other organizations that can
   mount pervasive attacks.  Because of the resources required to
   achieve pervasive scale, pervasive attacks are most commonly
   undertaken by nation-state actors.  For example, the Chinese Internet
   filtering system known as the "Great Firewall of China" uses several
   techniques that are similar to the QUANTUM program, and which have a
   high degree of pervasiveness with regard to the Internet in China.

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4.  Threat Model

   Pervasive surveillance aims to collect information across a large
   number of Internet communications, analyzing the collected
   communications to identify information of interest within individual
   communications or implied by correlated communications.  This
   analysis sometimes benefits from decryption of encrypted
   communications and deanonymization of anonymized communications.  As
   a result, these attackers desire both access to the bulk of Internet
   traffic and to the keying material required to decrypt any traffic
   which has been encrypted (though the presence of a communication and
   the fact that it is encrypted may both be inputs to an analysis, even
   if the attacker cannot decrypt the communication).

   The attacks listed above highlight new avenues both for access to
   traffic and for access to relevant encryption keys.  They further
   indicate that the scale of surveillance is sufficient to provide a
   general capability to cross-correlate communications, a threat not
   previously thought to be relevant at the scale of all Internet

4.1.  Attacker Capabilities

    | Attack Class             | Capability                          |
    | Passive                  | Capture data in transit             |
    |                          |                                     |
    | Active                   | Manipulate / inject data in transit |
    |                          |                                     |
    | Static key exfiltration  | Obtain key material once / rarely   |
    |                          |                                     |
    | Dynamic key exfiltration | Obtain per-session key material     |
    |                          |                                     |
    | Content exfiltration     | Access data at rest                 |

   Security analyses of Internet protocols commonly consider two classes
   of attacker: Passive attackers, who can simply listen in on
   communications as they transit the network, and "active attackers",
   who can modify or delete packets in addition to simply collecting

   In the context of pervasive attack, these attacks take on an even
   greater significance.  In the past, these attackers are often assumed
   to operate near the edge of the network, where attacks can be
   simpler.  For exmaple, in some LANs, it is simple for any node to
   engage in passive listening to other nodes' traffic or inject packets

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   to accomplish active attacks.  In the pervasive attack case, however,
   both passive and active attacks are undertaken closer to the core of
   the network, greatly expanding the scope and capability of the

   A passive attacker with access to a large portion of the Internet can
   analyze collected traffic to create a much more detailed view of user
   behavior than an attacker that collects at a single point.  Even the
   usual claim that encryption defeats passive attackers is weakened,
   since a pervasive passive attacker can examine correlations over
   large numbers of sessions, e.g., pairing encrypted sessions with
   unencrypted sessions from the same host.  The reports on the NSA
   XKEYSCORE system would make it an example of such an attacker.

   A pervasive active attacker likewise has capabilities beyond those of
   a localized active attacker.  Active attacks are often limited by
   network topology, for example by a requirement that the attacker be
   able to see a targeted session as well as inject packets into it.  A
   pervasive active attacker with multiple accesses at core points of
   the Internet is able to overcome these topological limitations and
   apply attacks over a much broader scope.  Being positioned in the
   core of the network rather than the edge can also enable a pervasive
   active attacker to reroute targeted traffic.  Pervasive active
   attackers can also benefit from pervasive passive collection to
   identify vulnerable hosts.

   While not directly related to pervasiveness, attackers that are in a
   position to mount a pervasive active attack are also often in a
   position to subvert authentication, the traditional response to
   active attack.  Authentication in the Internet is often achieved via
   trusted third party authorities such as the Certificate Authorities
   (CAs) that provide web sites with authentication credentials.  An
   attacker with sufficient resources for pervasive attack may also be
   able to induce an authority to grant credentials for an identity of
   the attacker's choosing.  If the parties to a communication will
   trust multiple authorities to certify a specific identity, this
   attack may be mounted by suborning any one of the authorities (the
   proverbial "weakest link").  Subversion of authorities in this way
   can allow an active attack to succeed in spite of an authentication

   Beyond these two classes (active and passive), reports on the BULLRUN
   effort to defeat encryption and the PRISM effort to obtain data from
   service providers suggest three more classes of attack:

   o  Static key exfiltration

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   o  Dynamic key exfiltration

   o  Content exfiltration

   These attacks all rely on a "collaborator" endpoint providing the
   attacker with some information, either keys or data.  These attacks
   have not traditionally been considered in security analyses of
   protocols, since they happen outside of the protocol.

   The term "key exfiltration" refers to the transfer of keying material
   for an encrypted communication from the collaborator to the attacker.
   By "static", we mean that the transfer of keys happens once, or
   rarely, typically of a long-lived key.  For example, this case would
   cover a web site operator that provides the private key corresponding
   to its HTTPS certificate to an intelligence agency.

   "Dynamic" key exfiltration, by contrast, refers to attacks in which
   the collaborator delivers keying material to the attacker frequently,
   e.g., on a per-session basis.  This does not necessarily imply
   frequent communications with the attacker; the transfer of keying
   material may be virtual.  For example, if an endpoint were modified
   in such a way that the attacker could predict the state of its
   psuedorandom number generator, then the attacker would be able to
   derive per-session keys even without per-session communications.

   Finally, content exfiltration is the attack in which the collaborator
   simply provides the attacker with the desired data or metadata.
   Unlike the key exfiltration cases, this attack does not require the
   attacker to capture the desired data as it flows through the network.
   The risk is to data at rest as opposed to data in transit.  This
   increases the scope of data that the attacker can obtain, since the
   attacker can access historical data - the attacker does not have to
   be listening at the time the communication happens.

   Exfiltration attacks can be accomplished via attacks against one of
   the parties to a communication, i.e., by the attacker stealing the
   keys or content rather than the party providing them willingly.  In
   these cases, the party may not be aware that they are collaborating,
   at least at a human level.  Rather, the subverted technical assets
   are "collaborating" with the attacker (by providing keys/content)
   without their owner's knowledge or consent.

   Any party that has access to encryption keys or unencrypted data can
   be a collaborator.  While collaborators are typically the endpoints
   of a communication (with encryption securing the links),
   intermediaries in an unencrypted communication can also facilitate
   content exfiltration attacks as collaborators by providing the
   attacker access to those communications.  For example, documents

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   describing the NSA PRISM program claim that NSA is able to access
   user data directly from servers, where it was stored unencrypted.  In
   these cases, the operator of the server would be a collaborator
   (wittingly or unwittingly).  By contrast, in the NSA MUSCULAR
   program, a set of collaborators enabled attackers to access the
   cables connecting data centers used by service providers such as
   Google and Yahoo.  Because communications among these data centers
   were not encrypted, the collaboration by an intermediate entity
   allowed NSA to collect unencrypted user data.

4.2.  Attacker Costs

     | Attack Class             | Cost / Risk to Attacker           |
     | Passive                  | Passive data access               |
     |                          |                                   |
     | Active                   | Active data access + processing   |
     |                          |                                   |
     | Static key exfiltration  | One-time interaction              |
     |                          |                                   |
     | Dynamic key exfiltration | Ongoing interaction / code change |
     |                          |                                   |
     | Content exfiltration     | Ongoing, bulk interaction         |

   In order to realize an attack of each of the types discussed above,
   the attacker has to incur certain costs and undertake certain risks.
   These costs differ by attack, and can be helpful in guiding response
   to pervasive attack.

   Depending on the attack, the attacker may be exposed to several types
   of risk, ranging from simply losing access to arrest or prosecution.
   In order for any of these negative consequences to happen, however,
   the attacker must first be discovered and identified.  So the primary
   risk we focus on here is the risk of discovery and attribution.

   A passive attack is the simplest attack to mount in some ways.  The
   base requirement is that the attacker obtain physical access to a
   communications medium and extract communications from it.  For
   example, the attacker might tap a fiber-optic cable, acquire a mirror
   port on a switch, or listen to a wireless signal.  The need for these
   taps to have physical access to a link exposes the attacker to the
   risk that the taps will be discovered.  For example, a fiber tap or
   mirror port might be discovered by network operators noticing
   increased attenuation in the fiber or a change in switch
   configuration.  Of course, passive attacks may be accomplished with
   the cooperation of the network operator, in which case there is a

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   risk that the attacker's interactions with the network operator will
   be exposed.

   In many ways, the costs and risks for an active attack are similar to
   those for a passive attack, with a few additions.  An active attacker
   requires more robust network access than a passive attacker, since
   for example they will often need to transmit data as well as
   receiving it.  In the wireless example above, the attacker would need
   to act as an transmitter as well as receiver, greatly increasing the
   probability the attacker will be discovered (e.g., using direction-
   finding technology).  Active attacks are also much more observable at
   higher layers of the network.  For example, an active attacker that
   attempts to use a mis-issued certificate could be detected via
   Certificate Transparency [RFC6962].

   In terms of raw implementation complexity, passive attacks require
   only enough processing to extract information from the network and
   store it.  Active attacks, by contrast, often depend on winning race
   conditions to inject pakets into active connections.  So active
   attacks in the core of the network require processing hardware to
   that can operate at line speed (roughly 100Gbps to 1Tbps in the core)
   to identify opportunities for attack and insert attack traffic in a
   high-volume traffic.

   Key exfiltration attacks rely on passive attack for access to
   encrypted data, with the collaborator providing keys to decrypt the
   data.  So the attacker undertakes the cost and risk of a passive
   attack, as well as additional risk of discovery via the interactions
   that the attacker has with the collaborator.

   In this sense, static exfiltration has a lower risk profile than
   dynamic.  In the static case, the attacker need only interact with
   the collaborator a small number of times, possibly only once, say to
   exchange a private key.  In the dynamic case, the attacker must have
   continuing interactions with the collaborator.  As noted above these
   interactions may real, such as in-person meetings, or virtual, such
   as software modifications that render keys available to the attacker.
   Both of these types of interactions introduce a risk that they will
   be discovered, e.g., by employees of the collaborator organization
   noticing suspicious meetings or suspicious code changes.

   Content exfiltration has a similar risk profile to dynamic key
   exfiltration.  In a content exfiltration attack, the attacker saves
   the cost and risk of conducting a passive attack.  The risk of
   discovery through interactions with the collaborator, however, is
   still present, and may be higher.  The content of a communication is
   obviously larger than the key used to encrypt it, often by several
   orders of magnitude.  So in the content exfiltration case, the

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   interactions between the collaborator and the attacker need to be
   much higher-bandwidth than in the key exfiltration cases, with a
   corresponding increase in the risk that this high-bandwidth channel
   will be discovered.

   It should also be noted that in these latter three exfiltration
   cases, the collaborator also undertakes a risk that his collaboration
   with the attacker will be discovered.  Thus the attacker may have to
   incur additional cost in order to convince the collaborator to
   participate in the attack.  Likewise, the scope of these attacks is
   limited to case where the attacker can convince a collaborator to
   participate.  If the attacker is a national government, for example,
   it may be able to compel participation within its borders, but have a
   much more difficult time recruiting foreign collaborators.

   As noted above, the "collaborator" in an exfiltration attack can be
   unwitting; the attacker can steal keys or data to enable the attack.
   In some ways, the risks of this approach are similar to the case of
   an active collaborator.  In the static case, the attacker needs to
   steal information from the collaborator once; in the dynamic case,
   the attacker needs to continued presence inside the collaborators
   systems.  The main difference is that the risk in this case is of
   automated discovery (e.g., by intrusion detection systems) rather
   than discovery by humans.

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5.  Responding to Pervasive Attack

   Given this threat model, how should the Internet technical community
   respond to pervasive attack?

   The cost and risk considerations discussed above can provide a guide
   to response.  Namely, responses to passive attack should close off
   avenues for attack that are safe, scalable, and cheap, forcing the
   attacker to mount attacks that expose it to higher cost and risk.

   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 Class             | High-level mitigations                 |
   | Passive                  | Encryption, anonymization              |
   |                          |                                        |
   | Active                   | Authentication, monitoring             |
   |                          |                                        |
   | Static key exfiltration  | Encryption with per-session state      |
   |                          | (PFS)                                  |
   |                          |                                        |
   | Dynamic key exfiltration | Transparency, validation of end        |
   |                          | systems                                |
   |                          |                                        |
   | Content exfiltration     | Object encryption, distributed systems |

   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

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   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.  Information that cannot be encrypted
   should be anonymized, i.e., it should be randomized so that it cannot
   be correlated with other information.  For example, the TOR overlay
   routing network anonymizes IP addresses by using multi-hop onion
   routing [TOR].

   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, as noted above, 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 include:

   o  Aggregation: Routing sessions for many endpoints through a common
      mid-point (e.g., an HTTP proxy).  Since the midpoint appears as
      the end of the communication, individual endpoints cannot 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 [TOR].
      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

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   An encrypted, authenticated session is safe from 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 in
   products or if malware is injected later on.

   The best defense against becoming an unwitting collaborator is thus
   to end systems are well-vetted and secure.  Transparency is a major
   tool in this process [secure].  Open source software is easier to
   evaluate for potential flaws than proprietary software.  Products
   that conform to standards for cryptography and security protocols are
   limited in the ways they can misbehave.  And standards processes that
   are open and transparent help ensure that the standards themselves do
   not provide avenues for attack.

   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 [key-recovery].  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

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   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 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 are thus to regard
   participants in the protocol as potential passive attackers

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   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"
   [snowden].  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.

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6.  Acknowledgements

   o  Trammel for ideas around pervasive passive attack and mitigation

   o  Thaler for list of attacks and taxonomy

   o  Security ADs for starting and managing the perpass discussion

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7.  TODO

   o  More thorough review of problem statement documents to ensure all
      bases are covered

   o  Look at better alignment with draft-farrell-perpass-attack

   o  Better coverage of traffic analysis and mitigations

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8.  Informative References

   [pass1]    The Guardian, "How the NSA is still harvesting your online
              data", 2013, <http://www.theguardian.com/world/2013/jun/

   [pass2]    The Guardian, "NSA's Prism surveillance program: how it
              works and what it can do", 2013, <http://

   [pass3]    The Guardian, "XKeyscore: NSA tool collects 'nearly
              everything a user does on the internet'", 2013, <http://

   [pass4]    The Guardian, "How does GCHQ's internet surveillance
              work?", n.d., <http://www.theguardian.com/uk/2013/jun/21/

   [dec1]     The New York Times, "N.S.A. Able to Foil Basic Safeguards
              of Privacy on Web", 2013, <http://www.nytimes.com/2013/09/

   [dec2]     The Guardian, "Project Bullrun - classification guide to
              the NSA's decryption program", 2013, <http://

   [dec3]     The Guardian, "Revealed: how US and UK spy agencies defeat
              internet privacy and security", 2013, <http://

   [TOR]      The Tor Project, "TOR", 2013,

   [TOR1]     Schneier, B., "How the NSA Attacks Tor/Firefox Users With
              QUANTUM and FOXACID", 2013, <https://www.schneier.com/

   [TOR2]     The Guardian, "'Tor Stinks' presentation - read the full
              document", 2013, <http://www.theguardian.com/world/

   [dir1]     The Guardian, "NSA collecting phone records of millions of
              Verizon customers daily", 2013, <http://

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   [dir2]     The Guardian, "NSA Prism program taps in to user data of
              Apple, Google and others", 2013, <http://

   [dir3]     The Guardian, "Sigint - how the NSA collaborates with
              technology companies", 2013, <http://www.theguardian.com/

   [secure]   Schneier, B., "NSA surveillance: A guide to staying
              secure", 2013, <http://www.theguardian.com/world/2013/sep/

   [snowden]  Technology Review, "NSA Leak Leaves Crypto-Math Intact but
              Highlights Known Workarounds", 2013, <http://

              Golle, P., "The Design and Implementation of Protocol-
              Based Hidden Key Recovery", 2003,

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

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

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

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

   [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.

   [RFC5750]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet

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              Mail Extensions (S/MIME) Version 3.2 Certificate
              Handling", RFC 5750, January 2010.

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

              Farrell, S. and H. Tschofenig, "Pervasive Monitoring is an
              Attack", draft-farrell-perpass-attack-00 (work in
              progress), November 2013.

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Authors' Addresses

   Richard Barnes

   Email: rlb@ipv.sx

   Bruce Schneier

   Email: schneier@schneier.com

   Cullen Jennings

   Email: fluffy@cisco.com

   Ted Hardie

   Email: ted.ietf@gmail.com

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